LA-ICP-MS U–Pb zircon geochronology of early Neoproterozoic mafic-intermediat intrusions from NW margin of the Yangtze Block, South China: Implication for tectonic evolution

Abstract

Proterozoic magmatism along the margin of the Yangtze Block is extensive and provides important evidence in the plate tectonic reconstructions for the Proterozoic. The Bikou Terrane on the northwestern margin of the Yangtze Block consists of complex Archean to Paleozoic metamorphosed sedimentary and volcanic sequences, intruded by several mafic-intermediate bodies. Three major intrusions are dated using laser ablation ICP–MS U–Pb of zircons. This dating yielded ages 884 ± 5.5 Ma, 884 ± 14 Ma and 877 ± 13 Ma for the Pingtoushan diorite, Guankouya diorite and Liujiping gabbro, respectively. These are the oldest intrusions found along the margin of the Yangtze Block. Geochemical analyses of these mafic-intermediate intrusions show they have negative Nb–Ta anomalies that are best interpreted as having been formed in an arc setting. The older magmatism and unique rock associations of the Bikou Terrane suggest that it was an isolated terrane that existed in a marginal sea to the Yangtze Block. The eastward subduction (present-day orientation) resulted in early arc magmatism (∼880 Ma) within the Bikou Terrane and lasted to ca. 750 Ma. This study supports the model that the Yangtze Block was surrounded by ocean and developed arc magmatism along its western margins in Neoproteroic time, with continent-directed subduction that commenced in the early Neoproterozoic. The Bikou Terrane was subsequently accreted onto the northwest margin of the Yangtze Block at about 750 Ma.