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.

Batholith recorded mesozoic multistage tectonic evolution of the South china block: A case study of the guandimiao intrusions

“…These results amply justify their geological affiliation. L4 of the Guandimiao batholith has low ε Nd (t) values (−10.18 to −9.62) but high I sr (0.71449-0.71784) and T DM2 values (1.75-1.70 Ga), which represents significant deviation from the Mesozoic EM II-type OIB in the SCB (6.8-10.8;Meng et al, 2012;Qin et al, 2020). The 207 Pb/ 204 Pb versus 207 Pb/ 204 Pb and 143 Nd/ 144 Nd versus 87 Sr/ 86 Sr diagrams show that L4 was derived from a Mesoproterozoic pelagic and/or terrigenous sediment source (Figures 10C,D).…”

“…Multiple stages of magmatic activity have been determined using accurate geochronological data (Zhou et al, 2006;Wang et al, 2010;Mao et al, 2011;Mao et al, 2014). Zhou et al (2006) proposed four main stages of granitic magmatism: Moreover, the mafic rocks contain three main clusters: the 190-160 Ma group (Zhao et al, 1998;Meng et al, 2012), the 160-110 Ma group (Wang et al, 2003;Wang et al, 2008;Ma et al, 2013) and the 110-80 Ma group (Chen et al, 2008a;Meng et al, 2012;Qin et al, 2020). Numerous models have been proposed for the systematic understanding of the evolutionary history of the regional geology.…”

“…Subsequently, the postcollisional and postorogenic regimes occurred 234-203 Ma and 190-170 Ma, respectively (Zhou et al, 2006;Mao et al, 2014;Shu et al, 2019). Two supportable mechanisms have been presented with the controversy of whether the subduction of the Pacific Ocean dominated the tectono-magmatic pattern of the SCB in the middle to late Mesozoic (Li, 1990;Gilder et al, 1996;Chen et al, 2014;Li et al, 2014;Qin et al, 2020;Yang et al, 2020).…”