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Home , Diabase, Slate Islands (Ontario)

Diatremes and shock features in rocks of the Slate Islands, northeastern

R. P. SAGE Geological Branch, Ministry of Natural Resources, Toronto, Ontario, Canada MSS 1B3

ABSTRACT province (Goodwin and others, 1972). Along the west coast of Pat- terson Island (Fig. 2), a small area of middle and upper Precam- The Precambrian rocks of the Slate Islands exhibit shock features brian supracrustal rocks of the Southern province of the Lake that have been attributed to impact. The islands occur at Superior basin is present. This sequence consists of a thin unit (less the intersection of two major regional faults, one of which controls than 21 m) of Gunflint iron formation of middle Precambrian age, the location of late Precambrian alkalic magmatism. Alkalic intru- overlain by a 120-m-thick sequence of upper Precambrian sive events north of Lake Superior occurred on the Slate Islands. volcanic flows of the Osier Formation. The upper Precambrian Port These alkalic intrusions are represented by a in the Coldwell alkalic rock complex is 24 km northeast of the Slate Is- southeast corner of the island and a set of alkalic dikes ex- lands. posed at several locations on the island. Diatreme cut these The lower Precambrian rocks of the Superior province are older alkalic rocks and are interpreted to be late-stage phases of this than 2,480 m.y. (Stockwell and others, 1970, p. 54), whereas the volatile-rich alkalic magmatism. Shatter cones appear spatially re- Gunflint (Animikie) and Osier volcanic rocks of the Southern prov- lated to the diatremes, and quartz grains displaying deformation ince have been dated at 1,650 to 1,800 m.y. and 900 to 1,100 m.y., lamellae are present in the matrix of the breccias. Shatter-cone respectively (Stockwell and others, 1972, p. 118-119). structures and deformation lamellae are considered to be indicative Faults beneath Lake Superior, indicated in Figure 1, were taken of shock events. On the basis of field observations and data, the from Hinze and others (1966), who have, on the basis of shock and diatreme events can be correlated and related to vol- aeromagnetic data, interpreted the presence of two major faults in canism or alkalic magmatic processes associated with major re- the northeastern Lake Superior basin intersecting immediately gional fractures and not to meteorite impact. south of the Slate Islands. These faults are (1) the northeast-striking Big Bay—Ashburton Bay fault, and (2) a curvilinear fault that passes INTRODUCTION north of and then to the west, passes im- mediately south of the Slate Islands, and then strikes northwest The Slate Islands, a group of 17 islands and islets covering a sur- toward Schreiber, Ontario. The positions of these faults have been face area of approximately 39 km2 in the northeast corner of Lake slightly modified from Hinze and others (1966) by contouring of Superior (Fig. 1), were mapped in 1974 (Sage, 1975). This work bathymetric maps published by the Department of Energy, Mines showed that the islands are geologically complex, with a variety of and Resources, Ottawa (Canada Department of Energy, Mines and rock types and structures ranging in age from early to late Pre- Resources, 1971b, 1973). Contoured water depth or lake-bottom cambrian. Among these complexities are numerous diatreme brec- topographic data indicate that a linear trench, with water depths cias, shatter cones, and associated deformation lamellae in quartz. approaching 240 m trends northeast approximately 1.6 km south- These phenomena have been accredited to meteorite impact (Halls, east of the Slate Islands, and a similar linear trench trends north- 1975; Halls and Grieve, 1976), but in my opinion, they are more west through a point approximately 1.6 km southwest of the is- likely caused by endogenous processes. lands, parallel to the trend of the faults inferred by Hinze and others (1966). REGIONAL The onshore extension of the Big Bay-Ashburton Bay fault of Hinze and others (1966) was interpreted on the basis of linear fea- The complexity of the Slate Islands cannot be fully appreciated tures evident on ERTS photographs (from the Canda Centre for without a general understanding of their regional geologic setting. Remote Sensing, Ottawa). Several interpretive possibilities exist for Figure 1 is a compilation of some of the pertinent geologic features joining the linear structures found north of the Port Coldwell found within the northeastern Lake Superior region. The general Alkalic Complex and the geophysically inferred faults of Hinze and geology of the region is taken from Ayres and others (1970); the others (1966), but only the most prominent trends are indicated in prominent linear passing through Chipman Lake is extended south Figure 1. to the Killala Lake alkalic rock complex and beyond to the north- The curvilinear fault south of Patterson Island does not appear ern flank of the Port Coldwell alkalic rock complex on the basis of on the mainland. Its eastern extension north of Michipicoten Island a topographic linear discernible on published topographic maps has been confirmed by Halls and West (1972). This fault appears (Canda Department of Energy, Mines and Resources, 1971a; On- similar to a number of geophysically inferred and/or geologically tario Ministry of Natural Resources, 1972). documented faults within and marginal to the Lake Superior basin Most of the islands are underlain by lower Precambrian supra- (Wold and Ostenso, 1966; Hinze and others, 1966). These cur- crustal rocks belonging to the Wawa subprovince of the Superior vilinear faults have strike lengths of many kilometres. The fault of

Geological Society of America Bulletin, v. 89, p. 1529-1540, 12 figs., 1 table, October 1978, Doc. no. 81008.

