Keywords
:
Seamounts; Archean; Slave Province; Mafic volcanic; Multiple dykes; Stratified hyaloclastite; Peperite; Shale
1. Introduction
Seamounts, also referred to as pillow mounds or pillow volcanoes, are mafic volcanic edifices
that form on the ocean floor. These subaqueous features, varying from 0.05 – 10 km thick and at-
taining diameters as large as 100 km, are com- monly associated with crustal-scale faults or rifts
Easton, 1984; Fornari et al., 1985; Chadwick and Embley, 1994; McPhie, 1995 and are generally
characterized by central feeder conduits Fisher, 1984; Head et al., 1996, in addition to predomi-
nant pillowed and sheet flows Chadwick and Embley, 1994; Orton, 1996. Pillow breccia and
hyaloclastite are commonly associated with pil- lowed and sheet flows on seamount flanks Fisher
and Schmincke, 1984; Staudigel and Schminke, 1984. The volcanic facies constituting seamounts
often overlie deep water sediments andor are interstratified with sedimentary material deposited
as suspension fallout during volcanism Fisher, 1984. Seamounts, although primarily associated
with mid-oceanic rift zones, have also been related to back-arc, arc, and hot spot volcanism. Distinc-
tion between mid-oceanic and back-arc seamounts is often problematic because mafic and felsic vol-
canic rocks in both tectonic settings display simi- lar geochemistry Thurston, 1994. MORB-type
signatures are commonly associated with both spreading centres, but tectonic reconstruction may
be facilitated where back-arc related seamounts contain rocks of arc-type compositions, as indi-
cated by island arc or calc-alkaline basalts and andesites Saunders and Tarney, 1984; Fryer,
1995.
Modern seamounts have been studied exten- sively to determine facies architecture and erup-
tion processes Smith and Batiza, 1989; Chadwick and Embley, 1994, possible conduits through
which magma is fed to the surface Fornari et al., 1985; Smith and Cann, 1992; Bryan et al., 1994,
petrological and geochemical variations on and off ridge axes Hekinian et al., 1989; Sinton et al.,
1991, and whether velocity at spreading centres plays a role in mafic flow type Hekinian, 1984;
Kennish and Lutz, 1998. Staudigel and Schminke 1984 documented the volcanic facies architecture
of a Pliocene seamount in the Canary Islands, McPhie 1995 discussed the facies associations
comprising a Pliocene seamount in Fiji, and Kano et al. 1993 described the volcanic facies of a
Miocene seamount in Japan, but examples of Archean seamount facies are lacking. Archean
greenstone belts compare favourably with modern volcano-sedimentary sequences in terms of lithol-
ogy, compositional changes with edifice evolution, and structure Ayres and Thurston, 1985; Taira et
al., 1992; Thurston, 1994. Greater inferred heat production, sea floor spreading, and eruption
rates during the Archean relative to modern regimes produced more volcanic rocks with
thicker tholeiitic basaltic sequences Taira et al., 1992; Windley, 1995, suggesting that seamounts
must have been prominent features on the Archean ocean floor.
This paper presents Archean mafic volcanic facies in the Slave Province, Northwest Territo-
ries, Canada, that resemble the facies comprising distinct portions of modern seamounts. Models of
seamount construction based on the facies associ- ations of two volcanic belts at three detailed
localities are provided. Although modern exam- ples contribute information concerning water
depth, composition of unaltered volcanic mate- rial, and location of the edifice with respect to a
spreading centre, seamount core exposure and contact relationships between facies are generally
absent. Cross sections through ancient rocks that demonstrate well-preserved volcanic structures
contribute substantially in recognizing the facies that form at specific levels during seamount
construction.
2. Slave Province geology
