Abstract

The early Paleozoic Kwangsian event, the first compressive orogeny after the Rodinia assemblage in the South China Block (SCB), is characterized by the angular unconformity between the Devonian and pre-Devonian sequences and widespread granitoids and small-voluminous mafic rocks. However, the mechanism triggering this event is highly debated and associated mantle source is poorly known. Our recent investigations identified the occurrence of the early Silurian high-Mg basalt-andesite-dacite in the Dashuang area along the northwestern Yunkai Domain in the SCB. This study presents a set of new zircon U-Pb geochronological, whole-rock geochemical and Sr-Nd isotopic data for these volcanic rocks. They yield weighted mean 206Pb/238U ages of c. 430–443 Ma, similar to the age-peaks of early-middle Paleozoic magmatism and metamorphism. These volcanic rocks have high MgO, Cr and Ni contents and Mg-numbers. They also exhibit arc-like geochemical affinities with crustal-like Sr-Nd isotopic compositions, similar to the Neoproterozoic high-Mg basalt-andesite in the eastern SCB. Such signatures, combined with other geological observations, suggest that the Dashuang high-Mg rocks were derived from an ancient (probably early Neoproterozoic) mantle wedge source. The early Paleozoic intra-continental reactivation in response to late-orogenic extension triggered the melting of this paleosubduction-modified mantle, which led to the formation of these volcanic rocks.

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