petrogenesis of felsic igneous rocks associated with the paleoproterozoic koillismaa ... ·...

1

Petrogenesis of felsic igneous rocksassociated with the Paleoproterozoic

Koillismaa layered igneous complex, Finland

by

Laura S. LauriDivision of Geology and Mineralogy

Department of GeologyUniversity of Helsinki

P.O. Box 64FI-00014 Helsinki, Finland

ACADEMIC DISSERTATION

To be presented, with the permission of the Faculty of Science of theUniversity of Helsinki, for public criticism in the small auditorium E204 of

Physicum, Kumpula, on May 28th, 2004, at 12 o’clock noon.

Helsinki 2004

2

Gummerus Kirjapaino Oy,Saarijärvi 2004

ISBN 952-91-7207-9 (paperback)ISBN 952-10-1853-4 (PDF)

Supervisors: Prof. O.Tapani RämöDepartment of GeologyUniversity of Helsinki, Finland

Prof. Emeritus Ilmari HaapalaDepartment of GeologyUniversity of Helsinki, Finland

Reviewers: Dr. Matti VaasjokiGeological Survey of FinlandEspoo, Finland

Prof. Eero HanskiDepartment of GeologyUniversity of Oulu, Finland

Opponent: Dr. Mikko NironenGeological Survey of FinlandEspoo, Finland

cover: granophyre, sample LSL-00-83, crossed nicols.

3

Dedicated to Hannu and Siljaand the little one in prep.

4

LAURI, L. S. 2004. Petrogenesis of felsic igneous rocks associated with the PaleoproterozoicKoillismaa layered igneous complex, Finland. Academic dissertation, University of Helsinki,Finland.

The Archean-Proterozoic transition at ~2.5–2.4 Ga was marked by worldwide anorogenicmagmatism that resulted in the emplacement of layered mafic intrusions, dike swarms, andsilicic plutonic rocks, as well as extrusion of volcanic rocks. These early Paleoproterozoicrift-related rocks are found throughout the northern Fennoscandian Archean craton. In theKoillismaa area, these rocks comprise the mafic Koillismaa layered igneous complex, theKynsijärvi quartz alkali feldspar syenite, and the volcanic Sirniö Group. In the adjoiningOulanka area in northwestern Russia, a similar suite consisting of the Oulanka layered igneouscomplex, the Nuorunen granite, and the Sumian volcanic rocks is found. U–Pb agedeterminations indicate that the main magmatic evolution of the Archean complex in Koillismaatook place at 2800–2700 Ma, whereafter the area was cratonized before the earlyPaleoproterozoic rifting began. The Koillismaa layered igneous complex and the Kynsijärviquartz alkali feldspar syenite were emplaced into the Archean crust at ~2.44 Ga, and theSirniö Group is thought to be of approximately the same age. The Kynsijärvi pluton and thefelsic volcanic rocks of the Sirniö Group show A-type mineralogical and geochemicalcharacteristics such as high Fe/Mg in mafic silicates, fluorite as a conspicuous minorcomponent, high total alkali content, high Fe/Mg, Ga/Al, Zr, REE, and a negative Eu anomaly.The Sirniö Group suffered from metamorphism and hydrothermal alteration that is thought tocoincide with the emplacement of the Koillismaa layered igneous complex beneath theSirniövaara Formation of the Sirniö Group. The Nd isotope composition of the Kynsijärviquartz alkali feldspar syenite (ε

Ndi of –4.8 and T

DM of ~3000 Ma) falls close to the evolution

path of the local Archean crust [εNd

(at 2440 Ma) between –5 and –8.5 and TDM

between 2900and 3100 Ma], whereas the rocks of the Sirniö Group show values (ε

Ndi of –1.9 and T

DM

between 2800 and 2900 Ma) that are close to those acquired from the mafic layered intrusions.The Nuorunen granite shows values (ε

Ndi of –1.8 and T

DM of 2750 Ma) that may reflect the Nd

isotope composition of local Archean crust in the Oulanka area. Geochemical modelingutilizing immobile elements indicates that the early Paleoproterozoic silicic rocks could havebeen derived from a mafic precursor by an AFC process involving a high-Mg parental magmaand lower crustal contaminants.

Keywords: Archean, Proterozoic, Koillismaa, Finland, A-type, layered mafic intrusion,continental rifting, granophyre, volcanic rocks, U–Pb geochronology, Nd isotopes

Laura S. Lauri, Department of Geology, P.O. Box 64, FI-00014 University of Helsinki,Finland

5

Table of contents

Original publications............................................................................................................61. Introduction......................................................................................................................62. Previous studies................................................................................................................73. Regional geology..............................................................................................................7

3.1. Kynsijärvi quartz alkali feldspar syenite............................................................83.2. Lithostratigraphy................................................................................................93.3. Sampling and analytical methods.....................................................................10

4. Petrography.....................................................................................................................114.1. Kynsijärvi quartz alkali feldspar syenite..........................................................114.2. Sirniö Group.....................................................................................................11

4.2.1. Sirniövaara Formation........................................................................114.2.2. Unijoki Formation..............................................................................13

4.3. Nuorunen granite..............................................................................................134.4. Archean rocks...................................................................................................13

5. U–Pb geochronology......................................................................................................145.1. Kynsijärvi quartz alkali feldspar syenite..........................................................145.2. Archean rocks...................................................................................................14

6. Geochemistry..................................................................................................................156.1. Whole-rock geochemistry.................................................................................15

6.1.1. Kynsijärvi quartz alkali feldspar syenite............................................156.1.2. Sirniö Group.......................................................................................156.1.3. Nuorunen granite................................................................................176.1.4. Archean rocks.....................................................................................18

6.2. Nd isotope geochemistry...................................................................................186.2.1. Kynsijärvi quartz alkali feldspar syenite............................................186.2.2. Sirniö Group.......................................................................................186.2.3. Nuorunen granite................................................................................196.2.4. Archean rocks.....................................................................................19

6.3. Post-crystallization disturbance........................................................................196.4. Geochemical modeling.....................................................................................20

6.4.1. Boundary conditions..........................................................................206.4.2. Partial melting....................................................................................216.4.3. Fractional crystallization....................................................................226.4.4. Combined assimilation-fractional crystallization...............................25

7. Discussion.......................................................................................................................267.1. Geochronology of the Koillismaa area.............................................................267.2. The granophyre problem...................................................................................287.2. Early Paleoproterozoic volcanism in the northern Fennoscandian shield.........287.3. Early Paleoproterozoic silicic plutonism in the northern Fennoscandian

shield............................................................................................................298. Conclusions.....................................................................................................................30Acknowledgments...............................................................................................................30References...........................................................................................................................31Appendix 1..........................................................................................................................36Papers I - III

6

Original publications

The thesis is based on following articles, referred to in the text by their Roman numerals.

I: Lauri, L.S., Mänttäri, I., 2002. The Kynsijärvi quartz alkali feldspar syenite,Koillismaa, eastern Finland – silicic magmatism associated with 2.44 Ga continentalrifting. Precambrian Res. 119, 121–140.

II: Lauri, L.S., Karinen, T., Räsänen, J., 2003. The earliest Paleoproterozoic supracrustalrocks in Koillismaa, northern Finland – their petrographic and geochemicalcharacteristics and lithostratigraphy. Bull. Geol. Soc. Finland 75(1–2), 29–50.

III: Lauri, L.S., Rämö, O.T., Huhma, H., Mänttäri, I., Räsänen, J. Petrogenesis ofsilicic magmatism related to ~2.44 Ga rifting of the Archean crust in Koillismaa,eastern Finland. Lithos (submitted).

L. S. Lauri’s contribution to I included everything except U–Pb dating, which was doneby the co-author. In II, L. S. Lauri was mainly responsible for the field geology, petrography,and geochemistry of the Sirniö Group, while the co-authors handled lithostratigraphy. In III,L. S. Lauri’s contribution included field geology, whole-rock geochemistry, a part of the Ndisotope analyses, and petrological modeling.

1. Introduction

Continental extension is often characterized by an anorogenic magmatic associationcomprising layered mafic intrusions and dike swarms as well as silicic plutonic and volcanicrocks. A world-wide phase of anorogenic, extension-related magmatism occurred at theArchean-Proterozoic transition, 2.5–2.4 Ga ago. At this time, many extensive mafic complexeswere emplaced in Archean terranes, including (1) the 2.47–2.45 Ga Hearst-Matachewan dikesin North America (Heaman, 1997), (2) the 2.42 Ga Jimberlana intrusion of Australia (Parkeret al., 1987), (3) the 2.42 Ga Scourie dikes of Scotland (Heaman and Tarney, 1989), (4) the2.42 Ga high-Mg tholeiite dikes of Vestfold Block, Antarctica (Collerson and Sheraton, 1986),and (5) the 2.50–2.44 Ga layered mafic igneous complexes of northern Fennoscandia (Alapietiet al., 1990). The 2.5–2.4 Ga event has been interpreted to indicate rifting of a late Archeansupercontinent (e.g., Windley, 1995; Heaman, 1997), although it is not clear how many of theabove mentioned Archean fragments were attached to each other at this time. Paleomagneticevidence suggests that at least the Superior and Wyoming cratons of Laurentia and the Kareliancraton of Fennoscandia were joined (e.g., Pesonen et al., 2003).

This work concentrates on the silicic rocks associated with early Paleoproterozoic riftingof the Archean nucleus of the Fennoscandian shield. The rocks include a quartz alkali feldsparsyenite pluton and a group of felsic and intermediate volcanic rocks that are closely associatedwith a ~2.44 Ga mafic layered intrusion. The aim of the study was to investigate the previouslylittle known silicic part of the early Paleoproterozoic magmatic system in Koillismaa, usingfield geology, petrography, lithostratigraphy, whole-rock geochemistry, and Nd isotopegeochemistry. The origin of the granophyric rocks associated with the Koillismaa layeredigneous complex was studied in detail. In the course of the study, Nd isotope data and newU–Pb ages were also acquired for the Archean crust that hosts the early Paleoproterozoicrocks. Based on these data, the origin of the ~2.44 Ga silicic rocks of Koillismaa was discussed

7

in the papers I, II, and III. A comparison was also made to other anorogenic magmatic systemsthat contain layered mafic intrusions and granophyric rocks and to known occurrences ofearly Paleoproterozoic volcanic and plutonic rocks. The main results of the study aresummarized in this synopsis.

2. Previous studies

Geological mapping of the Koillismaa area started already at the beginning of the 20th

century and the work was completed in the 1950’s, when geological map sheets withexplanations on the 1:400 000 scale were published from the area (Enkovaara et al., 1953;Matisto, 1958). In the late 1950’s, mining companies began to take interest in the ore potentialof the “Syöte-type gabbros” and, in the 1960’s, extensive geological mapping, geophysicalsurveys, diamond drilling, and pilot plant concentration tests on disseminated sulphideoccurrences were carried out by Outokumpu Co. Vanadium was quarried from an open pitmine (Mustavaara) in one of the layered intrusions in 1976–1985 by Rautaruukki Co. In the1970’s, the Koillismaa Project of the University of Oulu, focusing mainly on the study of thebasic magmatism in the area, produced several academic dissertations (see Piirainen et al.,1978). Alapieti (1982) collected the information produced by the Koillismaa Project into aPh.D. thesis that summarized the structural, mineralogical, and geochemical aspects of theKoillismaa layered igneous complex. Two new geological map sheets on the 1:100 000 scalewere also published from the area by the Geological Survey of Finland (GTK) (Honkamo,1979; Lahti and Honkamo, 1980). In the 1990’s, M.Sc. theses were compiled at the Universityof Oulu (Karinen, 1998) and the University of Helsinki (Landén, 1999), the latter in closecollaboration with the GTK, within the framework of a research project started in Koillismaain 1996.

So far, most of the publications from the Koillismaa area have dealt with the Koillismaalayered igneous complex and its relation to other ~2.44 Ga layered intrusions in the northernFennoscandian shield (e.g., Alapieti and Piirainen, 1984; Piispanen and Tarkian, 1984; Alapietiand Lahtinen, 1989; Alapieti et al., 1990; Saini-Eidukat et al., 1997; Alapieti and Lahtinen,2002). The works of Lauerma (1982) and Kontinen et al. (1992) include some samples fromthe Archean rocks of the Koillismaa area and present some isotope data on these samples.

3. Regional geology

The Koillismaa area (Fig. 1) is a part of the Archean Karelian province of the Fennoscandianshield (Korsman et al., 1997; Koistinen et al., 2001). The oldest rocks in the area are Archeanortho- and paragneisses and small granite bodies with ages around 3.0–2.7 Ga (Gaál andGorbatschev, 1987; III). At the beginning of the Proterozoic eon, the Archean cratonexperienced intermittent rifting that began at ~2.50 Ga, resulting in widespread volcanismand plutonism (e.g., Huhma et al., 1990; I; II). In the Koillismaa area, the onset of rifting at~2.44 Ga produced a sequence of volcanic rocks (II), the mafic Koillismaa layered igneouscomplex (Alapieti, 1982), and the silicic Kynsijärvi quartz alkali feldspar syenite pluton (I).At the same time, a similar rock suite consisting of the Oulanka layered igneous complex(e.g., Lavrov, 1979), the Sumian volcanic rocks, and the Nuorunen granite (Buyko et al.,1995) was formed in the adjoining Oulanka area in present northwestern Russia (Fig. 1). Theextension continued sporadically for the next several hundred million years and a large amountof mafic magma transected the fractured bedrock, forming numerous diabase dike swarms(Vuollo, 1994). However, the overall extension never proceeded to sea-floor spreading andwas halted at an intracratonic rift or aulacogen stage. Volcanic activity and sedimentation

8

continued in the N-NW-side of the Koillismaa area, forming the ~2.3–2.0 Ga supracrustalKuusamo belt (Silvennoinen, 1991).

3.1. The Kynsijärvi quartz alkali feldspar syenite

The Kynsijärvi quartz alkali feldspar syenite pluton (I) is a small (1 km by 5 km), EW-elongated intrusion that was emplaced in the Archean gneiss complex near the Kuusijärvi

AB

C

1 2

3

4

5

6

7

8

9

10

Proterozoic Lapland granulite complex

White Sea

Barents Sea

Gulf of Bothnia

Russia

FinlandSweden

Norway

Arctic circle

26°E

0 100 km

Archean rocks 2.50-2.44 Ga mafic layered intrusions

Proterozoic plutonic rocks

Proterozoic metasedimentary and metavolcanic rocks

Mesoproterozoic sedimentary rocks (1.4-1.3 Ga)

Caledonides

Paleozoic alkaline intrusions

2.44 Ga silicic intrusions:

A. Kynsijärvi quartz alkali feldspar syeniteB. Nuorunen graniteC. Kuhmo granites

1. Kukkola-Tornio2. Kemi3. Penikat4. Portimo5. Koillismaa6. Oulanka7. Koitelainen8. Akanvaara9. Monchegorsk10. Imandra11. Fedorova and Panski Tundra

national boundary

66.5 N°

N

11

Fig. 2

Kola

Belo

moria

n

Karelian

terrane boundary

Fig. 1. General bedrock map of the northern Fennoscandian shield and locations of the early Paleoproterozoicmafic and silicic intrusions (modified after Alapieti and Lahtinen, 1989; Korsman et al., 1997; Koistinen et

al., 2001).

