The Late Ordovician granitoids in the East Kunlun Orogenic Belt, Northwestern China: petrogenesis and constraints for tectonic evolution of the Proto-Tethys Ocean

Abstract

Petrology, whole-rock geochemistry, in situ zircon U–Pb ages, and Lu–Hf isotopic data are reported for granodiorite and monzonitic granite samples from the Harizha Region in the eastern section of the East Kunlun Orogenic Belt. The results of the LA-ICP-MS zircon dating indicate that the granodiorite and monzonitic granite were emplaced at 447.9 ± 3.0 Ma and 443.8 ± 3.5 Ma, respectively, i.e., in the Late Ordovician–Early Paleozoic periods. The granodiorite is characterized by relatively high MgO (4.52–5.91 wt%), Sr (541–712 ppm), Cr (218–357 ppm), Ni (56.7–80.7 ppm) and V (66.8–94.7 ppm), with low Y (10.40–13.60 ppm) and Yb (1.05–1.45) contents. This results in elevated Mg# (65–68), Sr/Y (39.78–58.84), and (La/Yb)N (10.21–18.30) ratios, common features for adakitic high-Mg andesite (HMA). The granodiorite is metaluminous to weakly peraluminous and high-K calc-alkaline in composition. In addition, the granodiorite is enriched in light rare-earth elements (LREE) and large ion lithophile elements (LILEs, Rb, K, Sr, and Pb) U, and Nd; and depleted in high-field strength elements (HFSEs, Nb, Ta, Zr, Hf, Ti) and P. The zircon grains from the granodiorites have eHf(t) values of − 5.5 to + 4.7, with two-stage Hf model ages (tDM2) ranging from 0.92 to 1.71 Ga. These features suggest that the granodiorites were formed by mixing processes of crustal melts and magma from a metasomatized mantle in a subduction setting. The monzonitic granite possesses high normative corundum values (2.34–4.10 wt%), is strongly peraluminous (A/CNK ratios of 1.16–1.32, with minor mineral muscovite), with high-K calc-alkaline affinity. Furthermore, enrichments in large ion lithophile elements (Rb, K, and Pb), U, Th, and Nd, depletions in high-field strength elements (Nb, Ta, and Ti) and Ba and P, and mainly right-inclined REE patterns with negative Eu anomalies, exhibits an S-type granite affinity. The magmatic zircons from the monzonitic granite yielded eHf(t) values between − 4.9 and − 2.7, corresponding to two-stage Hf model ages of 1.60–1.73 Ga. Therefore, this study suggests that the monzonitic granite magma was derived from crustal material (metagreywacke) in a syn-collision setting. Subsequently, by combining these findings with the previous research results, this study propose that a transverse diachronism closure model which entailed an “earlier on the sides–later in the middle” in an East–West direction in the East Kunlun Orogenic Belt for the Proto-Tethys Ocean’s evolution process.