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 melanocratic alkali gabbro to syenite. The pluton formation resulted from successive injections of numerous discrete portions of alkali-basaltic and syenitic magmas into the expanding system of steeply dipping subparallel fractures. This resulted in the formation of a plutonic sheeted complex, which represents a continuos alternation of phacoidal, plate-like, and dike-like concordant bodies. It is shown that alkali-feldspathic syenites abundant in the southern and southwestern periphery of the pluton do not belong to another, younger, magmatic suite, as considered earlier. They complete the alkali gabbro-syenite series making up the Oshurkovo Pluton. According to the data obtained by a Rb-Sr isotope 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 of the Early Cretaceous alkali basalts widespread in Transbaikalia. The common feature that distinguishes both plutonic and volcanic Cretaceous mafic rocks from the older and younger basalts and gabbro of the region is an enhanced content of phosphorus.