Petrogenesis and tectonic implications of Early Paleozoic granitoids in the Qiaerlong district of the West Kunlun orogenic belt: constraints from petrology, geochronology, and Sr-Nd-Hf isotope geochemistry

Abstract

ABSTRACT The West Kunlun orogenic belt (WKOB) is a complex orogenic belt that preserves the evidence of Early Palaeozoic and Late Palaeozoic orogenic events, with the former resulting from the closure of the Proto-Tethys Ocean. To better reveal the relationships between Early Palaeozoic granitoids in the WKOB and the tectonic evolution of the Proto-Tethys Ocean, we analysed the rocks of Akedala pluton, which is exposed within the Qiaerlong basin, and compared the results with contemporaneous magmatic rocks elsewhere in the WKOB. Integrated petrology, geochronology, and Sr–Nd–Hf isotope analyses of the rocks revealed the following: (1) The Akedala pluton is mainly composed of metaluminous–peraluminous I-type quartz monzonite and monzogranite; U–Pb dating of zircon indicates that quartz monzonite and monzogranite components is emplaced at 473–479 Ma and 462 Ma, respectively. (2) Both of them were enriched in large-ion lithophile elements and light rare earth elements and depleted in high field strength elements with negative Nb, Ta, Zr, P and Ti anomalies, indicating that they formed in a subduction zone environment. (3) Quartz monzonites were enriched in MgO concentrations (>3.10 wt.%), and show variable whole rock Sr-Nd isotopic ratios ((87Sr/86Sr)i = 0.7042–0.7058, εNd(t) = – 0.4 to +1.1), and had positive zircon εHf(t) (+0.0 to +6.8) values, which suggests they are derived from a mixed source of juvenile crust and older lower crust. (4) Monzogranites were characterized by high Sr (461–804 ppm) and low Y (2.00–3.68 ppm) and Yb (0.19–0.31 ppm) contents with high Sr/Y (164–325) and (La/Yb)N (12.6–46.7) ratios, indicating an adakitic affinity; they had relatively homogeneous whole rock (87Sr/86Sr)i (0.7044–0.7045) and positive εNd(t) (+2.0 to +2.9) and positive zircon εHf(t) (+1.1 to +4.3) values with low MgO, Cr, Ni and Nb contexts, suggesting that they were mainly generated from partial melting of thickened lower crust. Comparative analysis of these results with other Early Palaeozoic arc magmas in the WKOB shows that the Akedala pluton was formed due to the southward subduction of the Proto-Tethys Ocean. Divergent bidirectional subduction of the Proto-Tethys Ocean was initiated in the Early Cambrian, continued through the Middle Ordovician, and ended in the Early Silurian, due to terminal collision between the Tarim block and the West Kunlun terrane, which created the Early Palaeozoic orogenic belt.