Opening of the Proto-Tethys Ocean: Implications from the late Neoproterozoic mafic dike swarms in the South Qinling Belt, South China

Abstract

Magmatism in the passive margin records continuum rift of continental lithosphere and onset of seafloor spreading. The ca. 650 Ma ultramafic-mafic dike swarms are the latest magmatic episode in the South Qinling Belt during the Neoproterozoic, and therefore provide a window to address their mantle sources, and the continental rift to drift transition. These mafic dikes have distinct geochemical signatures, and can be grouped as N-MORB-, E-MORB-, and Arc-affinities. The Shiziyan amphibole gabbros have low Rb/Y (<0.75), Nb/Y (<0.15), Nb/Zr (<0.05), Th/Zr (<0.01) ratios, positive εNd (+4.3 to +5.4), low Sri (0.70650–0.70759) and moderate 207Pb/204Pb (15.48) and 206Pb/204Pb (17.57), similar to N-MORB, suggesting that they were derived from an asthenosphere mantle. The Yindongshan pyroxenites display enrichment of LILEs and HFSEs, similar to the E-MORB characters. They also have low Sri (0.70384–0.70410), high εNd (+4.2 to +5.4), and 207Pb/204Pb (15.53–15.54) and 206Pb/204Pb (18.18–18.37). They have high zircon εHf (+13.1 to +15.0), but low δ18O values (+4.43 to +5.17‰). These chemical features indicate an enriched mantle source with involvement of recycled component. The Duchongshan olivine gabbros show arc-like geochemical features, with high Nb/Zr (0.07–0.16), Nb/Yb (3.52–20.1) ratios, and high Sri (0.70545–0.70586), negative εNd (−6.1 to −5.2), large variable zircon εHf (−0.5 to +4.2) and high δ18O (+5.41 to +6.41‰). These features favor that they were derived from a subduction-modified lithosphere mantle followed by significant crustal contamination. The northern Yangtze Block experienced secular crustal extension and developed into a volcanic rift margin during the middle to late Neoproterozoic. Mafic dike swarms emplaced into the volcanic successions and unconformably overlayed by late Ediacaran neritic to abysmal deposition. The magmatic termination and sedimentary facies transition indicate that the northern Yangtze Block broke up during the late Neoproterozoic and the Proto-Tethys Ocean opened at ca. 650 Ma.