GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 18

Continental Alkaline Magmatism

March 9, 2009

ALKALINE IGNEOUS ROCKS

NephelineNa2Al2Si2O8

LeuciteKAlSi2O6

Alkaline rocks generally have more alkalis than can be accommodated by feldspars alone. The excess alkalis appear in feldspathoids, sodic pyroxenes-amphiboles, or other alkali-rich phases

In the most restricted sense, alkaline alkaline rocks are deficient in SiO2 with

respect to Na2O, K2O, and CaO to the extent that they become

“critically undersaturated” in SiO2, and Nepheline or Acmite appears in

the norm

Alternatively, some rocks may be deficient in Al2O3 (and not necessarily

SiO2) so that Al2O3 may not be able to accommodate the alkalis in

normative feldspars. Such rocks are peralkalineperalkaline and may be either silica undersaturated or oversaturated

ALKALINE ROCK SERIESOCEANIC VS. CONTINENTAL

Winter (2001) Figure 19-1. Variations in alkali ratios (wt. %) for oceanic (a) and continental (b) alkaline series. The heavy dashed lines distinguish the alkaline magma subdivisions from Figure 8-14 and the shaded area represents the range for the more common oceanic intraplate series. After McBirney (1993). Igneous Petrology (2nd ed.), Jones and Bartlett. Boston. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

WHAT’S IN A NAME1% OF IGNEOUS ROCKS ARE ALKALINE, BUT CONSTITUTE

>50% OF IGNEOUS ROCK NOMENCLATURE

Basanite feldspathoid-bearing basalt. Usually contains nepheline, but may have leucite + olivine

Tephrite olivine-free basanite

Leucitite a volcanic rock that contains leucite + clinopyroxene olivine. It typically lacks feldspar

Nephelinite a volcanic rock that contains nepheline + clinopyroxene olivine. It typically lacks feldspar. Fig. 14-2

Urtite plutonic nepheline-pyroxene (aegirine-augite) rock with over 70% nepheline and no feldspar

Ijolite plutonic nepheline-pyroxene rock with 30-70% nepheline

Melilitite a predominantly melilite - clinopyroxene volcanic (if > 10% olivine they are called olivine melilitites)

Shoshonite K-rich basalt with K-feldspar ± leucite

Phonolite felsic alkaline volcanic with alkali feldspar + nepheline. See Fig. 14-2. (plutonic = nepheline syenite)

Comendite peralkaline rhyolite with molar (Na2O+K2O)/Al2O3 slightly > 1. May contain Na-pyroxene or amphibole

Pantellerite peralkaline rhyolite with molar (Na2O+K2O)/Al2O3 = 1.6 - 1.8. Contains Na-pyroxene or amphibole

Lamproite a group of peralkaline, volatile-rich, ultrapotassic, volcanic to hypabyssal rocks. The mineralogy is variable, but most contain phenocrysts of olivine + phlogopite ± leucite ± K-richterite ± clinopyroxene ± sanidine.

Lamprophyre a diverse group of dark, porphyritic, mafic to ultramafic hypabyssal (or occasionally volcanic), commonly highly potassic (K>Al) rocks. They are normally rich in alkalis, volatiles, Sr, Ba and Ti, with biotite-phlogopite and/or amphibole phenocrysts. They typically occur as shallow dikes, sills, plugs, or stocks.

Kimberlite a complex group of hybrid volatile-rich (dominantly CO2), potassic, ultramafic rocks with a fine-grained matrix and macrocrysts of olivine and several of the following: ilmenite, garnet, diopside, phlogopite, enstatite, chromite. Xenocrysts and xenoliths are also common

Group I kimberlite is typically CO2-rich and less potassic than Group 2 kimberlite

Group II kimberlite is typically H2O-rich and has a mica-rich matrix (also with calcite, diopside, apatite)

Carbonatite an igneous rock composed principally of carbonate (most commonly calcite, ankerite, and/or dolomite), and often with any of clinopyroxene alkalic amphibole, biotite, apatite, and magnetite. The Ca-Mg-rich carbonatites are technically not alkaline, but are commonly associated with, and thus included with, the alkaline rocks.

