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

Abstract

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 εHf(t)values of 8.9 to 14.9 and slightly elevated δ18Ovalues of 4.6 to 6.8‰. The whole-rock yields low (87Sr/86Sr)i ratios of 0.7016–0.7031, positive εNd(t) values of 5.9 to 6.9 and positive εHf(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 εHf(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 εNd(t) values of 4.1 to 5.5 and highly positive εHf(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.