Paleocene (c. 62 Ma) leucogranites in southern Lhasa, Tibet: products of syn-collisional crustal anatexis during slab roll-back?

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, εNd(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 (