The Creighton Pluton: Introduction

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Abstract

The Creighton pluton, Ontario, is a small, composite granitic intrusive which intrudes mafic metavolcanic rocks at the base of the early Proterozoic Huronian Supergroup. Radiometric data indicate an age of about 2200 m.y. for the pluton. The Creighton pluton lies along the southern edge of the Sudbury Basin, and, together with the rocks of the Huronian Supergroup, was brecciated during the formation of the Sudbury Basin, possibly as a result of a meteor impact. A strong pre-brecciation foliation occurs within the Creighton pluton and the immediately adjacent wall rocks, but is not present elsewhere within the Huronian rocks. Mapping has shown that the foliation in the eastern portion of the Creighton pluton forms a complete elliptical loop, and indicates that the foliation formed during forceful intrusion of the pluton into a constricting collar of country rock.

A large inclusion is located within the center of the foliation loop. The rocks of this inclusion have been pervasively intruded by granitic material from the surrounding pluton and has undergone two episodes of pre-brecciation folding. Structures like those in the inclusion are not found in any of the rocks around the Creighton pluton. The location and unique structure of the inclusion indicate that it was deformed by constriction within the enclosing granitic rocks during the forceful emplacement of the Creighton pluton. Thus, the Creighton pluton must have been rigid enough to transmit stresses capable of deforming the rocks of the inclusion, at least during the final stages of emplacement.

The mafic intrusive ring which surrounds the Sudbury Basin, commonly called the "nickel irruptive", intrudes the Creighton pluton, and the intrusive contact of the irruptive is approximately parallel to the foliation within the pluton. It appears that the irruptive was intruded approximately along the northern contact of the Creighton pluton and another nearby granitic intrusive, the Murray pluton. This evidence for structural control of the irruptive, and the nondeformed dikes which radiate outward from the irruptive indicate that the irruptive and its wall rocks to the south have not moved relative to one another since the irruptive was emplaced. The south ward facing direction of the rocks south of the irruptive indicates that these rocks must have been overturned prior to the intrusion of the irruptive.

Although the structure and lithology of the Creighton pluton and its host rocks resemble those of the rocks of Archean greenstone belts, and an interpretation of the Creighton pluton as of Archean age cannot be entirely excluded, it is more likely that the Creighton pluton is related to early Proterozoic tectonic events. Events which may have been related to the intrusion of the Creighton pluton include metamorphic events in northern Michigan and the intrusion of a widespread suite of basic rocks, the Nipissing diabase. The radiometric ages of both of these events are approximately the same as that of the Creighton pluton (2100 to 2200 m.y.). Whether any of these events were related to plate-tectonic processes along the southern margin of the Superior Province craton is uncertain.

All field evidence within the Creighton pluton indicates that the brecciation within the Creighton pluton is the same age as brecciation elsewhere around the Sudbury Basin. No evidence which cannot be reconciled with the meteor-impact theory for the formation of the Sudbury Basin has been found.

Introduction

The Sudbury Basin, Ontario, is a possible Proterozoic meteor-impact site which lies along the contact between Archean granitic rocks of the Superior Province of the Canadian Shield and lower Proterozoic metavolcanic and metasedimentary rocks of the Huronian Supergroup in the Southern Province (Fig. 1). The Grenville Front, which separates relatively nondeformed and nonmetamorphosed Superior and Southern Province rocks to the northwest from their metamorphically reworked equivalents in the Grenville Province to the southeast (Wynne-Edwards, 1964), passes about 10 km southeast of the Sudbury Basin.

Figure 1. Geology of the Sudbury Basin. Inset: tectonic map of the Southern Province. Both maps compiled from numerous sources.

