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HANSKI, EERO 1980: Komati i t ic and tholeiitic metavolcanics of the Si ivikkovaara area in the Archean Kuhmo greenstone belt, eas tern Finland. Bull. Geol. Soc. Finland 52, 67—100. Komatii t ic metavolcanics are abundan t and wel l -preserved in the Siiv ikkovaara area, which lies in the middle par t of the Archean K u h mo greenstone belt. Together wi th the under lying tholeiites, these fo rm the Kellojärvi Group, which is divided into two formations, those of Pahakangas and Si ivikkovaara . The P a h a k a n g a s Format ion consists of pillowed and massive me ta lavas and intercalated banded iron formations. The meta lavas are chemically typical Archean low-potassium tholeiites. The overlying Si ivikkovaara Format ion has three subunits , the Siivikko, Mäkinen and Raivio Members . The metavolcanics of the Siivikko Member, which are mainly peridotit ic komati i tes (MgO ^ 18 °/o), have many s t ruc tures typical of ext rus ive rocks, including breccias, pillows, spinifex textures and polygonal jointing. The microspin i fextextured rocks r ichest in magnes ium contain about 20 °/o MgO. The Mäkinen Member consists of basaltic and pyroxeni t ic komati i tes (MgO < 18 °/o), which occur in the fo rm of variolitic pillow lavas. The Raivio Member is composed of u l t r amaf ic to mafic meta tuf f s . The spinifex tex tures in the Si ivikkovaara area were originally formed by clinopyroxene. Zones Ai, A-z, Bi and BL> can be dis t inguished in the sp in i fextex tured lava flows, and Ao has f u r t h e r subuni ts dominated by dendritic, platy or acicular hornblende pseudomorphs a f te r cl inopyroxene. The liquid composition of the spini fextex tured f lows is deduced to be in the range of pyroxenit ic komatii te. These f lows are d i f ferent ia ted mainly by olivine and minor pyroxene and chromite precipitat ion. Albite in the sp in i fextex tured rocks is formed through sodium metasomat ism caused by sea-water . There are m a n y indicators to support the notion of a deep-sea origin for the lavas of the Kellojärvi Group. Despite the relat ively low MgO content of the komati i tes in the Si ivikkovaara Format ion, two nickel-copper occurences have been found in the format ion.

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