Neoproterozoic active continental margin of the Cathaysia block: Evidence from geochronology, geochemistry, and Nd–Hf isotopes of igneous complexes

Abstract

Previous studies have suggested that northwest Cathaysia was a passive continental margin during Neoproterozoic time, given the absence of Neoproterozoic magmatism. However, this view has been challenged by recent discoveries of mafic and felsic magmatic rocks with geochemical affinities to continental arc or back-arc volcanic rocks from the 1.0–0.9Ga Wuyi–Yunkai belt, indicating the development of an early Neoproterozoic continental margin arc and back-arc system in the Cathaysia block. In this paper we present new age data for the Miaohou and Shanhou arc complexes, demonstrating that the active continental margin of the Cathaysia block lasted until ca. 830Ma. The Miaohou complex is composed of gabbro, diorite, and granite, the Shanhou complex of diorite and granite, and both complexes are bound by the Jiangshao Fault. Twelve samples from the Miaohou and Shanhou complexes give zircon U–Pb ages of ca. 830Ma, indicating that the magmatism of the Miaohou and Shanhou complexes were coeval. The Miaohou gabbro has high zircon ɛHf(t) (+2.8 to +9.6) and whole-rock ɛNd(t) (+3.9) values together with the geochemical features of volcanic arc basalts, while the Miaohou and Shanhou diorites and granites show large variations and high values of zircon ɛHf(t) and ɛNd(t) [ɛNd(t) (+1.1 to +5.5) and ɛHf(t) (−6.1 to +12.3) for the diorites, and ɛNd(t) (+0.3 to +3.1) and ɛHf(t) (−3.8 to +11.3) for the granites] together with a similar geochemistry to continental arc felsic rocks (e.g., Cordilleran batholiths). The geochemical and isotopic data suggest that the southeastwards subduction of an oceanic plate under the Cathaysia block caused the dehydration of sediments in the subducted slab, leading to the metasomatism of the mantle wedge by subduction-related fluids, partial melting of the mantle wedge, followed by varying degrees of assimilation and fractional crystallization (AFC) as the magmas ascended. These processes provide a reasonable explanation for the Neoproterozoic arc magmatism on the northwestern margin of the Cathaysia block, represented by the Miaohou and Shanhou complexes.