First report of phengites in the Longyou paragneiss in the northern Early Paleozoic Wuyi-Yunkai orogen, South China: P-T conditions, zircon U-Pb ages and tectonic implications

• Published in: Journal of Asian earth sciences Vol. 214; p. 104754
• Main Authors: Tong, Laixi, Li, Chao, Liu, Zhao, Zhai, Mingguo, Li, Wuxian
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-07-2021
• Subjects: Early Palaeozoic orogeny; High-pressure metamorphic phengites; Longyou paragneiss; P-T conditions; South China; High-pressure metamorphic phengites; Longyou paragneiss; P-T conditions; Early Palaeozoic orogeny; South China
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2021.104754

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•High-P phengites are identified in the Longyou paragneiss in South China.•P-T estimates indicate peak conditions of ~ 13.0 kbar and ~ 690 °C with post-peak ITD P-T path.•The study results support the Early Palaeozoic continent collisional orogeny in Cathaysia.
Our recent investigations into the Longyou paragneiss (country gneiss of the Longyou garnet amphibolite) in the northern Wuyi-Yunkai orogen of South China lead to a recognition of high-pressure metamorphic phengites in the paragneiss. The paragneiss consists of phengite, muscovite, biotite, K-feldspar, plagioclase, quartz, and minor graphite and ilmenite or magnetite. Mineral compositions show that Si contents in phengites are between 3.40 and 3.25 (p.f.u., per formula unit), belonging to high-pressure metamorphic phengites. P-T estimates suggest that peak high-pressure metamorphic stage (M2) has P-T conditions of ~ 13.0 kbar and ~ 690 °C, whereas post-peak retrograde stage (M3) has lower P-T conditions of ~ 6.0 kbar and ~ 695 °C. The two-stage P-T conditions define a clockwise P-T path of post-peak isothermal decompression, reflecting a typical collisional orogenic process. SIMS zircon U-Pb age data show a weighted mean 206Pb/238U age of 445 ± 3 Ma, consistent with the metamorphic age of the Longyou garnet amphibolite, suggesting that the peak high-pressure metamorphic event occurred in the Early Palaeozoic orogeny in the Cathaysia block in South China. Thus, we consider that both the Longyou paragneiss and enclosed garnet amphibolite together experienced a phase of high-pressure metamorphism during the Early Palaeozoic continent–continent collisional orogeny likely along the Zhenghe-Dapu fault between the South China block and an unknown block on the northern margin of eastern Gondwana continent.