Accretionary orogenesis of the Chinese Altai: Insights from Paleozoic granitoids

Abstract

Zircon U-Pb dating and whole-rock major oxide, trace element and Nd-Sr isotope compositions have been determined for four representative granitic intrusions in the SW Chinese Altai, in order to understand the orogenesis and history of crustal growth in the Central Asian Orogenic Belt (CAOB). The Ashile and Halatas bodies are relatively small intrusions formed in the Late Carboniferous (318 ± 6 Ma) and Permian (267 ± 4 Ma), respectively. The larger Tarlang (TB) and Keketuohai Batholiths (KB) have magmatic ages of 359 to 412 Ma. The granitoids consist chiefly of tonalite, granodiorite and granite, and are metaluminous to weakly peraluminous in composition (ASI = 0.8-1.1). The Ashile Pluton is characterized by relatively high MgO (2.73-3.54 wt.%) and Sr (395-456 ppm) contents, with low K 2O/Na 2O ratios (0.31-0.42) and mantle-like Nd-Sr isotopic compositions (εNd T = + 2.85 to+ 3.26; initial 87Sr/ 86Sr = 0.7047-0.7051). It probably formed by fractional crystallization of a mantle-derived magma, with limited assimilation of old crustal material. The other intrusions have near-zero or negative εNd T values (- 0.67 to- 4.41) and higher Sr isotopic compositions (initial 87Sr/ 86Sr = 0.7067-0.7092), indicating a mixture of juvenile and old components in their genesis. These intrusions are characterized by relatively low K, Rb, Cs contents and low Rb/Sr ratios (mostly < 0.6), and generally exhibit moderate to high heavy rare earth element (HREE) concentrations (Yb N = 8-26) and weak HREE fractionation (Dy/Yb N < 1.5), with or without negative europium anomalies (Eu/Eu * = 0.5-1.0). These characteristics are consistent with an origin by dehydration melting of a hornblende-bearing, mid-crustal source (above the garnet stability field) dominated by mafic to intermediate rocks. This suggests that the deep crust of the Chinese Altai may contain a considerable proportion of juvenile material. Mantle-derived magmatism, probably in an active extensional continental margin setting, played an important role in the formation of the Chinese Altai, acting not only as a heat source for crustal melting but as a source of juvenile components. The relatively small volumes and distinct geochemical compositions of the Ashile and Halatas Plutons may imply a substantial change in the geodynamic setting in the late Paleozoic. © 2007 Elsevier B.V. All rights reserved.