A new occurrence of porphyritic s yenite In the Oslo igneous province, southeast

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 -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 ­ 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 (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 . 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 , 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 , 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 . 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. The data were recalculated by a ZAF correc­ and Fe-Ti oxides. (Sample no. 81565, cross-polarized light, the section measures 2 cm from teft to right). (B) Primocryst of plagioclase (An40) is mantled by tion program. Cathodo1uminescence work was carried turbid cryptoperthitic ternary feldspar (Or30Ab55Anl5). The plagioclase core out at the Department of Geology and Geophysics, contains exsolved alkali feldspar (Or60Ab35An5). The matrix consists of turbid Grant Institute, University of Edinburgh. A Nuclide microperthitic alkali feldspar (Or43Ab53An4), quartz and Fe-Ti oxides. (Por· phyritic syenite, sample no. 81567, plane-polarized light). ELM2BX cold-cathode luminoscope was used. The ac­ celerating current varied between 0.6 and 0.8 rnA de­ pending on the vacuum. Voltage varied between 12 and sometimes with a rim of biotite, and inclusions of 16 kV. Optimum conditions were 14 kV, 0.7 rnA beam clinopyroxene are often present. These are usually fresh current and approximately 65-75 mTorr vacuum. and surrounded by a narrow rim of amphibole. Locally, there are grains of pyrite and chalcopyrite. The texture of the rocks most rich in plagioclase Feldspars phenocrysts indicates that these rocks have been formed In the larvikite and porphyritic syenite, large plagioclase by accumulation of plagioclase crystals. primocrysts have an anorthite content of from 20 to 51%. Smaller zoned grains ( only in the porphyritic syen­ ite) have an anorthite content of from lO to 32%. Repre­ Nordmarkitic syenites sentative compositions are shown in Table l and Fig. 4. This syenite is a reddish, fine- to coarse-grained, and Two types of alkali feldspar are present. One type is of a sometimes miarolitic nordmarkitic rock which generally ternary composition (l0-16% anorthite) and occurs as displays a granular texture and consists of 60-80% sub­ rims on the large plagioclase grains in larvikite and hedral, red, turbid, microperthitic alkali feldspar; 5-15% porphyritic syenite and interstitially in the larvikite. The interstitial quartz; 5-10% greenish-brown, subhedral, other type is anorthite-poor (less than 5% An) and partly altered edenitic amphibole, sometimes with re­ occurs only as interstitial grains and is thus later than the sorbed cores of augitic pyroxene; 5% anhedral Fe-Ti ternary feldspar. It is not present in the larvikite. The oxides; 2-10% anhedral, green-brown biotite; and 0-5% syenite and granite contain An-poor microperthite. Rep­ subhedral, unzoned oligoclase. There may be scattered resentative chemical compositions are presented in Table rectangular grains of a bluish-grey, cryptoperthitic alkali 2 and Fig. 4. NORSK GEOLOGISK TIDSSKRIFT 77 (1997) Porphyritic syenite, Oslo igneous province, Norway 127

Table l. Representative electron microprobe analyses of plagioclase. An Primocrysts

1 e 90 Larvikite Porph. syenite 20 80

456 595 548 559 561 582 o PJRPH. SYENJTE 30 70

O SYENiTE Si02 64.08 61.07 53.16 63.57 57.76 59.07 60

Al203 24.05 25.62 29.04 23.11 27.14 26.33 GRAN !TE 50 • 50 FeO 0.12 0.33 0.11 O.QI 0.18 0.28 o 60 Na20 9.22 7.69 5.67 2.75 7.03 7.03 o K20 0.65 0.78 0.23 0.50 0.37 0.58 30 CaO 4.31 6.69 11.01 4.96 8.30 7.53 TOTAL 102.42 102.19 99.27 94.89 100.96 101.00 20

Si 2.78 2.67 2.42 2.88 2.57 2.62 1 e Al 1.23 1.32 1.57 1.24 1.42 1.38 Fe2 O.oi 0.01 0.00 0.00 0.00 0.01 60 70 80 90 0 Or Fe3 0.00 0.00 0.00 0.00 0.00 0.00 Ca 0.20 0.31 0.52 0.45 0.40 0.36 Na 0.77 0.65 0.48 0.45 0.61 0.60 O.oi O.o3 K 0.04 0.04 0.03 0.02 Feldspars Fe3 0.00 0.00 0.00 0.00 0.01 O.oi Fig. 4. Chemical composition of feldspars of the rocks from the Mykle area. Or 3.56 0.46 1.25 5.65 2.05 3.27 Arrows indicate the variation from cores to rims of plagioclase grains. The Ab 76.66 64.62 47.63 47.25 59.26 60.75 analyses do not represent the true bulk compositions of perthitic grains because An 19.79 31.05 51.13 47.09 38.69 35.98 the areas of exsolved potassium feldspar have not been analysed.

Porph. syenite Zoned grains Pyroxenes 567 567 571 571 co re rim c ore rim The pyroxenes are usually homogeneous with regard to chemical composition (Table 3, Fig. 5). The contents of Si02 63.38 61.04 60.53 65.65 3 Na, AP+ and Fe + are generally low. In the larvikite the A1203 23.43 24.78 26.27 23.12 FeO 0.11 0.09 0.10 0.09 orthopyroxene is a ferrohypersthene and the clinopyrox­ Na20 9.46 8.42 7.69 10.31 ene ranges from to diopside. The syenites have K20 0.27 0.53 0.53 0.41 only augitic to diopsidic clinopyroxene with a slightly CaO 3.52 5.49 6.85 2.67 2 TOTAL 100.27 100.52 102.00 102.22 higher Fe + -content than in the larvikite.

