~(40)Ar/~(39)Ar Age and Geochemical Features of Syncollisional Gabbroids and Granites from the Western Baikal Region: Evidence from the Birkhin Massif and Its Folded Framing

摘要

When studying the deep-seated levels of the colli-sional systems, mafic-ultramafic rocks are traditionally considered the most probable thermal source of high-temperature metamorphism, while granites are considered indicators of retrograde metamorphism. This interpretation is based on the classical model of uninterrupted cooling during isostasy that was mathematically modeled by Dodson for isotope systems. At present, this approach is accepted by the majority of researchers and is used to reconstruct the thermal evolution of rocks based on "closure temperature" of isotope systems in mineral assemblages. However, detailed isotope-geo-chronological investigations in the Pamirs-Himalayas and Tibet made it possible to substantiate an alternative model (multistage thermal evolution of colli-sional systems). In these specific cases, an additional criterion ("pair criterion") has long been proposed to determine reliably the age of mineral formation or superposition: coincidence of K/Ar ages of two (or more) minerals with different capacities of radiogenic argon loss. This criterion can most efficiently be applied using the ~(40)Ar/~(39)Ar step heating method, which was tested for regional collisional-strike-slip zones with viscous and brittle-ductile flow of rocks (Irtysh fold zone and Sangilen collisional system). In this work, we attempted to estimate the primary age of syncollisional gabbroids and granites in the western Baikal region (with the Birkhin Massif and its folded framing as an example) based on the ~(39)Ar/~(40)Ar dating of amphiboles and biotites with different "closure temperatures."