Penokean Deformation and Associated Metamorphism in the Western Marquette Range, Northern Michigan

Penokean deformation and associated metamorphism in the western Marquette Range, northern Michigan

JOHN S. KLASNER Department of Geology, Western Illinois University, Macomb, Illinois 61455

ABSTRACT passive deformation of overlying middle the stratigraphie column (Fig. 2), mainly in Precambrian sedimentary rocks. Cannon the slates and graywackes of the Geologic studies of Precambrian rocks in also indicated that the middle Precambrian Michigamme Formation. The Michigamme northern Michigan indicate that the Peno- sedimentary rocks have undergone sub- Formation underlies more than 90% of the kean orogeny, which occurred about 1.9 stantial horizontal shortening as shown by area occupied by middle Precambrian rocks b.y. ago, consisted of four stages of defor- regional parallelism of smaller second-order and is best exposed at the Taylor mine (Fig. mation, three of which occurred during a structures. 3) and at Lake Michigamme (Fig. 4). Also, prolonged period of regional metamor- James (1955) outlined four nodes of re- Lake Michigamme lies within the staurolite phism. The first two stages of deformation gional metamorphism in northern Michi- zone and the Taylor mine lies within the were possibly caused by gravity sliding gan and northern Wisconsin. James indi- chlorite zone of James's (1955) Republic northward off an ancestral Penokean range cated that this metamorphism generally oc- node of regional metamorphism. For these located in central Wisconsin. The deforma- curred after deformation. Powell (1969, reasons, these two areas were studied in tion probably started while the sediments 1970, 1972) studied the time relationship greatest detail. were still soft, and it produced a pervasive between deformation and metamorphism Some previous stratigraphie studies have west-northwest-trending foliation in the and concluded that, in the Marquette stressed evidence for pre-Goodrich tec- middle Precambrian rocks. The third and Range area, metamorphism occurred after tonism (Boyum, 1964; Nordeng and fourth stages of deformation were caused the main deformational event and that it Spiroff, 1962; Tyler and Twenhofel, 1957), by uplift of rigid blocks of lower Precam- peaked just before a second but much less but although there are some data support- brian basement rocks; this uplift produced intense period of deformation. ing early slump folding within the Ne- prominent grabens such as the Marquette This study shows that, in the western gaunee Iron-Formation and older forma- and Republic troughs. Metamorphism Marquette Range area (Fig. 1), there were tions, the extent and intensity of the defor- began very early in the deformational se- at least three and probably four stages of mation are problematic. The present study quence, peaked during the third stage of de- deformation and that growth of metamor- involves mainly those rocks that are strati- formation, and ended in a period of ret- phic minerals began early in the deforma- graphically younger than the Goodrich rograde metamorphism. tional cycle and culminated at about the Quartzite; therefore, early folding that may time of uplift of fault-bounded basement have occurred is not an important consid- INTRODUCTION blocks. eration. The western Marquette Range area (Fig. Geologic studies together with radiomet- 1) is centered where the western ends of the STRUCTURE ric dating indicate that a major tecton- Marquette and Republic troughs open into othermal event, the Penokean orogeny, af- a rather broad sedimentary basin. The gen- Table 1 briefly summarizes the nature of fected lower and middle Precambrian rocks eral distribution of rock types and contacts the four phases of structural deformation

in Minnesota, northern Wisconsin, and were originally mapped by Van Hise and (Fl5 F/, F2, F3) that occurred in the western northern Michigan about 1.9 b.y. ago Bayley (1897). They named the granitic Marquette Range during the Penokean

(Aldrich and others, 1965; Banks and Cain, basement terrane north of the Marquette orogeny. F,, F2, and F3 were recognized in 1969; Banks and Van Schmus, 1971; Stuck- trough the northern complex and that south the field during the initial period of regional less and Goldich, 1972; Van Schmus, 1972, of the Marquette trough the southern mapping. The F/ phase was established 1974; Van Schmus and others, 1975). Can- complex. The Republic trough lies within later through detailed mapping at both the non (1973) outlined the historical develop- the southern complex. Taylor mine and Lake Michigamme and ment of ideas along with some of his ideas The western Marquette Range area was through thin-section studies primarily un- on the structural evolution of northern chosen for study because it contains an ex- dertaken to establish time relationships be- Michigan during the Penokean orogeny. He cellent record of the structural and meta- tween growth of metamorphic minerals and interpreted the wide divergence in trends of morphic evolution that occurred during the phases of deformation. first-order basement structures, such as the Penokean orogeny. In particular, the struc- In the following paragraphs the evidence Marquette and Republic troughs, as having tural fabric and the overprinting of succes- for each of the four deformational events resulted from vertically uplifted, fault- sive structural features are recorded exceed- will be discussed in terms of regional dis- bounded blocks of lower Precambrian ingly well in the pelitic rocks of the area. tribution and characteristics of structural granitic complexes. This uplift produced These are more abundant toward the top of elements, geometry and style of folding, and

Geological Society of America Bulletin, v. 89, p. 711-722, 10 figs., 1 table, May 1978, Doc. no. 80508.