Si 2.79 2.70 2.65 2.84 Al 1.22 1.29 1.36 1.17 Fe2 0.00 0.00 0.00 0.00 Amphiboles Fe3 0.00 0.00 0.00 0.00 Ca 0.17 0.26 0.32 0.13 In all the rock types the amphibole is edenite or ferroe­ Na 0.81 0.72 0.65 0.86 K O.OZ 0.03 0.03 0.02 denite (Hawthorne 1981). The amphibole is homoge­ neous on the sample scale and shows little variation Or 1.52 2.95 2.97 2.19 Ab 81.71 71.34 65.02 85.55 within each rock type. Selected chemical compositions An 16.78 25.70 32.00 12.26 are shown in Table 4 and Fig. 5. The amphiboles gener­ ally show a weak A-site occupancy (0.4-0.6). The amphibole replaces pyroxene, a process which has Table 2. Chemical composition of mantle- and interstitial feldspars. progressed furthest in the most evolved rocks. This re­ placement reaction resulted in the crystallization of cal­ Compositions of rims cite, which is present as minute, orange-luminescing Porph. syenite grains at the contacts between pyroxene and amphibole. Larvikite The reaction which formed the amphiboles may be illus­ 456 559 561 566 582 trated by the mineral assemblage: Or 35.89 29.86 29.82 33.73 33.43 Ab 48.70 56.88 55.46 50.91 56.01 5Ca(Mg, Fe)Si206 + 3C02 + (K, Na)A1Si308 + H20 An 15.41 13.26 14.72 15.36 10.56 au gite alkali feldspar

Compositions of interstitial feldspars = (K, Na) Ca2(Mg, Fe)5AlSi7022(0H)z Larvikite Porph. syenite edenite Syenite Gran i te 456 595 559 561 566 567 571 462 460 + 3CaC03 + 6Si02 calcite quartz Or 36.57 35.27 37.09 36.22 45.73 43.03 38.79 35.19 33.40 Ab 49.08 50.84 59.56 60.60 49.95 53.00 56.11 62.79 65.74 In rare cases the amphiboles are partly overgrown by An 14.37 13.89 4.35 3.18 4.32 3.97 5.10 2.02 0.86 annitic biotite and quartz. 128 N. W. Petersen & H. Sørensen NORSK GEOLOGISK TIDSSKRIFT 77 (1997)

Table 3. Representative electron microprobe analyses of pyroxene. Tab/e 4. Representative electron microprobe analyses of amphiboles.

Clinopyroxene Porph. syenite Orthopyroxene Larvikite Syenite Granite Larvikite Larvikite 456 566 582 582* 462 460 Porph. syenites 456 595 456 595 566 Si02 45.94 46.51 46.62 47.71 44.97 45.89 Ti02 1.20 1.75 1.46 1.07 1.74 1.06 Si02 52.08 51.77 52.46 52.30 51.78 Al203 7.00 6.12 6.21 7.54 6.57 4.10 0.25 0.54 0.57 0.17 Ti02 0.23 FeO 22.24 20.18 18.70 11.88 21.26 26.39 0.30 0.35 1.34 1.31 1.19 Al,o, MnO 0.56 0.61 0.65 0.33 0.72 1.14 0.00 0.00 0.43 0.26 0.46 Na20 MgO 9.04 10.06 l 1.36 15.02 9.13 5.64 1.41 1.27 0.64 0.53 0.84 Mn O Ca O 11.28 10.99 10.69 11.65 10.34 8.41 13.83 13.81 12.47 12.12 12.91 MgO Na20 1.61 2.25 1.79 1.93 2.28 2.73 2.35 2.02 18.97 19.09 16.75 CaO K20 0.96 1.05 0.95 1.06 0.95 0.88 FeO 30.31 30.96 13.17 13.88 15.65 Sum 99.82 99.53 98.42 98.18 97.96 96.23 TOTAL 100.51 10D.43 100.02 100.06 99.75 Si 6.90 6.97 6.99 6.94 6.89 7.32 Si 1.99 1.98 1.95 1.97 1.99 Al 1.24 1.08 1.10 1.29 1.19 0.77 AliV 0.01 O.D2 0.04 0.05 0.02 Ti 0.13 0.20 0.16 0.12 0.20 0.13 AlV! 0.01 0.00 O.D2 0.01 0.04 Fe2 2.75 2.53 2.25 1.45 2.72 3.51 Ti O .DI 0.01 0.02 0.02 O .DI Fe3 0.05 0.00 0.00 0.00 0.00 0.00 Fe2 1.01 1.02 0.38 0.42 0.44 Mn 0.07 0.08 0.09 0.04 0.09 0.15 Fe3 0.00 0.00 0.05 0.03 0.05 Mg 2.02 2.25 2.54 3.26 2.09 1.34 Mn 0.05 0.04 O.D2 0.02 0.03 Ca 1.82 1.76 1.72 1.82 1.70 1.44 Mg 0.85 0.85 0.72 0.71 0.72 0.47 0.65 0.52 0.54 0.68 0.84 Ca 0.10 0.08 0.78 0.77 0.69 Na 0.18 0.20 0.18 0.20 0.18 0.18 Na 0.00 0.00 0.03 0.02 0.03 K

En 42.61 42.71 38.18 36.92 38.36 • Amphibole enclosed in plagioclase. Fs 52.63 53.27 20.91 23.03 24.95 Wo 4.76 4.02 40.91 40.05 36.71

Clinopyroxenes Clinopyroxenes enclosed in plagioclase chemical compositions can be found in Tables 6 and 7. Porph. syenite Porph. syenite Syenite Larvikite In larvikite, magnetite ranges from Usp0 to Usp10, in 571 582 462 541 566 571 582 the syenites from Usp0 to Usp15• Ilmenite contains Hem0 to Hem8. The ilmenite of the red granite is most rich Si02 52.02 52.81 52.60 51.47 51.40 52.67 51.47 Ti02 0.25 0.09 0.14 0.69 0.52 0.37 0.81 in Mn. Al203 1.28 0.92 1.36 1.67 1.96 1.42 1.87 Na20 0.68 0.30 0.27 0.27 0.99 0.91 0.51 Mn O 0.89 0.81 1.05 0.61 0.69 0.65 0.67 MgO 10.70 11.68 10.35 13.03 11.40 10.76 12.98 Fluorite Ca O 18.34 18.54 16.21 19.02 19.72 16.99 20.55 FeO 16.70 15.13 17.81 13.03 13.86 16.14 11.55 Fluorides may have played an important role during the TOTAL 100.86 100.28 99.79 99.79 100.54 100.01 100.41 late- and post-magmatic stages of the consolidation pro­ Si 1.97 2.01 2.05 1.95 1.93 1.98 1.93 cess as indicated by the presence of scattered minute 0.04 0.00 0.00 0.06 0.07 0.02 0.07 AIIV interstitial grains of fluorite (identified by means of AlV! 0.02 0.04 0.06 0.02 0.01 0.04 0.01 Ti 0.01 0.00 0.00 0.02 O.D2 0.01 0.02 bright blue luminescence colours). Fe2 0.47 0.46 0.57 0.37 0.31 0.49 0.30 Fe3 0.06 0.03 0.00 0.05 0.12 0.04 0.06 Mn O.D3 0.03 0.04 0.02 0.03 0.02 0.02 Mg 0.61 0.66 0.58 0.74 0.64 0.63 0.73 Ca 0.75 0.76 0.68 0.77 o. 79 o. 70 0.82 Na 0.05 0.02 0.02 O.D2 0.07 0.07 0.04

