Petrogenesis of the Early-Middle Triassic high-Mg andesitic rocks in the southern margin of the South China Block: Implications for the convergence between the South China and Indochina Blocks

Abstract

Questions remain about whether the volcanic rocks and spatially associated mafic to intermediate intrusions in the Funing area, Southwestern China, are part of Emeishan large igneous province (ELIP) and formed in a plume-related extensional setting. Zircon U-Pb ages, geochemical and Sr-Nd-Hf isotopic data are reported for these volcanic rocks, in order to investigate their source, petrogenesis and tectonic setting. Zircon U-Pb dating yields an eruption age of 246 ± 2 Ma for the volcanic rocks, younger than that of the ELIP. They are calc-alkaline rocks, and are characterized by high MgO (>5 wt%) and Al2O3 contents at intermediate SiO2 contents (52.12–59.20 wt%), similar to those of high-Mg andesites, indicating a mantle source. They are enriched in large ion lithophile elements and light rare earth elements and are depleted in high field strength elements, with evidently negative Nb, Ta and Ti anomalies. They have relatively high Th/Nb and Th/La ratios and low Sr/Y and La/Yb ratios, with radiogenic initial Sr isotopic ratios (87Sr/86Sr i = 0.7080–0.7110) and negative εNd(t) (−8.8 to −6.5) and εHf(t) (−5.3 to −2.4) values. All of these geochemical features suggest that the Funing high-Mg andesitic rocks were derived from partial melting of an ancient, refractory lithospheric mantle that had been metasomatized by subducted sediment-derived fluids/melts before magma generation. In the context of other Early-Middle Triassic magmatism in the adjacent areas, we suggest that the Early-Middle Triassic volcanic rocks in the Funing area were generated in an arc setting, rather than by mantle plume-related melting. The spatial and temporal distribution of volcanic arc and the ophiolitic mélange in the China-Vietnam border region indicate Early to Middle Triassic northward subduction of the Babu (SW China)-Cao Bang (NE Vietnam) ocean basin beneath the southern margin of the South China Block.