摘要
North of Naran in the Kaghan Valley(NE Pakistan),the metabasites of the mélange units lying below the Kohistan Arc,contain glaucophane.Typically they reflect blueschist-metamorphic conditions(0.7 GPa,400℃).Associated graphite-rich metapelites with quartz veins underwent upper greenschist to amphibolite conditions.Near Naran we observed in quartz grains of type 3 veins first minute relics of Fe-Mg carpholite indicating earlier blueschist metamorphic conditions.P-T estimates indicate 1.2-1.6 GPa at 380-410℃,pressure and temperature values typical for blueschist metamorphic conditions.Changes in mineral assemblages and abundant sudoite component in associated chlorite point to a pressure drop after peak I conditions.We assign the observed changes to peak I conditions occurring during a Cretaceous subduction event.Temperatures estimated with Raman graphite-thermometry clearly indicate a significant subsequent rise of post-peak I temperatures up to 500℃.This is compatible with the amphibolite peak II assigned to the Tertiary continental collision that leads to subduction of the Indian Plate and ultra-highpressure metamorphism.During subduction the blueschist metamorphic metapelites underwent dehydration,which caused alteration in the overlying lithospheric mantle.In a hydrated lithospheric mantle density is significantly reduced which enhanced subduction of continental crust in the Higher Himalaya.This P-T evolution is typical for a collision orogen with a high plateau but remarkably contrasting findings from Eastern Anatolia,where plateau building is in“statu nascendi”(e.g.,Oberhänsli et al.,2010).