32.71 35.03 31.03 38.88 36.21 34.56 38.87 En l ø 9Ø Fs 26.99 25.46 32.74 20.47 19.13 27.79 17.11 Wo 40.30 39.52 36.23 40.66 44.66 37.65 44.03 20 \ 80

o PORPH. SHN l TE 3Ø 70

o SYENITE 40 60 Biotite GRAN l 1f sø 50

The grains are homogeneous and there is no significant 6Ø 4Ø

difference between the composition of the biotite rims 70 3Ø around Fe-Ti oxides and the interstitial bio tite (Ta ble 5). BØ 20 The most silicic biotite is found in the red granite. 9Ø !Ø

� Fe-Ti oxides ø l ø 20 3Ø 4Ø 50 60 70 Ø Ø \ø Mg B 9 f e All the Fe-Ti oxides belong to the magnetite-ulvospinel Fig. 5. Chemical compositions of pyroxenes and amphiboles plotted in the 2 and the hematite-ilmenite solid solution series. The Mg-Fe +-Ca system. NORSK GEOLOGISK TIDSSKRIFT 77 (1997) Porphyritic syenite, Oslo igneous province, Norway 129

Table 5. Representative electron microprobe analyses of biotite. fluorescence and glass discs prepared with a sodium tetraborate flux. Na was analysed by atomic absorption, Larvikite Porph. syenite 2 Syenite Gran i te Fe + by titration, and volatiles by loss on ignition. Trace 456# 456 566# 566 462 460 elements were analysed on pressed powder pellets at the Institute of Geology, University of Copenhagen by X-ray Si02 36.70 35.82 36.24 36.94 37.55 37.93 Ti02 3.31 4.17 3.08 4.31 5.33 3.51 fluorescence using a Philips PW1400 and the techniques Al203 13.52 13.14 11.96 12.07 12.07 10.21 of Norris & Chappell ( 1977). Results were corrected for FeO 25.64 28.08 21.04 23.63 21.52 25.39 background interference from tube and spectral lines, Mn O 0.08 0.33 0.00 0.29 0.28 0.79 MgO 8.78 6.72 11.08 10.17 10.49 7.49 and for matrix variation (using the major-element com­ Na20 0.01 0.06 0.00 0.23 0.24 0.06 position). USGS standard AGV-1 was used for calibra­ KP 9.42 9.35 9.34 8.66 9.01 9.54 tion (Gladney et al. 1983). In a number of samples, trace TOTAL 97.43 97.66 92.73 96.30 96.51 94.92 elements were determined by instrumental neutron acti­ 2.89 2.86 2.94 2.92 Si 2.95 3.10 vation analysis at the Institute of Geology, University of Al 1.26 1.24 1.14 1.13 1.12 0.98 Ti 0.20 0.25 0.19 0.26 0.32 0.22 Copenhagen. Fe2 1.69 1.88 1.43 1.56 1.42 1.74 Fe3 0.00 0.00 0.00 0.00 0.00 0.00 Mn 0.01 0.02 0.00 0.02 0.02 0.06 1.03 0.80 1.34 1.20 Mg 1.23 0.91 The Geochemical data Na 0.00 0.01 0.00 0.04 0.04 0.01 0.94 0.96 0.97 K 0.87 0.90 0.99 The major elements (Table 8) are plotted in Harker

# Biotite occurring as rims on oxide minerals. diagrams (Fig. 6). The components of the mafic minerals, MgO, FeO + Fe203 and Ti02, in the larvikite, por­ Table 6. Representative electron microprobe analyses of ilmenite. phyritic syenite and syenite plot in clusters, but forming two parallel trends, the syenites located a little higher Larvikite Porph. syenite Syenite Granite than the larvikites due to the higher content of Si02 in 594* 595 566 567* 462 460 the syenites. The plot of FeO + Fe203 indicates an en­ richment in Fe in the mafic minerals from larvikite to Ti02 50.94 51.53 48.98 49.54 51.49 51.35 Mn O 0.90 2.48 3.67 2.22 3.45 9.70 syenite, in accordance with the chemical analyses of the FeO 48.59 47.68 48.56 47.50 45.24 37.91 minerals. MgO, FeO + Fe203 and Ti02 are negatively TOTAL 100.43 101.82 101.21 99.43 100.37 99.33 correlated with Si02, as are P205, Al203 and CaO, while Ti 0.96 0.96 0.91 0.94 0.97 0.98 K2O is positively correlated with Si02• This clearly illus­ Fe2 0.94 0.91 0.84 0.89 0.90 0.77 trates the feldspar variation from predominantly plagio­ Fe3 0.08 0.08 0.17 O.l! 0.05 0.04 Mn 0.02 0.06 0.08 0.05 0.07 0.21 clase in the larvikite, to exclusively alkali feldspar in the

