Paleocene (c. 62 Ma) Leucogranites in Southern Lhasa, Tibet: Products of Syn-collisional Crustal Anatexis during Slab Roll-back?

Abstract

Voluminous peraluminous leucogranites are common in large-scale orogenic belts and are crucial in gaining a fuller understanding of the related geodynamic process. However, the origin of such syn-collisional leucogranites remains highly controversial. In this contribution, we report petrological and geochemical data for Paleocene (c. 63 Ma) garnet-bearing, two-mica granites and associated biotite granites from the Gangdese batholith in southern Tibet. The Zhengga biotite granites have high SiO2 (70–73 wt %) and low MgO (0·4–0·7 wt %) contents with initial 87Sr/86Sr ratios of 0·7049–0·7050, eNd(t) values of +0·5 to +1·2 and zircon δ18O values of 5·6–6·9‰, similar to most early Paleocene granitoids in southern Lhasa. These geochemical characteristics suggest that the Zhengga biotite granites were derived from a crustal source that mixed with variable amounts of Gangdese juvenile lower crust and minor ancient crust-derived melts. The Zhengga peraluminous, garnet-bearing, two-mica granites have similar Sr–Nd–O isotope compositions to the biotite granites (0·7037–0·7050, +0·4 to +0·8, 5·5–7·3‰, respectively) as well as higher SiO2 (73–76 wt %) and lower TiO2 (<0·06 wt %), MgO (<0·3 wt %), Fe2 OT3 (<2 wt %) and CaO (<0·7 wt %) contents. These most probably represent highly evolved biotite granite magmas that differentiated in the mid-crust. The first contact of India with Asia appears to have occurred in central Lhasa during the early Paleocene (65–63 Ma) and led to crustal thickening and cessation of magmatism. Early Paleocene slab roll-back would have significantly enhanced asthenospheric corner flow and supplied a long-lived heat source for coeval crustal anatexis and metamorphism in southern Lhasa during the early phase of continental collision. Similar interaction between continental collision and oceanic subduction may also occur in other large-scale convergence zones in which the lithosphere and crust are anomalously hot.