711

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/5/711/3418694/i0016-7606-89-5-711.pdf by guest on 27 September 2021 UPPER PRECAMBRIAN Figure 1. Regional map of western Marquette KEWEENAWAN Range showing structural geology and location of areas mapped in detail. Compiled from map- ROCKS Probably Includes ping by Klasner (1972) with data from Cannon Some Cambrian Rocks and Klasner (1972), Bodwell (1972), Cannon (1974), Simmons (1971), Boyum (1974), Taylor KEWEENAWAN DIKES (1967), S. C. Nordeng (1972, oral commun.), E3 and maps supplied by U.S. Steel Company. • MIDDLE PRECAMBRIAN BARAGA GROUP Meta- Sedimentary & Met a- Volcanic Rocks MENOMINEE GROUP Meta-Sedimentary Rocks LOWER PRECAMBRIAN TAYLOR MINE/ AREA NORTHERN COMPLEX CRYSTALLINE BASEMENT 4 f , V ROCKS Granite, Gneiss, , -, y |i /y j¿/J.'f-j^^^ SILVER LAKE Migmatite,& Pegmatite Trends of Structural Axes of Folds of Age

r"— ; Trends of Gneissic Foliation ^ ' i v ^ Isograds of Republic Node of Metamorphism

Quartziteat L^^ " Stereogram Projection of Poles to , Sturgeon i •* ' ^ Regional Foliation. Shows either 2? River plots of poles to foliation or contours of poles to foliation and orientation of regional foliation.

A-'L È* S |

fl" «/Ä 1 ! «« A « «*• H \ V 1 3 5 V, \\ Kilometers R33W R32W R31W I fr R29W R28W R27W

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possible cause of deformation. The time re- than 0.5 to 2 mm in length are formed in 2. Inasmuch as some of the quartz veins lationships between growth of porphy- black slate of the Michigamme Formation are isoclinally folded with S/ axial-plane roblasts and deformation are discussed later with their long axes parallel to S^ At the foliation, there must have been continued in this paper. Taylor mine, elongate masses of white shortening of the sedimentary section even quartz, about 100 m in length, are found in after the silica had been emplaced.

Fj-F/ Deformation the axial regions of Ft folds in cherty iron 3. Silica migration was accompanied by, formation (see Fig. 3). K. Spiroff (1962, un- and probably enhanced by, elevated tem- F, and F,' were probably caused by a pub. rept.) referred to these as quartz saddle peratures as shown by the presence of elon- single deformational event. F! denotes the reefs because of their similarity to saddle gate andalusite porphyroblasts parallel to initial phase of deformation; F,' is consid- reefs described by Stillwell (1918) and Hills S/ (see Fig. 9). This elongation must have

ered to be a renewed or continued pulse of (1963) from the Bendigo gold field, Austra- occurred before F2 deformation because S2 Fj, but differs from F, in the manner in lia. surfaces wrap around the porphyroblasts. which the deformation occurred, as ex- This migration of silica is interpreted to Thus the andalusite developed after F, and plained below. have occurred either during continued F, before F2. Regional Elements. In the western Mar- deformation, as a renewed pulse after Fj, or 4. The presence of elongate pods of quette Range, the middle Precambrian both; the renewed pulse is here termed F,'. quartz oriented with their long axes parallel rocks have a regional penetrative foliation Detailed analysis of the data indicates that to S/ and the presence of quartz veins (SO that trends N75°W and dips steeply the silica, which according to Van Schmus parallel to S/ suggest that silica was en- south. The orientation of the foliation is and Woolsey (1975) was probably derived trapped along S/ when the temperature consistent regardless of location relative to from the sediments, migrated along al- was lowered sufficiently for it to crystallize. basement. It is best developed in rocks of ready-existing Si and enhanced it, thus The general model proposed for F/ de- the Baraga Group, is less apparent in rocks forming a more pronounced foliation formation is similar to a model proposed by of the Menominee Group, and is absent in termed S/. The details of this interpretation Williams (1972) for the formation of slaty basement rocks. At both Lake Michigamme are as follows: cleavage through the migration of silica. In and the Taylor mine, S! is expressed by 1. Migration of the silica in such abun- his model, domains rich in layered-lattice parallelism of platy minerals such as chlo- dance probably would not occur during silicate minerals can be developed by selec- rite, biotite, and sericite. It is generally soft-sediment deformation (FJ. Thus, there tive removal of quartz from these domains. smooth except where it has been affected by must have been a later even^F/) that It is postulated that in the western Mar- later deformation. caused the silica to migrate. quette Range, relict connate water, which Grain size of the foliated rocks appears related to metamorphic grade. For example, SUPERGROUP , GROUP FORMATION, in the chlorite zone at the Taylor mine, the AND/OR ME MBER DESCRIPTION predominant rock types are slate and phyl- Diabase Nonmetamorphosed diabase dikes and lite, and in the staurolite zone at Lake stocks. Michigamme, the predominant rock types (Y ) Kewee - nawa n

are phyllite and schist. Black slate is the ex- Uppe r Precambria n ception and retains its slaty characteristic in Pegmatite Coarse-grai ned-m1crocli ne-quartz- the higher metamorphic zones. muscovlte rocks. Relatively rare. Metadiabase Amphibolitic rock, much with relict Powell (1972) suggested that cleavage in diabase texture. Mostly as sills in the Michigamme Formation was produced middle Precambrian rocks and dikes in lower Precambrian rocks. Much is by tectonic dewatering while the sediments probably associated with rocks of were still soft. One of his major arguments the Clarksburg Volcanics Member of the Michiqamme Formation. for tectonic dewatering is the presence of Upper slate member Metapelite and metagraywacke with clastic dikes that are parallel to regional calc-sllicate concretions. Also pyritlc black slate, and foliation. Clastic dikelets, similar to those e quartzltes at Sturgeon River are described by Powell, were found at Lake o Included 1n this member. +mJ Bijiki Iron Formation Mostly cherty silicate iron-formation. Michigamme (Fig. 5) and tend to support Supergrou p E Member Some qruneritlc schist. o Lower slate member Metapelitlc and metagraywacke with his hypothesis. Although Geiser (1975) Precambria n li- (X ) some pvritic carbonaceous slate. Grou p

