Recognition of early Carboniferous alkaline granite in the southern Altai orogen: post-orogenic processes constrained by U–Pb zircon ages, Nd isotopes, and geochemical data
The Altai orogen forms the southern part of the Central Asian Orogenic Belt (CAOB), the world’s largest accretionary orogen. However, its tectonic evolution, particularly during the late Paleozoic, is still not well understood. U–Pb zircon analyses for the Bulgen alkaline granite yield crystallization ages of 358 ± 4 Ma (SHRIMP) and 354 ± 4 Ma (LA-ICP-MS). These ages are significantly younger than published emplacement ages for subduction/collision-related syn-orogenic granitoids (460–375 Ma) in this region. The Bulgen granite has high SiO2, total alkalis, rare earth elements, HFSE (Th, Zr, Hf, Nb, and Ce), and low Ba, Sr with pronounced negative anomalies in Eu, Ba, Sr, P, and Ti, showing a clear A-type geochemical signature. The granite records high εNd(t) values of +6.3 to +6.4 and young model ages (TDM) of ca. 600 Ma. The Bulgen alkaline granite is largely undeformed as opposed to the early-middle Paleozoic counterparts, which form elongated deformed bodies parallel to the prevailing tectonic fabric (NW direction). Available data suggest that magmatism in the southern Altai region evolved from early-middle Paleozoic I-type tholeiitic and calc-alkaline granitoids to late Paleozoic A-type alkaline granitoids. The high εNd(t) values of the Bulgen alkaline granite indicate a homogeneous juvenile mantle source, whereas the early-middle Paleozoic granitoids are characterized by lower and more variable εNd(t) values (−2.6 to +4.2). These differences provide an important insight into the late Paleozoic orogenic processes of the Chinese Altai and indicate a significant change of the tectonic regime from a syn-orogenic regional compression setting to a post-orogenic extensional one. Major tectonic movements in this region ceased after the early Carboniferous.