Metamorphism And Deformation Of The Shangla Blueschist Along The Indus Suture Zone,Pakistan Himalaya

The Shangla blueschist,exposed along the Indus Suture Zone(ISZ),Pakistan Himalaya,have been petrologically investigated to elucidate their tectono-metamorphic evolution,and to understand the geodynamic scenario of an intra-oceanic subduction between the Indian Plate and Kohistan-Ladakh Arc.The Shangla blueschist consists mainly of amphiboles(glaucophane,magnesioriebeckite,winchite,actinolite,edenite,and magnesiohornblende),epidote,phengite,albite and chlorite,with minor amount of titanite,rutile,quartz and hematite.Three distinct metamorphic stages are constrained for the Shangla blueschist.An early prograde(pre–peak)metamorphic stage(M1)is characterized by presence of winchite as core part of zoned amphibole,together with inclusions of epidote,albite,chlorite,quartz,titanite and hematite in epidote porphyroblasts.This stage roughly reveals temperature range of 300–450°C and pressures of 0.4–0.7 GPa.The equilibrium minerals of peak(M2)metamorphic stage consist of matrix,schistosity-forming amphiboles(glaucophane,magnesioriebeckite),epidote and phengite,with fine-grained rutile,titanite and hematite in albite porphyroblasts,suggesting P–T condition of 350–520 °C and 0.9–1.2 GPa.The retrograde(M3)metamorphic stage involved overgrowth of winchite and actinolite along glaucophane rim,chlorite replacement of amphibole and growth of albite porphyroblasts,representing the greenschist facies metamorphic conditions.Our obtained peak P–T metamorphic condition(epidote-blueschist facie)is significantly higher than previously estimated P–T ranges of 400 °C/0.7 GPa.Moreover,the retrograde approximately reversed prograde trajectory defines a hairpin-type P–T path for the Shangla blueschist,similar to the Franciscan–type metamorphism that characterized by relatively slow exhumation after peak stage with the decompression accompanied by cooling process.Taking the published geochronological data into account,these new results provide insights into the metamorphic evolution of the Shangla blueschist during ca.80 Ma(Late Cretaceous)from burial of cold subduction of the Neo–Tethys to subsequent post peak exhumation.Furthermore,the exhumation mechanism of the Shangla blueschist is most likely driven by a combined effect of both underplating and later thrust faulting,probably during the collision of Indian Plate and Kohistan-Ladakh Arc.In addition,the results on deformation study shows that the lower angle to foliation(neighbor misorientation)and higher fabric strengths(perfectly aligned amphibole crystals)for glaucophane advocating strain partitioning into the glaucophane rich layer as obvious in specimen S13,and further indicating the weakness of glaucophane against other mineral phases to deformation.Thus,it could be evident that glaucophane could have possibly aided exhumation of these blueschists from mid-crustal levels to upper crustal depths.The Shangla blueschist(at depths of ~45 km)possibly have potentially triggered the intraplate earthquakes(dehydration embrittlement)and at such depths,brittle deformation of subducting Indian crust may have controlled mainly by the glaucophane.