suggested that the hypothesis of soft- Rang e Clarksburg Volcanic Mafic pyroclastic rocks with ra Member intercalated meta-argillite and sediment deformation for the origin of slaty Middl e CTI iron-formation Precambria n cleavage and related clastic dikes is still an Barag a JZu Greenwood Iron Cherty silicate Iron-formation iE Formation Member open question, this hypothesis and Powell's Marquett e interpretation are followed in this paper. Massive to banded protoquartzite Goodrich Quartzite with conglomerate near base. The presence of numerous quartz veins, quartz saddle reefs, and quartz segrega- Negaunee Iron-Formation Mostly cherty silicate Iron-formation with oxide iron-formation near top. tions, all of which are oriented parallel to Fi ne e Siamo Slate Laminated metapelite. fabric elements, indicate that silica migrated Ajlbik Quartzite Banded to massive, light colored

Grou p quartzite. during-Fj deformation. For example, at lenom i Gneiss of northern and Complex of granitic and mafic Lake Michigamme, several quartz veins are southern complexes gneiss, pegmatite, and greenstone. emplaced along Sj, some of which are isoclinally folded with Si axial-plane foliation. Some of these quartz veins have pegmatite (W ) Lowe r rinds of muscovite and pink feldspar. Also, Precambria n numerous elongate pods of quartz from less Figure 2. Stratigraphic column for western Marquette Range (after Cannon and Klasner, 1972).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/5/711/3418694/i0016-7606-89-5-711.pdf by guest on 27 September 2021 Figure 3. Structural geology of Taylor mine area. Lower-hemisphere stereographic plot A shows contours at 0.5,1,2,5, and 8 percent on 204 poles to bedding; B shows contours at 0.4, 2, 5, and 9 percent on 236 poles to cleavage (S,'). Drill-hole data for section A—A' supplied by Ford Motor Company.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/5/711/3418694/i0016-7606-89-5-711.pdf by guest on 27 September 2021 UPPER PRECAMBRIAN MIDDLE PRECAMBRIAN LOWER PRECAMBRIAN Geologic Contact n^I KEWEENAWAN DIABASE | METADIABASE SILLS AND UNDIFFERENTIATED GRANITIC Fault 1 62 DIKES AND STOCKS DIKES TERRANE -J- Strike and Dip of Bedding BARAGA GROUP I MICHIGAMME FORMATION: With Iron -Formation Units 82 Strike and Dip of Foliation 1 Upper Unit Is Bijiki Member ~65 I GOODRICH QUARTZITE Plunge of ^-F,. Folds I MENOMINEE GROUP INEGAUNEE IRON FORMATION 2~* Plunge of Small Folds of L2 Lineations Plunge Bands

Figure 4. Structural geology of Lake Michigamme area. Lower-hemisphere stereo- mation. Map after Klasner (1972) and Klasner and Cannon (1974b). * = location of island graphic plot A shows contours at 1, 5, and 10 percent on 252 poles to regional foliation referred to in Figure 7. ** = location of island referred to in Figure 5. (Si'); B shows poles of long axes of deformed calc-silicate concretions in Michigamme For-

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remained after soft-sediment deformation, axial-plane foliation. Folding is mostly due F^F/ deformation. The intermediate axes

migrated along zones of already-formed to Ft deformation with an unknown of the concretions reflect the intermediate penetrative foliation (S,) and enhanced that amount of enhancement during F/. They F,-F,' strain axis. Both the long and inter- foliation by widening the domains rich in are upward-facing folds because graded mediate axes of the concretions lie within layered-lattice silicate minerals. This was bedding indicates that younger beds are in- the S/ foliation planes. The shortest axes of accompanied by continued shortening of tersected by the axial plane in an upward the concretions reflect the axis of maximum the middle Precambrian section. The onset direction (Cummins and Shackleton, 1955). shortening during F^Fi' deformation and of metamorphism accompanying the mi- The calc-silicate concretions, which proba- are oriented perpendicular to S/. The initial gration of the fluids increased their ability bly formed from metamorphism of origi- shape of the concretions is not known. to dissolve quartz. As fluids migrated to nally calcareous concretions, occur in zones Pantin (1958) showed that concretion zones where temperatures were lower, silica that define bedding. These concretions are growth can be syngenetic or early diagene- precipitated as quartz veins and quartz important in this study because they show segregations parallel to Si'. In the cherty the relationship in orientation between the iron formation at the Taylor mine, the silica Fj-F/ strain axes, regional S/ foliation, and precipitated in zones of extension in the Fj-F/ fold geometry. The long axes of the axial regions of the F,-F/ folds to form concretions generally plunge 63° toward quartz saddle reefs. According to the model, N70°W as shown in Figure 4. This is inter- fluid migration must have been primarily preted as the orientation of the axis of vertical because it was controlled by the maximum extension that occurred during orientation of the near-vertical regional foliation. Fj-F,' Fold Geometry and Style of Fold- ing. The F^F,' folds described in this paper are the same as the uniformly oriented second-order folds previously discussed by Cannon (1973). They are small- scale folds, some isoclinal, that are generally visible in outcrop. F^Fj' folds can be

