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Two Early Proterozoic Successions in Central Wisconsin and Their Tectonic Significance: Alternative Interpretation and Reply

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Two Early Proterozoic Successions in Central Wisconsin and Their Tectonic Significance: Alternative Interpretation and Reply Two early Proterozoic successions in central Wisconsin and their tectonic significance: Alternative interpretation and reply Alternative interpretation R. S. MAASS Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 ù 1 Wisconsin Geological and Natural History Survey, 1815 University Avenue, Madison, Wisconsin 53705 J. K. GREENBERG I LaBerge and Myers (1984) proposed the existence of two early is recorded over a large area. Some of the youngest known rocks asso- Proterozoic sedimentary-volcanic-plutonic sequences in central Wisconsin ciated with the Penokean orogeny are metamorphosed to amphibolite that they claim are separated in time by a major orogenic event predating facies. These include (1) three tonalites between Stevens Point and Wis- the Penokean orogeny. They suggested that the two sequences can be consin Rapids that yield ages of 1,840 m.y. to 1,825 m.y., and mafic dikes distinguished on the basis of amphibolite-facies versus greenschist-facies that are intrusive into these tonalites (Maass and others, 1980; Van metamorphism and by contrasting styles of deformation. They also sug- Schmus, 1980); (2) 1,832 ± 9 m.y.-old foliated tonalite along the Black gested that lithologic and stratigraphic evidence supports a correlation of River at Greenwood (Maass and Van Schmus, 1980; Van Schmus, 1984); early Proterozoic rijcks in central Wisconsin with early Proterozoic rocks and (3) -1,840 m.y.-old tonalite located 15 km east of Eau Claire (Van in Upper Michigan, and Minnesota. As workers who are also involved in Schmus, 1980; Maass, unpub. data). Amphibolite-facies metamorphism of the difficult task of interpreting the Precambrian geology of this complex the above rocks is defined by the presence of coexisting hornblends and and poorly exposed terrane, we uphold all efforts to arrive at a simple, yet calcic plagioclase (An2o or greater) in combination with textural relation- comprehensive, tectonic model; however, we have great difficulty accept- ships that indicate that the mineral assemblages are metamorphic rather ing either the two-sequence-two-orogeny model or the proposed correla- than igneous. Coexisting calcic plagioclase and epidote in a thoroughly tion. Our data, and data of others, do not support the occurrence of a recrystallized, 1,838 ± 9 m.y.-old, biotite-bearing foliated tonalite located major early Proterozoic orogenic event predating the Penokean orogeny or 16 km northwest of Merrill (Van Schmus, 1984; Maass, unpub. data) the existence of two early Proterozoic rock sequences. We contest the suggest amphibolite-facies metamorphism (Turner, 1981) of this rock also. regional correlation of LaBerge and Myers on the basis of both age and Known and inferred older rocks have not reached amphibolite faces. A tectonic considerations. l,860-m.y.-old, or older, metavolcanic rock from northeastern Wisconsin It is our interpretation that most of the early Proterozoic deformation, has been metamorphosed to greenschist facies only (Banks and Cain, metamorphism, and igneous activity within the terrane occurred during 1969), and schist and iron-formation near Black River Falls in central the Penokean orogeny (-1,900-1,825 m.y. ago). Structural and meta- Wisconsin that are tentatively interpreted as being Archean are metamor- morphic evidence, in combination with radiometric age data, suggests that phosed to the chlorite zone (Maass, unpub. data). the Penokean orogeny involved multiple stages of deformation, and both To support the two-sequence concept, the statement is made (p. 251) amphibolite-facies and greenschist-facies metamorphism (Maass and oth- that "sharp contacts between amphibolite- and greenschist-facies rocks ers, 1980; Maass and Van Schmus, 1980; Greenberg and Brown, 1983a; occur in several localities in central Wisconsin. Some contacts are inter- Maass, 1983, unpub. data). Penokean deformation and metamorphism preted as faults but others ... appear to show an unconformable relation- appear to have affested a single, albeit complex, early Proterozoic group of ship between amphibolite- and greenschist-facies rocks." Amphitolite- rocks. Although earlier radiometric age data (Van Schmus, 1980) allowed facies and greenschist-facies rocks are typically exposed kilometres to tens the possibility of subdividing Penokean igneous activity, more recent data of kilometres from each other, however (Weidman, 1907; Dutton and and refinement of earlier data (W. R. Van Schmus, 1984 and also 1984, Bradley, 1970; Myers, 1980; LaBerge and Myers, 1983; Maass, unpub. personal commun. I present a more complex picture and suggest