Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
The Creighton pluton (Fig. 3) is a crudely tabular intrusive about 21 kilometers long and 6 kilometers wide, situated west of the city of Sudbury. Within the pluton are two large bodies of country rock, each about 5 kilometers long and 3 kilometers wide, and several smaller bodies. About 3 kilometers northeast of the Creighton pluton is a smaller intrusive, the Murray pluton. Both plutons intrude vertically dipping rocks of the Stobie Formation. The contact between the Creighton pluton and the Stobie Formation is obscured over nearly its entire length by a band of Sudbury breccia from one to 50 meters wide. The age relationships between the plutons and the irruptive have been debated for a long time. Granitic dikes locally intrude the irruptive, but the Copper Cliff Offset cuts the eastern end of the Creighton pluton (Figs. 3, 6, 7). It is now generally thought that the Creighton and Murray plutons antedate the irruptive, and that the intrusion of the irruptive caused local melting of the granitic rocks and rheomorphic intrusion into the irruptive (Collins, 1936b; Card, 1968a). Fairbairn (1960, 1965) obtained radiometric ages of 2140 m.y. for both the Creighton and Murray intrusives, and Gibbins (1972) obtained an age of 2230 m.y. for the Murray pluton.
Granitic intrusives contemporaneous with the Creighton and Murray plutons may occur elsewhere in the Sudbury region. Thomson (1961) described granites intruded into volcanic rocks west of Lake Wanapitei, near the southeastern corner of the Sudbury Basin, and Fairbairn (1965) reported a radiometric age of 2160 m.y. for granitic rocks from west of the Sudbury Basin. The presence of early Proterozoic granitic intrusives within the generally granitic Archean terrain of the Superior Province would be difficult to discern except by radiometric methods.
The Creighton pluton is a composite intrusive which ranges in composition from granodiorite and quartz monzonite to true granite (Card, 1968; Table III). Quartz is the most abundant constituent, followed by oligoclase, perthitic microcline, and biotite. Minor constituents include epidote, hornblende, and zircon. Although structural and isotopic evidence, to be discussed later, indicate that the Creighton pluton may have originated by remobilization of sialic crust, muscovite and garnet, two accessory minerals commonly found in such remobilized :granitic intrusive bodies, have not_been..found.in the rocks of the Creighton pluton. The oligoclase within the rocks of the Creighton pluton displays very narrow lamellae twinning, or is often not twinned, and is commonly highly poikilitic, with abundant euhedral epidote inclusions. Quartz grains commonly display serrate boundaries and undulose extinction; one specimen contains quartz with a markedly biaxial interference figure (2V = 20�). Large mineral grains have been recrystallized to domains of smaller grains. Consequently, the strong foliation defined in hand specimen by the parallel alignment of platy quartz and feldspar grains is not evident in thin section. Irregular patches of biotite define the only fabric visible in thin section; the individual mica plates are not aligned.
|Table III. Modal analyses of granitic rocks from the Creighton and Murray plutons. The analyses were obtained by the point-count method; 500 points were counted for each analysis. Staining was employed to distinguish the feldspars, but some nontwinned plagioclase may have been counted as quartz. Some of the high variability in the quartz contents of the analyzed rocks may be due to such errors, although such a large number of rock types occur within the Creighton pluton that much of the variability in quartz content must reflect actual differences in composition. |
ACC includes principally zircon, sphene, and opaque minerals. In specimen 908470, ACC also includes 5% muscovite. ALT includes principally chlorite and epidote. * denotes less than 1%. Specimen numbers are grid references referred to N.T.S. (Canada) topographic maps.
