The Oslo Region is a geographic term referring to an area of about 100 000 km2, reaching from Langesund and Brunlanes in the south, to the Mjøsa district in the north. The Oslo Region contains a sedimentary sequence of Cambrian, Ordovician and Silurian rocks as well as sediments and plutonic and volcanic rocks of Permian age. Sediments of late Carboniferous age are also represented. The Oslo Region is surrounded by Precambrian rocks, except in the north where the region adjoins the Caledonian nappe region and the late Precambrian to early Cambrian sparagmite sediments.
In late Carboniferous and early Permian time, the Oslo Region was the site of extensive magmatic activity. A wide variety of rocks of monzonitic composition make up about 40% of the magmatic rocks in the region. The genetic relations between the different rock types are complex, and several evolutionary series exist. Neumann (1978) suggests that the kjelsåsite/larvikites and rhomb-porhyry lavas are derived by fractional crystallization at the base of the crust from mantle-derived alkaline basalts. When fractionation of the residual magma reached monzonitic compositions, it extruded into the upper crust and crystallized under dry, reducing conditions, and without being significantly affected by the country rocks.
The southernmost part of the Oslo Region, the Larvik plutonic complex, is made up essentially of monzonitic rocks, mainly larvikite and its closely related varities, which occupies more than 1000 km2, stretching from Tønsberg in the east to the Langesundsfjord area in the west. It is bordered by lava flows and nordmarkite and ekerite to the north-east and north-west, respectively, and by Cambro-Silurian sediments and Permian lavas to the west and south-west. Petersen (1978) showed that the Larvik plutonic complex consists of 10 semicircular plutons arranged in a manner which suggests a general shift of focus towards the west with a progressive higher degree of undersaturation. The circular structures represent shifting of centres of igneous activity and a progressive younging of the structures (Fig. x). U-Pb isotopic age determinations of the different plutons indicate that the entire complex was emplaced during a period of 5-6 mill. years, from 297.3 ± 1.2 mill. years for pluton 2 to 292.0 ± 0.8 mill. years for pluton 9 (Dahlgren et al. 1996).
Larvikite is medium to coarse-grained, sometimes
with mafic and accessory minerals in interstices between the feldspar laths
of 1 cm or more in size. The larvikites commonly display an orientational
fabric caused by the parallel or subparallel alignment of the often characteristically
rhomb-shaped feldpar individuals. The mineral assemblage is dominated by
ternary feldspar An7Ab61Or32 - An15Ab76Or8,
sometimes with a core of partly resorbed plagioclase An35Ab60Or5
(Neumann 1978). The feldspar is unmixed to a variety of perthite with a
pronounced schillerizing effect which makes the rock an important and beautiful
ornamental and building stone. Larvikite also contains small quantities
of quartz or nepheline ± olivine (or olivine pseudomorphs) + Ti-poor
augite ± kaersutitic hornblende + ilmenite + magnetite + apatite
+ zircon ± biotite. Broadly stated, larvikite west of lake Farris
contains excess nepheline, whereas most of that to the east of the Lågen
valley contains neither free quartz nor free nepheline, and finally, around
Tønsberg, quartz is abundant. Red, quartz-bearing larvikite is called
tønsbergite. The colour is due to finely dispersed iron oxide. The
main part of the larvikite massif consists of a rather uniform rock type,
but as the western border is approached, essentially on the islands in
the Langesundsfjord, the Ormefjord, the Eidangerfjord, and their immediate
mainland, the larvikite is extensively penetrated by veins and bodies of
a medium-grained and often schistose nepheline syenite, the ditroite
by Brøgger (1890). Tectonic movements during crystallisation are
apparent. The nepheline syenite is slightly younger than the larvikite.
The relative amounts of nepheline syenite in larvikite increases towards
the western border of the district. An intrusion of homogeneous, medium-grained
nepheline syenite occurs from the southern point of Bjønnes peninsula
and stretches for 4 km towards north along the western part of the peninsula.
The lardalite complex is situated between Farris and Lågen, 10-30
km north of Larvik, and forms a composite ring-complex by two main, semicircular
sections (Oftedahl & Petersen 1978). It is the major occurrence of
undersaturated plutonic rocks in the Oslo Region, and represents the the
end member of the magmatic acticity in this region. Lardalite is the name
for a variety of fine- to coarse-grained rocks which consist of microcline
antiperthite (An2-10Ab55-85Or15-40) +
nepheline ± sodalite ± olivine (or olivine pseudomorphs)
+ Ti-poor augite + biotite + ilmenite + magnetite + apatite + zircon.
References
Brøgger, W. C. 1890: Die Mineralien der Syenitpegmatitgänge der südnorwegischen Augit- und Nephelinsyenite. Zeitschrift für Krystallographie, 16, 1-235 + 1-663.
Dahlgren, S., Corfu, F. & Heaman, L. M. 1996: U-Pb isotopic time constraints, and Hf and Pb source characteristics of the Larvik plutonic complex, Oslo paleorift. Geodynamic and geochemical implications for the rift evolution. Journal of Conference Abstracts, 1, 120.
Neumann, E.-R. 1978: Petrology of the plutonic rocks. Norges Geologiske Undersøkelse, 337, 25-34.
Oftedahl, C. & Petersen, J. S. 1978: Southern part of the Oslo rift. Norges Geologiske Undersøkelse, 337, 163-182.
Petersen, J. S. 1978: Structure of the larvikite-lardalite
complex, Oslo-Region, Norway, and its evolution. Geologischen Rundschau,
67, 330-342.