Paleozoic multi-stage accretionary evolution of the SW Chinese Tianshan: New constraints from plutonic complex in the Nalati Range

Abstract

We conducted field investigations, whole-rock geochemical, Sr-Nd and zircon U-Pb-Lu-Hf isotopic analyses on a suite of intrusive complex in the southern Nalati Range, SW Chinese Tianshan in order to better understand the Paleozoic tectonic and magmatic evolution of the belt. The intrusive complex comprises weakly foliated diorite, low-grade altered diabase, and deformed monzogranite; these plutonic rocks were in turn crosscut by undeformed coarse-grained diorite, granodiorite as well as granite stock. Foliated Late Silurian diorites (421±4Ma) show arc-type geochemical features, slightly negative whole-rock εNd(t) value (−1.7; TDM-Nd=1.52Ga) and variably positive zircon εHf(t) values (2.34 to 7.27; TDM-Hf: 0.95–1.26Ga). Deformed Early Devonian porphyritic monzogranites (411±4Ma) show geochemical features similar to A-type granite, and their zircon εHf(t) values range from −6.63 to 1.02, with TDM-Hf ages of 1.82 to 1.33Ga. Metamorphosed Early Devonian diabases (ca. 410Ma) have OIB-like REE patterns, εNd(t) values of −2.0~−0.8 and TDM-Nd ages of 1.37–1.25Ga. The undeformed Early Carboniferous diorite and granodiorite (353–344Ma) exhibit arc-type geochemical features, positive εHf(t) values of 6.11–7.91 with TDM-Hf ages of 0.97–0.86Ga, and positive εNd(t) value of 1.9 with TDM-Nd age of 1.04Ga. The Early Permian granite stock (292±5Ma) has highly differentiated REE pattern, slightly negative εNd(t) value (−4.4) and variable zircon εHf(t) values of −9.73–6.36. Combining with available data, Early Paleozoic (500–410Ma) arc-related magmatic rocks occurring on both sides of the suture zone along the southern Nalati Range, likely resulted from a bi-directional subduction of the Paleo-Tianshan Ocean beneath the Yili Block to the north and the Central Tianshan to the south. Occurrences of A-type granites and OIB-like diabases (ca. 410Ma) along the Nalati Range likely indicate a hot extensional regime probably induced by the break off of the northward subducting slab of the Paleo-Tianshan Ocean. The closure of the Paleo-Tianshan Ocean and subsequent amalgamation during Early Carboniferous resulted in the regional deformation and metamorphism of the Early Paleozoic arc-related magmatic rocks. From Early to Late Carboniferous, a magmatic arc that corresponded to the well-developed Late Paleozoic Balkhash-Yili active continental margin, superimposed upon the southern Yili Block, most likely resulted from the southward subduction of the Junggar-North Tianshan Ocean. After the closure of the North Tianshan Ocean in Late Carboniferous, the study area was dominated by post-orogenic magmatism.