distinguished from F2 and F3 folds by the presence of S/ axial-plane foliation, but in a few places near the basement contacts, it is difficult to determine the age of folding because S/ is lacking or poorly developed. At the Taylor mine (Fig. 3), the geometry of Fj-F,' folds is shown in both map view and cross section A—A' by the configuration of the contact between graphitic slate and cherty iron formation. On A—A', a line connecting the crests of the larger folds Figure 6. Diagrams to illustrate different styles plunges toward the south, which suggests a Figure 5. Clastic dikelets injected parallel to of folding in black slate and cherty iron formation. 15° rotation of the F,-F,' fold envelope. The St foliation during F, soft-sediment deformation. (A) Sketch of F,-F,' folding in graphite slate from cross section as well as the sketch of a fold These dikelets, found on an island in Lake Taylor mine area. (B) Sketch ofF^F/ fold in cherty in graphitic slate (Fig. 6A) show that some Michigamme, are from the upper slate member iron formation from Taylor mine area (P = pelit- of the folds are overturned with the axial of the Michigamme Formation. ic; C = cherty; B — brecdated). planes dipping south. Figure 6 shows that the style of folding TABLE 1. NATURE OF THE FOU PHASES OF DEFORMATION AND ASSOCIATED FABRIC ELEMENTS IN ESTERN MARQUETTE RANGE AREA differed in different rock types. In Figure 6A the black slate was intensely deformed by Deformational event Fabric elements passive slip along penetrative cleavage planes so that bedding was transposed S0 — bedding parallel to cleavage. Elsewhere, folding in F, — initial phase of soft-sediment S, — regional foliation the black slates was so intense that the rela- deformation, possibly caused by tionship between cleavage and bedding regional gravity slide cannot be determined. By contrast, the Fi' — continued regional deformation Si' — more pronounced regional foliation cherty iron formation shown in Figure 6B with associated migration of silica along formed by enhancement of Si consists of interbedded chert and pelite S, layers, which were deformed by flexure flow and flexure slip, respectively. F2 — deformation due to uplift of lower 52 — axial-plane fracture cleavage formed in F^F/ fold geometry and style of folding Precambrian basement as rigid blocks crests of crenulation folds with associated crenulation folding of L — lineations produced by intersection of is also illustrated in a detailed sketch of 2 S,' surfaces Sj' and S2 folds and accompanying calc-silicate concretions from an island in Lake Mich- F3 — deformation possibly due to con- 53 — kink bands that affect Si', S2, and L2 igamme (Fig. 7). The folds are interpreted tinued uplift of rigid blocks of lower L3 — minor lineations on S/ surface but hav- Precambrian basement ing different orientation than as Fj-F/ in age because they possess S/

tic, occurring while the sediments are still evidence for a gravity slide is not available middle Precambrian rocks of northern soft. I infer that the concretions at Lake in that décollement surfaces have not been Michigan indicates that the feature which Michigamme formed early in the sedimen- found within the Baraga Group rocks or be- caused this deformation must have been tary sequence, were deformed during the F[ tween underlying middle Precambrian truly regional. A mountain range in central phase of soft-sediment deformation, and rocks and basement. Nevertheless, several Wisconsin with an east-trending tectonic were then subjected to further F/ deforma- lines of evidence from previous studies and axis is one possible source for a gravity slide tion. Randomly oriented amphibole por- from this study support such a hypothesis that could have produced the regional phyroblasts within the concretions indicate and are presented below. FJ-FJ' deformational fabric in northern that they were metamorphosed after F,-F,' 1. Isoclinal folding and intense defor- Michigan. deformation. mation indicate that the middle Precam- 3. Radiometric dating studies in north- Cause of F^F/ Deformation. One pos- brian section was substantially shortened east Wisconsin (Banks and Cain, 1969) and sible cause of F1-F1' deformation is a gravity during F^F/ deformation. Yet the apparent in central Wisconsin (Van Schmus, 1972,

slide off an ancestral Penokean mountain absence of FrFi' structures in the underly- 1974; and Van Schmus and others, 1975) range located in central Wisconsin. In- ing lower Precambrian section (Cannon and show that the uranium-lead ages for plu- terpretations based on thin-skinned grav- Klasner, 1972; Klasner, 1972; Cannon, tonic rocks group around 1,875 to 1,900 ity-slide tectonics are not unprecedented in 1973) shows that the basement was not m.y. These ages agree with the time of geologic literature (Hamilton and Meyers, shortened at this time. Thus, it is a geologic Penokean deformation in northern Michi- 1967; Maxwell, 1962); such tectonic pro- necessity that one or perhaps many décol- gan (1.9 b.y. ago) and suggest plutonism cesses have previously been suggested as the lement surfaces separate the Michigamme and diastrophism, and possibly mountain cause of regional deformation in northern Formation and the basement. building. The exact cause of the diastroph- Michigan (Cannon, 1971; Cannon and 2. The presence of persistent and wide- ism is still an open question. Van Schmus Klasner, 1972; Klasner, 1972; Cannon, spread west-northwest—trending, steeply (1976) has tentatively proposed a plate 1973; Van Schmus, 1976). However, direct south—dipping foliation throughout the tectonic model.

