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Petrology and Metallogeny Associated with the Tryvann Granite Complex, Oslo Region

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Petrology and Metallogeny Associated with the Tryvann Granite Complex, Oslo Region Petrology and metallogeny associated with the Tryvann Granite Complex, Oslo Region ODD NILSEN Nilsen, O. 1992: Petrology and metallogeny associated with the Tryvann Granite Complex, Oslo Region. Nor. geol . unaers . Bull. 423, 1-18 . The Tryvann Granite Comple x (TGC) comprises four separate intrus ive bodies within the Oslo Graben . It constitutes a diff erentiated suite of young (240 - 245 Ma) alkaline quartz syenites and granites derived from a stron gly fractionated , hydrou s residual melt which was emplaced duri ng the collapse of the Brerum Cauldron. The emplacement occurred contemporaneously with the formation of the ring-dyke comple x. Intramagmatic vein-type molybden ite, pyrite and Fe-oxide mineralizations occur associated with a widespr ead albitic hydrothermal alteration confined chiefly to fracture zones within the TGC complex. On a regional scale, the geological pattern, as revealed by gravimetric anomali es, and the spatial metamorphic and rnetasomati c alter ation patterns adja­ cent to the southern part of the Brerum Cauldron , indicate the existenc e of a (partly) hidden, NE-SW trending , elongated plutonic comple x to which the TGC may be related. This complex apparently follows a central volcani c axis which constitutes a possible plutonic link between the Brerum and Nittedal Cauldrons of the Oslo Graben . O. Ni/sen, Institutt for Geolog i, Universitetet i Oslo , 0316 05/0 3, Norway. Introduction The Oslo Rift was magmat ically active for The Oslo Paleorift constitutes a well documen­ more than 60 million years. Magmatism began ted continental rift system of Permo-Carbonife­ at about 300 Ma (Late Carboniferous), and the rous age. On land, the Oslo Rift forms a rift went through several intrusive and vol­ major graben structure - the Oslo Graben canic periods up to about 240 Ma (Sundvoll (Ramberg 1976) - which has been extensive­ et aI.1990). The tectonomagmatic evolution ly studied for more than a century . Geophysi­ of the Oslo Rift has been subdiv ided into diffe­ cal data have shown that the rift continues to rent stages by Ramberg & Larsen (1978), the SSW into the Skagerrak sea, where it is Larsen & Sundvoll (1982) and Sundvoll et al. bounded by Mid to Late Precambrian gneisses (1990). After initial fissure eruptions of plateau and migmatites (Ro et al. 1990). Comprehensi­ lavas of basaltic and intermediate, trachytic ve summaries of the Oslo Rift geology have composition, a period of central volcano activi­ been presented by Oftedahl (1960, 1980), ty associated with cauldron collapses and ring­ Ramberg (1976), Dons & Larsen (1978), Neu­ dyke formation took place. This central vol­ mann & Ramberg (1978) and Neumann (1990). cano stage seems to have continued into the The Oslo Graben has been subdivided into main intrusive period with the emplacement two major graben segments, termed the Vest­ of large batholiths of monzonite (Iarvikite), fold Graben (southern) and the Akershus Gra­ syenite and granite . The petrogenesis of these ben (northern) . Within the graben the Precam­ rocks has recently been extensively studied brian basement is overlain by pre-rift Cambro ­ by Neumann (1978,1980,1988), Neumann et Silurian metasediments, which are folded in al.(1988) and Rasmussen et al.(1988). the central and northern part of the Oslo Regi­ Within the Oslo Graben magmatic rocks , on. They are inconformably overlain by a thin chiefly of the alkaline kindred , cover more horizon of predominantly continental, Upper than 75% of the area. Along with the petrologi­ Carboniferous metasediments. However, Per­ cal and geochemical research, the metalloge­ mian volcanites, metased iments and plutonic netic aspects of hydrothermal, late- and post­ rocks make up the main lithologies of the magmat ic processes assoc iated with the mag­ Oslo Graben. matic activity have been investigated. Compre - 2 Odd Ni/sen GU . BULL. 423. 1992 LEGEND : .,. + + Perm ian + + p lutonic roc ks ~... ... Perm ia n IIJ volcan ic rocks Camb ro - Siluri a n D meta sed imen ts 10 Precambria n I gnei sses I 1111111111111111111 1111111111 Fig. 1: Key map of the Oslo distr ict with main geological elements. From aterstad & Dons (1978). Inset map (A): The Oslo Rift wit h graben faults (stippled lines) and Permian igneous rocks (in black). hensive accou nts of the geological evolution intramagmatic Mo-deposits which include both and metallogeny of the Oslo Paleorift have vein- and stockwork-type mineralization. Ba­ recently been presented by Vokes (1 973, sed on their petrography and field and age 1988), Ihlen (1 978,1986), Ihlen & Vokes (1 978) relationsh ips, the biotite granites (BG) of the and Bjerlykke et al.