Geochronology, elemental and Nd–Hf isotopic geochemistry of Devonian A-type granites in central Jiangxi, South China: Constraints on petrogenesis and post-collisional extension of the Wuyi–Yunkai orogeny

Abstract

Abstract Details on processes of the early-middle Paleozoic Wuyi–Yunkai orogeny in South China remain poorly defined. Most Silurian–Devonian granites in South China are S-type or I-type granites, which are suggested to be petrogenetically related to the Wuyi–Yunkai orogeny. This paper firstly reported a systematic study on two Devonian A-type granites in the central Jiangxi Province. LA-ICP-MS zircon U–Pb dating results imply that the Huitong and Epo granites were emplaced at about 415 Ma. Both of the two granites have the petrographic and geochemical characteristics of A-type granites. Interstitial biotites occur along the boundary of euhedral plagioclase and quartz and they formed later than plagioclase and quartz. It implies that the primary magma could have been anhydrous. Biotites from the two granites are Fe-rich and have high Fe 2 + /(Fe 2 + + Mg 2 + ) ratios (0.60–0.74). The magmatic temperatures estimated from zircon saturation thermometer are 802–920 °C for the two granites, higher than common I-type and S-type granites. The two granites show high contents of total alkalis (Na 2 O + K 2 O = 6.96–9.39 wt.%), high field strength elements (e.g. Zr = 181–437 ppm, Y = 22.1–39.7 ppm, Nb = 18.6–30.3 ppm and Zr + Nb + Ce + Y = 324–555 ppm), rare earth elements (total REE = 155–312 ppm) as well as high Ga/Al ratios (10,000 × Ga/Al = 2.50–3.44). These geochemical characteristics are similar to those of A-type granites. The Huitong and Epo granites have relatively low e Nd (t) values of − 10.4 to − 7.7, and low zircon e Hf (t) values (peak value of − 8.0). Whole-rock two stage Nd isotopic model ages and zircon Hf isotopic model ages mostly vary from 1.78 Ga to 2.00 Ga. According to these data, we suggest that the two granites might have derived from partial melting of pre-Cambrian sedimentary rocks which had been granulitized during an earlier thermal event. The two granites contain abundant contemporaneous mafic microgranular enclaves (MMEs). The MMEs display igneous textures and contain acicular apatites, suggesting quenching of mafic magmas that have co-mingled with the host granites. The MMEs might have derived from the enriched lithospheric mantle source. This study on the Devonian Huitong and Epo A-type granites, together with previous studies on Paleozoic granites in South China, indicated that the Wuyi–Yunkai orogeny in South China had changed from syn-collisional crustal thickening to post-collisional extension at least from 415 Ma.