Cambrian Qimantagh island arc in the East Kunlun orogen: Evidences from zircon U-Pb ages, lithogeochemistry and Hf isotopes of high-Mg andesite/diorite from the Lalinggaolihe area

The Qimantagh belt is an important component of the East Kunlun orogen and plays a key role in reconstructing the early Paleozoic convergent history of the northern Tibetan Plateau. The Qimantagh belt used to be interpreted as part of early Paleozoci arc or back-arc basin, however, the geological records of the early-stage of subduction have seldom been reported and the age of subduction initiation still remains poorly constrained. In this paper, we studied the petrology, geochemistry, zircon U-Pb ages and Hf isotopes of volcanic rocks and diorite from the Qimantagh Group of the Lalinggaolihe area and discussed their petrogenesis and tectonic significance. Volcanic rocks are dominated by andesites with moderate SiO2 contents (54.02%-63.99%), low TiO2(0.19%-0.24%), Al2O3(10.67%-12.86%) and P2O5(0.03%-0.05%) contents and low FeOT/MgO ratios (0.80-1.04), high MgO contents (7.86%-11.44%) and Mg# (63-69). Diorites have moderate SiO2 contents (53.62%-55.81%), low TiO2(0.19%-0.25%), Al2O3(10.58%-11.52%) and P2O5(0.02%-0.04%) contents and low FeOT/MgO ratios (0.9-1.0) with high MgO contents (9.86%-11.63%) and Mg# (64-66). The high-Mg andesites and diorites share similar trace-elements compositions and have low Th(0.22-1.23 μg/g), Rb(4.11-16.11 μg/g) and Ba(66.7-166 μg/g) contents, high Cr(286-668 μg/g) and Ni(57-181 μg/g) contents, moderate δEu (0.91-1.47) and low Sr/Y ratios (19.7-36.1). They also have low total REE contents (11.8-27.3 μg/g) and (La/Yb)N values (1.3-4.3), indicating weak LREE/HREE fractionation. The primitive mantle-normalized trace-element diagram shows positive Sr, Ba and Pb anomalies and negative Ta, Nb and Ti anomalies. The LA-ICP-MS U-Pb zircon ages of high-Mg andesites and dorites are 519 and 520/516 Ma, respectively. Both zircons share similar Hf isotopic compositions with εHf(t) of +10.5 to +15.8. These results indicate that the andesites and diorites from the Lalinggaolihe area originated from partial melting of depleted mantle wedge in island arc setting during the early stage of subduction of the Proto-Tethyan Ocean. This study further demonstrates that the subduction of paleo-ocean basin in the Qimantagh belt predated 520 Ma, which improves the understanding on the early-stage subduction history of the Proto-Tethys Ocean in the East Kunlun orogen. © 2021, Editorial Office of Earth Science Frontiers. All right reserved.