Jurassic intraplate magmatism in southern Hunan-eastern Guangxi: ₄₀ Ar/ ₃₉ Ar dating, geochemistry, Sr-Nd isotopes and implications for the tectonic evolution of SE China

Abstract

The Mesozoic geology of SE China is characterized by intensive and widespread magmatism. However, the tectonic regime that accounted for the Mesozoic magmatism has been an issue with little consensus. A comprehensive study of 40Ar-39Ar dating, geochemistry and Sr-Nd isotopes has been conducted on basalts from southern Hunan and syenite intrusions from eastern Guangxi. Three episodes of Jurassic magmatism, i.e. alkaline basalts of c.175 Ma in age, syenitic intrusions of c.160 Ma and high-Mg basalts of c.150 Ma, are identified. The older, c.175 Ma alkaline basalts are characterized by low Sr (ISr = 0.7035–0.7040) and high Nd (εNd(T) = 5 to 6) isotopic compositions and OIB-like trace-element patterns (e.g. Nb/La > 1). In contrast, the younger, c.150 Ma high-Mg basalts have high Sr (ISrc.0.7054) and low Nd (εNd(T) c.-2) isotopic compositions and incompatible trace-element patterns of arc affinity. The c.160 Ma syenitic intrusions display a relatively large range of Sr and Nd isotopic compositions (ISr = 0.7032–0.7082, εNd(T) = 5.5 to −4.1), with the Qinghu syenites having the lowest ISr, highest εNd(T) and OIB-type incompatible trace-element patterns analogous to the c.175 Ma alkaline basalts. Such a secular variation in rock types and geochemical and isotopic characteristics reveals changes in melt segregation depth and mantle sources, which are inferred to have resulted from the post-Indosinian orogenic lithosphere extension and thinning. The c.175 Ma alkaline basalts are suggested to have formed by small degrees of decompression melting of the asthenosphere or an enriched lithospheric mantle source accreted by asthenosphere-derived melts during the initial extension. The c.160 Ma syenitic and c.150 Ma high-Mg basaltic rocks mainly originated from the enriched lithospheric mantle that melted owing to a raised geotherm caused by lithosphere thinning. This interpretation is at odds with the active continental margin related to the subduction of palaeo-Pacific plate, but consistent with continental rifting and extension for the Mesozoic of SE China.

Zircon U-Pb geochronology and heavy mineral composition constraints on the provenance of the middle Miocene deep-water reservoir sedimentary rocks in the Yinggehai-Song Hong Basin, South China Sea

“…Mesozoic igneous rocks associated with the collision between several microcontinents and the Yangtze Block yield U-Pb ages between 280 and 220 Ma (Hoa et al, 2008;Roger et al, 2012;Chen et al, 2014). Middle Jurassic to Cretaceous basaltic lavas and related mafic dikes ranging from 180 to 80 Ma are located in western and eastern Yangtze Block (Chen and Jahn, 1998;Wang et al, 2003;Li et al, 2004;Cheng and Mao, 2010). The Truong Son Belt of the Indochina Block is characterized by Silurian, Permian-Triassic granites and high-grade metamorphic rocks formed during the Caledonian and Indosinian Orogeny events, respectively.…”

Mesozoic large magmatic events and mineralization in SE China: oblique subduction of the Pacific plate

“…A-type granitic magma is generally taken as an indication of lithospheric extension (Collins et al 1982;Whalen et al 1987). A-type granites or alkaline intrusive rocks, for example, Sucun geode-like Kf-granite in Zhejiang (133 Ma) (Wang et al 2005b), Qianlishan Sn/W-bearing A-type granites in southwestern Hunan (Li et al 2004b), Pitou granite in Jiangxi (171 Ma) (Chen et al 2002), Fogang (159-165 Ma) and Nankunshan alkaline granites (158 Ma) in northern Guangdong (Li et al 2007c) with the occurrence of the early Yanshanian bimodal volcanic rock associations (Ningyuan, 173 Ma) (Li et al 2004a) in western Hunan, southern Jiangxi (Baimianshan, 172 Ma) , and eastern Guangxi (Huilongyu, 161-163 Ma) (Li et al 2004a), suggest that SE China was periodically in an extensional environment from approximately 180 Ma to the Cenozoic. Downloaded by [Guangzhou Institute of Geochemistry] at 00:00 09 February 2012…”

Petrogenesis of early Jurassic basalts in southern Jiangxi Province, South China: Implications for the thermal state of the Mesozoic mantle beneath South China

“…Furthermore, a mantle plume should have produced a large volume of flood basalts and continental-scale doming, neither of which has been observed in the Mesozoic of South China. Although a rifting and lithospheric extension model was widely accepted for explaining the petrogenesis of the within-plate magmatism (Chen et al, 1999a(Chen et al, , 1999bLi et al, 2003Li et al, , 2004Li and Li, 2007;Y.J. Wang et al, 2005Y.J.…”