ALKALINE ROCKS ASSOCIATED WITH

CONTINENTAL RIFTSEAST AFRICAN RIFT

Failed Arm of the Afar Triple Jct

MAGMA SERIES

Highly AlkalineAlkalineTholeiitic

MAGMA SERIES OF THE EAST AFRICAN RIFT

Series 4Oxide 1 2 3 4 5 6 7 8 9 10 11

SiO2 45.6 51.7 46.2 33.1 44.1 55.4 47.6 61.8 70.3 72.5 50.8

TiO2 2.4 0.9 1.6 2.6 2.8 0.5 2.0 1.0 0.3 0.2 1.4

Al2O3 15.6 19.3 18.6 11.3 17.0 20.8 14.8 14.2 7.6 10.3 14.9

FeO* 11.3 5.9 8.9 12.4 10.0 4.6 11.4 6.4 8.4 4.0 10.1 MnO 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.3 0.3 0.1 0.2 MgO 6.9 1.1 2.3 7.3 3.7 0.5 6.4 0.5 0.0 0.0 6.9 CaO 10.4 4.1 7.3 17.2 8.4 2.9 11.5 1.8 0.4 0.2 9.8

Na2O 3.2 8.9 9.3 3.2 4.3 9.2 2.7 6.2 7.3 5.9 2.6

K2O 1.3 4.6 4.2 3.6 7.2 5.5 0.8 5.2 4.3 4.4 0.4

P2O5 0.6 0.3 0.5 1.9 1.2 0.1 0.3 0.2 0.0 0.4

Total 97.5 97.0 99.1 92.9 98.9 99.7 97.7 97.6 98.8 97.6 97.4 CIPW NORMq 0.0 0.0 0.0 0.0 0.0 0.0 2.1 9.0 41.7 35.8 9.1 or 8.9 29.8 27.5 0.0 31.0 34.2 5.5 33.7 28.1 27.8 2.6 ab 31.4 28.6 8.0 0.0 0.0 30.2 26.5 48.3 16.7 30.4 25.2 an 28.3 0.0 0.0 7.3 6.5 0.0 30.0 0.0 0.0 0.0 31.9 lc 0.0 0.0 0.0 20.7 13.2 0.0 0.0 0.0 0.0 0.0 0.0 ne 0.0 28.3 39.1 18.2 22.2 27.1 0.0 0.0 0.0 0.0 0.0 di 14.2 6.5 13.7 15.0 16.7 2.8 20.8 2.9 0.1 0.0 12.7 hy 0.0 0.0 0.0 0.0 0.0 0.0 8.8 0.0 0.0 0.0 13.8 wo 0.0 3.9 5.7 0.0 0.0 4.1 0.0 0.9 0.6 0.0 0.0 ol 9.4 0.0 0.0 10.9 1.8 0.0 0.0 0.0 0.0 0.0 0.0 il 0.5 0.5 0.5 0.8 0.5 0.4 0.5 0.7 0.6 0.1 0.5 ti 4.7 0.0 0.0 0.0 0.0 0.0 5.0 1.8 0.1 0.5 3.3 ap 1.6 0.8 1.3 5.5 3.1 0.2 0.8 0.5 0.1 0.0 1.0 pf 1.0 1.3 2.6 4.8 4.9 0.5 0.0 0.0 0.0 0.0 0.0 ns 0.0 0.4 1.7 0.0 0.0 0.4 0.0 2.1 12.0 5.3 0.0 cs 0.0 0.0 0.0 16.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1. Ave. 32 alkaline basalts, Kenya (B) 2. Ave. phonolite (B) 3. Ave. Kenyan nephelinite (B) 4. Melilitite, W. Rift (KM) 5. Leucitite, W. Rift (KM)

6. Ave. of 55 phonolites, Uganda (B) 7. Ave. of 31 transitional basalts (B) 8. Ave. 40 trachytes (B) 9. Pantellerite (KM) 10. Comendite (KM)

11. Ave. of 26 Tholeiitic basalts (KM). KM = Kampunzu and Mohr (1991), B = Baker, 1987.

Series 3: Transitional Basalt-RhyoliteSeries 1: Alkaline Series 2: Ultra-alkaline

