Petrogenesis of Early Paleozoic adakitic granitoids in the eastern Qilian Block, northwest China: implications for the South Qilian Ocean subduction

The geodynamic mechanism responsible for the generation of Early Paleozoic magmatism within the Qilian Block, northwest China, remains controversial. In this paper, we present new geochronological, mineralogical, and geochemical data for the Hejiashan (HJS) granite and Lajishan (LJS) quartz diorite from the eastern Qilian Block, to constrain their origin and the regional evolutionary history. Laser abalation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating results show that the HJS granites and LJS quartz diorites were emplaced between 445 and 438 Myr ago. The HJS granites are high-K calc-alkaline, metaluminous to weakly peraluminous, with K2O/Na2O weight ratios of 0.84–1.12. The LJS quartz diorites are calc-alkaline and metaluminous with K2O/Na2O weight ratios of 0.34–0.46. Both have similar whole-rock Sr–Nd isotopic compositions. They are characterized by low heavy rare earth elements (HREEs) and Y concentrations, and high Sr/Y and La/Yb ratios, similar to adakitic rocks. Notably, the LJS quartz diorites are more depleted in HREEs and Y, and have much higher Mg# [= 100 × Mg/(Mg + Fe)], La/Yb, Sr/Y, and zircon εHf(t) values, and MgO, Cr, Co and Ni concentrations than those of the HJS granites. The HJS granites were most likely derived from partial melting of a thickened juvenile lower crust with amphibole and minor plagioclase as residual phases in the magma source. Conversely, the LJS quartz diorites were produced by partial melting of the subducted oceanic slab and minor sedimentary materials in the garnet and rutile stability field. Combined data from this and previous studies suggest that the Early Paleozoic magmatism within the Qilian Block mainly occurred ca. 464–402 Myr ago as a response to post-collisional extension. Break-off of the northward subducted South Qilian oceanic slab after continental collision between the Qaidam and Qilian Blocks was the primary dynamic mechanism responsible for the Early Paleozoic extensive crust-mantle interaction and magmatism within the Qilian Block.