9

and Lipeävaara blocks of the Koillismaa layered igneous complex (Fig. 2). The intrusion isvery poorly exposed and the size estimate is based on low altitude aeromagnetic maps anddiamond drilling that was carried out by the GTK in 1997. No contacts are exposed but bothArchean gneisses and gabbroic rocks were encountered in the drill cores close to the presumedcontact of the pluton.

3.2. Lithostratigraphy

The supracrustal rocks in the study area are divided into three groups, the oldest of whichprobably predates the layered intrusion. The other two groups are slightly younger but theage difference may be negligible. The lithostratigraphy (Fig. 3) is described in detail in II.

The oldest supracrustal sequence, the Sirniö Group, consists of two volcanic formations.The Sirniövaara Formation is up to several hundred meters thick, fine-grained, and on outcropscale relatively homogeneous, and consists of plagioclase crystal tuff with a granophyricgroundmass. Some breccia layers are also found within the unit. The overlying UnijokiFormation consists of several felsic and intermediate volcanic units that lack granophyrictexture. The internal stratigraphy of the Unijoki Formation cannot be investigated in detail as

N

0 5 10 Kilometers

Archean

granite gneiss

supracrustal rocks

granulite

Other

diabase dikes

fault

Proterozoic

< 2.44 Ga supracrustal rocks

Unijoki Formation

Sirniövaara Formation

quartz alkali feldspar syenite

gabbro (age unknown)

Koillismaa layered igneous complex

anorthosite

magnetite gabbro

olivine gabbronorite

gabbronorite

microgabbronorite

marginal series

Ab-Qtz-rock

Syöte

Porttivaara

Pyhitys

Kuusijärvi

LipeävaaraKynsijärvi

Kaukua

Fig. 2. General bedrock map of the western intrusion of the Koillismaa layered igneous complex and associatedrocks (modified after Alapieti, 1982; Räsänen et al., 2003).

10

the units are generally poorlyexposed. The Sirniövaara Formationcan be found overlying all the blocksof the Koillismaa layered igneouscomplex, whereas the rocks of theUnijoki Formation are observed in afew locations only in the study area(Fig. 2).

A thick conglomerate begins thesuccession in the Kynsijärvi Groupthat overlies the Sirniö Group. TheKynsijärvi Group that comprises thesedimentary Unikumpu Formationand the volcanic KuusijärviFormation has been tentativelycorrelated to the lowermost units in

the nearby Kuusamo belt (Juopperi, 1976; Karinen, 1998). The Unikumpu Formationcorresponds to the basal conglomerate of the Kuusamo belt. The Kuusijärvi Formation thatconsists of the hyaloclastic breccias of the Kivivaara Member and the basaltic flows of theLöytövaara member, resembles the Greenstone Formation I of the Kuusamo belt describedby Silvennoinen (1991) (cf., Karinen, 1998).

The youngest supracrustal unit in the study area is the Hautavaara Group that comprisesthe conglomerates of the Pajuniemi Formation and the quartzites of the LamminlehtoFormation. The Pajuniemi conglomerate contains abundant clasts from the underlyingKuusijärvi Formation, but also some fragments of the granophyres of the Sirniövaara Formationhave been found (Karinen, 1998). The quartzites of the Lamminlehto Formation grade fromorthoquartzites to subarkosic quartzites. Pelitic interlayers are found in the upper parts of theformation.

3.3. Sampling and analytical methods

The rocks of the Sirniö Group were sampled for whole-rock chemistry during the mappingof the study area in 1998–2000. Grab samples were collected from the outcrops. Diamonddrilling was also carried out at several locations (II) and the drill cores were logged andsampled. Most of the sampling was done by the author, additional samples were taken by T.Karinen and J. Räsänen. The sampling of the Kynsijärvi quartz alkali feldspar syenite wasdone mostly from drill cores recovered in 1997. A total of 216 whole-rock chemical sampleswere analyzed. Of these, 118 were from the Sirniövaara Formation, 74 from the UnijokiFormation (44 felsic and 30 intermediate volcanic rocks), 15 from the Kynsijärvi quartzalkali feldspar syenite, 6 from the Archean rocks in Koillismaa, and 3 from the Nuorunengranite near the Oulanka layered complex in Russia (Fig. 1; III). Polished thin sections wereprepared from most of the samples and petrographic and mineralogical determinations weremade from them (I, II). The whole-rock chemical analyses were made by the XRF method.Selected samples were also analyzed for the REE, Sc, Th, U, and Y by the ICP-MS method.The 29 samples analyzed for Nd isotope composition were chosen mostly on the basis ofwhole-rock chemistry (III). For U–Pb geochronology, one sample was analyzed from the

Archeanbasement complex

Koillismaa layeredigneous complex

Sirniö Group

Kynsijärvi Group

Hautavaara Group


Unijoki Formation

Unikumpu Formation

Kivivaara member

Löytövaara member

Kuusijärvi Formation

Pajuniemi Formation

Lamminlehto Formation

siltstone member

quartzite member

Kynsijärvi quartz alkalifeldspar syenite

Fig. 3. Lithostratigraphic column of thePaleoproterozoic supracrustal rocks ofKoillismaa (II).

11

Kynsijärvi quartz alkali feldspar syenite (I) and six from the surrounding Archean rocks inKoillismaa from which previous data was insufficient (III). Whole-rock chemical analyseswere made at the Geolaboratory of the GTK, the electron microprobe work was carried out atthe mineralogical laboratory of the GTK, and the U–Pb geochronological and Nd isotopeanalyses were performed at the isotope laboratory of the GTK. The analytical methods aredecribed in detail in I, II and III.

4. Petrography

4.1. Kynsijärvi quartz alkali feldspar syenite

A detailed petrographic description and microprobe data on the Kynsijärvi quartz alkalifeldspar syenite is presented in I. The quartz alkali feldspar syenite is red, medium-grained,and leucocratic. It is also isotropic, homogeneous, and undeformed with the exception of onedrill core that shows some brittle deformation. The dominant mineral is hypersolvus alkalifeldspar, the modal amount of which is 72.1 to 84.4 vol.%. Other major minerals are quartzand ferro-edenite. Accessory minerals include magnetite, titanite, zircon, fluorite, andsubsolidus stilpnomelane. The feldspar is a subhedral to euhedral mesoperthite with well-developed intergranular albite rims (Fig. 4a). The orthoclase and albite components of thefeldspar have exsolved quite effectively so that the amount of Or-component in the orthoclaseis ~98% and the Ab-component in the albite is >99%. An-component is negligible.

Quartz was crystallized in two stages. The first generation is present as euhedral shortprismatic quartz. The second generation is found as anhedral crystals in the interstices of thefeldspar crystals. The subhedral ferro-edenite is also interstitial relative to the feldspar and iscommonly associated with magnetite and titanite. The calculated Fe/(Fe+Mg) of the ferro-edenite is close to 0.9 (I). In the eastern parts of the pluton the rock is unaltered and undeformed,whereas in the western parts brittle deformation is found and the feldspar crystals are fracturedand ferro-edenite has been altered to stilpnomelane.

4.2. Sirniö Group

4.2.1. Sirniövaara Formation

The Sirniövaara Formation (II) consists of fine-grained, gray rhyodacite that is relativelyhomogeneous on outcrop scale. Weak to moderate schistosity can be observed in most outcropsand small shear zones are occasionally present. Tension veins and joints filled with quartz,epidote, chlorite, carbonate, and sulphides are also commonly found. The veins vary in widthfrom a few millimeters to several meters.

In thin section, the rhyodacite is microporphyritic with subhedral to euhedral plagioclasephenocrysts in a fine-grained groundmass that often shows a granophyric intergrowth texturebetween quartz and albite (Fig. 4b, c). The major rock forming minerals are plagioclase,quartz and biotite. Accessory minerals include chlorite, K-feldspar, zircon, apatite, fluorite,carbonate, epidote, clinozoisite, sericite, oxides, titanite, and sulphides. At least one generationof schistosity is seen in most thin sections. Thin veinlets of quartz, epidote, and carbonate arealso commonly present in the more altered samples.

Plagioclase phenocrysts are blocky or elongated, up to 2 mm in length. They are twinnedaccording to albite, Carlsbad, Baveno or pericline laws, the two first being the most common.Some of the crystals have been broken into several pieces. Most of the phenocrysts are albiteand some sericitisation and saussuritisation is commonly observed.

12

Groundmass plagioclase is anhedral albite that typically forms granophyric intergrowths withquartz. In most samples, the granophyric intergrowths form irregular radiating fringes aroundthe albitic phenocrysts and the granophyric albite shares crystallographic orientation with thealbite in the phenocrysts (Fig. 4b, c). In the more altered samples, a vermicular granophyricintergrowth is seen at the margins of larger quartz patches (Fig. 4d). In deformed samples, thegranophyric intergrowth is replaced by a fine-grained groundmass of granoblastic albite andquartz.

Fig. 4. Photomicrographs of (a) the Kynsijärvi quartz alkali feldspar syenite, crossed polars. Afs, alkalifeldspar; Q

1, primary quartz; Q

2, second generation quartz; Ab, albite; Hbl, ferro-edenite; F, fluorite; Mgt,

magnetite; S, stilpnomelane (I), (b) and (c) euhedral plagioclase phenocrysts in a groundmass that shows agranophyric texture of irregular radiating fringe type, samples LSL-98-18 (b) and R333 17.45 m (c), crossedpolars, (d) anhedral quartz crystal with vermicular granophyric intergrowths of albite, sample LSL-00-65,crossed polars, (e) felsic lava, sample LSL-98-112, crossed polars, (f) felsic tuffite, sample R336 56.90 m,crossed polars.

13

Brown biotite forms aggregates of small, randomly oriented crystals that commonly containsmall zircon inclusions surrounded by pleochroic haloes. Biotite compositions fall betweensiderophyllite and annite and show a trend toward phlogopite in the altered samples. Fe/Mgratio varies between 0.5 and 0.9. Green, Fe-rich chlorite forms lamellae in the biotite crystals.In the lower parts of the formation and in local shear zones, biotite is almost completelyreplaced by chlorite.

The most common oxide mineral is euhedral to subhedral ilmenite, but magnetite is alsofound. Some of the ilmenite crystals are skeletal. The oxides commonly have a very fine-grained rim of secondary titanite. Titanite also fills the interstices of the skeletal ilmenites.Apatite is found as long, thin, broken needles. Zircon occurs as small, euhedral, turbid crystals,whereas fluorite is found as small, anhedral crystals. Sulphides are present as small, anhedralto euhedral grains both disseminated through the rock and in quartz veinlets. Epidote andclinozoisite are found as anhedral to euhedral crystals of varying size, epidote also in veins.Anhedral carbonate also occurs as vein fillings and replaces other minerals, mostly plagioclase.Sericite is found as small, subhedral to euhedral crystals oriented along the plains of schistosity.

4.2.2. Unijoki Formation

The rocks of the Unijoki Formation (II) form a heterogeneous group of felsic to intermediatevolcanic rocks that mostly consist of pyroclastic or epiclastic material. Some lavas are alsofound, often containing fragments of coarse-grained plagioclase porphyry. Textures such asaccretionary lapilli and spherulites are present in some units.

The felsic varieties commonly have plagioclase phenocrysts in a fine-grained groundmassfrom which the granophyric intergrowth texture is lacking (Fig. 4e, f). The groundmass mainlyconsists of quartz, albite, and biotite. Schistosity is usually present and some samples alsoshow textures that to some degree resemble bedding and cross-bedding.

In the intermediate varieties, the major minerals are plagioclase, amphibole, epidote, andquartz. Accessory minerals include carbonate, biotite, chlorite, titanite, hematite, scapolite,and zircon. The intermediate rocks are interpreted to be porphyritic lavas that contain relicsof amphibole and plagioclase phenocrysts. The plagioclase is pervasively saussuritized andoften replaced by scapolite. The amphiboles commonly have a light green core of tremolite-actinolite and a darker green rim of hornblende. Twinning is seen in some of the amphibolecrystals.

4.3. Nuorunen granite

The description of the Nuorunen granite is based on Hackman and Wilkman (1929) and onthe three samples used in the Nd isotope work (III). The pluton consists of a somewhatdeformed pink, and medium- to coarse-grained granite. Associated with the main intrusionare quartz porphyry dikes that resemble the granite in mineral composition and geochemistry.The major minerals are microperthitic alkali feldspar, quartz, plagioclase, and amphibole.Accessory minerals include zircon, fluorite, oxides, allanite, and biotite. Micrographicintergrowths of quartz and alkali feldspar are common. The amphiboles are partly replacedby chlorite and secondary minerals such as sericite, stilpnomelane, and carbonate are alsocommon.

4.4. Archean rocks

The main rock types in the Archean crust of Koillismaa are medium- to coarse-grained,variably deformed gneisses and granitoids, although some small occurrences of greenstones

14

have also been found. Gneisses comprise both ortho- and paragneisses the latter of whichhave been strongly migmatized. Amphibolite fragments are found enclosed in the gneisses insome locations. The granitoids commonly occur as small, irregular bodies among the gneisses.Major minerals in the samples used for isotope geology in III are quartz, plagioclase, andalkali feldspar. The mafic minerals include biotite, hornblende, and in some casesclinopyroxene and orthopyroxene. Magnetite, sericite, epidote, zircon, and monazite arecommon as accessory phases.

5. U–Pb geochronology

5.1. Kynsijärvi quartz alkali feldspar syenite

Five zircon fractions were analyzed from the Kynsijärvi quartz alkali feldspar syenite(Fig. 5). Three of these were air-abraded. The zircon crystals were transparent and almostcolorless prisms with pyramidal edges. In BSE images they show typical magmatic zoningwith no evidence of inherited cores or metamorphic growths. The analytical results arepresented in I. The five-point discordia line calculated from the analyses interceps the concordiacurve at 2445±7 Ma and 415±70 Ma with a relatively high MSWD of 4.3. Ignoring thestrongly discordant point E the intercept ages are 2442±3 Ma and 341±47 Ma (MSWD = 1.8,n = 4). The upper intercept is interpreted as a close approximation of the emplacement age ofthe rock.

