Petrology of the Coyaguayma ignimbrite, northern Puna of Argentina: Origin and evolution of a peraluminous high-SiO 2 rhyolite magma

The Coyaguayma ignimbrite is a strongly peraluminous (SP), sillimanite, garnet-bearing silicic rhyolite which erupted in the northern Puna segment of the central Andean plateau in the Upper Miocene (~11Ma), a period that was characterized by the eruption of voluminous (100s to 1000s km 3) dacitic ignimbrites of high-K calc-alkaline affinity. In this region, the SP magmatic rocks are both rare and small, but their importance is potentially much greater as rocks of this type are usually interpreted as products of crustal melting and therefore useful for addressing mantle addition vs crustal recycling in the central Andes.The phenocryst assemblage of the Coyaguayma ignimbrite comprises plagioclase (An26-18), quartz, Ba-rich and Ba-poor sanidine, minor Al-rich ferromagnesian minerals (Al T- and Al VI-rich biotite, almandine-spessartine garnet) and sillimanite, as well as accessory zircon and monazite. Textural relations suggest that the accessory and ferromagnesian phases crystallized before quartz and feldspars. Mineral equilibria suggest that crystallization of the rhyolite magma began at ~5kbar and 800°C, and continued almost isobarically to 720°C, causing the residual liquid to increase H 2O contents from ~4-5% to ~7.5% before eruption.Most major features (e.g., high SiO 2, A/CNK >1.3, low CaO, MgO, TiO 2 and FeO) and trace element patterns (low Ba, Sr, Th, LREE and Eu/Eu*; high Rb, U, Y and Nb), along with the thermobarometric constraints on magmatic P, T and water contents are consistent with an origin by mica dehydration melting of metapelitic sources (e.g., typical biotite-muscovite gneisses from the outcropping S Puna basement). However, the relatively low initial 87Sr/ 86Sr (~0.7125), and high 143Nd/ 144Nd (~0.512200) ratios invalidate a pure crustal origin. Instead, we propose contamination of calc-alkaline dacitic magmas similar to typical Puna ignimbrites by metapelite at mid-crustal settings (≥18km depth). Geochemical modeling that satisfies the chemical and isotopic data suggests mixing of 70% dacite and ~30% of metapelite partial melts, followed by extensive (70%) fractionation of plagioclase, K-feldspar, quartz and biotite, with minor magnetite and apatite from the hybrid magma. Sillimanite in the Coyaguayma rhyolite is interpreted as a restite mineral or a product of incongruent melting of the metapelite, which was preserved intact in the hybrid melt due to local equilibrium.This petrogenetic model explains most characteristics of crystal-poor SP rhyolites from the Puna plateau (e.g., Tocomar, Ramadas rhyolites) and it may be more generally applicable to occurrences of SP magmas in Andean-type continental arcs dominated by calc-alkaline magmatism.Fil: Caffe, Pablo Jorge. Universidad Nacional de Jujuy; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Trumbull, Robert. German Research Centre for Geosciences; AlemaniaFil: Siebel, Wolfgang. Eberhard Karls Universität Tübingen; Alemani

The Grønnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility

The Grønnedal-Ika complex is dominated by layered nepheline syenites which were intruded by a xenolithic syenite and a central plug of calcite to calcite-siderite carbonatite. Aegirine-augite, alkali feldspar and nepheline are the major mineral phases in the syenites, along with rare calcite. Temperatures of 680-910°C and silica activities of 0·28-0·43 were determined for the crystallization of the syenites on the basis of mineral equilibria. Oxygen fugacities, estimated using titanomagnetite compositions, were between 2 and 5 log units above the fayalite-magnetite-quartz buffer during the magmatic stage. Chondrite-normalized REE patterns of magmatic calcite in both carbonatites and syenites are characterized by REE enrichment (LaCN-YbCN = 10-70). Calcite from the carbonatites has higher Ba (∼5490 ppm) and lower HREE concentrations than calcite from the syenites (54-106 ppm Ba). This is consistent with the behavior of these elements during separation of immiscible silicate-carbonate liquid pairs. εNd(T = 1·30 Ga) values of clinopyroxenes from the syenites vary between +1·8 and +2·8, and εNd(T) values of whole-rock carbonatites range from +2·4 to +2·8. Calcite from the carbonatites has δ18O values of 7·8 to 8·6‰ and δ13C values of −3·9 to −4·6‰. δ18O values of clinopyroxene separates from the nepheline syenites range between 4·2 and 4·9‰. The average oxygen isotopic composition of the nepheline syenitic melt was calculated based on known rock-water and mineral-water isotope fractionation to be 5·7 ± 0·4‰. Nd and C-O isotope compositions are typical for mantle-derived rocks and do not indicate significant crustal assimilation for either syenite or carbonatite magmas. The difference in δ18O between calculated syenitic melts and carbonatites, and the overlap in εNd values between carbonatites and syenites, are consistent with derivation of the carbonatites from the syenites via liquid immiscibilit

Tectono-metamorphic evolution of the Wadi Hafafit Culmination (central Eastern Desert, Egypt). Implication for Neoproterozoic core complex exhumation in NE Africa

The Neoproterozoic rock assemblages in the Wadi Hafafit Culmination (WHC) can be subdivided into two main units which are separated by the Nugrus Thrust. The structurally higher Nugrus unit is mainly composed of low grade micaschists, metavolcanic, serpentinites, and metagabbros. The overthrusted Hafafit unit forms the Hafafit domes and is composed of ortho- and para-gneisses associated with amphibolite and ultramafic rocks. Mineral chemistry and thermobarometry indicate that the WHC was affected by two main metamorphic phases. The first metamorphic phase (M1), observed in the micaschists of the Nugrus unit, is characterized by greenschist-facies conditions. Garnet-biotite and garnet-muscovite geothermometry, as well as temperatures calculated by means of the TWEEQU program yield temperatures of 400°-550°C, whereas the white mica geobarometer reveals pressure of 3.7-4.9 kbar for this metamorphic phase (M1). The second metamorphic phase (M2), observed in gneisses and amphibolites of the Hafafit unit, is characterized by amphibolite-facies conditions. Garnet-biotite, garnet-amphibole and amphibole-plagioclase geothermometry yield temperatures of 600°-750°C, whereas the garnet-hornblende-plagioclase-quartz geobarometer indicates pressures of 6-8 kbar for the second metamorphic phase (M2). Sm-Nd and Rb-Sr whole rock-mineral isochron ages around 590 Ma for gneisses and amphibolites probably represent cooling from the metamorphic thermal peak which was attained around 600 Ma or slightly earlier. A 3-stage geologic evolution model is proposed for the tectonic evolution of the WHC. The first stage started earlier than 680 Ma ago with rifting and ocean floor spreading at a time which is as yet unspecified. It was followed by a second stage of subduction and emplacement of subduction-related granitoids around 620-640 Ma. At this time, the Hafafit region has become an active margin with the production of large amounts of calc-alkaline subduction-related volcanic and plutonic sequences. Subduction was terminated by collision and NW-ward Nugrus Nappe thrusting under greenschist-facies conditions (M1) around 620-640 Ma. At this stage, rocks of Hafafit unit were subjected to intense deformation and metamorphism in amphibolite facies (M2). Next came the third stage of late-orogenic extension and crustal thinning that was controlled by the Najd transform faults (620-580 my) and that resulted in exhumation of the Hafafit domes through a combination of transpression and lateral extrusion