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Hinze and others (1966) south of Patterson Island is likely marginal topographic high or ridge that crosses Lake Superior in a to the Lake Superior basin and may be similar to the curvilinear northeast-southwest direction, from Big Bay, Michigan, to Ashbur- Keeweenaw fault in Michigan. ton Bay, Ontario, parallel to the geophysically inferred fault zone. Seismic studies in Lake Superior by Smith and others (1966) in- White (1972) divided the currently exposed Lake Superior basin dicate that the Mohorovicic discontinuity lies at a depth of approx- into five (possibly six) distinctly different volcanic basins. He inter- imately 60 km beneath Lake Superior at a position that lies approx- preted the western limits of the two most easterly basins as lying imately 80 km south of the Slate Islands, along the Big Bay- along the submarine ridge between Big Bay, Michigan, and Ashbur- Ashburton Bay fault. This 60-km depth is the most extreme depth ton Bay, Ontario, which is along the inferred fault of Hinze and in North America (Smith and others, 1966). O'Brien (1968) re- others (1966). Likewise the eastern limits of the two central basins examined the data of Hinze and others (1966) and calculated a lie along the same structural trend. This interpretation implies at depth slightly in excess of 50 km. Bottom topographic contours least minimum middle Precambrian age for the Big Bay—Ashburton compiled by Hough (1958) also indicate that the islands lie along a Bay structure, or that the ridge existed prior to the deposition of the

Figure 1. Regional geology of Slate Islands, after Ayres and others (1970); inferred faulting in Lake Superior after Hinze and others (1966), modified from Canadian Hydrographie Service maps (Canada Department of Energy, Mines and Resources, 1973, 1976). Inferred faulting between Killala Lake alkalic complex and Lake Superior interpreted from ERTS imagery.

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volcanic rocks within the basins east and west of the trend. The facies rank and consist of coarse pyroclastics, felsic to mafic possibility of this ridge being pre—late Precambrian was originally tuff, porphyry flows, volcanic slump deposits, and amyg- suggested by Halls (1972), who, on the basis of paleomagnetic daloidal, pillowed, and variolitic mafic flow rocks with thin inter- studies, postulated that the upper Precambrian Osier Group vol- bedded units of argillite and siltstone (Fig. 2). Several thin units of canic rocks pinch out toward the Slate Islands. He consequently in- bedded chert- iron formation are also present. This lower terpreted the Slate Islands to lie along an axis of a prevolcanic Precambrian volcanic sequence has been intruded by lower Pre- ridge. cambrian feldspar porphyry, quartz-feldspar porphyry, and mas- The results of many investigators working independently indi- sive intrusions and folded into a steeply southwesterly cate that the Slate Islands have a unique geologic setting. In addi- plunging (approximately 60°) fold (Sage, 1975). During this fold- tion, the extension of the Big Bay—Ashburton Bay fault and ing, considerable tectonic adjustment took place between Mortimer branches from this fault are the loci of several alkalic rock- and Patterson Islands, and a number of smaller fold structures were carbonatite complexes or areas of alkalic intrusive activity lying superimposed on the major structure. Relatively deep water, ap- north of Lake Superior (see Fig. 1). Of these complexes, the Prairie proaching 30 m, between Mortimer and Patterson Islands and Lake carbonatite yields a whole-rock Rb-Sr isochron age of 1,033 wide, steeply dipping zones along the south side of Mor- m.y. (K. Bell and H. D. Watkinson, in prep.); the Port Coldwell timer Island suggest that faults are likely to exist here. On the basis complex yields a similar age of 1,052 m.y. (Chaudhuri and others, of pillow-facing directions, Mortimer Island is interpreted to repre- 1971); and Coates (1970) reported a K-Ar isotopic age of 1,185 ± sent an anticlinal structure. 90 m.y. for the Killala Lake alkalic rock complex. Also, the exten- The dominant lithologic trend of the lower Precambrian rocks is sion of the curvilinear fault from Michipicoteri Island into the generally northeast parallel to the Big Bay-Ashburton Bay fault, mainland near Schreiber may be the controlling feature for an in- but along the west side of Patterson Island the trend is northwest trusive breccia reported by Harcourt (1938). parallel to the suggested fault zone from Michipicoten Island which passes near the south coast of the island and strikes northwest GEOLOGY OF SLATE ISLANDS toward the Schreiber area. The dominant northeast-southwest lithologic trend lies parallel to the isoclinally folded, regional The Slate Islands are composed predominantly of a sequence of lithologic trend previously mapped by Walker (1967) on the main- lower Precambrian volcanic and subvolcanic intrusive rocks that land to the north. The northwest trend along the west side of Pat- range in composition from calc-alkalic to tholeiitic . terson Island appears to be superimposed on this northeast regional These rocks have been regionally metamorphosed to greenschist- trend and thus may imply two periods of early Precambrian fold-

LATE PRECAMBRIAN

B883 Diatreme Breccia

^H Alkalic Diabase and Carbonatite

E3 Mafic Flow Rocks

Diabase Dikes and Sills

MIDDLE PRECAMBRIAN

Gunflint Iron Formation (not shown)

EARLY PRECAMBRIAN

ED Felsic to Intermediate

Intrusive Rocks

• Mafic to Intermediate

Intrusive Rocks

lllllllll Metasediments

Felsic to Intermediate

Volcanic Rocks VA

Mafic to Intermediate

Volcanic Rocks

Geophysically inferred faults, after Hinze and others (1966), (location approximate)

Figure 2. Generalized geologic map of Slate Islands (after Sage, 1975).