5.2. Archean rocks

Of the six sampleschosen for the U–Pbgeochronology from theArchean rocks inKoillismaa, two weregranulite facies gneissesand four wereorthogneisses and granitesthat, according to field

observations, represent the youngest rock types in the area. The details of the zircon andmonazite fractions analyzed from the samples are found in III. The zircon fractions weremostly discordant and showed evidence of mixing of different age populations (Fig. 6).Tentative zircon ages were acquired from only two samples. The Aholamminvaara granite(sample A1656) yielded a five-point discordia line that intercepts the concordia curve at2711±9 Ma and 270±450 Ma (MSWD = 2, n = 5). The Kaplaskumpu pyroxene tonalite(sample A1661), a granulite facies gneiss, gave an upper intercept age of 2808±20 Ma (MSWD= 3.9, n = 4). The four analyzed monazite fractions, all from different samples, gave concordantages close to 2695 Ma, regardless of the geological unit they were analyzed from. This age isinterpreted as the peak of the granulite facies metamorphism and, possibly, the emplacementage of the granites.

MSWD = 1.8; n=4

D

Intercepts (A,B,C,D) at

2441.9 ± 2.4 & 341 ± 47 Ma

A

B

C

E

A1585 Kynsijärvi quartzalkali feldspar syenite

TIMS U-Pb age data on zircons

U8

20

6P

b/2

3

6.5 7.5 8.5 9.5 10.5

207Pb/

235U

0.32

0.36

0.40

0.44

0.48

2100

2200

2300

2400

2500 Fig. 5. Concordia diagram for thefive analyzed zircon fractionsfrom the Kynsijärvi quartz alkalifeldspar syenite. Ellipses denotethe 2σ errors in the variables (I).

15

6. Geochemistry

Whole-rock chemical description of the Kynsijärvi quartz alkali feldspar syenite is foundin I and the geochemistry of the Sirniö Group is described in detail in II. This short compilationof the geochemistry of all the studied rock types is based on I and II, as well as (for thesamples chosen for isotope geology) III. Geochemical description of the Nuorunen graniteand the Archean samples utilized in the Nd isotope studies is also found in III.

6.1. Whole-rock geochemistry

6.1.1. Kynsijärvi quartz alkali feldspar syenite

The Kynsijärvi quartz alkali feldspar syenite pluton is geochemically quite homogeneousand shows A-type characteristics in the sense of Loiselle and Wones (1979), and Eby (1990).Its SiO

2 varies between 70.1 and 71.7 wt.% and the total amount of alkalies is higher than 9.9

wt.% with Na/K > 1 in all samples. The rock is metaluminous with an average A/CNK valueof 0.96 (Fig. 7a). Fe/Mg is high [FeO*/(FeO* + MgO) > 0.9; Fig. 7b] and the amounts ofMgO, CaO, and TiO

2 are quite low (~0.1, ~0.6, and ~0.2 wt.%, respectively).

Trace element data show high average values of Ba, Zr, Zn, Ga, and Nb and low values ofSc, V, Ni, and Co, most of which were under detection limit (I). Primitive mantle-normalizeddiagrams show deep negative anomalies of Sr, P, and Ti (Fig. 8a). Light REE are enrichedcompared to heavy REE with average (La/Yb)

N of 12.7 (Fig. 9a). A negative Eu-anomaly is

also present and Eu/Eu* averages at 0.28.

6.1.2. Sirniö Group

The rocks of the Sirniö Group can be divided into three geochemical groups. The SirniövaaraFormation is relatively homogeneous when compared to the rocks of the Unijoki Formation;

206

238

Pb

/U

207 235Pb/ U

0.56

0.52

0.48

0.4410 11 12 13 14 15

2500

2600

2700

2800

A1656 Aholamminvaara granite

A1657 Harjavaara granite

A1656A monaziteA1657E monazite

A1656 Intercepts at

270±450 & 2711±9 Ma

MSWD = 2, n = 5

A1657C

A1656E

A1656BA1657D

A1657BA1657A

A1656H

A1656CDFG

a)

A1661 Kaplaskumpu pyroxene tonalite0.57

0.55

0.53

0.51

0.49

0.47

20

62

38

Pb

/U

207 235Pb/ U

13.0 13.4 13.8 14.2 14.6 15.0 15.4

2820

2780

2740

2700A1661D monazite

A1661B

A1661EA1661A

A1661C

Intercepts at

702 850 & 2808 20 Ma

MSWD = 3.9, n = 4

± ±

c)

A1662 Matovaara tonalite0.57

0.55

0.53

0.51

0.49

0.47

20

623

8P

b/

U

207 235Pb/ U

13.0 13.4 13.8 14.2 14.6 15.0 15.4

2820

2780

2740

2700

A1662E monazite

A1662B

A1662C

A1662D

A1662A

d)

20

62

38

Pb

/U

207 235Pb/ U

0.54

0.50

0.46

0.42

0.38

0.34

0.307 9 11 13 15

A1642 Raatekalliot granite

A1644 Hanhimännikkö gneiss

2200

2300

2400

2500

2600

2700

2800

A1642C

A1642A

A1644A

A1644C

A1642B

A1644BA1644 Reference line

650 & 2650 Mab)

Fig. 6. Concordia diagrams for analysed zircon and monazite fractions from the Archean samples: (a) A1656Aholamminvaara granite (black ellipses) and A1657 Harjavaara granite (open ellipses), (b) A1642 Raatekalliotgranite and A1644 Hanhimännikkö gneiss, (c) A1661 Kaplaskumpu pyroxene tonalite, (d) A1662 Matovaaratonalite. Monazite fractions are marked with gray ellipses in a), c) and d). Ellipses denote the 2σ errors inthe variables (III).

16

the latter can be roughly divided intoa “felsic group” with SiO

2 > 63 wt.%

and an “intermediate group” withSiO

2 < 63 wt.%. The Sirniö Group as

a whole seems to follow a subalkalinedifferentiation trend (Fig. 10; cf.Winchester and Floyd, 1977). Therocks also show A-typecharacteristics, though not as stronglyas the Kynsijärvi quartz alkalifeldspar syenite.

The Sirniövaara Formation iscomposed of rhyodacitic rocks withSiO

2 values ranging from 64.4 to 70.4

wt.%. Many of the major elementsshow quite a variation but, throughoutthe unit, only TiO

2 (0.8–1.0 wt.%) and

Al2O

3 (10.6–13.4 wt.%) show trends that may indicate fractional crystallization (II). The

rocks are subalkaline and the less altered samples are metaluminous with A/CNK < 1.1 (Fig.7a). Fe/Mg is high in the less altered samples [FeO*/(FeO* + MgO) ~0.9; Fig. 7b]. P

2O

5

values concentrate around 0.3 wt.% and MnO values around 0.1 wt.%.Typical features in the trace element composition of the Sirniövaara rhyodacite are, e.g.,

relatively high average Zr value of ~270 ppm and low values of compatible elements such asCr and Ni (under the detection limit in most samples; II). Nb shows a clear negative anomalyin the primitive mantle-normalized diagrams (Fig. 8b). The Ga/Al is high (10000*Ga/Albetween 3 and 6). Rare earth element patterns show enrichment of light REE compared toheavy REE with average (La/Yb)

N of 7.4 (Fig. 9b). Most samples show a negative Eu anomaly

(Eu/Eu* between 0.45 and 0.85).The SiO

2 content ranges from 63.1 to 79.1 wt.% in the felsic group of the Unijoki Formation.

The rocks are subalkaline and metaluminous to peraluminous with A/CNK between 0.7 and1.9 (Fig. 7a). Fe/Mg is generally high [average FeO*/(FeO* + MgO) ~0.9] except for a fewsamples with values close to 0.6 (Fig. 7b). Trace element patterns are quite similar to theSirniövaara Formation with an average Zr content of 276 ppm and a negative Nb anomaly ina primitive mantle-normalized diagram (Fig. 8c). REE patterns (Fig. 9c) show enrichment oflight REE over heavy REE [average (La/Yb)

N ~9.2] and a negative Eu anomaly (Eu/Eu*

between 0.52 and 0.85).In the intermediate group of the Unijoki Formation, the SiO

2 content ranges from 50.7 to

60.9 wt.%. Fe/Mg is lower than in the felsic group [FeO*/(FeO* + MgO) between 0.55 and0.86; Fig. 7b]. Compatible trace elements (Cr, V, Ni, Zn) show higher values than in the felsicgroup, while the Zr values are lower, commonly close to 100 ppm. These rocks also show anegative Nb anomaly (Fig. 8c). The intermediate group shows some enrichment of light REEwith average (La/Yb)

N of ~9.0 (Fig. 9c). A small negative Eu anomaly is present (Eu/Eu*

between 0.63 and 1.00).

0.0

1.0

2.0

Metaluminous

Peraluminous

A/C

NK

Sirniövaara FormationUnijoki Formation felsicUnijoki Formation intermediateKynsijärvi quartz alkali feldspar syeniteNuorunen graniteArchean granitoids

a)

FeO

*/(

FeO

*+

MgO

)

0.6

0.7

0.8

0.9

1.0 b)

ferroan

magnesian

50 60 70 80SiO2

0.5

Fig. 7. Composition of the Archean granitoidsand the 2.44 Ga rocks of Koillismaa plotted in(a) A/CNK vs. SiO

2, and (b) FeO*/

(FeO*+MgO) vs. SiO2 diagrams. The line

separating the ferroan and magnesian fields inb) is from Frost et al. (2001).

17

6.1.3. Nuorunen granite

The Nuorunen granite is characterized by SiO2 contents of 75.1–75.7 wt.%. Like the

Kynsijärvi pluton, it shows A-type geochemical characteristics. The pluton is subalkalineand metaluminous with A/CNK ~0.99 (Fig. 7a). Fe/Mg is quite high [average FeO*/(FeO* +MgO) ~0.9; Fig. 7b]. Trace element data show Zr values close to 300 ppm and a small negativeNb anomaly in the primitive mantle-normalized diagrams (Fig. 8a). Ga/Al is high (10000*Ga/Al > 3) and (La/Yb)

N is ~19.0 in the granites of the pluton and 9.4 in an associated quartz

porphyry dike (Fig. 9a). The negative Eu anomaly varies from 0.32 in the granites of thepluton to 0.13 in the dike.

1000

100

10

1

0.1

Sam

ple

/Pri

mit

ive

man

tle

Rb Ba Th U Nb K La Ce Pb Pr Sr P Nd SmZr Eu Ti Gd TbDy Ho Er YbLuTmY

Sirniövaara Formation1000

100

10

1

0.1

Sam

ple

/Pri

mit

ive

man

tle


Kynsijärvi quartzalkali feldspar syenite

Nuorunen granite

1000

100

10

1

0.1

Sam

ple

/Pri

mit

ive

man

tle


Archean granitoids

under detectionlimit

1000

100

10

1

0.1

Sam

ple

/Pri

mit

ive

man

tle


Felsic

Intermediate

Unijoki Formation

a) b)

c) d)

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sam

ple

/C1

Ch

on

dri

te

1

10

100

1000b) Sirniövaara Formation

Sam

ple

/C1

Ch

on

dri

te


1

10

100

1000

Kynsijärvi quarz alkalifeldspar syenite

Nuorunen granite

a)

Sam

ple

/C1

Ch

on

dri

te


1000

100

10

1

0.1

Archean granitoidsd)

heavy REEunder detection limit

1

10

100

1000

Sam

ple

/C1

Ch

on

dri

te


c)

FelsicIntermediate

Unijoki Formation

Fig. 8. Primitive mantle-normalized trace elementdiagrams for (a) theKynsijärvi quartz alkalifeldspar syenite (grayfield = all samples) andthe Nuorunen granite,(b) the SirniövaaraFormation (gray field =all samples), (c) thefelsic (clear field = allsamples) and inter-mediate rocks (gray field= all samples) of theUnijoki Formation, and(d) the Archean rocks.The samples chosen forthe diagram were used inNd isotope determin-ations (III). Normalizingvalues after Sun andMcDonough (1989).

Fig. 9. C1 chondrite-normalized REE diagrams for (a) the Kynsijärvi quartz alkali feldspar syenite (grayfield = all samples) and the Nuorunen granite, (b) the Sirniövaara Formation (gray field = all samples), (c)the felsic (clear field = all samples) and intermediate rocks (gray field = all samples) of the Unijoki Formation,and (d) the Archean rocks. The samples chosen for the diagram were used in Nd isotope determinations(III). Normalizing values after Sun and McDonough (1989).

18

6.1.4. Archean rocks

The Archean rocks form aheterogeneous geochemical group, inwhich the SiO

2 content varies from 65

to 75 wt.%. The samples with SiO2 <69

wt.% are metaluminous whereas thosewith a higher SiO

2 content are

peraluminous (Fig. 7a). Some common features are, e.g., the generally low Fe/Mg ratio thatplaces the Archean rocks in the magnesian field of Frost et al. (2001); only one graniticsample (A1657) plots into the ferroan field (Fig. 7b). The Archean rocks also show a deepnegative Nb anomaly (Fig. 8d), and strong enrichment of light REE compared to heavy REE(Fig. 9d). The granitic samples A1656 and A1657 have heavy REE contents under detectionlimit. Eu anomaly is very small in most samples.

6.2. Nd isotope geochemistry

6.2.1. Kynsijärvi quartz alkali feldspar syenite

Four samples from the Kynsijärvi quartz alkali feldspar syenite were analyzed for Ndisotopes (III). The concentrations of Sm (~6.8 ppm) and Nd (~41.0 ppm), and the 147Sm/144Nd ratio (~0.10) are quite uniform in three of the samples, whereas the fourth sampleshowed markedly lower concentrations of Sm (3.6 ppm) and Nd (16.7 ppm) and a higher147Sm/144Nd (~0.13). The ε

Nd value of –4.8 of three samples is considered as representative of

initial composition of the pluton and the time-integrated evolution of these samples (TDM

~3000 Ma) falls close to the evolution path of the local Archean crust (Fig. 11). In the fourthsample, the initial ε

Nd value (–7.5) and the T

DM (3433 Ma) were clearly anomalous, probably

because of subsequent metamorphic disturbance.

