The 1.14 Ga mafic intrusions in the SW Yangtze Block, South China: Records of late Mesoproterozoic intraplate magmatism

Abstract

Abstract The late Mesoproterozoic geodynamic setting of the Yangtze Block, South China remains largely unknown due to the scarce geological records of this age. This study presents a new dataset consisting of petrology, whole-rock geochemistry, zircon U-Pb geochronology and Hf isotopes of newly identified late Mesoproterozoic diabases in the southwestern Yangtze Block. Two diabase samples give zircon U-Pb ages of 1138 ± 4 Ma and 1141 ± 4 Ma. The diabases are tholeiitic in composition and have consistent high TiO2 contents (2.18–2.93 wt%) but low Mg# values (41–47). They are slightly enriched in LREEs with (La/Yb)N ratios of 2.87–3.99 and show E-MORB-like trace element patterns with positive Nb-Ta anomalies. The diabase samples exhibit positive zircon eHf(t) values from + 9.4 to + 13.7 and whole-rock eNd(t) values from + 3.4 to + 4.0. These results suggest that they may have undergone negligible crustal contamination during the magma evolution and were most likely generated from partial melting of a depleted asthenosphere mantle source. The diabases possess consistent Nb/Yb (4.87–5.91) and Th/Yb (0.031–0.59) ratios, ruling out the possibility of interaction with subduction-related materials. They have high Ti/V (38–46) and Zr/Y (3.36–5.23) ratios, typical of within-plate basalts. Considering the contemporary A-type granites and alkaline basalts in the same region, intense continental rifting most likely occurred in the southwestern Yangtze Block during the late Mesoproterozoic. Widespread intraplate magmatism of 1.2–1.1 Ga in the Yangtze Block indicates that it may not have been involved in the Rodinia assembly at the time.