Newly discovered Neoproterozoic A-type granite in the Altun orogenic belt: A record of the initial breakup of Rodinia

摘要

The Altun orogenic belt, located on the northern margin of the Tibetan Plateau, records the assembly and breakup of the Rodinia supercontinent. In this paper, we report the petrography, zircon U-Pb ages and Hf isotope data, and bulk geochemistry of the Hongliugou alkali feldspar granites from the Altun orogenic belt to constrain their petrogenesis and tectonic implication. Zircon U-Pb dating yielded ages of 857-851 Ma, interpreted as the emplacement ages of the granites. The granites are metaluminous; have high SiO2, K2O, and rare-earth element (REE) contents; low CaO and P2O5 contents; high FeOt/MgO ratios; and high 10,000 x Ga/Al values. They show marked depletion in heavy REEs, enrichment in light REEs, and pronounced negative Eu anomalies. These rocks are enriched in high-field-strength elements, depleted in Ba, Sr, P, and Ti, and lack Nb and Ta anomalies. These petrological and geochemical characteristics are consistent with those of A-type granites. The samples plot in the A(1)-type field in a discrimination diagram for A-type granites. Zircon epsilon(Hf)(t) values vary from +1.76 to +7.40 with two-stage Hf model ages of 1.27 to 1.63 Ga, implying that the granites were derived mainly from juvenile crust. On the basis of these and previous results, we propose that the Hongliugou alkali feldspar granites formed in an intraplate rift setting related to the initial breakup of Rodinia.