Geochemical and Sr–Nd–Pb isotopic characteristics of basalt from eastern Hunan Province: New insight for the Late Cretaceous tectonic dynamic mechanism in South China

Abstract

During the Late Cretaceous period, massive magmatism occurred in South China. Eastern Hunan Province is situated in the inland part of South China. Comparing with the coastal area, the inland of South China is characterized by the absence of intermediate‐acid intrusive rocks, but the occurrence of multiple mafic rocks. To date, the emplacement and triggering mechanism of magmatism of inland South China is still very puzzling. In this article, we address this question by concentrating on the basalt exposed in eastern Hunan Province. Geochronologically, three stages of zircon grains, which include xenocrysts and authigenic crystals (rare), are recognized, and the last stage of zircon grains with a concordant age of 79.93 ± 0.47 Ma (MSWD = 1.9) represents the actual crystallization age of basaltic magma. Geochemically, the basalt in eastern Hunan Province has low TiO2 and K2O contents (1.11–2.63 and 0.28–2.88 wt%) but high alkali contents (3.75–6.58 wt%) and K2O/Na2O ratios (0.05–0.56). The basalts from eastern Hunan Province are enriched in large‐ion lithophile elements and light rare earth elements and obviously depleted in heavy rare earth elements with weak negative Eu anomalies (Eu/Eu* = 0.87–1.26); they lack significant Ti, Nb, Ta, etc., depletions. These features are much different from those of coastal basalt with strong arc‐related signatures. The incompatible element ratios and Sr–Nd–Pb isotope compositions commonly imply significant EMII‐type OIB‐like source region and within‐plate basalt characteristics. The basalt may have suffered from earlier partial melting of garnet‐spinel peridotite and later rapid fractional crystallization of mafic minerals (olivine and pyroxene). Significant geochemical and petrological discrepancies between inland basalt and coastal basalt suggest a subduction‐absent within‐plate setting for inland basalt but a strong subduction‐related signature for coastal basalt. The magmatic age of mafic rocks from inland South China hold a significant tendency to become younger to the west. The spatial and temporal distribution and variation of components of basalt in South China indicated a mantle plume model which is derived from the 670 km thermal boundary layer (670 TBL) between the lower and upper mantle. The trigger mechanism of this model could be attributed to the westward steep rollback subduction of the Palaeo‐Pacific Plate in the Late Cretaceous.