Middle Triassic lower crust‐derived adakitic magmatism: Thickening of the Dananhu intra‐oceanic arc and its implications for arc–arc amalgamation in the Eastern Tianshan (NW China)

Abstract

The timing and mechanism of the closure of the Palaeo-Asian Ocean are problematic and controversial. To help resolve these problems, we report geochronological, geochemical, and isotopic data from mid-Triassic adakitic intrusions in the Eastern Tianshan, NW China. U-Pb dating shows that the adakitic intrusions formed at 243-234 Ma. These mid-Triassic adakitic intrusions are characterized by high Sr/Y and La/Yb ratios, low Yb and Y, and positive ε Nd (t) (+3.12 − +4.71) and low ( 87 Sr/ 86 Sr) i (0.703956-0.704487) values. However, they have high K and low A/NK values (1.16-1.71), relatively low MgO (0.43-1.81 wt%) contents, and Mg # (44-61), low abundances of compatible elements (Cr = 4.09-16.69 ppm, Ni = 2.02-8.03 ppm), which are different from the typical slab-melting adakites. These features indicate that they were derived from the partial melting of the relatively depleted thickened lower crust. Our new geochronological, geochemical, and isotopic data integrated with the established amalgamation of nearby arcs lead us to conclude that the adakitic intrusions were most likely generated by partial melting of the tectonically thickened (>40 km) juvenile Dananhu intra-oceanic arc and, therefore, the Kanguer branch of the Palaeo-Asian Ocean closed since ca. 234 Ma. K E Y W O R D S accretionary orogeny, adakite, Eastern Tianshan, new crustal growth, thickened arc crustal melting Drummond, 1990) are andesitic to rhyolitic, extrusive, and intrusive rocks generated by slab melting in subduction zones. However, the compositions of some adakitic rocks are controversial, leading to alternative mechanisms of generation, for example, partial melting of the tectonically thickened mafic lower crust

Mesozoic exhumation of the Jueluotage area, Eastern Tianshan, NW China: constraints from (U–Th)/He and fission-track thermochronology

“…As a branch of the Palaeo-Asian Ocean, the youngest closure time of the North Tianshan Ocean indicates that final amalgamation of the Palaeo-Asian Ocean occurred in Middle-Late Triassic times (Xiao et al 2015(Xiao et al , 2018Ao et al 2021;Mao et al 2021). Several stratigraphic and low-temperature thermochronological studies have recorded closure of the Palaeo-Asian Ocean.…”

Late Paleozoic Southward Migration of the Dananhu Arc in the Eastern Tianshan (NW China)

“…(c). The geological map of the Eastern Tianshan marking the locations of Devionian to late Permian granites in the middle of the Dananhu arc in the eastern Tianshan orogen (Mao, Ao, Windley, Lü, et al., 2021; Mao, Ao, Windley, Wang, et al., 2021; Mao, Wang, Xiao, Windley, et al., 2021). The ages are listed in Table S4 in Supporting Information .…”

“…Decades of researches have shown that the Dananhu arc in the eastern Tianshan orogen is mainly an intra‐oceanic arc that was generated by subduction of the Kanguer ocean, an important branch of the Paleo‐Asian Ocean (Ma et al., 1997; Mao, Ao, Windley, Lü, et al., 2021; Mao, Ao, Windley, Wang, et al., 2021; Mao, Wang, Xiao, Windley, et al., 2021; Xiao et al., 2004; Y. Zhang et al., 2018). It experienced long‐lively complicated subduction accretionary processes from the Ordovician to the Triassic, and has discontinuous polyphase generations and types of granitic and volcanic magmatism (Figures 1b and 2; Du et al., 2019; C. M. Han et al., 2002; Lin et al., 2017; Mao, Ao, Windley, Lü, et al., 2021; Mao, Ao, Windley, Wang, et al., 2021; Q. G. Mao et al., 2019; Q. Mao et al., 2020; Mao, Wang, Xiao, Windley, et al., 2021; Sun, Wang, Wang, et al., 2019; Wang, Wang, & He 2006; Xiao et al., 2004; Y. Zhang et al., 2018; 2020; Zhou et al., 2010).…”

“…It experienced long‐lively complicated subduction accretionary processes from the Ordovician to the Triassic, and has discontinuous polyphase generations and types of granitic and volcanic magmatism (Figures 1b and 2; Du et al., 2019; C. M. Han et al., 2002; Lin et al., 2017; Mao, Ao, Windley, Lü, et al., 2021; Mao, Ao, Windley, Wang, et al., 2021; Q. G. Mao et al., 2019; Q. Mao et al., 2020; Mao, Wang, Xiao, Windley, et al., 2021; Sun, Wang, Wang, et al., 2019; Wang, Wang, & He 2006; Xiao et al., 2004; Y. Zhang et al., 2018; 2020; Zhou et al., 2010). Specifically these include: Ordovician‐Silurian and Carboniferous tholeiitic basalt in the Kalatage area and the Tuwu area, respectively (Mao, Ao, Song, Xiao, et al., 2021; Mao, Xiao, Windley, Yu, et al., 2021; Y. Zhang et al., 2018), early Carboniferous bimodal volcanic rocks (X. J. Chen et al., 2013); early basalt‐andesite‐rhyolite in Turpan basin (Mao, Ao, Song, Xiao, et al., 2021; Mao, Ao, Windley, Lü, et al., 2021; Mao, Ao, Windley, Wang, et al., 2021; Mao, Wang, Xiao, Windley, et al., 2021; Mao, Xiao, Windley, Yu, et al., 2021); Silurian to Triassic adakitic granitic rocks (L. Chen et al., 2017; Mao, Ao, Song, Xiao, et al., 2021; Mao, Ao, Windley, Lü, et al., 2021; Mao, Ao, Windley, Wang, et al., 2021; Mao, Wang, Xiao, Windley, et al., 2021; Mao, Xiao, Windley, Yu, et al., 2021; Qin et al., 2009; Wang, Xue, et al., 2015, 2016; Wang, Zhang, & Liu, 2016; L. Zhang et al., 2006); Odivician to Carboniferous granites in the Kalatage area (Du et al., 2018c; Mao, Ao, Song, Xiao, et al., 2021; Mao, Ao, Windley, Lü, et al., 2021; Mao, Ao, Windley, Wang, et al., 2021; Mao, Wang, Xiao, Windley, et al., 2021; Mao, Xiao, Windley, Yu, et al., 2021; Sun, Wang, Wang, et al., 2019); early Carboniferous granites in the Tuwu area (Figure 2a; Du et al.,…”

“…However, these publications did not discuss the spatial migration of the middle to late Devonian, Late Carboniferous and late Permian arc‐related granitic rocks, nor the constraints of the temporal‐spatial distribution and evolutionary processes of this evolving magmatism. Serveral models were proposed to explain one or more stages or single areas of the magmatism (Du et al., 2019; C. M. Han et al., 2002; Mao, Ao, Windley, Lü, et al., 2021; Mao, Ao, Windley, Wang, et al., 2021; Mao, Wang, Xiao, Windley, et al., 2021; Wang, Wang, & He 2006; Xiao et al., 2004; Y. Zhang et al., 2018). Also, there was much debate about the genesis and tectonic settings of Devonian to Permian rocks, including possible subduction, intra‐plate plumes, intra‐continental and post‐collisional mantle‐related extension, and mantle‐related syn‐collisional, compressional‐post‐collisional events, and a tectonic transition from multiple subduction accretion to collision.…”