Petrogenesis of the early Permian A-type granites in the Halajun region, southwest Tianshan, western Xinjiang, NW China: implications for geodynamics of Tarim large igneous province

Abstract

ABSTRACT Formation of a large igneous province, an explosive magmatic event in earth, is intimately related to geodynamic processes. The Tarim large igneous province, a famous representative in China, contains A-type granites. However, the petrogenesis of the A-type granites is debated. Some researchers think that the A-type granites are developed through extensive fractionation of magma derived from partial melting of newly underplated or gabbroic rocks. Others insist that the A-type granites are developed through intensive fractional crystallization of plume-derived mafic magma. Here, we present zircon U–Pb ages and whole-rock geochemical and Sr–Nd isotopic data for the Kezilekezitage, Keziertuo, Tamu, and Halajun II intrusions in northwest Tarim to elucidate their petrogenesis and geodynamic implications. Zircon U–Pb dating yielded crystallization ages of the Kezilekezitage, Keziertuo, and Halajun II intrusions of 275.1 ± 3.1 Ma (MSWD = 1.14), 274.8 ± 1.8 (MSWD = 0.46) and 275.6 ± 0.7 (MSWD = 1.04), respectively. The whole-rock geochemistry of the four intrusions is characterized by REE and HFSE enrichment, Ba, Sr, P, Ti, and Eu depletion, εNd (t) values of – 1.2 to +0.3, two-stage Nd model ages (T2DM) of 1.14–1.02 Ga, and an A1-type granite affinity. Whole-rock zircon geothermometry indicates a high formation temperature of 721–924°C. It is concluded that these A-type granites were generated by partial melting of Neoproterozoic OIB-like rocks, coupled with extensive fractional crystallization and minor crustal contamination. The production of the A-type granites at ca. 275 Ma represented the termination and greatest degree of lithospheric thinning during large igneous province formation.