Volcanic Rocks of the Early Proterozoic Flin Flon Domain in Saskatchewan 1

B.R. Watters 2, J. Dostal 3, W.L. Slimmon, D.J. Thomas, and B.A. Reilly

Watters, B.R., Dostal, J ., Slimmon, W.L. , Thomas, D.J., and Reilly, BA (1994): Volcanic rocks of the Early Proterozoic Flin Ron Domain in Saskatchewan; in Summary of Investigations 1994, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 94-4.

A significant portion of the Early Proterozoic Flin Flan Domain in adjoining parts of northern Saskatchewan

and Manitoba is formed by the Amisk Group. The lower part of the group consists mainly of pillowed basalt and

Amisk La.ke

~ Ordivician Dolomite

fo""'cl ls,·6 s.l Missi Group

EJ Welsh Lake Assemblage

Felsic Intrusions

Mafic Intrusions

t= · 3 Flin Flon Assemblage

subordinate basaltic pyroclastic rocks whereas the upper part in­cludes subaerial units with more volcaniclastic material and minor in­termediate and felsic lithologies. In

5 4' 45' the Amisk Lake area of Saskatche­wan (Figure 1 ), the Amisk Group is composed of three contrasting suites: 1) island arc tholeiite (IAT), 2) mid-ocean ridge basalt (MORB)­like rocks and/or oceanic Island ba­salt (018)-like rocks, and 3) calc-alkaline rocks.

54 '35'

E_=:_:::.:~ Athapapuskow Assemblage

L ~ , : l Mystic Lake Assemblage

~ Birch Lake Assemblage

~ Sandy Bay Assemblage

O West Amisk Assemblage

§ Muskeg Bay Assemblage

Figure 1 • Simplified geological map and regional setting of the Amisk Lake-Flin Flon area. Missi Island in Amisk Lake is left blank because revision mapping has not been completed.

The island arc tholeiitic suite com­prises volcanic rocks from the Flin Flon area (Flin Flon Assemblage) and the Amisk Lake area (Bi rch Lake Assemblage). Collectively, these rocks range from basalt through andesite to rhyolite without apparent discontinuities although most rocks have Si02<58 percent (Figure 2). The relative proportions of major rock types are similar to those of modern island arc tholeii­tic suites (Ewart, 1992) where there is a strong predominance of mafic members. The chondrite­normalized rare earth element (REE) patterns of the basaltic rocks (Figure 3) are flat or only slightly enriched or depleted in the light rare earth elements (LREE). The mantle-normalized trace ele­ment patterns of these rocks are depleted in high field strength ele­ments (HFSE: Nb, Zr, Hf, and Ti} relative to REE and have notice­able negative Nb and Ti anomalies (Figure 4). The felsic rocks have REE patterns similar to those of mafic rocks, although the rhyolites display negative Eu anomalies

(1) Project funded by an NSERC LITHOPROBE operating grant to B.R. Watters and J . Dostal. Rock samples for this wor1< were provided by Slimmon, Thomas, and Reilly.

(2) Department of Geology, University of Regina, Regina, Saskatchewan, S4S OA2. (3) Department of Geology. St. Mary's University, Hali fax. Nova Scotia, B3H 3C3.

Saskatchewan Geological Survey 93

80 O Flin ffo11 and Birch l ake Assemblages

• Sandy Bay Assemblage

• West Amisk Assemblage • • • 75

• 70 • ~ Cale-alkaline

·, .~. • • 0 • # ••• • ... 65 . . ·' • .! •• • •.. ~. • 0

" 60 ·~ •• ct O 0 0 ii) •• 41' •• O O o0 0 0

0 0 0 0

55 •• 0 Bo .~00~

0

°.8 0

0 0 0 Tholeiitic

50 ,~·· • • I •

0

45 • 0 2 3 4

FeO*/ MgO

Figure 2 - Variations of FeO•/MgO versus SiC>e (weight per­cent) in the island arc tholeiitic (open circle), MORB-like (filled circle), and ca/c-a/kaline (diamond) suites from the Saskatche­wan segment of the Flin Flon Domain. The line separating the calc-alkaline and tholeiitic fields is after Miyashiro (1974). The analyses are plotted on a LOI-free basis.

