MAGMATISM AND GEODYNAMICS OF EARLY CALEDONIAN STRUCTURES OF THE CENTRAL ASIAN FOLD BELT (ISOTOPE AND GEOLOGICAL DATA)

Abstract

The structure of the Caledonian area of the Central Asian fold belt is determined by Caledonian folded formations made up of Vendian-Cambrian ophiolite and island-arc complexes and by terrains or microcontinents of older crust, located between them. The corresponding structural zones differ in the Nd isotope composition of their crust, which is determined from analysis of granitoids. The crust of the Precambrian terrains is characterized by negative epsilon(Nd)(T) values. Most of the terrains (Hangayn, Barguzin-Vitim, Sangilen) have epsilon(Nd)(T)= +0.5...-10, thus corresponding to crust with a model Nd isotopic age T-Nd(DM-2st)=1000-1700 Ma. This allows uniting the above terrains into a "Riphean" isotopic province. The isotopic composition of granitoids from rare terrains with a pre-Riphean crust (Dzavhan and Gargan) is characterized by epsilon(Nd)(7) values of up to -17 and T-Nd(DM-2st) values of up to 2.9-3.3 Ma. The rocks of the Caledonian ophiolite zones are united into a "Caledonian" isotopic province. According to geochronological data (U-Pb, Ar-Ar), they formed mainly in the period 570-510 Ma. Basalts and plagiogranites of ophiolite and island-arc complexes have high positive epsilon(Nd)(T) values, +9.8...+7.1. The associated fine-clastic and chemogenic sedimentary rocks (pelites) are characterized by epsilon(Nd)(7)=+6.8...+5.3 and T-Nd(DM-2st)=0.94-0.76 Ga (the latter corresponds to Riphean age). This is, most likely, related to the supply of sedimentary material from the surrounding continental blocks into ocean basins. Syncollisional granitoids and those that formed later within the ophiolite zones throughout the Paleozoic and Early Mesozoic are characterized by a subsequent decrease in epsilon(Nd)(T) from +7.7...+5.2 (490 Ma) to +3.8...+0.9 (200 Ma) and a corresponding increase in T-Nd(DM-2st) from 0.6-0.71 to 0.96-0.77 Ga. The older model ages as compared with that of the earliest magmatic events are related to the partial melting of a mixed source, which consisted of a juvenile component of Vendian-Cambrian age with high positive epsilon(Nd)(T) values and a long-lived crustal matter with negative epsilon(Nd)(T) values and ancient (Middle Riphean-Archean) model Nd isotopic ages. Geochronological and Nd isotopic data indicate the juvenile composition of the Caledonides crust in Central Asia. The estimated rates of crust formation in Caledonian time are just a little lower than the average rates of the Earth's crust formation. These data helped to recognize the Caledonian stage of juvenile crust formation not only in Central Asia but on the whole Earth as well.