6.2.2. Sirniö Group

Of the 14 samples analyzed for Nd isotopes from the Sirniö Group, nine were from theSirniövaara Formation, three from the felsic unit, and two from the intermediate unit of theUnijoki Formation (III). The Sm concentrations vary from 6.1 to 8.8 ppm in the felsic samples(Sirniövaara and Unijoki Formations) and are around 3.5 ppm in the intermediate samples.The corresponding Nd concentrations are 31.1–46.5 ppm in the felsic rocks and 16.0 ppm inthe intermediate rocks and the 147Sm/144Nd ratios are 0.12 and 0.13, respectively. The preciseage of the volcanic rocks is unknown but the ε

Nd values were calculated at 2440 Ma. Three

U–Pb zircon analyses from an old sample from the Sirniövaara Formation (GTK, unpubl.)scatter randomly, but plot on the concordia diagram within the field of samples from theKoillismaa layered igneous complex (Alapieti, 1982) and are thus not inconsistent with aPaleoproterozoic, possibly 2.5–2.4 Ga age (H. Huhma, pers. comm., 2004). The rocks of theSirniö Group show average ε

Nd values of –1.9 (felsic rocks) and –2.5 (intermediate rocks)

.001 0.01 0.1 1 1040

50

60

70

80

Rhyolite

Rhyodacite-

Dacite

Andesite TrAn

Sub-Ab

AbBas-Trach-Neph

Com/Pan

Trachyte

Phonolite

Zr/TiO *0.00012

SiO

2

Kynsijärvi quartz alkalifeldspar syenite

Nuorunen granite

Fig. 10. The Sirniö Group follows a subalkalinedifferentiation trend in the classification diagramof Winchester and Floyd (1977). The Kynsijärviquartz alkali feldspar syenite and the Nuorunengranite are included for comparison. Symbols asin Fig. 7.

19

and TDM

ages between2800 and 2950 Ma(Fig. 11). One felsicsample from theUnijoki Formation andtwo samples from theSirniövaara Formationshow anomalousvalues compared to theother samples.

6.2.3. Nuorunen granite

The three samples from the Nuorunen granite have quite high elemental concentrations ofSm (9–10 ppm) and Nd (50–67 ppm) and the 147Sm/144Nd ratio is relatively low, close to 0.1.The average ε

Nd (at 2440 Ma) value is –1.8 and T

DM age is ~2750 Ma (Fig. 11). These values

differ from the Archean crust of Koillismaa but may reflect the (presumably different) Ndisotope composition of the local Archean crust in Oulanka area (III).

6.2.4. Archean rocks

The Archean rocks show varying contents of Sm (between 0.9 and 6.6 ppm) and Nd(between 8.7 and 64.2 ppm) and 147Sm/144Nd ratios between 0.059 and 0.121. The ε

Nd (at 2700

Ma) values of most samples range from –3.8 to –1.3 and TDM

ages are between 2900 and 3100Ma (Fig. 11). The one sample with a markedly higher ε

Nd (at 2700 Ma) value of +0.9 and

lower TDM

age of 2800 Ma was taken close to the Oulanka complex and Nuorunen granite.

6.3. Post-crystallization disturbance

The rocks of the Sirniö Group were metamorphosed in at least greenschist facies conditions,possibly clearly after the 2.44 Ga event. In the lower parts of the Sirniövaara Formation, thedegree of metamorphism may have reached upper amphibolite facies during the emplacementof the Koillismaa layered igneous complex. The vermicular granophyre intergrowths andstrong alteration of biotite to chlorite are found in those samples that also show geochemicalevidence of hydrothermal alteration (II). The alkalies seem to have been mobile in theSirniövaara Formation, as demonstrated by the varying contents of Na

2O (2.5–5.0 wt.%) and

K2O (0.2–4.0 wt.%). Higher than average values of MgO and CaO are also observed in the

samples that show evidence of alteration in thin section. The LILE such as Rb (15–134 ppm),Sr (14–297 ppm), and Ba (81–1087 ppm) seem to have followed the alkalies and also showscattered values (Fig. 8b). In general, the REE contents appear to be little affected by the

23002800 2600 2400

-12

-9

-6

0

3

Age/(Ma)

eNd

CHUR

DM (DePaolo, 1981)

Sirniövaara FormationUnijoki Formation:FelsicIntermediate

Kynsijärvi quartzalkali feldspar syeniteNuorunen graniteArchean granitoids

-3

2900 2700 2500

Koillismaa and Oulankalayered intrusions

disturbed samples

LSL-00-65

LSL-00-83

Kynsi

368-TTK-00

A1644

Archean gneisses andgranulitescalculated at 2800 Ma

Archean granitoidscalculated at 2700 Ma

Evolution of the Archean samples

in Koillismaa

Fig. 11. εNd

vs. age diagramfor the Archean granitoidsand the 2.44 Ga silicic rocksof Koillismaa. DM is theevolution of the depletedmantle by DePaolo (1981a).CHUR denotes thechondritic uniform reservoir(DePaolo and Wasserburg,1976) (III).

20

metamorphic processes, but Nd isotope data indicate that some of the samples (LSL-00-65,LSL-00-83; Fig. 11) have experienced disturbance of the magmatic Sm–Nd system (III).

Evidence of alteration is also seen in the rocks of the Unijoki Formation. The highest SiO2

values in the felsic group are probably caused by secondary silicification. The alkalies haveprobably also been mobile, as they show scattered values in both felsic and intermediaterocks. The alteration of plagioclase to scapolite in the intermediate group also suggests theinfluence of a Na-bearing fluid. The REE seem relatively unaffected also in the rocks of theUnijoki Formation, but at least one felsic sample (368-TTK-00; Fig. 11) shows evidence of adisturbed Sm–Nd system (III).

The only petrographic evidence of post-crystallization disturbance in the Kynsijärvi quartzalkali feldspar syenite is the replacement of ferro-edenite by stilpnomelane and the anomalousNd isotope systematics of sample Kynsi (Fig. 11). The anomalous values of ε

Nd (–7.5) and

TDM

(3433 Ma) are probably the result of Sm/Nd fractionation at ~1.8–1.9 Ga (III). Theappearance of stilpnomelane may be a subsolidus phenomenon associated with the formationof the intergranular albite rims – this probably happened during the cooling of the pluton andis unrelated to the process that caused the disturbance of the Sm–Nd system.

6.4. Geochemical modeling

6.4.1. Boundary conditions

Geochemical models outlined in III are expanded here to include assessment of the originof the involved mafic magmas in the subcontinental mantle. The modeling was focused onthe Sirniövaara Formation andthe Kynsijärvi quartz alkalifeldspar syenite, as these are thelargest and geochemically mostuniform units in the area. Forthe generation of thePaleoproterozoic silicic rocks,at least three possible scenariosexist. Considering the largeamount of magma that formedthe layered mafic intrusions at~2.44 Ga it would be possible(in terms of volume) tofractionate relatively smallamounts of silicic melts fromthem. The surrounding Archeancrust could also have been asource for the silicic rocks; inthis scenario the mafic magmaswould have acted as the heatsource. The third option is acombined assimilation-fractional crystallization (AFC)process.

The Nd isotope compositionof the Archean and

Table 1. Source rock compositions used in the partialmelting modeling of the Archean crust in Koillismaa

Average lower crust Pyroxene tonalite A1661Mineral Source modea Melt modeb Source modec Melt moded

Hornblende 0.05 0.06 0.15 0.166Biotite 0.10 0.13 0.10 0.11Plagioclase 0.29 0.32 0.40 0.20Alkali feldspar 0.007 0.02 0.02 0.06Clinopyroxene 0.05 0.03 0.03 0.005Orthopyroxene 0.29 0.16 0.035 0.001Quartz 0.17 0.24 0.25 0.45Oxide 0.04 0.03 0.013 0.001Zircon 0.0001 0.0005 0.0001 0.001Apatite 0.0005 0.001 0.001 0.005Allanite 0.0001 0.0005 - -Titanite - - 0.001 0.001

Trace element abundances (C0, in ppm) and D and P values:

C0

D P C0

D PNb 6.0 1.23 1.31 3.0 1.35 1.39Zr 70 0.86 2.70 270 0.94 4.98La 11.0 1.01 2.07 61.1 0.66 0.69Nd 12.7 0.53 1.20 41.73 0.60 0.82Eu 1.17 0.93 1.08 1.27 1.56 1.37Yb 2.2 0.65 0.71 0.81 0.90 1.25

aSource mode as the molecular normative composition of the average lower crustof Taylor and McLennan (1985), trace element abundances from Taylor andMcLennan (1985, Table 4.4).bMelt modes from Rämö (1991).cSource mode calculated by J. Paavola.dEstimated after Wyllie (1977).D denotes the bulk partition coefficient, P denotes the bulk partition coefficient ofthe melting phases in non-modal melting.

21

Paleoproterozoic rocks in Koillismaa places initial constraints on the three models. The εNd

(at 2440 Ma) values of the Archean rocks in Koillismaa vary between –5 and –8.5 (III),which indicates that the Sirniövaara Formation with average ε

Nd (at 2440 Ma) value of –1.9

was probably not derived by partial melting of the local Archean crust. The Koillismaa layeredigneous complex displays ε

Nd (at 2440 Ma) values between –1 and –2 (Rämö et al., 2004),

which in turn indicates that the Kynsijärvi pluton with an average εNd

(at 2440 Ma) value of–4.8 (III) was probably not derived by fractional crystallization of the mafic magma.

Geochemical modeling on the parental magma of the 2.44 Ga mafic layered intrusionswas done by Saini-Eidukat et al. (1997) who used trace elements and by Hanski et al. (2001)who used Sm–Nd and Re–Os isotopes. Their results have been utilized in constructing thefractional crystallization and AFC models. In the partial melting models, the sourcecompositions were limited to middle and lower crustal sources, an assumption based on theA-type geochemistry of the Kynsijärvi pluton. The mineral-melt partition coefficients usedin the models are found in Appendix 1.

6.4.2. Partial melting

For partial melting modeling, trace element modal and non-modal batch melting andfractional melting models were deviced. Batch melting is an approximation where the partialmelt is constantly reacting and equilibrating with the residue until the melt is able to escapethe site of melting (e.g., Allègre and Minster, 1978). The concentration of a trace element inthe melt, C

L, is described by equation

(1) CL = C

0/[D

0 + F(1–P)]

where C0 is the concentration of the element in the solid, D

0 is the bulk partition coefficient of

the element in the original solid, F is the weight fraction of the melt produced, and P is thebulk partition coefficient in the mineral assemblage that enters the melt. In modal melting,the minerals enter the melt in their modal proportions and P is substituted by D

0 in equation

(1) (Shaw, 1970).In fractional melting, a small amount of liquid is produced and instantly isolated from the

source. The concentration of a trace element in the melt, CL, is expressed by equation

(2) CL = [C

0(1 – PF/D

0)(1/P–1)]/D

0

The variables are the same as in batch melting. In the case of modal melting, P is substitutedwith D

0 and the equation (2) is simplified to

(3) CL = [C

0(1 – F)(1/Do–1)]/D

0

Batch melting is a commonly used model for the generation of granitic liquids that arerelatively viscous and may not have the ability to easily escape the site of melting. Fractionalmelting is used for the intermediate and mafic liquids that are more mobile. A-type silicicmelts are relatively hot and less viscous than the other types of granitic melts so fractionalmelting models were also constructed for the Kynsijärvi quartz alkali feldspar syenite.

Mineral parageneses and elemental concentrations of the average lower crust were takenfrom Taylor and McLennan (1985) and Rämö (1991). The pyroxene tonalite sample A1661from Koillismaa is considered to represent the possible local middle crustal source (III). Themode of the sample A1661 was calculated by J. Paavola (GTK). The mineral assemblage of

22

A1661 that enters the melt in non-modal melting was approximated afterWyllie (1977). The source and meltmodes for the sources and the bulkpartition coefficients are found in Table1. The modeling involved Nb, Zr, La,Nd, Eu, and Yb. These elements werechosen for their relative immobility inthe post-crystallization processes thathave affected some of the samples.

Non-modal fractional melting of theaverage lower crust producedacceptable Zr, Nd, and Yb contents with30% partial melting (Fig. 12a). Theother trace element contents and the(La/Yb)

N ratio of the modeled melts

indicate, however, that the Kynsijärviquartz alkali feldspar syenite was notderived by partial melting of theaverage lower crust. In particular, the high Nb content and the deep negative Eu anomaly ofthe Kynsijärvi pluton are not reproduced by the models. Non-modal melting of the pyroxenetonalite A1661 produces La, Zr, and Nd contents that are similar to the ones observed in theKynsijärvi pluton (Fig. 12b). However, this model also fails to produce the negative Euanomaly, and the Nb and Yb contents in the partial melts are too low in all models. Neither ofthese sources seem to offer a viable alternative for the Kynsijärvi quartz alkali feldspar syenite.

6.4.3. Fractional crystallization

To explore the fractionation hypothesis, both major element and trace element modelswere deviced. Major element fractional crystallization modeling was carried out using a semi-quantitative least-squares approximation approach (MAGFRAC; Morris, 1984). Althoughthe method is only a coarse approximation of the fractionating system, the modeling gives ageneral trend of the evolution of the liquid and the mode of the fractionating assemblage thatcan be used in the trace element models. The modeling was carried out in two stages. Theparental liquids were chosen utilizing previously published interpretations and modeling doneon the 2.44 Ga mafic layered intrusions and volcanic rocks in the Fennoscandian shield (Puchtelet al., 1997; Saini-Eidukat et al., 1997; Hanski et al., 2001). In the first stage, the komatiite ofLion Hills, Vetreny belt (Puchtel, 1997) was used as the parental liquid and the averagesiliceous high-Mg basaltic (SHMB) parental magma of the 2.44 Ga layered mafic intrusions(Saini-Eidukat et al., 1997) as the daughter. The second stage involved fractionation of theaverage Sirniövaara Formation from the gabbroic (SHMB) daughter of the first stage. In linewith general igneous evolution trends, the Mg/Fe and Ca/Na ratios of the fractionating mineralswere higher in the first stage (for details, see Appendix 1). The established major elementfractionates were then used in trace element modeling. Trace element behavior in fractionalcrystallization can be described by equation

A1661 fractional melting modal

1

10

100

1000

Nb La Nd Zr Eu Yb

sa

mp

le/P

rim

itiv

em

an

tle

5 %

10 %

30 %

50 % melting

Kynsijärvi

A1661

A1661 batch melting modal

1

10

100

1000

sa

mp

le/P

rim

itiv

em

an

tle

5 %10 %30 %50 % melting

Kynsijärvi

A1661

A1661 batch melting non-modal

5 %10 %15 % melting

Kynsijärvi

A1661

A1661 fractional melting non-modal

Nb La Nd Zr Eu Yb

5 %

10 %

15 % melting

Kynsijärvi

A1661

Lower crust batch melting modal1

10

100

sa

mp

le/P

rim

itiv

em

an

tle

5 %10 %30 %50 % melting

Kynsijärvi

Lower crust

Lower crust batch melting non-modal

5 %

10 %

30 % melting

Kynsijärvi

Lower crust

Lower crust fractional melting modal1

10

100

Nb La Nd Zr Eu Yb

sa

mp

le/p

rim

itiv

em

an

tle

5 %10 %30 %50 % melting

Kynsijärvi

Lower crust

Lower crust fractional meltingnon-modal

Nb La Nd Zr Eu Yb

5 %10 %20 %30 % melting

Kynsijärvi

Lowercrust

a)

b)

Fig. 12. Partial melting models with (a) lowercrust of Taylor and McLennan (1985) and (b)pyroxene tonalite A1661 as the sourcecompositions. Primitive mantle-normalizingvalues after Sun and McDonough (1989).

