ORTHOPYROXENE plus SILLIMANITE PREDATING SAPPHIRINE plus QUARTZ: A RARE CASE OF ULTRAHIGH-TEMPERATURE METAMORPHISM FROM THE CENTRAL ZONE, LIMPOPO COMPLEX, SOUTH AFRICA

Sapphirine + quartz and orthopyroxene + sillimanite occur in garnet from an Mg-Al granulite from the Central Zone of the Limpopo Complex in South Africa. Textural evidence and a chemical gradient in garnet between the zones preserving the inclusions argue for the formation of sapphirine + quartz after orthopyroxene + sillimanite. Petrological observations, pressure-temperature phase diagrams, and compositional and model proportion results on isopleths indicate the sapphirine + quartz + garnet + orthopyroxene (high-Al) assemblage as the peak metamorphic assemblage (similar to 1050 degrees C at similar to 8.5 kbars), whereas orthopyroxene (low-Al) + sillimanite represents the prograde stage (at ca. 900 degrees C at similar to 8.5 kbars). Our report of these two diagnostic ultrahigh-temperature mineral assemblages in garnet from an Mg-Al granulite is unique, given the rare occurrence of sapphirine + quartz postdating orthopyroxene + sillimanite assemblage in granulites.

In situ synthesis of Al2O3-ZrO2-SiCW ceramic matrix composites by carbothermal reduction of natural silicates

In situ formations of Al2O3 + ZrO2 + SiCW ternary composite powders have been obtained by carbothermal reduction of a mixture of Sillimanite. Kaolin and Zircon using two different carbon sources. Products formed were mixtures of alumina and zirconia along with silicon carbide in the form of whiskers. The effects of composition of the reactants, the role of fineness of the starting precursors and the nature of the carbon Source on the final product powder obtained are presented. XRD and SEM analyses indicate complete reaction of the precursors to yield Al2O3 + ZrO2 + SiCW as product powders, with the SiC having whisker morphology. It is also seen that zirconia could be stabilised to some extent in the tetragonal form without any stabilising agent by tailoring the starting materials and their composition. (C) 2002 Published by Elsevier Science B.V.

Multiple garnet growth in garnet-kyanite-staurolite gneiss, Pangong metamorphic complex, Ladakh Himalaya: New constraints on tectonic setting

Garnet-kyanite-staurolite gneiss in the Pangong complex, Ladakh Himalaya, contains porphyroblastic euhedral garnets, blades of kyanite and resorbed staurolite surrounded by a fine-grained muscovite-biotite matrix associated with a leucogranite layer. Sillimanite is absent. The gneiss contains two generations of garnet in cores and rims that represent two stages of metamorphism. Garnet cores are extremely rich in Mn (X(Sps) = 0.35-038) and poor in Fe (X(Alm) = 0.40-0.45), whereas rims are relatively Mn-poor (X(Sps) =0.07-0.08), and rich in Fe (X(Alm), = 0.75-0.77). We suggest that garnet cores formed during prograde metamorphism in a subduction zone followed by abrupt exhumation, during early collision of the Ladakh arc and Karakoram block. The subsequent India-Asia continental collision subducted the metamorphic rocks to a mid-crustal level, where the garnet rims overgrew the Mn-rich cores at ca. 680 degrees C and ca. 8.5 kbar. PT calculations were estimated from phase diagrams calculated using a calculated bulk chemical composition in the Mn-NCKFMASHT system for the garnet-kyanite-staurolite-bearing assemblage. Muscovites from the metamorphic rocks and associated leucogranites have consistent K-Ar ages (ca. 10 Ma), closely related to activation of the Karakoram fault in the Pangong metamorphic complex. These ages indicate the contemporaneity of the exhumation of the metamorphic rocks and the cooling of the leucogranites. (C) 2011 Elsevier B.V. All rights reserved.

