U-Pb AGE AND GEOCHEMICAL TYPIFICATION OF THE SARAM MASSIF ROCKS (WESTERN TRANSBAIKALIA): EARLY JURASSIC GRANITOID MAGMATISM

The paper presents data on the geological position, age and features of the material composition of the Saram massif granitoids, located in the northwestern part of the Malkhan ridge of Western Transbaikalia. The structure of the massif involves two-phase rocks corresponding to syenite, early-phase moderate-alkali granite and late-phase leucogranite families. The silica content (wt. %) varies from 63.8 to 71.2 in the early-phase granitoids and from 73.2 to 77.1 in the late phases. The early-to-late-phase rocks of the massif are mostly ferruginous (Fe*=0.77–0.88 and 0.80–0.93, respectively). Based on a high modified alkaline lime index (MALI) (8.75–9.97) and relatively low SiO 2 contents, the early-phase rocks can be referred to as alkaline rocks, and the late-phase rocks – as calc-alkaline rocks. According to the aluminum saturation index, the early-phase rocks (0.93–1.07) correspond to moderate-to-high-alumina rocks, and the late-phase rocks (1.09–1.13) – to high-alumina formations. Granitoids are geochemically and mineralogically different from typical agpaitic A-type granites and correspond to a special group of aluminous A-type rocks. The two phase magmatic zircon U-Pb dating yielded the 175–177 Ma age (Early Jurassic). The formation of granitoids in the Saram massif is temporally synchronous with intensive orogenesis in Transbaikalia, probably caused by the closure of the Mongol-Okhotsk Ocean.