Early Paleozoic tectonic evolution of the South China Block: Constraints from geochemistry and geochronology of granitoids in Hunan Province

Abstract The Early Paleozoic Wuyi–Yunkai orogeny in the South China Block (SCB) remains enigmatic. A geochemical and geochronological investigation was carried out for the four poorly-studied granitoid intrusions (i.e., Hongxiaqiao, Xuehuading, Baimashan and Wanyangshan granitoids) in Hunan Province, and within the SCB, to determine their petrogenesis and give enlightening insights into the tectonic evolution of the Wuyi–Yunkai Orogen. The Hongxiaqiao, Xuehuading and Baimashan granitoids have lower SiO2 contents, and total alkali (Na2O + K2O) and A/CNK values, but higher Fe2O3T, MgO and TiO2 contents, compared with those of the Wanyangshan granitoid. All the studied granitoids are strongly depleted in Ba, Nb, Zr, Ti, and HREEs, but are enriched in Rb, Th, U, K, and LREEs, with significant negative Eu anomalies, particularly in the Wanyangshan granitoid. Bulk-rock geochemical compositions indicate that the Hongxiaqiao, Xuehuading and Baimashan granitoids are metaluminous–weakly peraluminous I-type granitoids, whereas the Wanyangshan granitoid is peraluminous and moderately fractionated S-type granitoid. Zircon LA-ICP-MS U Pb dating results indicate that the Hongxiaqiao, Xuehuading, Baimashan and Wanyangshan granitoids were intruded at 451 ± 1 Ma (MSWD = 1.8), 412 ± 1 Ma (MSWD = 2.4), 415 ± 1 Ma (MSWD = 3.0) and 445 ± 1 Ma (MSWD = 3.9), respectively. The I-type granitoids have variable initial 87Sr/86Sr values of 0.71160–0.72362, high eNd(t) values (−6.5 to −8.1), and young T2DM ages of 1717–1894 Ma. In contrast, the S-type granitoid has consistent initial 87Sr/86Sr values of 0.71448–0.71887, slightly lower eNd(t) values (−9.8 to −10.3), and older T2DM ages (1977–2015 Ma), which suggests that they were derived from partial melting of different Paleoproterozoic basement rocks, without significant crustal contamination. The older Hongxiaqiao and Wanyangshan granitoids were formed in a compressional setting, whereas the younger Xuehuading and Baimashan granitoids were generated in a post-orogenic extensional setting. Integrated with geological evidence, we proposed that the Early Paleozoic Wuyi–Yunkai orogeny in the SCB resulted from intraplate orogenesis triggered by far-field tectonic response to the assembly of the northern East Gondwana, rather than from oceanic subduction–collision.