Early Cretaceous K-rich rhyolites from the Duolong Cu-Au deposit, southern Qiangtang, China: evidence for crustal growth

摘要

We report new zircon U-Pb age, Hf isotopic, and major and trace element data for rhyolites from the Duolong Ore Concentration Area of the Southern Qiangtang Terrane. Building on previous studies, we constrain the tectonic setting and propose a model to explain the geodynamics and crustal growth during regional magmatism in the Early Cretaceous. The analysed rhyolites yield laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb ages of 115 and 118Ma. The rocks are K-rich (K2O=6.66-9.93wt.%; K2O/Na2O=8.2-19.7wt.%), alkaline and peraluminous (A/CNK=1.02-1.46), and are characterized by high SiO2 contents (72.8-78.8 wt.%) similar to highly fractionated I-type granites. Fractionation of Fe-Ti oxides, plagioclase, hornblende, Ti-bearing phases, apatite, monazite, allanite and zircon contributed to the variations in major and trace element chemistry. High K2O contents are likely due to partial melting of the continental crust. The samples have positive zircon epsilon Hf(t) values ranging from +7.1 to +11.2. These features, together with young zircon Hf crustal model ages of 489-721Ma, indicate that the K-rich rhyolites were derived from juvenile lower crust with an input of a mantle-derived component. We suggest that the Early Cretaceous K-rich rhyolites formed in a continental arc setting during northward subduction of Bangong Co-Nujiang oceanic lithosphere. Basaltic magma underplating was responsible for vertical crustal growth, triggered by slab roll-back in the Duolong Ore Concentration Area in the Early Cretaceous.