MINERALOGICAL EVIDENCE for an OPHIOLITE from the OUTOKUMPU SERPENTINITES in NORTH KARELIA, FINLAND JOUNI VUOLLO and TAUNO PIIRAINEN
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MINERALOGICAL EVIDENCE FOR AN OPHIOLITE FROM THE OUTOKUMPU SERPENTINITES IN NORTH KARELIA, FINLAND JOUNI VUOLLO and TAUNO PIIRAINEN VUOLLO, JOUNI and PIIRAINEN, TAUNO, 1989: Mineralogical evidence for an ophiolite from the Outokumpu serpetinites in North Karelia, Finland. Bull. Geol. Soc. Finland 61, Part 1, 95—112. The serpentinites of the Outokumpu complex located in the early Proterozoic schist belt of North Karelia in Eastern Finland are investigated petrographically, mineralogically and chemically, with an emphasis on relict minerals, especially chrome spinel, and also olivine and orthopyroxene. The relict minerals and tex- tures provide evidence for an ophiolite of the Alpine type in the complex, and for the opinion that the serpentinites have originated from residual mantle perido- tites and overlying olivine cumulates. The mantle peridotites, being serpentinized to chrysotile rocks, contain relicts of olivine (Fo9|_93), orthopyroxene (100*Mg/ (Mg + Fe) = 92—94) and anhedral, euhedral or interstitial Cr-rich chrome spinel (Cr/(Cr +Al) 0.75—0.98 and Mg/(Mg + Fe-*) 0.1—0.4), while the cumulates have been serpentinized completely to antigorite rocks, being characterized by euhedral Al-rich chrome spinel (Cr/(Cr + AI) 0.65 and Mg/(Mg +Fe2 + ) 0.6). Key words: serpentinites, residual mantle peridotites, olivine cumulates, Alpine type ophiolite, Proterozoic, Outokumpu complex, Finland. Jouni VuoUo and Tauno Piirainen: Department of Geology, University of Oulu, Linnanmaa, SF-90570 Oulu, Finland. Introduction The research carried out to date provides two kinds of picture of the status of the complex with- The Outokumpu complex, assemblage, associ- in the North Karelia schist belt. One interpreta- ation or Steinmann trinity, whichever one may tion is that the zone is an autochthonous forma- choose to call it, is a zone of the North Karelia tion in a geosyncline into which magmas of man- schist belt in eastern Finland which is famous for tle origin have erupted (Vähätalo 1953; Huhma its Cu-Zn-(Co-Au) sulfide ores. The ore critical and Huhma 1970; Huhma 1975, 1976; Piirainen nature of this zone, together with the problems 1969, 1976; Peltola 1978; Simonen 1980; Park associated with it, has meant that it has been a and Bowes 1981; Mäkelä 1981; Bowes et al. 1984; focus of continuous research in the fields of Park 1984, 1988; Papunen 1987), while the oth- mineralogy, petrology, geochemistry, structural er maintains that it is an ophiolite complex, i.e. geology and ore geology ever since it was first dis- a tectonic allochthon derived from the oceanic covered in 1910 (Frosterus and Wilkman 1920; crust of 1.97 Ga age (Koistinen 1981, 1986, 1987; Trüstedt 1921). Ward 1987), a notion which has gained support 96 Jouni Vuollo and Tauno Piirainen from research into the Kainuu schist belt (Kon- to the discovery of amphibolites with relict struc- tinen 1987). Earlier work which also provides a tures allowing them to be interpreted as gabbros, basis for such a theory includes that of Wegmann pillow lavas and pyroclastites (Park et al. 1981; (1928), Väyrynen (1937, 1939) and Gaal et al. Rehtijärvi et al. 1985). A date of 1.97 Ga has been (1975). obtained from the formation from the zircon of It has been known for some time that the ser- the Horsmanaho metagabbro (Huhma 1986). pentinites of the Outokumpu complex contain ac- The above rock assemblage forms zones with- cessory chrome spinel and relict olivine and pseu- in the North Karelia schist belt which can be domorphs of orthopyroxene (Haapala 1936). traced for some 260 km (Fig. 1). The host rocks Haapala (op.cit.) claims that these serpentinites throughout are mica schists characterized by were originally mainly dunites and saxonites structures typical of turbidites (Huhma 1971). (harzburgites). These mica schists, which are regarded strati- A great deal of information has accumulated graphically as belonging to the Kaleva series at the Department of Geology, University of (Frosterus 1902; Frosterus et al. 1920; Väyry- Oulu, in recent years on the mineral compositions nen 1954; Simonen 1980) are bounded by Sario- of various basic and ultrabasic magmatic cumu- la and Jatuli quartzites (Gaal 1964, Piirainen lates and the chemical compositions of their 1968; Piirainen et al. 1974; Pekkarinen 1979; minerals (Alapieti et al. 1979a and b, 1984; Ala- Marmo et al. 1988), overlying the Archaean base- pieti 1982; Hanski 1986; Vuollo 1986), and vari- ment in a discordant manner. Younger than these ous ultramafiolites, including ophiolites and es- are the differentiated and spilitic sills and un- pecially chrome spinels, have also been subject differentiated tholeiitic dykes (Piirainen 1969; to intensive investigation elsewhere (see Green- Hanski 1986; Vuollo 1988), the age of which may baum 1977; Hoffman and Walker 1978; Leblanc be deduced from results obtained in other parts et al. 1983; Dick et al. 1984; Christodoulou et al. of