U-PB AGE OF ZIRCONS FROM PLAGIOGRANITE VEINS IN MIGMATIZED AMPHIBOLITES OF THE SHAMAN RANGE (IKAT-BAGDARIN ZONE, VITIM HIGHLAND, TRANSBAIKAL REGION)

Abstract

The Ikat–Bagdarin zone is considered as the north-eastern element of the Hercynian system in the western Transbaikal region. In the present-day structure, the zone consists of a system of outliers composed of metamorphosed rocks at the roof of the Angara–Vitim batholith. The largest of them is the Bagdarin synform representing a complicated fold–thrust structure with different rock complexes juxtaposed to form a packet of tectonic nappes. Its lower part (relative autochthon) is composed of the Sivokon–Tocher Complex. According to available data, the base of this complex in the Shaman Range is represented by apobasaltic albite-epidote–actinolite schists of the Sivokon Formation attributed conditionally to the Upper Riphean. The formation includes gabbroid, diorite, and plagiogranite bodies in the axial part of the range. Serpentinite veins occur in some places. All these rocks are united into an ophiolitic association. Recent Sm–Nd isotopic studies of amphiboles, which replace the dark mineral in gabbro of the Shaman Range, and of plagioclases from gabbro–pegmatites yielded an isochron age of 545 ± 19 Ma, εNd = 0.7 (MSWD = 0.72).