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

摘要

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 epsilon Hf(t) values of - 5.5 to + 4.7, with two-stage Hf model ages (t(DM2)) 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 epsilon Hf(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.