ANOROGENIC NATURE OF MAGMATISM IN THE NORTHERN BAIKAL VOLCANIC BELT -EVIDENCE FROM GEOCHEMICAL, GEOCHRONOLOGICAL (U-PB), AND ISOTOPIC (PB,ND) DATA

The Northern Baikal volcanic belt has an age of 1.82-1.87 Ga and exten ds along the boundary between the Siberian Platform and the Baikal fol dbelt. The volcanic belt is composed of volcanics of the Akitkan Group and granitic rocks of the Irel and Primorsk complexes. The geochemist ry of the rocks points to the intraplate anorogenic nature of the belt . U-Pb zircon dating of the Chuya granitoids revealed that they are ol der (2020-2060 Ma) than the Northern Baikal volcanic belt and, thus, c annot be regarded as its component. Data on the ph isotopic system of feldspars from the granitoids confirm the contemporaneity of all volca nic rocks of the belt except the volcanics of the upper portion of the Akitkan Group (Chaya Formation). Our data suggest its possibly younge r (similar to 1.3 Ga) age. The isotopic Nd and Pb compositions of the acid volcanic rocks provide evidence of the heterogeneity of their cru stal protoliths. The volcanics of the Malaya Kosa Formation have epsil on(Nd)(T) = -6.1, mu(2) = 9.36, and were most probably produced with t he participation of the U-depleted lower continental crust of Archean age. Other rocks of the complex show epsilon(Nd)(T) from -0.1 to -2.4, mu(2) = 9.78, and could have been formed by the recycling of the juve nile crust. The depletion of the Malaya Kosa volcanics in most LILEs a nd HFSEs compared with other acid igneous rocks of the belt possibly r eflects compositional differences between the Late Archean and Early P roterozoic crustal sources. The basaltic rocks of the Malaya Kosa Form ation (epsilon(Nd) varies from -4.6 to -5.4) were produced by either t he melting of the enriched Lithospheric mantle or the contamination of derivatives of the depleted mantle by Early Archean lower crustal roc ks, which are not exposed within the area.