Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol101.Issue2-4.2000:

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Precambrian Research 101 (2000) 211 – 235

Subaqueous, Paleoproterozoic, metarhyolite dome-flow-cone

complex, Flin Flon greenstone belt, Manitoba, Canada

L.D. Ayres

a,

_{*, A.S. Peloquin}

b

a_{Department of Geological Sciences}_,_Uni

6ersity of Manitoba,Winnipeg,Man.,Canada R3T2N2 b_{De´partement de Ge´ologie}_,_Uni

6ersite´ de Montre´al,CP6128,Succursale A,Montreal,Que´bec,Canada H3C3J7

Abstract

Two sparsely plagioclase-phyric rhyolite domes, 50 – 100 m thick and 125 – 250 m long as observed in 2-dimensional, vertically dipping exposures, are partly mantled by a rhyolite volcaniclastic complex that, on the south side of the domes, also forms a half-cone-like mound at least 60 m thick and 500 m long. The domes and mantling volcaniclastic complex are overlain by three aphyric to sparsely phyric rhyolite flows that are 70 – 120 m thick and 550 –\1200 m long. Domes and flows have a lower columnar-jointed, highly fractured, locally brecciated subfacies that grades upward into 40 – 95% crackled and disaggregated breccia. Crackled breccia is a highly fractured subfacies in which clast-like areas are bounded by closely spaced joints and minor matrix, but there is only limited rotation of clasts. Crackled breccia was produced by quench fracturing combined with hydraulic action of water converted to steam or supercritical fluid within the fractures. Overlying disaggregated breccia comprises rotated particles and 25 – 50% matrix. It is most abundant near flow margins and is a crumble breccia produced by flow advance or expansion. The volcaniclastic mound comprises lenticular, interlayered facies of (1) resedimented, phreatomagmatically generated, heterolithic, rhyolitic tuff and lapilli-tuff, (2) isolated to close-packed rhyolite lobes that are 0.5 – 5 m thick and 0.5 –\50 m long; the lobes are, at least in part, pillows, and (3) units comprising small (B2 m long) rhyolite lobes and blocks enclosed in a monolithic, probably hyaloclastic tuff and lapilli-tuff matrix. The volcaniclastic mound represents a series of thin lava tongues both extruded over, and intruded into, coeval hyaloclastite and resedimented, pyroclastic deposits, and it is part of a cone. Once initiated, the cone, and nearby early domes, acted as a barrier to further flow advance and progressively grew upward with eruption of successive flows. © 2000 Published by Elsevier Science B.V.

Keywords:Rhyolite; Subaqueous; Lava flows; Volcaniclastic rocks; Paleoproterozoic; Manitoba

www.elsevier.com/locate/precamres

1. Introduction

Felsic volcanism is uncommon in oceanic areas; thus, there are few modern examples of

sub-aqueous felsic lava flows and domes (e.g. Reynolds and Best, 1976; Lonsdale and Hawkins, 1985). There are, however, a number of examples from ancient volcanic sequences ranging in age from Archean to Quaternary. These include flows and domes erupted in both subglacial lakes (Fur-nes et al., 1980) and oceanic settings (Cas, 1978; * Corresponding author. Fax: +1-204-4747623.

E-mail address:_{ld –[email protected] (L.D. Ayres)}

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L.D.Ayres,A.S.Peloquin/Precambrian Research101 (2000) 211 – 235 212

Bevins and Roach, 1979; De Rosen-Spence et al., 1980; Yamagishi and Dimroth, 1985; Cas et al., 1990; Kano et al., 1991) and shallow intrusive domes that locally broke through to the surface to feed lava flows (Kokelaar et al., 1984; Allen et al., 1996).

Many of the described subaqueous felsic flows and domes are characterized, at least in part, by pillow-like lava lobes, pods, and tongues enclosed within a volcaniclastic component of the same composition. The volcaniclastic component is generally believed to be hyaloclastite produced by quench brecciation of the lobes, but there is some disagreement on the emplacement of the lobes. For example, De Rosen-Spence et al. (1980) and Yamagishi and Dimroth (1985) have proposed that the lobes were lava extrusions at the front of an advancing lava flow or on the surface of a growing dome. As lobe extrusion continued, quench brecciation produced a hyaloclastite en-velope surrounding the lobes. Furnes et al. (1980), on the other hand, stated that most lobes were intrusions into a preexisting hyaloclastite cone,

whereas Kano et al. (1991) described the lobe-vol-caniclastic facies as the subaqueous equivalent of block lava flows. In this paper, we describe a Paleoproterozoic rhyolitic flow-dome-cone com-plex, part of which is a discrete lobe-volcaniclastic facies that appears to be spatially separate from, although genetically related to, domes and flows.

2. Regional setting

The rhyolitic sequence is part of the 1.92 – 1.84-Ga, metavolcanic sequence of the Flin Flon greenstone belt of west-central Manitoba (Fig. 1). This basaltic to rhyolitic metavolcanic sequence is part of a tectonic collage that includes arc-like volcanoes, back-arc basin floor, and oceanic plateaus (Bailes and Syme, 1989; Stern et al., 1995a,b; Lucas et al., 1996).

The rhyolitic units are within a relatively nar-row fault sliver, the Grassy Narnar-rows zone (Bailes and Syme, 1989), which has a maximum width of 1300 m (Fig. 2). Metavolcanic units in the Grassy

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Narrows zone are a bimodal basalt – rhyolite suite that include MORB-like basalt and extension-re-lated rhyolite (Syme, 1998). All rock units have near-vertical dips, and maps are thus cross-sec-tions through the stratigraphy. Metamorphic grade is low to middle greenschist facies (Bailes and Syme, 1989), and many primary structures and textures of the volcanic units are well preserved.

The westward-younging, rhyolitic sequence ex-amined in the Grassy Narrows zone, and infor-mally termed the Grassy Narrows rhyolite (Syme, 1998), is :2 km long. It has a minimum thick-ness of 350 m, but the lower part of the sequence is covered by Manistikwan Lake, within which there is inferred to be a major fault (Fig. 2; Bailes and Syme, 1989). The Grassy Narrows rhyolite is separated by faults from other rhyolitic units to the south (Fig. 2), some of which have similar characteristics and may be a lateral extension. The study area is a single cross-section, albeit mostly well exposed, through the rhyolite complex. The orientation of the section relative to vent location is unknown, as is the vent location. The area was selected because of the excellent exposure of the rhyolite (Peloquin, 1981).

3. Overview of Grassy Narrows rhyolite

The rhyolite occurs in two distinct facies associ-ations: (1) northern brecciated and nonbrecciated facies that are inferred to represent domes and flows, and (2) lobe and volcaniclastic facies that are best developed in the south and are inferred to comprise both resedimented pyroclastic deposits and extrusive and intrusive lobes in a hyaloclastic matrix (Fig. 3). The southern lobe and volcani-clastic facies are largely separated from the north-ern brecciated and nonbrecciated facies by a bay of Manistikwan Lake, but some lobe and volcani-clastic facies also underlie, are intercalated with, and overlie the brecciated and nonbrecciated fa-cies. Stratigraphic correlation between the south-ern and northsouth-ern facies associations is facilitated by a distinctive, 25-m-thick, plagioclase-crystal-rich, intermediate-composition unit that com-prises pillowed flows and associated volcaniclastic Fig. 2. Simplified geology of the northern part of the Grassy

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units (Fig. 3). The subaqueous character of the rhyolites is documented by intercalated and over-lying pillowed basalt flows.