the Karelian schist belt to be 2.2 and 2.1 Ga 1985; Jankovic 1986; Obradovic 1986; Johan respectively (see Sakko 1971; Patchett et al. 1981; et al. 1986; Burgath et al. 1986). This research Huhma 1986). The dykes cut across the Sario- suggests that the primary minerals in the lan and Jatulian quartzites, but not the Kalevian ultramafiolites, olivine, pyroxene and above all schists (Väyrynen 1933; Piirainen et al. 1974). chrome spinel, are sensitive indicators of their Following the various sedimentary and mag- geological background. This makes the serpen- matic events, the North Karelia schist belt under- tinites of the Outokumpu complex, which have went a complex history of deformation and been studied surprisingly little since the work of metamorphism in association with the Sveco- Haapala cited above, a particularly interesting karelian orogeny around 1.9—1.8 Ga ago. The topic of investigation. peak of progressive metamorphism in the west- ern parts of the formation rose as far as condi- tions typical of a high amphibolite facies (Treloar Geological setting et al. 1981), whereas in the eastern parts it came to a halt at the boundary between the amphibo- The Outokumpu complex contains a rock as- lite and greenschist facies (Piirainen 1968). The semblage black schist — serpentinite — amphibo- deformations were also greater in number and de- lite — carbonate rocks — calc-silicate rocks — gree in the west than in the east. Six stages of quartz rock — Cu-Zn-(Co-Au) ore. It was deformation have been distinguished in the Outo- thought earlier that it did not contain any mag- kumpu area (Koistinen 1981), whereas only three matic derivatives apart from the serpentinites at are visible in the Koli-Kaltimo area (Kohonen all, but works carried out in recent years has led 1987). Also, the schists in the west are intersected Mineralogical evidence for an ophiolite from the Outokumpu serpentinites in North Karelia, Finland 97 LEGEND PROTEROZOIC ROCKS |x x I Granite I - f Mica schist/veined gneiss Black schist I • —*-1 Serpentinite and associated rocks Basic igneous rock ^ ] Quartzite ARCHAEAN ROCKS I ^ yJ Granitoids,augen gneiss, greenstone ® Drillhole, samples Fig. 1. Geological map of the Outokumpu area, modified after Huhma (1975), Koistinen (1981, 1987) and Parkkinen et al. (1985). The sampling sites are denoted by asterisks. by granitoids of age 1.86—1.87 Ga (Huhma Archaean mountain range with the RRR-system 1986), whereas these are absent on the eastern of the early Proterozoic and continuing to the edge of the schist belt. passive margin and finally a back geosyncline In summary, the North Karelia schist belt is (Piirainen 1968), in the same manner as described regarded as having gone through all the stages by Hoffman et al. (1974) for the Great Slave in the evolution model presupposed by plate tec- Lake area of Canada. Various interpretations tonics, beginning after the denudation of an have been put forward for the relation of the 7 98 Jouni Vuollo and Tauno Piirainen Outokumpu complex to this evolution, and we will return to these in the Discussion below. Serpentinites General Serpentinites constitute the principal rock type in the Outokumpu complex, forming bodies that interleave closely with the carbonate rocks, quartz rocks and calc-silicate rocks in a conform- able manner (Haapala 1936; Peltola 1978; Koisti- nen 1981). The individual bodies vary greatly in size and shape, the largest ones having a longitu- dinal axis of several kilometres and reaching thicknesses of several hundred metres. The con- tacts between the serpentinites and the surround- ing rocks are either tectonic or gradual, grading first to carbonate rocks, which then grade via calc-silicate rocks to quartz rocks (Haapala 1936; Peltola 1978). The carbonate rocks, calc-silicate rocks and quartz rocks bordering on the serpen- tinite bodies have typically high Cr, Ni and V concentrations (Eskola 1933; Huhma et al. 1970; Treloar 1987), and it is this that has led to the linking of their origins with the serpentinization event (Huhma et al. 1970; Gaal et al. 1975). Samples and petrography 125 samples for investigation of the serpen- tinites were selected from cores obtained by Outokumpu Oy and Myllykoski Oy in connec- tion with ore prospecting at Savonranta, Outo- Fig. 2. Photomicrographs of a) the Savonranta harzburgite, kumpu, Polvijärvi, Miihkali, Luikonlahti, Kok- plane polarized light, SR-PE-10, 250.00, b) the Polvijärvi ka and Poskijärvi, and also from the outcrops antigorite serpentinite, crossed polars, OKU-791, 436.50 at Mäntyjärvi in the commune of Kaavi. These c) the Miihkali cumulus-textured wehrlite, crossed polars, JU-MI-92, 298.00. Ol = olivine, Cr = chromite and Cx = sampling sites cover the whole length of the Outo- clinopyroxene pseudomorph. Photo length 4 mm. kumpu sequence (Fig. 1), the aim being to ob- tain as comprehensive a series of different types of serpentinite as possible. 170 m to 270 m, from one borehole, SR-PE-10. Savonranta The serpentinites are either dunites or harzbur- The samples (12) were taken from different gites in composition (Fig.