Late Paleozoic to early Triassic granitoids from the Rudny Altai, Central Asian Orogenic Belt: Petrogenesis and implications for continental crustal evolution

摘要

To better understand the continental crustal evolution in the Central Asian Orogenic Belt along the southwestern Siberian margin, a systematic study has been conducted on the late Paleozoic to early Triassic granitoids in the Rudny Altai. LA-ICP-MS zircon dating shows that granitoid magmatism in the study area can be divided into three episodes, i.e., middle to late Devonian, middle to late Carboniferous and early Triassic. Granitoids of the first two episodes have high SiO2(>70.0?wt.%), low MgO (<1.0?wt.%) and Ni (<3.1?ppm) contents, reflecting a crustal origin. Further, these samples exhibit variable Rb/Sr, CaO/Na2O and Al2O3/TiO2ratios and variable εNd(t) (1.7–5.5) values, which, along with their metaluminous to weakly peraluminous compositions, imply a mixed crustal source involving clay-poor sediments and meta-basaltic components. In contrast, magmatism of the third episode possesses relatively low SiO2(63.2?wt.%) and high MgO (1.6?wt.%) but similar Ni (3.9?ppm) and εNd(t) (2.7). Its low Rb/Ba, Rb/Sr and Al2O3/TiO2ratios and metaluminous feature indicate a derivation from pre-existing basaltic crust. By integrating regional data, it is suggested that the Rudny Altai was generated by Devonian to late Carboniferous northeastward subduction of the Ob-Zaisan oceanic lithosphere. In detail, a retreating episode of subduction may occur in the Devonian, followed by a middle to late Carboniferous (330–318?Ma) normal subduction. In the late Carboniferous to early Permian (308–290?Ma), granitoids vanished in the Rudny Altai but widely intruded farther south into the Irtysh–Zaisan belt, probably in response to its collision with the Kazakhstan collage, pointing to that the Rudny Altai entered a syn-/post-collisional stage. Granitoids in the Permo-Triassic boundary (253–251?Ma) display features of intraplate granites, recording a period of intraplate epeirogeny.