Late Carboniferous volcanic rocks in the North Tianshan Orogenic Belt, NW China: petrogenesis and tectonic implications

Abstract

The North Tianshan Orogenic Belt (NTB) is a crucial region for understanding the accretionary evolution of the Central Asian Orogenic Belt. However, its Late Carboniferous tectonic setting has long been controversial. This study presents new geochronological and geochemical data on volcanic rocks to shed light on this issue. The volcanic rocks are composed of basalt, andesite, dacite, and rhyolite. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yielded ages of 310.1 Ma (basalt), 304.8 Ma (andesite), 308.1 Ma and 307.0 Ma (two dacite samples), and 313.5 Ma (rhyolite). Geochemical data demonstrate that the calc-alkaline intermediate-mafic volcanic rocks (basalts and andesites) belong to co-genetic magmatic products, enriched in Ba, K, U, and Pb, and depleted in Nb, Ta, Ti, Zr, and Hf. They originated from fluid-metasomatized depleted mantle wedge. However, due to differences in subduction-related fluid input and the degree of partial melting, the intermediate-mafic volcanic rocks show slight differences in trace element composition. The I-type dacites, enriched in Ba, K, U, and Pb but depleted in Nb, Ta, and Ti, were derived from the partial melting of mafic lower continental crust. The A-type rhyolites are characterized by enrichment in Rb and Th and depletion in Ba, Sr, and P, consistent with derivation from partial melting of felsic upper continental crust. The spatiotemporal distribution patterns of regional magmatism and mineralization suggest that a ridge subduction setting was operating in the NTB during the Late Carboniferous.