The oldest rocks in the Sudbury region are Archean rocks of the Superior Province north and west of the Sudbury Basin (Fig. 1, Table 1) which yield radiometric ages in excess of 2500 m.y. (Aldrich and Wetherill, 1960; Van Schmus, 1965). The Archean rocks are uncomformably overlain by the Huronian Supergroup. The Stobie Formation, a volcanic unit that occurs along the southern rim of the Sudbury Basin, has been intruded by two small granitic bodies, (1) the Creighton pluton and (2) the Murray pluton. The Stobie Formation is considered by most authors to be the lowest unit of the Huronian Supergroup, but some workers have maintained that the Stobie Formation, and possibly the Creighton and Murray plutons, are of Archean age. Both Southern and Superior Province rocks were brecciated during the formation of the Sudbury Basin, possibly as a result of meteoritic impact. The rocks of the Southern Province were subsequently deformed and metamorphosed during the Penokean Orogeny, about 1700 m.y. ago. Throughout most of the Southern Province the rocks show only evidence of post-brecciation deformation. The only evidence of penetrative pre-brecciation deformation in the Southern Province occurs iimuediately south of the Sudbury Basin in and around the Creighton granite, where rotated blocks of foliated granite and country rock occur within the breccia.

EVENT LITHOLOGIC UNITS RADIMETRIC AGE:  INITIAL Sr87/Sr86 IN PARENTHESES REFERENCES
Penokean deformation and metamorphism Olivine diabase dikes 1200 m.y.

 
Van Schmus, 1965
 
  1600 Gates and Hurley, 1973
  1700 (approximately) Fairbairn and others, 1960, 1965; Van Schmus, 1965
"Sudbury event" Whitewater Group    
Nickel irruptive 1840 (.705) Krogh and Davis, 1974
2000 (.706) Gibbins and McNutt, 1972, 1975
Sudbury breccia   Speers, 1957
Metamorphism ?   1950 Fairbairn and others, 1969
  Creighton and Murray plutons 2140 (.712) Fairbairn and others,1960, 1965
Nipissing diabase 2150 (.706) Van Schmus, 1965
Murray pluton 2230 (.719) Gibbins, 1972
Deformation ?     Card and others, 1972
Huronian deposition Gowganda Formation 2290 Fairbairn and others, 1969
Copper Cliff rhyolite 2350 (.706) Fairbairn and others, 1960, 1965
Mafic volcanic rocks (.705) Fairbairn and others, 1969
Archean events Granitic basement of  Superior Province 2500 and older Aldrich and Wetherill, 1960; Van Schmus, 1965
Greenstone belts    
Stobie Formation ?   Cooke, 1946
Creighton pluton ?   Cooke, 1946

Table I. Geochronology and Geologic History of the Sudbury, Ontario, Region

The structural notation in this paper will denote pre-brecciation structures with the subscript (e) and Penokean structures with the subscript (p); thus, prebrecciation and Penokean foliations are denoted by Se and Sp, respectively. Two generations of prebrecciation deformation structures occur within one of the inclusions within the Creighton pluton. The earlier deformation within this inclusion will be denoted as Del and the later as De2.

The problem of interpreting the pre-brecciation foliation within the Creighton pluton centers upon the number of deformation and brecciation events in the Sudbury region. If there were only a single episode of brecciation, then the Creighton pluton and some of the surrounding rocks were subject to a strong but highly localized pre-Penokean deformation. By contrast, if there were several episodes of brecciation, then the foliation in the Creighton pluton could be explained in terms of a single (Penokean) regional deformation. The first interpretation poses the problem of explaining why a strong deformation is peculiarly localized, whereas the second conflicts with the meteor-impact theory, which requires a single brecciation event. Furthermore, because a variety of widely accepted indicators of shock metamorphism have been found in the rocks around the Sudbury Basin, any geologic evidence which conflicts with the impact interpretation of the Sudbury Basin would cast serious doubt upon the interpretation of other supposed impact structures. In an effort to resolve these problems, the author spent the 1973, 1974, and 1975 field seasons engaged in reconnaissance, structural studies, and detailed mapping in the Sudbury area.

