Petrology and U–Pb zircon geochronology of bimodal volcanic rocks from the Maierze Group, northern Tibet: Constraints on the timing of closure of the Banggong–Nujiang Ocean

Abstract

We present new zircon U–Pb dates, major and trace element chemistry, and Hf isotopic compositions for bimodal volcanic rocks of the Maierze Group (MG) in the Maierze area of the Southern Qiangtang–Baoshan block, northern Tibet. We discuss the implications of these data for the evolution of this region. The MG bimodal volcanic rocks consist of basalts and dacites that yield LA-ICP-MS zircon U–Pb ages of 122 and 120Ma, respectively. The MG basalts have light rare earth element (LREE)-enriched chondrite-normalized REE patterns (LaN/YbN=13–14), high Ti/V ratios (45–64), high Zr (190–270ppm) and Nb (22–41ppm) concentrations, and Zr/Y ratios (7–9) that are similar to those of within-plate basalts. The MG basalts also have low MgO and total Fe2O3 (TFe2O3) concentrations, significant enrichments in the LREE and the light ion lithophile elements (LILEs; Rb, Ba, Th, U and Pb), and weak depletions in the high field strength elements (HFSE; Nb, Ta, and Ti), all of which are clearly evident in primitive-mantle-normalized multi-element variation diagrams. The MG dacites are more enriched in the LILE (e.g., Rb, Ba, Th, U, K, and Pb) and more depleted in the HFSE (e.g., Nb, Ta, and Ti) than the MG basalts. Moreover, the dacites have variable zircon εHf(t) values (−6.3 to +6.3). These features indicate that the parental magma for the MG basalts was likely derived from an enriched lithospheric mantle source that was contaminated by subduction-related fluids or melts. In contrast, the MG dacites were derived from mixing of the MG basaltic magma with a second magma derived from partial melting of the continental crust. The geochemical and Hf isotopic characteristics of the MG bimodal volcanic rocks suggest that they formed during the initial stages of development of a back-arc basin. From south to north, the Bangong–Nujiang suture zone, the Duolong gold-rich porphyry copper deposit, and the Maierze bimodal rocks are interpreted to represent a remnant of a complete volcanic arc system that comprises the Bangong–Nujiang Ocean, an island arc, and a back-arc basin, respectively. The fact that this volcanic arc system was still active during the Early Cretaceous indicates that closure of the Bangong–Nujiang Ocean occurred later than the previously suggested period of between the Late Jurassic and Early Cretaceous.