Age and geochemistry of western Hoh-Xil-Songpan-Ganzi granitoids, northern Tibet: Implications for the Mesozoic closure of the Paleo-Tethys ocean

A geologic investigation was undertaken in the Hoh-Xil-Songpan-Ganzi (HXSG) complex, northern Tibet in order to better understand magma genesis and evolution during the late stages of Paleo-Tethys ocean closure. The HXSG complex is composed of vast accumulations of Middle-Upper Triassic marine gravity flow deposits that were extensively intruded by igneous rocks. These early Mesozoic rocks exposed in this area record a rich history of accretionary tectonics during the amalgamation of the Tibetan Plateau terranes. Eight plutons sampled from the western HXSG complex yield zircon U-Pb ages that range from 225 to 193Ma. Muscovite ⁴⁰Ar/³⁹Ar ages for the Hudongliang and Zhuonai Lake plutons yield ages of 210.7±2.5Ma and 212.7±2.5Ma, respectively. These plutonic rocks can be subdivided into two geochemically distinct groups. Group 1 (221-212Ma: Dapeng Lake, Changhong Lake and Heishibei Lake plutons) is composed of high-K calc-alkaline rocks that have strongly fractionated REE patterns with high (La/Yb)_N ratios (91-18) and generally lack Eu anomalies (Eu*/Eu=1.02-0.68). Rocks in Group 1 display pronounced negative Nb-Ta and Ti anomalies on primitive mantle-normalized spidergrams. Group 1 rocks exhibit high Sr (782-240ppm) and low Y (6.3-16.0ppm) contents with high Sr/Y ratios (84-20). Based on Sr-Nd-Hf isotopic data (⁸⁷Sr/⁸⁶Sr_i=0.7079-0.7090, εNd(t)=-7.7--4.7, εHf(t)=-5.7--0.8) and low MgO contents (MgO=1.10-2.18%), Group 1 rocks are geochemically similar to adakitic rocks and were probably derived from partial melting of the downgoing Paleo-Tethys oceanic slab and overlying marine sediments. Group 2 plutons (225-193Ma: Daheishan, Yunwuling, Zhuonai Lake, Malanshan and Hudongliang plutons) display lower P₂O₅ with increasing SiO₂ and are medium-K to high-K I-type calc-alkaline bodies with low Sr (14-549ppm) and high Y (22.3-10.5ppm) contents. Group 2 rocks have variable fractionated REE patterns ((La/Yb)_N=3-38) and negative Eu anomalies (Eu*/Eu=0.02-0.86). Together with Sr-Nd-Hf isotopes (⁸⁷Sr/⁸⁶Sr_i=0.7072-0.7143, εNd(t)=-6.6--2.0, εHf(t)=-0.6-+3.0), Group 2 rocks are most likely formed by partial melting of the juvenile crustal sources. Collectively, these data suggest that the Hoh-Xil turbidites were underlain by more continental arc crust than previously thought. We propose that rollback of the subducting Paleo-Tethys oceanic slab led to partial melting of overlying continental arc fragments which developed beneath the HXSG gravity flow deposits.