NEW DATA ON THE STRUCTURE AND AGE OF THE ALKALI GABBRO-SYENITE OSHURKOVO-PLUTON (TRANSBAIKALIA)

The Oshurkovo Pluton is made up of continuos rock series, from melanoc ratic alkali gabbro to syenite. The pluton formation resulted from suc cessive injections of numerous discrete portions of alkali-basaltic an d syenitic magmas into the expanding system of steeply dipping subpara llel fractures. This resulted in the formation of a plutonic sheeted c omplex, which represents a continuos alternation of phacoidal, plate-l ike, and dike-like concordant bodies. It is shown that alkali-feldspat hic syenites abundant in the southern and southwestern periphery of th e pluton do not belong to another, younger, magmatic suite, as conside red earlier. They complete the alkali gabbro-syenite series making up the Oshurkovo Pluton. According to the data obtained by a Rb-Sr isotop e analysis of rocks and minerals, the pluton age is 108+/-9 Ma, I-sr - 0,70523. These data point to the presence of a plutonic counterpart o f the Early Cretaceous alkali basalts widespread in Transbaikalia. The common Feature that distinguishes both plutonic and volcanic Cretaceo us mafic rocks from the older and younger basalts and gabbro of the re gion is an enhanced content of phosphorus.