F2 Deformation

Regional Elements. First-order basement structures and smaller second-order folds and cleavage within the middle Pre- cambrian strata comprise the major types of

F2 structures in the western Marquette Range. The Marquette and Republic troughs as well as the fault contact between lower and middle Precambrian rocks at the Taylor mine are the most prominent first- Bed of concretions order structures. Second-order features include structures that developed when the middle Precambrian sedimentary deposits Syncline were passively folded into the troughs. These features include folds with wave- Showing trace of axial plane lengths equal to the widths of the troughs Dashed where approximately located themselves, as well as numerous outcrop- — i— sized folds that occur throughout the west- Anticline, approximately located ern Marquette Range, generally within a Showing trace of axial plane few thousands of metres from the basement contact. These smaller folds can be iden-

tified as F2 because either they have a different orientation than F,-F/ folds or they fold Graded bed F/ surfaces. Arrow pointing toward top F2 structures generally are associated with Fj-Fj' structures, but within the tightly infolded middle Precambrian strata of the Zone of cleavage Republic trough, which is oriented about 30° from the strike of S/, only structures C that trend or strike parallel to the trough Folded quartz vein!et are observed, and these are interpreted as F2 structures. There are two possible expla- nations for the absence of Fi-Fj' structures: Strike and dip of cleavage (1) F2 deformation may have been so intense in the narrow troughs that it completely overprinted the Fi-F/ structures and Strike and dip of bedding no evidence of Fi-F/ deformation remains, Figure 7. Map and cross section showing F,-F,' generation folds in Michigamme Formation on or (2) the Republic trough may have started west shore of an island in Lake Michigamme. The location of this island is shown in Figure 4. forming before the onset of F^F/ deforma-

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tion and even before middle Precambrian within the quartzites and (2) some units evidence includes kink bands at Lake sedimentation ended. Thus, the sediments contain chlorite porphyroblasts (two chlo- Michigamme (see Fig. 4) and minor linea- were protected in some of the narrow rite porphyroblasts were observed, and they tion on S/ at the Taylor mine. All observed troughs and were not deformed during may be pseudomorphs after biotite as this F3 features are in the Michigamme Forma- Fj-F,' deformation. area is located within the biotite isograd). tion. There is no widespread distribution of

F2 Fold Geometry and Style of Folding. Although the metamorphic evidence is F3 structural elements. Some important aspects of F2 deformation weak, it appears that these rocks have been Fold Geometry and Style of Deformation. are shown in a hypothetical cross section affected by the Penokean orogeny and are Kink bands at Lake Michigamme affect all

(A—A') in Figure 4. This cross section, older than about 1.9 b.y. The exact corre- previous tectonic elements including SI , S2,

styled after gravity models (Klasner, 1972; lation with the middle Precambrian section and L2 and indicate a brittle style of F3 de- Klasner and Cannon, 1974a) and surface is uncertain. Because they do not show se- formation. The kink bands are about 5 cm geology, shows that faults which occur vere F2 folding as do those that lie close to wide, nearly vertical, and strike N20°E and along the contacts between middle and the faulted contact with the basement, I N50°W. lower Precambrian rocks are nearly verti- propose that the flat-lying quartzites are At the Taylor mine, pencil-line-thin line- underlain by a rigid uplifted block of gran- cal, and it suggests that fault orientation ations (L3) are found on S/ surfaces. They ite basement rock. They were not intensely was probably controlled by pre-existing are oriented at a large angle to F2 fold axes deformed during F uplift as were those that foliation within the basement rocks. Al- 2 and are interpreted as evidence for F3 de- though not shown in cross section A-A', in are located adjacent to the uplifted blocks formation. some places faults also occur in weaker of basement. Cause of F3 Deformation. As with F2, I layers within the middle Precambrian rocks. Although the cause of uplift of the lower attribute F3 deformation to uplift of lower Several parasitic folds are found in the Precambrian rocks is not known, it appears Precambrian basement. This interpretation middle Precambrian rocks near the base- to be associated with metamorphism be- is supported by the fact that F2 and F^F,' ment contact, as shown on A—A'. cause, as will be shown later, F2 deforma- fold axes at the Taylor mine and Silver Lake At Lake Michigamme and the Taylor tion occurred during the peak of thermal (located at opposite ends of the northern

mine, F2 deformation is characterized by metamorphism. Sims and Peterman (1976) complex as shown on Figure 1) plunge brittle crenulation folding of S{ slate and showed that in the Watersmeet node, the away from the basement contact, thereby

phyllite. Axial-plane fracture cleavage (S2) basement was partially melted and re- suggesting post-F2 uplift of the northern

creates numerous lineations (L2) where it in- mobilized during thermal metamorphism. complex.

