Oligocene magmatism in the eastern margin of the east Himalayan syntaxis and its implication for the India–Asia post-collisional process

Post-collisional high Sr/Y-type magmatism in the southern Lhasa terrane extends ~ 1500 km along the strike of the orogen. The origin and magma generation mechanism for these rocks remain controversial. We report a detailed study of such high Sr/Y-type magmatism in the east Himalayan syntaxis, SE Tibet, and explain its significance for understanding post-collisional geodynamical processes. The two-mica granites from the Beibeng, Damu, Bolonggong and Bomi plutons emplaced along the eastern margin of the east Himalayan syntaxis represent a high-K calc-alkaline suite (SiO2 = 67–75 wt.%, Al2O3 = 13.3–15.8 wt.%, K2O/Na2O = 0.7–1.9). An outstanding geochemical feature is that they have high Sr/Y ratios (19–88, mostly > 30), coupled with low Y and HREE, indicating that they were derived from partial melting of thickened lower crustal materials. Sr–Nd isotopic data show that the potential magma sources are the intermediate intrusive rocks related to the Jurassic–Eocene Gangdese magmatism and basement metasedimentary rocks. The contribution of the latter source increases from south to north. The two-mica granites from the Beibeng, Damu, Bolonggong and Bomi intrusions have magma crystallization ages of 29.9–27.5 Ma, 27.6 Ma, 27.1–26.5 Ma and 23.7 Ma, respectively. Our data show decreasing magma crystallization ages both from south to north along the eastern margin of the east Himalayan syntaxis and from east to west along the Gangdese belt. We suggest that such variations resulted from the diachronous breakoff of the subducted Indian continental slab. Beneath the east Himalayan syntaxis, the breakoff of the subducted Indian continental slab was initiated in the south and propagated towards north probably due to the thermal impact of a widening window opened by slab breakoff. Along the Gangdese belt, the breakoff of the subducted Indian continental slab commenced in east, and then propagated westwards.