Recognizing distinct portions of seamounts using volcanic
facies analysis: examples from the Archean Slave Province,
NWT, Canada
P.L. Corcoran *
Department of Earth Sciences,Dalhousie Uni6ersity,Halifax,NS,Canada B3H3J5
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. © 2000 Elsevier Science B.V. All rights reserved.
* Corresponding author.
E-mail address:[email protected] (P.L. Corcoran)
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 and/or 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
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
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 (B2605 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. 2(C)) or Point Lake Group as defined by Henderson (1998). Andesitic-dacitic vol-caniclastic deposits are locally interstratified with
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 SiO2
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).
3. Volcanic facies in the Point Lake and Beaulieu River belts
The study areas in the Peltier Formation, Point Lake belt, are composed of basaltic pillowed flows (50%), pillow breccia (20%), mafic dykes and sills (15%), massive flows (10%), and hyaloclastite (5%). Andesitic-dacitic volcaniclastic rocks are in-terstratified with massive flows (Fig. 3). The Beaulieu River volcanic belt study area is repre-sented by five basaltic volcanic facies: (1) pil-lowed-lobate flows (50%), (2) pillow breccia (20%), (3) massive flows (20%), (4) synvolcanic mafic dykes (5%), and (5) stratified hyaloclastite (5%). Mapping at scales of 1:100 and 1:300 in the Peltier Formation and 1:20 and 1:100 in the Beaulieu River volcanic belt was conducted to constrain lateral and vertical facies distribution and to document volcanic structures. Rocks in the
study areas are steeply dipping (75 – 90°) and have been affected by greenschist facies metamorphism, as indicated by the mineral assemblage chlorite9
epidote9albite9hornblende9carbonate, but the prefix ‘meta’ is omitted for simplicity.
3.1. Pillowed-lobate flows
Pillowed flow units at localities A and B in the Peltier Formation range from 1.5 to 32 m thick (Figs. 5 and 6) and contain 10 – 90 cm-size closely-packed pillows (Fig. 7(A)). Hyaloclastite charac-terizes chilled margins and is present in interstices between pillows. Percentage of vesicularity differs from 0 – 5% at locality A to 0 – 27% at locality B (Table 1) where vesicles range from 0.5 to 10 mm in diameter. Spherical to ovoid vesicles are con-centrically zoned from chlorite to quartz-albite or are entirely filled with calcite or chlorite. Pillowed flows in the Peltier Formation display hyalopilitic
Fig. 6. Vertical sections and lateral correlations based on contact relationships through pillowed and massive flows, pillow breccia, and hyaloclastite at locality B, Peltier Formation. Seven flow events are recorded upsection over 68 m. Note the interstratification of shale units between flows III, IV, and V.
and hyalophitic textures with plagioclase micro-lites, and 0.2 – 0.4 mm-size plagioclase phenocrysts and secondary hornblende, respectively, in chlori-tized sideromelane.
The 3 – 15 m-thick pillowed flows in the Beaulieu River volcanic belt contain closely-packed pillows, ‘isolated’ pillows, and lobe struc-tures (Fig. 8). Closely-packed pillows, similar to close-packed pillows illustrated by Yamagishi (1994; p. 66), range in size from 20 – 150 cm, are characterized by 1 – 2 cm-thick chilled margins and some demonstrate thermal contraction frac-tures (Fig. 7(B, C)). Lobe strucfrac-tures, 0.5 – 5 m long, are distinguished by discontinuous, 0.25 – 1 cm thick vesicular flow bands that are parallel to
Peltier Formation-Locality B
PLC-97-84 Massive flow (basal) 109 500 22
91 521
Pillow breccia 17
Photo 9b
Beaulieu Ri6er6olcanic belt
243 500
Pillowed flow (pillow rim) 49
PLC-98-36
PLC-98-37 Pillowed flow (pillow rim) 227 500 45
183 500
PLC-98-40 Pillowed flow (lobate part) 500 21
121 500
Pillowed flow (lobate part) 24
PLC-98-41
99 249
Photo 9a Pillowed flow (whole pillow) 40
135 362
Pillow breccia 37
Photo 9c
Massive flow (basal)
Photo 9d 192 434 44
aDetermined by visual estimate and by counting vesicles and amygdules (filled vesicles) using 100 point grids with 1 cm and 3 mm
spacing on photos, in addition to counting 500 points at every 2 mm microscopically.
pillow rims. The lobate part of the flow is charac-terized by microgranular plagioclase. Vesicles are either concentrically zoned with mineral assem-blages quartz+chlorite-calcite, chlorite-calcite, and chlorite-quartz or simply contain calcite (Fig. 7(E, F)).