semicon- data). tinuous or continuous igneous activity during the Penokean orogeny. An unconformity between older amphibolite-facies dioritic gneiss and younger greenschist-facies volcanogenic sediments is reported to exist METAMORPHISM at a small outcrop along the Eau Claire River (p. 251; Myers, 1980, p. 45-53). The volcanogenic sediments, which are -1,860 m.y. old (Van LaBerge and Myers reported that the older of their two rock se- Schmus, 1980), are <10 m thick and are exposed for <5 m along strike. quences was metamorphosed to amphibolite facies during a pre-Penokean They are flanked on both sides by dioritic gneiss. The contact between orogenic event. Tae younger rock sequence is reported to have been dioritic gneiss and the volcanogenic sediments is buried, but the two rock metamorphosed to lower greenschist facies during the Penokean orogeny; types are exposed to within a few metres of each other. Both rock types however, amphibolite-facies metamorphism during the Penokean orogeny appear equally deformed, both in the field and in thin section, and no evidence of different folding histories for the two rock types is apparent. There is no structural evidence of an unconformity, because layering and The article about which this alternative interpretation was written appeared in the Bulletin, v. 95, p. 246-253. penetrative foliation in both rocks strike to the northwest with nearly Geological Society oí America Bulletin, v. 96, p. 1340-1346, 3 figs., October 1985 1340 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/10/1344/3445051/i0016-7606-96-10-1344.pdf by guest on 25 September 2021 ALTERNATIVE INTERPRETATION AND REPLY 1341 vertical dips (Myers, 1980, Fig. 22; Maass, unpub. data). The volcanogenic canic rocks and metagraywackes. We have observed no evidence for a sediments have been assigned upper greenschist-facies metamorphism by major fault or other type of major boundary in the Athens area. The region LaBerge and Myers on the basis of the presence of garnet, epidote, musco- around Athens can be interpreted as being a single sequence of metavol- vite, and chlorite. Because chlorite occurs as pseudomorphs of garnet canic and metasedimentary rocks that record a gradual south-to-north (Myers, 1980, Figs. 24 and 25), chlorite is not necessarily a product of the transition from greenschist facies to amphibolite facies. first metamorphism recorded here. We do not consider the mineral as- If, for the sake of argument, juxtaposition of high-grade and low- semblage in the volcanogenic sediments to be diagnostic of greenschist- grade rocks along fault contacts is accepted, then in the absence of evi- facies metamorphism of the sediments during the first metamorphism, and dence of low-grade overprinting of the high-grade rocks, the relationship is the presence of garnet is suggestive of amphibolite-facies metamorphism. most reasonably interpreted as exposure of different levels of a single Dioritic gneiss, which is higher in Ca, Mg, and Fe content and lower in Si terrane. Metamorphic overprinting of Penokean assemblages occurs and A1 content than are the volcanogenic sediments (Myers, 1980, Table within central and northern Wisconsin; it has been interpreted as retro- 8), contains the amphibolite-facies assemblage hornblende-calcic andesine grade Penokean metamorphism or post-Penokean metamorphism (Brown (Maass, unpub. data). Retrograde metamorphism of dioritic gneiss is indi- and Greenberg, 1981; Greenberg and Brown, 1983a; Maass, 1983 and cated by the alteration of hornblende to actinolite and chlorite. Neither unpub. data; Olson, 1984). structural nor metamorphic evidence supports the presence of an uncon- In support of their separate successions, LaBerge and Myers (p. 249) formity at this locality, but even if one exists as proposed, no data have asserted that "the scarcity of transitional lithologies [s/c] between been presented to show that diorite gneiss is of early Proterozoic age rather amphibolite-facies rocks and lower greenschist-facies rocks is one of the than Archean age. The interpretation which we favor for this outcrop is most distinctive features of early Proterozoic rocks in central Wisconsin." that the small block of volcanogenic sediments represents a xenolith of We believe that this statement results from their lack of recognition of country rock in a Penokean dioritic pluton. In this scenario, both rock metamorphism to intermediate grades, rather than the absence of meta- types would have been subjected to Penokean folding, Penokean morphism to intermediate grades, and also from assignment of Penokean amphibolite-facies metamorphism, and a retrograde greenschist-facies or pre-Penokean ages to rocks that we interpret as post-Penokean. Our metamorphism of undetermined age. combined field and petrographic investigation of the terrane has provided An outcrop 0.8 km to the east (stop 7 of Myers, 1980) of the above
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