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A New Occurrence of Porphyritic Syenite in the Oslo Igneous Province, Southeast Norway

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A New Occurrence of Porphyritic Syenite in the Oslo Igneous Province, Southeast Norway A new occurrence of porphyritic s yenite In the Oslo igneous province, southeast Norway NIELS WESTPHAL PETERSEN & HENNING SØRENSEN Petersen, N. W. & Sørensen, H.: A new occurrence of porphyritic syenite in the Oslo igneous province, southeast Norway. Norsk Geolo gisk Tidsskrift, Vol. 77, pp. 123-136. Oslo 1997. JSSN 0029-196X. Small bodies of porphyritic syenite occur in the contact zone between larvikite and ekerite near Lake Mykle in the southwestem part of the Permian Oslo igneous province. Field relations, petrographical and geochemical data indicate that the plagioclase-bearing larvikite, the porphyritic syenite with plagioclase phenocrysts, and the associated nordmarkitic syenites have been formed by successive pulses of melts from a common source. The data may, however, also support the interpretation that the syenitic rocks have been derived directly from larvikitic melts by fractionation processes. The porphyritic texture is regarded to be a result of early nucleation and growth of plagioclase primocrysts throughout a super-cooled melt. N. W. Petersen, Mæ rsk Oil & Gas AS, Esplanaden 50, DK-1263 Copenhagen K; H. Sørensen, Institute of Geo/o gy, University of Copenhagen, Øster Vo ldgade 10, DK- 1350, Copenhagen K, Denmark. Introduction west coast of the lake host mafic bodies in the form of a net-veined complex (Morogan Sørensen Transitions between monzonitic and syemttc rocks are & 1994). rather common in the Permian Oslo igneous province 5. Ekerite (youngest). (cf. Neumann 1980). Various types of porphyritic syenite have been reported fr om these transitional zones. The The present paper treats the field relations, petrogra­ most prominent example is the 'larvikite 3' described by phy, geochemistry, and the origin of the porphyritic T. Andersen (1984, 1990) as an example of mixing and syenite and the other rocks occurring around the south remelting of already consolidated larvikite (monzonite) end of Lake Mykle. Preliminary results have been pre­ and a nordmarkitic magma ( nordmarkite is a quartz­ sented by Petersen (1992). bearing alkali syenite). Additional occurrences of por­ phyritic syenites have been discovered during the geological mapping of map sheet Siljan (Holm & Sørensen, in prep.). One of these, the subject of this Field relations paper, is located around the south end of Lake Mykle in the southwestern part of the Oslo province (Fig. 1), The body of porphyritic syenite located around the south about 45 km northwest of the town of Larvik. end of Lake Mykle (Fig. l) measures ca. 4. 5 x ca. 2 km The area around the south end of Lake Mykle is and appears to be dome-shaped with steep margins and mainly made up of larvikite belonging to the so-called a flat roof. The rock has phenocrysts of a bluish-grey Skrim batholith. Peralkaline granite (ekerite) is the major fe ldspar, in hand specimen similar to the fe ldspar of the rock type around the north end of the lake. In the larvikite, enclosed in a finer-grained matrix dominated by contact zone between these major rock bodies, a succes­ reddish alkali feldspar. sion of intrusive rocks has been distinguished during the The contact against the roof of larvikite has been mapping programme carried out in 1988-92 by the staff located in several places, always at an altitude of about and research students of the Geological Institute of the 540 m a.s.l. This upper contact is sharp, and sheet University of Copenhagen. These rocks are from the jointing in the porphyritic syenite is parallel to the oldest to the youngest (Holm & Sørensen, in prep.): contact. The exposed vertical thickness of the body is ca. 150 m. l. Larvikite ( oldest). The steep contacts with the larvikite are also generally 2. Diorite/gabbro and nepheline syenite, both of which sharp, but there are examples of gradual transitions in occur only as xenoliths in younger rocks, but which, the fo rm of an increasing density of phenocrysts toward from xenoliths and dykes, are seen to be younger the larvikite over a distance of a fe w centimetres. than the larvikite (Andersen & Sørensen 1994). Dykes of porphyritic syenite, up to 1-2 m wide, cut 3. The porphyritic syenite described in the present pa­ the larvikite and bodies of dioritic-gabbroic rocks (Figs. per. 2a,b). The syenite is locally rich in xenoliths of these 4. Nordmarkitic syenites and granites, which on the rocks. The diorite-gabbro xenoliths always have sharp 124 N. W. Petersen & H. Sørensen NORSK GEOLOGISK TIDSSKRIFT 77 (I997) ' '\ �< OSLO REGION l \ "_, Nepheline Larvikite Syenite N I>><:J syenite Net-veined Diori te -gabbro [t_] - complex Porphyritic Granites. o 2 1krn syenite incl. ekerite __:k -----.J Fig. l. Geological map of the area around the southem part of Lake Mykle compiled by P. M. Holm and O. Larsen. NORSK GEOLOGISK TIDSSKRIFT 77 (1997) Porphyritic syenite, Oslo igneous province, Norway 125 Petrography Larvikite This rock is bluish-grey, medium- to coarse-grained with a porphyritic