|TEXTURAL DESCRIPTION||SPECIMEN NUMBER||CONSTITUENT MINERALS (VOL %)||LOCATION|
|Coarse, pink, porphyritic||885433||48||27||12||10||3||2 km N of Lively|
|Coarse, gray, porphyritic||779425||39||37||5||12||1||6||Vermilion R. area|
|877463||35||25||21||11||4||1||3||3 km NE of Creigh ton|
|Coarse, pink, nonporphyritic||776419||38||29||32||1||*||Vermilion R. area|
|784424||24||51||20||2||3||Vermilion R. area|
|Medium-grained, dark pink, nonporphyritic||869450||35||27||18||13||*||7||1 km SE of Creighton|
|Medium-grained, light pink, nonporphyritic||939503||32||37||18||11||1||1||2 km N of Copper Cliff|
|948472||46||30||20||3||1||1 km SE of Pump L.|
|942476||32||37||27||2||1||1||1 km SE of Pump L.|
|Gray aplite||908470||62||23||5||4||6||Eastern Inclusion|
|Medium-grained, light pink, nonporphyritic||969508||35||42||19||2||2||SW end of pluton|
|004549||37||38||21||2||1||1||NE end of pluton|
A number of distinctive rock types occurs within the Creighton pluton. The contacts between individual granitic rock types are never sharp, but are gradational through a few meters. There does not appear to be any systematic relationship between the texture or color of a rock type and its composition (Table III). Most of the pluton is made up of pink to gray granitic porphyry, which consists of large (1-2 centimeters) microcline phenocrysts in a mediumgrained (2-4 millimeters) granitic groundmass. In inclusion contains several varieties of granitic intrusive rocks. Aplite is common within the eastern inclusion and along the southern contact of the Creighton pluton south of the eastern inclusion. Small pods of pegmatite also occur in places within the eastern inclusion. Lit-par-lit injection is widespread in the metavolcanic and metasedimentary rocks of the eastern inclusion and in the granitic rocks in and around the eastern inclusion (Fig. 4). Within the granitic rocks, narrow, subparallel veins of fine-grained quartzo-feldspathic aplite intrude the somewhat less silicic granitic host rock. Sometimes the intrusive veins are highly irregular, and the injected rocks grade into intrusion breccia. Some areas in which several varieties of granitic rock occur on a scale too small to be mapped individually have also been portrayed on the map (Fig. 4) with the same symbol as the lit-par-lit injected rocks. Within the metavolcanic rocks of the eastern inclusion, wellfoliated actinolite greenstone is intruded by granitic veins subparallel to the foliation. The granitic veins are usually about a centimeter wide, and typically make up 10 to 25 per cent of the rock.
Figure 4. Geology of the eastern half of the Creighton pluton.
The pervasive lit-par-lit injection in and near the eastern inclusion suggests that contamination of the granitic magma of the Creighton pluton by material from the eastern inclusion may have taken place. The dark pink nonporphyritic granite east of Creighton (Fig. 4) which is similar to the host material of the lit-par-lit injected granitic rocks, may owe its high content of biotite to such contamination. The Bouguer anomaly map of the Sudbury area (Fig. 5) corresponds closely to outcrop patterns. Mafic metavolcanic rocks of the Stobie Formation south of-the Creighton pluton and between the Creighton and Murray plutons are marked by positive anomalies. Negative anomalies over portions of the Creighton and Murray plutons can be interpreted as areas of deep granitic rocks. Popelar (1972) estimated depths of 4 kilometers of granitic rock beneath the low in the Vermilion River area, 2 kilometers in the eastern part of the Creighton pluton, and a maximum of 2.5 kilometers in the northeastern portion of the Murray pluton, and suggested that these areas represented centers of intrusive activity.
Figure 5 Gravity map of the Creighton and Murray plutons, based on data of Popelar (1971, 1972). Interpretation is that of the author.
Gravity and structural data indicate that the two large bodies of country rock within the Creighton pluton have radically different configurations. The more westerly of the two bodies (the "western inclusion") is made up of massive, weakly foliated or non P, foliated mafic metavolcanic rocks. which have been intruded by small, structurally simple granitic intrusives such as dikes, irregular granitic bodies, and intrusion breccias. A gravity high over the western inclusion is connected with the Stobie Formation by a positive anomaly "bridge" (Figs. 3, 5). It seems likely that the western inclusion is in situ rock of the Stobie Formation exposed through the floor of the pluton, and that the granitic rocks to the south form a thin roof above a basement ridge of Stobie rocks. The complex configuration of the floor of the pluton is not reflected at the surface in the petrography or texture of the granitic rocks or the attitude of the foliation. Several small inclusions which lie near the large western inclusion (Card, 1968, 1969) are not associated with any notable gravity anomalies, and are most likely small floating inclusions or minor protrusions from the basement ridge beneath the pluton.
