SUPERPOSITION OF REPLACEMENTS IN THE MAFIC GRANULITES OF THE JIJAL COMPLEX OF THE KOHISTAN ARC, NORTHERN PAKISTAN - DEHYDRATION AND REHYDRATION WITHIN DEEP ARC CRUST

A deep-level crustal section of the Cretaceous Kohistan are is exposed in the northern part of the Jijal complex. The occurrence of mafic to ultramafic granulite-facies rocks exhibits the nature and metamorphic evolution of the lower crust. Mafic granulites are divided into two r ock types: two-pyroxene granulite (orthopyroxene + clinopyroxene + pla gioclase +/- quartz [1]): and garnet-clinopyroxene granulite (garnet clinopyroxene + plagioclase + quartz [2]). Two-pyroxene granulite occ urs in the northeastern part of the Jijal complex as a relict host roc k of garnet-clinopyroxene granulite, where the orthopyroxene-rich host is transected by elongated patches and bands of garnet-clinopyroxene granulite. Garnet-clinopyroxene granulite, together with two-pyroxene granulite, has been partly replaced by amphibolite (hornblende +/- gar net + plagioclase + quartz [3]). The garnet-bearing assemblage [2] is expressed by a compression-dehydration reaction: hornblende + orthopyr oxene + plagioclase = garnet + clinopyroxene + quartz + H2O up arrow. Subsequent amphibolitization to form the assemblage [3] is expressed b y two hydration reactions: garnet + clinopyroxene + plagioclase + H2O = hornblende + quartz and plagioclase + hornblende + H2O = zoisite + c hlorite + quartz. The mafic granulites include pod- and lens-shaped bo dies of ultramafic granulites which consist of garnet hornblendite (ga rnet + hornblende + clinopyroxene [4]) associated with garnet clinopyr oxenite, garnetite, and hornblendite. Field relation and comparisons i n modal-chemical compositions between the mafic and ultramafic granuli tes indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet-cl inopyroxene granulite, The results combined with isotopic ages reporte d elsewhere give the following tectonic constraints: (1) crustal thick ening, through the development of the Kohistan are and the subsequent KKohistan-Asia collision caused the high-pressure granulite-facies met amorphism in the Jijal complex; (2) local amphibolitization of the maf ic granulites occurred after the collision. (C) 1998 Elsevier Science B.V. All rights reserved.