Table 19-2. Representative Chemical Analyses of East African Rift Volcanics

Q-Feither/or

THERMAL DIVIDE BETWEEN ALKALINE AND THOLEIITIC MAGMAS

Peritectic

Peritectic

Eutectic

1 atm Pressure

ISOTOPIC AND TRACE ELEMENT GEOCHEMISTRY OF EAR VOLCANICS

Figure 19-3. 143Nd/144Nd vs. 87Sr/86Sr for East African Rift lavas (solid outline) and xenoliths (dashed). The “cross-hair” intersects at Bulk Earth (after Kampunzu and Mohr, 1991), Magmatic evolution and petrogenesis in the East African Rift system. In A. B. Kampunzu and R. T. Lubala (eds.), Magmatism in Extensional Settings, the Phanerozoic African Plate. Springer-Verlag, Berlin, pp. 85-136. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 19-5. Chondrite-normalized REE variation diagram for examples of the four magmatic series of the East African Rift (after Kampunzu and Mohr, 1991), Magmatic evolution and petrogenesis in the East African Rift system. In A. B. Kampunzu and R. T. Lubala (eds.), Magmatism in Extensional Settings, the Phanerozoic African Plate. Springer-Verlag, Berlin, pp. 85-136. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Bulk Earth

MAGMA SUITESINTRA-SUITE HOMOGENEITY/INTER-SUITE

HETEROGENEITY

Figure 19-6a. Ta vs. Tb for rocks of the Red Sea, Afar, and the Ethiopian Plateau. Rocks from a particular area show nearly constant ratios of the two excluded elements, consistent with fractional crystallization of magmas with distinct Ta/Tb ratios produced either by variable degrees of partial melting of a single source, or varied sources (after Treuil and Varet, 1973; Ferrara and Treuil, 1974).

TECTONO-MAGMATIC MODEL FOR THE EAST AFRICAN RIFT

Pre-rift stage - an asthenospheric mantle diapir rises (forcefully or passively) into the lithosphere. Decompression melting (cross-hatch-green indicate areas undergoing partial melting) produces variably alkaline melts. Some partial melting of the metasomatized sub-continental lithospheric mantle (SCLM) may also occur. Reversed decollements (D1) provide room for the diapir.

Rift stage - development of continental rifting, eruption of alkaline magmas (red) mostly from a deep asthenospheric source. Rise of hot asthenosphere induces some crustal anatexis. Rift valleys accumulate volcanics and volcaniclastic material.

Afar stage- asthenospheric ascent reaches crustal levels. This is transitional to the development of oceanic crust. Successively higher reversed decollements (D2 and D3)

accommodate space for the rising diapir.

After Kampunzu and Mohr (1991), Magmatic evolution and petrogenesis in the East African Rift system.

CARBONATITES ASSOCIATED WITH THE EAR

Coarse Med.-Fine Calcite-carbonatite sövite alvikite Dolomite-carbonatite rauhaugite* beforsite Ferrocarbonatite Natrocarbonatite* Rarely used, beforsite may be applied to any grain size.

Table 19-3. Carbonatite Nomenclature

AlternativeName

Carbonatite: >50% carbonate minerals

Silico-carbonatite: 50-10% carbonate minerals

FIELD CHARACTERISTICS OF CARBONATITES

Winter (2001) Figure 19-11. Idealized cross section of a carbonatite-alkaline silicate complex with early ijolite cut by more evolved urtite. Carbonatite (most commonly calcitic) intrudes the silicate plutons, and is itself cut by later dikes or cone sheets of carbonatite and ferrocarbonatite. The last events in many complexes are late pods of Fe and REE-rich carbonatites. A fenite aureole surrounds the carbonatite phases and perhaps also the alkaline silicate magmas. After Le Bas (1987) Carbonatite magmas. Mineral. Mag., 44, 133-40.

• Commonly satellite intrusions to alkaline intrusive centers• Pipe-like, composite intrusions • < 25 km across• Ring-dike, cone sheets and plug forms common• Typically late in intrusive sequence• Emplacement T – 500-1000°C• Metasomatic halo – Fenite carbonatized wall rock

CHEMICAL ATTRIBUTES OF CARBONATITES

Calcite- Dolomite- Ferro- Natro-% carbonatite carbonatite carbonatite carbonatite

SiO2 2.72 3.63 4.7 0.16

TIO2 0.15 0.33 0.42 0.02

Al2O3 1.06 0.99 1.46 0.01

Fe2O3 2.25 2.41 7.44 0.05

FeO 1.01 3.93 5.28 0.23 MnO 0.52 0.96 1.65 0.38 MgO 1.80 15.06 6.05 0.38 CaO 49.1 30.1 32.8 14.0