The eastern inclusion is made up largely of complexly deformed and intruded metasedimentary and metavolcanic rocks. Except for a weak gravity high near the southwestern corner of the inclusion, the inclusion is unmarked by any gravity anomaly. This weak gravity signature may in part reflect the small density contrast between the inclusion and the surrounding granitic rocks, and may also mean that the inclusion is floating, and not connected with basement rocks beneath the pluton.
In the western half of the pluton, Se forms a broad northerly closure and swings from northeast to southeast in trend (Fig. 3). In the eastern half of the pluton, Se forms a complete elliptical loop, in the center of which is the eastern inclusion (Figs. 3, 6). In addition to Se, other planar structures, such as lit-par-lit injection veins or bedding and foliation within inclusions are also concordant with the curvature of the foliation loop. Bedding around the periphery of the eastern inclusion is generally concordant with Se in the granitic rocks. Se is generally parallel to the outer contact of the pluton; the eastern closure of the foliation loop follows the eastern contact of the pluton, and foliation appears to close around the western end of the pluton as well (Fig. 3). At the eastern end of the Creighton pluton, Se in both the pluton and the Stobie Formation is approximately vertical and parallel to the contact. The Copper Cliff Offset follows the eastern contact of the pluton for 2 kilometers, then crosses the pluton as a narrow, straight dike which clearly transects the closure at the eastern end of the foliation loop (Fig. 7). Thus the present form of the foliation loop clearly antedates the intrusion of the nickel irruptive.
Figure 6. Structure of the eastern half of the Creighton pluton. Structural data collected by author. Data on this map represents selected data from base maps at a scale of 1:7,290, copies of which are included in the Appendix.
Figure 7. Copper Cliff Offset transecting closure in Se at the eastern end of the Creighton pluton, about 1 km north of Copper Cliff.
A possible explanation for the structures within the Creighton pluton is pre-brecciation regional deformation. Such deformation, which could account for many of the features of the Creighton pluton, should also have resulted in widespread pre-brecciation deformation in rocks well removed from the contact zone of the Creighton pluton, and especially in the rocks in and near the Murray pluton, which is similar in age to the Creighton pluton. Instead, except for isolated examples, Se is entirely restricted to the Creighton pluton and the immediately adjacent rocks of the Stobie Formation. In addition, the completely closed form of the foliation loop is very difficult to explain satisfactorily in terms of a regional deformation.
The elliptical foliation loop indicates that the direction of maximum finite pre-brecciation flattening was radial about the eastern half of the Creighton pluton, and that the structure of the Creighton pluton is the result of forceful intrusion of the plu ton into a constricting collar of country rocks. This forceful intrusion also foliated the rocks of the Stobie Formation adjacent to the pluton. At the eastern end of the Creighton pluton the general northeasterly strike of bedding and foliation in the Stobie Formation is deflected to north-south along the contact (Fig. 6). West of the foliation loop, the deflection of Se from northeast to southeast in trend may also be an effect of forceful intrusion.
The structures within the Creighton pluton are similar to those found in forcefully emplaced plutons in many areas (Akaad, 1956; Martin, 1953; Reesor, 1954; Gastil and others, 1975). In such plutons, planar structures parallel the contact, and the intensity of deformation decreases away from the contact. Structures formed in the host rocks include flattening foliation, crenulation cleavage, and folds. Planar structures within the pluton include flow layering and foliation defined by platy mineral grains or flattened xenoliths. Often the attitudes of bedding or foliation in the host rocks are deflected near the contact of the intrusive. Joint sets systematically oriented with respect to the contact are sometimes noted; radial and tangential joints are most common. In general, the deformation structures associated with forceful emplacement reflect maximum finite shortening directed radially about the intrusive. Plutons with these structures range from simple, drop-shaped bodies obviously of diapiric origin to more complex intrusives in which the effects of several pulses of intrusion are apparent.
Field evidence indicates that the Creighton pluton was intruded in a highly consolidated state and that the Se fabric in the granitic rocks formed by flattening after partial or complete solidification of the intrusive, not as a result of magmatic fluid flow. The foliation clearly postdates the crystallization of the large microcline phenocrysts, because the phenocrysts are often oriented at high angles to the foliation and are enveloped by it augen-fashion (Plate 4). In addition compositional layering is sometimes crossed by the foliation, usually at low angles.
