Geochemical and zircon U-Pb-Hf isotopic study of volcanic rocks from the Yaolinghe Group, South Qinling orogenic belt, China: Constraints on the assembly and breakup of Rodinia

Abstract

The tectonic mechanism responsible for the Neoproterozoic magma generation at the northern margin of the Yangtze craton is crucial to understanding the assembly and breakup of the Rodinia supercontinent. Here, we present a systematic study of the petrology, whole-rock geochemistry and geochronology of mafic and felsic volcanic rocks from the Yaolinghe Group, which outcrops mainly in the Shangnan area (north) and the Wudang area (south) of the South Qinling orogenic belt along the northern margin of Yangtze craton. Basalts from the Shangnan area are tholeiitic and calc-alkaline and indicated an arc environment. They have a LA-ICP-MS zircon U-Pb age of 847 ± 5 Ma with positive εHf(t) values of + 6.93 to + 8.79 and TDM1 model ages of 1130–1053 Ma. The rhyolites from this area exhibit geochemical characteristics similar to S-type granites and have a zircon U-Pb age of 841 ± 7 Ma, with a wide range of εHf(t) values of + 1.02 to −11.40 and TDM2 model ages of 2453–1674 Ma. This is different from the rhyolites from the Wudang area, which reveal geochemical affinities similar to A2-type granites, with a zircon U-Pb age of 788 ± 28 Ma, uniform negative εHf(t) values of −8.75 to −0.15 and TDM2 model ages of 2214–1663 Ma. Both rhyolites were sourced from partial melting of Paleoproterozoic crust with minor input of mantle material, but were formed in an arc setting and a post-collision extension setting, respectively. The 663–644 Ma mafic volcanic rocks from the Wudang area are enriched in Th, U, Nb and Ta, similar to oceanic island basalts and have positive zircon εHf(t) values ranging from + 9.24 to + 4.89 with TDM1 model ages peaking at 1018 Ma. They were derived from partial melting of an asthenospheric mantle source that had been modified by slab-derived melts and erupted during late Neoproterozoic continental rifting. Combining our results with existing data, we suggest that the South Qinling orogenic belt resulted from bidirectional subduction of the South Qinling Ocean between the Douling block and the Yangtze craton at ca. 950–750 Ma, accompanied by the formation of a trench-arc-basin system along the Douling block and an Andean-type continental arc belt at the northern margin of the Yangtze craton. After accretion, the amalgamated Douling block and Yangtze craton were affected by oceanic subduction of the Mirovoi Ocean at ca. 750–680 Ma, and subsequent continental rifting at ca. 680–620 Ma. The model further poses that the Yangtze craton was assembled to the periphery of the Rodinia supercontinent.