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Precambrian Research 101 (2000) 237–261

Recognizing distinct portions of seamounts using volcanicfacies analysis: examples from the Archean Slave Province,

NWT, Canada

P.L. Corcoran *Department of Earth Sciences, Dalhousie Uni6ersity, Halifax, NS, Canada B3H 3J5

Abstract

Archean volcanic rocks in the mafic-dominated, ca. 2.66–2.69 Ga Point Lake and Beaulieu River belts, SlaveProvince, Northwest Territories, are significant in demonstrating the facies that characterize specific portions of pillowvolcanoes or seamounts, irrespective of tectonic setting. Three distinct localities mapped in detail display faciesconsistent with: (1) proximal, deep-water, (2) medial to distal, deep-water, and (3) medial, shallow-water seamountsettings. The proximal facies in the Point Lake belt include a 55-m-thick, non-vesicular pillowed sequence cut bynumerous mafic dykes and sills. Dykes contain multiple chilled margins, indicating successive magma pulses whichcontributed to edifice construction. Abundant feeder conduits, in addition to the absence of fragmental facies andvesicles, 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 pillowbreccia, and pillowed and massive flows with 5–27% vesicularity. Massive, non-vesicular hyaloclastite intermingledwith sedimentary material (fluidal peperite), in addition to thin shale units interstratified with pillow breccia andhyaloclastite, indicate that sedimentation and volcanism were contemporaneous. An increase in fragmental units andvesicularity relative to the proximal, deep water facies is suggestive of the medial to distal part of a seamount inshallower water. Bedded tuffs, laterally along strike with massive flows, are the results of turbidity current depositionimmediately following localized subaqueous eruptions. A medial, shallow water seamount setting is represented in theBeaulieu River belt, by a 5–85 m-thick sequence of vesicular lobate-pillowed and massive flows, stratified pillowbreccia and hyaloclastite, and mafic dykes. Vesicularity ranges from 21–49% in pillowed flows, 5–40% in massiveflows, and 20–35% in pillow breccia and hyaloclastite. Stratified pillow breccia developed along steep flow fronts inshallow water whereas bedded hyaloclastite formed during reworking and redeposition of autoclastic hyaloclastite onseamount flanks in shallow water. The volcanic facies associations in the study areas are analogous to those ofmodern seamounts associated with the Mid-Atlantic Ridge and East Pacific Rise, as well as Mesozoic-Cenozoicseamounts in the Canary Islands, Fiji, southwest Japan, the Sea of Japan, and Cyprus. Volcanological studies in thePoint Lake and Beaulieu River volcanic belts and subsequent comparisons with Phanerozoic analogues, demonstratethe manner in which distinct portions of ancient seamounts can be recognized in similar Archean terranes. © 2000Elsevier Science B.V. All rights reserved.

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* Corresponding author.E-mail address: corcoran@is2.dal.ca (P.L. Corcoran)

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P.L. Corcoran / Precambrian Research 101 (2000) 237–261238

Keywords: Seamounts; Archean; Slave Province; Mafic volcanic; Multiple dykes; Stratified hyaloclastite; Peperite; Shale

1. Introduction

Seamounts, also referred to as pillow moundsor pillow volcanoes, are mafic volcanic edificesthat form on the ocean floor. These subaqueousfeatures, 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 andEmbley, 1994; McPhie, 1995) and are generallycharacterized by central feeder conduits (Fisher,1984; Head et al., 1996), in addition to predomi-nant pillowed and sheet flows (Chadwick andEmbley, 1994; Orton, 1996). Pillow breccia andhyaloclastite are commonly associated with pil-lowed and sheet flows on seamount flanks (Fisherand Schmincke, 1984; Staudigel and Schminke,1984). The volcanic facies constituting seamountsoften overlie deep water sediments and/or areinterstratified with sedimentary material depositedas suspension fallout during volcanism (Fisher,1984). Seamounts, although primarily associatedwith mid-oceanic rift zones, have also been relatedto back-arc, arc, and hot spot volcanism. Distinc-tion between mid-oceanic and back-arc seamountsis often problematic because mafic and felsic vol-canic rocks in both tectonic settings display simi-lar geochemistry (Thurston, 1994). MORB-typesignatures are commonly associated with bothspreading centres, but tectonic reconstruction maybe facilitated where back-arc related seamountscontain rocks of arc-type compositions, as indi-cated by island arc or calc-alkaline basalts andandesites (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; Chadwickand Embley, 1994), possible conduits throughwhich magma is fed to the surface (Fornari et al.,1985; Smith and Cann, 1992; Bryan et al., 1994),petrological and geochemical variations on andoff 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 architectureof a Pliocene seamount in the Canary Islands,McPhie (1995) discussed the facies associationscomprising a Pliocene seamount in Fiji, and Kanoet al. (1993) described the volcanic facies of aMiocene seamount in Japan, but examples ofArchean seamount facies are lacking. Archeangreenstone belts compare favourably with modernvolcano-sedimentary sequences in terms of lithol-ogy, compositional changes with edifice evolution,and structure (Ayres and Thurston, 1985; Taira etal., 1992; Thurston, 1994). Greater inferred heatproduction, sea floor spreading, and eruptionrates during the Archean relative to modernregimes produced more volcanic rocks withthicker tholeiitic basaltic sequences (Taira et al.,1992; Windley, 1995), suggesting that seamountsmust have been prominent features on theArchean ocean floor.

This paper presents Archean mafic volcanicfacies in the Slave Province, Northwest Territo-ries, Canada, that resemble the facies comprisingdistinct portions of modern seamounts. Models ofseamount construction based on the facies associ-ations of two volcanic belts at three detailedlocalities are provided. Although modern exam-ples contribute information concerning waterdepth, composition of unaltered volcanic mate-rial, and location of the edifice with respect to aspreading centre, seamount core exposure andcontact relationships between facies are generallyabsent. Cross sections through ancient rocks thatdemonstrate well-preserved volcanic structurescontribute substantially in recognizing the faciesthat form at specific levels during seamountconstruction.