The rhyolite in brecciated, nonbrecciated, and lobe facies, as well as clasts in volcaniclastic facies varies from aphyric to phyric with as much as 15%, 0.1 – 2-mm plagioclase9quartz phenocrysts (Fig. 4) and microphenocrysts. Groundmass tex-tures vary from fine-grained, quartzofeldspathic spherulites produced by devitrification to very fine-grained quartzofeldspathic material that is probably the result of superimposed alteration and/or metamorphism; the devitrification textures imply that the rhyolite was largely vitric. Non-brecciated and lobe facies are highly fractured. Fractures are filled mostly with quartz but some contain carbonate9chlorite9sericite9potassic feldspar. In places spherical and tube pumice

tex-tures are preserved (Figs. 4 and 5), probably because vesicles were filled by quartz before compaction.

Element mobility during devitrification and al-teration hampers identification of the original composition of the rhyolite. This mobility is shown by chemical analyses of variably fractured and veined samples; these samples contain 75 – 80% SiO2, and Na2O/K2O of 0.02 – 30 (Bailes and Syme, 1989). Nevertheless, we, along with Bailes and Syme (1989), believe that the studied units are rhyolite because (1) some units contain quartz phenocrysts (Fig. 4; cf. Ewart, 1979), and (2) Zr/TiO2, a ratio of relatively immo-bile incompatible elements, ranges from 0.14 to 0.26 (Bailes and Syme, 1989). These are typical rhyolite values (cf. Winchester and Floyd, 1977).

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Fig. 4. Photomicrograph of embayed quartz phenocryst in pumice clast in heterolithic lapilli-tuff bed of southern rhyolite lobe and volcaniclastic facies association. Original glass re-placed by sericite+quartz+feldspar; vesicles filled by quart-zofeldspathic material. Plane polarized light; field of view is 4.2 mm.

and is only locally brecciated (Fig. 7); this grades rapidly upward into (2) a columnar-jointed subfa-cies (Figs. 8 – 10) that, in turn, grades over several metres into (3) a transition subfacies characterized by polyhedral joints but without columnar joints; the transition subfacies grades upward, again over several metres into (4) in situ crackled, jostled, or jigsaw-fit breccia subfacies (Fig. 11) overlain in turn by a discontinuous layer, of variable thick-ness, of (5) disaggregated breccia subfacies (Fig. 12) that is in relatively sharp contact with crack-led breccia and sharp contact with transition subfacies.

4.2. Facies distribution

Five distinct nonbrecciated to brecciated se-quences have been identified in the northern part of the Grassy Narrows rhyolite (1 – 5 of Fig. 3); however, not all subfacies are present in each sequence, and subfacies proportions are variable among sequences. Nonbrecciated rhyolite on a small island in Manistikwan Lake, east of se-quence 1, contains :8% plagioclase and quartz phenocrysts; it is inferred to be part of a sixth sequence that underlies the better exposed se-quences on the shore (Fig. 3).

The two lowermost sequences (1 and 2 of Fig. 3), the bases of which are covered by Manistik-wan Lake, have exposed lateral extents of 250 and 125 m and exposed thicknesses of 100 and 50 m, respectively. Rhyolite in both sequences is petro-graphically similar containing B1% plagioclase and rare quartz phenocrysts. Columnar jointing is well developed in both sequences, but column orientation between the two sequences differs by

:40° (Fig. 6). In both sequences, column orienta-tion is at a moderately high angle to subfacies boundaries, and, in sequence 2, to the discordant contact with sequence 1. In sequence 1, the disag-gregated subfacies is an asymmetric discordant unit that is in sharp contact with both crackled breccia and transition subfacies, and there is a spine-like projection of transition subfacies up-ward into disaggregated subfacies (Fig. 6). Disag-gregated subfacies was not observed in sequence 2. The two sequences are separated by an upward-thinning, nonbedded wedge of schistose lapilli-tuff Fig. 5. Photomicrograph of pumice in a 2.5-m-thick, aphyric

rhyolite lobe in the southern lobe and volcaniclastic facies association. White areas, which are now quartz, represent the original vesicles. Plane polarized light; field of view is 1.7 mm.

4. Northern brecciated and nonbrecciated rhyolite facies association — a dome-flow complex

4.1. Facies description

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Subfacies in rhyolite domes and flows of the dome-flow complexa

Boundary with overlying subfacies Comments Subfacies % of dome or Description

flow

Not always present; dis-Rounded to angular, apparently rotated clasts that are

0–95; but typi-Disaggregated breccia

subfa-cordant subfacies that mostly 0.5–5 cm long (Fig. 12), but are locally as much as

cally 0–50 cies of brecciated facies

overlies both crackled 3 m long; most clasts larger than 5 cm are internally

crack-led with some incipient separation of pieces along cracks breccia and transition

zone; in places envelops within which there are seams, as much as 2 cm wide, of

microbreccia; fine-grained sericite-rich matrix ranges in 1–10-m-thick,

discontinu-abundance from 25 to 50% and contains recognizable, ous, nonbrecciated and/

or columnar-jointed sharply bounded, rhyolite clasts as small as 0.05 mm; a

subfacies; origin contro-higher proportion of larger clasts in lower part of subfacies

versial (see text) than in upper part producing a downward increase in

aver-age clast size; there is a corresponding downward decrease in matrix abundance

0–45: but

typi-Crackled breccia subfacies Characterized by diversely oriented fractures, several cen- Relatively sharp contact over several tens Similar to subaerial

timeters apart, that produce a breccia-like appearance, but

cally 20–45 of centimetres with disaggregated breccia; jostle breccia of

Bon-of brecciated facies

nichsen and Kauffman local dike-like projections of disaggregated

there is no rotation of material bounded by joints, and

(1987) breccia, as much as 30 cm wide, extend

there is only minor matrix; fractured areas are interspersed

downward into crackled breccia with patches of disaggregated breccia that contains

appar-ently rotated, angular clasts, mostlyB1 cm in size and as

much as 50% matrix; the abundance of disaggregated brec-cia increases upward; in the lower part of the subfacies disaggregated breccia forms small patches within crackled breccia, but in the upper part of the subfacies, patches of

crackled breccia, 0.1–\1 m long, are surrounded by

disag-gregated breccia that forms:50% of the subfacies (Fig.

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Flow foliation is present in the lower half of this subfacies 0–40

Transition subfacies Somewhat undulating boundary marked Less well exposed than

other subfacies; absent by gradual change over several metres

and vague columns occur throughout, but the dominant

joints are polyhedral; areas between joints have variably into crackled breccia; change is defined by where columnar-jointed

disappearance of polyhedral joints and subfacies is poorly

devel-developed, diversely oriented fractures that, where closely

spaced (:5 cm), produce a crackled appearance; crackling appearance of ubiquitous crackling; in places, oped

crackled breccia is absent and there is a is not as pronounced as in the overlying crackled subfacies,

and there is no matrix in the fractures; near base of subfa- sharp contact with overlying disaggregated

breccia; local spine-like protuberances of cies there are local, 1–5-cm-wide pockets of breccia

con-taining rotated clasts less than 1 cm wide separated by transition subfacies, as much as 3 m wide

matrix; these breccia pockets are preferentially developed and 5 m long, into disaggregated breccia

along joints perpendicular to the original flow top; degree of brecciation increases upward with pockets and dike-like zones of breccia 20–30 cm wide occurring in upper part of

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Table 1 (Copntinued)

Boundary with overlying subfacies Comments Subfacies % of dome or Description

flow

Columnar-jointed subfacies of 5–40 Column development variable within and between flows and Undulating boundary; a relatively abrupt Columns have a relatively change over 1–2 m is marked by the

develop-domes; best developed in dome 1 (Fig. 8) and least developed in consistent orientation nonbrecciated facies

ment of polyhedral joints and the gradual disap- within individual domes flow 3; columns undulate slightly as shown by differences in

and flows, but the orienta-plunge within individual units (Fig. 6); columns pinch and swell, pearance of columnar joints; locally boundary is

tion differs from flow to and they are continuous across columnar subfacies; most defined

by a 0.5–1-m-thick zone of anastomosing platy flow reflecting differences columns are 10–20 cm in diameter (Figs. 8 and 9) with present

horizontal diameter consistently shorter than present vertical di- joints that parallel the boundary in orientation of cooling ameter; some columns as much as 30 cm in diameter, but these surfaces, apparently trolled by the basal con-are more poorly developed than smaller columns; columns have

both a variably developed internal cross fracturing with frac- tact; in flow 5, this subfacies is restricted to 1– tures 1–10 cm apart and 70–80° to column axis (Fig. 9), and

