THE GENESIS OF GARNET-PYROXENE SYENITES (SVIATONOSSITES) OF THE MALOBYSTRINSKY MASSIF (SLYUDYANKA COMPLEX, SOUTH BAIKAL REGION): RESULTS OF GEOCHEMICAL AND ISOTOPIC STUDIES

Abstract

This paper deals with the study of syenites and sviatonossites (andradite-bearing syenites) of the Malobystrinsky massif of the Slyudyanka complex (South Baikal region, Siberia), and a large monzonite dike similar in age and composition to the rocks of the massif considered. The studied rocks belong to a series of highly ferriferous and metaluminous A-type granitoids (ASI index <1). They are characterized by SiO 2 45–65 wt. %, K 2 O+Na 2 O up to 12 wt. %, MgO <4 wt. %, TiO 2 up to 2.5 wt. %, and Al 2 O 3 up to 17 wt. %. CaO varies in a wide range, from 2.2 to 14.7 wt. %. The rocks are similar to each other in trace element composition and show patterns with troughs for Th-U, Nb-Ta and Ti. Low-amplitude negative Eu anomaly is observed in the distribution spectra of rare earth elements for the entire rock complex. The obtained Sm-Nd age of sviatonossites in the Malobystrinsky massif is 487.1±6.1 Ma (MSWD=0.99). Our results indicate that syenites and monzonites have εNd (t) –1.9…–2.8, at εSr (t) 21–30, and sviatonossites have εNd (t) –3.8…–4.1 at εSr (t) – 26. Model ages T Nd (DM) for all rock types are Mesoproterozoic (1.3–1.4 Ga). Based on the chemical and Sr-Nd isotopic composition of the magmatic rocks studied, it can be assumed that they have been generated by partial melting of lower crustal rocks (amphibolites). Crystallization of andradite garnet in syenite magma can occur due to melt contamination with metamorphic host rocks of the Slyudyanka complex.