Ca. 830 Ma alkaline mafic dykes in the southeastern Guizhou Province, South China: New constraints on the Neoproterozoic evolution of the Yangtze Block

Abstract

The Neoproterozoic alkaline mafic rocks in the Yangtze Block are critical to investigate the magmatic and tectonic evolution of South China. However, these rocks are rarely exposed. We here present a detailed study of ca. 830 Ma alkaline mafic dykes newly identified from a drilled borehole at the Nage area in the southern Yangtze Block. The dykes have SIMS zircon U-Pb ages of 827 ± 8 Ma. They show coherent variations in major and trace elements, characterized by enrichment in LREE and incompatible elements and no significant Nb-Ta and Zr-Hf anomalies, suggesting that their parental magmas have experienced varying degrees of fractionation crystallization with negligible crustal contamination. Modeling calculations based on their εNd(T) values (-0.55–3.23), SiO2 contents (48.38–53.61 wt%) and elemental ratios (e.g., La/Ta and Nb/La) suggest that the Nage mafic dykes were probably generated from an asthenospheric mantle with previously input of a plume-derived enrich component occurred within the source region rather than during ascent. Rocks from the Nage dykes exhibit OIB-like trace element and REE patterns as well as trace elemental compositions, which are different from arc-related igneous rocks but similar to alkaline basalts and mafic dykes formed in mantle plume related rifting systems around the world. In combination with tectonostratigraphic evidences, the Nage dykes in this study are suggested to have been generated in an intraplate rift which was probably induced by a mantle plume beneath the Yangtze Block.