Petrogenesis and Sources for Rocks of the Rare-Metal Alkaline Burpalа Intrusion (Northern Baikal Region)

Abstract

The paper reports the results of petrologic and geochemical study of the Burpala alkaline intrusion composed of quartz syenites, alkaline and nepheline syenites, including ore-bearing varieties, which is a part of the Late Paleozoic North Baikal Alkaline Province. Chemically, the studied rocks belong to foid monzosyenites, foid syenites and syenites, ranging from agpaitic to miascitic varieties. Close Sr-Nd isotopic characteristics and geochemical patterns confirm the syngenetic nature of magmas that produced nepheline, alkaline, and quartz syenites. A negative Eu anomaly in the REE patterns and sufficiently low Mg# of rocks testify in favor of a prolonged fractional crystallization of rocks from an alkaline-basic melt. Sr-Nd-Pb isotopic and geochemical characteristics of rocks of the Burpala intrusion reflect the predominance of metasomatized lithospheric mantle in their source. The formation of the rocks of the intrusion, according to the trace element and isotopic data, was complicated by the upper crustal assimilation, which likely determined the genetic relationship between nepheline and quartz syenites within the intrusion.