,oo

,o

100

GI ... :§ i:: 0

.J::. (.J

..,. 10 0 0 a:

100

10

A. MORB-lilc:e Basalts

• 9 1-30-0041

0 91-30-00114

~ BHV0-1

~ B. IAT Basalts

• 89-12-0099

0 99-12-0138 . 91-30-0144

c _ IAT Andes1te, Dac:ile, Rh)l'olite . 89-12-0:2'26

0 8 9 - 12010S

"' 89-12-0224 . 6'9-12 0016

(Figure 3). Their mantle-normalized spider diagrams (Figure 4) resemble those of the basaltic rocks.

The MORB- or OIB-like basalts are represented by vol­canic rocks of the Sandy Bay Assemblage (from the area immediately east of Amisk Lake}. Compared to modern IAT as well as to the IAT basalts and basaltic andesites from the Flin Flon Domain, these basalts have higher Ti, higher TiN, fractionated heavy rare earth elements (HREE}, and for a given Mg#, a higher content of transition elements, particularly of Cr and Ni. Their REE and incompatible trace element patterns re­semble tholeiitic OIB and E-type MOAB (Figures 3 and 4). According to the ratios of incompatible trace ele­ments, the basaltic rocks can be subdivided into two dis­tinct groups. Group 1 has lower ZrN, TiN, and LaNb

5 ratios as well as lower abundances of Zr, Ti, and Nb. The Group 2 basalts are more enriched in LREE with the (LaNb}n ratio varying between 1 and 2 in Group 1 and between 1.7 and 4.2 in Group 2. The distribution of incompatible trace elements in basalts of Group 2 is similar to that of plume-related 018 (Figures 3 and 4).

The calc-alkaline suite of the West Amisk Assemblage in the Amisk Lake (west) area includes rocks mostly with Si02 ranging from 57 to 68 percent. Compared to

D_ CA Andes ites

• 1 197

a 1194

E. C A Oac i tes . 1219

0 121~

IAT rocks of equivalent Si02 con­tent, the calc-alkaline rocks are higher in some incompatible trace elements such as Th and LREE. Their trace element patterns (Fig­ure 4) are characterized by nega­tive Ti and Nb anomalies, typical of subduction-related rocks. The suite resembles modern calc-alkaline an­desitic suites from ensimatic arcs or arcs developed at thin sialic mar­gins.

1 . Discussion

Individual rock types of the IAT suite have a trace element compo­sition similar to their modern oceanic IAT correlatives. The IAT basalts were derived by relatively large degrees of partial melting at shallow depths. The isotopic as well as trace element data indicate that Archean continental crust was not involved in their genesis and suggest an intraoceanic arc setting for the Flin Flon Island arc tholeiitic suite.

Figure 3 - Chondrite-norrnalized REE abundances in volcanic rocks of the Saskatchewan segment of the Flin Flan Domain: A, MORB-like basalts; B, /AT basalts; C, /AT andesites and rhyolites; D, Cale-alkaline andesites; E, Cale-alkaline dacites; and F, Cale-alkaline rhyolites. Normalizing values after Sun (1982). Average of E-type MORB (Sun and McDonough, 1989) and USGS standard rock BHV0-1 (tholeiitic basalt from Hawaii) are shown for comparison.

The close spatial and temporal as­sociation between felsic and mafic units of the IAT suite as well as their Nd isotopic ratios imply that the rocks are genetically related. The felsic magmas were probably derived by extensive differentiation of the volumetrically more signifi­cant tholeiitic basalt magma. How-

94 Summary of Investigations 1994

A. M ORB-1,~e Basalts . 91 J0-0041 0 9 1 ·30·00 44 . E-M DAB

10

., -;: c ti! ~

100

8 IAT Basolts

• 89·12-0Dti8 0 89·1 2-0099 . 6 9 11-0 348

Flin Flon IAT are comparable to modem primitive island arc suites.

