Petrochemical features of rare metal alkaline massifs of the Northern Baikal Area

 Sotnikova I.A., Vladykin N.V.

  

The alkaline province of the North Baikal Area includes about nine alkaline massifs but only the Burpala and Akit massifs of this province possess rare-metal mineralization. The Burpala massif covers an area of 250 sq. km and is dated as 290 Ma; the Akit massif covers an area of 4.5 sq. km and has the age of 200 Ma. We proposed a new scheme of magmatism for the Burpala massif: schonkinites → nepheline syenites → alkaline syenites → quartz syenites → veined rocks: mariupolites, rare-metal pegmatites, apatite-fluorite rocks, alkaline rocks, carbonatites; while for the Akit massif this scheme is the following: nepheline syenites → alkaline syenites → quartz syenites → alkaline granites. On the classification Si-(Na2O+K2O) diagram there is a common trend of differentiation from basic to acid rocks for the Burpala massif  while for the Akit massif this trend goes from nepheline syenites to alkaline granites. On diagrams of binary correlations of petrogenic elements for both massifs one can observe common trends of composition change from basic to acid rocks that confirms the homodromic nature of all rocks from the massif.  There is a regular increase of SiO2, Al2O3, total Fe and alkaline elements and decrease in MgO and CaO. Similar correlation dependencies are found in the composition of rock-forming minerals of these massifs: mica, amphiboles and pyroxenes. Similar trends are also found on the ternary petrochemical diagrams of rock forming elements. On the whole petrochemical data confirm the formation of rocks from the common magma and our studies on geochemistry of Sr-Nd isotopes indicated the enriched EM-2 source that is typical of massifs formed in fold zones.