10-m-thick concordant do-internal diversely oriented tight fractures 0.5–5 cm apart defining

an incipient crackling with negligible clast separation or rotation mains surrounded by disaggregated breccia (Fig. 10); in some columns the spacing of diversely oriented

fractures decreases outward, and at the margin of some columns, crackling locally grades into disaggregated breccia that occurs along the common boundary of two columns; this brec-cia contains rotated angular clastsB1 cm long andB15% ma-trix; locally disaggregated breccia with angular clasts up to 5 cm long forms irregular pockets, 40–50 cm wide, crossing several columns; degree of crackling increases upward across the subfa-cies; where columns are poorly developed, disaggregated breccia forms local concordant 5–15-cm-thick lenses, 1–3-cm-wide zones related to original vertical joints (orientation given in relation to original flow top), and 5–30-cm-wide pockets; overall, breccia-tion in this facies is less than 5%; a slightly undulating flow foliation, which is parallel to the cross fracturing, is defined by 1–5-mm-long, flattened amygdules (Figs. 6 and 9)

1–2 Sharp lower, slightly undulating contact with local fine

breccia-Basal subfacies of nonbrec- Boundary defined by appearance of columnar Only exposed in flow 3,

joints; relatively rapid upward transition which overlies lobe and tion in basal 1–5 cm (Fig. 7); 10–20% quartz amygdules at base,

ciated facies

rapidly decreasing in abundance upward; in lower several me- volcaniclastic facies associ-tres, there are crudely concordant, somewhat anastomosing, 2– ation, and in dome 2

where this dome is adja-10-cm-thick, more vesicular layers; jointed, but lacks a coherent

joint pattern; generally less than several metres thick cent to dome 1

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Fig. 6. Detailed map of subfacies within the two domes of the northern dome-flow complex; rock units are subvertical and the younging direction is west-southwest. The base of both domes is covered by Manistikwan Lake. The steep contact between the domes represents overlap of dome 2 onto a tuff and lapilli-tuff, possibly hyaloclastic apron on the south side of dome 1. Note the difference in orientation of columns in the two domes. Orientation of columns in dome 2 has no apparent relationship to the orientation of the contact between the domes. The upper parts of the two domes are not exposed.

that is discordant to subfacies distribution in both domes. This wedge was originally interpreted as a tuff dike (Peloquin, 1981; Bailes and Syme, 1989), but the shape and lithology of the wedge are more compatible with a volcaniclastic unit deposited at the margin of sequence 1 prior to deposition of sequence 2.

Although the upper contact of sequences 1 and 2 are only locally exposed (Fig. 6), they appear to be overlain, and partly buried by fragmental units of the lobe and volcaniclastic facies association (Fig. 3). Overlying the lobe and volcaniclastic facies association are three, more laterally exten-sive, nonbrecciated to brecciated sequences (3, 4, and 5 in Fig. 3). Sequence 3 is aphyric whereas sequences 4 and 5 contain traces amount of pla-gioclase and quartz phenocrysts. In the present plane of exposure, these sequences range in length from 550 to 1200 m, but the two longer sequences are truncated on the north by a fault (Fig. 3).

Thickness ranges from 0 to 120 m. Sequence 3 wedges out both southward and northward where it overlaps sequence 1 and a pillowed basaltic

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Fig. 8. Longitudinal view of columnar joints in columnar-jointed subfacies of dome 1. Columns plunge about 20° east-erly, toward lower part of photograph, down the surface of the outcrop. Hammer for scale is 35 cm long.

absent and crackled breccia forms the upper part of the sequence. Farther south, where sequence 3 wedges out against sequence 1 (Fig. 3), disaggre-gated subfacies overlies crackled subfacies, and the combined thickness of the crackled and disag-gregated subfacies is greater than that of the crackled subfacies in the centre of the sequence. Where sequence 4 thins and wedges out south-ward, the combined thickness of the

columnar-Fig. 10. Detail of part of two columns in dome 1; boundary between columns is the slight depression within which the 8-mm-wide pencil is sitting. Closely spaced, diversely oriented fractures define a crackling within columns. In upper column, fracture spacing decreases inward (upward in photograph), and at column margin, the crackling has a breccia-like appear-ance but without any rotation of clasts.

Fig. 9. Detail of longitudinal section of three columns in dome 1. Alteration along columnar joints is defined by vertical bands of darker colour; within this alteration zone there is minor brecciation along column margins. A flow foliation, defined by flattened amygdules (small black lenses), is parallel to fractures that are approximately perpendicular to columnar joints. Pen-cil for scale is 8 mm wide.

Fig. 11. Upper part of crackled breccia subfacies of dome 1 is a mixture of crackled rhyolite with closely spaced, diversely oriented fractures, as shown in upper part of photograph, surrounded by disaggregated breccia in which rhyolite has broken into small angular fragments separated by slightly darker matrix, as shown in lower part. Pencil for scale is 8 mm wide.

mound, respectively (Fig. 3). Sequences 4 and 5, which cannot always be distinguished from each other, both wedge out southward in an area of poor exposure. The three upper sequences appear to be in direct contact, although the sequence boundaries were not observed.

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Fig. 12. Disaggregated breccia subfacies at top of flow 3. Note patchy variations in clast size and abundance. Pencil for scale is 8 mm wide.

of the domes and flows is unknown. The two lower sequences could represent either two domes, or two lobes of a single exogenous dome or lava flow. The latter interpretation is supported by the petrographic similarity of the two sequences and the apparent onlap of sequence 2 against sequence 1 (Fig. 6).

The variability in abundance of the columnar-jointed subfacies among the sequences is a func-tion of the amount of overlying brecciafunc-tion, and this, in turn, may reflect the third dimension of the flows and domes. Well developed, thick, columnar-jointed subfacies may represent cross-sections away from ends or edges of domes and flows. Thin, poorly developed zones of columnar joints, on the other hand, may represent cross-sec-tions near ends or edges of domes and flows. The asymmetric distribution of the disaggregated sub-facies in both dome 1 and flow 3 also appears to represent proximity to flow margins. From both of the preceding characteristics, flow 5 probably represents the margin of a flow.

Bailes and Syme (1989) have presented an alter-native model for the disaggregated breccia. They have proposed that the disaggregated breccia is a more highly developed equivalent of the crackled breccia with little interparticle movement; it would thus be a pseudobreccia. They have further stated that the interparticle material is the result of sericite-carbonate alteration along cracks rather than being matrix. We agree that much of the interparticle material is sericite and carbonate that is probably a metamorphic product of an original alteration assemblage. However, we reject the pseudobreccia concept because (1) there is evidence of clast rotation and (2) the contact between the sericite – carbonate material and par-ticles is sharp, with parpar-ticles as small as 0.05 mm clearly recognizable within sericite – carbonate ma-terial (Table 1). The sharp contacts are not com-patible with alteration of a solid component, but are compatible with alteration of a fine matrix component between particles. The sharp contact between disaggregated breccia and both crackled and transition subfacies, and the asymmetric and variable distribution of the disaggregated breccia also support a primary origin for this subfacies. jointed and transition subfacies remains relatively

constant, and most of the thinning occurs in the overlying breccia, which includes both crackled and disaggregated subfacies (Fig. 3). Sequence 5 is composed largely of disaggregated breccia, in the lower part of which there are discontinuous, columnar-jointed zones 1 – 10 m thick; the transi-tion subfacies is absent.

4.3. Interpretation

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5. Southern rhyolite lobe and volcaniclastic facies association — a partial cone?

