GEOCHEMISTRY AND GENESIS OF EARLY PRECAMBRIAN METABASITES IN THE EASTERN STANOVOI AREA, EASTERN SIBERIA

Archean and Proterozoic mafic-ultramafic complexes of the eastern Stan ovoi area (Stanovoi and Ust'-Gilyui groups) consist of geochemically d iverse rocks. On the basis of variations in major and trace element co ntents, petrographic features, and geologic settings, these rocks can be subdivided into five groups of ;Archean and two groups of Proterozo ic ages. The primary melts of the Archean rocks were highly magnesian basalts (14 - 15 wt % MgO). Three of the rock groups (amphibolites, fe rrous amphibolites, and hornblendites) formed due to the differentiati on of a single tholeiitic ma,oma, whereas the biotite amphibolites are metamorphosed basalts of the calc-alkalic series. The highly magnesia n serpentine-talc rocks are metamorphosed olivine cumulates. All of th e rocks are persistently enriched in ultimately incompatible elements; the calc-alkalic rocks are enriched much more strongly than the other rocks. This feature reflects the variable compositions of the mantle sources of the calc-alkalic and tholeiitic magmas, which were situated in the lithosphere. Similarly, the two groups of Proterozoic racks sh ow different contents of incompatible elements, including such moderat ely incompatible elements as Ti, P, and HREE. We suggest that these ro cks are also derivatives of two primary magmas, which melted out of ch emically diverse peridotites. The Archean mafic and ultramafic rocks s eem to have formed in an active continental margin, whereas the Proter ozoic rocks show features of continental magmatism.