3.1.1. Interpretation
Pillowed flows studied extensively in both Archean (Dimroth et al., 1978; Hargreaves and Ayres, 1979; Wells et al., 1979; Cousineau and Dimroth, 1982) and Phanerozoic (Moore, 1975; Moore and Lockwood, 1978; Yamagishi et al., 1989; Yamagishi, 1991; Walker, 1992) settings form when hot lava enters into or erupts under water. Pillow lava may resemble subaerial
pahoe-hoe flows (Ballard et al., 1979; Wells et al., 1979; Walker, 1992), but pillowed flows are distin-guished by radial contraction joints (Kennish and Lutz, 1998) and the association with hyaloclastite (McPhie et al., 1993). Closely-packed pillows in all study areas represent the normal, molded pil-lows of Dimroth et al. (1978) that are interpreted to develop when flow velocity and temperature has decreased. Isolated pillows are the results of cooling that was too rapid to allow for complete formation (Dimroth et al., 1978) or were buried rapidly by overlying lava during high eruption rates leading to incomplete chilling (Busby-Spera, 1987). Apparent massive areas between isolated pillows may mark the location of lava tubes (Busby-Spera, 1987) or welded megapillows
Fig. 8. Volcanic facies relationships in the Beaulieu River belt locality demonstrating four flow events. Flow I changes upsection from closely packed pillows (CPP) to isolated pillows that are subsequently intruded by a mafic dyke. Flow II is massive and illustrates the concentration of vesicles at flow base and top. Note the lateral transition from lobe structures to isolated pillows then pillow breccia in flow III. Massive flow IV is in sharp contact with flow III.
roth et al., 1978). Fractures in pillows in the Beaulieu River volcanic belt developed from ther-mal contraction during cooling (Wells et al., 1979; Yamagishi, 1991). Large vesicles at pillow centres represent coalesced bubbles that did not have enough time to migrate toward the upper pillow margin before it cooled and crystallized.
3.2. Pillow breccia
Pillow breccia at locality B, 5 – 35 m thick, is composed of disorganized pillow fragments and isolated pillows (Fig. 6). Pillow fragments, 2 – 30 cm in size, are angular to subrounded and gener-ally lack chilled margins (Fig. 10(A)). Whole, isolated, 20 – 60 cm pillows in pillow breccia are
subspherical, whereas isolated, 10 – 40 cm amoe-boid varieties are set in a matrix of massive hyaloclastite (Fig. 6, Sect. 3, flows IV and V). Vesicularity ranges from 5 – 17% with spherical vesicles B1 cm in size (Table 1, Fig. 9(B)). Thin,
B1.5 m-thick units of pillow breccia at locality A locally characterize pillowed flow tops (Fig. 5).
gener-plagioclase.
3.2.1. Interpretation
Pillow breccia generally develops during quench fragmentation resulting from lava-water interac-tions (Dimroth et al., 1978; Yamagishi, 1991). Alternatively, ‘pillow fragment breccia’ forms during mechanical disintegration of pillow lava due to slumping (Fisher and Schmincke, 1984; Staudigel and Schminke, 1984; Busby-Spera,
River volcanic belt suggests either gravity-induced deposition along seamount flanks in shallow wa-ter (Staudigel and Schminke, 1984; McPhie et al., 1993) or may represent flow-front deposits where pillow breccia formed the foresets of shallow wa-ter lava deltas (Dimroth et al., 1985). In contrast, disorganized pillow breccia at locality B is in-ferred to have formed by quench brecciation of pillowed flow tops and fronts (Dimroth et al., 1978).
Fig. 10. Characteristics of pillow breccia and massive flows in the Peltier Formation and Beaulieu River volcanic belt. Large arrow indicates younging direction. (A) Pillow breccia (PB) from locality B containing angular to subangular pillow fragments. Scale, pencil 14 cm. (B) Pillow breccia (PB) in the Beaulieu River volcanic belt containing subangular to subrounded pillow fragments and whole pillows with chilled margins (pillow just below ‘PB’ label). Scale, pencil 14 cm. (C) Amoeboid pillow (AP) in pillow breccia from the Beaulieu River volcanic belt. Note the mm-thick chilled margin and the tendency for the largest vesicles (LV) to occur in the pillow centre. Scale, pencil 14 cm. (D) Calcite amygdule (A) from the basal part of massive flow IV (MF), Beaulieu River volcanic belt (see Fig. 8). Scale, pencil 14 cm.
