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
The Point Lake and Beaulieu River volcanic belts are located in the Slave Province, a 500 ×
Fig. 1. Lithological map of the Slave Province SP in the Northwest Territories NT, Canada, illustrating the location of the Peltier Formation and Beaulieu River volcanic belt along the Beniah Lake fault. Modified from Corcoran et al. 1998.
700 km Archean craton in the Northwest Territo- ries of Canada Fig. 1. The 4.03 Ga Acasta
gneisses Bowring and Williams, 1998, and their \
2.8 Ga counterparts, including the Sleepy Dragon Complex and Augustus Granite Hender-
son et al., 1987; Northrup et al., 1999, are base- ment to overlying greenstone belts in the western
part of the craton. Volcanic rocks in the Slave Province are subordinate to sedimentary rocks
and are characterized by relatively high felsic mafic volcanic rock ratios Padgham and Fyson,
1992. Mafic and intermediate volcanic sequences, 2.66 – 2.72 Ga Isachsen and Bowring, 1997, char-
acterize greenstone belts in the western part of the province, whereas 2.67 – 2.7 Ga intermediate to
felsic rocks are more common in the east Padgham, 1985. The 2.66 – 2.69 Ga Point Lake
belt Mueller et al., 1998; Northrup et al., 1999, and Beaulieu River belt, inferred to be time-equiv-
alent with the 2663 Ma Cameron River belt Hen- derson et al., 1987; Lambert et al., 1992, can be
correlated with the 2722 – 2658 Ma Isachsen and Bowring, 1997 Yellowknife volcanic belt Fig. 2.
The Yellowknife volcanic belt is divided into the mafic flow-dominated Kam Group and the felsic
volcaniclastic-dominated Banting and Duncan
Fig. 2. Stratigraphy of the Yellowknife volcanic belt, Slave Province and correlations with the Peltier Formation and Beaulieu River volcanic belt. Age dates from: 1 Isachsen et al., 1991; Isachsen and Bowring, 1994, 1997; 2 Henderson et al., 1987; 3 Mueller
et al., 1998; 4 Northrup et al., 1999. JF, Jackson Lake Formation; BF, Burwash Formation; CL, Clan Lake felsic volcanic complex; BRF, Beaulieu Rapids Formation; Beaulieu River volcanic belt, Beaulieu River volcanic belt; SD, Sleepy Dragon
Complex; KF, Keskarrah Formation; SB, Samandre and Beauparlant formations; CF, Contwoyto Formation; AG, Augustus Granite. Modified from Corcoran et al. 1998.
Lake groups Helmstaedt and Padgham, 1986. The 10 – 15 km-thick basaltic Kam Group contains
massive, pillowed, and brecciated flows, intruded by gabbro sills and dykes MacLachlan and Helm-
staedt, 1995. Locally, metre-thick felsic volcani- clastic units are interstratified with the basalts. The
Banting and Duncan Lake groups, inferred to overlie the Kam Group unconformably, are repre-
sented mainly by felsic volcanic, felsic volcaniclas- tic, and turbiditic rocks. The latter, referred to as
the Burwash Formation Duncan Lake Group are associated with ca. 2661 – 2663 Ma felsic volcanic
centres Henderson et al., 1987; Mortensen et al., 1992. This assemblage is unconformably overlain
by the Jackson Lake Formation B 2605 Ma; Isachsen et al., 1991, an alluvial-marine sequence
Mueller and Donaldson, 1994, similar to the 2600 Ma alluvial-lacustrine Beaulieu Rapids Formation
Corcoran et al., 1999 overlying the Beaulieu River belt unconformably, and the 2605 Ma Isachsen
and Bowring, 1994 alluvial-marine Keskarrah Formation Corcoran et al., 1998 overlying the
Point Lake belt unconformably.
2
.
