GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY Lecture 15A Continental Alkaline Magmatism March 16, 2016 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 biotitephlogopite 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 HIGHLY ALKALINE MAGMA SERIES Highly Alkaline Alkaline Tholeiitic ROLE OF VOLATILES AND LOW DEGREES OF PARTIAL MELTING TO CREATE ALKALINE MAGMAS Figure 19.14. Grid showing the melting products as a function of pressure and % partial melting of model pyrolite mantle with 0.1% H2O. Dashed curves are the stability limits of the minerals indicated. After Green (1970), Phys. Earth Planet. Inter., 3, 221235. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. MAGMA SERIES OF THE EAST AFRICAN RIFT Table 19-2. Representative Chemical Analyses of East African Rift Volcanics Oxide SiO2 TiO2 Series 1: Alkaline 1 2 45.6 51.7 3 46.2 Series 2: Ultra-alkaline 4 5 33.1 44.1 6 55.4 Series 3: Transitional Basalt-Rhyolite 7 8 9 10 47.6 61.8 70.3 72.5 Series 4 11 50.8 2.4 0.9 1.6 2.6 2.8 0.5 2.0 1.0 0.3 0.2 1.4 15.6 11.3 0.2 6.9 10.4 3.2 19.3 5.9 0.2 1.1 4.1 8.9 18.6 8.9 0.2 2.3 7.3 9.3 11.3 12.4 0.3 7.3 17.2 3.2 17.0 10.0 0.2 3.7 8.4 4.3 20.8 4.6 0.2 0.5 2.9 9.2 14.8 11.4 0.2 6.4 11.5 2.7 14.2 6.4 0.3 0.5 1.8 6.2 7.6 8.4 0.3 0.0 0.4 7.3 10.3 4.0 0.1 0.0 0.2 5.9 14.9 10.1 0.2 6.9 9.8 2.6 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 Total 97.5 CIPW NORM q 0.0 or 8.9 ab 31.4 an 28.3 lc 0.0 ne 0.0 di 14.2 hy 0.0 wo 0.0 ol 9.4 il 0.5 ti 4.7 ap 1.6 pf 1.0 ns 0.0 cs 0.0 0.3 97.0 0.5 99.1 1.9 92.9 1.2 98.9 0.1 99.7 0.3 97.7 0.2 97.6 0.0 98.8 97.6 0.4 97.4 0.0 29.8 28.6 0.0 0.0 28.3 6.5 0.0 3.9 0.0 0.5 0.0 0.8 1.3 0.4 0.0 0.0 27.5 8.0 0.0 0.0 39.1 13.7 0.0 5.7 0.0 0.5 0.0 1.3 2.6 1.7 0.0 0.0 0.0 0.0 7.3 20.7 18.2 15.0 0.0 0.0 10.9 0.8 0.0 5.5 4.8 0.0 16.8 0.0 31.0 0.0 6.5 13.2 22.2 16.7 0.0 0.0 1.8 0.5 0.0 3.1 4.9 0.0 0.0 0.0 34.2 30.2 0.0 0.0 27.1 2.8 0.0 4.1 0.0 0.4 0.0 0.2 0.5 0.4 0.0 2.1 5.5 26.5 30.0 0.0 0.0 20.8 8.8 0.0 0.0 0.5 5.0 0.8 0.0 0.0 0.0 9.0 33.7 48.3 0.0 0.0 0.0 2.9 0.0 0.9 0.0 0.7 1.8 0.5 0.0 2.1 0.0 41.7 28.1 16.7 0.0 0.0 0.0 0.1 0.0 0.6 0.0 0.6 0.1 0.1 0.0 12.0 0.0 35.8 27.8 30.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.5 0.0 0.0 5.3 0.0 9.1 2.6 25.2 31.9 0.0 0.0 12.7 13.8 0.0 0.0 0.5 3.3 1.0 0.0 0.0 0.0 Al2O3 FeO* MnO MgO CaO Na2O K2O 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. Q-F either /or 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. 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 Carbonatite: >50% carbonate minerals Silico-carbonatite: 50-10% carbonate minerals Table 19-3. Carbonatite Nomenclature Name Calcite-carbonatite Dolomite-carbonatite Ferrocarbonatite Natrocarbonatite Alternative Coarse Med.-Fine sövite alvikite rauhaugite* beforsite * Rarely used, beforsite may be applied to any grain size. 