The detailed mapping of the eastern half of the Creighton pluton, upon which most of the conclusions in this paper are based, was performed in the. Following manner: structural data obtained in the field were plotted on overlay sheets superimposed on aerial photographs on loan from the Ontario Ministry of Natural Resources. The original scale of the photographs, 1:15,840 (4 inches = 1 mile) was insufficient for plotting data in structurally complex areas, so enlarged base maps at a scale of 1:7,920 (8 inches = 1 mile) were prepared from the aerial photographs, and data from individual overlay sheets were combined on these maps. Copies of these maps are included as Appendices I and II. Preparation of the enlarged base maps and the transfer of data onto the base maps were done by the author, using a cartographic opaque projector at the Lamont-Doherty Geological Observatory of Columbia University. On the original photographs, roads, trails, streams, and topographic features as small as 20 meters in size can easily be recognized, and most points were plotted with a maximum positional error of 20 meters. Comparable positional errors may have been introduced when compiling the aerial photographs into the enlarged base maps; these errors arise largely from perspective effects on the original aerial photographs. The largest single cause of positional error on the base maps is the need to present closely spaced data legibly; structural data from one outcrop might have to be dispersed over a small area on the map. The maximum positional error for structural data on the base maps is probably on the order of 50 meters (approximately 6 mm on the base maps at 1: 7,920 and proportionately less on maps of larger scale). In no case do these positional errors significantly affect the major findings of this paper.

The reproducibility of the data presented in this paper is variable. Even in areas of simple structure, the dip and strike of foliation or bedding commonly varies by 10 degrees within a few feet. Since most of the structures in the study area are nearly vertical, it is not uncommon for dips to change from steep northerly through vertical to steep southerly within a single outcrop. The author feels that, in the eastern half of the Creighton pluton, the most important elements in the structural geometry are the changes in strike of planar structures, and that the dips, and even the dip directions of these structures, are of virtually no significance. In some areas of inhomogeneous deformation, the attitude of a structure may change radically in only a few feet. In short, the broad structural trends outlined in this paper should be highly reproducible, but strict reproducibility at the outcrop level is impossible. Lithologic data are more subjective than structural data, both in terms of interpretation in the field, and in terms of locating contacts in areas of limited exposure or gradational lithologic changes. The reproducibility of the lithologic data presented in this paper should be good in terms of broad patterns, but significant variations in detail should be expected.

As originally stated, the problem of interpreting the structure of the Creighton pluton centered upon the number and order of brecciation and deformation events in the Sudbury area. As field work continued, it became evident that the apparently anomalous history of the Creighton pluton could be explained in terms of an early deformation of the Creighton pluton, a single regional brecciation related to the formation of the Sudbury Basin, and subsequent deformation during the Penokean orogeny. That is, one need not invoke the occurrence of several brecciation events to explain the structural relationships between deformation and brecciation within the Creighton pluton. Instead, the structure of the Creighton pluton can be explained in terms consistent with the meteor-impact theory for the formation of the Sudbury Basin. This finding is of considerable importance in interpreting the structural history of the Sudbury Basin.

Nevertheless, the unusual, and in some respects unique, deformation history of the Creighton pluton make the Creighton pluton a subject of wide geologic interest in its own right. Possibly the intrusion mechanics of the Creighton pluton are of more general geologic interest than the comparatively narrow problem of the relationship of the Creighton pluton to the formation of the Sudbury Basin.

Two problems are considered in this paper: the relationship between the Creighton pluton and the Sudbury Basin, and the intrusion mechanism of the Creighton pluton. The solution to the former problem can be stated in fairly simple terms, whereas the latter problem is considerably more complex. The author considers the two problems to be of nearly equal interest and importance. Much of this paper is concerned with structural analysis of the Creighton pluton and comparatively little to the relationships between the pluton and the breccia. This division reflects the relative complexity of the two topics, rather than their relative importance.

The Sudbury Basin and its surrounding region, one of the world's great mining areas, has been the subject of a vast amount of research, and the literature on the Sudbury region and the Southern Province is voluminous. The reader is referred to Thomson (1956), Card and others (1972), Card and Hutchinson (1972) and Brocoum and Dalziel (1974) for more details and further references.