3.3. Massi6e flows
Massive flows at locality B, 3 – 40 m thick, are up to 22% vesicular at flow bases, decreasing to 0% at flow centres and steadily increasing to 9% near flow tops (Table 1 and Fig. 6). Vesicles, 0.1 – 1 cm in size, are locally calcite-filled. Inter-granular and hyalopilitic textures characterize massive flows. Plagioclase phenocrysts, B0.2 mm long, are surrounded by plagioclase microlites, creating an intergranular texture, whereas hyalopilitic textures are represented by plagioclase microlites in a glassy matrix.
Individual massive flows in the Beaulieu River volcanic belt are 6 – 20 m thick (Fig. 8).
Vesicular-ity ranges from 5% at flow centres to 35 – 44% at flow tops and bases (Table 1 and Figs. 8 and 9(D)). Spherical to ovoid vesicles, 0.5 – 1.2 cm in size, are locally calcite-filled (Fig. 10(D)). Plagio-clase microlites in chloritized sideromelane pro-duce hyalopilitic textures.
3.3.1. Interpretation
during extrusion and solidification, the larger bubbles rise more quickly, and become trapped in the upper crystal front (Sahagian, 1985). Accord-ing to Sahagian (1985), the lower crystal front follows the direction of the upward migrating bubbles, such that it ‘chases’ and freezes them before continuing their ascent. With continued cooling, the central part of the flow eventually lacks bubbles because the lower crystal front freezes them as they rise more slowly with increas-ing viscosity. The absence of vesicles in flow cen-tres is dependent on flow thickness because thicker flows take longer to solidify, allowing more bubbles to migrate upwards (Walker, 1993).
3.4. Syn6olcanic dykes and sills
Pillowed flows at locality A are cut by synvol-canic mafic dykes and sills. Dykes, 2 – 15 m-thick, are generally perpendicular to pillow strike and to a 20 – 45 m-thick massive sill (Fig. 5). Numerous, mm-thick chilled margins, 10 – 75 cm apart, mark the presence of smaller intrusions within the cen-tral parts of most dykes. These dykes are charac-terized by as many as nine chilled margin contacts (Fig. 12(A, B)) with a decrease in grain size towards individual margins. Cross-cutting rela-tionships indicate that at locality A, the dykes were intruded after emplacement of the pillowed flows and sill. Subophitic and intersertal dykes and ophitic sills are composed of 0.3 – 1 mm-size secondary hornblende and 0.1 – 0.8 mm-size plagioclase.
Synvolcanic mafic dykes in the Beaulieu River belt, 0.03 – 0.4 m wide, cut pillow breccia and pillowed flows (Figs. 11 and 12(C)). The intru-sions begin and terminate within the same flow with local propagation into individual pillows (Figs. 11 and 12(D)). Transitions from dykes to overlying flows were nowhere evident, although the dykes are generally perpendicular to flow tops. Porphyritic and glomeroporphyritic textures are predominant with 0.2 – 2.5 mm-size plagioclase
Similarities in composition of flows and dykes, restriction of dykes within single flow units in the Beaulieu River belt, and multiple intrusions at locality A in the Peltier Formation, justify a syn-volcanic interpretation. Dyke discontinuity in the Beaulieu River belt is typical of narrow intrusions that inject for short distances before cooling and solidifying (Bruce and Huppert, 1990). Intrusions oriented perpendicular to bedding and terminat-ing with blunt ends or propagatterminat-ing into pillows are often classified as feeder dykes (Yamagishi, 1991; Kano et al., 1993). These parallel dykes indicate the manner in which overlying flows were fed, but cannot be used to interpret a specific seamount setting because of their restriction within single flow units. Multiple intrusions at locality A also support a synvolcanic, ‘feeder’ interpretation (Staudigel and Schminke, 1984; Mueller and Donaldson, 1992; Gibson et al., 1997), but unlike the dykes in the Beaulieu River volcanic belt, dykes at locality A cut sills and numerous pillowed flow units. Multiple dykes in-dicate successive magma pulses where a feeder conduit was used several times to supply magma higher up in the sequence (Mueller and Don-aldson, 1992). Multiple feeder dykes cutting nu-merous flow units associated with sills are typical of the central part of a volcanic edifice where construction is initiated (Easton, 1984).
