The earliest Jurassic A-type rhyolites and high-Mg andesites–dacites in southern Jiangxi Province, southeast China: Evidence for delamination of a flat-slab?

Abstract

Early Jurassic igneous rocks are abundant in the Nanling Range, southeastern China. They are closely related with economically significant deposits, but the tectonic-magmatic controls on their formation are still controversial. This paper reports zircon U–Pb–Hf–O isotopes for the rhyolites, and whole-rock major–trace element and SrNd isotope compositions for the rhyolites and andesites/dacites in the Changpu and Dongkeng basins located in the southern part of Jiangxi Province. The Changpu and Dongkeng rhyolites have high K2O contents (4.1–7.6 wt%) and K2O/Na2O ratios (> 1). Both groups are enriched in HFSE, LREE and show pronounced Nb–Ta–Eu–Sr–P–Ti negative anomalies. Geochemically, they can be classified as A-type and are similar to A2-type granitoids globally. Their highly evolved whole-rock SrNd isotopes and elevated zircon δ18O (Changpu rhyolites: (87Sr/86Sr)i = 0.7140 to 0.7156, εNd(t) = −7.2 to −8.7, δ18O = 8.76 to 9.93‰; Dongkeng rhyolites: (87Sr/86Sr)i = 0.7149 to 0.7162, εNd(t) = −6.4 to −7.9, δ18O = 7.04 to 9.78‰) are consistent with their generation dominated by remelting of exisiting crustal materials in an extensional environment. The slightly more radiogenic zircon Hf isotopes of the Dongkeng rhyolites, εHf(t) = −2.1 to −7.1 as compared with −6.1 to −8.3 of Changpu rhyolites, likely signify some mantle contribution. The Changpu high-Mg andesites–dacites are geochemically similar to high-Mg andesites (HMAs) of the Setouchi arc volcanic belt in Japan and the Piip Volcanos in the western Aleutian Arc. They have very low εNd(t) from −7.8 to −11.3 and highly radiogenic (87Sr/86Sr)i values from 0.7143 to 0.7154. Similar to HMAs in Setouchi, we suggest that they may be derived from melting of subducted sediments and followed by melt-mantle interaction.SIMS zircon UPb dating yielded extrusive ages of ~190 Ma for both Changpu and Dongkeng rhyolites. The new dating results constrain the minimum age of extrusion of rhyolites and andesites-dacites from the same volcanic sequence to be the earliest Jurassic period. Igneous rocks of this age have very limited distribution in south China. While those found in the coastal regions show features similar to arc rocks, these in the hinterland are rift related. We propose that formation of the Early Jurassic A-type rhyolites and high-Mg andesites–dacites may provide evidence for foundering/delamination of a previously subducted oceanic plateau beneath southeastern China.