(1990), and intramagmatic Oslo Graben have been classified by Gaut and related exocontact depos its of molyb­ (1981) into two major groups (BG1 and BG2). denite have been described as an important BG1 comprise the older (270-260 Ma) subset­ class of hydrothermal ore deposits in the vus granites which occur as large batholithic Oslo Graben (Geyti & SchOnwandt 1979, Ihlen bodies emplaced during the early batholith et al.1982, Schonwandt & Petersen 1983, Pe­ stage (e.g. the Finnemarka and Drammen gra­ dersen 1986). The molybdenite depos its are nites) (Fig.1). The BG2 granites are younger related to highly differe ntiated extrusive and (250-240 Ma), hypersolvus alkali-feldspar gra­ intrusive granitic rocks , temporally and spatial­ nites which in general form smaller, stock-like ly assoc iated with the calder a format ions and bodies. They probab ly evolved as late-stage batholith emplacements during different stages differentiation products of syenites, to which in the tecto nomagmatic evolution of the Oslo they have transitional contacts (e.g.in Hurdal) Rift. (Pedersen 1986). Biotite granites are the major host for the Both types occur in the central and north- NGU · BULL. 423. 1992 Petrology and metaffogeny 3 GEOLOGICA L MAP Of THE SOUTHERN OSLO NORDMARK REGION + + Explosion bre ccio + + Alka li-feldspar sve nite [ Nc rd mc rkite ] & G+ + + syen ite (G re fse n - sye nite] Rhyolitic a nd tra chytic fe lsites Coar se-grained monzod iori te o (Kje lso sile ) Gro nite - & sye nite po rphyry Fine- gra ined monzodiorite ( Ring - dyke com p lex ) [ Ake rite] A lkal i- fe ldspar gra nite TRYVA NN C7\l Trochytic (rhomb- porphyry) loves & ~ ba sol ts GRANITE Alko li- fel dsp'" COMPLEX Combro - Sil urion - quartz sye nite o me tas edimen ts Major fault tram wa y O .N . 19 91 Fig. 2: Geological map of the southern Oslo Nordmark region. 4 Odd Nilsen GU . BULL .23. 1992 ern part of the Oslo Graben and, together The Tryvann granite cons titutes a composite with alkali-feldspar syenites and alkali granites stock-shaped intrusion situated close to the (ekerite), const itute most of the pluton ic rocks eastern boundary of the Bcerum Cauldron. It in the Nordmarka-Hurdalen Syenite Complex includes an eastern segment of medium- to of the Akershus Graben Segment (Ramberg fine-grained, part ly porph yritic, alkali-feldspar 1976). In the Oslo district this plutonic comp ­ quartz syenite , and a western segment of fine­ lex borders the Cambro-Silurian metasedi­ grained, granophyric, alkali-feldspar granite ments of the lowland of Bcerum and Oslo and of a greyish-pink colour. In the field the bounda­ the Bcerum Cauldron (Fig.1). Geological maps ry betw een the two lithologies is transitional of the area under cons ideration at 1:50 000 over a distance of approximately 100 m. The scale have been compiled by Holtedahl & Skadalen, Aklungen and Hammeren Granites Dons (1952) and Naterstad et al.(1990). are satellite intrusions to the main Tryvann Between the lakes Bogstadvann and Mari­ granite body. They are all fine-grained, grano­ dalsvann four separate granite bodies intrud e phyric, alkali-feldspar granites with irregular the different syenitic rocks of the Nordmarka­ contacts. Hurdalen Syenite Complex (Fig.2). The Tryvann The Tryvann granite has an important spati­ granite is the largest of these; and tog ether al and tectonomagmatic relationship with the with the intrusions at Skadalen, Aklungen and Bcerum Cauldron (Figs.1 & 3). The geology Hammeren it forms the Tryvann Granite Comp­ of the Bce rum Cauldron has previously been lex (TGC), as revealed by the common field described by Holtedah l (1943), Seether (1945, relations, petrology and geochemical composi­ 1962) and Oftedahl (1952,1953,1978). It has a tion of these four bodies. semi-elliptic outline, measuring c. 8.5 x 10 km, and const itutes one of the best preserved The aim of this paper is to review the petro ­ cauldron structures of the Oslo Rift. It is com­ logy and geochemistry of the TGC, with particu­ posed of members of the younger lava series lar reference to the associated late- to post­ of the Oslo region which, in a pre-cauldron magmatic hydrothe rmal ore mineralization. The stage. were intersected by fine- and medium­ tecto nomagmatic relationship of the TGC with grained monzod iorites (akerite), syenitic plu­ the Bcerum Cauldron will be discussed . Some tons and dykes , as well as hypabyssal porphy­ preliminary ideas on the association between ritic rhyolitic and syenitic felsites (e.g.the Lat­ the TGC and adjacent ore mineralizations asso­ hus porphy ry (Oftedahl 1946,1957), and the ciated with the late-magmatic episodes in the Serked al porphy ry (Holtedahl 1943)). Subsequ­ development of the Oslo Rift have previously ent igneous activity led to the format ion of been presented by Nilsen (1990). numerous igneous breccia vents , ignimbrites and volcaniclastic metased iments. To the west the cauldron borders the lava series at Kolsas and Krokskogen; to the nort h and to the east the various monzon itic and syenitic plutonic Geological setting complexes of the Nordmarka-Hurda len Syenite Complex; and to the south the Cambro -Siluri­ The Tryva nn granite constitutes the major gra­ an metasediments of the Oslo region.
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