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

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 similar to 13.0 kbar and similar to 690 degrees C, whereas post-peak retrograde stage (M3) has lower P-T conditions of similar to 6.0 kbar and similar to 695 degrees 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 Pb-206/U-238 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.

Automated summary

Recent investigations in the Longyou paragneiss in the Wuyi-Yunkai orogen of South China revealed the presence of high-pressure metamorphic phengites, indicating an Early Palaeozoic high-pressure metamorphic event during continent-continent collisional orogeny. This study suggests a typical collisional orogenic process in the region, with peak high-pressure metamorphic conditions followed by post-peak retrograde conditions. The SIMS zircon U-Pb age data support the metamorphic age of the Longyou garnet amphibolite and suggest a continent-continent collision likely occurred along the Zhenghe-Dapu fault.