~ C. CA Amk'$•1es O IA T Andu11e, Oacrtt, Rh yolite

The Nd isotopic ratios for the IAT rocks of the Flin Flon Domain, as well as their trace element charac­teristics, support a derivation in an intraoceanic arc environment from a depleted mantle source with little, if any, contamination by signifi­cantly older continental crust. The presence of island arc calc-alkaline volcanic rocks in the Flin FJon Domain suggests that the island arc evolved to a relatively ad­vanced stage and indicates an in­creasing depth of melting with progressive evolution of the arc.

(J . 9 2- 23· 13ti4 • 89 -1 2- 10 9 0 O 92 23 ·0 20 9 0 89 ·1 2-001 6 a:

10

Figure 4 • Mantle-normalized patterns for volcanic rocks from the Flin F/on Domain:

The close association of IAT with MORB-like tholeiites in the East Amisk Lake area suggests that the suite was formed in a back-arc set­ting and the compositional vari­ations of the MORB-like basalts can be attributed to partial melting of a rising mantle diapir during

A. MORB-like basalts; B, IA T basalts; C, Cale-alkaline andesites; and D, /AT andesite, dacite and rhyo/ite. Normalizing values after Sun and McDonough (1989). Average of E· type MORB (Sun and McDonough, 1989) and USGS standard rock BHV0-1 (tholeiitic basalt from Hawaii) are shown for comparison.

ever, a simple process of fractional crystallization can­not account for the small differences in the abundances of incompatible trace elements between the basalts and rhyolites.

The MORB-like basalts are not directly related to the IAT suite and were derived from a different source. The Nd isotopic data for the MOAB-like basalts point to a de­pleted mantle source. The two groups of these basalts could have been generated by variable degrees of par­tial melting of a similar or common source. The Group 2 tholeiites were formed by a lower degree of melting than the Group 1 tholeiites. Assuming the mantle source had the composition of a primitive mantle, the differences between the two basalt groups suggest that the partial melting involved a garnet peridotite source. The basalts of Group 2 require a garnet-bearing source while some of the basalts of Group 1 may have been generated by the melting of spine! peridotite. Close simi­larities of the Group 2 basalts to the Hawaii 018 (Fig­ures 3 and 4) suggest that the rocks were derived from a mantle plume.

2. Conclusions In the Flin Flon Domain as a whole, the IAT suite is volumetrically far more important than the MOAB-like and calc-alkaline rocks. Compared to rocks of typical Proterozoic greenstone belts (Condie, 1989), the IAT suite is lower in abundances of incompatible trace ele­ments suggesting a more immature arc system and probably more depleted mantle sources. The low abun­dances of strongly incompatible trace elements in the

Saskatchewan Geological Survey

back-arc spreading. The composi­tional characteristics of the Group

2 MOAB-like tholeiites imply partial melting of a garnet peridotite source. These lavas were tapped from the diapir at relatively deep levels (at a depth of 100 to 60 km; McKenzie and O'Nions, 1991). Magmas gener­ated by melting of the diapir at shallower depth (in the spinel stability field) gave rise to the Group 1 basalts.

3. References Condie, K.C. (1989): Geochemical changes in basalts and an­

desites across the Archean-Proterozoic boundary: ldentifi· cation and significance; Lithos, v23, p1-18.

Ewart, A. (1992): The mlneralogy and petrology of Tertiary­Recent orogenic volcanic rocks: With special reference to the andesitic-basaltic compositional range; in Thorpe, R.S. (ed.), Andesites: Orogenic Andesltes and Related Rocks, John Wiley & Sons Lid., p25-98.

McKenzie, D. and O 'Nions, AK (1991): Partial melt distribu­tions from inversion of rare earth element concentrations; J . Petrol., v32, p1021-1091 .

Miyashiro, A. (1974 ): Volcanic rock series in island arcs and ac­tive continental margins; Amer. J . Sci., v274, p321-357.

Sun, S.S. (1982): Chemical composition and origin of the Earth's primitive mantle; Geochim. Cosmochim. Acta, v46, p179-192.

Sun, S.S. and McDonoogh, W.F. (1989): Chemical and iso­topic systematics of oceanic basalts: Implications for man· tie composition and processes; ir> Saunders, A.O. and Norry, M.J. (eds.), Magmatism in the Ocean Basins, Geol. Soc. Lon., Spec. Publ. 42, p313·345.

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