Eocene-Oligocene potassic high Ba-Sr granitoids in the Southeastern Tibet: Petrogenesis and tectonic implications

Abstract

Potassic high Ba-Sr granitoids are widely distributed over collisional orogenic belts worldwide, however, their magma sources and petrogenetic processes are still debated. To better understand them, we collected a suite of Paleogene high Ba-Sr granitoids in the Jianchuan Basin, southeast Tibet. Based on the mineralogical compositions, major and trace element geochemistry, and age of formation, we subdivide them into two groups: intermediate series and granitic series. LA-MC-ICP-MS zircon U-Pb analyses show that the intermediate series crystallized at 33.8–36.9 Ma, similar to or slightly earlier than the granitic rocks with crystallization ages of 33.1–34.0 Ma. New geochemical and zircon Hf isotopic results, in combination with previous results, support an enriched mantle source for the high Ba-Sr intermediate rocks, and a mixed source of enriched mantle and lower crust for the granitic ones. We suggest that partial melting of enriched mantle with fractional crystallization of mafic minerals produced the potassic high Ba-Sr intermediate series. Meanwhile, the juvenile heat from such magma induced melting of thickened lower crustal materials which together with magma from mantle accounted for source of the granite series. The high Ba-Sr intermediate rocks have nearly identical major and trace element, and isotopic signatures as Archean sanukitoids, thus offer a potential constraint on the formation mechanism for the sanukitoids. Our study supports that the emergence of the sanukitoids requires a sediment-metasomatized mantle and thus indicates the onset of crustal recycling into the mantle.