Petrogenetic and tectonic implications of Triassic granitoids in the Chinese Altay: the Alaer granite example

摘要

Petrogenesis of the Triassic Alaer granite in the Chinese Altay is important to understand the regional crustal evolution. This study found that the biotite granite is metaluminous and high-K calc-alkaline to shoshonitic. The rocks contain low SiO2 (64.3–68.7 wt.%), high Al2O3 (13.46–16.56%) and K2O (mostly 3.65–7.57 wt.%), and are featured by distinctly higher concentrations of HFSE (e.g., Th, Hf) and ΣREE than those of the nearby Devonian granites. REE patterns show right-inclining trend and weak negative Eu anomalies (δEu = mostly 0.68–0.94), whilst the spidergrams show consistent negative anomalies of Ba, Sr, P, Ti, Nb and Ta and positive anomalies of Th, K, La, Ce, Nd, Zr and Hf. Compared with the local Devonian granites, the Triassic granites have higher calculated initial ⁸⁷Sr/⁸⁶Sr ratios (0.70601–0.70920) and εNd(t) values −(−1.24 to −0.68). Their Nd model ages (average of 1.08 Ga for one-stage and 1.07 Ga for two-stage) are early Neoproterozoic to late Mesoproterozoic.We infer that the Triassic Alaer granite belongs to I-type granite and likely formed in an intraplate extensional setting under high temperature (829–885 °C) and pressure (depths of 30–50 km, corresponding to the middle-lower crust in the Chinese Altay region). Granitic magma was produced through the melting of old continental basement with substantial mantle input, triggered by extensional upwelling of mantle-derived magmas. Mixing equation of Nd isotope compositions yielded a 40% of mantle-derived juvenile components for the Alaer granite petrogenesis.