High density carbonic fluids in a slab window: Evidence from the Gangdese charnockite, Lhasa terrane, southern Tibet

Abstract

Charnockites, occurring in the well-studied Gangdese batholith in the southern Lhasa terrane, eastern Himalayan orogen, consist of orthopyroxene, clinopyroxene, plagioclase, and minor quartz and K-feldspar, and show adakitic affinity. Here we present a systematic study on the fluid inclusions in Gangdese charnockites which reveals that the primary fluid inclusions occur isolated or randomly distributed and as trails along intragranular fractures in quartz, and that the secondary ones occur along healed mircofractures and coexist commonly with mineral inclusions of calcite, magnetite and hematite within the host plagioclase and quartz. The fluid inclusions contain dominantly near-pure CO 2 with traces of N 2. Most of the primary fluid inclusions have low CO 2 homogenization temperatures and with densities of 1.138–1.013 g/cm 3 suggesting trapping pressures of 0.7–1.0 GPa at temperatures of 850–950 °C. We propose a model in which the southward subduction of Neo-Tethyan mid-oceanic ridge beneath the Lhasa terrane resulted in the release of heat and CO 2 from the upwelling asthenosphere through a slab window, providing high-temperatures (HT) and dry CO 2-rich fluids for the formation and stabilization of the adakitic charnockite as well as the associated HT granulite-facies metamorphic rocks in the Late Cretaceous, prior to the final collision of the Indian plate with the Asian continent. Our study provides new insights into the geodynamics of Andean-type orogeny in the southern Tibetan Plateau during the Late Mesozoic.