I-type granitoids associated with the early Paleozoic intracontinental orogenic collapse along pre-existing block boundary in South China

Abstract

The early Paleozoic Wuyi–Yunkai orogeny resulted in extensive magmatism in the Cathaysia and eastern Yangtze blocks, South China. Identifying the nature of related magmatism is essential for understanding the orogeny that remains enigmatic with regard to its tectonic setting and geodynamic driving force. The Zhangjiafang pluton (438±3Ma) in western Jiangxi province is composed of predominant granodiorite with abundant coeval mafic-intermediate microgranular enclaves (MMEs) (~433±5Ma). The granodiorite samples are weakly peraluminous (A/CNK=1.05–1.09) and have low SiO2 (61.9–64.9wt.%) and high Fe2O3 (4.6–5.6wt.%), MgO (2.2–2.8wt.%) and CaO (4.3–4.8wt.%), belonging to I-type suite due to abundant amphibole in the rocks. They exhibit strongly negative whole-rock εNd(t) values (−11 to −9) and zircon εHf(t) values (−14 to −4), similar to the basement of the Cathaysia Block, but distinguishable from simultaneous I-type granites of the Banshanpu and Hongxiaqiao plutons in eastern Yangtze Block in much lower Sr, Ba, Th and U. The MME samples show pronounced negative Nb–Ta–Ti anomalies and have overall less negative whole-rock εNd(t) (−9 to −7) and zircon εHf(t) values (−9 to −4) than the host granodiorite, which are best interpreted as products of mantle-derived melts that mixed insufficiently with crust-derived magma.The block boundary between the eastern Yangtze and Cathaysia blocks should pass through the nearby west area (i.e., Pingxiang City) of the Zhangjiafang pluton. The Early Paleozoic I-type granitic rocks near the block boundary have all negative εNd(t) and εHf(t) values, demonstrating an overall ancient basement of the two blocks prior to the Wuyi–Yunkai orogeny, preferring an intracontinental orogeny model. However, the early Paleozoic mafic rocks and I-type granites with coeval MMEs were frequently present along the Jiangshan–Shaoxing–Pingxiang Fault zone, illustrating widespread modification of the ancient basement adjacent to the block boundary by mantle-derived melt during the orogenic collapse. The pre-existing block boundary might have promoted asthenosphere upwelling and basaltic underplating during the intracontinental orogenic collapse.