Ultrahigh-temperature metamorphism from an unusual corundum plus orthopyroxene intergrowth bearing Al-Mg granulite from the Southern Marginal Zone, Limpopo Complex, South Africa

The Southern Marginal Zone of the Limpopo Complex is composed of granite-greenstone cratonic rocks reworked by a Neoarchean high-grade tectono-metamorphic event. Petrographic and mineral chemical characterization of an Al-Mg granulite from this zone is presented here. The granulite has a gneissic fabric with distinct Al-rich and Si-rich layers, with the former preserving the unusual lamellar (random and regular subparallel) intergrowths of corundum and symplectic intergrowth of spinel with orthopyroxene. The Al-rich layer preserves mineral assemblages such as rutile with orthopyroxene + sillimanite +/- A quartz, Al-rich orthopyroxene (similar to 11 wt%), spinel + quartz, and corundum in possible equilibrium with quartz, while the Si-rich layer preserves antiperthites and orthopyroxene + sillimanite +/- A quartz, all considered diagnostic of ultrahigh-temperature metamorphism. Application of Al-in-opx thermometry, ternary feldspar thermometry and construction of suitable pressure-temperature phase diagrams, compositional and model proportion isopleth results indicate P-T conditions as high as similar to 1,050-1,100 A degrees C, and similar to 10-12 kbars for the Al-Mg granulite. Our report of ultrahigh-temperature conditions is significant considering that the very high temperature was reached during decompression of an otherwise high-pressure granulite complex (clockwise P-T path), whereas most other ultrahigh-temperature granulites are linked to magma underplating at the base of the crust (counterclockwise P-T path).

Peraluminous sapphirine-cordierite pods in Mg-rich orthopyroxene granulite from southern India: Implications for lower crustal processes

Sapphirine-cordierite intergrowths occur as pods within garnet-absent, high-Mg orthopyroxene-granulite xenoliths in the Kambam valley, Madurai Block, southern India. Whereas the cores of the pods are composed of sapphirine (X-Mg = 0.871-0.897) - cordierite (X-Mg = 0.892-0.931) intergrowth along with rutile, zircon and monazite, the rims are characterized by cordierite, apatite, plagioclase, K-feldspar, quartz and minor calcite. The surrounding matrix comprises orthopyroxene (maximum Al2O3 4.1 wt.%, X-Mg 0.848-0.850), plagioclase, biotite and quartz, similar to the assemblage in the surrounding charnockites. Sapphirine in the Kambam rocks is characterized by high Al contents with an end-member composition in the range of 7:9:3 and 3:5:1. The occurrence of peraluminous sapphirine in association with cordierite and in the absence of phases such as sillimanite and garnet is distinct from ultrahigh-temperature assemblages in other localities within the Madurai Block. The peraluminous composition of the pods suggests that these domains could represent cryptic pathways through which aluminous melts migrated. The reaction of such peraluminous melts with Mg-rich orthopyroxene in the host granulite at temperatures of 1025 degrees C and pressures around 8 kbar as computed from phase equilibria modeling followed by an isobaric cooling is inferred to have generated the sapphirine-cordierite pods. The unusual high-Mg orthopyroxene granulite suggests interaction of supracrustal rocks with mafic magmas, which probably acted as the heat source for the partial melting of lower crust and UHT metamorphism.

Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications

The occurrence of high-pressure mafic-ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet-clinopyroxene-quartz coronas that formed after the breakdown of orthopyroxene-plagioclase-ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system indicate a pressure-temperature (P-T) evolution from an isothermal (780 degrees C) pressure up to c. 24 kbar to decompression and cooling. Such a P-T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U-Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625 Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818 Ma and Th/U ratios vary from 1.08 to 2.09. K-Ar dating of muscovite and biotite from biotite-kyanite-sillimanite gneiss and garnet-biotite gneiss yields age of 486 +/- 9 Ma and 459 +/- 9 Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43 km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal formation of the Betsimisaraka suture in north-eastern Madagascar. (C) 2015 Elsevier B.V. All rights reserved.