RECURRENT MIXING AND MINGLING OF COEXISTING SYENITE AND BASALT MAGMASIN THE UST-KHILOK MASSIF, TRANSBAIKALIA, AND ITS PETROLOGIC SIGNIFICANCE

The Ust'-Khilok pluton, which is 60 by 10 lan in size, crops out south of the town Ulan-Ude. The pluton is predominantly composed of monzosy enite and syenite of an Early Permian alkalic syenite-granite series. The plutons of this series occur as a belt that is more than 1500 lan long. The massif was formed by a number of subsequent syenitic intrusi ons, each of which was accompanied by one or two injections of trachyb asaltic melts into the felsic magmas during different stages of their consolidation. At least six subsequent injections of this type were di stinguished. The primary basalt compositions were computed on the basi s of the rock-forming mineral chemistries and the basite rock geochemi stries, which reflect the variable influence of the host felsic magmas . The components were differentiated into groups according to their be havior: most mobile (K and Rb), relatively mobile (Sr, Ba, Zr, and REE ), and relatively inert (Si, Ti, Al, Mg, Ca, Cr, Ni, Co, and V). These groupings were further used in the geochemical classification of the synplutonic basites. The latter rocks were determined as subalkalic co ntinental basalts. The mixing and mingling of the syenite and trachyba salt magmas were accompanied by fractional crystallization in the magm a chamber. As was demonstrated by geochemical modeling, the early monz osyenites were formed by the mixing of 75 wt % syenite and 25 wt % tra chybasalt magma. The stage-II syenite melts originated due to the appr oximately 35% fractional crystallization of plagioclase, mafic, and ac cessory minerals from the hybrid monzosyenitic magma. Further fraction ation of the magma probably produced an aplitic residue. At moderate d epths, the compositionally contrasting magmas experienced only minglin g and formed syenite-basite and aplite-basite composite dikes. The dat a obtained from the Ust'-Khilok pluton provide a persuasive example of the long-term coexistence of two magmatic reservoirs with composition ally contrasting melts and of the active role of mantle magmas in the genesis of felsic rocks.