This association is well exposed only on the south side of the dome-flow complex although similar facies occur between the domes and over-lying flows on the north (Fig. 3). Because all sequences have the same defining characteristic, namely the ubiquitous presence of cogenetic rhyo-lite lobes and clasts within rhyolitic volcaniclastic material of ash to lapilli size, our description will focus on the well-exposed southern association. In the following sections we use the terms tuff, lapilli-tuff, and tuff-breccia in the context of Fisher and Schmincke (1984) to denote only parti-cle size, and not genetic processes.

The southern association is at least 60 m thick and 500 m long, but the original thickness is unknown because the base is covered by Manis-tikwan Lake and the top has been intruded by a gabbro sill (Figs. 3 and 13). The lobe and volcani-clastic facies associations vary from stratified to chaotic. Stratification, which is typically lenticu-lar, is defined by interlayering of four, partly intergradational facies, and by internal structures within facies; the facies include (1) isolated rhyo-lite lobes that are as much as 50 m long and 5 m

thick and occur mostly within the bedded het-erolithic facies, (2) close-packed lobes, some of which appear to be well-formed pillows, (3) monolithic, unbedded tuff and lapilli-tuff contain-ing smaller, randomly dispersed rhyolite lobes; and (4) bedded heterolithic tuff, lapilli-tuff, and minor tuff-breccia (Fig. 13). Facies are described in Table 2. In the southern facies association, the strike of stratification is variable over short dis-tances, both vertically and laterally (Fig. 13), and this variation appears to be a primary feature of the deposit. The strike ranges from 145 to 180°, but the average strike is about the same as that of flows in the northern association.

5.1. Rhyolite lobes — general characteristics

The defining parameter of the lobe and volcani-clastic facies association is the presence of white-to yellow-weathering, rhyolite lobes. Lobes range in thickness from 0.5 to 5 m, and in length from 0.5 to \50 m. Some lobes, particularly the smaller ones, are relatively equidimensional in cross-sectional outcrop exposures (Figs. 14 and 15), but larger lobes are lenticular, and many have an aspect ratio (thickness over length) of B0.1 (Figs. 13 and 15 – 17). Lobe terminations are

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Facies of the southern lobe and volcaniclastic facies associationa

Comments

Facies Shape and dimensions Description Internal features

White-weathering with most lobes having distinctly pointed

Low aspect ratio; 0.5–5 m thick, Highly fractured, with fractures best de- Vary from being iso-Isolated rhyolite lobes within

and 0.5–\50 m long; concordant

heterolithic volcaniclastic fa- terminations; many lobes are in sharp contact with the host veloped at nonbrecciated lobe margins; lated lobes, at least on the present outcrop sur-volcaniclastic facies, which contains only rare clasts or small

with enclosing heterolithic facies

cies fractures generally healed by narrow

face, to being in contact quartz and carbonate veins; lobe margins

lobes that could have been derived from the larger lobes by

with adjacent lobes typically nonbrecciated, but there is local

breakage, spalling, or injection; other lobes, however, have

(Fig. 15) to being spa-a mspa-arginspa-al zone thspa-at is 10 cm to severspa-al metres thick spa-and matrix-poor, marginal breccia that

rapidly decreases in intensity inward, and tially associated with either surrounds the lobe or is confined to the upper part of

forms discontinuous zones that are

gener-the lobe (Fig. 18); this marginal zone, which has sharp to other lobes in units 1–2 allyB10 cm, but locally as much as 30 m thick (Fig. 16); best gradational contacts with both the lobe and the host

het-erolithic volcaniclastic facies, is composed of smaller lobes cm thick (Fig. 21); sharp contact between developed in north and marginal breccia and host heterolithic central part of southern and blocks within a matrix of monolithic tuff and

lapilli-tuff, the particles of which have the same phenocryst popu- unit facies association (Fig. 13)

lation as the lobe (Fig. 18); many of the smaller lobes are irregular in shape, possibly as a result of plastic deforma-tion; in large lobes, the marginal zones appear to represent injection of small lava lobes upward from the main lobe into fragmental material produced by simultaneous quench brecciation

Close-packed rhyolite lobes Lenticular lobe units are as much Most lobes are lenticular with pointed ends, but some have Lobe margins vary from nonbrecciated to Concentrated in central distinctly rounded ends and appear to be pillows (Fig. 22); and south part of

as 35 m thick and\100 m long; brecciated, with the amount of

breccia-lobes vary from touching, with overlying breccia-lobes moulded tion variable among lobes, and around southern facies associa-individual lobes range in length

against lower lobes, to separated by tuff-breccia, lapilli-tuff, the circumference of single lobes; where tion (Fig. 13) away from 0.5 to 6 m, but most are

and tuff screens containing variable amounts of angular to

several metres long present, the zone of marginal brecciation from the flows and

is generallyB10 cm wide and it typically domes of the northern irregular blocks and locally small rounded lobes (Figs. 22

and 23); many touching lobes have yellow-weathering mar- has a sharp contact with adjacent lobes facies association or interlobe tuff; where well exposed,

ginal zones that are 2–30 cm wide and have a sharp contact

lobes have an apparent selvage defined by with lobes; these touching lobes are partly separated by

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Facies Shape and dimensions Description Internal features

Monolithic, unbedded tuff and Thin lenses to almost equidimen- Mostly tuff and less abundant lapilli-tuff containing sparse, No internal bedding although clast size is Best developed in the variable across some units; lobes highly upper part of the facies sional areas that range in

thick-lapilli-tuff containing small large, white-weathering, isolated lobes that only rarely

ex-association; gradational, tend beyond the facies boundary (Figs. 13, 15 and 17), and fractured, with fractures marked by

nar-ness from 1 to 20 m (Figs. 13 rhyolite lobes

interfingering 25–60%, smaller, rounded to lenticular, white- and yellow- row zones of very fine-grained

quartzo-and 15); includes both mappable

boundaries with adja-lobes as much as 2 m thick and weathering lobes that vary considerably in trend over dis- feldspathic recrystallization rather than

15 m long, and smaller unmap- tances of 5–10 m within units, and vary in distribution from veins; lobe margins vary from brecciated cent heterolithic tuff to nonbrecciated, the degree of

breccia-pable lobes 0.5–2 m long closely packed to isolated; where the matrix is lapilli-tuff, and lapilli-tuff contain-there is commonly a complete range in size from 2-m-long tion is more intense in yellow-weathering ing isolated lobes that lobes to 1-cm clasts, with the decimeter-size population in- than in white-weathering lobes and over- only rarely extend

all it is greater than in isolated lobes;

cluding both small, round to lenticular lobes and angular across the boundary (Figs. 13 and 15); sharp many of the small, yellow-weathering

blocks apparently broken from lobes; textures in the tuff

matrix are poorly preserved, and the tuff is a very fine- lobes are surrounded by breccia zones as contacts with isolated much as 30 cm thick, and these lobes lobes and closely grained quartz+plagioclase+sericite aggregate in which

packed lobe units variations in sericite content outline ash- to small lapilli- grade outward into, and are almost

indis-tinguishable from, the adjacent yellow-size, lenticular to locally equant areas that appear to

repre-weathering monolithic matrix sent original clasts

0–40%, white- to yellow-weathering, aphyric to porphyritic,

Bedded, heterolithic tuff, Discontinuous and lenticular to Unbedded to internally bedded with Irregular map distribu-lapilli-tuff, and minor tuff- irregular units that range in locally flow-layered, generally nonamygdaloidal rhyolite sharply to gradationally bounded, thick tion (Fig. 13) that is a function of the spatial to very thick, mostly ungraded beds

lapilli; lapilli are mostly 0.2–5 cm, but locally up to 20 cm breccia thickness from several metres to

defined by variations in crystal content, relationship with the \25 m long, and are angular to rounded to oval and lenticular in

