Precambrian Rhyolites and Granites in South-Central Wisconsin: Field Relations and Geochemistry

Precambrian rhyolites and granites in south-central Wisconsin: Field relations and geochemistry

EUGENE I. SMITH Division of Science, University of Wisconsin - Parkside, Kenosha, Wisconsin 53141

ABSTRACT Baraboo rhyolites and granites were used to crop out in the Baraboo area (Stark, 1932; subdivide these rocks into four chemical Dalziel and Dott, 1970). Early geologic Isolated exposures (inliers) of Precam- groups: (1) fine-grained granite at Baxter work on the inliers and on the igneous rock brian rhyolites and granites (1,765 m.y. Hollow and coarse-grained rhyolite dikes at in the Baraboo area was done by Buckley old) crop out in the Fox River valley and in Observatory Hill are characterized by high (1898), Weidman (1898, 1904), Hobbs and the Baraboo area of south-central Wiscon- CaO and low Rb/Sr; these rocks intrude Leith (1907), and Pretts (1895). These sin. The geochemical characterization of rhyolite flows and may represent an intru- studies were followed by geologic investi- rock units, in addition to field and petro- sive event that occurred after the major ex- gations by Gram (1947), Stark (1932), graphic studies, was used to unravel the trusive episode; (2) fine-grained and Gates (1942), and Asquith (1964). Because complex geology of two of these inliers, the porphyritic rhyolite at the Marquette expo- of the renewed interest in Canadian Shield Marquette and Marcellon rhyolites, and to sure and the fine-grained plagioclase- geology and the economic geology of Wis- determine in a preliminary fashion the bearing rhyolite at Marcellon have inter- consin, considerable field research is now geology of the Precambrian igneous terrain mediate CaO and Rb/Sr; (3) porphyritic being done in south-central Wisconsin between exposures, which is covered by a rhyolite and granophyric granite are char- (Smith and Hartlaub, 1974; Smith, 1975a, section of Paleozoic sedimentary rocks and acterized by low CaO and high Rb/Sr; 1975b, 1976), and in north-central and Pleistocene glacial deposits of varying similarities in chemistry between these two northeastern Wisconsin (for example, thickness. rock types suggest that the granophyric LaBerge and Myers, 1973; Van Schmus and The rhyolite at Marquette consists of a granites are the subvolcanic equivalents of others, 1975a, 1975b). Sims (1976) pre- series of ash-flow tuffs, interbedded with the rhyolites; and (4) rhyolites at the Mar- sented a good summary of the Precambrian mud-flow breccia, which are broadly folded cellon and Baraboo exposures are inter- geology of Wisconsin and Minnesota. Also, into a series of normal and overturned mediate in chemistry between groups 2 and Rb/Sr and U/Pb dating of Precambrian asymmetric folds. The rhyolite is cut by a 3. The four groups are chemically related rocks, including the granites and rhyolites 100-m-thick andesite dike that intruded and show calc-alkalic affinities. of the inliers, is currently being done by along a northeast-trending normal fault. Chemical correlation and geologic map- W. R. Van Schmus. The top of the exposed section is a por- ping show that the chemical groups occur U/Pb dating of zircons in the Fox River phyritic rhyolite containing quartz, alkali geographically as northeast-trending bands valley rhyolite and granite inliers feldspar, and plagioclase phenocrysts. This across south-central Wisconsin. Gran- (W. R. Van Schmus, 1976, personal com- unit is underlain by ash-flow tuffs ranging ophyric granite lies to the northwest of mun.) indicates an average age of 1,765 ± from porphyritic quartz, plagioclase, alkali group 3 porphyritic rhyolites; the por- 20 m.y. for these rocks. Except for recent feldspar rhyolite to rhyolite with only pla- phyritic rhyolites lie in general to the isotopic work, no modern geological or gioclase phenocrysts. Plagioclase units are northwest of the texturally variable rhyo- geochemical study has been done on these phenocryst poor and have Rb/Sr ratios lites of groups 2 and 4. Structural trends rhyolites and granites. Besides the Fox greater than 1; the three mineral units are within exposures parallel contacts between River valley inliers, other exposed 1,765- phenocryst rich and have Rb/Sr ratios less chemical groups, thus strengthening the m.y.-old rocks are granite near Monico and than 1. thesis that the chemical trends reflect the the Arnberg Quartz Monzonite, both in The rhyolite at Marcellon consists of four geology of the large area of buried Pre- northeastern Wisconsin (Van Schmus and ash-flow tuffs folded into a northeast- cambrian rock in south-central Wisconsin. others, 1975b). Several granites in central trending asymmetric antiform. Lithologi- Wisconsin (for example, a granite near cally, the Marcellon units are spherulitic, INTRODUCTION Mosinee) are texturally similar to the gran- flow-banded, brecciated, and massive. Min- ites in the Fox River valley and may also eralogically, they vary from porphyritic Numerous small Precambrian rhyolite belong to this group (Van Schmus and quartz, plagioclase, alkali feldspar rhyolite and granite outcrops in the Fox River valley others, 1975b). Also, a porphyritic granite to plagioclase-bearing rhyolite. Geochemi- in south-central Wisconsin lie to the south at Radisson in northwest Wisconsin is cal correlation was used to relate units from of the main area of Precambrian outcrop in 1,765 m.y. old (W. R. Van Schmus, 1976, one part of the inlier to the other and thus north-central and northeastern Wisconsin personal commun.). Rhyolites and granites establish the existence of the antiform. This (Fig. 1). Each exposure (inlier) is sur- in the Baraboo area are most likely the same exposure is cut by andesite and basalt dikes. rounded and partially covered by Cambrian age as the Fox River valley rocks (Fig. 1). Forty new major- and trace-element sandstone and Pleistocene drift and out- The dates for the Fox River valley rocks analyses for the Fox River valley and wash. Rhyolite, granite, and diorite also are distinctly younger than those obtained

