Metamorphic evolution of the Precambrian South Badakhshan block, based on mineral reactions in metapelites and metabasites associated with whiteschists from Sare Sang (Western Hindu Kush, Afghanistan)

Textural relations and metamorphic fabrics of metapelites and associated me tabasites from the Precambrian Sare Sang series, which is known to contain lapis lazuli deposits and whiteschists, indicate an increase in metamorphic pressures and temperatures to the transition of eclogite-amphibolite facie s boundary and a subsequent retrograde PT trajectory to amphibolite facies conditions. The metapelites are characterized by the presence of garnet, bi otite, plagioclase, K-feldspar and Al2SiO5 phases, showing the P-T path inv olved change by compression from sillimanite to kyanite stability fields an d subsequent decompression along an overall clockwise loop. The metabasites contain garnet, hornblende and plagioclase+/-biotite and clinopyroxene+pla gioclase symplectites that might represent a replacement product after omph acite. Maximum P-T conditions of 12-13 kbar at 700-750 degrees C were deduc ed from thermobarometric calculations. Peak metamorphism was accompanied by dehydration melting of muscovite, but not biotite, followed by minor back reaction of garnet + K-feldspar + H2O = Al2SiO5 + biotite + plagioclase. Re trograde phases in metapelites and metabasites are muscovite, calcite and f luorine- and/or chlorine-rich amphiboles and biotite which probably resulte d from infiltration of fluid that had equilibrated with surrounding metacar bonates and metaevaporites. (C) 1999 Elsevier Science B.V. All rights reser ved.