Much of the Southern Province in Ontario, including the western half of the Creighton pluton (Card, 1968) has been mapped at a scale of two inches to the mile by geologists of the Ontario Ministry of Natural Resources. The eastern half of the Creighton pluton and much of the area to the east have been mapped at a scale of one inch to the mile (Collins and others, 1938; Cooke and Collins, 1947a; 1947b; Card, 1968) except for some areas which were mapped in more detail by Burrows and Rickaby (1934) and Phemister (1956). McKim Township, which lies just east of the Creighton granite and contains most of the Murray granite, is presently being mapped by D.G. Innes of the Ontario Ministry of Natural Resources. This paper contains the first detailed structural and geologic maps of the eastern half of the Creighton granite.

Because the Sudbury region has been so intensively studied, it is important to distinguish the author's contributions from those of other workers. The geologic and structural maps of the eastern portion of the Creighton pluton (Figs. 4,6,8) are the first detailed geologic maps of this area to be published, and are based entirely on previously unpublished data collected by the author. Card (1968a) suggested that the foliation within the Creighton pluton was formed during forceful intrusion of the pluton; however, the discussions within this paper of the structure and mechanism of emplacement of the Creighton pluton (pp.29-38) are based upon the author's interpretations and data of Popelar (1971, 1972). The discussion of the structural relationships between the nickel irruptive, the Creighton pluton, and the lower Huronian rocks (pp. 52-53) draws heavily on published literature, but the discussion of geometric constraints on models of the Sudbury Basin (pp. 52 ff.) is based wholly on ideas of the author which, as far as is known, have not been previously published elsewhere.

Geologic Setting

The Southern Province (Fig. 1) represents an exposed remnant of an early Proterozoic mobile belt, the Penokean Fold Belt, along the southern margin of the Archean craton of the Superior Province. The term "Penokean orogeny" has been the subject of some debate (Card and others, 1972). In this paper, the term "Penokean orogeny" will refer to deformation and metamorphic events which took place in the Southern Province between about 1900 and 1600 m.y. ago; this orogeny may have included several tectonic events. The Southern Province also includes late Proterozoic volcanic rocks of the Keeweenawan Series, which will not be discussed in this paper. The present limits of the Southern Province are largely defined by the outcrop of Archean rocks on the north and west, and the onlap of Paleozoic cover rocks to the south.

The Southern Province is one of several early Proterozoic mobile belts which nearly surround the craton of the Superior Province. The contrast between the Superior and Southern provinces exemplifies the contrast between Archean and later mobile belts in general. Archean mobile belts are dominated by socalled greenstone belts, which are synformal troughs tens of kilometers wide and hundreds of kilometers long, filled with thick accumulations of ultramafic, mafic and felsic volcanic rocks and clastic sedimentary rocks. Greenstone belts are commonly cuspate in form, and wrap around or have been intruded by elliptical, diapiric or dome-shaped granitic intrusive bodies (Anhaeusser and others, 1969; Salop and Scheinmann, 1969). In contrast, Proterozoic and later mobile belts are geosynclinal, and consist of shallow-water, shelf facies sedimentary assemblages along the margin of a craton and distal deep-water flysch and volcanic assemblages. These mobile belts are generally hundreds of kilometers wide and thousands of kilometers long. The numerous types of geosynclinal mobile belts can be fairly well explained in plate-tectonic terms (Dewey and Bird, 1970). The change in tectonic style at the end of the Archean may reflect a change from an early, highly unstable crust to a large-scale plate-tectonic regime. The early Proterozoic mobile belts of the Canadian Shield seem to be the earliest examples of orogens that are directly comparable to mobile belts of Mesozoic and Cenozoic age, and thus these early mobile belts are of special interest in studying the early history of plate tectonics.

Precambrian radiometric age provinces beneath the Paleozoic cover rocks of the United States (Goldich and others, 1966a, 1966b; Lidiak and others, 1966; Muehlberger and others, 1966) show a general pattern of elongate, northeast trending belts which decrease in age southward and eastward from the margin of the Superior Province craton. It appears likely that sialic crustal material has been accreted to the southeastern margin of the North American craton during successive orogenic events since the early Proterozoic, and that the Penokean orogeny represents the earliest accretionary event.