2. Slave Province geology

The Point Lake and Beaulieu River volcanicbelts are located in the Slave Province, a 500×

P.L. Corcoran / Precambrian Research 101 (2000) 237–261 239

Fig. 1. Lithological map of the Slave Province (SP) in the Northwest Territories (NT), Canada, illustrating the location of the PeltierFormation and Beaulieu River volcanic belt along the Beniah Lake fault. Modified from Corcoran et al. (1998).

P.L. Corcoran / Precambrian Research 101 (2000) 237–261240

700 km Archean craton in the Northwest Territo-ries of Canada (Fig. 1). The 4.03 Ga Acastagneisses (Bowring and Williams, 1998), and their\2.8 Ga counterparts, including the SleepyDragon Complex and Augustus Granite (Hender-son et al., 1987; Northrup et al., 1999), are base-ment to overlying greenstone belts in the westernpart of the craton. Volcanic rocks in the SlaveProvince are subordinate to sedimentary rocksand 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 theprovince, whereas 2.67–2.7 Ga intermediate tofelsic rocks are more common in the east(Padgham, 1985). The 2.66–2.69 Ga Point Lakebelt (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 becorrelated with the 2722–2658 Ma (Isachsen andBowring, 1997) Yellowknife volcanic belt (Fig. 2).The Yellowknife volcanic belt is divided into themafic flow-dominated Kam Group and the felsicvolcaniclastic-dominated Banting and Duncan

Fig. 2. Stratigraphy of the Yellowknife volcanic belt, Slave Province and correlations with the Peltier Formation and Beaulieu Rivervolcanic belt. Age dates from: (1) Isachsen et al., 1991; Isachsen and Bowring, 1994, 1997; (2) Henderson et al., 1987; (3) Muelleret al., 1998; (4) Northrup et al., 1999. JF, Jackson Lake Formation; BF, Burwash Formation; CL, Clan Lake felsic volcaniccomplex; BRF, Beaulieu Rapids Formation; Beaulieu River volcanic belt, Beaulieu River volcanic belt; SD, Sleepy DragonComplex; KF, Keskarrah Formation; S/B, Samandre and Beauparlant formations; CF, Contwoyto Formation; AG, AugustusGranite. Modified from Corcoran et al. (1998).

P.L. Corcoran / Precambrian Research 101 (2000) 237–261 241

Lake groups (Helmstaedt and Padgham, 1986).The 10–15 km-thick basaltic Kam Group containsmassive, pillowed, and brecciated flows, intrudedby gabbro sills and dykes (MacLachlan and Helm-staedt, 1995). Locally, metre-thick felsic volcani-clastic units are interstratified with the basalts. TheBanting and Duncan Lake groups, inferred tooverlie the Kam Group unconformably, are repre-sented mainly by felsic volcanic, felsic volcaniclas-tic, and turbiditic rocks. The latter, referred to asthe Burwash Formation (Duncan Lake Group) areassociated with ca. 2661–2663 Ma felsic volcaniccentres (Henderson et al., 1987; Mortensen et al.,1992). This assemblage is unconformably overlainby the Jackson Lake Formation (B2605 Ma;Isachsen et al., 1991), an alluvial-marine sequence(Mueller and Donaldson, 1994), similar to the 2600Ma alluvial-lacustrine Beaulieu Rapids Formation(Corcoran et al., 1999) overlying the Beaulieu Riverbelt unconformably, and the 2605 Ma (Isachsenand Bowring, 1994) alluvial-marine KeskarrahFormation (Corcoran et al., 1998) overlying thePoint Lake belt unconformably.

2.1. Local geology

The Slave Province is characterized by north-trending lineaments along which several volcanicbelts and most of the 2.6 Ga late-orogenic sedimen-tary rocks are exposed (Fig. 1). The Point Lake andBeaulieu River volcanic belts are located along thenorth-trending Beniah Lake fault (Fig. 1), a linea-ment previously interpreted to coincide with amajor tectonic break between an older, westernterrane containing a sialic basement and a younger,eastern terrane (Padgham and Fyson, 1992). Previ-ous studies have demonstrated the crucial role ofthe Beniah Lake fault in the development of ca. 2.6Ga conglomeratic sequences (Corcoran et al., 1998,1999), but the significance of this structure in theformation of 2.66–2.69 Ga volcanic belts remainsdebatable.

The Peltier Formation, a subaqueous mafic-dominated succession located in the north-centralSlave Province, comprises part of the Point Lakebelt (this paper; Fig. 2(C)) or Point Lake Group asdefined by Henderson (1998). Andesitic-dacitic vol-caniclastic deposits are locally interstratified with

the basaltic Peltier formation, but the majority ofthe intermediate and felsic volcanic rocks comprisethe Samandre and Beauparlant formations, respec-tively (Fig. 2(C)). The sedimentary ContwoytoFormation is time-equivalent with the Peltier For-mation as indicated by interstratified turbiditicdeposits 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 overliethe mafic-felsic volcanic rocks unconformably (Fig.2). Preliminary geochemical data indicate that thePeltier Formation, over a regional area of 12.5×17.5 km contains tholeiitic basalts and subordinatecalc-alkaline basalts and andesites with SiO2 con-tents ranging from 46–59% (Dostal and Corcoran,1998). Two study areas composed of tholeiiticbasalts and referred to as localities A and B, wereselected for detailed work (Fig. 3). Although thetrue thickness of the Peltier Formation remainsenigmatic due to structural complexity in the PointLake region (Henderson, 1998), the most extensivehomoclinal sequence identified is :1.5 km thick,of which locality B constitutes the basal part of theuppermost 700 m (Fig. 3). A northwest-southeasttrending, northeast-dipping reverse-slip fault sepa-rates localities A and B (Henderson, 1988).

