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)

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 εNd (t) (+3.12 − +4.71) and low (87Sr/86Sr)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. In the Middle Triassic (243–234 Ma), the Kanguer Ocean, a branch of the Palaeo‐Asian Ocean, remained open. Partial melting of the lower parts of thickened Dananhu arc crust (thickness > 40 km, ~1.2 GPa) consisted of eclogite facies, or garnet‐bearing amphibolite facies mafic rocks produced the adakitic rocks in the subduction zone.