tersects Si. The axes of the F2 crenulation

folds and L2 are generally parallel to the F3 Deformation TIME RELATIONSHIPS BETWEEN strike of the nearest contact between base- REGIONAL METAMORPHISM

ment and Precambrian X rocks. Because the Regional Elements. Firm evidence for F3 AND DEFORMATION strikes of S2 and S/ are nearly the same deformation is found on only a few out- (although dips are in opposite directions), crops in the western Marquette Range. This Similar to Bayley's (1959) findings in the most L2 lineations are nearly horizontal. Peavy node area of metamorphism about Deformational events Further details of F2 deformation are seen 80 1cm to the south, regional metamorphism in the stereographic plots from the Taylor in the western Marquette Range started F| F | * F2 F3 mine area (Fig. 3B). The great-circle plot of early and peaked rather late in the sequence

poles to Si' shows that the axes of the F2 of deformation. The time relationships be- Andalusite folds are oriented east-west and plunge to tween growth of porphyroblasts and the the west. Also the great circle labeled 2 on Garnet various deformational events as interpreted Figure 3A is interpreted as an F2 overprint Staurolite from thin sections of rocks from the stauro- of the FrF/ fabric pattern. lite zone near Lake Michigamme (see Fig. 1) Actinolite — Cause of F2 Deformation. Previous are shown in Figure 8. studies (Cannon and Klasner, 1972; Klas- Grunerite Figure 8 is not a petrogenetic model, but ner, 1972; Cannon, 1973) have shown that rather an interpretation of the observed Biotite the structural troughs in the western Mar- fabric relationships from the staurolite zone quette Range, as well as elsewhere in north- Sericite of regional metamorphism. Time relation- ern Michigan, were formed by uplift of ships between deformation and por- Chlorite - rigid blocks of of basement. Further evi- phyroblast growth may be different in other dence in support of this concept is found in zones. Evidence for the time of growth of

the flat-lying quartzites at Sturgeon River Fabric elements S| S|' S2 L2 S3 L3 each metamorphic mineral is shown in Fig- (see Fig. 1). Because these rocks lack the ures 9 and 10 and discussed in the following obvious structural features present in sur- Time paragraphs. rounding rocks (tight folds, steeply dipping Figure 8. Relative time relationships between That metamorphism started early, before beds, regional foliation), it may be reasoned deformational events and growth of por- F,' deformation, is shown by the deforma- that the quartzites represent a younger Pre- phyroblasts in the staurolite zone near Lake tion of andalusite porphyroblasts by F,' Michigamme. The varying intensity of thermal cambrian sedimentary section. Yet, in spite metamorphism is shown schematically by the dot (See Fig. 9). The evidence for time of of marked differences in external features pattern. The vertical lines denote periods of de- growth of andalusite is as follows: between these rocks and surrounding mid- formation and can be considered as approximate 1. Rather than being euhedral in form, dle Precambrian rocks, they are assigned a time lines for the relatively small Lake andalusite porphyroblasts are generally middle Precambrian age because (1) folia- Michigamme area. The horizontal lines (dashed where uncertain) indicate the time during which augen-shaped in those rocks that have tion parallel to the regional S{ fabric pat- the particular mineral grew in relation to the de- prominent Si foliation (James, 1955, p. tern is developed in some thin pelitic layers formational events. 1483). Hie short axes of the augen are gen-

Figure 9. Micrographic sketch, photograph, and micrographs from samples in the Lake Michigamme area that show relationships between metamorphic minerals and structural fabrics. (A) Andalusite and chlorite porphyroblasts from a staurolite-andalusite schist from Lake Michigamme (sec. 31, T. 48 N., R. 30 W.). An- dalusite porphyroblasts are partially retrograded to se- riate. They are deformed with long axes parallel to S/. They enclose detrital ilmenite-leucoxene grains that are randomly oriented within the andalusite but aligned parallel to Sj outside the andalusite. The nonretrograded andalusite shows strongly developed cleavage oriented perpendicular to S,'. The andalusite has been rotated coun- 2,54 cm terclockwise by S2. The chlorite porphyroblasts grow parallel to S/. (B) Photo of crenulated (S/) staurolite schist. The L2 lineations wrap around the staurolite porphyroblasts. Sample from sec. 36, T. 48 N., R. 31 W. (C) Rotated staurolite from sample 8. The section was cut perpendicular to L2 (the b tectonic axis). Helicitic staurolite rotated by S2. Note the enclosed ilmenite crystal aligned parallel to Si' within the staurolite. Staurolite has been retrograded to sericite. Crossed nicols. (D) Rotated helicitic staurolite, not retrograded. Shows same fabric relationship to S/ and S2 as sample C. Section was cut from the same specimen as sample A. Uncrossed nicols.

erally perpendicular to S/, and the long to be more randomly oriented inside the outside the andalusite porphyroblasts than axes lie within the plane of S/, suggesting porphyroblasts than they are in the matrix they are inside the porphyroblasts, which

that the mineral was deformed during F/ and in the accompanying post-F,', pre-F2 suggests they were protected within the deformation. staurolite porphyroblasts (compare Fig. 9 A porphyroblasts during F/ deformation. I 2. In those andalusites that have not with Fig. 9C). These grains were incorpo- infer that Fi deformation started before the been retrograded, a prominent fracture set rated into andalusite before F/ deforma- onset of metamorphism because field evi- occurs perpendicular to S/ foliation (Fig. tion. dence suggests soft-sediment deformation 9A). Although these fractures may repre- I interpret these data as indicating that prior to metamorphism.

sent the natural cleavage of andalusite, it is metamorphism started after Ft and before F/ Staurolite porphyroblasts in samples clear that extension parallel to S/ during deformation. The andalusite porphyroblasts from the Michigamme Formation are F/ deformation has emphasized the cleav- were deformed at the time of the F/ defor- clearly rotated by F2 crenulation folding age sets perpendicular to S,'. mation, which indicates pre-F/ growth of the (See Figs. 9B and 9C). These samples were 3. Detrital ilmenite-leucoxene grains andalusite. The long axes of the ilmenite- taken at least 3.2 km away from the contact enclosed by andalusite porphyroblasts tend leuxocene grains are more strongly aligned with either of the lower Precambrian granit-

