Shoshonitic- and adakitic magmatism of the Early Paleozoic age in the Western Kunlun orogenic belt, NW China: Implications for the early evolution of the northwestern Tibetan plateau

Abstract

The Western Kunlun orogenic belt in the northwestern margin of the Tibetan plateau contains two magmatic belts; early Paleozoic belt in the northern part of Western Kunlun Terrane (WKT), and early Mesozoic belt in the southern part of WKT. Both formed from northward subduction of the Paleo-Tethys. The early Paleozoic belt contains large Datong and Qiukesu igneous complexes and many smaller plutons. The Datong complex is mainly composed of dark-colored porphyritic syenite and monzonite with minor light-colored dykes of granite and monzonite. The dark-colored rocks are characterized by moderate SiO2 (58.2–69.3wt.%), and high Al2O3 (15.3–17.1wt.%), total alkali (Na2O+K2O=8.07–10.2wt.%) and ratios of K2O/Na2O (0.77–1.83). They plot in “shoshonite” field, and show high abundances of LILE including LREE ((La/Yb)n=15.4–26.2; mean 20.2) with pronounced negative anomalies of Nb–Ta–P–Ti in normalized trace elemental patterns and weak negative anomalies of Eu (δEu=2Eun/(Smn+Gdn)=0.68–0.80). The light-colored rocks contain slightly higher concentrations of SiO2 (60.3–72.0wt.%), similar Al2O3 (14.7–17.6wt.%), and slightly lower total alkalis (6.57–9.14wt.%) than dark-colored rocks. They show adakitic geochemical signatures with low Y (5.80–17.2ppm) and Yb (0.63–1.59ppm), and high Sr/Y (>40). U–Pb zircon dating indicates that shoshonitic rocks and adakitic dykes formed at ~444Ma to ~443Ma, and a separate small adakitic plug at ~462Ma. The mean εHf(t) values of zircon range from −1.6 to −0.94 (n=14) with TDM2 of ~1.5Ga for shoshonitic rocks and εHf(t) values from −1.8 to +0.72 (n=12) with TDM2 of 1.4 to 1.5Ga for adakitic rocks. Shoshonitic rocks show initial 87Sr/86Sr and εNd(t) of 0.7092–0.7100 and −3.9 to −3.2, respectively, and adakitic rocks yield initial 87Sr/86Sr and εNd(t) of 0.7099–0.7134 and −3.6 to −3.1, respectively. Similar Sr, Nd, and Hf isotope compositions for the shoshonitic and adakitic rocks suggest similar ancient rocks as their sources.Combined with the geological development of the area, we propose that a local extension in the WKT over the subducting Paleo-Tethys has induced the upwelling of asthenospheric mantle and partial melting in the lower crust during mid-Ordovician to early Silurian. The primary shoshonitic melt evolved into the parental magmas for the shoshonitic rocks and adakitic rocks through fractional crystallization of hornblende.