clast size, abundance of lapilli, and

abun-shape; clast elongation parallels both bedding and a ubiqui- two other facies; in places, this facies partly tous metamorphic foliation in the matrix; many lapilli are dance of intermediate to mafic clasts; rare

spherulitic and were probably originally vitric, and they are normal grading, reverse grading, and to completely envelops texturally and compositionally identical to the larger lobes, cross-bedding; beds vary from relatively the other two facies from which they were probably derived; matrix composed continuous and\50 m in strike length to

lenticular and only several metres long; largely of somewhat elongated, felsic ash and small lapilli

that vary from nonamygdaloidal to pumiceous (Fig. 4); in some lenticular beds probably occupy scours (Fig. 16). In places, bed strike can matrix, primary clast shapes largely masked by

metamor-phic recrystallization, but better preserved, small lapilli were change by 10–25° over distances of only several tens of metres with lower beds originally angular; in any given bed, these felsic clasts are

texturally variable and range from aphyric to porphyritic truncated by overlying beds along ero-sional surfaces; local interbeds of other with as much as 15% phenocrysts; plagioclase phenocrysts

are generally more abundant than quartz phenocrysts; most facies of these clasts lack the spherulitic textures characteristic of the larger lapilli; other components include trace to 10% plagioclase and quartz pyrogenic crystals, and sparse inter-mediate to mafic volcanic clasts; some interinter-mediate to mafic clasts are tourmaline rich and were possibly derived from a source that was altered prior to brecciation and incorpora-tion in the heterolithic unit

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Fig. 14. Small, equidimensional, rounded, rhyolite lobe within monolithic unbedded tuff and lapilli-tuff facies of southern lobe and volcaniclastic facies association. Lobe has sharp boundaries with, and is distinguished from, enclosing foliated lapilli-tuff by slightly whiter colour and better developed frac-tures; adjacent to lobe, foliation in lapilli-tuff parallels lobe boundary. This is a possible cross-section through a tubular lobe. Drawn from a photograph.

Many lobes that are \1 m thick have discon-tinuous, thin internal domains, defined by a higher degree of fracturing, incipient brecciation, and a more yellowish weathering that typically accompanies a higher degree of sericitic alteration (Figs. 16 – 18). These domains, which pinch and swell, are generally only several centimetres thick, but locally they swell into pockets, as much as 50 cm thick. These pockets contain 0.5 – 20-cm-long, round to angular clasts of rhyolite, apparently broken from adjacent lobes, in a schistose tuffa-ceous matrix (Fig. 18). These domains, which are most common in the upper part of lobes, outline lenticular to ovoid to irregular areas of massive rhyolite, generally B1 m across (Figs. 16 – 18), that appear to be closely packed, small lobes within the larger lobe. These small internal lobes differ from lobes of close-packed lobe subfacies (Table 2; Figs. 13 and 15) in being smaller and generally less well defined.

The lobes vary from pumiceous (Fig. 5) to nonvesicular; in some lobes amygdules are re-stricted to flow margins (Bailes and Syme, 1989), but in other lobes they appear to occur through-out the lobes. Lobes vary from aphyric to plagio-clase-phyric to quartz9plagioclase-phyric; the groundmass is uniformly spherulitic throughout the lobes, attesting to its original vitric nature, and there is only limited evidence of marginal quenching. Lobes in the lower 15 m of the south-ern facies association contain 10 – 15% quartz+

plagioclase phenocrysts (Fig. 13), and many clasts in spatially associated heterolithic tuff, lapilli-tuff and minor tuff-breccia facies have similar phe-nocryst contents. Most lobes higher in the associ-ation, but below the plagioclase-crystal-rich unit, and clasts in spatially associated heterolithic tuff, lapilli-tuff and minor tuff-breccia facies are aphyric or contain sparse quartz9plagioclase phenocrysts, although a few lobes and lapilli-tuff beds here have higher phenocryst contents. The upward change in phenocryst population is com-parable to that in the northern facies association. The phenocryst-rich lobes and volcaniclastic rocks are comparable to the nonbrecciated facies ex-posed on the small island in Manistikwan Lake and the aphyric to phenocryst-poor lobes and volcaniclastic rocks are comparable to flows and domes of sequences 1 – 5.

mostly pointed but some are rounded, lobate, or blunt (Figs. 14 – 20). Upper and lower margins of lobes vary from relatively smooth and curving to undulating to lobate. Generally the lower margins of lobes are smoother than the upper margins. Lobe margins also vary from sharp and nonbrec-ciated, to sharp and brecciated with breccia confined within the lobe (Fig. 21), to gradational and brecciated with the broken pieces intimately mixed with enveloping finer volcaniclastic material.

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Many lobes contain sparse, centimetre-size pla-gioclase-phyric xenoliths that have a very fine-grained groundmass containing felted plagioclase laths. Texturally the xenoliths do not resemble any of the lobes or flows and the more crystalline nature of the xenoliths suggests slower cooling at depth. The xenoliths were probably derived from an earlier magma that was emplaced and crystal-lized at shallow crustal depths.

5.2. Isolated rhyolite lobes within a heterolithic

6olcaniclastic host

5.2.1. Description

These typically white-weathering rhyolite lobes form :15% of the southern facies association, and they are concordant with, and readily

distin-guished from enclosing yellowish heterolithic vol-caniclastic facies. Characteristic features include low aspect ratio (Figs. 13 and 15), wide range in size, distinctly pointed terminations, generally sharp, nonbrecciated contacts with heterolithic tuff and lapilli-tuff, and, in some lobes, a mar-ginal zone of smaller lobes and blocks, some of which are irregular in shape as if plastically de-formed while still hot (Fig. 18; Table 2). In places, several lobes of varying sizes occur at the same horizon, where they are partly enveloped by tuff-breccia containing both small lobes, some of which are internally brecciated, and blocks and lapilli (Fig. 16). These spatially associated lobes and associated tuff-breccia form concordant stratigraphic units 1 – 2 m thick that are in sharp contact with both underlying and overlying

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Fig. 16. A large aphyric rhyolite lobe and numerous smaller lobes occur at the same horizon within massive, crystal-poor heterolithic tuff; younging direction is towards top of figure. White lines represent thin domains that appear to define internal lobe boundaries. The lobes are connected, and partly completely enveloped, by monolithic, apparently cogenetic tuff-breccia that contains angular to irregular blocks, some of which appear to have been plastically deformed while hot. The tuff-breccia is in sharp contact with the enclosing heterolithic tuff, which contains only rare small lobes and lapilli. This figure also shows the lenticular nature of bedding in the bedded heterolithic tuff, lapilli-tuff, and minor tuff-breccia facies. The variable thickness of the heterolithic tuff immediately beneath the lobes suggests that the largest lobe either depressed the tuff, or ploughed into and removed some of the tuff. From mapping by L. Slezak.

Fig. 17. Several spatially associated aphyric rhyolite lobes are enveloped in unbedded, crystal-free, monolithic rhyolite tuff and lapilli-tuff; younging direction is towards top of figure. White lines represent thin domains within lobes that appear to define internal lobe boundaries. Lobe terminations vary from smoothly pointed to complexly lobate with inderdigitating lenses of apparently cogenetic tuff-breccia. The largest lobe is overlain by apparently cogenetic tuff-breccia that contains small lobes and angular to irregular broken pieces of rhyolite. The contact between tuff-breccia and the enveloping tuff and lapilli-tuff is sharp. From mapping by L. Slezak.

erolithic tuff and lapilli-tuff (Fig. 16). Some iso-lated lobes have tails, as much as 10 m long, composed of small lobes and angular to irregular blocks within a tuff matrix (Fig. 19). The irregular blocks also appear to have been plastically de-formed. These lobes with tails are also concordant stratigraphic units.