The Penokean Fold Belt (Fig. 1) is made up of lower and middle Proterozoic supracrustal rocks which unconformably overlie sialic Archean basement rocks. The lower Proterozoic rocks include the Huronian Supergroup in eastern Ontario and the basal portions of the Marquette Range Supergroup in northern Michigan and the Animikie Series in Minnesota and western Ontario. The Huronian rocks appear to be the oldest of the lower Proterozoic rocks; the upper Huronian has been correlated with rocks in Michigan and Minnesota, but more recent data indicate that the Huronian rocks could be up to 200 m. y. older that the lower Proterozoic rocks elsewhere in the Southern Province (Hanson and Malhotra, 1971; Morey, 1973; Van Schmus, 1976).

With the exception of volcanic rocks at the base of the Huronian Supergroup and minor volcaniclastic rocks in Minnesota, the lower Proterozoic supracrustal rocks of the Southern Province predominantly consist of shallow-water, stable-shelf sedimentary rocks, made up mostly of clastic detritus derived from the craton with some chemically-precipitated carbonate and ironstone. The Huronian rocks thicken abruptly south of a line roughly coincident with present Archean outcrop limits, which suggests that the basin in which they were deposited was steep sided and probably fault-bounded. The considerable thickness of the Huronian rocks and the lack of rocks of similar age elsewhere in the Southern Province also indicate that the Huronian rocks were preserved from erosion within a structural depression. Except for the syndepositional downfaulting of the Huronian depositional basin, the Southern Province appears to have been a tectonically stable, shallowwater shelf environment throughout most of early Proterozoic time. The downfaulting of the Huronian depositional basin might have occurred during the development of a rifted continental margin.

During the middle Proterozoic, rapid subsidence took place in the southern part of the Southern Province, and deep-water sediments of the Rove and Thomson formations were deposited in central Minnesota (Morey, 1973), while thick basaltic and deep-water sedimentary rocks of the Baraga and Paint River Groups were deposited in northern Michigan (Cannon and Gair, 1970; Cannon, 1973). The change from shallow-water shelf sedimentation to a deep-water volcanic-arc environment (James, 1954; Van Schmus, 1976) may mark the onset of subduction and reflect the change from a stable, "Atlantic" continental margin to a consuming "Andean"margin, which was followed by deformation, metamorphism and intrusive activity during the Penokean orogeny.

The Penokean orogen, as presently preserved, contains several easterly-trending tectonic elements (Fig. 1). The northernmost element, a nonmetamorphosed, nearly nondeformed foreland, is made up of flat-lying Huronian rocks of the Cobalt Plain in eastern Ontario, slightly folded Huronian rocks at the western end of Lake Huron, and the Port Arthur Homocline at the western end of Lake Superior. South of the foreland, a metamorphosed fold belt includes folded rocks of the Huronian Supergroup in the Sudbury region, the Marquette Range Supergroup in northern Michigan and the Animikie Series in Minnesota. The metamorphic grades within the fold belt display a nodal pattern (Card and others, 1972; James, 1955), with amphibolite facies nodes in a predominately greenschist facies terrain. Small granitic plutons intrude rocks of the southern part of the fold belt in eastern Ontario, Michigan, northern Wisconsin, and Minnesota. In Ontario, small Penokean plutons occur, in reworked form, within the Grenville Province (Lumbers, 1975). Finally, a granitic batholith terrain covers much of central Wisconsin and Minnesota (Van Schmus and others, 1975).

The geology of the Southern Province is comparable to many other mobile belts and suggests that the exposed portion of the Southern Province represents the foreland or cratonic side of a continental margin mobile belt. A plate-tectonic interpretation in terms of an east-west-trending consuming plate margin somewhere to the south of present Proterozoic outcrop limits appears plausible, though alternative interpretations are possible.

