Age and composition of Cu–Au related rocks from the lower Yangtze River belt: Constraints on paleo-Pacific slab roll-back beneath eastern China

Cu–Au mineralization in the Yangtze River belt is closely related to the formation of adakitic rocks. The origin of these rocks is still controversially discussed. In this study, we report zircon U–Pb ages, major-trace element and Pb–Sr–Nd isotopes of Cu–Au related adakitic rocks from the Anqing and Tongling areas of the lower Yangtze River belt (LYRB). Geochemically, the rocks show similarities to adakites derived from slab melting. Statistical evaluation of existing zircon U–Pb data from the LYRB reveals a protracted magmatic activity from 148Ma to 106Ma with peak activity at ~139Ma. Cu–Au related magmatism tends to decrease in age from the southwest (Gan-Hang belt) to the northeast (LYRB). This trend was probably imposed by a roll-back process of the paleo-Pacific plate. The Cu–Au related adakitic rocks in the Gan-Hang belt and the middle and lower Yangtze River belt show systematic variation in geochemical and Pb–Sr–Nd isotopic compositions, pointing to a change in magma composition with time. The rocks have features in common with Cenozoic slab-derived adakites, but have experienced different extents of melt peridotite interaction and complex crustal assimilation processes. AFC modeling shows that input of crustal material increased with time. Combined with the decrease of estimated copper reserves, it is concluded that this records a diminishing influence of slab-derived components during the course of the paleo-Pacific subduction process.