The Ultramafites and layered Gabbro Sequences

On Stjernöy, Seiland and the neighbouring peninsulas of Öksfjord and Bergsfjord ultramafic bodies of peridotite and pyroxenite with associated layered gabbro sequences occur within a complex of highly metamorphic gabbro gneisses, rocks akin to pyroxene-granulites and mafic charnockites. As is shown on the basement map in insert of backflap the layered gabbro is generally concordant with the foliation of the surrounding gneisses but locally shows transgressive features. Within this mafic province occurrences of hornblendite, carbonatite, and nephelinesyenite are also found, the latter affiliation of rocks probably being the result of metasomatic processes. The petrography and general mineralogy of the ultramafic, gabbroic, and metamorphic types of rocks are described in detail. In the layered gabbro sequences on Stjernöy there exists no gravitational crystallization differentiation such as is found in other stratiform gabbro-peridotite complexes; the variation in the composition of the characteristic minerals is too small, and irregular at that. The textural relations point to incipient fracturing along grain-boundaries and, especially, to deuteric processes which can be inferred from the abundance of spinel-pyroxene symplectites. The characteristic rims of symplectite around olivine are clearly demonstrated in the photographs 2 and 3 on plate C. In texture and structure the gabbro gneisses and intercalated syenitic and meta-sedimentary rocks on Stjernöy exhibit considerable clastic and plastic deformation. Their mineral association points to an intense regional metamorphism near or within the granulite facies; by means of the ACF diagrams, fig. 9a and 9b, it is shown that the plotted mineral parageneses may be stable in the hornblendegranulite and the pyroxene-granulite subfacies. The investigation of co-existing clinopyroxene, orthopyroxene, and olivine in the layered gabbro-peridotite sequence on Stjernöy indicates that high pressures and temperatures prevailed at the time of their formation. In tie-line trends of co-existing pyroxenes the layered gabbros from Stjernöy are remarkably similar, not to the normal type of stratiform gabbro-peridotite complexes, but to peridotite nodules in certain basaltic lavas. (compare fig. 5a and 6a). As is shown in fig. 6b the tie-lines joining co-existing orthopyroxene and clinopyroxene from gabbro gneisses exhibit similar trends; these data most probably point to an extreme metamorphism of anhydrous rocks at great depth within the earth’s crust. The chemical analyses of samples of the ultramafic sequence, as plotted in the QLM diagram of fig. 8a, exhibit a linear trend of differentiation; this suggests a simple process of fractional melting or crystallization. The projection points of the gabbro gneiss occupy a central position in this differentiation trend on Stjernöy; thus the gabbro gneiss provides a suitable material for such a process. High lime content is another chemical characteristic of gabbro and related ultramafic rocks on Stjernöy (fig. 8b); hence the major femic mineral is a calcium-rich clinopyroxene, whereas magnesium-rich orthopyroxene predominates in the stratiform type of layered complex. Clearly, then, the layered gabbro and related ultramafite within the mafic province of western Finnmark differs considerably in major features from the wellknown layered gabbro-peridotite occurrences such as Bushveld, Stillwater or Skaergaard. Finally the results of the mineralogical and chemical investigations are discussed from the petrogenetic point of view. A hypothesis is offered for the emplacement of layered gabbro and ultramafite on Stjernöy involving differential anatexis with local palingenesis in a deepseated zone of the Caledonian orogen.