Abstract
The Neoproterozoic (~723–716 Ma) Franklin Large Igneous Province exposed on Victoria Island in the Canadian Arctic is comprised of a sill-dominated magma plumbing system overlain by the coeval Natkusiak flood basalts. We have investigated three sections, separated by a total of >50 km of distance, of a sill (the Fort Collinson Sill Complex) emplaced just above a prominent sedimentary marker unit. The sill is characterized by a basal olivine-enriched layer (OZ: up to 55 % olivine) and an upper gabbroic unit. The observed diversity of olivine compositions in the OZ implies that bulk-rock MgO versus FeO arrays reflect accumulation of a heterogeneous olivine crystal cargo. We suggest that the OZ was formed as a late olivine slurry replenishment in a partially crystallized gabbroic sill, propagating for over 50 km along strike. This interpretation is consistent with Pb-isotope data, which show that at least three geochemically distinct magmas were emplaced into the Fort Collinson Sill Complex. The OZs exhibit a gradual westward evolution toward more Fe-rich bulk compositions. This is best explained by progressive mixing of the replenishing olivine slurry with a resident gabbroic mush during westward flow. Pb-isotopic signatures suggest that magmas near the inferred conduit feeder assimilated small amounts (<10 %) of dolostone country rock, which may have locally buffered olivine compositions to high-Fo contents.
Highlights
To understand the geochemical evolution of basalts, we need to constrain the processes active in the associated magmatic plumbing system
Examples of crystal slurry emplacement have been documented in the Ferrar sills of Antarctica (Bédard et al 2007); the Shiant Isles sills in NW Scotland (Gibb and Henderson 2006); and the Franklin sills on Victoria Island (Hayes et al 2015)
The samples were prepared into solutions for inductively coupled plasma–mass spectrometry (ICP–MS) and inductively coupled plasma–atomic emission spectrometry (ICP–OES) analyses using the lithium metaborate fusion method described in McDonald and Viljoen (2006)
Summary
To understand the geochemical evolution of basalts, we need to constrain the processes active in the associated magmatic plumbing system. Sill-dominated plumbing systems are prominent in Proterozoic and Phanerozoic continental flood basalt provinces (Baragar 1976; Francis and Walker 1986; Hawkesworth et al 1995; Chevallier and Woodford 1999; Bédard et al 2007), oceanic crust formed by seafloor-spreading (Bédard 1991; Lissenberg et al 2004), oceanic plateau crust (Kerr et al 1998), and Archean greenstones (Bédard et al 2009). Documenting lateral changes in bulk-rock and mineral chemistry in the constituent sills of such basaltic plumbing systems would shed light on the mechanisms of magma emplacement in the crust, of magma interaction with host rocks, on igneous differentiation processes, and may enable the reconstruction of magma flow directions (Marsh 2004). In addition to constraining fundamental properties of magmatic systems, this type of information is an important element in the search for Noril’sk-type Ni–Cu–PGE sulfide deposits (Naldrett 1992)
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