Hem 2.00 2.10 4.65 2.95 1.35 0.98 syenite coupled with fractionation of mafic minerals. The lim 98.00 97.90 95.35 97.05 98.65 99.02 high contents of CaO and Al203 in two samples of larvikite (541 and 595) most probably reflect a feldspar • llmenite enclosed in plagioclase. cumulus origin. The possibility that these high contents could be caused by the presence of a more basic plagio­ Table 7. Representative electron microprobe analyses of magnetite. clase is unlikely, since the An component of the larvikite Porph. syenite feldspars only exceptionally exceeds 35%. Larv. Syenite Na20 varies in a non-systematic way, most probably 595 566 567* 462 as a result of late- and post-magmatic processes. Two Ti02 7.92 7.43 2.13 8.40 analyses of dioritic-gabbroic rocks (Pedersen 1994) are Mn O 0.55 0.73 0.16 0.68 plotted as examples of the most primitive igneous rocks FeO 87.43 88.68 90.33 86.18 TOTAL 97.63 96.84 93.71 95.26 found in the Mykle area, and some analyses of granite as examples of the most evolved rocks. Ti 0.22 0.21 0.06 0.24 Fe2 1.20 1.19 1.06 1.22 The plots of Sr and Rb in Fig. 7 illustrate the evolu­ Fe3 1.49 .58 1.84 1.52 tion from larvikite, rich in plagioclase, to syenite rich in 0.02 0.02 0.01 0.02 Mn alkali feldspar. The incompatible elements Nb, Th and Us 10.12 9.54 3.02 10.82 Zr (and Rb) of larvikite, porphyritic syenite and syenite Mt 89.88 90.46 96.98 89.18 are positively correlated with Si02, that is increasing • Magnetite enclosed in plagioclase. from larvikite over porphyritic syenite to syenite. Additional information about the behaviour of the trace elements is obtained from Figs. 8 and 9. The Geochemistry chondrite normalized REE pattems of porphyritic syen­ ite and syenite in Fig. 8 are practically identical and Analytical methods higher in REE than the larvikite. All these rocks are The major-element analysis was carried out at the Geo­ enriched in the light REE. The two types of syenite show logical Survey of Greenland, Copenhagen, using X-ray slight, and the granites pronounced, negative Eu anoma- 130 N. W. Petersen & H. Sørensen NORSK GEOLOGISK TIDSSKRIFT 77 (I997)

Table 8. Chemical analyses of the rock types of the Mykle area. Data for the gabbros supplied by L. Pedersen ( unpublished data).

Gabbro Larvikite Porph. syenite

Analysis #755 796# 456 541 594 595 559 561 566

Si02 51.99 52.94 58.62 57.30 59.91 57.53 60.74 59.80 58.81 Ti02 2.42 2.10 1.36 1.25 I. l l 1.33 1.33 1.43 1.42 Al203 15.98 16.42 17.39 18.77 17.64 18.09 16.17 16.16 16.36 Fe203 3.29 2.93 1.81 1.43 1.20 1.46 1.80 2.02 2.26 FeO 6.33 5.89 3.52 3.63 3.55 4.02 3.67 3.90 3.73 Mn O 0.17 0.16 0.13 0.12 0.12 O.l l 0.15 0.15 0.15 MgO 3.62 3.05 1.44 1.36 1.12 1.50 1.28 1.47 1.49 CaO 5.85 6.19 4.22 5.16 3.56 5.17 3.38 3.87 3.80 Na20 4.48 4.80 5.48 5.54 5.87 4.89 5.08 5.01 5.37 K20 2.81 2.88 4.42 3.27 4.14 3.43 4.61 4.28 4.68 P20s 0.72 0.77 0.55 0.60 0.45 0.60 0.47 0.56 0.54 Volat. 1.51 1.25 0.63 0.69 0.75 1.00 0.67 0.71 0.68 Total 99.17 99.38 99.57 99.12 99.42 99.13 99.35 99.36 99.29

Rb 114 120 144 109 155 112 211 201 215 Cs n.a. n.a. n.a. 3.08 3.65 n.a. 4.52 n.a. 2.65 Sr 990 1237 488 573 430 519 329 390 370 Ba 910 986 915 728 948 780 627 640 604 Ga 22 24 26 27 26 26 27 26 26 Cr lO 13 3.35 5.77 5 7.67 lO 8.40 Ni 31 32 4 5 3 6 2 4 3 V 163 148 40 45 32 52 38 55 54 Cu 45 35 lO 4 2 2 3 5 5 Pb 5 9 13 12 15 9 17 17 12 Zn 118 Il l 101 104 93.60 84 106 105 93 Zr 724 599 907 939 819 750 1190 1010 1300 Hf n.a. n.a. n.a. 24.20 19.80 n.a. 33.60 n.a. 31.90 Nb 91 81 137 119 117 99 165 151 204 Ta n.a. n.a. n.a. 7.92 7.77 n.a. 11.10 n.a. 13.20 Se 16 14 8 7.98 8.29 8 9.70 12 10.30 Ce 202 189 223 204 202 188 273 264 288 La 100 88 Il l 104 106 95 138 124 147 Nd 93 189 98 85.20 85 86 127 112 134 Sm n.a. n.a. n.a. 14.30 14.10 n.a. 19.20 n.a. 20 Eu n.a. n.a. n.a. 4.20 4.41 n.a. 3.69 n.a. 3.65 Tb n.a. n.a. n.a. 1.94 1.97 n.a. 2.85 n.a. 3.05 Yb n.a. n.a. n.a. 5.70 5.65 n.a. 8.97 n.a. 9.92 L u n.a. n.a. n.a. 0.80 0.77 n.a. 1.18 n.a. 1.26 y 68 64 66 57 59 59 87 83 95 Th 17 18 21 17.10 19.50 16 38.30 39 41.20 u 5 4 n.a. 5.03 4.54 n.a. 11.30 lO 8.68

Porph. syenite Syenite Alk. granite Gran i te Analysis 567 571 577 582 461 462 460 084# 097#

Si02 60.52 61.24 59.19 59.55 61.61 60.80 68.54 72.84 70.99 Ti02 1.30 1.28 1.52 1.46 1.14 1.27 0.54 0.29 0.37 Al203 16.17 16.01 16.25 16.12 15.81 15.83 14.56 13.00 13.88 Fe203 1.67 1.84 2.15 2.18 2.13 2.40 1.94 1.12 1.15 FeO 3.75 3.57 3.87 3.80 3.13 3.45 1.09 1.10 1.39 Mn O 0.14 0.15 0.15 0.16 0.14 0.15 0.09 0.09 0.10 MgO 1.29 1.30 1.57 1.48 1.12 1.31 0.32 0.16 0.24 CaO 3.35 3.26 4.08 3.85 2.07 2.29 0.46 0.19 0.30 Na20 5.13 4.87 4.96 4.73 5.23 5.05 5.11 4.40 4.85 K,O 4.71 4.60 4.25 4.47 5.47 5.33 6.22 5.31 5.67 P20s 0.45 0.44 0.62 0.60 0.35 0.42 O.Q7 0.02 O.Q3 Volat. 0.68 0.73 0.71 0.88 0.86 0.86 0.69 0.42 0.38 Total 99.16 99.29 99.32 99.28 99.06 99.16 99.63 98.94 99.35 Rb 237 218 191 206 270 250 306 282 265 Cs n.a. n.a. n.a. n.a. n.a. 4.83 8.40 n.a. n.a. Sr 328 326 425 426 212 216 61 10 10 Ba 582 594 695 788 657 700 847 43 66 Ga 27 27 25 27 27 28 28 27 27 Cr 4 3 4 5 6 4.95 4.34 3 15 Ni 2 4 5 4 5 5 2 4 4 V 42 40 55 51 30 38 4 <3 C u 2 4 2 2 6 3 3 <2 <2 Pb 14 17 15 13 18 18 29 Il lO Zn 94 117 103 119 108 103 77.70 38 80 Zr 1300 1270 1040 941 1430 1630 886 616 704 NORSK GEOLOGISK TIDSSKRIFT 77 (1997) Porphyritic syenite, Oslo igneous province, Norway 131

Table 8. (Continued).

Porph. syenite Syenite Alk. granite Granite Analysis 567 571 577 582 461 462 460 084# 097#

Hf n.a. n.a. n.a. n.a. n.a. 39.60 25.70 n.a. n.a. Nb 174 170 136 151 217 205 209 214 199 Ta n.a. n.a. n.a. n.a. n.a. 13.60 14.50 n.a. n.a. Se lO Il 13 lO 7 9.55 5.07 6 7 Ce 287 278 253 262 314 300 300 203 348 La 131 133 117 ll9 !58 148 142 102 165 Nd ll8 ll8 109 ll3 124 127 136 81 126 Sm n.a. n.a. n.a. n.a. n.a. 19.40 16.40 n.a. n.a. Eu n.a. n.a. n.a. n.a. n.a. 3.53 1.74 n.a. n.a. Tb n.a. n.a. n.a. n.a. n.a. 3.02 2.57 n.a. n.a. Yb n.a. n.a. n.a. n.a. n.a. 9.55 9.84 n.a. n.a. L u n.a. n.a. n.a. n.a. n.a. 1.36 1.31 n.a. n.a. y 89 93 81 87 87 90 77 78 95 Th 38 38 32 38 61 56 58.80 45 47 u n.a. n.a. n.a. 9 n.a. 12.40 12.40 n.a. n.a.

# = analyses supplied by L. Pedersen.

Iies indicating that the syenites and granites may have the main rocks of the province to be members of a been formed by feldspar fractionation processes. The differentiation series (e.g. Barth 1946). larvikite has a small positive Eu anomaly in accordance The field relations of the rocks from the Mykle region with the interpretation that the larvikite is partly of indicate a definite order of emplacement. The larvikite cumulate origin. crystallized first, it occurs as xenoliths in and/or is in­ The spidergrams of Fig. 9 again show practically iden­ truded by dykes and sheets of all other rocks; the por­ tical patterns for the two types of syenite, the only phyritic syenite is younger than both the larvikite and the marked differences are in Sr, as a result of the presence dioritic-gabbroic rocks, and it predates the nordmarkitic of plagioclase in the porphyritic type, and in Th, Zr and syenites and granites. Hf, which rnay be explained by a higher con tent of zircon The straightforward explanation of the origin of this in the syenite. When compared with the larvikite the sequence of rocks is a formation from successive pulses syenites are enriched in K, Rb, Th, Nb, Ta, Ce, Nd, Sm, of melts derived from a common source. Tb, Yb, Y, Zr and Hf, and impoverished in the compo­ Pedersen et al. (1995) have obtained precise U-Pb ages nents of the feldspars, Sr and Ba. from zircon in larvikite and ekerite from the Skrim area ranging from 281.2 ± 0.6 to 279.8 ± 0.7 Ma, that is em­ placement of the magmas within a very short period of time. Discussion No isotopic data are available for the rocks discussed It is generally accepted that the present level of exposure in this paper. It should, however, be pointed out, that M. of the Oslo igneous province represents a depth of 1-3 K. Andersen (1990) and Pedersen & Holm (pers. comm.) km below the land surface at the time of its formation have undertaken studies of the strontium isotope rela­ (Neumann et al. 1990). This means that the plutonic tions of larvikites and nordmarkitic syenites located, rocks crystallized at total pressures of 0.3 to l kbar. The respectively, to the west and to the south of the Mykle larvikites crystallized under rather dry conditions, while region. It is shown that the larvikites and nordmarkitic the miarolitic syenities and granites were formed under syenites in both cases have practically identical initial water-saturated conditions. Furthermore, all the plutonic 87Sr/86Sr ratios in the range of 0.7023 to 07039, and rocks of the Mykle area contain hypersolvus feldspars. exceptionally O. 7045, which is consistent with derivation The larvikites of the Oslo igneous province are consid­ of the larvikites and syenites from a common source. ered to have been formed from magmas derived by the These data also indicate that crustal contamination of fractional crystallization of basaltic melts which again the melts has been of minor importance. Field observa­ were formed by partial melting of a mildly depleted, tions in the area immediately to the west of the region somewhat heterogeneous mantle source region (e.g. Neu­ considered here have demonstrated, that the larvikite of mann 1980; Neumann et al. 1988; and Rasmussen et al. the Skrim batholith is underlain and injected by nor­ 1988). These authors also suggest that the syenites and markitic syenites. In the Mykle area the larvikite is granites of the province are not derived from the underlain by granitic rocks. larvikitic melts by simple fractionation, but that crustal The geochemical data of Table 8 and Figs. 6 and 7 contamination has played a major role in the formation may be taken as support of the derivation of the of these rocks. This is contrary to the views held by larvikitic and syenitic rocks from pulses of magma from previous investigators of the province, who considered a common source. It should, however, be horn in mind 132 N. W. Petersen & H. Sørensen NORSK GEOLOGISK TIDSSKRIFT 77 (1997) 20 5 /:;. 18 /:;. 4 �:;.!:>. A ;;R.o ;;R.o A 3 A � 16 A C') � � o o X � • �2 <( 14 • Æ'�OCJ 12 1

X • 10 o • 45 50 55 60 65 70 75 45 50 55 60 65 70 75 Si02wt% Si02wt%

3 10 A A A #.a � #.2 A C') � '&6 C\1 Q) �:o�/:;. o LL /r§o + 4 i= /:;.. o 1 X Q) • LL • X 2 • • o o 45 50 55 60 65 70 75 45 50 55 60 65 70 75 Si02wt% Si02wt%

