A Comparison of Two Archean Ultramafic Pyroclastic Rock Units from the Superior Province, Northwestern, Ontario

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 northern side of the Quetico fault within the Wabigoon Subprovince of the Superior Province. It is part of a supracrustal sequence (the Steep Rock Group) that overlies an Archean unconformity. 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 Quetico Fault is a dextral fault with a history of transpressive tectonics. The Dismal Ashrock is steeply inclined, little deformed and has undergone greenschist facies metamorphism, and it is divided into komatiitic lapilli tuff, komatiitic volcanic breccia, komatiitic volcaniclastic rocks and a mafic pillowed flow. The GUP outcrops form an arcuate fold interference pattern. The GUP is strongly deformed and has undergone amphibolite facies metamorphism. It is divided into komatiitic lapilli tuff and komatiitic volcanic breccia. The Dismal Ashrock and the GUP contain cored and composite lapilli - unequivocal 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 association with pillowed lava flows in the case of the Dismal Ashrock indicate phreatomagmatic volcanic activity. Explosive water-magma interaction was probably initiated by modified eruption characteristics produced from exsolving volatiles. The Dismal Ashrock and GUP are similar in chemical composition and plot on the border between peridotitic and basaltic komatiites on a Jensen AFM diagram. They are high in MgO, Cr and Ni; however, they are unusually enriched in Fe, Ti, Zr, Mn, P, Ba, Nb, Rb and Sr compared to other komatiites. Several lines of evidence indicate that this unusual composition could not have been caused by alteration or assimilation, and an enriched mantle source region is the likely cause. Many of the characteristics that Dismal Ashrock and GUP share are rare or unique on a global scale, indicating that the Dismal Ashrock and GUP are correlative in some manner.