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ing, since major deformation structures are lacking in the middle structures and a matrix containing clastic quartz grains with de- Precambrian rocks. formation lamellae. Rocks correlated with the middle Precambrian Gunflint iron The islands were subsequently glaciated during Pleistocene time. formation occur along the west coast of Patterson Island and lie be- The massive bulk of Mortimer Island, the lower Precambrian tween lower and upper Precambrian metavolcanic rocks (not dioritic intrusions, and the upper Precambrian diabase intrusions shown in Fig. 2). The middle Precambrian iron formation is about have all undoubtedly helped shield the more fissile lower Precam- 21m thick, dips approximately 50° west, and lies in angular uncon- brian rocks found on Patterson Island from Pleistocene glacial ero- formity on the underlying lower Precambrian rocks. It has been sion. Plotting of trends of glacial striae indicate a southwest trend folded but lacks the penetrative schistosity characteristic of the and local deflection of the ice around Mortimer Island. Glacial lower Precambrian rocks. striae are present, but uncommon, on Patterson Island rocks of On the basis of attitudes of bedding and spatial distribution of similar competency to those of Mortimer Island. The apparent outcrop, an angular unconformity separates the middle Precam- circular nature of the island complex can be accounted for by (1) brian strata from an upper Precambrian sequence of tholeiitic, folding within the lower Precambrian sequence, and (2) protection amygdaloidal to massive mafic flow rocks containing local thin in- from Pleistocene glacial erosion of the schistose rocks found on Pat- terbeds of red siltstone and sandstone. This sequence is approxi- terson Island by the massive edifice of Mortimer Island and the mately 120 m thick and dips 80° west in the lower part of the sec- massive lower and upper Precambrian mafic intrusions. tion and 25° west in the upper part. The upper Precambrian rocks have been extensively deformed in the lower part of the sequence. DIATREME BRECCIA DIKES This deformation makes determination of the true stratigraphic thickness of the upper-Precambrian section difficult if not impossi- Breccia dikes, forming an anastomosing network, cut all rock ble. The angle of possible discordance with the middle Precambrian types found on the islands and are exposed along the western, rocks is also difficult to determine but is believed to be relatively southern, and eastern coasts of Patterson Island, on shoals off the low. Halls (1974), on the basis of paleomagnetic data, indicated coast of Patterson and Dupuis Islands, and on Spar Island. Along that the upper Precambrian mafic flow rocks found on the Slate Is- the east coast of Patterson Island, intrusive breccias were recog- lands are equivalent in age to mafic volcanic rocks found in the nized beneath the waters of Lake Superior, where they are best ob- lower part of the Osier Group farther west. Flow directions in the served in the early spring before the algal growth has covered the upper Precambrian mafic flows were determined from the ropy sur- lake bottom. East of Dupuis Island, the breccia dikes are present on faces of many of the flows. These determinations can be considered submerged shoals. The relative abundance of the breccias along the only approximate because of the arcuate nature of the individual coast is in part attributable to the excellent exposure there. Dikes ridges on a ropy surface. The readings were rotated back to hori- are present within the interior of Patterson Island, but they are zontal on a stereonet projection and plotted on a rose diagram, either not as well exposed or, perhaps, as well developed. In addi- which indicates that the source for the flow rocks was toward the tion, along the west and south coasts of Patterson Island, the upper east, in the direction of the interior of Patterson Island (Fig. 3). Precambrian—lower Precambrian contact may have had a strong Mapping indicates that individual flow units are lensoid in plan view and relatively thin, rarely exceeding 10 m in thickness. The relatively strong orientation of flow directions implies strong con- trol of flow directions by topography on the paleoslope. ^50 % The pervasive diabase that cuts Patterson Island is 40% similar in age to the upper Precambrian flow rocks and helps form some of the precipitous cliffs found along the coast. One large 30% diabase dike was observed to cut the lower but not the upper part of the upper Precambrian flow sequence, and some of these dikes are probably feeders for the mafic flows. I consider Patterson Island 20% to be a center of late Precambrian volcanism. The diabase dikes consist of four easily recognizable types: (1) red alkalic diabase, (2) 10% feldspar porphyry diabase, (3) porphyritic () diabase, and (4) massive equigranular diabase. Massive equigranular diabase dominates over the other three types. An upper Precambrian lamprophyre dike with carbonatitic affinities cuts across the southeast corner of Patterson Island. This dike approaches a maximum width of 30 m and consists of an aligned series of apparently disconnected tectonically undeformed segments that strike northeast, generally parallel to the trend of the Big Bay-Ashburton Bay fault of Hinze and others (1966). The lam- prophyric dike consists of , phlogophite, magnetite, , pyroxene, garnet, perovskite, and in widely varying propor- tions. Its emplacement is tentatively considered to be coeval with the emplacement of other alkalic rock-carbonatite intrusions along the Big Bay—Ashburton Bay fault trend. This entire sequence of lower to upper Precambrian rocks has been intruded by a ramifying Figure 3. Rose diagram of Keweenawan flow directions, based network of diatreme breccias with spatially associated shatter-cone on 18 flow-direction determinations from ropy lava surfaces.