Na2O 0.29 0.29 0.39 32.2

K2O 0.26 0.28 0.39 8.38

P2O5 2.10 1.90 1.97 0.85

H2O+ 0.76 1.20 1.25 0.56

CO2 36.6 36.8 30.7 31.6

BaO 0.34 0.64 3.25 1.66 SrO 0.86 0.69 0.88 1.42 F 0.29 0.31 0.45 2.50 Cl 0.08 0.07 0.02 3.40 S 0.41 0.35 0.96

SO3 0.88 1.08 4.14 3.72

Table 19-5. Representative Carbonatite Compositions

Figure 19-15. Silicate-carbonate liquid immiscibility in the system Na2O-

CaO-SiO2-Al2O3-CO2 (modified by Freestone and Hamilton, 1980, to

incorporate K2O, MgO, FeO, and TiO2). The system is projected from CO2

for CO2-saturated conditions. The dark shaded liquids enclose the

miscibility gap of Kjarsgaard and Hamilton (1988, 1989) at 0.5 GPa, that extends to the alkali-free side (A-A). The lighter shaded liquids enclose the smaller gap (B) of Lee and Wyllie (1994) at 2.5 GPa. C-C is the revised gap of Kjarsgaard and Hamilton. Dashed tie-lines connect some of the conjugate silicate-carbonate liquid pairs found to coexist in the system. After Lee and Wyllie (1996) International Geology Review, 36, 797-819.

ORIGIN OF CARBONATESIGNEOUS, METAMORPHIC, OR

METSOMATIC

ULTRAPOTASSIC ROCKS LAMPROITES AND KIMBERLITES

Lamproite*

SiO2 33.0 27.8-37.5 35.0 27.6-41.9 45.5

TiO2 1.3 0.4-2.8 1.1 0.4-2.5 2.3

Al2O3 2.0 1.0-5.1 2.9 0.9-6.0 8.9

FeO* 7.6 5.9-12.2 7.1 4.6-9.3 6.0 MnO 0.14 0.1-0.17 0.19 0.1-0.6MgO 34.0 17.0-38.6 27. 10.4-39.8 11.2 CaO 6.7 2.1-21.3 7.5 2.9-24.5 11.8

Na2O 0.12 0.03-0.48 0.17 0.01-0.7 0.8

K2O 0.8 0.4-2.1 3.0 0.5-6.7 7.8

P2O5 1.3 0.5-1.9 1.0 0.1-3.3 2.1

LOI 10.9 7.4-13.9 11.7 5.2-21.5 3.5

Sc 14 20 19V 100 95 66Cr 893 1722 430Ni 965 1227 152Co 65 77 41Cu 93 28Zn 69 65Ba 885 3164 9831Sr 847 1263 3860Zr 263 268 1302Hf 5 7 42Nb 171 120 99Ta 12 9 6Th 20 28 37U 4 5 9La 150 186 297Yb 1 1 1Data from Mitchell (1995), Mitchell and Bergman (1991)

* Leucite Hills madupidic lamproite

Table 19-8. Average Analyses and Compositional Ranges of Kimberlites, Orangeites, and Lamproites.

Kimberlite Orangeite Lamproites – Mafic mineralogyKimberlites/Orangites – Ultramafic mineralogy

ULTRAPOTASSIC ROCKS LAMPROITES

Old Nomenclature

wyomingite diopside-leucite-phlogopite lamproiteorendite diopside-sanidine-phlogopite lamproitemadupite diopside madupidic lamproitecedricite diopside-leucite lamproitemamilite leucite-richterite lamproitewolgidite diopside-leucite-richterite madupidic lamproitefitzroyite leucite-phlogopite lamproiteverite hyalo-olivine-diopside-phlogopite lamproitejumillite olivine diopside-richterite madupidic lamproitefortunite hyalo-enstatite-phlogopite lamproitecancalite enstatite-sanidine-phlogopite lamproite

From Mitchell and Bergman (1991).

Table 19-6. Lamproite Nomenclature

Recommended by IUGS

Lamproites• K/Na > 3 (ultrapotassic)• K/Al > 1 (perpotassic)• (K+Na)/Al > 1 (peralkaline)• mg# > 70• Incompatible element-enriched

ULTRAPOTASSIC ROCKS KIMBERLITES/ORANGITES