Geological Society of America Bulletin, v. 89, p. 875-890, 11 figs., 8 tables, June 1978, Doc. no. 80607.

875

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for volcanic rocks in the Wausau area many of the rhyolite units are lithogically areas by using similarities in relative con- (1,850 ± 30 m.y.; Van Schmus, 1976), and similar, correlations between and within centrations of Sr, Zr, and Rb, of Ba, Ti, and for volcanic rocks and granites in north- inliers cannot be accomplished by tra- Mn, and of Y, Nb, and Pb. In the study de- eastern Wisconsin (1,850 to 1,900 m.y.; ditional field and petrographic techniques. scribed here, elements that have been espe- Banks and Cain, 1969). On the other hand, The geochemical characterization of rhyo- cially useful to characterize units are Ba, La, the inliers are significantly older than the lite makes unit-by-unit correlation possible Y, Zr, Rb, Sr, K, Na, and Ca. Wolf River batholith in northeastern Wis- within inliers and enables correlation of Besides using the rock chemistry for cor- consin, which is dated at 1,500 m.y. B.P. chemically similar rock types between in- relation purposes, the chemical data were (Van Schmus and others, 1975a). Volcanic liers. The latter is especially important be- also instrumental in addressing the follow- and plutonic rocks in northeastern and cen- cause it establishes regional subsurface ing key questions concerning the Fox River tral Wisconsin show calc-alkalic affinities trends in the Precambrian basement of Valley inliers and the igneous rock in the and were formed during the major phase of south-central Wisconsin. Baraboo area. (1) Are the rhyolite inliers the Penokean orogeny, whereas the Wolf The geochemical characterization or chemically related? (2) Are the rhyolites and River batholith is alkalic in nature and is fingerprinting of volcanic rocks was dem- granites comagmatic, with the granites the anorogenic (Van Schmus and others, onstrated by Cann and Renfrew (1965) and subvolcanic equivalents of the rhyolites? 1975a; Anderson and others, 1975). Jack and Carmichael (1968). Jack and Hobbs and Leith (1907), and Asquith Precambrian inliers in south-central Wis- Carmichael successfully used the finger- (1964) assumed that rhyolite is genetically consin are widely spread, and even within printing technique to distinguish silicic vol- related to granite and cited the close prox- an individual inlier exposures may be sepa- canic rocks erupted from several source imity of outcrop and the gross similarities in rated by as much as 2 km. Because much of chemistry as evidence. Van Schmus and the Precambrian basement is buried and others (1975b) cited the similar ages as

0 40 80 MILES I I 40 80 120 KILOMETERS

EXPLANATION

PALEOZOIC

Paleozoic Cover

LATE PRECAMBRIAN

Keweenawan Lavas and Sedimentary Rocke

Wolf River Batholith

Quartzlte

MIDDLE PRECAMBRIAN

Granites, Metasedlmentary Rocks, Metavolcanlc Rocks and Iron Formations

EARLY PRECAMBRIAN

Gnelssic Rocks, Granitic Rocks and Metavolcanlc Rocks

FAULT Figure 1. Location of Fox River valley inliers. Other possible 1,765-m.y.-old exposures (Van Schmus and others, 1975b) include Arnberg Quartz Mon- zonite (Am), granite near Monico (M), granophyric granite near Mosinee (Mo), and porphyridc granite at Radisson (R) (W. R. Van Schmus, 1976, personal commun.). Geology of northern Wisconsin modified from Sims (1976) and LaBerge (1977).