7 6 /:;. A t;.ll D 6 A 5 A �:;. X • ... 'ffb 5 � • ;;R.o 4 ;;R.o � 4 o ' � 3 ca 3 o (.) o &2 2 o z 1 X • • o o 45 50 55 60 65 70 75 45 50 55 60 65 70 75 Si02wt% Si02wt%

6 X • 1.0 5 r:P • �fu 0.8 A A #.4 ;;R.o � � 0.6 It::.lb 3 fr LO � '& AA o � c[' 0.4 � 2 o 0.2 X o 0.0 • 45 50 55 60 65 70 75 45 50 55 60. 65 70 75 Si02wt% Si02wt% "-Gabbro t;. Larvikite o Porph. syenite o Syenite x Red granite • Alkali granite

Fig. 6. Variation diagram of the major elements in the rocks from the Mykle area plotted against silica. Data of gabbros supplied by L. Pedersen (1994). NORSK GEOLOGISK TIDSSKRIFf 77 (1997) Porphyritic syenite, Oslo igneous province, Norway 133

1800 70 1600 o o 60 X o 1400 o o o0 50 • E 1200 o • a. 0 E 40 6\sb 0.1000 6. a...... 6. X a. N '8 o 800 6. .c Å 6. • 30 1- 600 Å • 20 ÅÅ 6.6. 400 � 200 10 o o 45 50 55 60 65 70 75 45 50 55 60 65 70 75 Si02wt% Si02wt%

250 350 o • X o o X • 300 • 200 o • 250 o � o E 150 a> Og,cP a. l:.o §_ 200 o a. a. .o 6. 6. .o 6. z Å 6. 150 6. 100 a: Å Å Å di> 100 50 50

o o 45 50 55 60 65 70 75 45 50 55 60 65 70 75 Si02wt% Si02wt%

1000 Å 1400 Å 6. 6. Å X 1200 800 6. o 6. o o 1000 Å 0 o o0 E 600 oo E a. a. 800 a. a. ro ...... 600 co 400 en i6. 400 oaa(Il) 200 200 DO • • X o o 45 50 55 60 65 70 75 45 50 55 60 65 70 75 Si02wt% Si02wt%

Fig . 7. Variation diagram of selected trace elements in the rocks from the Mykle area plotted against silica. Data of gabbros supplied by L. Pedersen ( 1994).

that only the massive types of larvikite and syenite may have almost identical contents of MgO, while the latter represent liquid compositions, which is definitely not the rock has slightly higher contents of FeO + Fe203 and case for the porphyritic and cumulative rock types. Ti02 than the larvikite (Table 8, Fig. 6). The clinopyrox­ The plots of CaO, Al203, K20, Ba and Sr in the enes of the porphyritic syenite have slightly higher con­ variation diagrams of Figs. 6 and 7 indicate that feldspar tents of FeO + Fe203 than those of the larvikite (Table fractionation has played a major role in the evolution of 3). Orthopyroxene is only found in the larvikite. The the rock series larvikite - porphyritic syenite - syenite. plots of MgO, FeO + Fe203, Ti02 and P205 of larvikite This view is supported also by the Eu data of Fig. 8, with and porphyritic syenite show a negative correlation with a slight positive Eu anomaly in the larvikite, weak nega­ Si02• They form two parallel trends, the porphyritic tive anomalies in the two syenites and a strong negative syenite with the highest content of Si02 lying above the anomaly in the granite. plots of the larvikite. The plots of the syenite continue The components of the mafic rninerals show a less the trend of the porphyritic syenite. All these features clear picture. The larvikite and the porphyritic syenite and the diagrams of Figs. 8 and 9 may be taken as 134 N. W. Petersen & H. Sørensen NORSK GEOLOGISK TIDSSKRIFT 77 (1997)

!:::, LRRV IKITE and the granites are respectively older and younger than the syenites. The gabbrojdiorite contains xenoliths of 0 PORPH. SYENITE larvikite. Furthermore, the Harker diagrams of Fig. 7 show a positive correlation between the contents of Rb, 0 SYENITE Zr, Th, Nb and Si02 in larvikite, porphyritic syenite and GRRNITE syenite, while gabbro/diorite and the granites do not plot X along this trend. The distinct incompatible behaviour of some trace elements of the larvikite, porphyritic syenite and syenite is demonstrated in Figs. l O and 11, where some of the element ratios and concentrations discussed by Ras­ mussen et al. ( 1988) are plotted against respectively Si02 and Th, giving additional support to the view that there is a genetic relation beween larvikite and the two syenites 10 studied in this paper. Lo Ce Nd Sm Eu Tb Yb Lu The porphyritic syenite presents, however, an enig­ Fig. 8. Chondrite normalized REE diagrams for the Lake Mykle rock types, ' matic case. It only occurs in the proximity of the large except gabbro. (Norrnalization values from Hanson (1978)). larvikite massif and forms an independent intrusion with generally sharp contacts against the larvikite. Locally t::. Lorvikite there are, however, transitions between these rocks, the O Porph. Syenite plagioclase phenocrysts of the syenite giving the impres­ O Sy enite sion of representing the feldspars of disintegrated X Red Gronite larvikite. The mineralogical composition of the matrix of the porphyritic syenite is practically identical to that of 10000 10000 the nordmarkitic syenite. "' "' Andersen (1984) has interpreted one particular occur­ ... "U 1000 1000 ... c "O rence of porhyritic syenitic rocks, the larvikite 3 of the o c � o Sande cauldron, as being hybrid rocks in the contact u �· 100 100 u ' zone between consolidated larvikite and a nordmarkitic ., .::< u .::< o 10 10 u a:: ·� o 55 a:: o D 50 o Do Bo RbTh K NbTo loCeSrNd PSmZr Hf Ti Tb Y Yb o 45 Fig. 9. Spider diagrams for the Lake Mykle rocks, norrnalization according to o X Thompson et al. ( 1984). �4ctl 0 o support of the idea that fractionation of plagioclase, o pyroxene, iron-titanium oxides and perhaps apatite 35 has played a major role in the crystallization of the 30 t:.. melts from which larvikitic and porphyritic syenite lh t:.. were formed, while the scarce data on the syenite 25 indicate a genetic relationship with the porphyritic syen­ 55 60 65 70 ite. Rasmussen et al. ( 1988) discussed the petrogenetic Si02 wt% relations of the intermediate and silicic magmatism of the Oslo rift. They concluded ( op. cit., p. 302) that the 6 trace-element relations of syenites and granites do not 5 support the assumption of fractional removal of feldspar. X There is, for instance, a wide variation in element ratios 4 such as Rb/Sr, K/Ba, Zr/Th and Ta/Th in rocks with .... . similar concentrations of Si02 or Th. The lack of distinct � 3 trends in the various trace-element diagrams also ex­ a: 2 cludes the possible effect of removal of minor phases in addition to feldspar. Therefore, in their view, the trace­ D 1 D element relations have also been affected by processes o CO o � l r:f! other than fractional crystallization. Our data support o the conclusion that there is no direct genetic relation 55 60 65 70 between the syenites and the gabbro/diorite and the Si02wt% granites in the region studied by us. The gabbro/diorite Fig . JO. Variation diagram of the K/Ba and Rb/Sr ratios plotted against Si02 • NORSK GEOLOGISK TIDSSKRIFT 77 (1997) Porphyritic syenite, Oslo igneous province, Norway 135

