Komatiitic and tholeiitic metavolcanics of the Siivikkovaara area in the Archean Kuhmo greenstone belt, eastern Finland

Abstract

Komatiitic metavolcanics are abundant and well-preserved in the Siivikkovaara area, which lies in the middle part of the Archean Kuhmo greenstone belt. Together with the underlying tholeiites, these form the Kellojärvi Group, which is divided into two formations, those of Pahakangas and Siivikkovaara. The Pahakangas Formation consists of pillowed and massive metalavas and intercalated banded iron formations. The metalavas are chemically typical Archean low-potassium tholeiites. The overlying Siivikkovaara Formation has three subunits, the Siivikko, Mäkinen and Raivio Members. The metavolcanics of the Siivikko Member, which are mainly peridotitic komatiites (MgO ≥ 18 %), have many structures typical of extrusive rocks, including breccias, pillows, spinifex textures and polygonal jointing. The microspinifex-textured rocks richest in magnesium contain about 20 % MgO. The Mäkinen Member consists of basaltic and pyroxenitic komatiites (MgO < 18 %), which occur in the form of variolitic pillow lavas. The Raivio Member is composed of ultramafic to mafic metatuffs. The spinifex textures in the Siivikkovaara area were originally formed by clinopyroxene. Zones A1, A2, B1 and B2 can be distinguished in the spinifex-textured lava flows, and A2 has further subunits dominated by dendritic, platy or acicular hornblende pseudomorphs after clinopyroxene. The liquid composition of the spinifex-textured flows is deduced to be in the range of pyroxenitic komatiite. These flows are differentiated mainly by olivine and minor pyroxene and chromite precipitation. Albite in the spinifex-textured rocks is formed through sodium metasomatism caused by sea-water. There are many indicators to support the notion of a deep-sea origin for the lavas of the Kellojärvi Group. Despite the relatively low MgO content of the komatiites in the Siivikkovaara Formation, two nickel-copper occurences have been found in the formation.