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additional evidence that the granites may be MARQUETTE AND MARCELLON tocene and Paleozoic sedimentary rock (Fig. the intrusive equivalents of the rhyolites. (3) INLIERS (TEXTURALLY 3). The extensive cover prevents reliable What is the relationship of the 1,765-m.y. VARIABLE RHYOLITE) field correlation of units from hill to hill, B.P. event to volcanism and plutonism oc- and since most contacts are obscured, rela- curring earlier during the Penokean orog- Texturally variable rhyolite flows and tive age of the Marquette units can only be eny (1,850 to 1,900 m.y. ago) and to the ash-flow tuffs (Table 1; Fig. 2) are charac- inferred by noting their stratigraphic posi- later anorogenic plutonism (1,500 m.y. terized by the change from phenocryst-rich tion within major folds. Correlations de- ago?). Anderson and others (1975) specu- quartz-bearing rhyolites to phenocryst-poor picted on the geologic map (Fig. 3) are lated that the l,765-m.y.-old rocks formed rhyolites with a paucity of quartz and based primarily on chemical and petro- during the waning phase of the Penokean abundant plagioclase. Rocks in the inliers graphic similarities. orogeny. Here, I support this suggestion by are black to dark gray in color and vary The inlier is formed by seven mineralog- showing that the l,765-m.y.-old rhyolites considerably in texture. The rock may be ically and chemically distinct volcanic and granites are more closely related to the banded, brecciated, spherulitic, or massive. flows, ash-flow tuffs, and breccias. The calc-alkalic 1,900-m.y. B.P. event than to Inliers at Marquette and Marcellon ex- units, designated A to G from southeast to the 1,500-m.y. B.P. alkalic plutonism. emplify this lithological type. northwest, are broadly folded into a series In this paper, I describe the general geol- of normal and overturned anticlines and ogy of the inliers, with special emphasis on Marquette Inlier synclines with an average wavelength of the Marquette and Marcellon localities, 300 m. The fold axes strike N20° to 40°E where detailed field and chemical work has The rhyolite at Marquette (Pretts, 1895; and plunge to the northeast. These folded been completed. I also present the first Hobbs and Leith, 1907; Smith and rhyolite units are cut by a 100-m-thick modern major- and trace-element data for Hartlaub, 1974; Smith 1975a, 1975b) oc- andesite dike that was intruded along a the l,765-m.y.-old rocks. cupies seven small hills surrounded by Pleis- northeast-trending normal fault. The fault

TABLE 1. SUMMARY OF GEOLOGY OF FOX RIVER VALLEY RHYOLITES AND GRANITES AND BARABOO AREA IGNEOUS ROCKS

Exposure Location Important rocks types Comments References

Montello Sec. 9, T. 15 N., R. 10 E. Granophyric granite, metabasalt Rock contains less than 2% Buckley (1898) chlorite and opaque minerals Redgranite Sec. 1, T. 17 N., R. 11 E.; Granophyric granite, metabasalt, Rock contains less than 5% Buckley (1898), NWy-t, T. 18 N., R. 12 E.; porphyritic granite chlorite and opaque minerals Weidman (1904) NE1/», T. 18 N., R. 11 E. Bedin SEVi, Sec 3, T. 17 N., Porphyritic rhyolite Rhyolite is locally sheared Buckley (1898) R. 13 E. Marquette Sec. 34-35, T. 15 N., Porphyritic and fine-grainted Rhyolite is folded and faulted This paper R. 11 E.; Sec. 1-2, rhyolite, breccia, andesite dike T. 14 N., R. 11 E. Observatory Hill SWV4, sec. 8, T. 14 N., Porphyritic rhyolite, dikes of Little or no textural variation Hobbs and Leith (1907) R. 10 E. course-grained rhyolite and metabasalt Taylor Farm NE1/., sec. 13, T. 14 N., Porphyritic rhyolite Location incorrect in many earlier Hobbs and Leith (1907) R. 9 E. references Endeavor SVz, sec. 5, NVa, sec. 8, Porphyritic rhyolite Little or no textural variation Hobbs and Leith (1907) T. 14 N., R. 9 E. Marcellon Sec. 7, T. 13 N., R. 10 E. Texturally variable rhyolite, Folded into a northeast-trending This paper metabasalt antiform Udey NV2, sec. 36, T. 15 N., Porphyritic rhyolite, felsic and Zones of spherulites and Gram (1947) R. 13 E. metabasalt dikes lithophysae trend N50°W Baraboo Sees. 21, 22, 23, T. 12 N., Texturally variable rhyolite Lies below Baraboo Quartzite Dalziel and Dott (1970) R. 7 E. Caledonia Church NE1/., sec. 3, T. 11 N., Fine-grained rhyolite and breccia Lies below Baraboo Quartzite Dalziel and Dott (1970) R. 8 E. Baxter Hollow SW1/», sec. 33, T. 11 N., Fine-grained granite Intrusive into rhyolite; relation- Gates (1942) Granite R. 6 E. ship to Baraboo Quartzite unclear Denzer rhyolite SE'/i, sec. 11, T. 10 N., Fine-grained bedded rhyolite Volcaniclastic sandstone? Stark (1932) R. 5 E. Denzer diorite Sees. 9 and 10, T. 10 N., Coarse-grained diorite Contacts with adjacent units not Stark (1932) R. 5 E. exposed Note: Dutton and Bradley (1970) showed rhyolite inliers near Pardeeville, Lake Mills, and Cambria in south-central Wisconsin. These exposures could not be located in the field and probably do not exist.