The Beaulieu River volcanic belt, adjacent to thenorth-trending Beniah Lake fault in the south-cen-tral part of the Slave Province (Fig. 4), is inferredto overlie the Sleepy Dragon Complex uncon-formably. Sedimentary rocks of the 2.6 Ga(Mueller et al., 1998) Beaulieu Rapids Formationoverlie the volcanic succession unconformably(Fig. 2(B)). Tholeiitic basalts and calc-alkalinebasalts and andesites predominate, but minor felsictuffs, breccias, and flows also characterize thesequence (Lambert et al., 1992). One study area, 85m thick and composed of tholeiitic basalts, wasselected for detailed study and comparison with thelocalities in the Peltier Formation because: (i) thePoint Lake and Beaulieu River belts are spatiallyrelated in that they are adjacent to the 600 km-longBeniah Lake fault (Fig. 1), (ii) both belts occupysimilar stratigraphic positions with respect to sur-rounding rock types (Fig. 2), and (iii) all detailedlocalities display a variety of comparable maficvolcanic facies (Figs. 3 and 4).

P.L. Corcoran / Precambrian Research 101 (2000) 237–261242

Fig. 3. Location of the Peltier Formation at Point Lake relative to the Contwoyto and Keskarrah formations and the basementAugustus granite. A 55×160 m schematic section through locality A and an 80×230 m schematic section through locality Billustrate the facies architecture of volcanic edifices in the Peltier Formation. Note the location of bedded tuff interstratified withmassive flows at locality B. Modified from Corcoran et al. (1998).

P.L. Corcoran / Precambrian Research 101 (2000) 237–261 243

Fig. 4. (A) Location of the Beaulieu River volcanic belt relative to the Beniah Lake fault and 2.8–2.9 Ga plutono-gneissic SleepyDragon complex. (B) Location of the study area in the Beaulieu River volcanic belt. (C) Schematic section through the study areademonstrating the vertical and lateral facies changes over 85×300 m.

P.L. Corcoran / Precambrian Research 101 (2000) 237–261244

3. Volcanic facies in the Point Lake and BeaulieuRiver belts

The study areas in the Peltier Formation, PointLake 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). TheBeaulieu 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) synvolcanicmafic dykes (5%), and (5) stratified hyaloclastite(5%). Mapping at scales of 1:100 and 1:300 in thePeltier Formation and 1:20 and 1:100 in theBeaulieu River volcanic belt was conducted toconstrain lateral and vertical facies distributionand to document volcanic structures. Rocks in the

study areas are steeply dipping (75–90°) and havebeen affected by greenschist facies metamorphism,as indicated by the mineral assemblage chlorite9epidote9albite9hornblende9carbonate, butthe prefix ‘meta’ is omitted for simplicity.

3.1. Pillowed-lobate flows

Pillowed flow units at localities A and B in thePeltier 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 intersticesbetween pillows. Percentage of vesicularity differsfrom 0–5% at locality A to 0–27% at locality B(Table 1) where vesicles range from 0.5 to 10 mmin diameter. Spherical to ovoid vesicles are con-centrically zoned from chlorite to quartz-albite orare entirely filled with calcite or chlorite. Pillowedflows in the Peltier Formation display hyalopilitic

Fig. 5. Mafic volcanic facies at locality A in the Peltier Formation. Three north-south trending dykes with multiple intrusions cutan east-west trending sill that has intruded a pillowed sequence.

P.L. Corcoran / Precambrian Research 101 (2000) 237–261 245

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 interstratificationof shale units between flows III, IV, and V.

and hyalophitic textures with plagioclase micro-lites, and 0.2–0.4 mm-size plagioclase phenocrystsand secondary hornblende, respectively, in chlori-tized sideromelane.

The 3–15 m-thick pillowed flows in theBeaulieu River volcanic belt contain closely-packed pillows, ‘isolated’ pillows, and lobe struc-tures (Fig. 8). Closely-packed pillows, similar toclose-packed pillows illustrated by Yamagishi(1994; p. 66), range in size from 20–150 cm, arecharacterized by 1–2 cm-thick chilled marginsand some demonstrate thermal contraction frac-tures (Fig. 7(B, C)). Lobe structures, 0.5–5 mlong, are distinguished by discontinuous, 0.25–1cm thick vesicular flow bands that are parallel to

upper and lower flow margins and are locallyfolded (Fig. 7(D) Fig. 8). Isolated pillows, B20cm in size, analogous to incompletely-formed orwelded pillows of Dimroth et al. (1978) or pillow‘ghosts’ as described by Busby-Spera (1987), areformed along lobe margins (Fig. 8; flow III),resembling the lava lobes of Yamagishi (1994; p.42). Vesicles, comprising 21–49% of pillowedflows (Table 1), are spherical to ovoid and B0.5–15 mm in size. Calcite and chlorite fill vesicles,and although common throughout pillows, vesi-cles tend to be concentrated in pillow centres (Fig.9(A)). Pillow centres are chiefly composed of mi-crogranular plagioclase, whereas plagioclase mi-crolites in a glassy matrix are predominant near

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Fig. 7.

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Table 1Percentage of vesicularity in massive and pillowed flows and pillow breccia in the Peltier Formation and Beaulieu River volcanicbelta

Volcanic facies Number of vesicles Total countSample/outcrop num- Percentage vesicularity (%)ber

Peltier Formation-Locality B135 500 27PLC-96-78 Massive flow (basal)44 500Massive flow (middle) 9PLC-97-82

PLC-97-84 109Massive flow (basal) 500 2291 521Pillow breccia 17Photo 9b

Beaulieu Ri6er 6olcanic belt243 500Pillowed flow (pillow rim) 49PLC-98-36

PLC-98-37 227Pillowed flow (pillow rim) 500 45183 500Massive flow (basal) 37PLC-98-38

Pillowed flow (pillow centre)PLC-98-39 103 500 21104PLC-98-40 500Pillowed flow (lobate part) 21121 500Pillowed flow (lobate part) 24PLC-98-4199 249Photo 9a 40Pillowed flow (whole pillow)

135 362Pillow breccia 37Photo 9cMassive flow (basal)Photo 9d 192 434 44

a Determined by visual estimate and by counting vesicles and amygdules (filled vesicles) using 100 point grids with 1 cm and 3 mmspacing 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 areeither 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. InterpretationPillowed flows studied extensively in both

