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

Abstract

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/86Sri=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/86Sri=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.