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

摘要

Early Jurassic igneous rocks are abundant in the Nanling Range, southeastern China. They are closely relatedwith 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, andwhole-rock major-trace element and Sr-Nd 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 A(2)-type granitoids globally. Their highly evolved whole-rock Sr-Nd isotopes and elevated zircon delta O-18 (Changpu rhyolites: (87Sr/86Sr)(i) = 0.7140 to 0.7156, epsilon(Nd)(t) = -7.2 to -8.7, delta O-18 = 8.76 to 9.93%; Dongkeng rhyolites: (Sr-87/Sr-86) i = 0.7149 to 0.7162, epsilon(Nd)( t) = -6.4 to -7.9, delta O-18 = 7.04 to 9.78%) are consistent with their generation dominated by remelting of exisiting crustalmaterials in an extensional environment. The slightly more radiogenic zircon Hf isotopes of the Dongkeng rhyolites, epsilon(Hf)(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 epsilon(Nd)(t) from -7.8 to -11.3 and highly radiogenic (Sr-87/Sr-86) 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.