Early Neoproterozoic assembly and subsequent rifting in South China: Revealed from mafic and ultramafic rocks, central Jiangnan Orogen

Abstract

The assembly time and process of the South China Block and its subsequent breakup remain controversial. This paper presents a set of new mineral chemical, zircon U-Pb geochronological, elemental and Sr-Nd-Hf-O isotopic data of the Group 1 Yiyang pillow basalt, Group 2 Aikou and Xinyu mafic–ultramafic rocks, and Group 3 Yuanling-Anjiang gabbro-diabase from the central Jiangnan Orogen. Group 1 is ∼830 Ma high-Mg subalkaline basalt with the mineral assemblage of clinopyroxene, epidote, chlorite and minor plagioclase. It shows arc-like geochemical signatures and derived from a wedge source recently infiltrated by recycled component. Group 2 is mainly characterized by peridotite, pyroxenite, gabbro and diabase and crystallized at ∼791–780 Ma. It has zircon εHf(t) and δ18O values of −0.4∼+5.6 and +4.99∼+5.90‰, respectively. The Group 2 rocks geochemically resemble OIB-like high-Nb/Nb-enriched basalt and originated from an OIB-like origin with the ultramafic rocks being the cumulates of the mafic magma. Group 3 contains normal zoned clinopyroxene and mainly formed at ∼780–786 Ma. It has 87Sr/86Sr(t) = 0.7066–0.7116, εNd(t) = −2.5∼−0.3, zircon εHf(t) = −6.6∼−1.0 and δ18O = +6.06∼+6.65‰, originated from the lithosphere-asthenosphere interaction. These data synthetically reflect the South China Block being created by an early Neoproterozoic accretionary orogen along the periphery of Rodinia and transformed into lithospheric extensional regime since ∼785 Ma in response to Rodinia breakup. Along the Jiangnan Orogen, the Paleo-Huanan Ocean was completely closed by ∼830 Ma. The Yangtze with Cathaysia blocks finally amalgamated at ∼810 Ma and subsequent rifting of the united SCB occurred at ∼785 Ma, respectively.