The age and number of tectonic events in the Southern Province are uncertain. Van Schmus (1976) reports that pre-Marquette Range Supergroup basement in Michigan was deformed, metamorphosed and intruded by granitic rocks about 2100 m.y. ago, and suggested than an orogenic episode occurred between the deposition of the Huronian Supergroup and the deposition of lower Proterozoic rocks in Michigan and Minnesota. This event is not reflected in the sedimentary lithology of either the Huronian or the lower Marquette Range rocks. However, most traces of the proposed 2100 m.y. old orogenic event could have been obliterated during the interval, nearly 200 m.y. in length, which intervened between the 2100 m.y. old event and the onset of the Penokean orogeny.

Metamorphism, intrusive activity and vulcanism occurred in Minnesota, Michigan, and Wisconsin about 1900 m.y. ago (Goldich, 1968; Van Schmus and Woolsey, 1975 Van Schmus and others, 1975; Van Schmus, 1976), followed by thermal overprinting 1600-1700 m.y. ago. The main deformation and metamorphism in the Huronian rocks postdate the formation of the Sudbury Basin and yield radiometric ages of about 1700 m.y. Pre-main phase deformation events and a thermal event 1950 m.y. ago have been proposed (Table I). On the other hand, Brocoum and Dalziel (1974) suggest that only a single deformation event occurred in the Sudbury area. It is possible that the 1700 m.y. ages in the Sudbury region represent the time of final closure of isotopic systems, that the actual age of the deformation and metamorphism is older, and possibly was penecontemporaneous with the 1900 m.y. old events elsewhere in the Southern Province. The Huronian Supergroup consists of minor amounts of volcanic rocks overlain by thick cyclic successions of argillite, quartzite and conglomer ate which may represent glacio-marine sedimentary cycles (Coleman, 1907; Young, 1970). The Huronian rocks are intruded by sills of Nipissing diabase (Table I), whose age of 2150 m.y. places a lower limit on the age of Huronian deposition. This paper is principally concerned with the rocks of the Elliot Lake Group (Table II), which make up the lowest portion of the Huronian Supergroup. In the SUdbury area the lowest unit of the Elliot Lake Group is a thick succession of mafic volcanic and intercalated psammitic rocks sometimes known as the Stobie Formation (Cooke, 1946), which consists predominantly of dark green to black, commonly pillowed mafic metavolcanic rocks. West of Sudbury thin interbeds of psammitic rock snake up a minor portion of the Stobie Formation, but east of Sudbury about half of the formation is made up of metasedimentary rocks. The Copper Cliff Formation crops out south of and overlies the Stobie Formation, and consists of pinkish felsic pyroclastic rocks, rhyolite flows and possibly rhyolite intrusives. The Copper Cliff Formation is thickest near Sudbury, and thins and disappears to the east and west. The uppermost unit of the Elliot Lake Group is the McKim For mat.ionr whi.eh.. typically consists of alternating gray pelitic and psammitic interbeds a few centimeters thick. To the west, the McKim Formation intertongues with the quartzitic Matinenda Formation. The stratigraphy of the Huronian above the McKim Formation is beyond the scope of this paper. A summary is given in Table II; detailed accounts may be found in Frarey and Roscoe (1970) and Card and others (1972).

Table II. Stratigraphy of the Huronian Supergroup

GROUP NAME FORMATION NAME DOMINANT LITHOLOGY APPROXIMATE THICKNESS
COBALT GROUP Bar River Orthoquartzite 1000 m
Gordon Lake Siltstone 500 m
Lorrain Sandstone 2000 m
Gowganda Conglomerate (Tillite) Argillite 2500 m
QUIRKE LAKE GROUP Serpent Sandstone 600 m
Espanola Calcareous Argillite 400 m
Bruce Conglomerate 200 m
HOUGH LAKE GROUP Mississagi Sandstone 1500 m
Pecors Argillite 500 m
Ramsay Lake Conglomerate 200 m
ELLIOT LAKE GROUP McKim Argillite , Greywacke 1000 m
Matinenda Sandstone 1000 m
Copper Cliff Rhyolite 500 m
Stobie Basalt, Greywacke 2000 m

Modified from Card and others (1972), Frarey and Roscoe (1970), and Young (1968).