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influence on the location of diatreme intrusions. The development dikes and red of the larger dikes. The west coast breccia dikes are of diatremes has been traced onto the mainland to the vicinity of generally deep red and derived their coloration in part from the the east flank at the Port Coldwell alkalic complex, where I re- presence of comminuted Gunflint iron formation and in part from examined an Animikie conglomerate unit, mapped by Walker hydrothermal activity related to their emplacement. The east coast (1967), and reinterpreted it as a northeast-striking breccia dike in- breccias are a much lighter shade of red. This color is from a pul- trusion. This diatreme is poorly exposed and relatively small com- verulent fine-grained hematite likely exclusively related to hydro- pared to the Slate Island intrusions. I found neither shatter-cone thermal activity at the time of emplacement. Clasts of Gunflint iron structures in outcrop nor deformation lamellae, in thin sections formation are absent in the east coast breccias and common in prepared from samples, at this dike. This northeast-striking breccia those along the west coast. dike is on strike with a northeasterly extension of the Big Bay— Ashburton Bay fault of Hinze and others (1966) and in all proba- Fragment Alteration bility is related to the Slate Island diatreme event, even though it lacks apparent shock features. Little visual evidence exists in the vast majority of breccias exam- The Slate Island breccias consist of sharply angular to sub- ined for any significant alteration of the fragments in the hematite- rounded fragments, as much as 4 m in maximum dimension, de- impregnated matrix, suggesting that the fluids accompanying dia- rived locally from the lower to upper Precambrian rocks and set in treme emplacement were low temperature and/or nonreactive with a matrix of comminuted rock debris of similar derivation (Fig. 4). the fragments. One notable exception is in one diatreme on the The breccias are indurated but are easily eroded and can be sam- west coast of Patterson Island, approximately 0.8 km north of the pled with little effort, as they readily fall apart upon being ham- southwest corner of the island. At this location, angular blocks of mered. The breccias are chaotic or generally lack any evidence for upper Precambrian diabase incorporated into the central part of fragment sorting and thus are homogeneous in appearance across this diatreme have bleached rims (Fig. 5). This feature is best seen at any given exposure. Dike widths locally approaching 30 m were the waterline, where the fragments have been polished by wave ac- observed. Except for one occurrence, the breccias are undeformed tion. The bleached margins vary in width from 3 to 7 cm, and the and lack any evidence of having undergone postemplacement fragments break easily when hammered, suggesting that the altera- tectonism. The one exception is an intrusive breccia found along tion has penetrated deeper than the bleached area rimming the the upper surface of a diabase cutting the lower part of the sec- fragment. During the course of mapping, numerous large breccia tion of upper Precambrian flow rocks found along the west coast of samples, including specimens from the diatreme displaying Patterson Island. The sill and intrusive breccia have undergone bleached rims, were collected and subsequently slabbed. The minor faulting. This breccia is thought to be of the same age, and slabbed surfaces failed to disclose visible evidence of fragment related to, the diatreme breccias. alteration. The visible alteration along the rims of fragments from In outcrop areas where dikes are especially abundant, large bed- the center of this one diatreme is presumably due to its large size rock blocks between and within the dikes have undergone little or and the greater volume of fluid or gases accompanying it during no rotation. Unrotated to slightly rotated blocks of bedrock en- emplacement. The lack of significant fragment alteration in all closed by dikes of breccia as much as 15 m on a side have been ob- other dikes would not support the thesis that the hematitic colora- served. Because of the lack of any significant rotation of the large tion of these breccias is a product of alteration of the clastic debris. blocks, it is possible to trace lithologic units and structural trends The contact between the breccia dikes and host rocks lacks any through areas of extensive breccia-dike intrusion. visual evidence of alteration or metamorphism. However, as a check on the interpretation that alteration or metamorphism did Source of Breccia Dike Color not accompany breccia-dike emplacement, a breccia dike cutting an unmetamorphosed lithologic unit was selected for a comparative The Diatreme breccias are brick-red, gray, and greenish gray. study. One thin section was prepared of a red alkalic diabase in The gray and greenish gray color is more typical of the smaller

Figure 4. Typical appearance of diatreme breccias in one of Figure 5. Diabase clasts in center of one of larger diatreme in- larger diatreme intrusions approximately one-third of the way up trusions approximately one-third of the way up west coast of Pat- west coast of Patterson Island. terson Island. Note bleaching and alteration along clast margins.

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contact with an intrusive diatreme breccia dike, and a second thin Island, where a breccia dike of 72 cm wide displays a strong grada- section was prepared from an unaltered-appearing center of an tion of fragment size from fine pulverized rock debris along its identical alkalic diabase dike. This was done to compare micro- flanks to a coarse breccia core, where fragments as large as 5 cm are scopically the contact effects of the diatremes. The red alkalic present (Fig. 6). diabase of the contact is significantly altered relative to the sample A second form of fragment sorting was observed in a diatreme of red alkalic diabase taken from the center of a similar dike. The breccia along the northeast coast of Patterson Island; here the alkalic diabase lying in contact with the breccia dike displays saus- breccia contains several large angular blocks of lower Precambrian suritized and a mafic assemblage altered to chlo- rocks, approximately 1 m along a side. The breccia along the side rite and a reddish-brown iron oxide. Some trace amounts of car- of the blocks displays a size gradation in comminuted rock matrix bonate and small flakes of are present in the altered rock. over a width of 5 cm, with the finest fragmental material in direct The sample from the center of the red alkalic diabase dike consists contact with the block. The size-graded rock fragments grade im- of unaltered feldspar (An45_54), , hypersthene, perceptibly into unsorted breccia away from the edge of the block. apatite, and magnetite. Other than a relatively low-temperature, These forms of fragment distribution are interpreted to be the result low-pressure, -epidote-—facies type of contact of the local presence of lam inar flow, similar to the model proposed metamorphism, no other difference existed between the two sam- by Bhattacharji (1967). ples. The alteration of the red diabase is essentially one of hydra- A breccia intrusion exposed midway up the west coast of Patter- tion and oxidation. Turner (1968, p. 257, 258, 366) indicated that son Island contains a small, very fine-grained irregular dikelet of the albite-epidote hornfels facies of contact metamorphism takes comminuted rock debris, which is interpreted to be an autointru- place at approximately 300 to 400 °C. This temperature range is sion of the fluidized matrix. also typical of the greenschist facies of regional metamorphism (Turner, 1968, p. 366). Because the lower Precambrian rock repre- Diatreme-Wall-Rock Contact sents a water-rich and carbon dioxide—rich sequence of greenschist-facies metamorphism, the incorporation of fragments All country-rock—diatreme breccia contacts are vertical to of these rocks into the diatremes under conditions similar to their steeply dipping, have wedge-shaped terminations, and have sharp original metamorphism would not leave a metamorphic overprint. angular corners and edges along the flanks of the dikes, giving the This is likely the basis, in part, for the observed alteration rims on appearance of brittle fractures (Fig. 7). The dikes commonly form a previously unmetamorphosed upper Precambrian diabase clasts in ramifying network, and dike orientation is not controlled by the the large diatreme along the west coast of Patterson Island, whereas excellent cleavage in the lower Precambrian host rocks. This fea- associated lower Precambrian clasts within the same dike did not ture was particularly evident on the southeast coast of Patterson Is- display such visible alteration. land, where the ramifying nature of the dikes is well displayed be- The lack of fragment alteration indicates that the hematite pres- neath the shallow waters of Lake Superior. At this location, the an- ent in the breccia is probably not a product of alteration of the gular fracture pattern is without any obvious control by the fragments entrapped in the diatreme intrusions. In addition, the strongly anisotropic host rocks, and this implies rapid, sudden general lack of extensive fragment alteration would further suggest emplacement and/or random, nonoriented force of sufficient inten- that the fluids that fluidized the breccia either (1) were not at an sity to ignore existing anistrophy in the rocks. The well-developed elevated temperature, (2) escaped rapidly, or (3) were nonreactive foliation of the lower Precambrian felsic metavolcanic host rocks at — or a combination of any of these factors. this location would be expected to exert more structural control on diatreme emplacement if the emplacement was at all passive in na- Fragment Sorting ture.

