Abstract

Two Archean komatiitic pyroclastic rock units occur on opposite sides of the Quetico Fault in northwestern Ontario. The eastern unit, the Dismal Ashrock, is located 3 km north of Atikokan, Ontario, on the north side of the Quetico Fault within the Wabigoon Subprovince of the Superior Province. It is part of a suprascrustal sequence, the Steep Rock Group. The Grassy Portage Bay ultramafic pyroclastic rock unit (GUP) is located 100 km to the west, on the south side of the Quetico Fault, and is part of an overturned succession comprising mafic metavolcanic rocks, GUP, and metasedimentary rocks. The Dismal Ashrock dips steeply, is little deformed, has undergone greenschist metamorphism, and is divided into komatiitic lapilli tuff, komatiitic volcanic breccia, komatiitic volcaniclastic rocks, and a mafic pillowed flow. GUP outcrops form an arcuate fold interference pattern, are strongly deformed, and have undergone amphibolite metamorphism. GUP is divided into komatiitic lapilli tuff and komatiitic volcanic breccia. Both pyroclastic units contain cored and composite lapilli, evidence for explosive volcanism. Locally, some of the lapilli fragments are highly vesicular (up to 30% by volume), greater than reported for any other komatiites. Other fragments show no vesicularity. The low vesicularity of some of the pyroclasts and, in the case of the Dismal Ashrock, their association with pinowed lava flows may indicate explosive hydrovolcanic activity. The Dismal Ashrock and GUP are high in MgO, Cr, and Ni and are unusually enriched in Fe, Ti, Zr, Mn, P, Ba, Nb, Rb, and Sr compared with other komatiites. These unique geochemical compositions are not understood at this time.

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