23

(4) CL = C

0F(D–1)

where CL is the concentration of element in the residual melt, C

0 is the concentration of

element in the initial melt, F is the fraction of melt left, and D is the bulk partition coefficientof the crystallizing mineral assemblage for element. This model is a modification of theRayleigh distillation law (Rayleigh, 1896) and equilibrium is assumed only between the surfaceof the crystallizing minerals and the melt.

Major element modeling of stage one indicates that when 31% olivine (Fo86.2

), 7%clinopyroxene, 7% plagioclase (An

76.3) and <2% oxide fractionate from the komatiitic parent,

the evolved liquid is close in composition to the average SHMB parental magma of Saini-Eidukat et al. (1997), with the sum of squared residuals value of 0.324. The amount of theliquid remaining is 53% of the original. The estimated komatiitic parental liquid shows

Table 2. Fractional crystallization model for the Sirniövaara Formation.

STAGE 1 STAGE 2

Fractionating assemblage:

Olivine 30.7% Orthopyroxene 43.3%Clinopyroxene 7.2% Clinopyroxene 15.2%Plagioclase 6.9% Plagioclase 33.0%Oxide 1.9% Ilmenite 0.3%

Magnetite -1.0%

F 53% 8%ΣR2 0.324 1.209

Model liquid compositions:

Initial Initial Daughter Initial Initial DaughterAssumeda Calculated Assumedb Assumedb Calculated Assumedc

SiO2

48.0 48.0 54.1 54.1 54.2 69.9TiO

20.45 0.37 0.31 0.31 0.31 0.97

Al2O

38.51 8.54 11.6 11.6 11.6 12.3

FeO* 10.8 10.8 8.66 8.66 8.66 7.91MnO 0.18 0.17 0.18 0.18 0.17 0.09MgO 22.9 22.9 13.5 13.5 13.4 0.83CaO 7.02 7.04 8.41 8.41 8.35 1.51Na

2O 1.64 1.24 1.93 1.93 1.64 4.02

K2O 0.30 0.70 1.28 1.28 0.23 2.13

P2O

50.07 0.02 0.04 0.04 0.02 0.27

Trace elements:

STAGE 1 STAGE 2D C

0C

LD C

0C

L

Nb 0.06 1.325 2.41 0.55 2.41 7.51Zr 0.03 40.3 74.5 0.24 74.5 508La 0.15 5.1 8.7 0.53 8.7 28.5Nd 0.05 6.31 11.5 0.23 11.5 80.6Eu 0.03 0.48 0.90 0.91 0.90 1.13Yb 0.12 1.02 1.78 0.56 1.78 5.41

aAverage of four samples (91103-91106) from Lion Hills, Vetreny Belt, Russia (Puchtel et al., 1997; Table 2).bAverage SHMB parental magma (Saini-Eidukat et al., 1997; Table 3)cSirniövaara Formation, average of 30 analyses (L. Lauri, unpubl.).F = amount of residual liquid, ΣR2 = sum of squared residuals, FeO* = total Fe calculated as Fe2+, D = bulk partitioncoefficient, C

0 = concentration of element in the initial melt, C

L = concentration of element in the residual liquid.

24

somewhat lower contents of TiO2, Na

2O, and P

2O

5 and higher contents of K

2O than the original

parent (Table 2). A parental composition with lower Ti could perhaps be used (e.g., Saini-Eidukat et al., 1997) to eliminate the TiO

2 anomaly. The anomalies in Na, K, and P are explained

by the facts that mobility of alkalies is common in these rocks and the model did not involvea phosphorus-bearing phase such as apatite. However, the model cannot be consideredquantitative, as the only mineral that can be expected to substantially fractionate from akomatiitic liquid is olivine. However, it suggests that gabbroic magmas associated with themafic layered intrusions can be derived from a komatiitic parent via major fractionation ofolivine (cf. Hanski et al., 2001). In this model, the SiO

2 content changes from 48 wt.% to 54

wt.%. A more realistic approach would be to involve several stages with different fractionatingminerals between the komatiite parent and the SHMB daughter. It is also questionable whetherthe oxide that fractionates from a mafic magma at this early stage is magnetite, as required bythis model. However, the fractionation of chromite could not be modeled with MAGFRACand ilmenite was not compatible with the model.

Stage two involved fractionation of the average Sirniövaara Formation from the averageSHMB parental magma of Saini-Eidukat et al (1997). The fractionating assemblage was 33%plagioclase (An

63.1), 15% clinopyroxene, 43% orthopyroxene, and 0.3% ilmenite and the

amount of residual liquid was approximately 8%. The sum of the squared residuals was >1.2,which indicates that the model is not very accurate. The estimated parental liquid showslower contents of MgO, CaO, Na

2O, K

2O, and P

2O

5 than the original parent. Other weaknesses

of the model are that the amount of orthopyroxene fractionation is quite high compared tothat of plagioclase and the melt requires approximately 1% added magnetite to balance the Fecontent.

The mineral parageneses of stages one and two were used in trace element modeling thatinvolved basaltic partition coefficients in stage one and dacitic-rhyolitic partition coefficientsin stage two. Nb and La contents produced by fractionation of the komatiitic parental magmain stage one correspond quite well to thecontents in the SHMB magma (Fig. 13a).However, the REE fractionation in themodeled melt is much smaller than in theSHMB, as seen in the (La/Yb)

N ratio (3.5

and 7.4, respectively). Zr contents are alsohigher in the modeled melt than in theSHMB.

The daughter trace element contents ofstage one were fractionated further in stagetwo, now with dacitic-rhyolitic partitioncoefficients. Nb, La, and Eu contentscomparable with those of the SirniövaaraFormation are produced by 92%fractionation of the SHMB magma (Fig.

Stage 1, F=53%

1

10

100

Nb La Nd Zr Eu Yb

Sa

mp

le/P

rim

itiv

em

an

tle Parent (komatiite)

Daughter (high-Mggabbro)

average SHMB(Saini-Eidukat etal., 1997)

Stage 2, F=8%

1

10

100

Parent (high-Mg gabbro of Stage 1)

Daughter (SiO = 69.9 wt.%)2


Sa

mp

le/P

rim

itiv

em

an

tle

Nb La Nd Zr Eu Yb

a)

b)

Fig. 13. Trace element fractional crystallizationmodel for the Sirniövaara Formation, (a) Stage 1:fractionation of the average SHMB (Saini-Eidukatet al., 1997) from the komatiite of Lion Hills, Vetrenybelt, Russia (Puchtel et al., 1997), (b) Stage 2:fractionation of average Sirniövaara Formation (L.Lauri, unpubl.) from the daughter high-Mg gabbroof Stage 1. Primitive mantle-normalizing values afterSun and McDonough (1989).

25

13b). In the modeled melt, the negative Eu anomaly is deeper and the Nd, Zr, and Yb contentsare markedly higher than in the Sirniövaara Formation.

The fractional crystallization model is not inconsistent with the derivation of the SirniövaaraFormation from the mantle-derived magmas that produced the 2.44 Ga layered intrusions butthe model is rather inaccurate. The amount of felsic magma derived by fractional crystallizationof mafic magma is also very small. The observed thickness of the Sirniövaara Formation issome hundreds of meters, which implies that the amount of the mafic parental magma shouldhave corresponded to an overall thickness of ~10 km, if the rocks of the Sirniövaara Formationwere derived by fractional crystallization.

6.4.4. Combined assimilation-fractional crystallization

The principle of AFC modeling is based on Bowen (1928), who first proposed that wallrockassimilation is driven by the latent heat of crystallization during fractional crystallization.The equations that describe the concentration of a trace element in a melt relative to theoriginal magma composition during an AFC process were derived by DePaolo (1981b) andPowell (1984). AFC process is described with equation

(5) CL = C

0f’+rC

A(1–f’)/(r–1+D)

where CL is the concentration of the element in the residual liquid, C

0 is the concentration of

the element in the initial liquid, CA is the concentration of the element in the assimilant, r is

the ratio of the assimilation rate to the fractional crystallization rate, D is the bulk partitioncoefficient for the element and f’ is described by the relation

(6) f’ = F–(r–1+D)/(r–1)

where F is the amount of residual liquid and r and D are as in (5).Previous modeling done on the 2.44 Ga mafic layered intrusions (Saini-Eidukat et al.,

1997; Hanski et al., 2001) is in favor of an AFC model with a komatiitic depleted mantle-derived melt and an Archean crustal contaminant for the generation of the mafic magmas.These models were used as a basis for the AFC modeling that was carried out to examine therole of Nd isotope composition and variation of trace element contents in the 2.44 Ga silicicrocks of Koillismaa. Fractionation of the komatiitic magma [ε

Nd (at 2440 Ma) +2.6] was

modeled with the pyroxene tonalite A1661 and the lower crust of Taylor and McLennan(1985) as the assimilants. The lower crust was assumed to have an ε

Nd(at 2440 Ma) value of

–5, which is the most radiogenic value measured from the Koillismaa Archean samples and isconsidered to comply with an Archean intermediate to mafic lower crust characterized by arelatively high time-integrated Sm/Nd ratio (cf. Rudnick and Fountain, 1995). The pyroxenetonalite A1661 has an ε

Nd(at 2440 Ma) value of –7.6 and is thus one of the least radiogenic

samples analyzed from the Archean rocks of Koillismaa. The rate of assimilation (r; DePaolo,1981b) was varied between 0.1 and 0.7. Basaltic partition coefficients were used until theamount of residual liquid was 50%, after which the partition coefficients were changed todacitic-rhyolitic.

Assimilation of the average lower crust of Taylor and McLennan (1985) produced the Ndisotope composition and some of the trace element characteristics of the Sirniövaara Formationwith an assimilation rate of r = 0.5 (Fig. 14a). However, the model is inconsistent with thederivation of either the parental magma of the Koillismaa layered igneous complex or theKynsijärvi quartz alkali feldspar syenite from the contaminated komatiite, as the ε

Nd of the

26

modeled melt is still higher than in the 2.44 Ga plutonic rocks. The local pyroxene tonaliteA1661 as assimilant produced a better fit for all Paleoproterozoic rock types (Fig. 14b).Consistently with the model of Hanski et al. (2001) for the Koitelainen and Akanvaara layeredintrusions, the Nd isotope composition of the Koillismaa layered igneous complex (~ –1.5;Rämö et al., 2004) is derived by 10–15% assimilation of Archean crust with the ε

Nd(at 2440)

value of –7.6. The trace element and the Nd isotope composition of the Sirniövaara Formationand the Kynsijärvi quartz alkali feldspar syenite are produced by ~80–90% fractionation ofthe mantle-derived magma with assimilation rates of r = 0.1 and r = 0.3, respectively (Fig.14b). However, the model shows that both the Sirniövaara Formation and the Kynsijärvipluton would require an assimilant with a higher Nb content. One weakness of the model isthat the fractionating assemblages were the same that were used in the fractional crystallizationmodeling and may thus not be altogether realistic. Also, the residual liquid value of 50% forthe modification of the partition coefficients was also chosen according to the fractionalcrystallization model. Nevertheless, AFC seems to offer the best explanation for the generationof the 2.44 Ga silicic rocks of Koillismaa.

7. Discussion

7.1. Geochronology of the Koillismaa area

The ages determined for the Archean rocks of the Fennoscandian shield range from rareoccurrences of >3.1 Ga rocks (Kröner et al., 1981; Paavola, 1986; Lobach-Zhuchenko et al.,1993; Mutanen and Huhma, 2003) to the prominent group of 2.85 to 2.68 Ga old rocks (e.g.,Vaasjoki et al., 1999 and references therein; Hölttä et al., 2000; Luukkonen, 2001; Evins etal., 2002; Mänttäri and Hölttä, 2002). The Archean rocks in Koillismaa belong to the mainNeoarchean phase of crustal growth based on the age determinations in III. The gneisses, inwhich the metamorphic degree may rise up to granulite facies conditions, are at least 2800Ma old. The 2900 to 3100 Ma T

DM ages of the Archean rocks suggest that even older rocks

-6

-5

-4

-3

-2

-1

0

1

2

3

0 100 200 300 400 500 600

Zr

ep

sil

on

Nd 0.9

0.9

0.9

0.9

0.7

0.7

0.7

0.7

0.5

0.5

0.5

0.5

0.4

0.4

0.4

0.4

0.3

0.3

0.3

0.3

0.2

0.2

0.2

0.2 0.1

0.1

0.1

0.1

r=0.1

r=0.3

r=0.5

r=0.7

K

KS

SFSHMB

LC

0 20 40 60 80 100 120

Nd

0.9

0.90.9

0.9

0.2

0.7

0.7

0.7

0.7

0.5

0.5

0.5

0.5

0.4

0.4

0.4

0.4

0.3

0.3

0.3

0.30.1

0.2

0.2

0.2

0.1

0.1

0.1

K

KS

SHMBSF

LC

r = 0.7

r = 0.5

r = 0.3

r = 0.1

-8

-6

-4

-2

0

2

4

0 200 400 600 800 1000

Zr

ep

sil

on

Nd

0.9

0.9

0.9

0.9

0.7

0.7

0.7

0.7

0.1

0.5

0.5

0.5

0.5

0.4

0.4

0.4

0.4

0.3

0.3

0.3

0.3

0.2

0.2

0.20.1

K

KS

SF

SHMB

A1661

r = 0.1

r = 0.3

r = 0.5

r = 0.7

0 20 40 60 80 100

Nd

0.9

0.9

0.9

0.9

0.7

0.7

0.7

0.7

0.5

0.5

0.5

0.4

0.4

0.4

0.3

0.3

0.3

0.2

0.20.1

K

KS

SF

SHMB

A1661

r = 0.1

r = 0.3

r = 0.5

r = 0.7

a)

b)

Fig.14. The evolution of Zr and Nd concentrations in an AFC model with (a) the lower crust of Taylor and McLennan(1985) and (b) the pyroxene tonalite A1661 as assimilant. K = average of four samples of the komatiite of LionHills, Vetreny belt (Puchtel et al., 1997), SHMB = average parental magma of the 2.44 Ga layered intrusions (Saini-Eidukat et al., 1997), SF = Sirniövaara Formation [trace element data average of 30 analyses (L. Lauri, unpubl.), ε

Nd

average of the unaltered samples in III], KS = Kynsijärvi quartz alkali feldspar syenite (I), LC = average lowercrust of Taylor and McLennan (1985).

