Geol 2312 Igneous and Metamorphic Petrology Lecture 18 Continental Alkaline Magmatism March 9, 2009

Alkaline Igneous Rocks 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 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 peralkaline and may be either silica undersaturated or oversaturated Nepheline Na2Al2Si2O8 Leucite KAlSi2O6

Alkaline Rock Series Oceanic 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 Name 1% 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 Rifts East African Rift Failed Arm of the Afar Triple Jct

Magma Series Highly Alkaline Alkaline Tholeiitic

Magma Series of the East African Rift Q-F either /or

Thermal Divide Between Alkaline and Tholeiitic Magmas 1 atm Pressure Peritectic Eutectic Peritectic

Isotopic and Trace Element Geochemistry of EAR Volcanics Bulk Earth 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.

Magma Suites Intra-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 >50% carbonate minerals Silico-carbonatite: 50-10% carbonate minerals

Field Characteristics of Carbonatites 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 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.

Chemical Attributes of Carbonatites 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 Carbonates Igneous, Metamorphic, or Metsomatic

UltraPotassic Rocks Lamproites and Kimberlites Lamproites – Mafic mineralogy Kimberlites/Orangites – Ultramafic mineralogy

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

UltraPotassic Rocks Kimberlites/Orangites