Archean (Dimroth et al., 1978; Hargreaves andAyres, 1979; Wells et al., 1979; Cousineau andDimroth, 1982) and Phanerozoic (Moore, 1975;Moore and Lockwood, 1978; Yamagishi et al.,1989; Yamagishi, 1991; Walker, 1992) settingsform when hot lava enters into or erupts underwater. 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 andLutz, 1998) and the association with hyaloclastite(McPhie et al., 1993). Closely-packed pillows inall study areas represent the normal, molded pil-lows of Dimroth et al. (1978) that are interpretedto develop when flow velocity and temperaturehas decreased. Isolated pillows are the results ofcooling that was too rapid to allow for completeformation (Dimroth et al., 1978) or were buriedrapidly by overlying lava during high eruptionrates leading to incomplete chilling (Busby-Spera,1987). Apparent massive areas between isolatedpillows may mark the location of lava tubes(Busby-Spera, 1987) or welded megapillows (Dim-

Fig. 7. Characteristics of pillows and lobe structures in the Peltier Formation and Beaulieu River volcanic belt. Large arrowsindicate younging direction. (A) Closely packed pillows in a pillowed flow (PF) from locality A. Scale, fieldbook 18.5 cm. (B)Vesicles (V) and a chilled margin (CM) of a pillow from the Beaulieu River volcanic belt. Scale, pencil, 14 cm. (C) Thermalcontraction fractures (F) in a pillow from the Beaulieu River volcanic belt. Scale, pencil 14 cm. (D) Lobe structures (small arrows)and vesicular laminae (VL) that laterally becomes pillowed in the Beaulieu River volcanic belt. Scale, pencil 14 cm. (E)Photomicrograph of chlorite-filled (CH) and calcite-filled (CA) vesicles near the rim of a pillow, Beaulieu River volcanic belt. (F)Photomicrograph of a calcite-filled (CA) vesicle and a concentrically zoned amygdule with chlorite+quartz (CH+Q) along the rimand calcite (CA) in the core.

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Fig. 8. Volcanic facies relationships in the Beaulieu River belt locality demonstrating four flow events. Flow I changes upsectionfrom closely packed pillows (CPP) to isolated pillows that are subsequently intruded by a mafic dyke. Flow II is massive andillustrates the concentration of vesicles at flow base and top. Note the lateral transition from lobe structures to isolated pillows thenpillow breccia in flow III. Massive flow IV is in sharp contact with flow III.

roth et al., 1978). Fractures in pillows in theBeaulieu River volcanic belt developed from ther-mal contraction during cooling (Wells et al., 1979;Yamagishi, 1991). Large vesicles at pillow centresrepresent coalesced bubbles that did not haveenough time to migrate toward the upper pillowmargin before it cooled and crystallized.

3.2. Pillow breccia

Pillow breccia at locality B, 5–35 m thick, iscomposed of disorganized pillow fragments andisolated pillows (Fig. 6). Pillow fragments, 2–30cm 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 massivehyaloclastite (Fig. 6, Sect. 3, flows IV and V).Vesicularity ranges from 5–17% with sphericalvesicles B1 cm in size (Table 1, Fig. 9(B)). Thin,B1.5 m-thick units of pillow breccia at locality Alocally characterize pillowed flow tops (Fig. 5).

In the Beaulieu River volcanic belt, crudelystratified pillow breccia, 0.5–6 m thick, containswhole pillows and pillow fragments (Fig. 10(B),Fig. 11). Whole pillows with 0.5–1.5 cm-thickchilled margins, are either 10–15 cm long andsub-spherical, or 10–25 cm long and amoeboid(Fig. 10(C)) and are locally associated with B30cm-thick hyaloclastite. Pillow fragments, 1–18 cmin size, are subangular to subrounded and gener-

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ally lack chilled margins. Vesicles, B1.5 cm insize, are common in whole pillows and pillowfragments, in which vesicularity ranges from 20 to37% (Table 1 and Fig. 9(C)). The matrix of pillowbreccia is chiefly composed of microgranularplagioclase.

3.2.1. InterpretationPillow breccia generally develops during quench

fragmentation resulting from lava-water interac-tions (Dimroth et al., 1978; Yamagishi, 1991).Alternatively, ‘pillow fragment breccia’ formsduring mechanical disintegration of pillow lavadue to slumping (Fisher and Schmincke, 1984;Staudigel and Schminke, 1984; Busby-Spera,

1987). In addition, where lava flows into shallowwater, wave or tide action combined with a steepflow front may be sufficient to break pillows intofragments (Moore, 1975; Kokelaar, 1986).Crudely bedded pillow breccia in the BeaulieuRiver volcanic belt suggests either gravity-induceddeposition along seamount flanks in shallow wa-ter (Staudigel and Schminke, 1984; McPhie et al.,1993) or may represent flow-front deposits wherepillow 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 ofpillowed flow tops and fronts (Dimroth et al.,1978).

Fig. 9. Vesicularity index of massive and pillowed flows and pillow breccia in the Peltier Formation and Beaulieu River belt (resultsalso in Table 1). Vesicle percentage (V) and number of counts (N) are indicated. (A) Vesicular pillow in the Beaulieu River volcanicbelt with a marked chilled margin and concentration of larger vesicles towards the pillow core. (B) Vesicles in subangular tosubrounded pillow fragments in pillow breccia from locality B, Peltier Formation. (C) Vesicles in subangular pillow fragments inpillow breccia from the Beaulieu River volcanic belt. (D) Vesicles at the base of a massive flow, Beaulieu River volcanic belt.

P.L. Corcoran / Precambrian Research 101 (2000) 237–261250

Fig. 10. Characteristics of pillow breccia and massive flows in the Peltier Formation and Beaulieu River volcanic belt. Large arrowindicates 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 andwhole pillows with chilled margins (pillow just below ‘PB’ label). Scale, pencil 14 cm. (C) Amoeboid pillow (AP) in pillow brecciafrom the Beaulieu River volcanic belt. Note the mm-thick chilled margin and the tendency for the largest vesicles (LV) to occur inthe pillow centre. Scale, pencil 14 cm. (D) Calcite amygdule (A) from the basal part of massive flow IV (MF), Beaulieu Rivervolcanic belt (see Fig. 8). Scale, pencil 14 cm.

3.3. Massi6e flows

Massive flows at locality B, 3–40 m thick, areup to 22% vesicular at flow bases, decreasing to0% 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 characterizemassive flows. Plagioclase phenocrysts, B0.2 mmlong, are surrounded by plagioclase microlites,creating an intergranular texture, whereashyalopilitic textures are represented by plagioclasemicrolites in a glassy matrix.

Individual massive flows in the Beaulieu Rivervolcanic belt are 6–20 m thick (Fig. 8). Vesicular-

ity ranges from 5% at flow centres to 35–44% atflow tops and bases (Table 1 and Figs. 8 and9(D)). Spherical to ovoid vesicles, 0.5–1.2 cm insize, are locally calcite-filled (Fig. 10(D)). Plagio-clase microlites in chloritized sideromelane pro-duce hyalopilitic textures.

3.3.1. InterpretationMassive basalts represent non-channelized sheet

flows that form during the initial stages of sub-aqueous eruption (Ballard et al., 1979; Cousineauand Dimroth, 1982) and are characteristic ofhigher lava effusion rates and temperatures com-pared to pillowed flows (Yamagishi, 1991). Abun-dant vesicles at flow bases and tops with

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vesicle-poor centres are common features of sub-aqueous massive flows (Sahagian, 1985; McMillanet al., 1987; Aubele et al., 1988). Vesicles developwhen gases are trapped as bubbles during coolingof a flow (Aubele et al., 1988). The bubbles riseduring extrusion and solidification, the largerbubbles rise more quickly, and become trapped inthe upper crystal front (Sahagian, 1985). Accord-ing to Sahagian (1985), the lower crystal frontfollows the direction of the upward migratingbubbles, such that it ‘chases’ and freezes thembefore continuing their ascent. With continuedcooling, the central part of the flow eventuallylacks bubbles because the lower crystal frontfreezes them as they rise more slowly with increas-ing viscosity. The absence of vesicles in flow cen-tres is dependent on flow thickness becausethicker flows take longer to solidify, allowingmore 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 toa 20–45 m-thick massive sill (Fig. 5). Numerous,mm-thick chilled margins, 10–75 cm apart, markthe 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 sizetowards individual margins. Cross-cutting rela-tionships indicate that at locality A, the dykeswere intruded after emplacement of the pillowedflows and sill. Subophitic and intersertal dykesand ophitic sills are composed of 0.3–1 mm-sizesecondary hornblende and 0.1–0.8 mm-sizeplagioclase.

Synvolcanic mafic dykes in the Beaulieu Riverbelt, 0.03–0.4 m wide, cut pillow breccia andpillowed flows (Figs. 11 and 12(C)). The intru-sions begin and terminate within the same flowwith local propagation into individual pillows(Figs. 11 and 12(D)). Transitions from dykes tooverlying flows were nowhere evident, althoughthe dykes are generally perpendicular to flow tops.Porphyritic and glomeroporphyritic textures arepredominant with 0.2–2.5 mm-size plagioclase

phenocrysts and 1–3.5 mm-size phenocryst clus-ters, respectively, in matrices of feldsparmicrolites.

3.4.1. InterpretationSimilarities in composition of flows and dykes,

restriction of dykes within single flow units in theBeaulieu River belt, and multiple intrusions atlocality A in the Peltier Formation, justify a syn-volcanic interpretation. Dyke discontinuity in theBeaulieu River belt is typical of narrow intrusionsthat inject for short distances before cooling andsolidifying (Bruce and Huppert, 1990). Intrusionsoriented perpendicular to bedding and terminat-ing with blunt ends or propagating into pillowsare often classified as feeder dykes (Yamagishi,1991; Kano et al., 1993). These parallel dykesindicate the manner in which overlying flows werefed, but cannot be used to interpret a specificseamount setting because of their restrictionwithin single flow units. Multiple intrusions atlocality 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 Rivervolcanic belt, dykes at locality A cut sills andnumerous pillowed flow units. Multiple dykes in-dicate successive magma pulses where a feederconduit was used several times to supply magmahigher up in the sequence (Mueller and Don-aldson, 1992). Multiple feeder dykes cutting nu-merous flow units associated with sills are typicalof the central part of a volcanic edifice whereconstruction is initiated (Easton, 1984).

3.5. Hyaloclastite

Massive non-vesicular hyaloclastite at localityB, 5–16 m thick, is characterized by isolated,amoeboid pillows (Fig. 12(E)) and is locally sepa-rated by thin, 1–4 m-thick shale units (Fig. 6).The hyaloclastite consists of two components: (1)chloritized sideromelane and (2) very fine-grainedsedimentary material, and thus may be referred toas intrusive or peperitic hyaloclastite (McPhie etal., 1993). Convex-concave sideromelane shardsrange from 0.2 to 3 cm in size and contain 0.02–0.05 mm-size opaque spherules that acted as cores

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around which chalcedony nucleated, and 0.1–0.35mm-size plagioclase microlites (Fig. 12(F)). Veryfine-grained sandstone and siltstone composed ofmicrogranular quartz and feldspar, are containedin 0.5–5 cm-size globules that envelop glassshards, giving the hyaloclastite a fluidal texture(Fig. 12(E, F)).

Stratified hyaloclastite in the Beaulieu River

volcanic belt is characterized by 1–4 cm-thickvesicular layers in 0.5–3 m-thick units (Fig.12(G)). Principal components include 0.3–4.5mm-size scoria containing quartz-albite-filled vesi-cles (Fig. 12(H)), 0.1–0.35 mm-size plagioclasecrystals, and B0.1 mm-size bubble-wall and cus-pate glass shards in a matrix of microgranularfeldspar.

Fig. 11. Outcrop sketch of the relationship between mafic feeder dykes, pillow breccia, and a pillowed flow in the Beaulieu Riverbelt locality. Crudely bedded pillow breccia overlies a pillowed flow. Feeder dykes demonstrate chilled margins and locally propagateinto individual pillows.