3.5. Hyaloclastite
around which chalcedony nucleated, and 0.1 – 0.35 mm-size plagioclase microlites (Fig. 12(F)). Very fine-grained sandstone and siltstone composed of microgranular quartz and feldspar, are contained in 0.5 – 5 cm-size globules that envelop glass shards, giving the hyaloclastite a fluidal texture (Fig. 12(E, F)).
Stratified hyaloclastite in the Beaulieu River
volcanic belt is characterized by 1 – 4 cm-thick vesicular layers in 0.5 – 3 m-thick units (Fig. 12(G)). Principal components include 0.3 – 4.5 mm-size scoria containing quartz-albite-filled vesi-cles (Fig. 12(H)), 0.1 – 0.35 mm-size plagioclase crystals, and B0.1 mm-size bubble-wall and cus-pate glass shards in a matrix of microgranular feldspar.
Fig. 11. Outcrop sketch of the relationship between mafic feeder dykes, pillow breccia, and a pillowed flow in the Beaulieu River belt locality. Crudely bedded pillow breccia overlies a pillowed flow. Feeder dykes demonstrate chilled margins and locally propagate into individual pillows.
3.5.1. Interpretation
Hyaloclastite generally develops as a response to thermal contraction at flow tops and fronts (Dimroth et al., 1978; McPhie et al., 1993) or may form as a result of subaqueous lava fountaining (Smith and Batiza, 1989). The fluidal texture re-sulting from the combination of devitrified sideromelane containing microlites and granules and sedimentary material in the Peltier Formation suggests that the hyaloclastite formed by auto-brecciation when hot lava came into contact with cool, wet sediment, akin to the formation of peperite (Schminke, 1967). The fluidal texture of the hyaloclastite is attributed to en-trainment of very fine-grained wet sediment in a vapor film at the magma-sediment interface (Busby-Spera and White, 1987). Stratified hyalo-clastite in the Beaulieu River volcanic belt is attributed to resedimentation of reworked pillow breccia and autoclastic hyaloclastite (McPhie et al., 1993).
3.6. Bedded tuffs
Bedded, fine- to medium-grained volcaniclastic deposits are the on-strike equivalents of massive flows at locality B (Fig. 3). The 10 – 35 m-thick, andesitic-dacitic volcaniclastic rocks are locally massive, but are generally characterized by 10 – 50 cm-thick planar beds. The rocks are poorly sorted and contain 0.1 – 1.2 mm, euhedral, subangular, and broken plagioclase crystals, B
1.6 mm subangular to subrounded quartz crystals, 0.3 – 0.8 mm relic, euhedral hornblende crystals, and 0.2 – 2 mm subangular volcanic lithic frag-ments.
3.6.1. Interpretation
The andesitic-dacitic volcaniclastic rocks at lo-cality B are referred to as tuffs, based on the grain size classification of Fisher (1961, 1966). The tuffs are interpreted as Bouma Ta divisions (Bouma, 1962) or S3 beds (Lowe, 1982), the results of
turbidity current deposition (McPhie, 1995). Vol-caniclastic rocks are typically the direct or redeposited products of subaerial and/or sub-aqueous eruptions, or are deposited following erosion and remobilization (reworking) of
erup-tion products. Distinguishing between primary, redeposited, and reworked deposits is often problematic, but the abundance of angular and broken crystals in addition to lithic fragments in the tuffs argues for a primary or redeposited pyroclastic origin. Subaerial eruptions that settle through the water column are typically well-sorted and are distributed over an extensive area (McPhie et al., 1993). In contrast, the poor sorting, generally unmodified to slightly modified crystal and lithic fragment shapes, and the limited extent of the bedded tuffs at locality B are consistent with deposition or redeposition from a nearby subaqueous eruption (McPhie, 1995).
4. Vertical and lateral facies transitions
as a result of increasing surface area (Cas, 1992). The lobe to pillow transition in the Beaulieu River volcanic belt indicates decreasing flow rate (Griffiths and Fink, 1992). Autoclastic pillow breccia and disorganized hyaloclastite in the Peltier Formation is interpreted to have developed at flow tops and fronts from quench fragmenta-tion during the late stages of an erupfragmenta-tion in a remote part of a volcanic edifice where lava sup-ply was sufficiently decreased (Dimroth et al., 1978; Busby-Spera, 1987). Stratified hyaloclastite, or hyalotuff, in the Beaulieu River volcanic belt represents the reworked deposits of pillowed flows and autoclastic pillow breccia and hyaloclastite (Dimroth et al., 1978). Bedded tuffs interstratified with massive flows indicate syn-volcanic deposi-tion of localized subaqueous erupdeposi-tion material. Shale units intercalated with pillow breccia and hyaloclastite mark the boundaries between sepa-rate flow events, indicating periods of volcanic quiescence.