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is downthrown to the north, and the dis- respondence to those to the south, except poor structural control to the north of the placement calculated by estimating the that they are displaced to the southwest. fault. amount of structural shortening probably Although the map shows simpler structural The youngest rock in the inlier is a does not exceed 600 m. The structures to patterns to the north of the fault than to the northeast-trending fine-grained massive the north of the fault have a one-to-one cor- south, this difference is probably due to rhyolite dike, 35 m thick, which cuts the fault and the andesite dike. The dike is similar to unit C in lithology (fine-grained with plagioclase as the dominant phenocryst), but it is distinguished from unit C on chemical grounds (Fig. 4). Unit G is a thick (1,000 m) quartz (10%), alkali feldspar (commonly perthitic; 16%), WAUSHARA and plagioclase (7%) rhyolite porphyry.

MARQUETTE BERLIN • 8 Except for minor variations in phenocryst abundance and faint banding, the unit is texturally homogeneous. Shardlike forms were observed in the matrix of unit G, indi- WINNEBAGO cating that it is an ash-flow tuff. A fine- grained rhyolite is interbedded with unit G and crops out near the south end of the unit GREEN LAKE G exposure. The six units lying to the southeast of unit G are texturally variable, with banded, PUCK AWAY LAKE fine-grained, and porphyritic varieties common. Most of the units show evidence <¿a.6 O of brecciation and microbrecciation. Coarse breccia is found on the southeast margin of Ingall's Knob, where clasts of porphyritic and fine-grained red to black rhyolite exceed 10 m in size. Unit D on Cluppert's Hill GREEN LAKE FOND DU LAC is also extensively brecciated. Eutaxitic tex- COLUMBIA §> ture is well displayed in several of the units. DODGE Each unit in the Marquette inlier has dis- tinguishing chemical and mineralogical characteristics that are used to correlate o |0 20 30 —^^HIM^MHMai^^ MILES units between exposures (Fig. 4; Table 5). Units B, D, and F are porphyritic plagio- 0 10 20 30 40 ^^ 1 KILOMETERS clase (18% to 28%), quartz (2% to 8%), and alkali feldspar (1%) rhyolites with 20%to 36% total phenocrysts. Unit B is distinguished from the other quartz-bearing

rhyolites by Na20/K20 ratios greater than 1.0 and low Rb/Sr ratios (0.64 to 0.92). Unit D and Unit F are similar in both major-element and minor-element chemistry. Unit D, however, contains in its upper part a 100-m-thick massive phase; a similar massive rock is not associated with unit F. Units C and E are fine grained (10% to 15% phenocrysts), with plagioclase as the dominant phenocryst. Unit E is distinguished from unit C by lower CaO. Care

must be taken when using Na20/K20 ratios to correlate Marquette rhyolite units because several samples probably have had

Na20 contents lowered during hydration. Noble (1967, 1970) reported typical losses Figure 2. Index map of rhyolite and granite inliers and associated Precambrian exposures in Fox River valley (upper map) and Baraboo area (lower map). 1, Endeavor rhyolite; 2, Taylor Farm rhyo- of as much as 0.5% by weight Na20 (aver- lite; 3, Marcellon rhyolite; 4, Observatory Hill rhyolite; 5, Montello granite; 6, Marquette rhyolite; 7, age loss 0.4% by weight) and changes in

Utley rtiyolite; 8, Berlin ihyolite; 9, granite at Redgranite; 10, granite at Pine Bluff; 11, Lower Nar- K20 content varying from +0.4% to rows rhyolite; 12, Caledonia Church rhyolite; 13, South Limb rhyolite; 14, Baxter Hollow granite; — 0.1% by weight when comparing 15, