Emplacement ages and petrogenesis of intermediate‐acid intrusive rocks in the Lajishan region of the Central Qilian Belt, China: Constraints from zircon U–Pb ages, Hf isotopes, and whole‐rock geochemistry

Abstract

As a major tectonic unit along the southwestern rim of the Central Qilian Belt, the Lajishan region plays an important role in the amalgamation of continental plates in the Qilian Orogenic Belt. In this study, zircon laser ablation inductively coupled plasma mass spectrometry for U–Pb dating and Hf isotope analysis, as well as geochemical analysis of intermediate‐acid intrusive rocks from the Lajishan area were conducted. Results show that formation ages of the intermediate‐acid intrusive rocks are 438–452 Ma. Whole‐rock geochemistry results show that the diorite and monzodiorite are metaluminous, low‐ to medium‐K series, and characterized by obvious Nb, Ta, and Ti depletions as well as large‐ion lithophile element and light rare earth element enrichments. The εHf(t) values of zircons from monodiorite range from +2.71 to +10.59 and the two‐stage model ages range from 751 to 1,253 Ma. A large transversal span of εHf(t) values suggests that the source of depleted mantle was mixed with a small amount of older crustal material. The diorite and monzodiorite show a covariant relationship between εHf(t) values and Th/U ratios, which imply the existence of source heterogeneity. The diorites and monzodiorites were most likely produced in an arc setting. The granites are low K in composition and metaluminous to weakly peraluminous. In view of obvious Nb, Ta, and Ti depletions as well as large‐ion lithophile element and light rare earth element enrichments, parent magmas of the granites are suggested to have formed with the participation of crustal source materials. The εHf(t) values of zircons from granite range from +5.82 to +8.90, and the two‐stage model ages range from 861 to 1,057 Ma, indicating that magmas of the granites were mainly derived from partial melting of depleted mantle or juvenile thickened lower crust. The granites were most likely produced in an arc setting and share a similar source with the diorite and monzodiorite. Correlation between major oxides and SiO2 show that during the formation of the three kinds of rocks, crystallization and differentiation both played important roles. Combined with published data from the Central Qilian Belt and its vicinity, our results conclude that the northward subduction of oceanic crust contributed to the formation of an Early Palaeozoic subduction–accretion system in the Lajishan region. This model reveals a tectonic–magmatic thermal event and provides important insights into the tectonic evolution process in the Qilian region, as well as the Central Orogenic Belt during the Early Palaeozoic.