Early Paleozoic arc magmatism and metamorphism in the northern Qilian Block, western China: Petrological and geochronological constraints

The Datong-Menyuan Complex, located in the northern margin of the Qilian Block, is composed dominantly of high-grade metamorphic rocks covered by a Paleozoic-Mesozoic sedimentary sequence. In this study, we present petrographic observation, conventional thermobarometry and P-T pseudosection modelling, whole-rock major and trace geochemistry, zircons U-Pb dating and Hf isotopic data together, to reveal the relationship between the magmatism and metamorphism along the northern margin of the Qilian Block. The anticlockwise P-T paths of gneiss and amphibolites are obtained using the garnet isopleths thermobaromety combined with phase equilibria modelling. The P-T pseudosection also shows that the gneiss and amphibolite underwent similar retrograde metamorphism with slightly different peak metamorphic conditions at similar to 720 degrees C and similar to 6.4kbar for the former and similar to 700 degrees C and similar to 7.1kbar for the latter. This indicates the Datong-Menyuan Complex recorded an amphibolite-granulite-facies metamorphism. The zircon U-Pb analyses of 3 intrusive rocks including granitic dike, diorite, and granite are dated at ca. 499.8 +/- 4.3, 495.9 +/- 3.3, and 505.5 +/- 3.2Ma, and 3 representative metamorphic rocks underwent contemporaneous metamorphism at ca. 498.9 +/- 4.1, 504.4 +/- 3.9, and 499.3 +/- 2.9Ma. Zircon Hf isotopic analyses show that 505-Ma zircons from the granitoid have positive epsilon(Hf)(t) values ranging from +8.5 to +12.8, indicating a depleted mantle source. Based on the penecontemporaneous magmatic and metamorphic event, we suggest that the southward subduction of the north Qilian Ocean triggered the activity of mantle-derived magma and coeval metamorphic event. A subsequent compression is attributed to crustal thickening, indicating counter clockwise P-T paths. Combining these data with previous studies, it suggests paired metamorphic belts: the penecontemporaneous high-temperature metamorphic belt related to an arc along the northern Qilian Block and high-pressure/low-temperature metamorphic belt existed in the north Qilian suture.