Pyroxenite
Corse (pegmatic) Pyroxenite from the Brattåsen plug, Vestby.
Fine to medium grained Pyroxenite from the Knalstad plug, Vestby.
Some green pyroxene mineral, red arrow in the picture above.
Gabbro (Oslo-essexit)
Oslo-essexite from the Brattåsen plug, Vestby.
Weathered surface of Oslo-Essexite from Vestby.
Plagioclase crystal in the Oslo-essexite from Vestby.
Husebyitt
A type of Oslo-essexite found in Mærradalen (vally) in Oslo.
Monzonitter
Akerite
From just west of Songsvann, Oslo.
Kjelsåsite
Kjelsåsite gets its name from the Kjelsås farm in Sørkedalen. The rock is a plutonic rock, meaning the magma has solidified in magma chambers deep underground and never reached the surface. According to Oftedahl (1960), the rock contains plagioclase (mainly andesine) and alkali feldspars in a ratio of 2:1. Other dark minerals include diopsidic augite, biotite and olivine as well as other trace minerals. A little quartz and hypersthene may also occur. The definition used in NGU's maps is: "A light monzodiorite-monzonite with rectangular feldspar crystals and with aegirine-augite, biotite and brown hornblende." (Gjelle & Sigmond, 1995). The rock is closely related to the more famous Larvikite, the difference being that the andesine proportion of the feldspar in the rock must be above 30 % for it to be a Kjelsåsite. If it is below that, it is Larvikite (Neumann, 1978). Since this is difficult to see, they are often grouped together.
In some places, olivine is altered to serpentine and chlorite due to metamorphism.
The Slottet Kjelsåsite pluton (a magma chamber around the younger Oppkuven caldera), from which this piece comes, is dated by Corfu & Larsen (2020) to be between 275.9 and 276.7 Ma.
Syenite
Grefsen syenite
Grefsen syenite from Grefsenkollen in Oslo
Alkalifeltspatsyenitt
Nordmarkite
Nordmarkite is named after Nordmarka woodlands in the north of Oslo. Nordmarkite is also the official county stone of Oslo. Originally, the name Nordmarkite was introduced as a collective term for all syenites in Nordmarka. Oftedahl (1960) divides these into three categories, first the true syenites containing plagioclase (Grefsen syenite) and alkali feldspar syenite. He further divides the alkali feldspar syenites according to how alkaline they are, based on which dark silicates the rock contains. The less alkaline ones contain biotite and hornblende, while the name Nordmarkite goes to the most alkaline rocks with ægirite and arfvedsonite. So Nordmarkite is an ægirite and/or arfvedsonite bearing alkali feldspar syenite. With increasing quartz content, the rock gradually transitions to the alkali feldspar granite Ekeritt.
Age dating has shown that the syenites are among the youngest rocks in the Oslo rift. Measurements from Nittedal show an age of 252±3 Ma for the Nordmarkites, slightly younger than the surrounding Grefsen syenite (Sundvoll et al., 1990).
Alkali feldspar granite
Drammen granite
Drammen granite is named after the city of Drammen, since it frequently occurs in the mountains surrounding Drammen. Historically, Drammen granite has been among the most important industrial rocks in Norway. As a beautiful red and weathering resistant stone, it has been used in various construction projects, as paving stones and for headstones. Today, there is only one active quarry for Drammen granite at Høgåsen, just outside Spikkestad, about 8 km west-southwest of Drammen.
Chemically, Drammen granite is a Biotite alkali feldspar granite, where biotite is the only important dark mineral (Oftedahl, 1960). Drammen granite is usually coarse-grained (grain sizes in the range of 3-5mm). Of which 60-65% of is perthitic feldspar, 3-5% oligoclase feldspar (both alkali feldspars) and 30-35% is quartz. The oligoclase crystals are often bleached and appear white, and the quartz is white to shiny. Perthitic feldspar is reddish, and in addition to that, small amounts of red iron oxide (hematite) are often precipitated along the grain boundaries. Overall, we get a white-mottled pattern in a reddish matrix (NGU, 2015). Haug (2007) has determined the age of the Drammen granites to be between 272 and 287 Ma (Corfu & Larsen, 2020).
References
NGU. (2015), Granitt. https://web.archive.org/web/20160302193626/https://www.ngu.no/emne/granitt
Granite
An older granite, found as a lose rock along Lake Mjøsa.
Diabase
Diabase dike at Karihaugveien, in Oslo, at the intersection with Edvard Munch road.
Fine grained (Aphanitic) diabase, from the dike above. The fine grained nature is a sign of rapid cooling.
A corse grained diabase from Hovedøya, Oslo.
Microsyenite
Mæanite
Microsyenite porphyry from the Ekeberg hill in Oslo.
Microsyenite with albite and orthoclase .
Grorudite (Aegirine granite porphyry)
Grorudite-dike in Ravnkollen, Grorud, that cuts
through the Nordmarkite. Described by Brøgger (1894).
Grorudite was first described and named in 1894 by Brøgger. He describes a green, approximately 5-meter wide dike that cuts through the nordmarkite in Ravnkollen in Grorud, Oslo. The name comes from the district of Grorud where that dike is. Brøgger describes the rock as Aegirine granite porphyry, where aegirine is the dark green mineral that gives the characteristic color. The composition is approximately 1/5 aegirine, 2/5 feldspar and 2/5 quartz (Brøgger, 1894).
Brøgger (1894) and later geologists have found several Grorudite passages in the Oslo field, but on a world scale it is quite rare rock (Thuesen, 1977). The two perhaps best known dikes are the dike at Grorud, which Brøgger followed up to Steinbruvannet and which Kvamsdal (1999) has followed further, and a dike from Stig (Southwest of Grefsenkollen, Oslo) up past Storhaug.
Historically, Groudite was used to make axes in the Neolithic Age. Worldwide, flint and obsidian were the preferred rocks for that purpose, but unfortunately they are not found in the Oslo field. Flint was therefore imported from Denmark and perhaps Skåne in Sweden, but it must have been much more difficult to import in the Stone Age than today. So local stones was probably used as materials when possible. And the tough Groudite could be sharpened into axes that stood up to wear and tear. Around 15 such axes have been found, but exactly where they came from is somewhat uncertain. Chemical comparisons may indicate that they come from the Groudite dike up from Stig, near Svartputten (Kapteinsputten) or from a Groudite dike that crosses the railway in the north of Maridalen between Svingen and Sandermosen (possibly a northern exposure of the same passage) (Thuesen, 1977; Nyland, 2013).
These dikes represents the last, final phase of the Oslo Field. With the dyke at Storhaug dating to 249±3 Ma, which makes it one of the youngest rocks in the Oslo Field (Sundvoll & Larsen, 1993).
Downfall from the Grorudite dike in Ravnkollen. White phenocrysts of K-feldspar and a matrix of quartz and many small green aegirine crystals.
Grorudite dike at Storhaug. A possible site for a Stone Age quarry of Grorudite, but it is impossible to determine now, everything we see today is a result of natural processes and the construction of the gravel road (Nyland, 2013).
A close up photo of the dike at Storhaug. Not as porphyritic as the dike at Ravnkollen.
Basalt
Kvartstholeiitt
Basalt the B1 lava flow, from Kolsås. This rock is showing vesicular texture, characterized by numerous small cavities created by gas bubbles trapped in the lava. A clear indication that the lava solidified at the surface, where the lower pressure allowed gas bubbles to develop in the lava.
Basalt the B1 lava flow, from Kolsås.