1
. Local geology The Slave Province is characterized by north-
trending lineaments along which several volcanic belts and most of the 2.6 Ga late-orogenic sedimen-
tary rocks are exposed Fig. 1. The Point Lake and Beaulieu River volcanic belts are located along the
north-trending Beniah Lake fault Fig. 1, a linea- ment previously interpreted to coincide with a
major tectonic break between an older, western terrane containing a sialic basement and a younger,
eastern terrane Padgham and Fyson, 1992. Previ- ous studies have demonstrated the crucial role of
the Beniah Lake fault in the development of ca. 2.6 Ga conglomeratic sequences Corcoran et al., 1998,
1999, but the significance of this structure in the formation of 2.66 – 2.69 Ga volcanic belts remains
debatable.
The Peltier Formation, a subaqueous mafic- dominated succession located in the north-central
Slave Province, comprises part of the Point Lake belt this paper; Fig. 2C or Point Lake Group as
defined by Henderson 1998. Andesitic-dacitic vol- caniclastic deposits are locally interstratified with
the basaltic Peltier formation, but the majority of the intermediate and felsic volcanic rocks comprise
the Samandre and Beauparlant formations, respec- tively Fig. 2C. The sedimentary Contwoyto
Formation is time-equivalent with the Peltier For- mation as indicated by interstratified turbiditic
deposits and mafic flows; both overlie the 3.22 Ga Northrup et al., 1999 Augustus Granite uncon-
formably. Late-orogenic, clastic sedimentary de- posits of the 2.6 Ga Keskarrah Formation overlie
the mafic-felsic volcanic rocks unconformably Fig. 2. Preliminary geochemical data indicate that the
Peltier Formation, over a regional area of 12.5 × 17.5 km contains tholeiitic basalts and subordinate
calc-alkaline basalts and andesites with SiO
2
con- tents ranging from 46 – 59 Dostal and Corcoran,
1998. Two study areas composed of tholeiitic basalts and referred to as localities A and B, were
selected for detailed work Fig. 3. Although the true thickness of the Peltier Formation remains
enigmatic due to structural complexity in the Point Lake region Henderson, 1998, the most extensive
homoclinal sequence identified is : 1.5 km thick, of which locality B constitutes the basal part of the
uppermost 700 m Fig. 3. A northwest-southeast trending, northeast-dipping reverse-slip fault sepa-
rates localities A and B Henderson, 1988.
The Beaulieu River volcanic belt, adjacent to the north-trending Beniah Lake fault in the south-cen-
tral part of the Slave Province Fig. 4, is inferred to overlie the Sleepy Dragon Complex uncon-
formably. Sedimentary rocks of the 2.6 Ga Mueller et al., 1998 Beaulieu Rapids Formation
overlie the volcanic succession unconformably Fig. 2B. Tholeiitic basalts and calc-alkaline
basalts and andesites predominate, but minor felsic tuffs, breccias, and flows also characterize the
sequence Lambert et al., 1992. One study area, 85 m thick and composed of tholeiitic basalts, was
selected for detailed study and comparison with the localities in the Peltier Formation because: i the
Point Lake and Beaulieu River belts are spatially related in that they are adjacent to the 600 km-long
Beniah Lake fault Fig. 1, ii both belts occupy similar stratigraphic positions with respect to sur-
rounding rock types Fig. 2, and iii all detailed localities display a variety of comparable mafic
volcanic facies Figs. 3 and 4.
Fig. 3. Location of the Peltier Formation at Point Lake relative to the Contwoyto and Keskarrah formations and the basement Augustus granite. A 55 × 160 m schematic section through locality A and an 80 × 230 m schematic section through locality B
illustrate the facies architecture of volcanic edifices in the Peltier Formation. Note the location of bedded tuff interstratified with massive flows at locality B. Modified from Corcoran et al. 1998.
Fig. 4. A Location of the Beaulieu River volcanic belt relative to the Beniah Lake fault and 2.8 – 2.9 Ga plutono-gneissic Sleepy Dragon complex. B Location of the study area in the Beaulieu River volcanic belt. C Schematic section through the study area
demonstrating the vertical and lateral facies changes over 85 × 300 m.
3. Volcanic facies in the Point Lake and Beaulieu River belts