In general, little sorting of the breccia fragments in the dikes is recognizable. One exception was found on the north side of Spar

Figure 7. One of smaller breccia dikes displaying sharply cross- cutting relations. Breccia dike cuts lower Precambrian felsic meta- volcanic rocks and is located approximately one-third of the way Figure 6. Unusually well-developed clast size gradation in dia- up west coast of Patterson Island. Note lack of offset or rota- treme dike on north side of Spar Island. Photograph taken at tion of lower Precambrian rocks. Area is pervasively diked with lake level. breccia.

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The diatreme—wall-rock contact relations therefore suggest sud- some of the breccia matrix of the diatremes, suggesting the addition den and rapid emplacement of the breccias, such as would be neces- of some carbon dioxide. sary for the accompaniment and development of shock features The chemistry of the upper Precambrian mafic flow rocks, thin found in the host rocks. sections of the breccia matrix, and thin sections of the host rocks flanking the breccia dikes suggest the introduction of water and Breccia Transporting Fluids carbon dioxide, indirectly implying that these fluids accompanied diatreme emplacement. Fluid charged with carbon dioxide would Only indirect inferences can be drawn about the nature of the thus appear to be the most plausible transporting medium for the transporting medium for the breccia dikes. These inferences must breccia. be made from chemical and mineralogical changes or alterations in the host rocks. A fluid composed essentially of water and carbon Diatreme Breccia and Associated Magmatism dioxide is likely the most probable transporting medium, and the process of fluidization as proposed by Reynolds (1954) is the most A recognizable igneous matrix is lacking in all samples studied in likely emplacement mechanism. thin section, with the exception of a breccia occurrence located ap- The suggestion that carbon dioxide and water were the medium proximately midway up the west coast of Patterson Island. At this for diatreme breccia emplacement is inferred from (1) the high location, a breccia with a diabase matrix lies along the upper con- water and carbon dioxide content of the upper Precambrian flow tact of a diabase sill, and this indirectly implies a relationship be- rocks that have been cut by the breccia dikes; (2) presence of car- tween diatreme emplacement and mafic (diabasic) magmatism. bonate in the diatremes; and (3) even though not visually apparent, At this location, a west-dipping sill-like intrusion of diabase the development of hornfelsic contact metamorphic effects along forms a prominent hill within the upper Precambrian section. Co- the diatreme contacts, essentially hydration and oxidation effects. lumnar jointing is well developed, and along the upper surface of Breccia dikes are abundant in the lower part of the upper Pre- this sill a zone of massive breccia approximately 4 m thick is pres- cambrian volcanic sequence near the upper Precambrian—lower ent (Fig. 8). The true breccia thickness is difficult to measure, be- Precambrian contact. Even though they were the freshest samples cause of the oblique erosion surface. Rare amygdules are present in available, analysis of carefully selected amygdule-free, massive the upper and lower surface of the sill. These amygdules are likely mafic volcanic rocks from this sequence show that the combined the result of local accumulations of steam generated at the time of water and carbon dioxide for the five samples analyzed was an av- emplacement of the mass into the water-saturated upper Precam- erage of 8.29% by weight (Sage, in prep.) This water and carbon brian volcanics and interbedded sediments. The intense vesicularity dioxide content is three times that reported by Mcllwaine and Wal- characteristic of the other upper Precambrian flows is not charac- lace (1976) from similar rocks of the Osier Group located farther teristic of this unit, whose structure is more typical of the numerous west; these, on the basis of 15 samples, averaged 2.58% by weight diabase dikes found on the island. I do not consider it likely that combined water and carbon dioxide. The upper Precambrian mafic this unit is an extrusive flow rock as suggested by Halls (1977, per- volcanic rocks have thus been extensively impregnated with water sonal commun.). and carbon dioxide. A logical source is the diatremes found along The breccia contains rounded fragments of fine-grained diabase the base of the section. In thin section, carbonate veinlets and min- and chert, which have undergone partial assimilation, in a matrix eral replacements of feldspar fragments and chips are present in of very fine-grained chilled diabase. The fine-grained diabase frag- ments were probably derived from the chilled margin of the sill and the chert from the Gunflint iron formation, which in this locality would lie some distance below the sill. The rounded clasts of chert are as large as 48 by 26 cm and coated with very fine-grained chilled diabase. The diabase clasts are much smaller than the chert clasts. The rounding of the Gunflint clasts likely results from mil- ling of the fragments during ascent at the time of breccia-dike emplacement immediately preceding the intruding diabase. In thin section, polycrystalline, recrystallized, and nonrecrystallized grains of chert have been assimilated by the diabase. These observations are consistent with the fact that the intruding diabase would be ex- pected to be more reactive with the quartz than the more refractory diabase of the same composition as the intruding . Some of the partly assimilated quartz fragments display a weak concentra- tion of mafic around their periphery, and one grain dis- plays well-developed fine-grained radiating acicular of an unidentified mafic mineral. Those chert grains that have been re- Figure 8. Diabase sill cutting upper Precambrian flows on west crystallized lack a strong development of undulatory extinction, coast of Patterson Island, which has had a diatreme emplaced along and none of the grains, either recrystallized or nonrecrystallized, its upper surface. Matrix of breccia is composed of very fine- contain lamellae. Diabase fragments in the chilled diabase matrix grained chilled diabase, which is reacting with some of the clastic contain plagioclase feldspar crystals with well-developed cleavages quartz grains. This was the only breccia containing recognizable and very weakly developed cuspate or embayed margins where they igneous matrix. Presumably, a diatreme breccia dike was emplaced lie along the edge of the fragment in contact with the enclosing and shortly followed by diabase intrusion. Intrusion breccia now diabase matrix. The finer grained diabase of the matrix was ob- consists of clasts of fine-grained diabase Gunflint iron formation set served to penetrate into the margins of the fragments as lobate in- in very fine-grained chilled diabase. trusions. The lack of well-developed cuspate or embayed fragment