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ic complexes and are from an area of prom-

inent F,' fabric development. The F2 deformation that is expressed as crenulation fold-

ing of S/ is a part of the widespread F2 deformational pattern and cannot be attrib- uted to a minor, postmetamorphic, recur- rent movement of the granitic blocks. The staurolite crystals have helicitic S/

fabric that shows their rotation by F2 crenulation folds (Figs. 9C and 9D). Also the axes

of the F2 folds appear to wrap around the staurolite porphyroblasts. Detrital ilmenite-leucoxene grains are strongly aligned parallel to S/ foliation both within and outside of the porphyroblasts. Based on these observations, staurolite crystals are shown in Figure 8 as developing after F,'

deformation but mainly before F2 deformation. Garnets form at different pressures and temperatures, depending on rock type and chemical composition of the host rock. This is shown diagrammatically in Figure 8 by the growth of garnets during much of the deformational sequence. Figure 10A shows a garnet porphyroblast that has formed in

chlorite schist that was produced by F2 fault movements in the Goodrich Quartzite. The garnet exhibits a rotated, syntectonic texture, which indicates that it grew during the

development of the S2 foliation. Figure 10B shows undeformed garnet

porphyroblasts that have grown in S2-

foliated gruneritic schist. The S2 lineations

are due to F2 fracture cleavage that has formed in the crest of a parasitic fold on the north limb of the Lake Michigamme synclinorium. Because the garnets crosscut

S2 foliation and are not deformed, they are

interpreted as being of post-F2 age. Powell (1970, p. 23) showed garnets from the Lake Michigamme area that are both prekinematic and synkinematic to prominent foliation. Although Powell interpreted these as being rotated by late "sporadic" deformation during meta- Figure 10. Photomicrographs showing fabric relationships between metamorphic minerals and morphism, my work at Lake Michigamme structural fabric from the Lake Michigamme area. See text for interpretation. (A) Syntectonic garnet in suggests that the garnets have been rotated an S2-foliated chlorite schist. Sample taken from chlorite schist shear zone in Goodrich Quartzite, SE V* sec. 19, T. 48 N., R. 30 W. Crossed nicols. (B) Garnetiferous-gruneritic schist from Bijiki Iron by F deformation because they occur in the 2 Formation Member along north shore of Lake Michigamme (sec. 23, T. 48 N., R. 30 W.). Garnet (Ga) zone that has prominently developed F 2 porphyroblasts are undeformed by S2 foliation. Grunerites (Gr) mimic the S2 foliation. Uncrossed structural fabric. nicols. (C) Biotite (B) and actinolite (Act) porphyroblasts from mafic sill. Both the biotite and actino- Radiating grunerite porphyroblasts are lite mimic the S2 fabric direction, and both show evidence of strain. The biotites are dark brown. abundant in the Negaunee Iron-Formation Sample taken from small sill along the southeast shore of Lake Michigamme. Uncrossed nicols. (D) Chlorite (Ch) and biotite (B) porphyroblasts that have grown along S/ foliation and around an S2 fold and in gruneritic schist of the Bijiki Iron in the Si' foliation. Neither is strained. Uncrossed nicols. Formation Member. Especially in the Bijiki, the radiating grunerites have grown mimet-

ically parallel to S2 fracture cleavage (Fig. Figure 10C shows the relationship of ac- rock generally grew with its long dimen-

10B), which indicates that the grunerites tinolite and biotite porphyroblasts to S2 sions parallel to the S2 fabric. The actinolite formed either during or after F2 deforma- foliation. They occur in biotite-amphibole crystals show strong undulatory extinction; tion. No evidence was found to indicate the schist that forms a thin mafic sill that has the biotite, in a few instances, shows very time relationship of the formation of the been sheared during F2 and possibly F3 de- minor undulatory extinction. Because the

grunerite porphyroblasts to F3 deformation. formation. The dark-brown biotite in this actinolite and biotite crystallized mimeti-

cally parallel to S2 foliation, they are inter- periods of deformation (F,, F,', F2, and F3) affected by Keweenawan tectonism and preted as forming after F2 deformation. The and by one prolonged thermal metamor- igneous activity. This is represented by undulatory extinction is interpreted as evi- phic event. The deformational sequence mafic dikes and stocks that intrude the dence of strain due to late F3 uplift of the started with a phase of soft-sediment de- middle and lower Precambrian rocks and lower Precambrian complexes; therefore, formation (Ft), possibly caused by thin- by a series of north-trending faults that both minerals are shown as forming before skinned tectonism that produced regional show up as prominent lineaments on topo- F3 deformation. folds and a penetrative N75°W-trending graphic, geologic, and geophysical maps. James (1955) used dark-brown biotites regional foliation (SO in the middle Pre- from mafic rocks as index minerals for cambrian sedimentary deposits. It is pos- ACKNOWLEDGMENTS high-grade (garnet, staurolite, and tulated that this deformation may be related sillimanite) regional metamorphism. Thus to a décollement off a rising Penokean During this study I received financial the biotites are interpreted as having mountain range in central Wisconsin. support in the form of a National Science formed during the most intense period of Selective migration of silica during con- Foundation Traineeship through the De- regional metamorphism. tinued or reactivated deformation (F/) partment of Geology and Geological En- Quartz grains in the Michigamme area along the already-formed S, slaty cleavage gineering at Michigan Technological Uni- tend to show less undulatory extinction and enhanced this cleavage and formed numer- versity. Ford Motor Company provided the less evidence of strain than do those at the ous quartz veins and quartz saddle reefs. diamond drill data for the Taylor Mine

