Geology of Moskenesøy, Lofoten, North Norway.

Abstract

Moskenesøy, Lofoten, forms part of the Lofoten-Vesterålen Precambrian high grade granulite metamorphic province. The dominant basal rock type of the island is a porphyroblastic monzonitic gneiss interbanded with, and grading into, a dioritic gneiss and a leucocratic quartz monzonitic gneiss. Chemically, it is distinct from the widespread massive intrusive porphyritic mangerites occurring on other islands in Lofoten. The subordinate rock type constituting the basal gneiss consists of a series of veined and layered gneissess of variable composition (dioritic-monzonitic) and mineralogy (granulite-amphibolite facies mineral assemblage) together with minor occurrences of thin quartz-magnetite bands. The basal gneiss sequence is intruded by small gabbroic and ultramafic masses. In the south a dome-like anorthosite occurs in the core of an anticline. Late stage dolerite and pegmatite dykes are commonly found. The pegmatites represent the last igneous event recorded and may be related to the widespread retrograde metamorphism of the granulite facies assemblages. This retrogression varies from microscopic garnet corona formation to complete recrystallization. Chemically the rocks show high K\/Rb ratios, generally >300. Extreme K\/Rb values (>2000) occur in the anorthosite and the gneisses in contact with the anorthosite. Small mangerite veins, dykes and intrusions with K\/Rb>1000 possibly represent melting of the gneisses at the time of the anorthosite emplacement.