Origin of plagioclase-megacrysitic, orthopyroxene amphibolites within a Precambrian banded gneiss suite, Flekkefjord area, Vest-Agder, South Norway.

Abstract

Within a banded gneiss suite belonging to the high-grade Mesoproterozoic Flekkefjord complex in southern Norway, thin lenses of orthopyroxene amphibolites with medium- to large-size plagioclase megacrysts are located at roughly the same stratigraphic level in an area covering more than 200 km2. The big feldspars are either more or less altered, single plagioclase megacrysts up to decimetre size or anorthositic aggregates measuring almost 1 metre across. The rock has suffered several phases of deformation and high-grade metamorphism during the Sveconorwegian orogeny, obscuring the primary origin of this unusual rock-type. Most rocks in the area have retained their granulite-facies mineral assemblage, whereas some zones within the southwestern area have been retrograded to amphibolite facies. Although several alternative modes of origin are possible for these big-feldspar rocks, a sill-like emplacement of jotunitic magma bearing plagioclase megacrysts which had previously crystallized in a deep-seated magma chamber is favoured. Intrusion of the big-feldspar amphibolites as sills took place before1.16 Ga. Geochemical similarities with jotunitic rocks in the Rogaland anrothosite complex, emplaced at around 932 Ma, and the hunendalen dykes, which were intruded 100 million years later, suggest that similar magma-producing processes where in operation sporadically over a period of more than 325 million years.