Plate 4. Coarse porphyritic quartz monzonite from the Creighton pluton. The large light phenocrysts are microcline crystals up to 3 centimeters long. The light portions of the groundmass consist of feldspar; dark portions consist of gray quartz and biotite. Note that some phenocrysts are oriented at various angles to the foliation, and that many phenocrysts are enveloped, augen fashion, by the foliation. Photo location 5 kilometers west of Creighton.
The foliation loop in the eastern half of the Creighton pluton is superimposed on a lithologically heterogeneous suite of granitic and inclusion rocks without regard to lithologic boundaries (Figs. 4,-6). Contacts are crossed at all angles by the foliation, which clearly postdates all intrusive events within the pluton. Another intrusive with relationships be tween intrusive and deformation events similar to those of the Creighton pluton is the White Creek Batholith of British Columbia (Reesor, 1954). These relationships indicate a complex history of intrusive events, not forceful in nature, followed by forceful emplacement of the entire intrusive complex as a single unit after considerable (but not total) solidification had taken place. Thus, the Creighton pluton appears to have been emplaced by a mechanism similar to the diapiric intrusion mechanism of Ramberg (1967).
The Creighton pluton contrasts strongly with the nearby Murray pluton, which is a homogeneous intrusive that consists of weakly foliated equigranular granite (Table III). The nearness and similarity in age of the Creighton and Murray plutons, and the textural and petrographic similarity between the rocks of the Murray pluton and some of the rock types at the eastern end of the Creighton pluton argue strongly that the two plutons are or were connected, even though they are separated at their present level of exposure. There does not appear to be any connection between the Creighton and Murray plutons at shallow levels, because a pronounced gravity high over metabasalts of the Stobie Formation between the two intrusives (Popelar, 1971, 1972; Fig. :5) indicates a considerable thickness of dense, mafic rock in that area.
A regional gravity high south and west. of the Sudbury Basin indicates that mafic rock underlies the northern margin of the Southern Province in the Sudbury area. Popelar (1972) estimated that mafic volcanic rocks south of the Sudbury Basin extend to a depth of 6 kilometers. The Creighton and Murray plutons could be cupolas extending from a large felsic body below this depth. Alternatively, a former connection between the two plutons may once have existed above the present level of erosion. The rocks between the two plutons might represent an unroofed basement ridge similar to the ridge inferred to exist beneath the Creighton pluton south of the western inclusion.(Fig. 5).
Apart from foliation in the matrix of the Sudbury breccia, there is almost no evidence of postbrecciation deformation within the Creighton pluton. To some extent Penokean deformation may have been masked by the coarse texture of the granitic rocks and the strong Se foliation. A few examples of folding of Se occur, but in general the absence of postDe structures in the pluton and its host rocks is striking. Foliation is common in the matrix of the breccia, and generally strikes northeast, but with considerable variation in attitude. In small breccia veins the foliation is nearly parallel to the vein walls. S foliation in the blocks within the breccia is e nearly randomly oriented, with a very weak concentration around the attitude of Se in the nearby unbrecciated rock. Foliation in the breccia matrix shows a similar distribution. Several possible mechanisms could have produced the foliation within the breccia. The frequent occurrence of northeasterly striking foliation within the breccia suggests that much of the post-brecciation foliation is of Penokean age. Residual De stresses within the pluton and processes related to the brecciation might also have formed some foliation. The variable attitude of the foliation may have resulted from differences in the rheologic properties of the matrix material, clasts, and host rock, which resulted in local inhomogeneous deformation. In general it is not possible to ascertain which mechanism produced the foliation at any given location.
A northwest-directed Penokean flattening deformation, such as proposed by Brocoum and Dalziel (1974), might also have flattened the foliation loop within the Creighton pluton. The present ratio of the major to the minor axis of the foliation loop is approximately 2.5 to one (Fig. 6). A northwesterly directed flattening of 30%, as suggested by Brocoum and Dalziel, would imply an initial axial ratio of 1.75 to one for the foliation loop. Thus, even allowing for possible deformation effects of the Penokean orogeny, the foliation loop must still have had a pronounced elliptical original form.
