Geochemical and zircon U–Pb study of the Huangmeijian A-type granite: implications for geological evolution of the Lower Yangtze River belt

摘要

The Early Cretaceous Huangmeijian Pluton is an A-type granite located on the northern bank of the Lower Yangtze River in Anhui Province, east-central China. It intruded the SE edge of the Early Cretaceous Luzong volcanic basin. The moderate- to coarsegrained granite is mainly composed of alkali feldspar, plagioclase, and quartz and has a typical A-type geochemical signature. LA-ICP-MS zircon dating yielded a weighted mean ~(206)Pb/~(238)U age of 127.1 ± 1.4 Ma, similar to other A-type granites in the Lower Yangtze River belt, indicating an Early Cretaceous extensional environment. Temperatures calculated using the Ti-in-zircon thermometer suggest that the magma formed under high-temperature conditions (720–880℃). The low calculated Ce(IV)/Ce(III) ratio based on zircon rare earth element patterns indicates low oxygen fugacity for this A-type magma. Previous studies suggested that eastern China was an active plate margin related to the Early Cretaceous subduction of the Pacific and Izanagi plates. The ridge between these two plates probably passed under the Lower Yangtze River belt, forming A-type granites and adakites. The Huangmeijian Pluton is roughly the same age within error but is marginally older than the Baijuhuajian A-type granite in the eastern part of the Lower Yangtze River belt. A-type granite genesis in the Lower Yangtze River belt only lasted for 2–3 million years and slightly predates the transition from regional extension to compression. All these can be plausibly interpreted by the ridge subduction model, that is, A-type granites formed because of mantle upwelling through the slab window during subduction of the ridge separating the Pacific and Izanagi plates.