27

might be found with more extensive sampling. The granitic magmatism and migmatizationtook place at ~2700 Ma or even slightly later, if the monazite age of ~2695 Ma is taken torepresent the emplacement age of the granites. The monazites in the ~2800 Ma gneisses ofKoillismaa also record the 2695 Ma age, which is thought to coincide with the peak of granulitefacies metamorphism. In some parts of the Archean craton, granulite facies metamorphismoccurred at ~2.63 Ga (e.g., Hölttä et al., 2000; Mänttäri and Hölttä, 2002); however, theArchean magmatic evolution of the Koillismaa area seems to have ceased at ~2.69 Ga. TheArchean K–Ar ages of hornblendes and biotites also indicate that these later processes didnot markedly affect the lithologic units of the area (Kontinen et al., 1992).

The Archean crust of Koillismaa experienced a period of cratonization that lasted until~2.44 Ga, at which time upwelling of mantle beneath the Archean craton and resultant extensionstarted. The cause of the upwelling is debated – both a mantle plume model and passiverifting controlled by pre-existing weaknesses of the lithosphere have been proposed (e.g.,Amelin et al., 1995; Puchtel et al., 1997; Hanski et al., 2001). The Paleoproterozoic magmatismin Koillismaa was connected to the rifting, the mafic magmas originating in the mantle andthe felsic magmas being produced as the mafic magmas interacted with the Archean lowercrust.

This tectonothermal event was quite short in duration, as shown by the U–Pb mineral datapublished previously and in this work. The age of the Koillismaa layered igneous complex is2436±5 Ma (Alapieti, 1982) and that of the Kynsijärvi quartz alkali feldspar syenite 2442±3Ma (I). The Sirniövaara Formation is also presumably ~2.4–2.5 Ga old (see page 16 fordetails). Furthermore, no substantial Archean inheritance has been detected in the zirconpopulation of the Paleoproterozoic rocks (Fig. 15).

The extensional tectonics continued intermittently in the Koillismaa area for the durationof the Paleoproterozoic until ~2.0 Ga, resulting in the emplacement of several mafic dikeswarms (Vuollo, 1994; Vuollo et al., 2000). Also, the sill-like western intrusion of theKoillismaa layered igneous complex was also broken into several fragments during theextensional phase (Alapieti, 1982; Karinen, 1998; Karinen and Salmirinne, 2001).Compressional tectonics prevailed in Koillismaa area at ~1.8–1.9 Ga, when the blocks of thewestern intrusion of the Koillismaa complex were rotated to their present location and the

Fig. 15. Concordia diagram for comparison of the Archean and Proterozoic rocks in Koillismaa (III).

2200

2400

2600

2800

0.36

0.40

0.44

0.48

0.52

0.56

6 8 10 12 14

207Pb/

235U

206P

b/2

38U

TIMS U-Pb age data on

zircons and monazites

from Koillismaa, Finland

Kynsijärvi quartz alkali feldspar syenite2442±3 Ma (Lauri and Mänttäri, 2002)

Koillismaa layered igneous complex2436±5 Ma (Alapieti, 1982)

filled ellipsoids: granite A1656zircons 2711

open ellipsoids: pyroxene tonalite A16612808

±9 Ma (MSWD = 2, n = 5),monazite 2695±2 Ma

±20 Ma (MSWD = 3.9, n = 4),monazite 2696±2 Ma

Soilu granodiorite (Lauerma, 1982)

A1656 andA1661monazite

Proterozoic

Archean

28

hanging-wall supracrustal rocks (including the Sirniö Group) were sheared and folded(Karinen, 1998; Karinen and Salmirinne, 2001; II). Fluid flow in the shear zones at this timemay also have affected some of the samples used in the Nd isotope determinations (III).After the Paleoproterozoic, the Koillismaa area experienced a period of magmatic and tectonicquiescence that was briefly broken at ~370 Ma, when the alkaline intrusion of Iivaara wasemplaced (Kramm et al., 1993).

7.2. The granophyre problem

Granophyre refers both to an irregular microscopic intergrowth texture between quartzand alkali feldspar and to a porphyritic rock type displaying this kind of texture in thegroundmass (Bates and Jackson, 1987). The interpretations about the origin of the texturevary from magmatic processes such as simultaneous crystallization of quartz and alkali feldsparfrom a ternary eutectic granitic melt (e.g., Hughes, 1971) to metamorphic solid-staterecrystallization or devitrification (e.g., Walraven, 1985; McPhie et al., 1993). Granophyricrocks form a part of many igneous suites, both mafic and felsic, and the depth of formationvaries from volcanic to plutonic (e.g., Hughes, 1971; Irvine, 1975; Pankhurst et al., 1978;Agron and Bentor, 1981; Dickin and Exley, 1981; Walraven, 1985; Bloxam and Dirk, 1988;Hirschmann, 1992; Lipman et al., 1997; Lowenstern et al., 1997; Mutanen, 1997). Layeredmafic intrusions are commonly associated with granophyric rocks. Besides Koillismaa,extensive granophyres occur, e.g., in the Bushveld complex, South Africa (Walraven, 1985),the Skaergaard intrusion, Greenland (Hirschmann, 1992), and the Koitelainen and theAkanvaara intrusions, northern Finland (Mutanen, 1997). The genesis of the granophyres inthe mafic complexes is in many cases complicated and, in general, relatively little studiedcompared to the efforts on the layered intrusions themselves. Interstitial granophyric materialis common in the upper cumulates of many mafic intrusions (e.g., Alapieti, 1982), but mostof the larger granophyre occurrences are considered to represent partial melts or recrystallizedparts of the roof and wall rocks of the intrusions (e.g., Smith, 1974; Irvine, 1975; Walraven,1985).

Based on the data collected by the Koillismaa Project of the University of Oulu, Alapieti(1982) interpreted the western intrusion of the Koillismaa layered igneous complex to be asill-like body that intruded a subhorizontal unconformity between the Archean basement andoverlying volcanic rocks the age of which is uncertain. He stated that the intrusion of themafic magmas resulted in the crystallization of the mafic layered sequence and the granophyre,but he did not discuss the genesis of the latter in detail.

The Sirniö Group overlies the gabbroic and anorthositic cumulates of the Koillismaa layeredigneous complex. The contact is unexposed but no evidence of erosion of the upper cumulatesof the layered intrusion exists, contrary to what has been observed in the Penikat layeredintrusion in the west (e.g., Alapieti et al., 1990). The rocks of the Unijoki Formation displayvolcanic textures such as a very fine grain size, amygdules, and shattered plagioclasephenocrysts. The Sirniövaara Formation displays similar textures and is also considered to bevolcanic in origin, although the volcanic traits are not as clear as in the Unijoki Formation.The absence of typical textures of intrusive granophyres (e.g., Hughes, 1971; Smith, 1974)also argues for the volcanic origin of the Sirniövaara rhyodacite (II).

7.3. Early Paleoproterozoic volcanism in the northern Fennoscandian shield

In addition to the emplacement of the mafic layered intrusions, the early Paleoproterozoicextension in the Fennoscandian shield was manifested by voluminous volcanism that ranged

29

from komatiitic to rhyolitic (e.g., Manninen, 1991; Puchtel et al., 1997; Lehtonen et al., 1998;II). In northern Finland, the ~2.44 Ga volcanic rocks are combined into the Salla Group(Lehtonen et al., 1998), in Russia they are commonly referred to as the Sumi Group in Kareliaor the Strelna Group in the Kola area (e.g., Zagorodny et al., 1982; Gorbunov et al., 1985).The rocks of the Salla Group are found throughout northern Finland, commonly as thelowermost volcanic formation lying unconformably on the Archean basement, or formingthe roofs of some of the 2.44 Ga layered intrusions (Mutanen, 1997; Lehtonen et al., 1998).The U–Pb zircon ages indicate that the Salla Group is 2.48 to 2.44 Ga old (Manninen et al.,2001; Mutanen and Huhma, 2001; Räsänen and Huhma, 2001). Age determinations availablefor the Sumian volcanic rocks in the Paanajärvi area near the Oulanka layered complex inRussia show that these rocks are also ~2.44 Ga old (Buyko et al., 1995). It has been suggestedthat some of the Paleoproterozoic volcanic rocks (e.g., the Vetreny belt, Russia; Puchtel et al.,1997) could represent extruded equivalents of the mafic layered intrusions, but the questionremains open.

The Sirniö Group in Koillismaa is an example of Paleoproterozoic volcanism intimatelyassociated with a layered intrusion and is tentatively correlated to the Salla Group in northernFinland (II). The Sirniö Group is interpreted to be slightly older than the Koillismaa layeredigneous complex and thus to represent the original roof of the mafic intrusion (II). Thehydrothermal alteration of the Sirniövaara rhyodacite and the formation of the granophyricintergrowth texture are thought to result from the heating and fluid flow associated with theemplacement of the gabbroic rocks beneath the volcanic pile. Trace element data and Ndisotope composition of the rocks of the Sirniö Group suggest that these rocks could representmelts formed from the mafic magmas in the lower crust, possibly by an AFC process (III).The absence of inherited Archean zircons also indicates a more juvenile source for theserocks. The rift-related magmatism was probably initiated with the volcanism, as the relativelylight silicic melts were able to rise through the crust via zones of extension.

7.4. Early Paleoproterozoic silicic plutonism in the northern Fennoscandian shield

The data on silicic plutonism associated with the ~2.44 Ga rifting of the Fennoscandianshield is relatively sparse. In the Karelian Province, the three known occurrences of silicicplutonic rocks of this age are the 2435±12 Ma Kuhmo granites in eastern Finland (Luukkonen,1988), the 2450±72 Ma Nuorunen granite in Oulanka area, northwestern Russia (Hackmanand Wilkman, 1929; Buyko et al., 1995), and the 2442±3 Ma Kynsijärvi quartz alkali feldsparsyenite in Koillismaa (I). These intrusions are situated ~100 km apart from each other (Fig.1). Some small occurrences of A-type granites of similar age are also known in the Belomorianbelt (Lobach-Zhuchenko et al., 1998). The Kynsijärvi quartz alkali felsdpar syenite and theNuorunen granite are both intimately associated with mafic layered intrusions, the Koillismaaand Oulanka complexes, respectively, and the Kuhmo granites are thought to be connected toa diabase dike swarm, also indicating an extensional emplacement environment for thesegranites (Luukkonen, 1988). All these ~2.44 Ga silicic intrusions show A-type geochemicalcharacteristics such as high alkali abundances, high FeO*/MgO, Zr, Nb, Zn, Ga, REE and anegative Eu anomaly. Other geochemical traits indicate unique sources for each of the threeintrusions. The Kynsijärvi quartz alkali feldspar syenite shows considerably lower SiO

2

contents of ~71 wt.% and higher alkali abundances compared to the high-silica (>75 wt.%),metaluminous Nuorunen granite and the peraluminous Kuhmo granites (I). Nd isotope datafor the Kynsijärvi and Nuorunen plutons also indicate that these rocks had different sources(Fig. 11; III).

A-type magmas are thought to originate either from the partial melting of the lower crust,mainly amphibolitic or granulitic, in some cases ferrodioritic rocks (e.g., Collins et al., 1982;

30

Rudnick and Fountain, 1995; Frost and Frost, 1997) or to be the products of extremefractionation of mantle-derived mafic melts (e.g., Collerson, 1982; Foland and Allen, 1991;Turner et al., 1992; Smith et al., 1999). The petrological modeling on the Kynsijärvi quartzalkali feldspar syenite indicates that the most probable alternative in this case is the AFCprocess involving the mafic mantle-derived magma and a lower crustal assimilant (III). AnA-type geochemical character is often associated with an anorogenic and extensional tectonicsetting (e.g., Eby, 1990). In the northern Fennoscandian shield, the A-type silicic plutonicrocks, the mafic layered intrusions and the rift-related volcanic rocks, which also show A-type geochemical characteristics, form a typical magmatic association marking the rupture ofthe Archean craton in the early Paleoproterozoic.

8. Conclusions

This work leads to the following conclusions:1. The Archean nucleus of Fennoscandian shield was rifted during the early Paleoproterozoic,the rifting was caused by mantle upwelling beneath the craton.2. During the rifting, an anorogenic magmatic association consisting of mafic layeredintrusions, silicic plutons, and rift-related volcanic rocks was emplaced throughout the northernFennoscandian Archean craton. In the Koillismaa area, these rocks comprise the 2436±5 Mamafic Koillismaa layered igneous complex, the 2442±3 Ma Kynsijärvi quartz alkali feldsparsyenite, and the volcanic Sirniö Group. In the adjoining Oulanka area in Russia, a similarmagmatic association of the Oulanka complex layered intrusions, the Nuorunen granite, andthe Sumian volcanic rocks is found.3. The age of the Archean complex that hosts the early Paleoproterozoic rocks in Koillismaais between 2800 and 2700 Ma. The youngest Archean granites were emplaced in the gneissicbasement at ~2.69 Ga, which also coincides with the peak granulite facies metamorphism ofthe gneisses.4. The early Paleoproterozoic silicic intrusions and the felsic volcanic rocks show A-typegeochemical characteristics such as high total amount of alkalies, high Fe/Mg, Ga/Al, Zr, andREE, and a negative Eu anomaly.5. The rocks of the Sirniö Group suffered metamorphism and hydrothermal alteration that isinterpreted to have been caused by the emplacement of the Koillismaa layered igneous complexunder the Sirniövaara Formation.6. The Nd isotope composition of the Kynsijärvi quartz alkali feldspar syenite [ε

Nd (at 2440

Ma) of –4.8 and TDM

age of ~3000 Ma] falls close to the evolution path of the Archean rocksin Koillismaa [ε

Nd (at 2440 Ma) between –5 and –8.5 and T

DM ages between 2900 and 3100

Ma]. In the Sirniö Group, the εNd

(at 2440 Ma) values between –1.9 and –2.5, and TDM

agesbetween 2800 and 2950, are close to the values acquired from the layered mafic intrusions.The Nuorunen granite shows ε

Nd (at 2440 Ma) values of –1.8 and T

DM age of 2750 Ma, which

may reflect the Nd isotope composition of the local Archean crust.7. The early Paleoproterozoic silicic rocks in Koillismaa may all have been derived frommafic mantle-derived magmas by AFC processes involving lower crustal contaminants.

