Geochemistry of dolerite and metadolerite dykes from Varanger Peninsula, Finnmark, North Norway.

Abstract

Dolerite dykes provide the only manifestation of igneous activity in the thick, Riphean to Lower Cambrian, weakly metamorphosed sedimentary sequence of Varanger Peninsula. Major and trace element abundances and ratios from two distinctive generations of mafic dyke, one metamorphosed and cleaved and the other post-metamorphic and fresh, reveal disparate grouping and trends in several variation diagrams. Older metadolerites show an overall major-element compositional similarity with abyssal tholeiites, a relationship which is confirmed by most trace element concentrations and ratios. Certain element abundances, together with the tectonic environment, suggests however thet these dykes are representative more of magmas transitional to those producing abyssal and continental tholeiites. The younger dolerites are also subalkaline though less typically tholeiitic than the metadolerites. Comparison of their element contents and ratios reveals calc-alkaline affinities, as well as similarities with data for continental tholeiites. The trend of relative enrichment in Si, Ba, Zr, Rb and alkalies in the dolerites may possibly signify a greater depth of magma generation than that which produce the metadolerites. The petrogenesis of the dyke sets is discussed in the light of the regional geology and recently published K-Ar whole-rock ages which place the dolerites in the uppermost Devonian and date the metadolerites, provisionally, as Riphean.