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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)
Author(s) -
Mao Qigui,
Ao Songjian,
Windley Brian F.,
Wang Jingbin,
Li Yuechen,
Xiao Wenjiao
Publication year - 2021
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.4095
Subject(s) - adakite , partial melting , geology , magmatism , geochemistry , crust , oceanic crust , subduction , island arc , petrology , paleontology , tectonics
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.