Evolution of the Kolyvan-Tomsk granitoid magmatism (Central Siberia): Insights into the tectonic transition from post-collision to intraplate settings in the northwestern part of the Central Asian Orogenic Belt

Abstract

Late Paleozoic – early Mesozoic geological evolution of the Kolyvan-Tomsk Fold Zone (KTFZ) located in the northwestern part of the Central Asian Orogenic Belt is significant to understanding the tectonic transition from post-collision to intraplate settings. We present new whole-rock geochemical and Sr-Nd isotopic data, and in situ zircon U-Pb ages for granitic rocks from the KTFZ basement to constrain their petrogenesis and to characterize changing of the tectonic setting and geodynamic evolution. Zircon U-Pb ages reveal extended multi-stage emplacement of the KTFZ granitoids for about 20 Ma from ~260 to ~240 Ma, that can be divided into two stages: the Permian-Triassic (~260 to 250 Ma) and the Triassic (~250 to 240 Ma). The Permian –Triassic granitic rocks with SiO2 between 63% and 74% have high LREE, HFSE (except Nb and Ta), Sr, Mg#, Cr, Ni concentrations, Sr/Y and (La/Yb)N ratios and remarkably low Y and Yb contents and 87Sr/86Sr(T) values (0.704682 and 0.703368). They are magnesian and show metaluminous to weakly peraluminous natures, indicating their formation through high degree fractionation of mantle-derived magmas. The Triassic granitoids exhibit extremely high SiO2 (73–77%), HFSE, HREE, Rb contents, Rb/Sr ratios and elevated negative Eu and Sr anomalies, low Mg#. They are ferroan and have a peraluminous nature with the absence of aluminium-rich minerals, all of which indicate the presence of the crustal substance in the magma source. Isotopic Nd compositions show that the both the Permian –Triassic and Triassic granitoids have uniform magma's, which were derived from partial melting of juvenile Neoproterozoic sources.These two stages of granitoid magmatism are interpreted as reflecting the transition from post-collisional magmatism, related to the final closure of the Paleo-Asian Ocean due to collision between the Baltica and Siberian paleocontinents during the Late Permian – Early Triassic and hence, to the intraplate magmatism. The formation of the KTFZ granitic rocks was induced by an upwelling asthenosphere in an extensional geodynamic setting. During the post-collisional process, the large-scale lithospheric delamination plays a key role in the genesis of the Permian-Triassic granitoids and provide the magma generation for them. In the Triassic, we propose additional source of heat from mantle plume beneath the KTFZ, which promote extending of the granitoid magmatism in intraplate setting.