The Huronian rocks which crop out south of the Stobie Formation dip steeply southward and have relatively narrow outcrop bands. By contrast, the Mississagi Formation crops out over a wide area south of the steeply dipping lower Huronian rocks. About 20 kilometers south of the Sudbury Basin, a large inlier of lower Huronian rocks is exposed beneath the Mississagi Formation (Frarey and Cannon, 1968; Card and others, 1971). Therefore, the steeply dipping lower Huronian rocks must flatten out at a fairly shallow depth (no greater that the thickness of the Mississagi Formation, or 2 to 3 kilometers) to form a subhorizontal structural terrace (Fig. 2). It is worth speculating whether the upturn of the Huronian rocks near the Sudbury Basin represents part of the upturned rim or central uplift of an impact crater; this problem will be discussed in more detail in a later section (pp. 60-61) of this paper.

Figure 2 (A-C). Restored north-south sections through the Sudbury Basin and vicinity. Line of section is approximately along line X-Y in Figure 1.

In addition to the Stobie and Copper Cliff formations, volcanic rocks of generally mafic composition occur at the base of the Huronian Supergroup near Thessalon (Fig. 1), in Baldwin Township (Thomson, 1952) and at several other localities. The Baldwin and Thessalon volcanic rocks are clearly intercalated with lower Huronian metasedimentary rocks, and probably were erupted during the initial faulting of the Huronian depositional basin (Card and others, 1972). Most authors regard the Stobie and Copper Cliff formations as similar in origin to these other volcanic rocks; a Proterozoic radiometric age of 2350 m.y. has been obtained for the Copper Cliff Formation (Table I). On the basis of lithologic and structural similarities to the rocks of Archean greenstone belts, and the proximity of the Archean rocks of the Superior Province, other workers haveconsidered that part or all of the Elliot Lake Group in the Sudbury area is of Archean age (Collins, 1936a; Cooke, 1946). The age relationships of the Stobie Formation will be considered in a later section (pp.52ff) of this paper.

An elliptical intrusive ring, the so-called "nickel irruptive", surrounds the Sudbury Basin. The irruptive has been dated between about 1800 and 2000 m.y. old (Table I); radiometric ages of about 1700 m.y. have also been obtained (Fairbairn and others, 1968) but are probably Penokean metamorphic ages. The irruptive, which grades from norite on the outer contact to nearly granitic on the inner contact, is shaped in three dimensions like a northward-tipped funnel, and encloses a doubly-plunging synclinorium made up of the rocks of the Whitewater Group, which underlie the interior of the Sudbury Basin. The Whitewater Group includes, in ascending stratigraphic order, the Onaping Formation, interpreted as a welded tuff (Williams, 1956; Stevenson, 1972) or meteoritic fallback breccia (Peredery, 1972), the Onwatin Slate, and the Chelmsford Formation, a carbonaceous turbidite (Rousell, 1972). The Whitewater rocks have no known correlatives outside the Sudbury Basin. The Basin has been interpreted as a lopolith (Wilson, 1956), volcanic collapse structure (Williams, 1956), and more recently as a meteor-impact structure (Dietz, 1964). The presence of shatter cones (Dietz and Butler, 1964; Guy-Bray, 1966) and the occurrence of various indicators of shock metamorphism in the rocks around the Basin and in the Onaping Formation (French, 1967, 1968, 1972) lend strong support to the impact hypothesis. On all but its southern side, the outer contact of the nickel irruptive is an intrusive contact with the Archean granitic rocks of the Superior Province. Along its southern side the nickel irruptive intrudes the Stobie volcanic rocks and the Creighton and Murray plutons. Large dikes, sometimes referred to as "offsets", radiate outward from the nickel irruptive into the Archean rocks and stratigraphically as far upward into the Huronian Supergroup as the Mississagi Formation. Along its inner contact, the nickel irruptive intrudes the base of the Onaping Formation (Thomson, 1956).

