Is the West Karmøy complex igneous or metasedimentary?

Abstract

The island of Karmøy in southwestern Norway is famous among geologists for the Ophiolite, one of the first ophiolites to be recognized and described in the Scandinavian Caledonides. Much of the island is underlain by the West Karmøy Complex, presently interpreted as an igneous complex that intrudes the Karmøy Ophiolite. There is a striking resemblance of some rocks of the Complex with the Sykesville “granite” of Maryland, which was shown by Cloos and by Hopson (1964) to be a metadiamictite. After local examination of the rocks and after comparison with the 1980 report of Ledru, the conclusion was drawn that — except for the Risdal granodiorite, pegmatite and aplite dikes — the Complex is a metasedimentary, compositionally variable succession of meta-arkose (the “quartz-augen gneiss”—Ledru's Diorite quartzique et Granodiorite du nord) and metadiamictite (his various inclusion-rich “granite” units). Both units were derived by erosion from advancing thrust sheets, including the Karmøy Ophiolite, which then overrode the sedimentary succession. The high modal quartz and normative corundum contents of the “granitic” rocks resemble those of the Sykesville and favor a metasedimentary origin; the chemistry is also similar and rather far from a granite minimum melt, though certain mixtures of quartz-sandy matrix and mafic blocks may have melted locally while being overridden by the Ophiolite. Reports that quartz diorite of the West Karmøy Complex intrudes metagabbro of the Karmøy Ophiolite could not be confirmed, although trondhjemite dikes belonging to the Ophiolite do intrude the metagabbro; the contact of the Ophiolite with the Complex was faulted wherever observed. As recognized by almost all observers, both Ophiolite and Complex are overlain unconformably by Upper Ordovician sediments of the Skudeneset Group, which was later deformed and metamorphosed in the greenschist facies; at least a great part of the fault separating the Karmøy Ophiolite and the West Karmøy Complex dates from that later episode.