Multistage reaction textures in xenolithic high-MgAl granulites at Anakapalle, Eastern Ghats Belt, India: examples of contact polymetamorphism and infiltration-driven metasomatism

摘要

High-MgAl rocks occur as xenoliths (up to 2 in in diameter) in mafic granulites at a newly discovered locality near Anakapalle. Following an early phase of deformation, ultrahigh-temperature (UHT) metamorphism and near-isothermal decompression, the rocks were intruded in a lit-par-lit manner by felsic melts (charnockite), which caused local-scale metasomatism. A subsequent deformation produced isoclinal folds and the distinct gneissic foliation of the charnockite still at granulite facies conditions. The sequence of multiphase reaction textures in the high-MgAl xenoliths reflects the changes of physicochemical conditions during the polyphase evolution of the terrane, UHT metamorphism (stage 1, >1000 degreesC, c. 10 kbar) is documented by relics of extremely coarse grained domains with the assemblage orthopyroxene (opx)(1) + garnet (grt)(1) + sapphirine (spr)(1) + spinel (spl)(1) + rutile (rt). A subsequent phase of near-isothermal decompression in the order of 1-2 kbar (stage 2) resulted in extensive replacement of grt 1 and opx I megacrysts by lamellar (opx(2) + spr(2)) symplectites. The intrusion of felsic melt (stage 3) led to the development of a narrow metasomatic black wall reaction zone (bt + sil + plg(3) + opx(2,3) + rt) at the immediate contact of the xenoliths and in melt infiltration zones to the partial replacement of (opx(2) + spr(2)) symplectites by biotite and sillimanite and/or plg(3), mainly at the expense of orthopyroxene, with concomitant coarsening of the intergrowth texture. The subsequent deformation (stage 4) further modified the symplectite textures through polygonization, recrystallization and grain-size coarsening. The deformation was followed by a period of cooling and decompression (stage 5, c. 800 degreesC, 4-7 kbar) as indicated by local growth of late garnet (grt(5),) at the expense of (opx + spr + plg) domains at static conditions, Recently published isotope data suggest that the multistage evolution of the high-MgAl granulites at Anakapalle followed a discontinuous P-T trajectory that may be related to heating of the crust through magmatic accretion culminating in deep-crustal UHT metamorphism at 1.4 Ga (stage 1), fast uplift of the UHT granulites into mid-crustal levels as a consequence of extensional tectonics (stage 2), emplacement of felsic magmas in the Grenvillian (at c. I Ga, stage 3) resulting in reheating of the crust to high-T conditions followed by a phase of compressional tectonics (stage 4) and a period of cooling to the stable geotherm (stage 5) still in the Grenvillian. [References: 64]