Komatiitic Chlorite-amphibole rocks and mafic metavolcanics from the Karasjok greenstone belt, Finnmark, northern Norway: a preliminary report.

Abstract

A general account is presented of the field relationships, geochemistry and petrology of mafic and ultramafic metavolcanics within the Precambrian Karasjok greenstone belt. These metavolcanic rocks make up the major part of this greenstone belt, and variably deformed and metamorphosed in middle greenschist to lower amphibolite facies. Based on their unusual chemistry (21-39% MgO volatile-free) and extrusive textures, the ultramafic chlorite-amphibole rocks are considered to belong to the komatiite association. The mafic metavolcanics (amphibolites) form a tholeiitic series (5-8% MgO volatile-free), and are chemically similar to intermediate and iron-rich tholeiites of Archaean age. The mutual occurence of these komatiitic and tholeiitic series as well as the presence of exhalative-sedimentary Fe-Mn formations in association with the komtiites, would suggest that the Karasjok greenstone belt forms an extention of the 2.6-2.75 Ga Archaean greenstone belts of eastern and northern Finland.