Taylor mine. This is probably due to the The F2 period of deformation is marked area, and Cleveland Cliffs Iron Company higher temperatures reached in the stauro- by the formation of fault-bounded basins and United States Steel gave access to their lite zone of metamorphism at Lake Mich- and troughs such as the Marquette and Re- files and company reports for the Lake igamme. In order to release strain in the public troughs, the Lake Michigamme Michigamme area. I especially want to quartz grains, temperatures must have been synclinorium, and the Taylor mine syncline, thank J. Kalliokoski and W. F. Cannon, high enough after deformation to cause an- which is fault-bounded on its south flank. who spent considerable time with me in the nealing recrystallization within the strained Folding of the middle Precambrian rocks field and office discussing and reviewing my quartz grains (see Pitcher and Flinn, 1965, resulted from the vertical rise of the lower work and manuscripts. I would also like to p. 110). Precambrian granitic complexes as rigid thank A. P. Ruotsala for originally suggest- Figure 10D shows chlorite and biotite fault-bounded blocks and was accompanied ing the petrofabric studies relating defor- porphyroblasts that have crystallized by faulting within the less competent mid- mation and metamorphism. I am also grate- mimetically parallel to S/ foliation that has dle Precambrian strata and brittle deforma- ful to N. L. Archbold, R. J. Borchers, and D. G. Hill for reviewing final copies of the been crenulation folded during F2 deformation of the Fj-Fj' tectonites to produce tion. These long-bladed minerals occur crenulation folding and S2 fracture cleav- manuscript. along the folded S/ foliation but do not age.

show any evidence of strain. Therefore, The last period of deformation (F3) was REFERENCES CITED they are interpreted as having formed after also probably related to uplift of the granit- F2 deformation and most likely formed dur- ic blocks. Evidence for this event is found in Aldrich, L. T., Davis, G. L., and James, H. L., ing retrograde metamorphism. kink bands and tilted fold axes, both of 1965, Ages of minerals from metamorphic and igneous rocks near Iron Mountain, which affect F/-F structural elements. The Another example of retrograde 2 Michigan: Jour. Petrology, v. 6, pt. 3, metamorphism is the replacement of an- tilted fold axes are found at either end of p. 445-472. dalusite and staurolite porphyroblasts with the northern complex where F/ and F2 fold Banks, P. O., and Cain, J. A., 1969, Zircon ages sericite and the replacement of garnet with axes dip away from the basement complex. of Precambrian granitic rocks, northeastern chlorite. During the Penokean deformation, only Wisconsin: Jour. Geology, v. 77, p. 208- 220. James (1955, p. 1486) pointed out that the middle Precambrian rocks were folded. Banks, P. O., and Van Schmus, W. R., 1971, retrograde metamorphism has been exten- Regional structural trends within the granit- Chronology of Precambrian rocks of Iron sive, affecting nearly one-fourth of the ic complexes diverge from the structural and Dickinson Counties, Michigan [abs.]: metamorphic minerals. The widespread na- trends within the middle Precambrian 17th Ann. Inst, on Lake Superior Geology, ture of the retrograde effects and the fact rocks, and mafic dikes of pre-Penokean age Duluth, Minn., p. 9. Bayley, R. W., 1959, Geology of the Lake Mary that the retrograde minerals have not been that intrude the lower Precambrian com- quadrangle, Iron County, Michigan: U.S. deformed indicates that retrograding oc- plexes were not appreciably deformed dur- Geol. Survey Bull. 1077, 112 p. curred after all of the Penokean deforma- ing Penokean deformation other than by Bodwell, W. A., 1972, Geologic compilation and tional sequence. It is not known, however, shearing along their contacts (Cannon, non-ferrous metal potential, Precambrian section, northern Michigan [M.S. thesis]: 1973). how long after deformation the retrograde Houghton, Michigan Tech. Univ., 73 p. metamorphism occurred, but it must have Contrary to previous studies that suggest Boyum, B. H., 1964, The Marquette mineral dis- occurred before the intrusion of the Kewee- that metamorphism almost completely trict, Michigan: Guidebook, 10th Ann. Inst, nawan diabase dikes (1,100 m.y. ago) be- postdated deformation (James, 1955; on Lake Superior Geology, Ispheming, Mich., 13 p. cause these have not been metamorphosed. Powell, 1969,1972), the present study indi- Cannon, W. F., 1971, The Penokean orogeny in cates that the thermal regional metamor- northern Michigan [abs.]: Geol. Assoc. A SUMMARY OF phic event began before F/ deformation, Canada Ann. Mtg., Sudbury, Ontario, May THE GEOLOGIC HISTORY probably peaked about the time of F2 de- 1971 Program, p. 9-10. formation, and ended in a phase of ret- 1973, The Penokean orogeny in northern Michigan, in Huronian stratigraphy and The data discussed above indicate that rograde metamorphism. sedimentation: Geol. Assoc. Canada Spec. middle Precambrian rocks in the study area After the cessation of Penokean defor- Paper 12, p. 251-271. were affected by three and possibly four mation and metamorphism, the area was 1974, Bedrock geologic map of the Green-

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