Ultra high temperature metamorphism of mafic granulites from South Altyn Orogen, West China: A result from the rapid exhumation of deeply subducted continental crust

The South Altyn orogen in West China contains ultra high pressure (UHP) terranes formed by ultra-deep (>150-300 km) subduction of continental crust. Mafic granulites which together with ultramafic interlayers occur as blocks in massive felsic granulites in the Bashiwake UHP terrane, are mainly composed of garnet, clinopyroxene, plagioclase, amphibole, rutile/ilmenite, and quartz with or without kyanite and sapphirine. The kyanite/sapphirine-bearing granulites are interpreted to have experienced decompression-dominated evolution from eclogite facies conditions with peak pressures of 4-7 GPa to high pressure (HP)-ultra high temperature (UHT) granulite facies conditions and further to low pressure (LP)-UHT facies conditions based on petrographic observations, phase equilibria modelling, and thermobarometry. The HP-UHT granulite facies conditions are constrained to be 2.3-1.6 GPa/1,000-1,070 degrees C based on the observed mineral assemblages of garnet+clinopyroxene+rutile+plagioclase+amphibole +/- quartz and measured mineral compositions including the core-rim increasing anorthite in plagioclase (X-An = 0.52-0.58), core-rim decreasing jadeite in clinopyroxene (X-Jd = 0.20-0.15), and TiO2 in amphibole (Ti-M2/2 = 0.14-0.18). The LP-UHT granulite facies conditions are identified from the symplectites of sapphirine+plagioclase+spinel, formed by the metastable reaction between garnet and kyanite at GPa/940-1,030 degrees C based on the calculated stability of the symplectite assemblages and sapphirine-spinel thermometer results. The common granulites without kyanite/sapphirine are identified to record a similar decompression evolution, including eclogite, HP-UHT granulite, and LP-UHT granulite facies conditions, and a subsequent isobaric cooling stage. The decompression under HP-UHT granulite facies is estimated to be from 2.3 to 1.3 GPa at similar to 1,040 degrees C on the basis of textural records, anorthite content in plagioclase (X-An = 0.25-0.32), and grossular content in garnet (X-Grs = 0.22-0.19). The further decompression to LP-UHT facies is defined to be >0.2-0.3 GPa based on the calculated stability for hematite-bearing ilmenite. The isobaric cooling evolution is inferred mainly from the amphibole (Ti-M2/2 = 0.14-0.08) growth due to the crystallization of residual melts, consistent with a temperature decrease from >1,000 degrees C to similar to 800 degrees C at similar to 0.4 GPa. Zircon U-Pb dating for the two types of mafic granulite yields similar protolith and metamorphic ages of c. 900 Ma and c. 500 Ma respectively. However, the metamorphic age is interpreted to represent the HP-UHT granulite stage for the kyanite/sapphirine-bearing granulites, but the isobaric cooling stage for the common granulites on the basis of phase equilibria modelling results. The two types of mafic granulite should share the same metamorphic evolution, but show contrasting features in petrography, details of metamorphic reactions in each stage, thermobarometric results, and also the meaning of zircon ages as a result of their different bulk-rock compositions. Moreover, the UHT metamorphism in UHP terranes is revealed to represent the lower pressure overprinting over early UHP assemblages during the rapid exhumation of ultra-deep subducted continental slabs, in contrast to the cause of traditional UHT metamorphism by voluminous heat addition from the mantle.