Abstract:Garnet glaucophane schist coexistent with Permian limestone, quartzite and muscovite schist was recognized below Permian (meta)basalts and altered gabbro in the vicinity of Pana Village within the southern central Lhasa terrain. The blueschist consists of corroded garnet and amphibole porphyroblasts situated in a matrix of finegrained muscovite, epidote, chlorite, biotite, albite, quartz, and accessory rutile, titanite and apatite. Mn and Mg contents of the corroded garnet enclosing aegirinerich clinopyroxene, rutile and quartz decrease and increase, respectively, from core to rim. Amphibole porphyroblasts grew in several stages, as evidenced by two different galucophane components in the cores and NaCa amphibole and/or Ca amphibole in the rims. The glaucophane surrounded by late glaucophane is richer in Fe and poorer in Na than the late one that is close to the pure glaucophane endmember. The Si contents of phengite are high in the center but low along the rim. The p-t path, starting above 2.5 GPa450℃ and showing subsequently first a temperature increase to 500℃ and then a pressure release via blueschist conditions to 0.6 GPa, was reconstructed using a p-t pseudosection calculated for the p-t range 0.4~2.8 GPa and 250~650℃. Such a low gradient is typical for the subduction of a cold oceanic plate along a continental margin. In combination with early Triassic Chaqupu andesite, late Triassicearly Jurassic peraluminous granites and Permian fossils within the metamorphic complex, this finding has led to the conclusion that, prior to Gondwana dispersion in early Mesozoic, the subduction of an ocean plate beneath the northeastern Gondwana margin took place during the PermoTriassic period and that, subsequent outgrowth of NEtrending Gondwana triggered off the final formation of the Lhasa terrain.
