MINERALOGICAL EVIDENCE for an OPHIOLITE from the OUTOKUMPU SERPENTINITES in NORTH KARELIA, FINLAND JOUNI VUOLLO and TAUNO PIIRAINEN
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.
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