Archean volcanoes in southwestern Abitibi Belt, Ontario and Quebec: form, composition, and development

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Abstract
Southwestern Abitibi Volcanic Belt is composed of numerous deformed volcanoes that were originally circular to subcircular in outline, 100–200 km in diameter, and 10–16 km in stratigraphic thickness. Tholeiitic (TH) lava flows with local komatiitic (KM) flows and intrusions predominate in the lower parts of each volcano and calc-alkalic (CA) lava flows or pyroclastic rocks in the upper parts. Subsidence of the volcanoes more or less kept pace with magma extrusion such that volcano slopes remained mainly horizontal to subhorizontal. The lower TH parts accumulated rapidly at substantial water depths. The upper CA parts also accumulated rapidly but at decreasing water depths with possible local, brief, island emergence.The lower TH division is dominated by thick, uniform TH basalt lava flows. This changes abruptly at about volcano mid-thickness to the conformably overlying CA andesite-rich division characterized by irregularly recurring basalt–andesite– dacite–rhyolite flow or pyroclastic alternations that become increasingly felsic upwards. Multicyclic volcanoes include second-order internal TH–CA subdivisions. The generalized Abitibi succession is represented by a simplified, composite Abitibi volcano.The salient control in volcano development is attributed to diapiric ascent within a heterogeneously layered Archean mantle. TH and CA components are attributed to direct mantle sources. The major TH–CA volcano pattern represents a full Archean thermal cycle. The TH–CA discontinuity reflects the switch from an earlier, mainly depleted mantle source to a later, mainly undepleted mantle source. Second-order volcano subdivisions (subgroups) represent thermal pulses in diapiric ascent. Such a system of recurring, migrating mantle plumes gave rise, in due course, to the volcano-dominated greenstone belt.

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