CHROMITES FROM THE EARLY PROTEROZOIC OUTOKUMPU-JORMUA OPHIOLITE BELT - A COMPARISON WITH CHROMITES FROM MESOZOIC OPHIOLITES

Chromites from the ultramafic rocks of the early Proterozoic Outokumpu-Jormua Ophiolite Belt were found to possess textural and Cr/(Cr + Al) and Mg/(Mg + Fe2+) ratios that resemble those described in Mesozoic ophiolite complexes. Chromites of cumulus texture were found at all locations along the Belt, whereas residual and podiform chromites were found only in the Outokumpu complex. The chromites in the Outokumpu complex originated in a supra-subduction zone environment, whereas those in the Jormua ophiolite complex originated in a mid-ocean ridge environment. The main difference between the early Proterozoic ophiolitic chromites and those of Mesozoic age is the relatively high Zn content in the former, which is a metasomatic feature related to the presence of large amounts of Cu-Co-Zn-Au ore, The data and observations presented provide evidence contributing to the concept of Proterozoic plate tectonics.