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, 13340. CHEMICAL ATTRIBUTES OF CARBONATITES Table 19-5. Representative Carbonatite Compositions % SiO2 CalciteDolomiteFerroNatrocarbonatite carbonatite carbonatite carbonatite 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 FeO MnO MgO CaO Na2O 2.25 1.01 0.52 1.80 49.1 0.29 2.41 3.93 0.96 15.06 30.1 0.29 7.44 5.28 1.65 6.05 32.8 0.39 0.05 0.23 0.38 0.38 14.0 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 BaO SrO F Cl S SO3 36.6 0.34 0.86 0.29 0.08 0.41 0.88 36.8 0.64 0.69 0.31 0.07 0.35 1.08 30.7 3.25 0.88 0.45 0.02 0.96 4.14 31.6 1.66 1.42 2.50 3.40 3.72 Figure 19-15. Silicate-carbonate liquid immiscibility in the system Na2OCaO-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 Table 19-8. Average Analyses and Compositional Ranges of Kimberlites, Orangeites, and Lamproites. SiO2 Kimberlite 33.0 27.8-37.5 Orangeite 35.0 27.6-41.9 Lamproite* 45.5 TiO2 1.3 0.4-2.8 1.1 0.4-2.5 2.3 Al2O3 FeO* MnO MgO CaO Na2O 2.0 7.6 0.14 34.0 6.7 0.12 1.0-5.1 5.9-12.2 0.1-0.17 17.0-38.6 2.1-21.3 0.03-0.48 2.9 7.1 0.19 27. 7.5 0.17 0.9-6.0 4.6-9.3 0.1-0.6 10.4-39.8 2.9-24.5 0.01-0.7 8.9 6.0 11.2 11.8 0.8 K2O 0.8 0.4-2.1 3.0 0.5-6.7 7.8 P2O5 LOI 1.3 10.9 0.5-1.9 7.4-13.9 1.0 11.7 0.1-3.3 5.2-21.5 2.1 3.5 Sc V Cr Ni Co Cu Zn Ba Sr Zr Hf Nb Ta Th U La Yb 14 100 893 965 65 93 69 885 847 263 5 171 12 20 4 150 1 20 95 1722 1227 77 28 65 3164 1263 268 7 120 9 28 5 186 1 Data from Mitchell (1995), Mitchell and Bergman (1991) * Leucite Hills madupidic lamproite 19 66 430 152 41 9831 3860 1302 42 99 6 37 9 297 1 Lamproites – Mafic mineralogy Kimberlites/Orangites – Ultramafic mineralogy ULTRAPOTASSIC ROCKS LAMPROITES Lamproites • K/Na > 3 (ultrapotassic) • K/Al > 1 (perpotassic) • (K+Na)/Al > 1 (peralkaline) • mg# > 70 • Incompatible element-enriched Table 19-6. Lamproite Nomenclature Old Nomenclature Recommended by IUGS wyomingite orendite madupite cedricite mamilite wolgidite fitzroyite verite jumillite fortunite cancalite diopside-leucite-phlogopite lamproite diopside-sanidine-phlogopite lamproite diopside madupidic lamproite diopside-leucite lamproite leucite-richterite lamproite diopside-leucite-richterite madupidic lamproite leucite-phlogopite lamproite hyalo-olivine-diopside-phlogopite lamproite olivine diopside-richterite madupidic lamproite hyalo-enstatite-phlogopite lamproite enstatite-sanidine-phlogopite lamproite From Mitchell and Bergman (1991). ULTRAPOTASSIC ROCKS KIMBERLITES/ORANGITES Figure 19.21. Hypothetical cross section of an Archean craton with an extinct ancient mobile belt (once associated with subduction) and a young rift. The low cratonal geotherm causes the graphite-diamond transition to rise in the central portion. Lithospheric diamonds therefore occur only in the peridotites and eclogites of the deep cratonal root, where they are then incorporated by rising magmas (mostly kimberlitic“K”). Lithospheric orangeites (“O”) and some lamproites (“L”) may also scavenge diamonds. Melilitites (“M”) are generated by more extensive partial melting of the asthenosphere. Depending on the depth of segregation they may contain diamonds. Nephelinites (“N”) and associated carbonatites develop from extensive partial melting at shallow depths in rift areas. GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY Lecture 15B Anorthosites and the Anorthosite Problem March 16, 2016 GLOBAL OCCURRENCES OF ANORTHOSITE 1. Archean anorthosite 2. Proterozoic “massif-type” anorthosite plutons 3. Centimeter-to-100m thick layers in layered mafic intrusions 4. Thin cumulate layers in ophiolites/oceanic crust 5. Small inclusions in other rock types (xenoliths and cognate inclusions) 6. Lunar highland anorthosites (after Ashwal, 1993) Streikeisen (1991) ARCHEAN ANORTHOSITES • Hi-Ca plagioclase (An >80) • Equant crystal habit • Pl megacrysts in a gabbroic matrix • Sill-like bodies • Associated with gabbroic intrusions in greenstone belts • Commonly metamorphosed and deformed MASSIF-TYPE ANORTHOSITES Proterozoic age (1.7-0.9 Ga) Intermediate Pl composition (An 65-40) Tabular crystal habit Pl-adcumulates common (minor interstitial mafics) Large pluton-scale bodies Anorogenic setting Associated with K-/Fe-rich gabbros (jonuites) and “dry” felsic rocks (charnokites) Grenville Province ANORTHOSITE LAYERS/INCLUSIONS IN MAFIC LAYERED INTRUSIONS Bushveld Complex Skaergaard Intrusion Stillwater Complex ANORTHOSITIC ROCKS OF THE DULUTH COMPLEX AND RELATED ROCKS OF NE MN Anorthositic Rock–bearing Units ANORTHOSITE INCLUSIONS OF THE BEAVER BAY COMPLEX •90-100% Plagioclase •An 70-90 •Mafic phases – Ol, Opx •Single grains to 500m block •Isotopes indicate older crustal component •Enclosing diabase NOT chilled against inclusions LAKE SUPERIOR ANORTHOSITES A 150 YEAR PUZZLE Norwood (1852) Lawson (1893) Winchell (1899) BEAVER RIVER DIABASE HOST OF ANORTHOSITE INCLUSIONS Leveaux Porphyry Lutsen Tofte Silver Bay Carlton Peak Split Rock PORPHYRITIC FE-DIORITE SILLS LEVEAUX PORPHYRY & CABIN CREEK PORPHYRITIC DIORITE ANORTHOSITIC SERIES OF THE DULUTH COMPLEX troctolitic anorthosite poikilitic olivine anorthosite olivine leucogabbro gabbroic anorthosite olivine-oxide leucogabbro pOGA (PPocf) TA (PpO) “An Igneous Breccia” R. Taylor, 1964 An (mPP) mTA (mPpO) GA (cPpcf) TA (cPPOc) Well developed, but erratic foliation Summary of Lithologic and Structural Relationships of Anorthositic Series rocks in the Snowbank Lake Quadrangle Fo-An Relationships Plagioclase Zonation Patterns Complex Patterns Outer jump in An – Decompression? Hi-P Lo-P MODELS OF ANORTHOSITE PETROGENESIS Weiblen and Morey (1980) Model for the formation of the Anorthositic and Layered (Troctolitic) Series Major Problem: No Evidence for Significant Upper Crustal Ultramafic Rocks AS Parent Magma = Plagioclase Crystal Mush GEOCHRONOLOGY OF MCR INTRUSIONS ANORTHOSITIC SERIES ≈ LAYERED SERIES GENERAL MODEL OF ANORTHOSITE PETROGENESIS ASHWAL (1993)