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boundaries and the generally straight edges are taken to indicate and others (1970) concluded that the gas system escaping at the that the fine-grained diabase fragments have undergone only minor surface would transport wallrock xenoliths and fill the vents with resorption. an agglomeration of volcanic and xenolithic material. However, The very fine-grained diabase matrix consists of plagioclase volcanic fragments that could possibly be related to the diatreme

(An43) and an unidentified reddish-brown alteration product after event are lacking in the Slate Islands breccias, even though the pyroxene. Minor amounts of very fine-grained intergranular mechanism proposed by Harris and others (1970) would otherwise granophyre are also present. seem to me to be applicable to the Slate Island diatremes. The lack Halls (1977, personal commun.) has also suggested that this of volcanic fragments in the breccia that can be related to the dia- breccia unit may be a rubbly flow top and that the flow top repre- treme event suggests that fluid activity accompanying the diatreme sents an incorporated gravel deposit. Presumably, this conglomer- event may have had little surface manifestation other than the de- ate would be similar to the basal conglomerates found between the velopment of chaotic breccias marginal to the diatreme vents. Any and Gunflint iron formation in the Sibley Penin- such deposits that were originally present have subsequently been sula (Franklin and Kustra, 1970). A conglomerate unit was not ob- removed by erosion. It would seem that diatreme emplacement served to lie along the contact between the upper Precambrian need not be accompanied by extensive volcanism and that magmat- flows and middle Precambrian sedimentary rocks on the Slate Is- ic activity associated with the diatremes need not have a significant lands. The close spatial distribution of outcroppings of these two surface expression. In the case of the Slate Island diatremes, the sequences precludes the likelihood that such a conglomerate exists. present level of exposure is above that of any associated magmatic If the unit was a flow, as suggested by Halls (1977, personal com- activity. mun.), a problem exists as to source, because the Gunflint would The presence of alkalic-rock—carbonatite magmatism along have been covered at this location by earlier flows, preventing ero- major faults (for example, Big Bay-Ashburton Bay) is characteris- sion of the underlying middle Precambrian strata. tic of low degrees of partial melting at deep crustal levels (Bailey, Thin sections of interflow sediments between the upper Pre- 1974a; Gast, 1968; Hart and others, 1970). In addition, alkalic cambrian flows indicate a well-sorted, fine-grained arkosic siltstone are also a characteristic feature of continental rifting and and sandstone consisting of subrounded to subangular grains of are noted for their volatile enrichment (Bailey, 1974a, 1974b). quartz, microcline, and plagioclase and minor chert. The mineral- ogy suggests a type of source area. Neither the implied SHATTER CONES source area nor the relatively low-energy environment suggested by the fine-grained sediments is consistent with conglomerate deposi- During the mapping along the west coast of Patterson Island, tion consisting of such large fragments of the Gunflint formation. several shatter-cone structures were noted within the upper Pre- The possibility of the breccia being a rubbly flow top therefore cambrian mafic flow rocks and interbedded siltstones and must be rejected. sandstones (Fig. 9). These shatter cones are better developed in the The breccia lying on top of the sill is unique among the breccias lower parts of the upper Precambrian section in close proximity to found on the island, for it has a clearly recognizable igneous ma- breccia dikes. The shatter cones are fewer and more poorly defined trix. It is thought that this is an intrusive breccia accompanying the in outcrops on reefs and shoals farthest from the coast and highest emplacement of a diabase dike coeval with breccia-dike emplace- stratigraphically. This trend towards poorer development in shatter ment. This breccia may represent the intrusive edge of a diabase cones is in the direction away from the larger and better developed dike emplaced along the upper contact of an earlier diabase intru- diatreme structures found in the lower part of the upper Precam- sion. It is possible that diatreme breccias and diabase dikes are re- brian section close to the lower Precambrian—upper Precambrian lated and that the observed presence of a diabase dike or breccia dike may be a function of the present level of exposure. The north end of this diabase sill and accompanying intrusive breccia has been offset approximately 10 m by a small fault. This small fault is thus the latest recognizable tectonic event on the islands, even though it is of minor importance.