texture. It consists of 70% feldspar, 10% din o- and orthopyroxenes, 5% amphibole, 5% Fe-Ti oxides and 10% minor minerals induding biotite, quartz, calcite, apatite and zircon. Allanite, fiuorite and chev­ kinite are occasionally present. The feldspar forms large, bluish-grey, rectangular crys­ tals of unzoned plagiodase rimmed by a colourless cryp­ toperthite. In addition, there are two types of interstitial feldspar. One of these is a microperthite consisting of (A) potassic feldspar and anorthodase. This type appears to be most turbid dose to the porphyritic syenite. The other, less common type, is an irregular intergrowth between oligodase and potassic feldspar. This resembles the 'symplectite' reported by Parsons & Brown (1983) from the Klokken intrusion in the Gardar province, southwest Greenland. Its origin is not fully understood. The rock resembles the plagiodase-rich type of larvikite that has been described under the name of kjelsåsite from other parts of the Oslo area. The mafic silicates and Fe-Ti oxides occur interstitially and as indusions in the feldspars. There are separate grains of dino- and orthopyroxenes, which are sur­ rounded by narrow rims of green-brown, edenitic amphi­ bole. The dinopyroxene is dearly dominant relative to (B) the orthopyroxene. Orthopyroxene in larvikite from the Oslo Region has been reported previously only from the Sande central Pluton (Neumann 1976; Andersen 1984). The Fe-Ti oxides have rims of green-brown biotite. Porphyritic syenite The porphyritic syenite consists of rectangular phe­ nocrysts of bluish-grey plagiodase ( andesine-oligodase), which make up 30-70% of the rock (Fig. 3a). They are embedded in a fine-grained matrix dominated by 70- 80% subhedral, turbid microperthite or dear cryptop­ erthite, usually without indusions, although strongly resorbed cores of plagioclase may be present. There is 10-15% greenish-brown, subhedral, edenitic amphibole, in some cases with resorbed cores of augitic dinopyrox­ ene, 5% subhedral Fe-Ti oxides, occasionally with rims (C) of greenish-brown biotite, and 5% interstitial quartz. The Fig. 2. (A) Thin dykes of porphyritic syenite cutting larvikite. SW shore of Lake accessories are zircon, calcite, fluorite, allanite, chevkinite Mykle. (B) Dyke of porphyritic syenite in larvikite showing gradual contacts. Xenocrysts of larvikite feldspar 'float' in the porphyritic syenite. SW shore of Lake and apatite. Biotite occurs as large grains and as rims Mykle. (C) Xenolith of gabbro in porphyritic syenite. SE shore of Lake Mykle. around Fe-Ti oxides. The plagiodase phenocrysts are typically up to 12-15 contacts with the syenite (Fig. 2c). The larvikite xenoliths mm in size, a few reaching 20-25 mm. There are two may also have sharp contacts, but they are commonly types of plagiodase. One type resembles the large crystals disintegrated into dusters of grains of larvikite feldspar. of the larvikite, but each 'grain' is made up of dusters of The nordmarkitic syenite intrudes the larvikite, diorite­ several albite-twinned grains, each duster being rimmed gabbro and porphyritic syenite in the form of sheets and by microperthite. The other type generally forms smaller dykes, which often terminate in pegmatitic bodies. It single grains of reversely or - more rarely - normally contains xenoliths of all the earlier rocks. Dykes of red, zoned plagiodase with rims of microperthite (Fig. 3b). fine-grained granite and ekerite cut all other rocks. Indusions of Fe-Ti oxides are abundant in both types, 126 N. W. Petersen & H. Sørensen NORSK GEOLOGISK TIDSSKRIFT 77 (1997) feldspar. Accessories are titanite, fluorite, apatite, calcite and zircon. Granites Ekerite (peralkaline granite) is predominant in the north­ em part of the area covered by this study. It is medium­ to coarse-grained and made up of sub- to euhedral grains of microperthitic alkali feldspar with interstitial quartz, aegirine and/or sodic amphibole, and magnetite. A red granite is the predominant type of granite around the southern end of Lake Mykle. It is finer grained than the ekerite and has a granular texture. It contains 65% subhedral, turbid, microperthitic alkali feldspar, 25% interstitial quartz, 5% anhedral ilmenite. There are rare phenocrysts of a bluish-grey, cryptop­ erthitic alkali feldspar, a feature resembling some vari­ eties of the nordmarkite. The accessories are biotite, zircon and green, ferroedenitic amphibole. Miarolitic cavities are common. Mineral chemistry Analytical methods Minerals were analysed at the Institute of Geology, University of Copenhagen, by means of a JEOL electron Fig. 3. (A) Microphotograph illustrating the texture and the feldspar phases of microprobe equipped with wave-length and energy-dis­ the porphyritic syenite. Large grains of plagioclase are present on the teft- and right-hand sides. Smaller zoned grains of plagioclase are common in the central persive spectrometers. The standard acceleration voltage part. The plagioclase grains are mantled by microperthitic alkali feldspar and are was 15 kV, beam current was 15 JJ.A and counting times embedded in a matrix of turbid microperthitic alkali feldspar, quartz, amphibole 60 seconds.
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