5. Discussion
Volcanic facies characteristics and associations are contingent on eruption style, depositional pro-cesses and environment, and tectonic setting. In contrast to studies of modern seamounts, identify-ing and describidentify-ing Archean pillow volcano or seamount sequences is facilitated by cross-sec-tional exposures through the edifice and unam-biguous contact relationships. Problems may arise in areas that have undergone extensive metamor-phism, deformation, and erosion of the seamount summit. Notwithstanding, a model reconstructing a seamount based on the facies architecture in all three localities is attempted because (1) examples of Archean seamounts are lacking, (2) using a model facilitates interpretation of eruptive pro-cesses in addition to lateral and vertical facies distribution, and (3) the study areas contain well-exposed homoclinal sequences.
aqueous environment in which effusion rates were generally low (Dimroth et al., 1978; Ballard et al., 1979). Although all three study areas document mafic subaqueous volcanism, differing facies types, structures, and percentage of vesicularity, are characteristic of specific locations on a typical seamount.
An increase of pillow breccia and hyaloclastite with interstratified sedimentary deposits at local-ity B represents growth of a seamount outward from the source (Fisher, 1984 and Fig. 13(B)). Following quench fragmentation of pillowed flow tops and fronts, periods of volcanic quiescence ensued, resulting in the accumulation of very fine-grained sedimentary deposits between pillow brec-cia and hyaloclastite (Fig. 13(B)). Massive hyaloclastite units typically accumulate at vents relatively remote from the magma source where discharge temperatures are significantly less (Lonsdale and Batiza, 1980; Busby-Spera, 1987). Effusive volcanism ensued following hyaloclastite emplacement, resulting in the deposition of mas-sive flows. Local interstratified bedded tuffs were deposited from turbidity currents following lim-ited subaqueous pyroclastic eruptions (Fig. 13(B)). Volcaniclastic deposits are typically more common in remote parts of a volcanic edifice, away from the near-vent setting (McPhie, 1995; Orton, 1996). The increase in vesicularity (0 – 27%) with respect to locality A (0 – 5%) suggests that the pillowed and massive flows, pillow breccia, and hyaloclastite at locality B were deposited in water
B500 – 800 m deep, but no shallower than 200 m based on the presence of non-stratified pillow breccia and interstratified shale, indicating a be-low wave base setting (Fig. 13(B)).
Stratified pillow breccia and hyaloclastite and increased vesicularity in the Beaulieu River vol-canic belt are characteristic features of the upper-most part of a subaqueous edifice (Staudigel and Schminke, 1984, Fig. 13(C)). Scoriaceous, glass shard-rich, stratified hyaloclastite formed by wave reworking and redeposition along the flanks of an edifice in shallow water (B200 m). Based on the lateral and vertical facies associations with coher-ent pillowed flows, the stratified pillow breccia is
interpreted to have developed at the head of steep flow fronts in shallow water (Dimroth et al., 1978, Fig. 13(C)). In addition, high vesicularity (20 – 49%) in pillowed-lobate and massive flows, and in pillow breccia, supports a shallow water setting (Moore and Schilling, 1973). The study area in the Beaulieu River volcanic belt is a good repre-sentative of the topmost portion of a seamount (Fig. 13(C)).
5.2. Analogues
Volcanic facies in the Peltier Formation and Beaulieu River volcanic belt resemble those of Phanerozoic seamounts or pillow volcanoes that develop on the ocean floor in mid-oceanic or back-arc rift zones. Mid-oceanic analogues in-clude modern seamounts associated with the Mid-Atlantic (Smith and Cann, 1992; Head et al., 1996) and Juan de Fuca (Chadwick and Embley, 1994) ridges, and the East Pacific Rise (Lonsdale and Batiza, 1980; Hekinian et al., 1989; Smith and Batiza, 1989), in addition to Mesozoic-Cenozoic seamounts in the Canary Islands (Staudigel and Schminke, 1984) and Cyprus (Eddy et al., 1998). Further comparisons can be made to modern arc-backarc-related seamounts in the Mariana Trough (Fryer, 1995), and the Lau Basin (Hawkins, 1995), and to Cenozoic seamounts in southwest Japan (Kano et al., 1993) and in the Japan Sea (Sohn, 1995).
