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)

Abstract

Textural relations and metamorphic fabrics of metapelites and associated metabasites 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 facies boundary and a subsequent retrograde P– T trajectory to amphibolite facies conditions. The metapelites are characterized by the presence of garnet, biotite, plagioclase, K-feldspar and Al 2SiO 5 phases, showing the P– T path involved change by compression from sillimanite to kyanite stability fields and subsequent decompression along an overall clockwise loop. The metabasites contain garnet, hornblende and plagioclase±biotite and clinopyroxene+plagioclase symplectites that might represent a replacement product after omphacite. Maximum P– T conditions of 12–13 kbar at 700–750°C were deduced from thermobarometric calculations. Peak metamorphism was accompanied by dehydration melting of muscovite, but not biotite, followed by minor back reaction of garnet+K-feldspar+H 2O=Al 2SiO 5+biotite+plagioclase. Retrograde phases in metapelites and metabasites are muscovite, calcite and fluorine- and/or chlorine-rich amphiboles and biotite which probably resulted from infiltration of fluid that had equilibrated with surrounding metacarbonates and metaevaporites.