A slab breakoff model for the Neogene thermal evolution of South Karakorum and South Tibet

On the South Karakorum margin, Neogene high-temperature–medium-pressure (HT–MP) gneisses define an east–west trending thermal anomaly. These rocks have been heated from 600 to 750°C during a slight pressure drop from 0.7 to 0.5 GPa. Their retrogressive path cross-cuts the relaxed geotherm of tectonically thickened crust. Such a P–T evolution occurs only if an advective source of heat is involved. Involvement of an advective heat source is also implied by the occurrence of Neogene granitoids and lamprophyres within the HT–MP gneiss area. These rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies. We interpret these geochemical characteristics to be the result of melting of metasomatized Asian lithospheric mantle. The Nd and Sr isotopic compositions of the South Karakorum Neogene magmatic rocks (ϵNd=−12 to −7 and 87Sr/86Sr=0.705–0.725) further suggest they could have originated from mixing between Asian variously metasomatized mantle and Precambrian crust. By contrast, the origin of the youngest magmatic rocks (<10 Myr), here exemplified by the Hemasil syenite and associated lamprophyres, requires involvement of a depleted mantle. The combined ϵHf–ϵNd signature of these rocks (ϵHf=+10.4–+11.5 and ϵNd=+3.4–+4.3) suggests that the source of the Hemasil syenite could have been depleted mantle contaminated by oceanic sediments, likely during the earlier subduction of the Tethyan ocean. Neogene magmatic rocks with the same geochemical characteristics and evolution as those of South Karakorum have previously been described in South Tibet. Based on their location and the geochemical evolution of their source region, we here propose that the Neogene magmatic and metamorphic evolution of the South Asian margin was controlled by slab breakoff of the subducting Indian continental margin starting at about 25 Ma. This model is supported by available geophysical data from South Karakorum and South Tibet.