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

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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 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 PENOKEAN DEFORMATION AND ASSOCIATED METAMORPHISM, NORTHERN MICHIGAN 713 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.
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