Fig. 12. Characteristics of mafic feeder dykes and hyaloclastite in the Peltier Formation and the Beaulieu River volcanic belt. (A)Nine multiple intrusions in dyke I, locality A (see Fig. 5). Scale, chisel 20 cm. (B) Six multiple intrusions in dyke II, locality A (seeFig. 5). Scale, hammer 40 cm. (C) Dyke (D) cutting pillowed flow (PF) I in the Beaulieu River volcanic belt (see Fig. 8) Large arrowindicates younging direction. Scale, pencil 14 cm. (D) Dyke (D) propagating into a pillow (P) in pillow breccia (PB), Beaulieu Rivervolcanic belt (see Fig. 11) Large arrow indicates younging direction. Scale, pencil 14 cm. (E) Disorganized hyaloclastite (H) or fluidalpeperite from section 3, locality B, composed of sideromelane shards, sedimentary material, and isolated amoeboid pillows (AP).Large arrow indicates younging direction. Scale, pencil 14 cm. (F) Photomicrograph of disorganized hyaloclastite (globular peperite)containing sedimentary material (S), chloritized sideromelane (C), and oxide granules (O). (G) Stratified hyaloclastite (H) in theBeaulieu River volcanic belt. Large arrow indicates younging direction. Scale, knife 9 cm. (H) Scoria lapillus (S) in stratifiedhyaloclastite, Beaulieu River volcanic belt. Note the coalescing vesicles (V).

P.L. Corcoran / Precambrian Research 101 (2000) 237–261 253

Fig. 12.

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3.5.1. InterpretationHyaloclastite generally develops as a response

to thermal contraction at flow tops and fronts(Dimroth et al., 1978; McPhie et al., 1993) or mayform as a result of subaqueous lava fountaining(Smith and Batiza, 1989). The fluidal texture re-sulting from the combination of devitrifiedsideromelane containing microlites and granulesand sedimentary material in the Peltier Formationsuggests that the hyaloclastite formed by auto-brecciation when hot lava came into contactwith cool, wet sediment, akin to the formationof peperite (Schminke, 1967). The fluidaltexture of the hyaloclastite is attributed to en-trainment of very fine-grained wet sediment in avapor film at the magma-sediment interface(Busby-Spera and White, 1987). Stratified hyalo-clastite in the Beaulieu River volcanic belt isattributed to resedimentation of reworked pillowbreccia and autoclastic hyaloclastite (McPhie etal., 1993).

3.6. Bedded tuffs

Bedded, fine- to medium-grained volcaniclasticdeposits are the on-strike equivalents of massiveflows at locality B (Fig. 3). The 10–35 m-thick,andesitic-dacitic volcaniclastic rocks arelocally massive, but are generally characterized by10–50 cm-thick planar beds. The rocks arepoorly sorted and contain 0.1–1.2 mm, euhedral,subangular, and broken plagioclase crystals, B1.6 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. InterpretationThe andesitic-dacitic volcaniclastic rocks at lo-

cality B are referred to as tuffs, based on the grainsize classification of Fisher (1961, 1966). The tuffsare interpreted as Bouma Ta divisions (Bouma,1962) or S3 beds (Lowe, 1982), the results ofturbidity current deposition (McPhie, 1995). Vol-caniclastic rocks are typically the direct orredeposited products of subaerial and/or sub-aqueous eruptions, or are deposited followingerosion and remobilization (reworking) of erup-

tion products. Distinguishing between primary,redeposited, and reworked deposits is oftenproblematic, but the abundance of angular andbroken crystals in addition to lithic fragments inthe tuffs argues for a primary or redepositedpyroclastic origin. Subaerial eruptions thatsettle through the water column are typicallywell-sorted and are distributed over an extensivearea (McPhie et al., 1993). In contrast, thepoor sorting, generally unmodified to slightlymodified crystal and lithic fragment shapes, andthe limited extent of the bedded tuffs at locality Bare consistent with deposition or redepositionfrom a nearby subaqueous eruption (McPhie,1995).

4. Vertical and lateral facies transitions

Volcanic facies in the Peltier Formation andBeaulieu River volcanic belt generally conform tothe ‘standard’ sequence of Dimroth et al. (1978),where massive parts of flows laterally and verti-cally become pillowed, overlain by pillow breccia,and capped by hyaloclastite or hyalotuff. Mostflows display only one or two facies transitions,rarely recording the entire sequence of divisions.Vertical changes from pillowed to pillow brecciawere identified at locality B (Fig. 6; flows I, V), inaddition to lateral transitions from pillowed topillow breccia, locally grading into hyalo-clastite (Fig. 6; flows IV, V). Discrete shale units,1–4 m thick, separate pillow breccia and hyalo-clastite (Fig. 6; flows III, IV, V). Similar lateraltransitions from lobate to pillowed (isolated pil-lows), to pillow breccia (Fig. 8; flow III), andvertical changes from pillowed to pillowbreccia (Fig. 11), were identified in the BeaulieuRiver volcanic belt. All visible contacts betweenthe massive facies and overlying pillows and pil-low breccia are sharp (Figs. 6 and 8). Bedded,fine- to medium-grained tuffs are interstratifiedwith the summital massive flow unit at locality B(Fig. 3). Stratified hyaloclastite in the BeaulieuRiver volcanic belt, located immediately below theunconformity with the overlying sedimentary se-quence, sharply overlies a massive flow (Fig.4(C)).

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Vertical transitions from pillowed to pillowbreccia are explained by changing discharge ratesduring single eruptive events, whereas lateralchanges represent slope differences and/or dimin-ishing magma supply at the distal parts of flowsas a result of increasing surface area (Cas, 1992).The lobe to pillow transition in the Beaulieu Rivervolcanic belt indicates decreasing flow rate(Griffiths and Fink, 1992). Autoclastic pillowbreccia and disorganized hyaloclastite in thePeltier Formation is interpreted to have developedat flow tops and fronts from quench fragmenta-tion during the late stages of an eruption in aremote 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 beltrepresents the reworked deposits of pillowed flowsand autoclastic pillow breccia and hyaloclastite(Dimroth et al., 1978). Bedded tuffs interstratifiedwith massive flows indicate syn-volcanic deposi-tion of localized subaqueous eruption material.Shale units intercalated with pillow breccia andhyaloclastite mark the boundaries between sepa-rate flow events, indicating periods of volcanicquiescence.