Acknowledgments

This study was initiated in 1998 when I started my M.Sc. work in the “Layered igneouscomplexes in northern Finland” projent of the GTK under the supervision of Dr. MarkkuIljina. Although my Ph.D. work has been done at the Department of Geology, University ofHelsinki with the funding of the Academy of Finland (project #47690) and the University ofHelsinki, the fruitful collaboration with the GTK has continued over the years. Most of the

31

References

Agron, N., Bentor, Y.K., 1981. The volcanic massif of Biq’at Hayareah (Sinai-Negev): a case of potassiummetasomatism. J. Geol. 89, 479–495.Alapieti, T., 1982. The Koillismaa layered igneous complex, Finland – its structure, mineralogy and geochemistry,with emphasis on the distribution of chromium. Geol. Surv. Finland, Bull. 319.Alapieti, T.T., Lahtinen, J.J., 1989. Cu-Ni-PGE mineralization in the Koillismaa layered complex. In: Alapieti,T.T. (Ed.), 5th international platinum symposium; Guide to the post-symposium field trip; I, The Tornio-Näränkävaaraintrusion belt; II, Kuhmo-Koli-Enonkoski sites. Geol. Surv. Finland, Guide 29, 189–198.Alapieti, T.T., Lahtinen, J.J., 2002. Platinum-group element mineralization in layered intrusions of northern Finlandand the Kola Peninsula, Russia. In: Cabri, L.J. (Ed.), The geology, geochemistry, mineralogy and mineral beneficiationof platinum-group elements. Special Volume - Canadian Institute of Mining and Metallurgy (1982) 54, 507–546.Alapieti, T., Piirainen, T., 1984. Cu-Ni-PGE mineralization in the marginal series of the early Proterozoic Koillismaalayered igneous complex, Northeast Finland. In: Buchanan, D.L., Jones, M.J. (Eds.), Sulphide deposits in maficand ultramafic rocks. Inst. Min. and Metall., London, United Kingdom (GBR).Alapieti, T.T., Filén, B.A., Lahtinen, J.J., Lavrov, M.M., Smolkin, V.F., Voitsekhovsky, S.N., 1990. EarlyProterozoic layered intrusions in the northeastern part of the Fennoscandian Shield. Mineral. Petrol. 42, 1–22.Allègre, C.J., Minster, J.F., 1978. Quantitative models of trace element behavior in magmatic processes. EarthPlanet. Sci. Lett. 38, 1–25.Amelin, Yu.V., Heaman, L.H., Semenov, V.S., 1995. U–Pb geochronology of layered mafic intrusions in theeastern Baltic Shield; implications for the timing and duration of Paleoproterozoic continental rifting. PrecambrianRes. 75, 31–46.

material used in this study has been provided by the GTK and many people have contributedto this work. The thin sections and geochemical analyses were made at the GTK. LassiPakkanen and Bo Johanson provided the mineral chemical analyses and most of the thinsection pictures. The whole staff of the Rovaniemi office of the GTK is acknowledged andspecial thanks go to Jorkki Räsänen, a competent field geologist and a reliable co-author. Asfor the other co-authors, I have been very lucky to have had a chance to work with peoplesuch as Dr. Hannu Huhma, Dr. Irmeli Mänttäri, Tuomo Karinen, and Prof. O. Tapani Rämö.Hannu, Irmeli, and the other people (Tuula Hokkanen, Marita Niemelä, and Arto Pulkkinen)at the isotope laboratory of the GTK always welcomed me warmly to their lab and wereincredibly patient with a field geologist who was more used to wielding a hammer than doingprecision work with the laboratory equipment. During the years, numerous discussions anddays spent doing fieldwork with Tuomo have improved my knowledge on the geology of thelayered intrusions in general and of the Koillismaa area in particular. Tapani has been a greatsupervisor who, despite his busy schedule, has always been ready to help and encourage. Ialso want to acknowledge my other supervisor, Prof. Emeritus Ilmari Haapala, who providedthe original funding for the project. The people at the Department of Geology, University ofHelsinki, have provided a good working environment and refreshing coffee breakconversations. Kirsi-Marja Äyräs is thanked for keeping the administrational things of myproject in law and order. Dr. Paula Kosunen and Dr. Arto Luttinen have helped with many,sometimes absurd-sounding, scientific problems that came up during the study. This workwas thoroughly reviewed and greatly improved by Dr. Matti Vaasjoki (GTK) and Prof. EeroHanski (University of Oulu).

Beside the academic community, I want to acknowledge the help and support of my family,both alive and dead. The three people I miss the most are my father Hannu, my grandfatherL.O.Kuitunen, who, despite his commercial background, was very interested in naturalsciences, and my aunt Annukka, who taught me to appreciate both books and nature. But I amespecially indebted to my mother Nora Landen, my mother-in-law Sirkka Lauri and my dearhusband Hannu, who have taken care of the important things - such as our little daughterSilja - while I have been busy with geology. Thank you all!

Espoo, 26.4.2004 Laura S. Lauri

32

Bates, R.L., Jackson, J.A., 1987. Glossary of Geology, Third Edition. American Geological Institute, Alexandria,Virginia.Bloxam, T.W., Dirk, M.J.H., 1988. The petrology and geochemistry of the St. David’s granophyre and the CwmBach rhyolite, Pembrokeshire, Dyfed. Mineral. Magazine 52, 563–575.Bowen, N.L., 1928. The evolution of the igneous rocks. Princeton Univ. Press.Buyko, A.K., Levchenkov, O.A., Turchenko, S.I., Drubetskoi, Je.R., 1995. Geologija I izotopnoje datirovanijeranneproterozoiskogo sumiisko-sarioliiskogo kompleksa Severnoi Karelii (Paanajarvi-Tsipringskaja struktura).Stratigrafija Geologicheskaja korreljatsija, T.3, No 4, 16–30 (in Russian).Collerson, K.D., 1982. Geochemistry and Rb–Sr geochronology of associated Proterozoic Peralkaline andSubalkaline Anorogenic Granites from Labrador. Contrib. Mineral. Petrol. 81, 126–147.Collerson, K.D., Sheraton, J.W., 1986. Age and geochemical characteristics of a mafic dike swarm in the ArcheanVestfold Block, Antarctica: inferences about proterozoic dike emplacement in Gondwana. J. Petrol. 27(4), 853–886.Collins, W.J., Beams, S.D., White, A.J.R., Chappell, B.W., 1982. Nature and origin of A-type granites withparticular reference to southeastern Australia. Contrib. Mineral. Petrol. 80, 189–200.DePaolo, D.J., 1981a. Neodymium isotopes in the Colorado Front Range and crust–mantle evolution in theProterozoic. Nature 291, 684–687.DePaolo, D.J., 1981b. Trace element and isotopic effects of combined wallrock assimilation and fractionalcrystallization. Earth Planet. Sci. Lett. 53, 189–202.DePaolo, D.J., Wasserburg, G.J., 1976. Nd isotopic variations and petrogenetic models. Geophys. Res. Lett. 3,249–252.Dickin, A.P., Exley, R.A., 1981. Isotopic and Geochemical Evidence for Magma Mixing in the Petrogenesis of theCoire Uaigneich Granophyre, Isle of Skye, Scotland. Contrib. Mineral. Petrol. 76, 98–108.Eby, G.N., 1990. The A-type granitoids: a review of their occurrence and chemical characteristics and speculationson their petrogenesis. Lithos 26, 115–134.Enkovaara, A., Härme, M., Väyrynen, H., 1953. Kivilajikartan selitys C5–B5, Oulu-Tornio. English summary:Explanation to the map of rocks. General Geological Map of Finland, 1:400 000. Geol. Surv. Finland.Evins, P.M., Mansfeld, J., Laajoki, K., 2002. Geology and Geochronology of the Suomujärvi Complex: a newArchaean gneiss region in the NE Baltic Shield, Finland. Precambrian Res. 116, 285–306.Foland, K.A., Allen, J.C., 1991. Magma sources for Mesozoic anorogenic granites of the White Mountain magmaseries, New England, USA. Contrib. Mineral. Petrol. 109, 195–211.Frost, C.D., Frost, B.R., 1997. Reduced rapakivi-type granites: the tholeiite connection. Geology 25, 647–650.Frost, B.R., Arculus, R.J., Barnes, C.G., Collins, W.J., Ellis, D.J., Frost, C.D., 2001. A geochemical classificationof granitic rock suites. J. Petrol. 42, 2033–2048.Gaál, G., Gorbatschev, R., 1987. An outline of the Precambrian evolution of the Baltic Shield. Precambrian Res.35, 15–52.Gorbunov, G.I., Zagorodny, V.G., Robonen, W.I., 1985. Main features of the geological history of the BalticShield and the epochs of ore formation. Geol. Surv. Finland, Bull. 333, 2–41.Hackman, V., Wilkman, W.W., 1929. Suomen geologinen yleiskartta, lehti D 6, Kuolajärvi, kivilajikartan selitys.Suomen geologinen toimikunta (in Finnish).Hanski, E., Walker, R.J., Huhma, H., Suominen, I., 2001. The Os and Nd isotopic systematics of c. 2.44 GaAkanvaara and Koitelainen mafic layered intrusions in northern Finland. Precambrian Res. 109, 73–102.Heaman, L.M., 1997. Global mafic magmatism at 2.45 Ga: remnants of an ancient large igneous province. Geology25(4), 299–302.Heaman, L.M., Tarney, J., 1989. U–Pb baddeleyite ages for the Scourie dike swarm, Scotland: evidence for twodistinct intrusion events. Nature 340, 705–708.Hirschmann, M., 1992. Origin of the transgressive granophyres of the Skaergaard intrusion, East Greenland. J.Volcan. Geothermal. Res. 52, 185–207.Hölttä, P., Huhma, H., Mänttäri, I., Paavola, J., 2000. P-T-t development of Archaean granulites in Varpaisjärvi,central Finland. II. Dating of high-grade metamorphism with the U–Pb and Sm–Nd methods. Lithos 50(1–3), 121–136.Honkamo, M., 1979. Rytinki. Geological map of Finland 1:100 000, Pre-Quaternary rocks, sheet 3541. Geol. Surv.Finland.Hughes, C.J., 1971. Anatomy of a granophyre intrusion. Lithos 4, 403–415.Huhma, H., Cliff, R.A., Perttunen, V., Sakko, M., 1990. Sm –Nd and Pb isotopic study of mafic rocks associatedwith early Proterozoic continental rifting: the Peräpohja schist belt in northern Finland. Contrib. Mineral. Petrol.104, 369–379.Irvine, T.N., 1975. Crystallization sequences in the Muskox intrusion and other layered intrusions – II. Origin ofthe chromitite layers and similar deposits of other magmatic ores. Geochim. Cosmochim. Acta 39, 991–1020.Juopperi, H., 1976. Kuusijärven–Lipeävaaran alueen kallioperä. M.Sc. Thesis, Department of Geology, Universityof Oulu (in Finnish).Karinen, T., 1998. Posion Kuusijärven alueen stratigrafia ja rakennegeologia. M.Sc. Thesis, Department of

