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 ± 3 Ma) in western Jiangxi province is composed of predominant granodiorite with abundant coeval mafic-intermediate microgranular enclaves (MMEs) (~433 ± 5 Ma). The granodiorite samples are weakly peraluminous (A/CNK = 1.05-1.09) and have low SiO 2 (61.9-64.9 wt.%) and high Fe 2 O 3 (4.6-5.6 wt.%), MgO (2.2-2.8 wt.%) and CaO (4.3-4.8 wt.%), belonging to I-type suite due to abundant amphibole in the rocks. They exhibit strongly negative whole-rock ε Nd (t) values (−11 to −9) and zircon ε Hf (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 ε Nd (t) (−9 to −7) and zircon ε Hf (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 ε Nd (t) and ε Hf (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.

Geochronological, geochemical, and Nd–Hf isotopic constraints on the origin of magmatism in the Dabaoshan ore district of South China

“…Our samples of dacite (ca. 438 Ma), together with coeval mafic intrusions and I‐type granites (e.g., Huang, Yu, Li, Tong, & Chen, ; Y. J. Wang, Zhang, Fan, Zhang, & Zhang, ; Yao et al, ; Yu, Huang, He, & Li, ), indicate the development of an extensional setting in the SCB at ca. 440 Ma, during the Caledonian period.…”

“…An increasing number of studies have demonstrated that the Caledonian Orogen in the SCB (>460–415 Ma), also known as the Wuyi–Yunkai Orogen, was an intraplate orogenic event (e.g., Charvet et al, ; Faure et al, ; Z. X. Li et al, ; J. H. Li et al, ; Shu et al, ; Y. J. Wang, Zhang, et al, ; Yao et al, ) rather than a continental or arc collisional event involving the closure of the proposed Huanan Ocean within the SCB (Guo et al, ; Hsü, ; Hsü et al, ). Models of crustal anatexis (L. Wang et al, ) and lithospheric delamination or orogenic collapse (Z. X. Li et al, ; Shu et al, ; Y. J. Wang, Zhang, et al, ; Yao et al, ; Yu et al, ) have been put forward to explain the Caledonian structural elements, metamorphism, and magmatism. The Caledonian metamorphic rocks exhibit decompressional clockwise P–T paths (e.g., Faure et al, ; Huang et al, ; Z. X. Li et al, ) and yield metamorphic ages that cluster at 460–423 Ma (Y. J. Wang et al, ; Y. J. Wang, Fan, et al, ).…”

“…The crustal thickening that took place from 460 to 440 Ma resulted in the breakdown of hydrous minerals and the formation of synorogenic granites during prograde metamorphism (Y. J. Wang et al, ; Y. J. Wang, Fan, et al, ). The subsequent lithospheric delamination or orogenic collapse, and the upwelling of the asthenosphere and underplating by basalts (440–415 Ma), may have resulted in partial melting of the lower crust and subcontinental lithospheric mantle (Z. X. Li et al, ; Shu et al, ; Y. J. Wang, Zhang, et al, ; Yao et al, ; Yu et al, ), leading in turn to the formation of widespread granites and the scattered mafic–ultramafic rocks and volcanics.…”

The early Paleozoic oceanic island seamount in the Chencai area, Zhejiang Province: Implication of the Yangtze–Cathaysia amalgamation

“…It has been suggested that the South China Sea closed before 820 Ma, with the intracontinental orogeny commencing in the Caledonian (Charvet, ; Li et al, ; Song, Shu, & Santosh, ; Wang, Zhang, Fan, Zhang, & Zhang, ; Yu, Huang, & He, ). However, we believe that the amalgamation between the Cathaysia and Yangtze blocks was completed during the Caledonian; the evidences are as follows.…”

“…The Chencai Complex has been the subject of intensive research since the 1970s, and significant advances in understanding its age and origins have been achieved (Chen et al, 2018;Li, Lin, Li, He, & Ge, 2018;Sun et al, 2018;Xia, Xu, Niu, & Liu, 2018 (Charvet, 2013;Song, Shu, & Santosh, 2015;Wang, Zhang, Fan, Zhang, & Zhang, 2013;Yu, Huang, & He, 2016). However, we believe that the amalgamation between the Cathaysia and Yangtze blocks was completed during the Caledonian;…”