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

Abstract

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 ~720 °C and ~6.4 kbar for the former and ~700 °C and ~7.1 kbar 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 ± 3.2 Ma, and 3 representative metamorphic rocks underwent contemporaneous metamorphism at ca. 498.9 ± 4.1, 504.4 ± 3.9, and 499.3 ± 2.9 Ma. Zircon Hf isotopic analyses show that 505‐Ma zircons from the granitoid have positive εHf(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.

Petrogenesis of Early Paleozoic high Sr/Y intrusive rocks from the North Qilian orogen: Implication for diachronous continental collision

“…Sr-Nd-Pb isotopes and trace elemental geochemistry of the basaltic rocks from the Yushigou and Dongcaohe ophiolites suggested that the North Qilian Ocean has a tectonic affinity with the Proto-Tethyan Ocean (Hou et al, 2006;Tseng et al, 2007). Studies on Early Cambrian volcanic-intrusive rocks show that northward subduction of the North Qilian oceanic slab initiated at ~520 Ma (Xia et al, 2012;Chen et al, 2014), although some researchers also inferred a southward subduction for the North Qilian Ocean (Peng et al, 2017a(Peng et al, , 2017b. Geochronological and geochemical data of the Jiugequan ophiolite imply that the extension of the back-arc basin in the North Qilian initially occurred at ~490 Ma (Xia and Song, 2010).…”

A paired metamorphic belt in a subduction‐to‐collision orogen: An example from the South Qilian–North Qaidam orogenic belt,NWChina

“…Zircon U–Pb geochronology revealed that these magmatic arc rocks intruded between c . 510 and 460 Ma (Huang et al., ; Peng et al., ). Its Neoproterozoic magmatic–tectonometamorphic history is similar to that of the South China Block, which suggests a close affinity between them (e.g.…”

“…It was extensively intruded by early Palaeozoic mafic and granitic magmatism with arc-related geochemical features (e.g. Huang et al, 2015;Peng et al, 2017;Yan et al, 2015). Zircon U-Pb geochronology revealed that these magmatic arc rocks intruded between c. 510 and 460 Ma (Huang et al, 2015;Peng et al, 2017).…”

Central China Orogen along the Silk Road (Part I): Tectono‐thermal evolution and its links

“…A much wider theme is addressed in the paper by Wu et al () who examine the sedimentary sequences in cratonic basins, especially Yangtze Craton, during the break‐up of Rodinia. In the paper by Peng et al (), the focus of the study was on Early Paleozoic arc magmatism and metamorphism in the northern Qilian Block. The final three papers in this region by H. Feng et al (); Shu et al (); and Z. Tang et al () all concentrate on aspects of Jurassic basin sedimentation characteristics, and provenance in the northern Qaidam Block.…”