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

Abstract.

Scapolites developed variably in the deep-seated metagabbros of the Kohistan Arc, north Pakistan, were investigated to evaluate the distribution and migration of C–O–H–S fluid and origin of carbon in the lower crust. Scapolites 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 mid-crustal levels and were formed by an increase in the CaO activity in the country rocks and simultaneous decarbonation of calcite. Predicted equilibrium CO₂ mole fraction in fluid $\left( {X_{CO_2}} \right)$ is sufficiently low to be consistent with removal of CO₂. Sulfate-rich scapolites occur in the garnet-bearing granulites at the lower crustal level and were formed by breakdown reactions of garnet and plagioclase under relatively higher CO₂ activity $\left( {a_{CO_2}} \right)$ and oxygen fugacity. The carbon isotopic composition (δ¹³C) in carbonate scapolite is equivalent to the juvenile mantle (–5 to –6‰), whereas sulfate-rich scapolite has heavy carbon stable isotope values close to 0‰. These results reveal that carbonate scapolite was formed by decarbonation and metasomatism of igneous carbonate pods with the host metagabbros under granulite facies conditions, and sulfate-rich scapolites were formed by infiltration of CO₂-rich fluid derived from decarbonation of carbonate-bearing sediments in the subducting slab.