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Komatiites occur in many Archean and Proterozoic greenstone belts worldwide, provide critical information about the nature of early Earth volcanism and the stratigraphy, structure, and crustal evolution of greenstone belts, and locally host Ni-Cu-(PGE) sulfide deposits. Despite their importance, many aspects of the physical volcanology of komatiites are not well understood. The Western Abitibi Greenstone Belt (WAGB) contains some of the best exposed and well-preserved komatiites on Earth, including several areas with spectacular glacially-polished and well-preserved outcrops, and is one of the best places to address some of the fundamental questions regarding the physical volcanology of komatiites. Komatiites form a wide range of volcanic facies (e.g., textural, morphological, lithological, and volcanological facies) and geochemical types (e.g., ADK: Al-depleted, TEK: Ti-enriched, AUK: Al-undepleted, TDK: Ti-depleted) that occur sporadically throughout the Superior Province. The western Abitibi greenstone belt is one of the youngest volcano-plutonic greenstone belts in the Superior Province and is characterized by approximately equal proportions of supracrustal (53%) and intrusive rocks (47%). Komatiitic volcanism spaned over 40 Ma and occurs as minor component in 4 volcano-sedimentary assemblages: the Pacaud assemblage (2750-2735 Ma) contains ∼4% komatiites with abundant differentiated flows (ortho- to mesocumulate lower zones and poorly developed spinifex-textured upper zones) and less abundant undifferentiated flows (olivine-phyric or ortho- to mesocumulate) characterized by TDK geochemical type, the Stoughton-Roquemaure assemblage (2723-2720 Ma) contains ∼3% komatiites with undifferentiated flows, pillowed flows with lesser differentiated flows characterized by ADK-TEK≈AUK geochemical types, and the Kidd-Munro assemblage (2719-2711 Ma) with ∼11% komatiites and the Tisdale assemblage (2710-2704 Ma) with ∼7% komatiites both contain undifferentiated flows with abundant ortho- to adcumulate rocks with fewer olivine-phyric rocks, and differentiated flows with thick orthocumulate lower zones and thin spinifex-textured upper zones characterized by AUK > ADK-TEK. Thus, the younger assemblages (Kidd-Munro and Tisdale) contain larger volumes of komatiitic rocks, in particular larger volumes of thick, highly magnesian cumulate lava channels and channelized sheet flows than the older assemblages (Pacaud and Stoughton-Roquemaure). This might reflect higher magma discharge rates, emplacement closer to the eruptive site, and/or selective preservation. Substrate erosion by modern lava flows and Archean komatiite lava flows has been debated for many years, but is now recognized by most workers to be one of the most dominant mechanisms in the genesis of komatiite-associated Ni-Cu-(PGE) deposits worldwide. Field relationships at the Alexo Mine in Dundonald Township provide unequivocal evidence for thermomechanical erosion of andesite by komatiite. The evidence includes: (1) nested footwall embayments occur on scales from 100s of meters to a few centimetres; (2) the morphologies of the embayments vary from concave to re-entrant; (3) the basal contact is very sharp but scalloped and clearly transgresses the footwall rocks without any evidence of a regolith, shearing, or folding; (4) there is evidence for contact metamorphism along the entire length of the contact, but it appears to be thicker and more intense around embayments; (6) xenoliths are more common within embayments; and (7) komatiitic dikes penetrate downward into the host andesites primarily along the lateral margins of embayments. The spatial association between thermomechanical erosion and Fe-Ni-Cu sulfide mineralization indicates a clear relationship between those two critical mechanisms in the genesis of komatiite-associated Ni-Cu-(PGE) deposits. (Abstract shortened by UMI.) |