The pillowed flow-dominated sequence at local-ity A in the Peltier Formation is similar to B45 m-thick pillow mounds on the Cleft segment of the Juan de Fuca Ridge (Chadwick and Embley, 1994), 50 – 650 m-high pillow-dominated seamounts in the rift valley of the Mid-Atlantic Ridge (Smith and Cann, 1992; Head et al., 1996), and the 200 m-high Alestos Hill seamount in an
inter-graben zone of the Troodos ophiolite, Cyprus (Eddy et al., 1998). Edifices in these set-tings are either inferred or proven to be fed by magma migrating through feeder dyke/sill com-plexes. Mafic edifices characterized by abundant feeder dykes and sills indicate extension associ-ated with rifting (Walker, 1993).
Extensive disorganized autoclastic pillow brec-cia and hyaloclastite assobrec-ciated with coherent massive and pillowed flows at locality B resemble the Trachyte I lithofacies of the inferred 2000 m-high, back-arc Tok Island volcano, Korea, which represents a subaqueous effusive episode before the onset of explosive shallow-water to emergent volcanism (Sohn, 1995). The interstrat-ification of bedded tuffs and massive flows at locality B is comparable to the relationship be-tween distal volcaniclastic deposits and volcanic flows in the \1500 m-thick, inferred island-arc seamount on the Shimane Peninsula, southwest Japan, as described by Kano et al. (1993). Inter-stratified sedimentary deposits recording volcanic quiescence are comparable to those presently ac-cumulating on seamount summits 800 – 2500 m deep near the East Pacific Rise (Lonsdale and Batiza, 1980; Smith and Batiza, 1989).
Stratified hyaloclastite and pillow breccia asso-ciated with coherent pillowed and massive flows in the Beaulieu River volcanic belt are analogous to the intermediate-shallow water flank deposits of the 1800 m-thick, Pliocene seamount sequence at La Palma, Canary Islands (Staudigel and Schminke, 1984). The Archean Slave Province examples are best compared with the seamount at La Palma, which is characterized by a deep water, basal pillowed sequence intruded by dykes and sills, overlain by in-situ hyaloclastite and pillow breccia, changing up-section into intermediate-shallow water, stratified, reworked deposits (Staudigel and Schminke, 1984). The paucity of mafic explosive debris in the Peltier Formation and Beaulieu River volcanic belt indicates that either (1) none of the seamounts breached the water surface, or (2) the emergent portions of the seamounts have been eroded.
Comparisons with Phanerozoic mafic sub-aqueous edifices have shown that distinguishing between Archean seamounts forming in different
tectonic settings based solely on volcanology is problematic. Elucidating the tectonic setting would be facilitated by integrating volcanology and geochemistry. On a regional scale, the Point Lake and Beaulieu River belts are tentatively interpreted to represent remnants of arc systems that developed on continental crust based on geo-chemical results from Lambert et al. (1992) and preliminary results from Dostal and Corcoran (1998) which indicate that the rocks range from mainly tholeiitic to calc-alkaline in composition, and in the Point Lake belt display both negative and positiveond values. On a more detailed scale,
the tholeiitic basalts in each of the study areas are similar to MORB, indicating that the seamounts may be associated with mid-oceanic or back-arc rifting. More in-depth geochemical results and implications from the Peltier Formation are the focus of a subsequent manuscript in preparation.
6. Conclusions
parable to subaqueous volcanic sequences in mod-ern and Archean settings, indicating that seamounts may have been common features on ancient ocean floors.
Acknowledgements
This project was made possible by operating grants to J. Dostal from the Geology Division of the Department of Indian Affairs and Northern Development (contribution no. 99-002) and LITHOPROBE (contribution no. 1024). Great appreciation goes out to Jarda Dostal, James White, C.J. Northrup, Clark Isachsen, Becky Jamieson, John Waldron, and Nick Culshaw for their valuable input and to Wulf Mueller for his help in the field and endless stream of informa-tion. Many thanks to Clarence Picket and Michael Coˆte´ for their assistance in the field and especially to Rene´e-Luce Simard for her patience and diligence. Incisive reviews by Mike Easton and Harald Stollhofen significantly improved the manuscript.
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