33

Geosciences, University of Oulu (in Finnish).Karinen, T., Salmirinne, H., 2001. Koillismaan kerrosintruusiokompleksin läntisen osan geologinen evoluutiomalli.Geol. Surv. Finland, Rep. M19/3543/2001/2 (in Finnish).Koistinen, T., Stephens, M.B., Bogatchev, V., Nordgulen, Ø., Wennerström, M., Korhonen, J., 2001. Geologicalmap of the Fennoscandian Shield, scale 1:2 000 000. Geological Surveys of Finland, Norway and Sweden and theNorth-West Department of Natural Resources of Russia.Kontinen, A., Paavola, J., Lukkarinen, H., 1992. K–Ar ages of hornblende and biotite from Late Archean rocksof eastern Finland – interpretation and discussion of tectonic implications. Geol. Surv. Finland, Bull. 365.Korsman, K., Koistinen, T., Kohonen, J., Wennerström, M., Ekdahl, E., Honkamo, M., Idman, H., Pekkala,Y. (Eds.), 1997. Suomen kallioperäkartta – Berggrundskarta över Finland – Bedrock map of Finland 1:1 000 000.Geol. Surv. Finland, Espoo, Finland.Kramm, U., Kogarko, L.N., Kononova, V.A., Vartiainen, H., 1993. The Kola alkaline province of the CIS andFinland: precise Rb–Sr ages define 380–360 Ma age range for all magmatism. Lithos 30, 33–44.Kröner, A., Puustinen, K., Hickman, M. 1981. Geochronology of an Archaean tonalitic gneiss dome in northernFinland and its relation with an unusual overlying volcanic conglomerate and komatiitic greenstone. Contrib. Mineral.Petrol. 76, 33–41.Lahti, S.I., Honkamo, M., 1980. Loukusa. Geological map of Finland 1:100 000, Pre-quaternary rocks, sheet3543. Geol. Surv. Finland.Landén, L.S., 1999. Koillismaan kerrosintruusiokompleksin yhteydessä esiintyvät felsiset kivilajit. M.Sc. Thesis,Department of Geology, University of Helsinki (in Finnish).Lauerma, R., 1982. On the ages of some granitoid and schist complexes in northern Finland. Bull. Geol. Soc.Finland 54(1–2), 85–100.Lavrov, M.M., 1979. Giberbasity I rasslojennie peridotit-gabbro-noritovie intrusii dokembria Severnoi Karelii.NAUKA, Leningrad, Soviet Union (in Russian).Lehtonen, M., Airo, M.-L., Eilu, P., Hanski, E., Kortelainen, V., Lanne, E., Manninen, T., Rastas, P., Räsänen,J., Virransalo, P., 1998. Kittilän vihreäkivialueen geologia. Geol. Surv. Finland, Rep. Invest. 140 (in Finnish, withEnglish summary).Lipman, P., Dungan, M., Bachmann, O., 1997. Comagmatic granophyric granite in the Fish Canyon Tuff, Colorado:Implications for magma-chamber processes during a large ash-flow eruption. Geology 25, 915–918.Lobach-Zhuchenko, S.B., Chekulayev, V.P., Sergeev, S.A., Levchenkov, O.A., Krylov, I.N., 1993. Archaeanrocks from southeastern Karelia (Karelian granite greenstone terrain). Precambrian Res. 62, 375–397.Lobach-Zhuchenko, S.B., Arestova, N.A., Chekulaev, V.P., Levsky, L.K., Bogomolov, E.S., Krylov, I.N., 1998.Geochemistry and petrology of 2.40–2.45 Ga magmatic rocks in the north-western Belomorian Belt, FennoscandianShield, Russia. Precambrian Res. 92, 223–250.Loiselle, M.C., Wones, D.R., 1979. Characteristics and origin of anorogenic granites. GSA Abstr. Progr. 11, 468.Lowenstern, J.B., Clynne, M.A., Bullen, T.D.,1997. Comagmatic A-type Granophyre and Rhyolite from the AlidVolcanic Center, Eritrea, Northeast Africa. J. Petrol. 38, 1702–1721.Luukkonen, E.J., 1988. Moisiovaaran ja Ala-Vuokin kartta-alueen kallioperä. Suomen geologinen kartta 1.100000, lehdet 4421 ja 4423 + 4441. Explanation to the maps of Pre-Quaternary Rocks, sheets 4421 and 4423 + 4441.Geol. Surv. Finland (in Finnish, with English summary).Luukkonen, E.J., 2001. Lentiiran kartta-alueen kallioperä. Suomen geologinen kartta 1:100 000, lehdet 4414 +4432. Explanation to the maps of Pre-Quaternary Rocks, sheets 4414 + 4432. Geol. Surv. Finland (in Finnish, withEnglish summary).Manninen, T., 1991. Sallan alueen vulkaniitit: Lapin vulkaniittiprojektin raportti. Geol. Surv. Finland, Rep. Invest.104 (in Finnish, with English summary).Manninen, T., Pihlaja, P., Huhma, H., 2001. U–Pb geochronology of the Peurasuvanto area, northern Finland. In:Vaasjoki, M. (Ed.), Radiometric age determinations from Finnish Lapland and their bearing on the timing ofPrecambrian volcano-sedimentary sequences. Geol. Surv. Finland, Spec. Paper 33, 189–200.Mänttäri, I., Hölttä, P., 2002. U–Pb dating of zircons and monazites from Archean granulites in Varpaisjärvi,Central Finland: Evidence for multiple metamorphism and Neoarchean terrane accretion. Precambrian Res. 118,101–131.Matisto, A., 1958. Kivilajikartan selitys D5, Suomussalmi. English summary: Explanation to the map of rocks.General Geological Map of Finland, 1:400 000. Geol. Surv. Finland.McPhie, J., Doyle, M., Allen, R., 1993. Volcanic textures: a guide to the interpretation of textures in volcanicrocks. Centre for Ore Deposits and Exploration Studies. University of Tasmania.Morris, P.A., 1984. MAGFRAC: A basic program for least-squares approximation of fractional crystallization.Computers and Geosciences 10(4), 437–444.Mutanen, T., 1997. Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and theKeivitsa-Satovaara layered complex, northern Finland. Geol. Surv. Finland, Bull. 395.Mutanen, T., Huhma, H., 2001. U–Pb geochronology of the Koitelainen, Akanvaara and Keivitsa layered intrusionsand related rocks. In: Vaasjoki, M. (Ed.) Radiometric age determinations from Finnish Lapland and their bearing onthe timing of Precambrian volcano-sedimentary sequences. Geol. Surv. Finland, Spec. Paper 33, 229–246.Mutanen, T., Huhma, H., 2003. The 3.5 Ga Siurua trondhjemite gneiss in the Archean Pudasjärvi Granulite Belt,

34

northern Finland. Bull. Geol. Soc. Finland 75(1–2), 55–68.Paavola, J., 1986. A communication on the U–Pb- and K–Ar-age relations of the Archaean basement in the Lapinlahti– Varpaisjärvi area, central Finland. Geol. Surv. Finland, Bull. 339, 7–15.Pankhurst, R.J., Walsh, J.N., Beckinsale, R.D., Skelhorn, R.R., 1978. Isotopic and other geochemical evidencefor the origin of the Loch Uisg Granophyre, Isle of Mull, Scotland. Earth. Planet. Sci. Lett. 38, 355–363.Parker, A.J., Rickwood, P.C., Baillie, P.W., McClenaghan, M.P., Boyd, D.M., Freeman, M.J., Pietsch, B.A.,Murray, C.G., Myers, J.S., 1987. Mafic dike swarms of Australia. In: Halls, H.C., Fahrig, W.F. (Eds.), Mafic DikeSwarms. Geol. Assoc. Can. Spec. Pap. 34, 401–417.Pesonen, L., Elming, S.-Å., Mertanen, S., Pisarevsky, S., D’Agrella-Filho, M., Meert, J., Schmidt, P.,Abrahamsen, N., Bylund, G., 2003. Paleomagnetic configuration of continents during the Proterozoic.Tectonophysics 375, 289–324.Piirainen, T., Hugg, R., Aario, R., Forsström, L., Ruotsalainen, A., Koivumaa, S., 1978. Koillismaanmalmikriittisten alueiden tutkimusprojektin loppuraportti 1976. English summary: The report of the KoillismaaResearch Project. Geol. Surv. Finland, Rep. Invest. 18.Piispanen, R., Tarkian, M., 1984. Cu-Ni-PGE mineralization at Rometölväs, Koillismaa layered igneous complex,Finland. Mineral. Deposita 19(2), 105–111.Powell, R., 1984. Inversion of the assimilation and fractional crystallization (AFC) equations; characterisation ofcontaminants from isotope and trace element relationships in volcanic suites. J. Geol. Soc. Lond. 141, 447–452.Puchtel, I.S., Haase, K.M., Hofmann, A.W., Chauvel, C., Kulikov, V.S., Garbe-Schönberg, C.-D., Nemchin,A.A., 1997. Petrology and geochemistry of crustally contaminated komatiitic basalts from the Vetreny Belt,southeastern Baltic Shield: Evidence for an early Proterozoic mantle plume beneath rifted Archean continentallithosphere. Geochim. Cosmochim. Acta 61(6), 1205–1222.Rämö, O.T., 1991. Petrogenesis of the Proterozoic rapakivi granites and related basic rocks of southeasternFennoscandia: Nd and Pb isotopic and general geochemical constraints. Geol. Surv. Finland, Bull. 355.Rämö, O.T., Karinen, T., Iljina, M., Lauri, L.S., 2004. Nd and Sr isotope composition and origin of a 2.44 Galayered mafic intrusion in Koillismaa, Finland. Goldschmidt 2004 Abstract volume. Geochim. Cosmochim. Acta(in press).Räsänen, J., Huhma, H., 2001. U–Pb datings in the Sodankylä schist area of the Central Lapland Greenstone Belt.In: Vaasjoki, M. (Ed.), Radiometric age determinations from Finnish Lapland and their bearing on the timing ofPrecambrian volcano-sedimentary sequences. Geol. Surv. Finland, Spec. Paper 33, 153–188.Räsänen, J., Iljina, M., Karinen, T., Lauri, L., Salmirinne, H. and Vuollo, J. 2003. Geologic map of the Koillismaaarea, northeastern Finland, 1:200 000. Geological Survey of Finland (in electronic format, data available at thedatabase of the Geological Survey of Finland, Rovaniemi Unit).Rayleigh, J.W.S., 1896. Theoretical considerations respecting the separation of gases by diffusion and similarprocesses. Philosophical Magazine 42, 493–498.Rudnick, R.L, Fountain, D.M., 1995. Nature and composition of the continental crust: a lower crustal perspective.Rev. Geophys. 33(3), 267–309.Saini-Eidukat, B., Alapieti, T.T., Thalhammer, O.A.R., Iljina, M.J., 1997. Siliceous, high-magnesian parentalmagma compositions of the PGE-rich early Paleoproterozoic layered intrusion belt of northern Finland. Proc. 30th

Int’l. Geol. Congr., Vol. 9, 177–197.Shaw, D.M., 1970. Trace element fractionation during anatexis. Geochim. Cosmochim. Acta 34, 237–243.Silvennoinen, A., 1991. Kuusamon ja Rukatunturin kartta-alueiden kallioperä. Suomen geologinen kartta 1:100000, lehdet 4524 + 4542 ja 4613. Explanation to the maps of Pre-Quaternary Rocks, sheets 4524 + 4542 and 4613.Geol. Surv. Finland (in Finnish, with English summary).Smith, J.V., 1974. Feldspar minerals, Volume 2, Chemical and Textural Properties. Springer-Verlag, Berlin,Heidelberg, New York.Smith, D.R., Noblett, J., Wobus, R.A., Unruh, D., Douglass, J., Beane, R., Davis, C., Goldman, S., Kay, G.,Gustavson, B., Saltoun, B., Stewart, J. 1999. Petrology and geochemistry of late-stage intrusions of the A-type,mid-Proterozoic Pikes Peak batholith (Central Colorado, USA): implications for petrogenetic models. PrecambrianRes. 98, 271–305.Sun, S.S., McDonough, W.F. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantlecomposition and processes. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in Ocean Basins. Geological SocietySpecial Publications 42, 313–345.Taylor, S.R., McLennan, S.M., 1985. The continental crust: its composition and evolution. Blackwell, Oxford.Turner, S.P., Foden, J.D., Morrison, R.S. 1992. Derivation of some A-type magmas by fractionation of basalticmagma: an example from the Padthaway Ridge, South Australia. Lithos 28, 151–179.Vaasjoki, M., Taipale, K., Tuokko, I. 1999. Radiometric ages and other isotopic data bearing on the evolution ofArchaean crust and ores in the Kuhmo-Suomussalmi area, eastern Finland. In: Kähkönen, Y., Lindqvist, K. (Eds.),

35

Studies related to the Global Geoscience Trasects/SVEKA Project in Finland. Bull. Geol. Soc. Finland 71(1), 155–176.Vuollo, J., 1994. Palaeoproterozoic basic igneous events in eastern Fennoscandian Shield between 2.45 Ga and1.97 Ga, studied by means of mafic dike swarms and ophiolites in Finland. Acta Univ. Ouluensis, Ser. A 250.Vuollo, J., Huhma, H., Pesonen, L.J., 2000. Mafic Dyke Swarms – Geological Evolution of the Paleoproterozoicin the Fennoscandian Shield. In: Pesonen, L.J., Korja, A., Hjelt, S.-E. (Eds.), Lithosphere 2000 – A Symposium onthe Structure, Composition and Evolution of the Lithosphere in Finland. Programme and Extended Abstracts, Espoo,Finland, October 4–5, 2000. Institute of Seismology, University of Helsinki, Rep. S-41, 107–111.Walraven, 1985. Genetic Aspects of the Granophyric Rocks of the Bushveld Complex. Econ. Geol. 80, 1166–1180.Winchester, J.A., Floyd, P.A., 1977. Geochemical discrimination of different magma series and their differentiationproducts using immobile elements. Chem. Geol. 20, 325–343.Windley, B.F., 1995. The Evolving Continents, 3rd ed. Wiley, Chichester, England.Wyllie, P.J., 1977. Crustal anatexis: an experimental review. Tectonophysics 43, 41–71.Zagorodny, V.G., Predovsky, A.A., Basalaev, A.A. et al., 1982. Imandra–Varzuga Zone of Karelides. NAUKA,Leningrad (in Russian).

36

Appendix 1. Mineral compositions and mineral/melt partition coefficients used forpetrological modeling.

Mineral compositions used in Stage 1 of fractional crystallization

Olivine Clinopyroxene Plagioclase MagnetiteSample 6Nä Sample 6Nä Bushveld bytownitea Sample 16Po

SiO2

38.96 54.25 49.18 -TiO

2- 0.15 - 9.93

Al2O

3- 2.07 32.22 0.21

FeO* 13.61 4.36 0.24 89.66MnO 0.18 0.16 - 0.14MgO 47.18 17.84 0.20 0.06CaO 0.07 20.65 15.42 -Na

2O - 0.52 2.57 -

K2O - - 0.17 -

Total 100 100 100 100

Mineral compositions used in Stage 2 of fractional crystallization

Clinopyroxene Orthopyroxene Plagioclase Magnetite IlmeniteSample 14Po average of 18 analyses labradoriteb Sample 16Po average of 13 analyses

SiO2

51.71 54.41 52.63 - 0.07TiO

20.53 0.22 0.09 9.93 47.85

Al2O

31.69 1.21 29.82 0.21 0.07

FeO* 11.01 16.09 0.45 89.66 50.19MnO 0.26 0.28 - 0.14 1.52MgO 14.16 25.69 0.08 0.06 0.25CaO 20.64 2.09 12.70 - 0.25Na

2O - 0.02 4.02 - -

K2O - - 0.21 - -

Total 100 100 100 100 100

Mineral analyses from the Koillismaa layered igneous complex (Alapieti, 1982), excepting the plagioclase:aKracek, F.C., Neuvonen, K.J., 1952. Amer. J. Sci., Bowen vol., 93–318.bGutman, J.T., Martin, R.F., 1976. Schweitz. Min. Petr. Mitt. 56, 55–64.Analyses recalculated to 100%. FeO* denotes total Fe2+, - not detected.

Rhyolitic and dacitic Kd’s

Element Hbl Bt Pl Afs Cpx Opx Qtz Ox Zrc Apa All Ti Ilm

Nb 4.00 6.367 0.06 - 0.80 0.80 - 2.50 - 0.1 - 6.3 -Zr 1.40 2.00 0.10 0.10 0.60 0.20 - 0.80 4500 0.10 2.00 - 1.00La 0.90 3.18 0.38 0.072 0.015 0.78 0.015 0.89 2.00 20.0 2600 4.00 7.10Nd 2.89 0.044 0.17 0.025 0.166 0.22 0.016 1.65 2.20 57.1 1620 - 7.60Eu 3.44 0.145 2.11 1.13 0.411 0.17 0.056 0.47 3.14 30.4 110 - 2.50Yb 4.89 0.179 0.077 0.012 0.64 0.86 0.017 1.10 270 23.1 30 - 4.10

Basaltic Kd’s

Element Cpx Pl Ol Ox

Nb 0.10 0.01 0.01 1.00Zr 0.10 0.048 0.012 0.10La 0.56 0.148 0.067 0.015Nd 0.23 0.055 0.007 0.026Eu 0.474 1.125 0.007 0.025Yb 0.542 0.023 0.049 0.018

Kd’s: see references in Rämö (1991), Rollinson (1993) and Elliott (2003).

petrogenesis of felsic igneous rocks associated with the paleoproterozoic koillismaa ... ·...

Documents