Identification of Indian crustal melting beneath Gangdese arc during India-Asia collision

The continental collision between the Indian and Asian plates constitutes one of the most distinguished convergent systems on a global scale. Geophysical data have revealed that subducted Indian slab lies beneath the Lhasa terrane of the Asian plate. However, whether, when, and how the Indian continental crust contributed to the magmatism in the Lhasa terrane remain unclear. In this study, we present petrological, geochronological, and geochemical evidence for Indian crustal melting beneath the Lhasa terrane in the Ayilari range (western Gangdese). Zircon and monazite U-(Th)-Pb dating and whole -rock Sr-Nd-Hf isotope analysis show that the Ayilari granitoids can be divided into three stages: (i) Paleocene -Eocene granites (60-45 Ma) with juvenile isotopic compositions (87Sr/86Sr(t) = 0.706474-0.709635, epsilon Nd(t) = +0.3 to +1.0, epsilon Hf(t) = +7.2 to +8.1); (ii) Oligocene -early Miocene granites (29-21 Ma) with slightly evolved isotopic compositions (87Sr/86Sr(t) = 0.707809-0.719349, epsilon Nd(t) = -11.2 to -6.7, epsilon Hf(t) = -4.8 to -1.5); and (iii) Miocene leucogranites and porphyritic granites (18-12 Ma) with isotopic compositions that show clear signature of evolved crustal material (87Sr/86Sr(t) = 0.725465-0.745704, epsilon Nd(t) = -15.2 to -12.0, epsilon Hf(t) = -13.7 to -11.3). Comparing our findings with existing data on the Himalayan leucogranites and intermediate-silicic rocks in southern Lhasa, we find that the Miocene leucogranites and porphyritic granites align with the characteristics of the Himalayan leucogranites, but differ from the southern Lhasa rocks. Furthermore, the elevated whole -rock beryllium concentrations and the absence of a zircon age peak around 1170 Ma in the inherited zircons imply the incorporation of Indian crustal material into the magma source of the Oligocene -early Miocene granites. Isotopic data for the Miocene granites also indicate that these rocks were derived from the Indian continental crust. We posit that the newly identified Oligocene and Miocene granites record the Indian crustal melting beneath the Lhasa terrane. Integration of Ayilari geologic data with orogen-scale magmatic and tectonic data suggests that changes of subduction geometry and periodic underthrusting of the Indian continental slab perhaps explain the alter of magmatic sources.