Slide 1Alkaline Igneous RocksAlkaline Rock Series Oceanic vs. ContinentalWhat’s in a Name 1% of Igneous Rocks are Alkaline, but constitute >50% of Igneous Rock NomenclatureAlkaline Rocks Associated with Continental Rifts East African RiftMagma SeriesMagma Series of the East African RiftThermal Divide Between Alkaline and Tholeiitic MagmasIsotopic and Trace Element Geochemistry of EAR VolcanicsMagma Suites Intra-suite homogeneity/Inter-suite HeterogeneityTectono-Magmatic Model for the East African RiftCarbonatites Associated with the EARField Characteristics of CarbonatitesChemical Attributes of CarbonatitesOrigin of Carbonates Igneous, Metamorphic, or MetsomaticUltraPotassic Rocks Lamproites and KimberlitesUltraPotassic Rocks LamproitesUltraPotassic Rocks Kimberlites/OrangitesGEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 18Continental Alkaline MagmatismMarch 9, 2009ALKALINE IGNEOUS ROCKSNephelineNa2Al2Si2O8LeuciteKAlSi2O6Alkaline 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 phasesIn 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 normAlternatively, 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 oversaturatedALKALINE ROCK SERIESOCEANIC VS. CONTINENTALWinter (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 NOMENCLATUREBasanite 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 RIFTFailed Arm of the Afar Triple JctMAGMA SERIESHighly AlkalineAlkalineTholeiiticMAGMA SERIES OF THE EAST AFRICAN RIFTSeries 4Oxide 1 2 3 4 5 6 7 8 9 10 11SiO245.6 51.7 46.2 33.1 44.1 55.4 47.6 61.8 70.3 72.5 50.8 TiO22.4 0.9 1.6 2.6 2.8 0.5 2.0 1.0 0.3 0.2 1.4 Al2O315.6 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 P2O50.6 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 NORMq0.0 0.0 0.0 0.0 0.0 0.0 2.1 9.0 41.7 35.8 9.1 or8.9 29.8 27.5 0.0 31.0 34.2 5.5 33.7 28.1 27.8 2.6 ab31.4 28.6 8.0 0.0 0.0 30.2 26.5 48.3 16.7 30.4 25.2 an28.3 0.0 0.0 7.3 6.5 0.0 30.0 0.0 0.0 0.0 31.9 lc0.0 0.0 0.0 20.7 13.2 0.0 0.0 0.0 0.0 0.0 0.0 ne0.0 28.3 39.1 18.2 22.2 27.1 0.0 0.0 0.0 0.0 0.0 di14.2 6.5 13.7 15.0 16.7 2.8 20.8 2.9 0.1 0.0 12.7 hy0.0 0.0 0.0 0.0 0.0 0.0 8.8 0.0 0.0 0.0 13.8 wo0.0 3.9 5.7 0.0 0.0 4.1 0.0 0.9 0.6 0.0 0.0 ol9.4 0.0 0.0 10.9 1.8 0.0 0.0 0.0 0.0 0.0 0.0 il0.5 0.5 0.5 0.8 0.5 0.4 0.5 0.7 0.6 0.1 0.5 ti4.7 0.0 0.0 0.0 0.0 0.0 5.0 1.8 0.1 0.5 3.3 ap1.6 0.8 1.3 5.5 3.1 0.2 0.8 0.5 0.1 0.0 1.0 pf1.0 1.3 2.6