Amalgamation of South China into Rodinia during the Grenvillian accretionary orogeny: Geochemical evidence from Early Neoproterozoic igneous rocks in the northern margin of the South China Block

In order to gain insights into the amalgamation process of South China into Rodinia, a combined study of whole-rock geochemistry, zircon U-Pb ages and Hf-O isotopes was carried out for igneous rocks from the Mianlue ophiolite in the northern margin of the South China Block. In-situ zircon U-Pb dating yields ²⁰⁶Pb/²³⁸U ages of 905 ± 8 Ma for plagiogranite, 936 ± 6 Ma for andesite and 950 ± 5 Ma for gabbro-diorite. The plagiogranite is characterized by high Na₂O contents of 6.6–7.0 wt%, low TiO₂ contents of 0.2–0.4 wt%, and enrichment in LREE and LILE but depletion in Nb, Ta and Ti. The zircons have positive ε_Hf(t) values of 8.9 to 14.9 and slightly elevated δ¹⁸O values of 4.6 to 6.8‰. The whole-rock yields low (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7016–0.7031, positive ε_Nd(t) values of 5.9 to 6.9 and positive ε_Hf(t) values of 11.1 to 11.2. This indicates that the plagiogranite was produced by partial melting of juvenile oceanic crust that experienced seawater-hydrothermal alteration. The age of 905 ± 8 Ma for the plagiogranite marks the obduction of the ophiolite on continental margin during Rodinia assembly. The andesite and gabbro-diorite are enriched in LILE and LREE but depleted in HFSE. The andesite shows highly positive zircon ε_Hf(t) values of 8.6 to 15.8 and mantle-like zircon δ¹⁸O values of 4.1 to 5.9‰. In addition, both andesite and gabbro-diorite whole-rocks are depleted in Sr-Nd-Hf isotopes, with positive ε_Nd(t) values of 4.1 to 5.5 and highly positive ε_Hf(t) values of 12.1 to 16.0. Such geochemical characteristics indicate that these mafic igneous rocks were derived from partial melting of arc-type mantle sources. In this regard, the ages of 936 ± 6 Ma and 950 ± 5 Ma for the andesite and gabbro-diorite date mafic arc magmatism above a Grenvillian oceanic subduction zone. Taken altogether, the subduction of the Grenvillian oceanic crust was continuous till the obduction of ophiolite at ∼900 Ma. Therefore, the Grenvillian oceanic subduction system was developed along the northern margin of the Yangtze Craton in the Early Neoproterozoic. This implies that South China was assembled into Rodinia through a Grenvillian accretionary orogeny.