The eastern inclusion is about 5 kilometers long and 3 kilometers wide, and is made up of steeply to vertically dipping mafic metavolcanic rocks, impure psammitic rocks and minor amounts of pelitic rocks. In all probability these rocks belong to the upper part of the Stobie Formation, which crops out just east of the Creighton pluton and is lithologically very similar to the rocks of the eastern inclusion.
A wide variety of complex intrusion structures, including intrusion breccia, irregular granitic intrusives, and lit-par-lit injection, occur within the eastern inclusion. Where lit-par-lit injection or foliated granitic rocks occur within bedded rocks, the injection veins or foliation are approximately subparallel to bedding. Plates 5 to 8 show typical structures within the eastern inclusion.
|Plate 5. Fine-grained gray granite showing folds in lit-par-lit injection banding. The outcrop is located in a large granitic pod within the eastern inclusion, 3 kilometers northeast of Creighton and one kilometer south of the northern contact of the inclusion.|
Plate 6. Sudbury breccia cutting an Fe fold in psammitic metasedimentary rocks within the eastern inclusion. This outcrop is located near the southern contact of the inclusion, about 4 kilometers east-northeast of Creighton.
Plate 7. Refolded fold in psammitic metasedimentary rocks in the hinge area of the central closure within the eastern inclusion. An Fel isoclinal fold is refolded by open Fe2 flexures. The width of the picture area is about two meters.
|Plate 8. Refolded fold from the eastern limb of the central closure within the eastern inclusion. The rock is gray, fine-grained granite intruded by lighter granitic veins. The granitic vein shown here has been isoclinally folded by an Fel fold and subsequently refolded by a Z-shaped Fe2 fold whose asymmetry is in agreement with the overall geometry of the large central closure.|
The metamorphic mineral assemblages in the rocks of the eastern inclusion consist of quartz, plagioclase, biotite, and sometimes actinolite or hornblende. Relict garnet is occasionally present, and possible staurolite is now represented by pseudomorphs of sericite. Epidote and chlorite are common. These assemblages seem more characteristic of middle greenschist- to lower amphibolite grade regional meta- morphism than contact metamorphism. In large part the mineral assemblages within the eastern pluton probably reflect Penokean metamorphism, although some of the staurolite pseudomorphs appear to be oriented parallel to the axial surfaces of Fe folds and thus may be related to De. It is also possible that the inclusion was metamorphosed prior to being incorporated in the Creighton pluton, although in view of the lithologic similarity of the rocks of the inclusion to upper Stobie Formation rocks exposed just a few kilometers away to the east of the pluton, there is no compelling reason to believe that the rocks of the inclusion are not identical with the upper Stobie Formation, with a similar metamorphic history. The problem of separating the effects of the different thermal events which have taken place in the Sudbury area will be covered in the' next section of this paper (pp. 46-47). Two phases of pre-brecciation deformation can be recognized within the eastern inclusion. Del is represented by steep to vertical foliation, steeply or vertically plunging tight to isoclinal minor folds (Plates 5, 7, 8), and lit-par-lit injection veins. De2 is represented by several large, steeply plunging folds and occasional small structures, including small folds whose axial surfaces are subparallel to those of the major Fe2 folds, folded foliations and occasional refolded Fel isoclines (Plates 7, 8). Pre-brecciation folds are not known from any of the rocks outside the Creighton pluton.
In addition to mappable Fe2 folds, inhomogeneous De2 deformation which can be accurately described as chaotic occurs in several areas. In these areas fold styles, vergences and bedding-cleavage relationships vary from outcrop to outcrop in no systematic way. Open flexures with wavelengths from 10 to 100 meters are the dominant major structures. Boudinage is common throughout the inclusion and is especially widespread in the chaotic areas. The eastern inclusion can be divided, from west to east, into three sectors (Figs. 6, 8). In the western and eastern sectors Del is the dominant deformation phase, whereas the central sector is a large Fe2 closure. Chaotic deformation areas mark the transition from the central sector to the sectors on either side.
Figure 8. Structural sketch map of the eastern inclusion within the Creighton nluton.
