Continental vertical growth in the transitional zone between South Tianshan and Tarim, western Xinjiang, NW China: Insight from the Permian Halajun A1-type granitic magmatism

Abstract

The South Tianshan Collisional Belt (STCB) and northern margin of the Tarim Block (NTB) are key areas for understanding the prolonged tectonic evolution of the Central Asian Orogenic Belt (CAOB). The Halajun region in Xinjiang province, NW China is located within the tectonic transition zone between STCB and Tarim Blocks. Several granitic intrusions and one mafic–ultramafic complex (Piqiang complex) are exposed in this region. Zircon U–Pb dating, whole-rock major oxide, trace element and Nd isotopic data are presented for the Huoshibulake, Tamu, Kezi'ertuo and Halajun II granitic intrusions in this area. New LA-ICP-MS U–Pb age for Kezi'ertuo intrusion, coupled with previously published SHIRMP U–Pb ages for Huoshibulake and Halajun II intrusions and Piqiang complex, reveals that all the igneous rocks in the Halajun region are coeval (~275Ma). Geochemically, the four granitic intrusions show high contents of SiO2, K2O and total alkalis and possess trace element patterns characterized by Rb, Nb, Ta, Zr and Hf enrichment and significantly negative Ba, Sr, P, Eu and Ti anomalies. These features strongly favor an A1-type affinity for the Halajun granitic intrusions. Among the four intrusions, the Kezi'ertuo, Tamu and Halajun II intrusions possess positive to slightly negative εNd(t) values ranging from −0.9 to +0.6, whereas the Huoshibulake intrusion displays less depleted εNd(t) values of −2.6 to −2.9. Our new elemental and isotopic data suggest that the four granitic intrusions were generated by the partial melting of a common Neoproterozoic gabbroic source, probably as a result of the ~275Ma underplating of the asthenosphere mantle-derived magmas. The variable involvement of the mantle components accounts for the range of εNd(t) values. After the generation of the parental magma, alkali feldspar, arfvedsonite, biotite, Fe–Ti oxides and zircon seem to have fractionated prior to the final emplacement of the granitic magmas. In combination with the regional geological history, we propose that the ~275Ma A1-type granitic magmatism in the Halajun region and other areas of the NTB provides a good proxy record for the vertical continental crustal growth in the southern margin of the CAOB during the Permian. Our study, in combination with other geological evidence, indicates that these A1-type felsic and OIB-like mafic–ultramafic rocks, with ages from ~282Ma to ~275Ma, in the southern margin of the CAOB are parts of the Permian Tarim large igneous province and could be genetically related to the Tarim mantle plume.