Recognizing distinct portions of seamounts using volcanic facies analysis: examples from the Archean Slave Province, NWT, Canada

Abstract

Archean volcanic rocks in the mafic-dominated, ca. 2.66–2.69 Ga Point Lake and Beaulieu River belts, Slave Province, Northwest Territories, are significant in demonstrating the facies that characterize specific portions of pillow volcanoes or seamounts, irrespective of tectonic setting. Three distinct localities mapped in detail display facies consistent with: (1) proximal, deep-water, (2) medial to distal, deep-water, and (3) medial, shallow-water seamount settings. The proximal facies in the Point Lake belt include a 55-m-thick, non-vesicular pillowed sequence cut by numerous mafic dykes and sills. Dykes contain multiple chilled margins, indicating successive magma pulses which contributed to edifice construction. Abundant feeder conduits, in addition to the absence of fragmental facies and vesicles, are typical of the central, deep water portion of seamounts where growth is initiated. The medial to distal, deep water facies in the Point Lake belt are represented by a 30–80 m-thick assemblage of disorganized pillow breccia, and pillowed and massive flows with 5–27% vesicularity. Massive, non-vesicular hyaloclastite intermingled with sedimentary material (fluidal peperite), in addition to thin shale units interstratified with pillow breccia and hyaloclastite, indicate that sedimentation and volcanism were contemporaneous. An increase in fragmental units and vesicularity relative to the proximal, deep water facies is suggestive of the medial to distal part of a seamount in shallower water. Bedded tuffs, laterally along strike with massive flows, are the results of turbidity current deposition immediately following localized subaqueous eruptions. A medial, shallow water seamount setting is represented in the Beaulieu River belt, by a 5–85 m-thick sequence of vesicular lobate-pillowed and massive flows, stratified pillow breccia and hyaloclastite, and mafic dykes. Vesicularity ranges from 21–49% in pillowed flows, 5–40% in massive flows, and 20–35% in pillow breccia and hyaloclastite. Stratified pillow breccia developed along steep flow fronts in shallow water whereas bedded hyaloclastite formed during reworking and redeposition of autoclastic hyaloclastite on seamount flanks in shallow water. The volcanic facies associations in the study areas are analogous to those of modern seamounts associated with the Mid-Atlantic Ridge and East Pacific Rise, as well as Mesozoic-Cenozoic seamounts in the Canary Islands, Fiji, southwest Japan, the Sea of Japan, and Cyprus. Volcanological studies in the Point Lake and Beaulieu River volcanic belts and subsequent comparisons with Phanerozoic analogues, demonstrate the manner in which distinct portions of ancient seamounts can be recognized in similar Archean terranes.