5.2.2. Interpretation

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Fig. 18. The well exposed, northern end of the largest isolated rhyolite lobe in the southern lobe and volcaniclastic facies association (see Fig. 13 for location) shows the complexity of external and internal contact relations; younging direction is towards top of figure. The rhyolite lobe contains quartz and less abundant plagioclase phenocrysts. The lower boundary of the large lobe varies from smooth and gently curving to lobate with pillow-like projections downward into the underlying, very thick bed of crystal-rich heterolithic tuff; there is only local marginal brecciation and only rare, apparently isolated small lobes below the large lobe. Within the large lobe, discontinuous, thin domains or boundaries defined by increased fracture intensity, increased alteration, and incipient brecciation locally merge with pockets of monolithic tuff and lapilli-tuff containing pieces broken from adjacent lobes; these boundaries, only the most obvious of which are shown on the figure, outline smaller closely packed lobes. The upper boundary of the large lobe also varies from smooth and curving to lobate, but it is overlain by a discontinuous zone, as much as several metres thick, of small, rounded to irregular, isolated lobes that are separated by apparently monolithic tuff that contains angular clasts broken from the adjacent lobes. Some of the irregular lobes appear to have been plastically deformed while hot. This zone is sharply overlain, in turn, by heterolithic lapilli-tuff that lacks lobes (not shown). From mapping by C. Nikols.

for some lobes, particularly the few lobes that cross the boundary between heterolithic and monolithic tuff and lapilli-tuff facies (e.g. Fig. 15). The flow interpretation is supported by the con-cordant attitude of both single lobes and lobe plus tuff-breccia units, the concordant tails of small lobes and blocks that occur with some lobes, paucity of cross-cutting relations (Fig. 15), and sharp, commonly smooth basal contacts of many of the isolated lobes with enclosing heterolithic tuff and lapilli-tuff. Unlike many of the isolated lobes in the Grassy Narrows rhyolite, intrusive rhyolite lobes described in subglacial deposits in Iceland are discordant (Furnes et al., 1980).

Flow may have been by a single pulse, in which case, considering the thinness of many lobes, the lava must have had a low viscosity and deposition was probably on a slope, or by pillow-like lava fingers erupted through fractures in the chilled

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Fig. 20. Exposure of close-packed rhyolite lobes on a vertical face, oriented at right angles to the outcrop strike of lobes, shows that these lobes rapidly terminate downward into the outcrop; younging direction is to right. This suggests that, in three dimensions, the lobes are tubular. Note the variation in lobe terminations, which range from pointed to rounded to undulating.

lobes. Pillow-like morphology is also found in some close-packed lobes (Figs. 22 and 23).

In some of the larger lobes, there are spatially associated smaller, typically irregular lobes and blocks, most of which occur above the main lobe. These small lobes and blocks occur in a matrix that is monolithic rather than heterolithic and has the same crystal content as the lobe (Fig. 18; Table 2), and they probably represent upward injection of flow lobes from the main lobe into a thin hyaloclastite blanket that developed by quenching as the main lobe advanced. The blocks may have been derived by quench fragmentation of the small lobes. The irregular shape of many of these lobes and blocks may be a result of plastic deformation within a weak, but insulating hyalo-clastite blanket. Thus, there is some evidence of upward intrusion, but only into a cogenetic hyalo-clastite, and not into heterolithic tuff and lapilli-tuff.

A puzzling aspect of many lobes is the distinctly pointed terminations. Similar-sized lobes de-scribed in other subaqueous rhyolite flows gener-ally have rounded terminations (De Rosen-Spence et al., 1980; Furnes et al., 1980), although many lobes depicted by Yamagishi and Dimroth (1985), (Fig. 2) have pointed or both pointed and

Fig. 21. Brecciation at lower margin of a rhyolite lobe. Pen (:13 cm long) is close to margin of lobe; the 8-cm-wide clast immediately below pen is in the enclosing lapilli-tuff and is not part of the lobe. Note variable nature and degree of develop-ment of breccia along margin. Breccia grades upward into nonbrecciated rhyolite at top of photograph.

Fig. 22. Left side of photograph is rounded end of a 5.5-m-long rhyolite pillow (lowermost large pillow of Fig. 23); younging direction is towards top of photograph. The chilled margin is indicated by the slightly paler colour of the pillow edge, which is in contact with interpillow tuff. Small rounded rhyolite areas below, above, and to right of pillow are either small pillows or deformed blocks in which deformation must have occurred while the block was still hot. Pencil for scale is 8 mm wide.

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Fig. 23. Rhyolite containing sparse quartz and plagioclase microphenocrysts forms close-packed pillow-like lobes within the southern lobe and volcaniclastic facies association (location is on Fig. 13); younging direction is towards top of figure. Lobes vary from touching and moulded against each other to separated by apparently cogenetic tuff-breccia containing 40 – 50% angular to irregular blocks and local small rounded lobes. East-west dimension slightly distorted because outcrop face slopes 45 – 60° towards lake. From mapping by C. Nikols.

rounded ends. Superficially, the pointed termina-tions look like deformational flattening, but the good preservation of textures in the lobes and in adjacent volcaniclastic beds suggests that tectonic deformation had a minimal effect on the se-quence. We believe that the lobe shapes are largely primary, and any flattening must be the result of extrusion processes possibly combined with load generated by rapidly deposited overly-ing units; flattenoverly-ing must have occurred before devitrification because spherulitic devitrification textures are well preserved. The pointed termina-tions may be the result of necking and severance of lobes, enclosed within a plastic skin, from a main feeder lobe advancing down a relatively steep slope; narrow necks connecting rhyolite ‘blocks’ have been described by Kano et al. (1991). Such severed lobes could then slide farther down the slope producing trains of lobes and associated broken material (Fig. 16). In Fig. 16, the change in thickness of the underlying het-erolithic tuff as a direct function of thickness of overlying lobes and the slight depression of the contact between this tuff and underlying lapilli-tuff could be the result of lobes sliding down a slope. This process would be analogous to some of the stone streams produced at the front of pillowed basaltic flows on the flanks of seamounts (Lonsdale and Batiza, 1980).

5.3. Close-packed rhyolite lobes

These units, which form :25 – 35% of the southern facies association, differ from the iso-lated lobes in the more obvious composite nature (Fig. 23) and the higher aspect ratio of both lobes and lobe units (Fig. 13; Table 2). Most units occur in the central and south part of the southern facies association where there is a crude inverse relationship between the distribu-tion of lobe units and of large isolated lobes (Fig. 13). Relative to the flows and domes of the northern facies association, the lobe units would be more distal than many of the large isolated lobes.

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lobe units suggests that the axis of flow advance was at some angle to the present erosion surface, and the more distal location could represent accu-mulation in topographically lower areas, possibly base of slope. The close-packed lobes, particularly those in the 4-m-thick unit shown in Fig. 23, resemble rhyodacite pillows described by Bevins and Roach (1979).

5.4. Monolithic unbedded tuff and lapilli-tuff containing small rhyolite lobes

This facies occurs throughout and forms :25 – 35% of the southern facies association, although it is best developed in the upper part. The facies occurs as thin lenses to almost equidimensional units interlayered and interfingered with other facies (Figs. 13 and 15; Table 2). Isolated lobes that occur within this facies only rarely cross the gradational boundary with heterolithic tuff and lapilli-tuff facies. The facies differs from other units in lack of bedding, abundance of small, unmappable lobes, many of which grade into the tuff matrix across a wide zone of brecciation, and poorly preserved but relatively uniform textures in the tuff matrix (Table 2). The lobes are typically aphyric and the tuff matrix generally lacks crystals.

The poor preservation of textures in the rhy-olitic tuff and lapilli-tuff matrix of this facies precludes a specific genetic interpretation of the matrix component. However, this poor preserva-tion may also be a clue to the genesis. The much better textural preservation in adjacent rhyolitic heterolithic tuff and lapilli-tuff (Table 2) suggests that the difference in textural preservation be-tween the two types of tuff and lapilli-tuff is a reflection of primary differences in rhyolitic clasts with the better preservation in heterolithic units being a function of textural variability among clasts. Accordingly, we infer that the poor preser-vation, yet relatively consistent textures of the matrix of this facies, reflects an original mono-lithic, vitric tuff.