600 350 - .6. .6. 300 X 500 .6. • • o .6. o o 250 o o 400 o E o � o <9 [ 200 o 8: a. .... 300 .6. ..c 150 .6. Cf) a: o o 200 100 �

100 50 X o 0 15 20 25 30 35 40 45 15 20 25 30 35 40 45 50 55 60 65 50 55 60 65 Th ppm Th ppm

170 1700 • o 160 o 1500 150 o o o X 140 o 1300 8 o E [ 130 8 o a. o §: .... 11 00 ttl 120 o o o o N .6. .6. .6. o ...J 11 0 .6. 900 X .6. • .6. 100 .6. .6. 700 • 90 • ao+-�--��--r-���--�-- 500+-�--r-��--��--�� 15 20 25 30 35 40 45 50 55 60 65 15 20 25 30 35 40 45 50 55 60 65 Th ppm Th ppm

Fig . 11. Variation diagram of Sr, Rb, La and Zr plotted against Th.

magma. This explanation cannot, in our opinion, be The similarity between the matrix of the porphyritic applied directly to the Mykle occurrence. The porphyritic syenite and the nordmarkitic syenite suggests that syenite at Mykle forms a true intrusion with generally syenitic magmas may also develop by differentiation of sharp contacts with the larvikite. The hybridization larvikitic melts. Such an origin of the syenite is in accor­ mechanism presented for the Sande larvikite 3 cannot dance with the geochernical data of Table 8 and Figs. 6 explain that disintegrated grains of larvikitic feldspar and 7, which indicate that fractional crystallization may have been distributed evenly in a large volume of the have played an important role in the formation of the intruding syenitic melts, and that they are even found in syenites. thin dykes cutting the larvikite. This interpretation has one inherent difficulty: the fact The textural, mineralogical and geochemical data that the already consolidated larvikite was intruded by could be explained as follows: The plagioclase phe­ dioritic-gabbroic rocks before the emplacement of the nocrysts of the porphyritic syenite are so similar to the porphyritic syenite. The objection that there was an plagioclase of the larvikite that a common origin may be emplacement of dioritic-gabbroic melts between the fo r­ envisaged. The porphyritic syenite may therefore also be mation of the larvikite and the later syenites may be met regarded as a product of extended crystallization of the with the assumption that there were several intrusive larvikitic magma caused by an increase in the contents of phases of larvikite with an interrnittent intrusion of water, other volatiles and incompatible elements in re­ dioritic-gabbroic rocks. This is in accordance with the sponse to the crystallization of the anhydrous minerals: observation that there are several intrusive phases of feldspars, pyroxenes, iron-titanium oxides, and perhaps larvikitic rocks a few kilometres to the south of the apatite. Such melts may then have collected in 'pockets' Mykle area (Holm & Sørensen, in prep.), and with the in the larvikitic magma chamber. In places where the demonstration of the successive emplacement of larvikite was still semi-molten, there was a transition to larvikitic rocks in the Larvik area further to the south the porphyritic syenite. Where the larvikite was already (Petersen 1977). The exposures in the Mykle area are so completely crystallized, but still hot, some distance from few and poor that it has been impossible to distinguish the roof and walls, the residual magma would break up separate phases of larvikite. There are, however, dis­ the larvikite and form dykes with sharp contacts (but tinctly different types of larvikite. One type with basic without any chilled margins), and pick up xenoliths inclusions is located immediately to the south of the which could disintegrate. porphyritic syenite. This indicates that a forma tion of the 136 N. W. Petersen & H. Sørensen NORSK GEOLOGISK TIDSSKRIFT 77 (1997) porphyritic syenite by accumu1ation of vo1ati1e-rich melts of Copenhagen. The contributions by the heads of these laboratories, Jørgen Kystol, John Bailey, Raymond Gwozdz and Jørn Rønsbo are gratefully acknowl­ in cupo1as at the top of a 1arvikitic magma chamber and edged. The Danish Natura! Science Research Council covered the expenses of the in fractures in the a1ready consolidated larvikite cannot be field work, and the study visit in Edinburgh was funded by an ERASMUS grant excluded. awarded to Niels Westphal Petersen.

