MEZOPROTEROZOIC BASITE MAGMATISM OF THE BASHKIRIAN MEGANTICLINORIUM (SOUTHERN URALS): AGE CONSTRAINTS, PETROLOGICAL AND GEOCHEMICAL FEATURES

Abstract

Among the Riphean–Vendian dyke complexes of the basic composition, which intersect the Precambrian strata of the Bashkirian meganticlinorium (Southern Urals), one of the most common is the Kurgas gabbro-dolerite complex dated to the Early – Middle Riphean. This article presents the isotopic dating, petrological and geochemical features of the rocks belonging to the Kurgas complex. It gives the first description of the regional geochemical zoning that is reflected in a decrease in SiO 2 contents from the north to the south along the Bashkirian meganticlinorium, and an increase in MgO contents in the same direction, which is attributed by the authors to an increase in the permeability of the crust to the south during opening of the Mashak riftogenic structure. New data on isotopic ages were obtained for the dyke that cuts the Satka formation of the Lower Riphean – 1318±10 ( 40 Ar/ 39 Ar), the intrusion in the exocontact zone of the Berdyaush rapakivi granite massif – 1349±11 Ma (U-Pb), and the andesite dyke among the metamorphic rocks of the Taratash complex – 1365.6±6.6 Ma (U-Pb). These ages, in combination with the previously obtained data, suggest that the complex formed during a rather long period of time (from 1385 to 1318 Ma, as a minimum), which corresponds to the beginning of the Middle Riphean. Isotopic U-Pb age of the dyke that cuts the contact zone of the Berdyaush massif, suggests that the major portion of the massif had already crystallized by that time and was exhumed into the shallow zone of brittle deformation.