Subduction and rollback of the paleo-Pacific plate during the Early Cretaceous: Insights from the Jiangmiao gabbroic intrusion in the Lower Yangtze River Belt, central-eastern China

Abstract

The dynamics of Mesozoic tectonism recorded by the Lower Yangtze River Belt (LYRB), located in central-eastern China, are controversial. Between 148 and 123 Ma, three major and continuous stages of magmatism occurred in the LYRB: 1) high-K calc-alkaline intrusions, 2) andesitic shoshonitic volcanism, and 3) A-type granitic intrusions. The Jiangmiao gabbroic intrusion, which is located at the eastern end of the LYRB, is composed of gabbro and diorite and was emplaced at ~124 Ma (Dong et al., 2017; Sun et al., 2017a), simultaneous with the A-type granites of the third stage of Mesozoic magmatism in the LYRB. Petrography and mineral chemistry indicate that the Jiangmiao intrusion was formed by the intense crystal fractionation of an oxidized (ΔNNO ca. +1.0) and hydrous (3.5–4.5 wt% H2O) basaltic magma that produced the evolved dioritic magma and gabbroic cumulate. The accumulation of magnetite, olivine, clinopyroxene, and plagioclase resulted in abnormally high Fe2O3 (12.44–16.30 wt%) and low SiO2 (40.75–45.88 wt%) contents in the gabbro, as well as the enrichment of TiO2, Sr, Cr, Ni, Co, and V in the gabbro compared with the diorite. Intense crystal fractionation coupled with the fluid transport led to the enrichment of highly fluid-mobile elements (Rb, Th, U, and Pb) in the evolved diorite but depletion of these elements in the gabbro. The mineral chemistry of clinopyroxene and whole-rock Sr−Nd−Pb isotopic compositions indicate that the primitive basaltic magma originated from the partial melting of an enriched lithospheric mantle source in a subduction environment, but more asthenosphere mantle components were incorporated into the magma source compared with the earlier Mesozoic magmatic rocks in the LYRB. The coevality of the Jiangmiao gabbroic intrusion and the A-type granites supports the tectonic model of Mesozoic rollback of the westward-subducting paleo-Pacific plate. The Jiangmiao gabbroic intrusion likely represents the edge of the back-rolling slab at ~124 Ma, whereas the simultaneous A-type granites in the middle of the LYRB were formed as a result of intraplate extension generated by rollback.