BATHOLITHS AND GEODYNAMICS OF THEIR FORMATION IN THE CENTRAL ASIAN FOLD BELT

Abstract

Three large batholiths formed in the Central Asian fold belt in the period 310-190 Ma: Angara-Vitim, Hangayn, and Henteyn-Daurian. They are made up of granitoids of variable composition - from tonalities and plagiogranites to granosyenites and rare-metal granites, with a predominance of normal granites. The formation of each batholith lasted about 20-30 myr. For example, the Angara-Vitim batholith formed in the period 320-290 Ma, the Hangayn one, 275-250 Ma, and the Henteyn batholith, 225-195 Ma. The batholiths are localized in zonal magmatic areas and are framed by rifting zones with basaltic, bimodal, and peralkaline-granite magmatism. According to isotope-geochemical data, the batholith granitoids have the same isotopic Nd composition as the host crust. The rocks of the Angara-Vitim and Hangayn batholiths, formed within terrains with "Riphean" isotopic crust, have εNd(T) = -8...-2. The granitoids of the Henteyn-Daurian batholith, formed within a "Caledonian" isotopic province, have εNd(T) = -2...+2. The rocks of the batholiths are similar in geochemistry and have a composition close to that of the continental crust, which points to the crustal source of anatectic magmas during the batholith formation. Mantle sources of magmas contributed to the formation of the peripheries of magmatic areas and synplutonic basites of the batholiths. The isotopic compositions of basic rocks (εNd(T) = -1...+4 and εSr(T) = -5...+25) evidence that their melts were generated with the participation of moderately depleted mantle (PREMA or HIMU) and enriched mantle (EM-II). The batholiths resulted from the overlapping of a series of hot points of the Paleo-Asian ocean mantle by the North Asian continent and the subsequent interaction of mantle plumes with lithosphere within the active continental margin. Mantle plumes initiated within-plate magmatic activity and favored rift breaking and the formation of alkaline-basic and peralkaline-salic associations. Under stresses that appeared on collisions (continent-continent, continent-continental terrain) within the active margin, mantle melts ascended no higher than the lower crust. Their thermal effect on the crustal rocks caused anatexis and the formation of batholiths.