Sr–Nd–Pb isotopic constraints on multiple mantle domains for Mesozoic mafic rocks beneath the South China Block hinterland

Abstract

The Sr–Nd and Pb isotopic compositions of middle Jurassic to late Cretaceous mafic rocks across the Chenzhou–Linwu Fault within the South China Block are presented to constrain the characteristics and evolution of Mesozoic mantle source. These data, in combination with geochronological results, can be divided into four groups. Group 1 (~ 175–170 Ma) occurs on either side of the Chenzhou–Linwu Fault and has moderately depleted Sr–Nd and radiogenic Pb isotopes, akin to a FOZO-like source and is inferred to have originated from deep asthenospheric mantle with delaminated lower crustal signatures. Group 2 (~ 175–145 Ma) is restricted to the west of the fault and shows a significant Dupal anomaly and a binary mixing array between a component from an enriched mantle source (EM1) with low ε Nd( t) values and 206Pb/ 204Pb, and intermediate 87Sr/ 86Sr ratios and a FOZO-like component characterized by Group 1. Group 3 (~ 175–125 Ma) occurs to the east of the fault and exhibits low ε Nd( t) values and high Sr–Pb isotopic ratios, similar to EM2 component with minor DMM component. The Sr–Nd–Pb isotopic systematics for Group 4 (~ 93–80 Ma) resembles those of SE China Cenozoic basalts, suggestive of DMM-EM2 reservoir in response to the uppermost asthenospheric mantle source. These temporal and spatial variations, together with other geologic and paleomagnetic data, suggest that (1) the multiple mantle reservoirs include FOZO-, DMM-, EM1- and EM2-like components beneath the South China Block; (2) DMM-EM2 sources have dominated the mantle domains beneath SE Asia since late Cretaceous; (3) the Chenzhou–Linwu Fault corresponds with the early Mesozoic lithospheric boundary between the Yangtze and Cathaysia blocks within the South China Block. The Mesozoic mafic rocks within the South China Block developed in an active rift setting in response to the delamination of partially over-thickened lithospheric keel and upwelling of asthenosphere rather than a supra-subduction environment.