Author: Xiaoqi Zhang, Hongfu Zhang, Haibo Zou
Abstract
Neoproterozoiclayered maficintrusions occur along the northern margin of the Yangtze Block inSouth China. In this paper, we report the chemical compositions ofclinopyroxene, plagioclase, and olivine from two major maficintrusions at Bijigou (BJG) and Wangjiangshan (WJS), to constrain theparental magma characteristics and geodynamic setting. The olivineand plagioclase compositions indicate that the parental magma for BJGand WJS was an evolved tholeiitic magma with high FeOT/MgOratios. An origin of arc magmatism is not supported by the followingmineralogical and petrological observations: (1) the strongrift-affinityindicated by olivine, plagioclase and clinopyroxene geochemistry, (2)the absence of high anorthite in the cumulate rocks, (3) the paucityof abundant cumulus hornblende crystals, and (4) the lack ofvoluminous intermediate rocks in the rock associations. We proposethat the BJG and WJS mafic-ultramaficintrusions formed under a rift extensional setting during 820–780Ma.The subduction process ceased before 820Ma and did not directlycontribute to the generation of the BJG and WJS mafic-ultramaficintrusions. The arc-like Nb-Ta depleted trace element pattern in theparental magma is inherited from the mantle metasomatized by earliersubduction processes.
Keywords: Mineralchemistry; Mafic-ultramaficintrusion; Neoproterozoic; Rodinia supercontinent; South China