5. Discussion

Volcanic facies characteristics and associationsare contingent on eruption style, depositional pro-cesses and environment, and tectonic setting. Incontrast to studies of modern seamounts, identify-ing and describing Archean pillow volcano orseamount sequences is facilitated by cross-sec-tional exposures through the edifice and unam-biguous contact relationships. Problems may arisein areas that have undergone extensive metamor-phism, deformation, and erosion of the seamountsummit. Notwithstanding, a model reconstructinga seamount based on the facies architecture in allthree localities is attempted because (1) examplesof Archean seamounts are lacking, (2) using amodel facilitates interpretation of eruptive pro-cesses in addition to lateral and vertical faciesdistribution, and (3) the study areas contain well-exposed homoclinal sequences.

5.1. Seamount construction

Abundant pillowed flows, pillow breccia, andhyaloclastite in the Peltier Formation andBeaulieu River volcanic belt attest to a sub-aqueous environment in which effusion rates weregenerally low (Dimroth et al., 1978; Ballard et al.,1979). Although all three study areas documentmafic subaqueous volcanism, differing faciestypes, structures, and percentage of vesicularity,are characteristic of specific locations on a typicalseamount.

Initial seamount construction in water depthsbelow 500–2000 m is represented by locality A inthe Peltier Formation, as suggested by the thickpillowed sequence, dyke/sill complex, absence ofexplosive debris, and low vesicularity (Moore andSchilling, 1973; Fisher, 1984; Staudigel andSchminke, 1984; Kokelaar, 1986; Cas, 1992).Dykes and sills, salient components of seamountarchitecture, are interpreted to represent settingsproximal to the magma source (i.e. vent) becausedyke percentage decreases with increasing dis-tance from a volcanic centre (Walker, 1993; Mc-Phie, 1995). These intrusions have highpreservation potential, especially in ancient rockswhere erosion of the subaerial portions ofseamounts is extensive (Walker, 1993; Sohn,1995). Primary eruption in seamount developmentnecessitates a point source that may be fed di-rectly by pipes or from an initial fissure that hascollapsed, leaving only a few central conduitsthrough which lava is supplied to the edifice(Smith and Cann, 1992; Chadwick and Embley,1994). At locality A, magma originating from apoint source is inferred to have been distributedand emplaced through a complex dyke-sill system,leading to the development of a pillow volcano ormound (Fig. 13(A)). The minimal pillow brecciaand hyaloclastite component at locality A sup-ports a proximal setting with respect to themagma source (Wells et al., 1979; Fisher, 1984;Busby-Spera, 1987). The low vesicularity of flowsis suggestive of a deeper water environment thanthose envisaged for locality B and the BeaulieuRiver volcanic belt because vesicularity typicallydecreases with increasing depth (Moore andSchilling, 1973; Staudigel and Schminke, 1984).

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An increase of pillow breccia and hyaloclastitewith interstratified sedimentary deposits at local-ity B represents growth of a seamount outwardfrom the source (Fisher, 1984 and Fig. 13(B)).Following quench fragmentation of pillowed flowtops and fronts, periods of volcanic quiescenceensued, resulting in the accumulation of very fine-grained sedimentary deposits between pillow brec-cia and hyaloclastite (Fig. 13(B)). Massivehyaloclastite units typically accumulate at ventsrelatively remote from the magma source wheredischarge temperatures are significantly less(Lonsdale and Batiza, 1980; Busby-Spera, 1987).Effusive volcanism ensued following hyaloclastiteemplacement, resulting in the deposition of mas-sive flows. Local interstratified bedded tuffs weredeposited from turbidity currents following lim-ited subaqueous pyroclastic eruptions (Fig.13(B)). Volcaniclastic deposits are typically morecommon 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 thepillowed and massive flows, pillow breccia, andhyaloclastite at locality B were deposited in waterB500–800 m deep, but no shallower than 200 mbased on the presence of non-stratified pillowbreccia and interstratified shale, indicating a be-low wave base setting (Fig. 13(B)).

Stratified pillow breccia and hyaloclastite andincreased vesicularity in the Beaulieu River vol-canic belt are characteristic features of the upper-most part of a subaqueous edifice (Staudigel andSchminke, 1984, Fig. 13(C)). Scoriaceous, glassshard-rich, stratified hyaloclastite formed by wavereworking and redeposition along the flanks of anedifice in shallow water (B200 m). Based on thelateral and vertical facies associations with coher-ent pillowed flows, the stratified pillow breccia is

interpreted to have developed at the head of steepflow fronts in shallow water (Dimroth et al., 1978,Fig. 13(C)). In addition, high vesicularity (20–49%) in pillowed-lobate and massive flows, and inpillow breccia, supports a shallow water setting(Moore and Schilling, 1973). The study area inthe 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 andBeaulieu River volcanic belt resemble those ofPhanerozoic seamounts or pillow volcanoes thatdevelop on the ocean floor in mid-oceanic orback-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 (Lonsdaleand Batiza, 1980; Hekinian et al., 1989; Smith andBatiza, 1989), in addition to Mesozoic-Cenozoicseamounts in the Canary Islands (Staudigel andSchminke, 1984) and Cyprus (Eddy et al., 1998).Further comparisons can be made to modernarc-backarc-related seamounts in the MarianaTrough (Fryer, 1995), and the Lau Basin(Hawkins, 1995), and to Cenozoic seamounts insouthwest Japan (Kano et al., 1993) and in theJapan Sea (Sohn, 1995).