The central sector is a large teardrop-shaped southerly closure, which can be clearly traced by by following a distinctive banded marker unit (Plate 9). Foliation in the marker and other units, and the axial planes of isoclinal Fel S-folds (Plates 7, 8) curve around the hinge of the closure, thus demonstrating its De2 age. Minor folds along the western limb of the closure are S-shaped, whereas on the eastern limb S- and Z-folds occur. The geometry of the minor folds appears to have resulted from overprinting of S-shaped Fel folds by S- and Z-shaped Fe2 minor folds related to the main closure.
Plate 9. The banded marker unit found in the central closure of the eastern inclusion. The bands average 5 centimeters in width. The dark bands are actinolite greenstone; the light bands are biotite-chlorite schist.
The central sector is flanked on both sides by areas of markedly different structural trend and style. To the west, northeast-trending structures of the central sector. No simple lithologic or structural break exists between the two areas. Instead, there is a structurally complex zone of small-scale folds and some faults (Figs. Northeast-trending Del structures predominate in the western sector (Fig. 8). The western end of the inclusion is a westerly Fel closure concordant with the closure in Se in the surrounding granitic rocks. The northern limb of the closure within the inclusion appears to be continuous along the northern side of the western sector, but the structure of the structure of the southern limb is complicated by De2 folding. The axial trace of the closure can be followed about a kilometer eastward from the contact of the inclusion before apparently being truncated, possibly by a fault. There is no evidence for a major closure in the rocks in the eastern half of the western sector.
The transition between the central and eastern sectors is a chaotically deformed and extensively intruded area where structural trends appear to fan from north-south along the eastern limb of the central closure to nearly east-west in the eastern sector (Fig. 8). The eastern sector is an easterlytrending terrain which was dominately affected by Del. Bedding and foliation along the northern contact of the eastern sector are subparallel to the foliation in the granitic rocks. Minor folds and bedding-cleavage relationships are predominantly in the Z-sense in this area, but numerous S-shaped folds occur farther south. It is not clear whether these reversals represent Fel fold axes or merely reflect inhomogeneous deformation of a homoclinal sequence of rocks in which no large folds formed.
The chaotic areas which flank the central closure pose a problem in correlating the rocks of one sector with those of another. A distinctive light green cloritic marker, probably a metatuff, occurs at the southern edge of the eastern sector, as discontinuous bodies in the chaotic area east of the central closure, and in several areas in the western sector (Fig. 8). The occurrence of this marker, and the S-shaped Fel folds in both the western and central sectors, suggest that the central sector was once structurally continuous with the sectors on either side. After the initial folding of the central closure, continued deformation tightened the closure into its present isoclinal form, while the rocks on either side of the closure were crushed against the rocks of the eastern and western sectors to form the chaotic areas.
Several faults cut the eastern inclusion, including a family of faults which trend slightly south of east (Fig. 4). One of these faults dextrally offsets the banded marker on the western side of the central closure. The faults truncate Fe2 closures but probably antedate brecciation. None of the faults offset the outer contact of the pluton; they probably do not extend far into the granitic rocks, and are related to the final stages of deformation of the inclusion. The eastern inclusion is the most distinctive and unusual feature of the Creighton pluton. The occurrence of lit-par-lit injected rocks and two generations of pre-brecciation structures within the inclusion, and the total lack of comparable structures in the rocks outside the Creighton pluton, indicate that the deformation of the inclusion was related to forceful emplacement of the pluton. Therefore the granitic rocks must have been sufficiently rigid to transmit stresses capable of deforming the rocks of the inclusion. The location of the inclusion in the center of the foliation loop suggests constriction as the dominant deformation mechanism for the inclusion. The outcrop pattern in the eastern end of the Creighton pluton invites comparison with that of a ring dike, though the forceful emplacement of the Creighton pluton contrasts markedly with the style of emplacement, generally not associated with deformation, which characterizes most ring dikes. It is conceivable that the eastern inclusion is a downdropped roof block, surrounded by a ring dike, but the available evidence is insufficient to prove or disprove this hypothesis.
Created 4 August 2004, Last Update 11 January 2020
Not an official UW-Green Bay Site