Mineralogical evidence for an ophiolite from the Outokumpu serpentinites in North Karelia, Finland

Abstract

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 textures provide evidence for an ophiolite of the Alpine type in the complex, and for the opinion that the serpentinites have originated from residual mantle peridotites and overlying olivine cumulates. The mantle peridotites, being serpentinized to chrysotile rocks, contain relicts of olivine (Fo91‒93), 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 + Fe2+) 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)