Geodynamics of Late Paleozoic batholith-forming processes in western Transbaikalia

Abstract

Isotopic dates newly obtained for the northwestern portion of the Angara–Vitim batholith are consistent with preexisting data on the duration of the Late Paleozoic magmatic cycle: 55–60 Ma (from 325 to 280 Ma). These data also indicate that alkaline mafic magmatism in western Transbaikalia began simultaneously with the transition from crustal granite-forming processes to the derivation of granites of a mixed mantle–crustal nature, with gradual enrichment of the juvenile component in the source of the magmas. Analysis of the currently discussed geodynamic models of Late Paleozoic magmatism shows that a key role in all models of extensive granite-forming processes in the region is assigned to mafic mantle magmas, which can be generated in various geotectonic environments: subduction, delamination, decompression, and a mantle plume. The plume model is most consistent with the intraplate character of the Angara–Vitim batholith. The derivation of the vast volume of granitic material (approximately 1 million km3) should have required a comparable volume of mafic magma that should have been pooled in the middle crust of the Baikal fold area. However, the density structure of the region does not provide evidence of significant volumes of mafic rocks. This suggests that the mechanism of plume–lithospheric interaction that should have induced extensive crustal melting and the origin of vast granite areas was more complicated than simply conductive melting of crustal protoliths in contact with mafic intrusions.