Sr, Nd and O isotopic characters of quartz syenite in the Weiya magmatic complex from eastern Tianshan in NW China: Melting of the thickened juvenile lower crust

Abstract

New Sr-Nd-O isotopic data are reported for the early Triassic (early Indosinian) Weiya quartz syenite from eastern Central Tianshan, northwestern China, with the aim of documenting its possible source(s), constraining its petrogenesis and discussing its tectonic implications. Whole-rock Rb-Sr isochron of the Weiya quartz syenite rock yielded an age of 246.5 ± 4.8 Ma, which is well consistent with zircon SHRIMP U-Pb age. Additionally, the rock exhibits positive ε Nd (t) value (+1.36 to +1.66), medium I sr (0.7088 to 0.7090), "normal" δ 18 O value (6.9 to 8.1‰) and young Nd model ages (T DM = 0.88 to 0.91 Ga). Combined with their petrogaphic and geochemical characters, the Weiya quartz syenite is suggested to have been generated by partial melting of juvenile lower crust rocks at high temperature. The partial melting is attributed to intra-continental subduction, which resulted in thickening of the continental crust, delamination of the lithosphere root, underplating of mantle-derived magma and subsequent regional extension. Heat from underplated mantle magma caused melting of the overlying lower crustal rocks and consequent generattion of the quartz syenitic liquid. It is supposed that the intra-continental subduction was induced by tectonic conversion from the Paleo-Asian subduction-collision system to the Paleo-Thethys regime during the latest Permian or earliest Triassic in the eastern Tianshan.

Late Carboniferous crustal evolution of the Chinese Central Tianshan microcontinent: Insights from zircon U–Pb and Hf isotopes of granites

“…In addition, the final closure of the North Tianshan Ocean was probably in the Latest Permian–Early Triassic, which was marked by the collision between the Dananhu arc and the Yamansu–Central Tianshan arc along the Kangurtag belt (Figure b; Chen et al, , ). This geodynamic model is rather consistent with intensive Triassic (250–227 Ma) magmatisms in the CTM (Deng et al, ; Lei et al, ; Wu, Zhang, Gu, Tang, & Lei, ; Zhang et al, ; Zhang et al, ; Zhao et al, ), which were accompanied by synchronous contact metamorphism and regional tectonic activities, such as the ~247–244 Ma Weiya high‐temperature, low‐pressure granulites (Mao et al, ) and the ~240–235 Ma Xingxingxia sinistral fault (Wang, Sun, & Li, ; Zhang & Cunningham, ). This Triassic magmatic–metamorphic event was probably related to the post‐collisional stage after the closure of the North Tianshan Ocean (Figure b; Chen et al, , ; Mao et al, ; Xiao et al, ; Xiao, Huang, Han, Sun, & Li, ).…”

Latest Permian–early Triassic arc amalgamation of the Eastern Tianshan (NW China): Constraints from detrital zircons and Hf isotopes of Devonian–Triassic sediments

“…The above Permian–Triassic extensional events have been explained by the coeval far‐field tectonic effects from the Paleo‐Tethyan collisional region (Y. Wang et al, ; Wu, Zhang, Gu, Tang, & Lei, ) in the Eastern Tianshan. However, this model has a difficulty to explain the magmatism before 250 Ma, because the closure of the Paleo‐Tethyan Ocean is dated as 246.1 ± 3.8 Ma by Th–U–Pb on metamorphic monazite from garnet‐mica schists (Chen et al, ) and by the Jinshajiang ophiolite of 246.2 ± 5.18 Ma (Li, Wang, & Yi, ).…”

Evolving Mantle Sources in Postcollisional Early Permian‐Triassic Magmatic Rocks in the Heart of Tianshan Orogen (Western China)

“…In the second stage, during the middle Triassic, which is after the termination of collision between the Tarim Craton and Yili‐Central Tianshan arc, the enriched lithospheric source is replaced by depleted asthenospheric mantle through convective thinning or delamination (Bird, ; Houseman et al, ) (Figure ). In addition to the Triassic mafic dikes in the Awulale Mountains, Triassic mafic rocks are occur sporadically in the east and southwest Tiansan regions (e.g., Wu et al, ), which is consistent with lithosphere thinning in a postcollisional setting.…”

“…Contemporaneous with the mafic volcanism in this region are abundant Atype granites and subordinate adakites, which are attributed to crustal melting as a result of enhanced heat input into the crust triggered by upwelling asthenosphere (Long et al, 2011;Tang et al, 2017bTang et al, , 2010Zhao et al, 2008). Most research into the tectonic evolution of the Tianshan Orogen has focused on the Paleozoic granitoids (Long et al, 2011;Tang et al, 2014Tang et al, , 2010. However, the mantle evolution processes of the Tianshan Orogen, postcollision, are poorly constrained.…”