Within the facies, the lack of bedding, abun-dance of small lobes, variable distribution of these lobes, apparently monolithic character of the en-closing tuff and lapilli-tuff, presence of lapilli and blocks that are texturally similar to the lobes, and overall shape of units (Table 2) all support a cogenetic relationship of the lobes and enclosing tuff and lapilli-tuff. From the preceding character-istics, we also infer that the tuff and lapilli-tuff are a hyaloclastite produced by extensive quench frag-mentation from advancing lobes.

Unlike the minor hyaloclastite blankets associ-ated with isolassoci-ated lobes interlayered with het-erolithic volcaniclastic units (Fig. 18), hyaloclastite is the dominant component of the monolithic units. Furthermore, the lenticular to almost equidimensional, cross-sectional shape of the monolithic units, some of which have thick, blunt interfingering relations with adjacent bed-ded heterolithic tuff and lapilli-tuff (Figs. 13 and 15), is not compatible with extrusion on a deposi-tional surface of volcaniclastic deposits. The shapes and relations with adjacent units, however, are compatible with near-surface intrusive com-plexes where intrusion of lobes into preexisting water-saturated, heterolithic volcaniclastic units resulted in concomitant development of the sur-rounding hyaloclastite by quench brecciation. This intrusion model is similar to that proposed by Furnes et al. (1980) for subaqueous rhyolite units in Iceland where the intrusive lobes and hyaloclastite envelopes are interlayered with ex-plosively generated tephra units. Intrusion is also supported by the variable attitude of lobes within individual monolithic units and the irregular shape of some lobes (Fig. 17).

5.5. Bedded heterolithic tuff, lapilli-tuff and minor tuff-breccia

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dominantly felsic, there is a wide variation in internal textures. The larger clasts are spherulitic and are texturally very similar to the lobes, but the smaller clasts lack spherulitic textures and clasts within individual beds have a wide variation in phenocryst populations (Table 2). The het-erolithic character of the beds is shown by both textural variation of the felsic clasts and the sparse mafic and intermediate clasts (Table 2).

In the lower 15 m of the association, felsic clasts are mostly porphyritic with many contain-ing 10 – 15% phenocrysts. These beds also contain 2 – 10% pyrogenic quartz and plagioclase crystals. Higher in the association, felsic clasts in most lapilli-tuff beds have a much lower phenocryst content, and many clasts are aphyric; there are only rare pyrogenic crystals. There are, however, isolated beds that are more comparable to those in the lower 15 m of the association. The boundary between the lower phenocryst-rich and upper phenocryst-poor parts of the association is relatively sharp (Fig. 13).

The textural variation in felsic clasts of this facies indicates that the clasts were derived from a variety of sources and were mixed together either during eruptive or transportation processes. The larger felsic lapilli, which are texturally similar to the lobes, flows and domes, were probably derived from brecciated portions of lobes. However, the texturally dissimilar and texturally variable, smaller felsic clasts must have been derived from a different source, possibly by phreatomagmatic eruptions from a vent that was outside the present plane of exposure. An eruptive origin is supported by the felsic composition of most clasts, the angu-larity of some clasts, the presence of some pumiceous clasts, and the general similarity of crystal contents of lobes and spatially associated tuff and lapilli-tuff. Phreatomagmatic eruptions are supported by the mixture of various textural types of felsic clasts, by the low vesicularity of many clasts (Houghton and Wilson, 1989), and the relative paucity of pyrogenic crystals.

Once erupted, tephra fell through the water column to form pyroclastic deposits, which then were transported and resedimented, at least

partly, by water currents or other downslope transport. Such resedimentation is indicated by the lenticularity of some beds, by rapid changes in bed attitude with truncation of lower beds by overlying beds, by the widespread distribution of spherulitic clasts derived from lobes, and by the mixing of textural types. The mixing of clast types is also compatible with clast derivation from ero-sion of texturally different felsic flows outside the present plane of exposure. However, a strictly erosional origin of clasts is unlikely considering the similar upward change in crystal content of both lobes and heterolithic tuff and lapilli-tuff, the widespread basaltic units elsewhere in the sequence but paucity of basaltic clasts, and the submarine setting of the rhyolite flows in the exposed sequence. The wide distribution of the heterolithic units, both laterally and within the stratigraphic sequence (Fig. 13), combined with the upward change in crystal content, suggests that explosive activity persisted for a relatively long period of time, although it was periodically interrupted by more rapid lobe and flow emplace-ment events.

6. Stratigraphic relations between lobe and volcaniclastic facies association and dome-flow complex

The well exposed southern lobe and volcani-clastic facies association is separated from the well exposed northern brecciated and nonbrecciated facies association, which is interpreted to be a sequence of five flows and domes, by a bay of Manistikwan Lake (Fig. 3). Exposure of contacts between these two facies associations is restricted to two locations north of the bay. Flow 3 overlies a lobe and volcaniclastic sequence with a sharp contact (Figs. 3 and 7), and a lobe and volcani-clastic sequence overlaps dome 2 with a sharp contact (Fig. 6). Flow 4 is inferred to overlie a lobe and volcaniclastic sequence, although the actual contact was not observed (Fig. 3). All other contacts are covered either by overburden or Manistikwan Lake.

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sequence immediately above the dome is thick-bedded, lobe-free, heterolithic tuff and lapilli-tuff. The contact with crackled breccia of the dome is sharp, but, in the lower 2 – 3 m of the heterolithic unit, there are 5 – 10%, 1 – 5 cm long, white-weath-ering clasts that resemble pieces of, and may have been derived from the crackled breccia. The first lobes were observed :50 m above the dome.

7. Discussion

Except for the lowermost units, the Grassy Narrows rhyolite lava flows, lobes, and volcani-clastic units have a very low phenocryst content (Fig. 13). This low phenocryst content, and, in places, lack of phenocrysts, suggests that, when erupted, much of the magma was close to liquidus temperatures, and thus had a relatively low vis-cosity. Similar interpretations have been made for other subaqueous rhyolite flows and associated lobe units (Yamagishi and Dimroth, 1985). The eruptions apparently tapped a zoned chamber that had more crystal-rich, possibly cooler magma at the top. This early magma contained 8 – 15% quartz and plagioclase phenocrysts and is repre-sented by the lowermost large lobe and underlying heterolithic beds of the southern lobe-volcaniclas-tic facies association (Figs. 13 and 18), and by the incomplete flow on the small island in Manistik-wan Lake east of dome 1 (Fig. 3). Pillows in intercalated and overlying basaltic lava flows indi-cate that the rhyolitic units were extruded sub-aqueously, but water depth is uncertain.

7.1. Origin of flow facies

Although the basal contacts of domes and flows are exposed only locally, the paucity, and where present, the thinness, of basal breccia supports the concept of a relatively fluid lava that, as the flow advanced, quenched and locally brecciated on contact with the sea floor. Further advance of the still-liquid flow interior occurred above this quenched base with limited brecciation produced by flow movement. Some laterally extensive, sub-aerial and subaqueous rhyolite flows also lack basal breccia (Cas, 1978; De Rosen-Spence et al.,

1980; Bonnichsen and Kauffman, 1987; Green and Fitz, 1993).

