Origin of scapolite in deep-seated metagabbros of the Kohistan Arc, NW Himalayas

Scapolites developed variably in the deep-seated metagabbros of the Kohista n Are, north Pakistan, were investigated to evaluate the distribution and m igration of C-O-H-S fluid and origin of carbon in the lower crust. Scapolit es can be classified, based on their distinct difference in occurrence and chemical composition, into carbonate type and sulfate-rich type. Carbonate- type scapolites occur around carbonate pods in the layered gabbros at the m id-crustal levels and were formed by an increase in the CaO activity in the country rocks and simultaneous decarbonation of calcite. Predicted equilib rium CO2 mole fraction in fluid (X-CO2) is sufficiently low to be consisten t with removal of CO2. Sulfate-rich scapolites occur in the garnet-bearing granulites at the lower crustal level and were formed by breakdown reaction s of garnet and plagioclase under relatively higher CO2 activity (a(CO2)) a nd oxygen fugacity. The carbon isotopic composition (delta C-13) in carbona te scapolite is equivalent to the juvenile mantle (-5 to -6 parts per thous and), whereas sulfate-rich scapolite has heavy carbon stable isotope values close to 0 parts per thousand. These results reveal that carbonate scapoli te was formed by decarbonation and metasomatism of igneous carbonate pods w ith the host metagabbros under granulite facies conditions, and sulfate-ric h scapolites were formed by infiltration of CO2-rich fluid derived from dec arbonation of carbonate-bearing sediments in the subducting slab.