Neoproterozoic adakitic rocks from Mopanshan in the western Yangtze Craton: Partial melts of a thickened lower crust

Abstract

The tectonic setting of South China during the Neoproterozoic remains a subject of debate. Neoproterozoic adakites from Xuelongbao and Datian along the western margin of the Yangtze Craton have been used to argue for an arc setting assuming that these rocks are the melting products of a subducted oceanic slab (Zhou, M.F., Yan, D.P., Wang, C.L., Qi, L., Kennedy, A., 2006a. Subduction-related origin of the 750 Ma Xuelongbao adakitic complex (Sichuan Province, China): Implications for the tectonic setting of the giant Neoproterozoic magmatic event in South China. Earth and Planetary Science Letters 248, 286–300.; Zhao, J.H., Zhou, M.F., 2007. Neoproterozoic Adakitic Plutons and Arc Magmatism along the Western Margin of the Yangtze Block, South China. The Journal of Geology 115, 675–689.). However, the slab-related petrogenetic model is unsuitable for the Neoproterozoic (782 ± 6 Ma) adakites from Mopanshan. The Mopanshan adakites are characterized by low MgO, Cr and Ni contents, high Rb/Sr and Sr/Y ratios and negative ε Nd( t) values (− 2.06 to − 0.43). These features, plus the presence of relict zircons (1.16–0.83 Ga) and Mesoproterozoic zircon Hf model ages (1.06–1.27 Ga), are inconsistent with a slab-melting origin, but favour an origin by melting of a thickened continental lower crust. It is likely that the crust associated with magma generation may have experienced continuous growth and reworking since the Sibao Orogeny. Sharing the main geochemical features with the Mopanshan rocks, the Xuelongbao and Datian adakites may also have been derived from a thickened lower crust. This interpretation casts doubts on the Neoproterozoic arc setting proposed for south China. Instead, the Neoproterozoic in south China may represent an intra-continental extensional setting.