Diatreme Genesis

Harris and others (1970) considered that diatremes are the result of volatile-rich magmas formed at depths of 35 km or greater which have risen to shallower depths and exsolved a gas phase. Be- cause the two phases (gas-magma) of the system occupy a greater volume than the initial magma, a gas pressure greater than load pressure can be created that could be released by failure of the crus- tal rocks (Harris and others, 1970). On the basis of experimental data, Wyllie and Huang (1976) have predicted a phase transition in

C02 from liquid to gas in carbonated alkaline magmas ascending from the asthenosphere. This transition would take place at depths Figure 9. Well-developed shatter cones in amygdaloidal upper of 80 to 100 km. Precambrian flow rocks. This is an exceptionally well-developed Adiabatic expansion and exsolution of the gaseous vapors exposure, located approximately 15 m from a breccia dike. Cone (water, carbon dioxide) would cool the magma, which would so- development decreases in intensity and perfection of development lidify before reaching the surface (Harris and others, 1970). Harris as one moves away from diatreme intrusions.

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contact. It thus appears that the shatter cones and diatremes are 25° west in the upper part, it is suggested that the shatter cones spatially related. The shatter cones are best developed in the amyg- were developed after the flows were rotated into their present posi- daloidal tops of the mafic flows and in the fine silty layers of the tion. This interpretation could be somewhat suspect because cone- sandstone interbeds. The presence of shatter-cone structures is gen- axes bearing and plunge measurements were made only in a limited erally attributed to the passage of intense shock waves through part of the stratigraphic section immediately above the intense rock (Dietz, 1968). Recognizable cone structures occur in lower shearing and diatreme intrusions where the cones were available Precambrian rocks but are much more poorly developed. for examination. These measurements are thus confined to approx- Cones within the upper Precambrian rocks are generally about imately 60 m of the 120-m upper Precambrian section that is the 20 cm long and have a well-developed splay of lineations along more shallowy dipping. their flanks. The cones are poorly developed in the sandstone inter- The cone apices in Figure 10 were plotted on a rose diagram beds, which consist of easily disaggregated sand-size grains. One which indicates that the cone-forming event occurred due east or poorly defined cone about 0.3 m long was observed in a specimen just north of east and toward the interior of Patterson Island (Fig. of upper Precambrian sandstone float found on the west coast of 11). the island. Shatter cones are recognizable only in the less schistose lower Within the upper Precambrian section on the west coast of Pat- Precambrian rocks, and nowhere are they as well developed as in terson Island, strike and plunge were measured on a large number the upper Precambrian section. The schistose nature of the lower of cone axes. Cone axes were measured using a Brunton compass Precambrian rocks of Patterson Island is not conducive to the de- and not by the method described by Manton (1965). Neither time velopment of good cone structures. The presence of shatter cones in nor facilities were available to undertake such a detailed procedure. the lower Precambrian rocks is indicated by splaying lineations on All recognizable cone structures were measured, and all axes are some fracture surfaces, the crudely conical fracturing of some rock plotted in Figure 10 on a lower-hemisphere stereonet and con- units, and flattened conical fractures with splaying lineations. toured. The resulting diagram displays a strong maximum approx- These structures seldom exceed 4 or 5 cm in length, and their poor imately 10° below the horizontal and a scatter of data that would development, combined with imperfect cone shape, makes mea- average at best into a horizontal position (Fig. 10). The cone axes surements difficult if not impossible. do not fall along a great circle; thus, considering that the upper Pre- Poorly developed or crudely inferred shatter-cone structures cambrian flows dip from 80° west in the lower part of the section to were observed on the southwest coast of Mortimer Island, on McColl Island, and along the south coast of Patterson Island. Some crudely developed shatter cones were identified on the east coast of Patterson Island. No attempt was made to measure the features in the lower Precambrian rocks, but the visual impression is of a highly variable if not somewhat random orientation. In hand sam- ples, as many as four different cone orientations are recognizable, some of which point in opposite and apparently conflicting direc-

I I 0.0-1.0% ill 1.0-1.6% ~~ 1.6-3.3% 3.3-6.6% 6.6-9.8% Figure 10. Contoured stereoplot showing 9.8-13.1% spatial relationships of shatter-cone axis based Figure 11. Rose diagram of shatter-cone apex orientations on on 61 measurements. basis of 61 measurements.

