Early Paleozoic intracontinental orogeny and post-orogenic extension in the South China Block: Insights from volcanic rocks

Abstract

The early Paleozoic orogeny represents the first extensive Phanerozoic tectono-thermal event in the South China Block (SCB). Two distinct orogeny models, subduction–collision orogeny and intracontinental orogeny, have been proposed, and one of the key controversies is the nature and the tectonic implications of the associated early Paleozoic volcanic rocks in the SCB, which have not yet been systematically investigated. Zircon U-Pb dating results show that these volcanic rocks formed at 445–435Ma, coeval with large-scale intrusive magmatism (446–420Ma). The felsic volcanic rocks, which include the Mashan and Hekou dacites and rhyolites, show high SiO2, low MgO and low Fe2O3 contents. Whole-rock trace-element and isotopic compositions of the felsic volcanic rocks suggest that they were generated by partial melting of a Paleoproterozoic crustal component. The mafic volcanic rocks are represented by the Chayuanshan basalts, which are characterized by low SiO2, high MgO, Cr and Ni contents, enrichment in LILEs and depletion in HFSEs. The low Nb/La ratios, high Th/Yb ratios and negative whole-rock ɛNd(t) values suggest that the basalts were derived from partial melting of a metasomatized subcontinental lithospheric mantle (SCLM). The “subduction signature” (calc-alkaline affinity, enriched LILEs and depleted HFSEs) of the Chayuanshan basalts was supposed to be inherited from the source and it didn’t reflect their generation in a subduction-related arc setting. Asthenospheric mantle upwelling and basaltic magma underplating may have been responsible for the partial melting of the crust and the metasomatized SCLM, which produced the Hekou and Mashan dacites and rhyolites and Chayuanshan basalts, respectively. Activation of the pre-existing suture zones, asthenospheric mantle upwelling and extensive partial melting of the crust caused the intensive magmatism of the SCB in early Paleozoic.