Geochemistry and tectonic implications of Early Permian granitic rocks in the Xingxingxia area of Chinese Central Tianshan Arc Terrane

Abstract

The Chinese Central Tianshan Arc Terrane (CTA) in the southern Central Asian Orogenic Belt is characterized by voluminous Palaeozoic to Mesozoic (500–240 Ma) granites, with a magmatic “flare‐up” at Early Permian time (ca. 280 Ma). However, the tectonic setting of the Early Permian granites in the CTA is highly debated. In this study, Early Permian (ca. 280 Ma) granitic intrusions, including quartz diorite and dioritic enclave, amphibole granite, biotite granite, dioritic porphyry dyke, and a weakly mylonitic granitic dyke, were revealed from the Xingxingxia area of the CTA by LA‐ICP‐MS zircon UPb dating. These granitic rocks show magnesian and calc‐alkaline affinities and are characterized by Nb, Ta, P, Ti, and MREE depletion [(Dy/Yb)N = 1.02–1.51] with no significant Eu anomalies, suggesting geochemical characteristics of typical active continental margin magmatic rocks. Zircon trace element geochemistry of these Early Permian granitic rocks also indicates a continental arc setting with high U/Yb and Gd/Yb ratios. Among these granitic intrusions, the dioritic porphyry dyke has negative zircon εHf(t) values (−8.0 to −5.7), suggesting that the magma was produced by partial melting of the ancient crustal basements of the CTA. The rest intrusions show positive but largely varied zircon εHf(t) values of +4.1 to +10.9 (5 samples), suggesting that the magmas were probably derived from the mantle wedge with variable contributions from ancient crustal component during the magma evolution. These granitic intrusions were coeval with high‐temperature metamorphic rocks, mafic‐ultramafic rocks, and Cu‐Ni sulphide deposits in the CTA. Therefore, we propose a geodynamic model of oceanic ridge subduction of the South Tianshan Ocean during Early Permian involving upwelling of asthenospheric mantle through slab window and partial melting of the mantle wedge. The multiple‐derived magmas, including those from mantle wedge and/or ancient crustal basement, and their blendings formed the studied Early Permian granitic associations in the CTA.