The geochemistry of Lower Proterozoic mafic to felsic igneous rocks, Rombak Window, North Norway.

Abstract

The supracrustal sequence of the Rombak Basement Window, consisting of vocanic rocks and quartzites, was intruded by mafic dykes, mafic to intermediate plutons and variety of granitoid batholiths c. 1.8-1.7 Ga aga. The region has experienced amphiobolite-grade metamorphism, followed by retrogression to greenschist facies along Caledonian shear-zones.On the basis of their petrographic and geochemical characteristics the volcanic rocks can be divided into 3 suites: (1) high-Mg basalts; (2) mafic to felsic volcanites with fairly high potassium contents and with calc-alkaline affinities; and (3) low-potassium, calc-alkaline felsic volcanites. Based on major element geochemistry the evolution of the potassic volcanites is interpreted to have been controlled, in the case of mafic-intermediate varieties, by early fractionation of Fe, Mg-rich minerals, and by plagioclase crystallisation for the felsic varieties. Suites 2 and 3 are similar to associated granites and granodiorites in their chemical composition.It is concluded that the volcano-sedimentary and intrusive rocks were formed in an lower Proterozoic mature magmatic arc environment at the southern margin of a continent composed predominantly of Archean tonalitic granitoid rocks and Lower Proterozoic greenstone terranes.