Geochemistry and zircon U–Pb–Hf isotopic systematics of the Ningshan granitoid batholith, middle segment of the south Qinling belt, Central China: Constraints on petrogenesis and geodynamic processes

Abstract

The Ningshan Granitoid Batholith (NGB) is located in the middle segment of the South Qinling Belt (SQB) section of the Qinling orogenic belt, in Shaanxi Province, China, and consists of the Laocheng, Yanzhiba and Lanbandeng intrusions. The Laocheng intrusion is dominated by quartz diorites, granodiorites and monzogranites, with the Yanzhiba intrusion being granodiorite dominated, and the Lanbandeng intrusion predominantly consisting of two-mica monzogranites. LA–ICP–MS zircon U–Pb isotopic dating indicates that the NGB formed during two distinct episodes of magmatism, with an earlier episode at 222–216Ma and a later episode at ∼210Ma. Whole-rock geochemistry and geochronology data have allowed us to divide the NGB into three groups, with Group #1 rocks consisting of Laocheng intrusion quartz diorites and granodiorites, and Yanzhiba intrusion granodiorites that were produced in the early magmatic episode. Group #1 samples have high-K calc-alkaline compositions, and are characterized by high Mg# values, high Sr, Cr, and Ni concentrations, and high (La/Yb)N and Sr/Y ratios, but low Yb and Y concentrations, implying that these rocks were formed from a mixed magma derived from melting of Neoproterozoic crustal basaltic rocks and magmas sourced from a region of depleted mantle. Group #2 rocks are composed of the mid-K calc-alkaline monzogranites in the Laocheng intrusion that were formed in the later magmatic episode, and are characterized by lower Mg# values, and lower Cr, Ni, and Co concentrations than the Group #1 samples, but with high Sr concentrations and (La/Yb)N and Sr/Y ratios, low Y and Yb concentrations, negligible Eu anomalies, and a significant range in zircon εHf(t) values. The highest zircon εHf(t) values of the Group #2 samples plot close to the depleted mantle evolutionary line, indicating that these rocks were derived from a mixed magma with both crustal- and mantle-derived components. Group #3 samples consist of high-K calc-alkaline two-mica monzogranites of the Lanbandeng intrusion that were formed during the later magmatic episode. These rocks are characterized by high SiO2, Al2O3, and K2O concentrations, with low MgO and Sr concentrations, low (La/Yb)N and Sr/Y ratios, Nb, Ta, P and Ti depletions, and significantly negative Eu anomalies, implying that the magmas from which these rocks formed were derived from partial melting of sedimentary rocks and minor amount of basalt. The early magmatic episode associated with formation of the NGB occurred in a continental arc environment at 222–216Ma, with ∼210Ma magmatism during a geodynamic transition from syn-collisional to post-collisional tectonics across the SQB.