Physical volcanology, stratigraphy, and lithogeochemistry of an Archean volcanic arc: evolution from plume-related volcanism to arc rifting of SE Abitibi Greenstone Belt, Val d'Or, Canada

Abstract

The eastern segment of the Southern Volcanic Zone (SVZ) of the Abitibi greenstone belt is a complex sequence of volcano-sedimentary rocks interleaved with syn- to post-volcanic plutonic suites. The eastern segment can be broken into two stratigraphic groups based on regional tectonics and volcano-sedimentary stratigraphy, the basal Malartic Group (MG), composed of the La Motte-Vassan, Dubuisson and Jacola Formations (JFs), and the upper Louvicourt Group (LG), containing the Val d'Or (VDF) and Héva formations (HF). The MG 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 were still operating, the LG signaled the change to subduction-related processes featuring a deep-marine volcanic arc. This complex volcano-sedimentary sequence evolved between 2714±2 and 2702±2 Ma. U–Pb age determinations of 2704±2 Ma for the Val d'Or Formation (VDF) and 2702±1 Ma for the HF constrain the duration of the 5–7 km-thick Val d'Or Arc to ca. 5 My. The JF 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 JF and intermediate-felsic VDF is gradational and indicated by andesitic volcaniclastic debris deposited via mass flow processes within the lower VDF. The 3–5 km-thick VDF is a complex subaqueous volcano-sedimentary sequence with rapid lateral and vertical changes in the depositional environment 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 and 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 areal 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 HF is defined by effusive mafic and felsic volcanism with local volcaniclastic deposits. A basal spherulitic dacite unit within the HF, traceable for 40 km along strike, serves as the marker horizon for the LG. 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 JF 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 VDF indicate subduction-related volcanism. Fragment-dominated calc-alkaline volcanism of the VDF marks arc construction forming numerous small and overlapping subaqueous volcanic centers. The tholeiitic HF 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.