PETROGENESIS OF ULTRAHIGH-PRESSURE ROCKS AND THEIR COUNTRY ROCKS AT SHUANGHE IN DABIESHAN, CENTRAL CHINA

UHP metamorphic rocks at Shuanghe occur as a thrust slab within the country-rock granitic gneisses. They consist of eclogites, jadeite quartzite, eclogite nodule-bearing marble, and structurally concordant ''in situ'' schists and gneisses - sometimes with relict garnets and phengites. Coesite inclusions have been found in garnets and other UHPM phases within the eclogites. Detailed petrological and mineralogical studies indicate a complex polymetamorphic history for the UHPM slab. A possible P-T path for the UHPM slab involving the subduction of sedimentary and volcanic protoliths and their ensuing exhumation can be outlined as follows: surface protoliths - amphibolite facies (phengite and barroisitic amphibole inclusions in garnet) - peak UHP metamorphism (coesite-eclogite subfacies) - quartz-eclogite subfacies - symplectite of amphibolite facies (garnet replaced by amphibole-plagioclase coronas, clinopyroxene by amphibole-sodic plagioclase symplectite) - post-symplectite of greenschist facies (biotite, epidote and chlorite in the matrix of strongly retrograded eclogites). The path is clockwise, with substantial post-peak near-isothermal decompression, as with those defined for other UHPM terranes. No demon strably ultrahigh pressure mineral phases have yet been found in the country-rock gneisses encompassing the tectonic slab of UHPM rocks at Shuanghe. The prevailing metamorphic P-T conditions in these gneisses correspond to the amphibolite facies. Accordingly, tectonic juxtapositioning of UHPM rocks and country-rock gneisses at high crustal levels late in the tectonometamorphic cycle is favoured.