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

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

4 . 1 . Facies description The brecciated facies is spatially associated with, and typically overlies the nonbrecciated fa- cies Fig. 3. These two facies are subdivided, in turn, into five subfacies Fig. 6 that are described in Table 1. From base to top of a typical nonbrec- ciated to brecciated sequence, the subfacies are a 1 thin basal subfacies that lacks columnar joints L .D . Ayres , A .S . Peloquin Precambrian Research 101 2000 211 – 235 Table 1 Subfacies in rhyolite domes and flows of the dome-flow complex a 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 Similar to subaerial Relatively sharp contact over several tens Characterized by diversely oriented fractures, several cen- 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, mostly B1 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. 11 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 subfacies but degree of brecciation is B15 L .D . Ayres , A .S . Peloquin Precambrian Research 101 2000 211 – 235 217 Table 1 Copntinued Boundary with overlying subfacies Comments Subfacies of dome or Description flow Columnar-jointed subfacies of 5–40 Columns have a relatively Column development variable within and between flows and Undulating boundary; a relatively abrupt 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- in orientation of cooling joints that parallel the boundary ameter; some columns as much as 30 cm in diameter, but these surfaces, apparently con- 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 clasts B1 cm long and B15 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- Only exposed in flow 3, Boundary defined by appearance of columnar 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 a Description is based mainly on well exposed dome 1, supplemented with information from the other domes and flows. 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 Fig. 7. Base of flow 3 arrow, defined by thin breccia zone immediately above arrow, overlies thin-bedded volcaniclastic unit of the lobe and volcaniclastic facies association. End of 8-mm-wide pencil for scale. 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. In the upper three, nonbrecciated to brecciated sequences, development of subfacies is variable. For example, in sequence 3, there is a lateral facies change from the centre of the sequence southward. In the centre, disaggregated breccia is 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 brecciation, 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 particles 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 Based on the sequence and characteristics of subfacies, the variable development and distribu- tion of subfacies, the differences in phenocryst populations among sequences, and the lateral and vertical dimensions of the nonbrecciated to brec- ciated sequences, we interpret these sequences to represent one or two domes and three lava flows Fig. 3 that were erupted on the sea floor. Al- though upper contacts of individual domes and flows are poorly exposed, there is no evidence to support an intrusive origin for the Grassy Nar- rows rhyolite units cf. Allen et al., 1996. Extru- sive origin is supported by the consistent repetition of subfacies within sequences, the dis- cordant boundary between sequences 1 and 2, and the stacking of sequences 3, 4, and 5 without apparent intervening units. Because of the 2-di- mensional nature of the exposure, the present lateral dimension of the domes and flows could be length, width, or some dimension intermediate between length and width, and the original extent 5. Southern rhyolite lobe and volcaniclastic facies association — a partial cone?