Larvik plutonic complex

The Oslo Rift in south-eastern Norway is part of a system of late Carboniferous to Permian continental rifts in northern Europe. Rift-related igneous rocks range in composition from alkaline basalt to granite and nepheline syenite, with a large proportion of intermediate, monzonitic intrusive rocks (larvikite) and compositionally equivalent latitic lavas, known as rhomb porphyry.
The plutonic rocks of the Oslo graben are distributed in three main complexes, geographically separated by outcrops of Permian lavas and Cambro-Silurian sedimentary rocks (Fig. 1). The northern graben segment is occupied by the Nordmarka-Hurdalen batholith of mostly syenitic and granitic alkaline rocks and some intrusions of biotite granite and monzonite. Biotite granites of the Drammen and Finnemarka batholiths dominate the central part of the Oslo Graben; these batholiths cover areas of 650 km² and 125 km² respectively, and, the Drammen batholith is the largest granitic complex in the rift. The largely monzonitic batholiths Larvik pluton and the compositionally more diverse Siljan-Mykle complex occupy the southern and central parts of the graben.

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Fig.1:Simplified geological map of the Oslo Graben. From Andersen, T. (2008).

The Larvik Plutonic Complex (Fig.2) is a large, composite intrusion making up the southernmost onshore part of the Oslo Rift. The complex is built up by 10 arc-shaped or circular intrusions of hypersolvus monzonite (larvikite) and nepheline syenite with internal intrusive contacts marked by chill-zones. The intrusive center migrated westwards and northwards with time, at the same time the composition changed from quartz normative, through silica saturated to olivine- and nepheline bearing varieties. The youngest intrusive members of the complex are nepheline monzonites (zones IX and X in Fig.2).

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Fig.2: Simplified geological map of the Larvik Plutonic complex and its surroundings. From Andersen, T. (2013).

Trace element distribution patterns and radiogenic isotopic signatures of larvikite and rhomb porphyry point towards an origin from the lithospheric mantle. The rocks of the Larvik Plutonic Complex range from mildly quartz normative in the early ring intrusions, to strongly nepheline normative in the younger intrusions. In the north and west, the complex is penetrated by slightly younger nepheline syenite intrusions, some of which also carry sodalite. The range in composition of the Larvik Plutonic Complex is explicable by polybaric fractionation of mildly alkaline, mafic parent magma in deep- to middle-crustal magma chambers, combined with a density filtering mechanism. U-Pb dating of magmatic zircon suggests a relatively short period of emplacement (299-292 Ma), overlapping in time with the main rifting episode with widespread rhomb porphyry volcanism. Neumann 1980.

The petrographic nomenclature used for plutonic rocks in the Oslo graben has been heavily influenced by W.C. Brøgger, who introduced a large number of locally defined rock names. Some of his more obscure terms have mercifully been discarded even by local geologists. Definitions of the local petrographic terms are summarized by Le Maitre et al. (2002):

• Ekerite: Peralkaline granite with arfvedsonite or aegirine as charactieristic dark minerals. The name originates from Eikeren pluton.

• Grefsen syenite: A medium-grained syenite to quartz syenite which contains grey plagioclase and/or anorthoclase feldspar rimmed by pink microcline; its characteristic dark silicate minerals are biotite and amphibole. The name originates from Grefesnåsen hill, Oslo.

• Hedrumite: A white, medium-grained nepheline syenite, commonly containing minor amounts of blue to pale bluish grey sodalite. The name originates from Hedrum, Oslo.

• Kjelsåsite: A plagioclase-rich larvikite. The name originates from Kjelsås, Sørkedal, Oslo.

• Lardalite: A nepheline monzonite carrying a ternary feldspar similar to that of the larvikite. Normative nepheline contents are in the range 20-25 %. The name originates from Lardal, Oslo.

• Larvikite:Variety of augite syenite or monzonite consisting of rhomb-shaped ternary feldspars (with a distinctive adularescence), dark amphibole, Ti-augite and Fe-rich micas. Minor nepheline, iron-rich olivine or quartz may be present.

• Nordmarkite: A aegirine-augite and/or alkali amphibole bearing quartz alkali feldspar syenite. The name originates from Lake Katnosa, Nordmarka, Oslo.

• Tønsbergite: A red quartz-bearing larvikite. The name originates from Tønsberg, Oslo.

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Ekerite: plagioclase (white), K-feldspar (pink) and arfvedsonite (dark). Oslo graben, Norway. From Hildegard Wilske

Ekerite: plagioclase (white), K-feldspar (pink) and arfvedsonite (dark). Oslo graben, Norway. From Hildegard Wilske

Grefsen syenite: plagioclase rimmed by pink microcline. Oslo graben, Norway. From Hildegard Wilske

Grefsen syenite: plagioclase rimmed by pink microcline. Oslo graben, Norway. From Hildegard Wilske

Hedrumite: white, medium-grained nepheline syenite. Oslo graben, Norway. From Hildegard Wilske

Kjelsåsite: grossi cristalli di feldspati ternari (grigio-blu) e cristalli di plagioclasio (bianco). Oslo graben, Norway. From Hildegard Wilske

Kjelsåsite: large ternary feldspar (grey-blue) and plagioclase crystals (white). Oslo graben, Norway. From Hildegard Wilske

Lardalite: large ternary feldspar (grey-blue) and nepheline crystals (grey-yellow). Oslo graben, Norway. From Hildegard Wilske

Lardalite: large ternary feldspar (grey-blue) and nepheline crystals (grey-yellow). Oslo graben, Norway. From Hildegard Wilske

Larvikite: ternary feldspar (grey-blue) and dark femic minerals. Oslo graben, Norway. From Hildegard Wilske

Larvikite: ternary feldspar (grey-blue) and dark femic minerals. Oslo graben, Norway. From Hildegard Wilske

Nordmarkite: alkali feldspar (pink) and aegirine (dark). Oslo graben, Norway. From Hildegard Wilske

Tønsbergite: red quartz-bearing larvikite. Oslo graben, Norway. From Hildegard Wilske

Bibliography

• Andersen, T., Trønnes, R. G., & Nilsen, O. Alkaline rocks of the Oslo Rift, SE Norway: A field trip with emphasis on felsic to intermediate intrusive rocks and their associated mineralizations. August 1st to 5th, 2008.
• Andersen, T., Erambert, M., Larsen, A. O., & Selbekk, R. S. (2013). Petrology of nepheline syenite pegmatites in the Oslo Rift, Norway: Zr and Ti mineral assemblages in miaskitic and agpaitic pegmatites in the Larvik Plutonic Complex. Mineralogia, 44(3-4), 61-98.