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 Narrows zone Bailes
and Syme, 1989, which has a maximum width of 1300 m Fig. 2. Metavolcanic units in the Grassy
Fig. 1. Location and simplified geology of the Flin Flon – Snow Lake FFSN greenstone belt modified from Bailes, 1971; Whitaker and Pearson, 1972; Lucas et al., 1996. Star shows location of study area.
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 northern 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 Narrows zone modified from Bailes and Syme, 1989. Young-
ing directions, based on pillow shape, are from Bailes and Syme 1989.
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 plagioclase 9 quartz 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
andor 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 carbonate 9 chlorite 9 sericite 9 potassic 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 SiO
2
, and Na
2
OK
2
O 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 ZrTiO
2
, 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.
Fig. 3. Spatial relationship of brecciated and nonbrecciated rhyolite facies association, which represent rhyolite domes 1 and 2 and flows 3, 4, and 5, and rhyolite lobe and volcaniclastic facies association in the Grassy Narrows rhyolite. The small island in
Manistikwan Lake represents an older sixth flow. Units are subvertical and young to southwest Fig. 2. The map is a cross-section through the rhyolite sequence. Because flow 5 largely consists of breccia, the boundary with flow 4 could be defined only in areas
of good exposure. Brecciated rhyolite includes both in situ crackled breccia and disaggregated breccia subfacies.
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 B 1 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
