Huiying He

Carboniferous Arc Setting in Central Hainan: Geochronological and Geochemical Evidences on the Andesitic and Dacitic Rocks

Volcanic rocks in the Bangxi-Chenxing tectonic zone provide important carries for better understanding the Late Paleozoic tectonic evolution in Hainan and its temporal-spatial pattern of the eastern Paleotethyan evolution. This paper presents a set of new geochronological and geochemical data on the andesitic and dacitic rocks along the Bangxi-Chenxing tectonic zone in central Hainan. The representative andesitic and dacitic samples yield similar zircon U-Pb ages of 353±3 and 351±7 Ma, respectively, being of Early Carboniferous origin. These volcanic rocks are characterized by low TiO2 and high Al2O3 contents and are enriched in LILEs and LREEs but depletion in HFSEs, along with negative εNd(t) values of -1.4– -4.7 and high 87Sr/86Sr(i) ratios of 0.707 2–0.710 1. Geochemical signatures suggest that the andesitic and dacitic samples might originate from a metasomatized wedge modified by the slab-derived component in a continental arc setting. In combination with the available data, it is proposed for the development of a Carboniferous continental arc in response to the eastern Paleotethyan evolution. The Bangxi-Chenxing tectonic zone might westerly link with the Jinshajiang-Ailaoshan-Song Ma suture zone, constituting an assemblage boundary between the South China and Indochina blocks.

Petrogenesis of the early Silurian Dashuang high-Mg basalt–andesite–dacite in eastern South China: origin from a palaeosubduction-modified mantle

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-volume mafic rocks. However, the mechanism triggering this event is highly debated and the associated mantle source is poorly known. Our recent investigations identified the occurrence of 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 in the SCB. 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 palaeosubduction-modified mantle, which led to the formation of these volcanic rocks.