Chrome spinel compositions as evidence for an Archaean ophiolite in the Kuhmo greenstone belt in Finland

Abstract

Investigations into chromite compositions in Archaean ultramafic rocks are rare. Most Archaean ultramafic rocks have undergone a multi-stage history of deformation and metamorphism, and therefore fresh primary chromite is rarely preserved. The preserved cores nevertheless offer a method for studying the petrogenesis of highly altered Archaean ultramafic rocks. Hundreds of electron microprobe analyses have been performed on chrome spinels in the serpentinites from the Archaean Kuhmo greenstone belt to constrain the origin of the serpentinite massives. Best preserved chromites can be divided into two groups on the basis of their composition. Those of the first group share the characteristics of Alpine-type complexes in terms of Cr/(Cr+Al) values from 0.58 to 0.76 and Mg/(Mg+Fe2+) values from 0.53 to 0.60, while those of the second type have altered to ferritchromite and have their Mg/(Mg+Fe2+) values from 0.00 to 0.37. These observations suggest that lithological unit composed of serpentinites, and pillow lavas of tholeiitic (Thl), komatiitic, and basaltic komatiitic composition, known as the Kellojärvi group, includes parts of an Archaean ophiolite.

Keywords

• greenstone belts
• serpentinite
• chrome spinel
• chromite
• electron probe data
• ophiolite
• Archaean
• Kuhmo
• Finland