Abstract
AbstractNew zircon UāPb ages and whole-rock chemical data from four adakitic and two non-adakitic igneous rocks as tectonic blocks in the southern West Junggar accretionary complexes, northwestern China and one gabbro enclave in adakitic block provide further constraints on the initial subduction and following rollback process of the Junggar Ocean as part of southern Palaeo-Asian Ocean. The oldest adakitic monzonite in Tangbale is intruded by the non-adakitic quartz monzonite at 549 Ma, and the youngest adakitic diorite in Tierekehuola formed at 520 Ma. The EdiacaranāCambrian magmatism show a N-wards younger trend. The high-SiO2 adakitic rocks have high Sr (300ā663 ppm) and low Y (6.68ā12.2 ppm), with Sr/Y = 40ā84 and Mg no. = 46ā60, whereas the non-adakitic rocks have high Y (13.2ā22.7 ppm) and Yb (2.32ā2.92 ppm), with Mg no. = 36ā40. The gabbro has high MgO (14.81ā15.11 wt%), Co (45ā48 ppm), Cr (1120ā1360 ppm) and Ni (231ā288 ppm), with Mg no. = 72ā73. All the samples show similar large-ion lithophile element (LILE) and light rare earth element (LREE) enrichment and Nb, Ta, Ti and varying Zr and Hf depletion, suggesting that they were formed in a subduction-related setting. The adakitic rocks were produced by partial melting of subducted oceanic slab, but the melts were modified by mantle wedge and slab-derived fluids; the non-adakitic rocks were likely derived from partial melts of the middle-lower arc crust; and the gabbro originated from the mantle wedge modified by slab-derived fluids. The magmatism could have been generated during the Ediacaran initial subduction and Cambrian slab rollback of the Junggar Ocean.
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