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tions. The visual randomness of the cones within the island, in con- 15 composite grains best displaying lamellae. These lamellae are trast to the consistent orientation found in the upper Precambrian similar if not identical to lamellae in quartz grains whose formation rocks on the west coast of Patterson Island, may be a function of has been attributed to a shock event (Carter, 1965, 1968a, 1968b; the geometry of diatreme dike emplacement. The east and west Bunch, 1968; Robertson and others, 1968). Several angular pla- coast pattern of breccia dike emplacement forms a wedge-shaped gioclase grains, some within kinked albite twin planes, were noted. mass relative to Patterson Island. The explosive emplacement of the These grains are generally extensively saussuritized. dikes would make Patterson Island the site of shock-wave collision and reverberations from the west and east coast. Such a shock- ORIGIN OF SHOCK FEATURES wave pattern would account for the random-appearing cone struc- tures within and along the east coast of Patterson Island. The dia- Although a detailed theoretical or historical discussion of the tremes along the west side of the island lie at the base of the upper significance of shock features in a rock suite is beyond the scope of Precambrian section and outside of the wedge-shaped mass form- this paper, a brief discussion of the controversial nature of shock ing the bulk of Patterson Island. The consistent pattern in the upper features is warranted. Precambrian rocks is thus accountable by being outside this The recognition of shatter cones and deformation lamellae in wedge-shaped mass and thus subjected to only one shock wave quartz grains is taken by several investigators to indicate the pres- without significant reverberations. ence of a shock event that they correlate a priori with hypervelocity meteorite impact (Carter, 1965, 1968a, 1968b; Bunch, 1968; MICROSTRUCTURES Dietz, 1968). Bucher (1963) and Currie (1965) have documented the fact that A few breccia samples were selected for thin-section examination several proposed meteorite craters occur on or along major re- of the finer grained matrix parts. The texture is best described as gional geologic structures; this cannot be accounted for by ex- mortar with angular rock fragments and mineral chips set in a traterrestial meteoric impact. Several structures hematite-rich pulverized rock matrix. Some fibrous chlorite is pres- within the Canadian Shield typify the present debate between the ent in the hematite-rich groundmass, and numerous tiny irregular opposing points of view. Opposing meteorite impact—endogenous stringers of carbonate cut through the breccia matrix. With the one process views are given for the , Ontario, by Hartang previously described exception, no igneous matrix was recognized, and others (1971), Dence (1968), and Currie (1971b). Recent nor was glass or divitrified glass present in the sections examined. mapping of the area of the Brent crater by the Ontario Division of Several thin sections display composite quartz grains, likely chert Mines supports Currie's (1971b) interpretation (S. Lumbers, 1977, fragments derived from the Gunflint iron formation, with well- personal commun.). Lumbers (1977, personal commun.) and Cur- developed lamellae, and in one section an abundance of such grains rie (1971b) related the Brent structure to alkalic magmatism along is evident under high magnification (Fig. 12). These lamellae appear the Ottawa-Bonnechere graben system. as a series of parallel lines on the individual quartz grains. At least The astrobleme-endogenous debate over the , On- two sets of lamellae are present on individual quartz grains in the tario, has been summarized by Dence (1972), French (1968, 1972), thin section displaying the best development of the feature. These Dietz (1972), and Card and Hutchinson (1972). Card and Hutch- lines appear to have a spacing varying between approximately inson pointed out that the regional tectonic and stratigraphic set- 0.005 and 0.04 mm. Measurements of the attitude of lamellae rela- ting of the Sudbury structure involves a number of features that are tive to the C crystallographic axis of quartz (Table 1) were made on inconsistent with a meteorite impact origin. A summary of opposing views regarding the formation of the Clearwater complex, near Quebec, can be found in the articles by Bostock (1969), Dence and others (1974), and Dence (1968). Bos- tock (1969) proposed updoming over a magma chamber, explosive devolatization of a volatile-rich magma, and collapse along radial and ring fractures for the Clearwater Lake structures. Currie (1972) investigated the Manicouagan resurgent caldera in Quebec, a structure classified by some as an astrobleme (Robertson

TABLE 1. CRYSTALLOGRAPHIC ORIENTATION OF LAMELLAE IN QUARTZ FROM DIATREME BRECCIA OF SLATE ISLANDS

No. of Crystallographic orientation grains of lamella relative to quartz c-axis

9 (0001) 3 (1013) 1 (0001) and (1013) 1 (1013) and (1011) 1 (1011) and (0001)

Note: These measurements do not display the same relative frequency of Figure 12. Quartz lamellae in composite quartz grains within occurrence as those reported by Halls and Grieve (1976). The difference matrix of one of larger breccia intrusions located approximately may be a function of sample location. one-third of the way up west coast of Patterson Island. Photo- Measurements by Darko Sturman, research mineralogist, Royal Ontario graphy by A. J. Naldrett. Museum.

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and Grieve, 1975), and concluded that the structure was related to ACKNOWLEDGMENTS faulting, alkaline magmatism, and caldera-style collapse. The disagreement between the two groups of thought is a result, I thank D. Pyke, and S. Lumbers for encouragement during prep- in part, of different approaches to the same problem. Those propos- aration of this paper. K. Card, Pyke, and Lumbers have given freely ing endogenic processes base their conclusions on field observations of their editorial comment. K. Treacher, D. Meloche, D. Bathe, and and field data, while those proposing astrobleme origins support B. Campbell aided me in mapping the Slate Islands in the summer their hypothesis with the recognition of shock metamorphic fea- of 1974. Meloche and Bathe mapped and recorded the tures and the a priori assumption that such features can be caused orientations in the upper Precambrian rocks found on the west only by meteoric impact. coast of Patterson Island. D. Sturman completed detailed studies of Proposals for the meteoric impact hypothesis lack an explanation lamellae in quartz grains within the diatreme breccia matrix. I for closely associated local and regional structures, discussed by the thank V. Milne for reviewing this paper. Published by permission previously cited authors, and associated evidence for magmatic ac- of E. G. Pye, Director, Ontario Geological Survey, Ministry of tivity. Natural Resources. The correlation between geologic structure and cryptoexplosion structures was first summarized by Bucher (1963), and an adequate REFERENCES CITED explanation for this in the context of meteoric impact has yet to be made. The debate, astrobleme verses endogenic process, applies di- Ayres, L. D., Lumbers, S. B., Milne, V. G., and Robeson, D. W., 1970, On- rectly to the Slate Islands. Halls (1975, 1976), Robertson and tario geological map, west central sheet: Ontario Div. 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