The pillowed flow-dominated sequence at local-ity A in the Peltier Formation is similar to B45m-thick pillow mounds on the Cleft segment ofthe Juan de Fuca Ridge (Chadwick and Embley,1994), 50–650 m-high pillow-dominatedseamounts in the rift valley of the Mid-AtlanticRidge (Smith and Cann, 1992; Head et al., 1996),and the 200 m-high Alestos Hill seamount in an

Fig. 13. Models illustrating the inferred location of Peltier Formation and Beaulieu River belt facies on a seamount. (A) Initialseamount construction on the ocean floor, represented by locality A in the Peltier Formation. Magma originating from a pointsource was transported through the edifice using dykes and sills as conduits and was extruded as pillowed flows in deep water. (B)Continued effusive volcanism of a seamount in shallower water is represented by locality B. Disorganized pillow breccia andhyaloclastite accumulated on the medial portions of the seamount. Interstratified shale units indicate periods of volcanic quiescence.During deposition of massive flows, local andesitic-dacitic eruptions occurred, depositing bedded tuffs. (C) A shallow water settingis represented by the volcanic facies in the Beaulieu River belt where stratified pillow breccia and hyaloclastite were deposited underwave action along the flanks of a seamount.

P.L. Corcoran / Precambrian Research 101 (2000) 237–261 257

Fig. 13.

P.L. Corcoran / Precambrian Research 101 (2000) 237–261258

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 bymagma migrating through feeder dyke/sill com-plexes. Mafic edifices characterized by abundantfeeder dykes and sills indicate extension associ-ated with rifting (Walker, 1993).

Extensive disorganized autoclastic pillow brec-cia and hyaloclastite associated with coherentmassive and pillowed flows at locality B resemblethe Trachyte I lithofacies of the inferred 2000m-high, back-arc Tok Island volcano, Korea,which represents a subaqueous effusive episodebefore the onset of explosive shallow-water toemergent volcanism (Sohn, 1995). The interstrat-ification of bedded tuffs and massive flows atlocality B is comparable to the relationship be-tween distal volcaniclastic deposits and volcanicflows in the \1500 m-thick, inferred island-arcseamount on the Shimane Peninsula, southwestJapan, as described by Kano et al. (1993). Inter-stratified sedimentary deposits recording volcanicquiescence are comparable to those presently ac-cumulating on seamount summits 800–2500 mdeep near the East Pacific Rise (Lonsdale andBatiza, 1980; Smith and Batiza, 1989).

Stratified hyaloclastite and pillow breccia asso-ciated with coherent pillowed and massive flowsin the Beaulieu River volcanic belt are analogousto the intermediate-shallow water flank depositsof the 1800 m-thick, Pliocene seamount sequenceat La Palma, Canary Islands (Staudigel andSchminke, 1984). The Archean Slave Provinceexamples are best compared with the seamount atLa Palma, which is characterized by a deep water,basal pillowed sequence intruded by dykes andsills, overlain by in-situ hyaloclastite and pillowbreccia, changing up-section into intermediate-shallow water, stratified, reworked deposits(Staudigel and Schminke, 1984). The paucity ofmafic explosive debris in the Peltier Formationand Beaulieu River volcanic belt indicates thateither (1) none of the seamounts breached thewater surface, or (2) the emergent portions of theseamounts have been eroded.

Comparisons with Phanerozoic mafic sub-aqueous edifices have shown that distinguishingbetween Archean seamounts forming in different

tectonic settings based solely on volcanology isproblematic. Elucidating the tectonic settingwould be facilitated by integrating volcanologyand geochemistry. On a regional scale, the PointLake and Beaulieu River belts are tentativelyinterpreted to represent remnants of arc systemsthat developed on continental crust based on geo-chemical results from Lambert et al. (1992) andpreliminary results from Dostal and Corcoran(1998) which indicate that the rocks range frommainly tholeiitic to calc-alkaline in composition,and in the Point Lake belt display both negativeand positive ond values. On a more detailed scale,the tholeiitic basalts in each of the study areas aresimilar to MORB, indicating that the seamountsmay be associated with mid-oceanic or back-arcrifting. More in-depth geochemical results andimplications from the Peltier Formation are thefocus of a subsequent manuscript in preparation.

6. Conclusions

Ancient subaqueous, mafic-dominated volcanicsequences in the Slave Province are valuable forstudying facies associations, volcanic processes,and seamount construction. Volcanic facies andfacies associations at three localities in the PointLake and Beaulieu River belts resemble proximal,deep water (\500–2000 m), medial-distal, mod-erate depth (200–800 m), and proximal-medial,shallow water (B200 m) portions of modernseamounts. Paleogeographic reconstruction illus-trates that each of the three study areas representsa specific phase of seamount development. Theproposed phases include: (1) initial, proximal,deep water emplacement of effusive, non-vesicularpillowed flows associated with feeder dykes andsills transporting magma higher up in the se-quence (locality A; Point Lake belt), (2) medial todistal, intermediate water-depth eruptions of thinvesicular flows, pillow breccia and non-vesicularhyaloclastite, contemporaneous with sedimenta-tion and deposition of reworked pyroclastic mate-rial (locality B; Point Lake belt), and (3) shallowwater eruptions of well-vesiculated massive andpillowed-lobate flows with hyaloclastite in pillowinterstices and pillow brecciated flow tops being

P.L. Corcoran / Precambrian Research 101 (2000) 237–261 259

reworked along seamount flanks and deposited asstratified fragmental deposits (Beaulieu Riverbelt). The three study areas in the Point Lake andBeaulieu River belts, which are similar in terms ofvolcanology and stratigraphic position, are com-parable to subaqueous volcanic sequences in mod-ern and Archean settings, indicating thatseamounts may have been common features onancient ocean floors.

Acknowledgements

This project was made possible by operatinggrants to J. Dostal from the Geology Division ofthe Department of Indian Affairs and NorthernDevelopment (contribution no. 99-002) andLITHOPROBE (contribution no. 1024). Greatappreciation goes out to Jarda Dostal, JamesWhite, C.J. Northrup, Clark Isachsen, BeckyJamieson, John Waldron, and Nick Culshaw fortheir valuable input and to Wulf Mueller for hishelp in the field and endless stream of informa-tion. Many thanks to Clarence Picket andMichael Cote for their assistance in the field andespecially to Renee-Luce Simard for her patienceand diligence. Incisive reviews by Mike Eastonand Harald Stollhofen significantly improved themanuscript.

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