Vein breccia, the so-called Sudbury breccia (Speers, 1957; Plates 1-3), intrudes the Creighton and Murray plutons, the Nipissing diabase, and all older rocks within about 50 kilometers of the Sudbury Basin, but not the nickel irruptive or the rocks of the Whitewater Group. The nickel irruptive postdates both brecciation (Speers, 1957) and the deposition of at least part of the Whitewater Group and may be a product of impact-triggered magmatic activity (French, 1970, 1972). The relationships between the breccia, the Nipissing diabase, and the irruptive bracket the time of brecciation between 1800 and 2150 m.y. ago (Table I). The Sudbury breccia consists of veins or irregular patches in which disoriented blocks of country rock are enclosed in a matrix of comminuted rock fragments. The composition of the matrix is essentially the same as that of the enclosing host rock (Speers, 1957). Breccia veins range from thin seams to bodies hundreds of meters wide and several kilometers long, and the blocks within the breccia range in size from millimeters to tens of meters. Some breccia veins were emplaced along lithologic contacts. The breccia has been considered as volcanic or diatreme breccia (Fairbairn and Robson, 1942; Speers, 1957) and meteorimpact breccia (Dietz, 1964).

Plate 1. Typical appearance of Sudbury breccia; at a locality 5 kilometers west-northwest of Whitefish and 2.5 kilometers north of Highway 17. The dark, uniform-appearing blocks within the breccia consist of Nipissing diabase, and the light, banded blocks consist of Matinenda quartzite. The length of the largest block is about 50 centimeters.
Plate 2. Sudbury breccia within the McKim Formation. The outcrop is located within the core of the LorneLouise anticline, a large fold of Penokean age. The matrix of the breccia and the blocks within the breccia are penetrated by a strong Penokean foliation. This outcrop is located 8 kilometers west of Whitefish and 0.5 kilometers south of Highway 17. The box measures about 10 by 15 centimeters.
Plate 3. Sudbury breccia within the Creighton pluton. Strong pre-brecciation foliation is disoriented in the blocks within the breccia. Photo location 4 kilometers northeast of Creighton.

Some authors (Thomson, 1952; Speers, 1957) have held that the Sudbury breccia formed over an appreciable span of time or in several separate events, whereas the impact hypothesis requires a single, brief brecciation event. The relationship between brecciation and deformation in and near the Creighton pluton is anomalous; nowhere else within the Southern Province was brecciation preceded by penetrative deformation. With only local exceptions, foliation occurs throughout the Creighton pluton, and everywhere predates the breccia. Possible evidence of more than one brecciation event, such as intersecting breccia veins of different ages or blocks of early breccia within later breccia, has not been observed, and no breccia is present within the nickel irruptive immediately adjacent to the Creighton pluton. It will be shown later that the pre-brecciation deformation was the result of forceful intrusion of the Creighton pluton prior to the intrusion of the nickel irruptive. Therefore, the field evidence is consistent with the idea that a single brecciation event preceded the intrusion of the nickel irruptive.

The Huronian rocks and the rocks of the Sudbury Basin were deformed and metamorphosed during the Penokean orogeny, about 1700 m.y. ago (Fairbairn, 1960, 1965; Van Schmus, 1965; Fairbairn and others, 1969). Tight concentric folds and steeply dipping foliation formed in the Huronian rocks, where metamorphism of up to almandine-amphibolite grade occurred (Card, 1964). The Whitewater rocks were gently folded and foliated, and the Sudbury Basin may have been flattened from a more nearly circular original form to its present elliptical shape (Brocoum and Dalziel, 1974). The last event of regional extent in the Sudbury region was the intrusion of northwest trending olivine diabas dikes. The nondeformed and nonmetamorphosed rocks from this dike swarm yield radiometric dates of 1200 to 1600 m.y. (Gates, 1971, 1972; Gates and Hurley, 1973).


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