Physical volcanology, stratigraphy, and lithogeochemistry of the Val d'Or Archean volcanic arc, South-East Abitibi Greenstone Belt, Val d'Or, Québec

Craig R. Scott, Ph.D. thesis.

Ph.D. thesis in mineral ressources under the supervision of Dr. Wulf U. Mueller.

Project description

The eastern segment of the Southern Volcanic Zone of the Abitibi greenstone belt is a complex assemblage of volcano-sedimentary rocks interleaved with syn- to post-tectonic plutonic suites. The eastern segment can be broken into two groups based on regional tectonics and volcano-sedimentary stratigraphy: the basal Malartic Group, composed of the La Motte-Vassan, Dubuisson and Jacola Formations, and the upper Louvicourt Group, containing the Val d'Or and Héva Formations. The Malartic Group represents a Archean oceanic floor controlled by extensional mantle plume tectonics and characterized by effusive komatiites and basalts, and intrusive dykes and sills. Although plume processes are still operating, the Louvicourt Group signals the change to subduction-related processes featuring a deep-marine volcanic arc. This complex volcano-sedimentary sequence evolved between 2714 ± 2 Ma and 2702 ± 2 Ma. U-Pb age determinations of 2704 ± 2 Ma for the Val d'Or and 2702 ± 1 Ma for the Héva Formations constrain the age of the 5-7 km-thick Val d'Or Arc to ca. 5 Ma.

The Jacola Formations is characterized by the interstratification of komatiitic, plume-related volcanism and basaltic, subduction-related volcanism based on stratigraphy, geochemistry, and geochronology. The contact between the mafic-ultramafic Jacola Formations and Val d'Or is gradational and indicated by andesitic volcaniclastic debris deposited via mass flow processes. The 3-5 km-thick Val d'Or is a complex subaqueous volcano-sedimentary sequence with rapid lateral and vertical changes of volcaniclastic deposits and associated andesitic to rhyolitic lavas. The 1-50 m-thick volcaniclastic deposits are normal- to reverse-graded beds composed of angular to subrounded tuff- to breccia-sized clasts, that locally have a vesicularity index between 15-35%. The 1-100 m-thick massive, pillowed/lobate and brecciated lava flows are variably vesicular (1-30%). In addition, numerous small felsic-dominated volcanic centers of limited extent, containing massive sulfide deposits define the Val d'Or Arc. Effusive eruptions and their autoclastic to hydroclastic counterparts form these centers. The 2-3 km-thick tholeiitic Héva Formations is defined by effusive mafic and felsic volcanism with local volcaniclastic deposits. A basal spherulitic dacite unit traceable for 40 km along strike serves as the marker horizon for the Val d'Or Arc. Up section, massive to pillowed mafic flows with gabbroic dykes and sills are dominant, suggesting more fissural-type volcanism. Local tuff turbidites and reworked scoriaous pyroclastic deposits attest to explosive eruption(s).

The Jacola Formations represents a deep marine ridge-like oceanic setting controlled by mantle plume volcanism with intercalated volcanics originating from a proto-arc. Andesitic volcaniclastic rocks and lavas at the base of the Val d'Or indicate subduction-related volcanism. Fragment-dominated calc-alkaline volcanism of the Val d'Or marks arc construction forming numerous small and overlapping subaqueous volcanic centers. The tholeiitic Héva Formations indicates a return to an extensional regime with fissure-type volcanism during arc dissection. The Val d'Or Arc forms a homoclinal, south-facing volcano-sedimentary succession evolving from plume- to subduction-related volcanism.

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