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Middle Neoproterozoic syn-rifting volcanic rocks in Guangfeng, South China: petrogenesis and tectonic significance

WU-XIAN LI^*, XIAN-HUA LI^*,, and ZHENG-XIANG LI

^* Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Institute of Geoscience Research, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia

Author for corresponence: lixh{at}gig.ac.cn

Middle Neoproterozoic igneous rocks are widespread in South China, but their petrogenesis and tectonic implications are still highly controversial. The Guangfeng middle Neoproterozoic volcani-sedimentary succession was developed on a rare Sibaoan metamorphic basement (the Tianli Schists) in the southeastern Yangtze Block, South China. This paper reports geochronological, geochemical and Nd isotopic data for the volcanic rocks in this succession. The volcanic rocks consist of alkaline basalts, andesites and peraluminous rhyolites. SHRIMP U–Pb zircon age determinations indicate that they were erupted at 827 ± 14 Ma, coeval with a widespread episode of anorogenic magmatism in South China. Despite showing Nb–Ta depletion relative to La and Th, the alkaline basalts are characterized by highly positive Nd(T) values (+3.1 to +6.0), relatively high TiO₂ and Nb contents and high Zr/Y and super-chondritic Nb/Ta ratios, suggesting their derivation from a slab melt-metasomatized subcontinental lithospheric mantle source in an intracontinental rifting setting. The andesites have significantly negative Nd(T) values (–9.3 to –11.1) and a wide range of SiO₂ contents (57.6–65.6 %). They were likely generated by the mixing of fractionated basaltic melts with felsic melts derived from the Archaean metasedimentary rocks in the middle to lower crust. The rhyolites are highly siliceous and peraluminous. They are characterized by depletion in Nb, Ta, Sr, P and Ti and relatively high Nd(T) values (–3.0 to –4.8), broadly similar to those of the adjacent c. 820 Ma peraluminous granitoids derived from the Mesoproterozoic to earliest Neoproterozoic sedimentary source at relatively shallow levels. We conclude that the Guangfeng volcanic suite is a magmatic response of variant levels of continental lithosphere (including lithospheric mantle and the lower-middle to upper crust) to the middle Neoproterozoic intracontinental rifting possibly caused by mantle plume activity.

Key Words: Neoproterozoic • geochemistry • volcanic rocks • rift basin • South China