The Kummitsoiva komatiite complex and its satellites in northern Finland

The Kummitsoiva komatiite complex and its satellites, regarded either as Archean or Proterozoic in age, consist principally of amphibole- chlorite rocks corresponding chemically to basaltic komatiite of the Geluk type in South Africa. The rocks are mainly pyroclastic and vary from agglomerates to tuffs with epiclastic amphibole-chlorite debris. The lavas are massive flows, autobrecciated lavas and, sometimes, pillow lavas. The Kummitsoiva complex also contains komatiites proper as a few serpentine-olivine rock interlayers made up of lavas, and of pyroclastic and epiclastic rocks. Erupting basaltic komatiite lavas were mainly very viscous whereas those of the komatiites proper were fluidal. The Kummitsoiva complex, originally a large isolated volcano, erupted through a volcanic conduit and a fissure net. The satellites deposited around and upon separate volcanic vents. They were all located at the margin, or on islands of an interior basin in a continental environment. Euxinic-epiclastic sedimentation preceded the komatiite eruptions, and mafic or intermediate volcanism took place or continued after the explosive komatiite volcanism. The alignment of the volcanic vents implies a geotectonic fault in a northwesterly direction coeval with the euxinic-epiclastic sedimentation. Displacements of fault blocks can be inferred from tectonicstratigraphic features. The komatiites belong to the upper part of the Lapponian supracrustal sequence, which forms a greenstone belt-like rock association in the schist area of central Lapland in the Baltic Shield