Combined, the upper crackled and disaggre-gated breccia subfacies are much thicker than the upper breccia found on many subaerial rhyolite flows (Christiansen and Lipman, 1966; Bonnich-sen and Kauffman, 1987; Dadd, 1992; Manley, 1996) or reported from other subaqueous rhyolite flows (Cas, 1978; De Rosen-Spence et al., 1980). In the Grassy Narrows rhyolite, these two subfa-cies form 40% or more of the five domes and flows (Figs. 3 and 6; Table 1), all of which have roughly similar thicknesses. The breccia is just the ultimate manifestation of the dominant character-istic of the flows and domes; namely, the ubiqui-tous, closely spaced, diversely oriented fractures that occur in all subfacies but are most pro-nounced in the upper subfacies (Table 1). We believe that the great thickness of the two breccia subfacies and the ubiquitous fracturing are proba-bly a result of two simultaneous processes, (1) extended interaction of the lava with the sur-rounding water column, combined with (2) dislo-cations produced by flow advance and dome growth.

As domes expanded upward and outward, and lava flows advanced, cooling of the upper surface in contact with water led to an increase in viscos-ity, and ultimately to cracking and brecciation as the crust shrank in volume and the liquid interior of the flow continued to move under the quenched crust (cf. subaerial rhyolite flows; Christiansen and Lipman, 1966). This initiated the develop-ment of crackled breccia. Continued ingress of water downward along cracks resulted in repeated episodes of cooling and crack development that continued long after the lava solidified. Cracks propagated downward and new generations of cracks propagated inward from the face of older cracks, resulting in progressively smaller un-cracked areas (cf. Yamagishi and Dimroth, 1985). The final stage of crack development is related to water ingress along columnar joints; this pro-duced the diversely oriented fracture pattern that characterizes columns.

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supercritical fluid. Hydraulic action of this expan-sion would have separated the quenched lava along cracks and rotated the separated pieces (Allen et al., 1996). This produced disaggregated breccia with low matrix content, which is mixed with crackled breccia in the crackled breccia sub-facies and is a less common component of the transition and columnar-jointed subfacies (Table 1). The final stage of hydraulic action was the localized breccia that occurs along the margins of joint columns.

Continuing dome growth or flow advance would have resulted in breaking and crumbling of crackled breccia, particularly on the steep front or sides of a flow or dome. This process produced a second type of disaggregated breccia or crumble breccia, which forms the separate, but variably developed, disaggregated breccia subfacies. Early development of this subfacies is supported by the local upward projection of spines of transition subfacies into disaggregated breccia and down-ward projection of disaggregated subfacies into crackled breccia. The relationship of the disaggre-gated subfacies to flow margins is indicated by the discordant nature of the subfacies, particularly in dome 1 (Fig. 6), the spatial relationship of the subfacies to flow margins in dome 1 and flow 3, the sharp contact between this subfacies and other subfacies, and the large volume of the subfacies in flow 5.

7.2. Relationship of lobe and 6olcaniclastic facies

association to domes and flows

The rhyolite lobe and volcaniclastic facies asso-ciation is somewhat similar to lobes that have been described from a number of subaqueous rhyolite flows elsewhere, but the facies relations are different. Dimroth et al. (1979) and De Rosen-Spence et al. (1980), for example, proposed that lobes in Archean flows at Noranda were pillows that formed at the front of advancing, thick, submarine rhyolite flows. However, unlike the Grassy Narrows rhyolite lobes, the Noranda lobes occur within a monolithic hyaloclastite produced during flow advance, not a heterolithic volcani-clastic sequence, and they typically show foreset cross-bedding (De Rosen-Spence et al., 1980).

They are a medial to distal facies of unbrecciated lava flows. As described above, there are both vertical and lateral facies changes within the Grassy Narrows rhyolite flows and domes. How-ever, all facies are integral parts of the flows and domes, and there is no evidence that the flows and domes grade laterally into the lobe and volcani-clastic facies association. Instead, the lobe and volcaniclastic facies association appears to be a distinct genetic entity that is interlayered with flows and domes, not a facies of the flows and domes. Where the lobe and volcaniclastic facies association immediately overlies dome 2, there is a gradational zone in which fragments apparently derived from the dome are incorporated in overly-ing heterolithic tuff; the boundary between the two facies associations is, however, sharp, and the gradation is the result of mechanical mixing, pos-sibly as a result of some downslope sliding.

Furnes et al. (1980) described lobes in Quater-nary rhyolite deposits in Iceland that are not associated with lava flows. They proposed that the lobes were intruded into rhyolite pyroclastic deposits, and, during intrusion, quench fragmen-tation produced a hyaloclastite envelope that sur-rounds the lobes. This mechanism is similar to that envisaged for the Grassy Narrows monolithic unbedded tuff and lapilli-tuff facies containing small rhyolite lobes. This facies resulted from intrusion of lobes into resedimented pyroclastic deposits and concomitant development of a hyaloclastite envelope. Bailes and Syme (1989) have proposed a similar origin for all of the Grassy Narrows lobes, but we believe that the genesis of the lobe and volcaniclastic facies associ-ation is more complex.

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the domes and flows were erupted farther north. The coeval nature of the southern and northern facies association is also supported by the plagio-clase-crystal-rich unit that occurs in both associations.

We believe that the southern lobe and volcani-clastic facies association represents an incomplete subaqueous tuff cone produced by phreatomag-matic eruptions; the cone was greatly modified by resedimentation and lobe intrusion. Evidence in support of the cone model includes the concomi-tant development of the two facies associations, the marked difference in thickness of the lobe and volcaniclastic facies associations beneath the pla-gioclase-crystal-rich unit in the south and north, the low aspect ratio of rhyolite lobes, which sug-gests eruption on a slope, and the more distal location of the close-packed lobe units. Stratifica-tion in the cone is variable over short distances with evidence of current activity and erosion.

The inferred cone grew upward as domes and flows were erupted to the north. In this model, domes 1 and 2 were erupted close to the inner wall of the cone (Fig. 3), and the domes may have prevented destruction of the cone by advancing younger flows, and also partly ponded the younger flows. The various types of lobes within the cone may represent overtopping of the cone (isolated lobes) and injection into the cone (mono-lithic units) by thin lava tongues produced from thick, partly ponded lava flows. The overtopping is a smaller-scale and subaqueous equivalent of subaerial lava tongues associated with the Bad-lands rhyolite flow of Idaho (Manley, 1996).

8. Conclusions

The Grassy Narrows subaqueous rhyolite lava flows, domes, and associated volcaniclastic units are similar to subaqueous rhyolite flows and domes described elsewhere in terms of the spatial and genetic association of lobes with volcaniclas-tic units. However, in many other aspects the Grassy Narrows sequence differs from other de-scribed flows and domes. (1) With an average thickness of 100 m, Grassy Narrows flows are thinner than the only other laterally extensive

flows; namely, the Archean examples described by De Rosen-Spence et al. (1980). (2) The upper breccia is thicker than that described in other flows and domes, and was probably largely the result of interaction with seawater. (3) The lobe and volcaniclastic facies association is not a facies of the lava flows and domes, but is, at least in part, a coeval tuff cone greatly modified by resed-imentation. (4) Some of the lobes were emplaced as discrete, low-volume, miniflows on the outer surface of the cone as thick flows ponded against the inner surface of the cone. (5) Some lobes are pillows.

The Grassy Narrows rhyolite domes, flows, and lobes provide some constraints on the nature and setting of the volcanism. Critical features are the phenocryst-poor character, paucity of basal brec-cia, presence of pumice, shape and aspect ratio of the lobes, and presence of pillowed rhyolite. These features indicate that the rhyolite magma was hot and fluid, and was erupted in an oceanic setting at shallow to moderate depths. This may indicate, in turn, a relatively deep source for the rhyolite magma (Ekren et al., 1984), which is part of a bimodal basalt-rhyolite sequence.

Acknowledgements

At various times this research has been sup-ported by the Manitoba Department of Energy and Mines, and by grants from the Natural Sci-ences and Engineering Research Council, and Ri-ocanex. Capable field assistance was provided by M. Boulet, J. Fredericks, C. Nikols, and L. Slezak. The final manuscript has benefited from reviews by A. Bailes and J. McPhie.

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