A problem common to all attempts at explaining the Manuscript received July 1995 origin of the porphyritic syenite is the fact that the density of the dry melt of the composition of this rock, according to the method of Bottinga & Weill ( 1970), varies from 3 2.50 to 2.54 gfcm at temperatures ranging from 1100 to References 900°C. This is a maximum value taking the volatile Andersen, M. K. 1990: En petrologisk og geokemisk undersøgelse af Fjellvannkom­ con tent of the melt into consideration. The density of the plekset: Sydlige Oslofelt, Norge. cand.scient.dissertation, University of Capen­ 3 hagen, l 06 pp. plagioclase phenocrysts, 2.6 to 2.7 gfcm , is distinctly Andersen, T. 1984: Crystallization history of a Permian composite monzonite-alkali higher than the density of the melt. This is difficult to syenite pluton in the Sande cauldron, Oslo rift, southem Norway. Lithos 17, reconcile with the even distribution of the phenocrysts 153-170. Andersen, T. 1990: Melt-mineral-fiuidinteraction in peralkaline silicic intrusions in even in thin dykes. the Oslo Rift, southeast Norway. IV: Fluid inclusions in the Sande cauldron. The most likely explanation of the porphyritic texture Norges geologiske Undersøkelse Bulletin 41 7, 41-54. is that the magma was on its liquidus at the time of Andersen, T. & Sørensen, H. 1994: Crystallization and metasomatism of nepheline syenite xenoliths in quartz-bearing intrusive rocks in the Permian rift,SE Norway. emplacement, resulting in an initial rapid crystallization Norsk Geologisk Tidsskrift 73, 250-266. of plagioclase throughout the melt. The fine-grained Barth, T. F. W. 1946: Studies on the complex of the Oslo region. Il. matrix is an indication that crystallization of the intercrys­ Systematic petrogra ph y of the plutonic rocks. Sk r. Norske Videnskaps-Akadem i, Oslo . l. Ma t.-Naturv. Kl. 9, 1-104. tal melt prevented the phenocrysts from sinking. Bottinga, Y. & Weill, D. F. 1970: Densities of liquid silicate systems calculated from partial mola! volumes of oxide components. American Journal of Science 269, 169-182. Gladney, E. S., Bums, C. E. & Roelandts, I. 1983: 1982 compilation of elementa! Conclusion concentrations in eleven United States Geological Survey rock standards. Geostandards Newsletter 7, 3-226. The new occurrence of porphyritic syenite in the Oslo Hanson, G. N. 1978: The application of trace elements to the petrogenesis ofigneous igneous province described in this paper is located in the rocks of granitic composition. Earth and Planetary Science Letters 38, 26 -43. boundary zone between a southern massif dominated by Hawthorne, F. C. 1981: Crystal chemistry of amphiboles. In Veblen, D. R. (ed.): Arnphiboles and other hydrous pyriboles - mineralogy. Reviews in Mineralogy . larvikitic rocks and a northern massif of ekeritic granites. Vo l. 9A . Mineralogical Society of Arnerica. It is only found in the immediate vicinity of a larvikite rich Holm, P. M. & Sørensen, H. (in prep.): The geological map sheet Siljan, southeast in plagioclase and containing orthopyroxene. The phe­ Norway. Morogan, V. Søresen, H. 1994: Net-veined complexes in the Oslo Rift, southeast nocrysts of the porphyritic syenite consist of plagioclase & Norway. Lithos 32, 21-45. very similar to the plagioclase of the larvikite. The Neumann, E.-R. 1976: Composition'al relations among pyroxenes, amphiboles and porphyritic syenite is associated with nordmarkitic syenite other mafic phases in the Oslo Region plutonic rocks. Lithos 9, 85-109. Neumann, E.-R. 1980: Petrogenesis of the Oslo Region larvikites and associated and is intersected by nordmarkite and by red granite and rocks. Jo urnal of Petrology 21, 499-531. ekerite. The petrological and geochemical data indicate Neumann, E.-R., Tilton, G. R. & Tuen, E. 1988: Sr, Nd and Pb isotope geochemistry that fractional crystallization has played a major role in of the Oslo rift igneous province, southeast Norway. Geochimica Cosmochimica Acta 52, 1997-2007. the formation of the larvikite, porphyritic syenite and Neumann, E.-R., Sundvoll, B. & Øverli, P. E. 1990: A mildly depleted upper mantle nordmarkite, white crustal contamination of the melts has beneath southeast Norway: evidence from basalts in the Permo-Carboniferous been of minor importance. The geochemical data indicate Oslo Rift. Tectonophysics 178, 89-107. Norris, K. & Chappell, B. W. 1977: X-ray fiuorescence spectometry. In Zussman, J. that the granitic rocks cannot be derived from the syenitic (ed.): Physical Methods in Determinative Mineralogy, 2nd ed., 201-272. Aca­ melts by simple fractionation processes. The larvikite, demic Press, London. porphyritic syenite and nordmarkite are interpreted to Parsons, I. & Brown, W. L. 1983: A TEM and microprobe study of a two-perthite alkali gabbro: implications for the temary feldspar system. Contributions to have been formed by crystallization of pulses of magma Mineralalogy and Petrology 82, 1-12. from a common source or, alternative1y, the porphyritic Pedersen, L. 1994: Gabbroic-dioritic rocks in the Mykle area, southeast Norway. syenite and the nordmarkite may have been formed by Cand. scient. dissertation, University of Copenhagen. Pedersen, L. E., Heaman, L. M. & Holm, P. M. 1995: Further constraints on the extended crystallization of a larvikitic melt enriched in tempora! evolution of the Oslo Rift from precise U-Pb zircon dating in the volatiles. The texture of the porphyritic syenite is a result Siljan-Skrim area. Lithos 34, 301-315. of earl y nucleation and growth of plagioclase primocrysts Petersen, J. S. 1977: Structure of the larvikite-lardalite complex, Oslo region, Norway, and its evolution. Geologische Rundschau 67, 330-342. throughout a super-cooled melt. Petersen, N. W. 1992: An investigation of feldspars in monzosyenitic rocks near Lake Mykle, the Oslo Region, southeast Norway. 20th Nordic Geological Acknowledgements. - This paper presents some of the results of a thesis work Win termeeting, Reykja vik 1992, p. 132. Abstr. carried out by the first-named author at the Geological Institute, University of Petersen, N. W. (in prep.): The evolution of feldspars in larvikite from the Mykle Copenhagen and at the Department of Geology and Geophysics, University of area, the Oslo rift, southeast Norway. Edinburgh, Scotland. Professor I. Parsons, Edinburgh, Professors T. Andersen Rasmussen, E., Neumann, E.-R., Andersen, T., Sundvoll, B., Fjerdingstad, V. & and E.-R. Neumann, Oslo, Professor Brian Robins, Bergen, and Dr. Ø. Nordgu­ Stabel, A. 1988: Petrogenetic processes associated with intermediate and silicic len, Trondheim, are thanked for helpful discussions and for constructive criticism magmatism in the Oslo rift, southeast Norway. Mineralogical Magazine 52, of the manuscript. Cand. scient. L. Pedersen provided chemical analyses of 293-307. gabbros. The chemical analyses of the rocks were carried out at the Geological Thompson, R. N., Morrison, M. A. , Hendry, G. L. & Parry, S. J. 1984: An Survey of Greenland, Copenhagen (now GEUS), and the trace element analyses assessment of the relative ro les of crust and mantle in magma genesis: an elementa! and the electron microprobe analyses at the Geological Institute, the University approach. Philosophical Transactions Royal Society, London, AJ/O, 549-590.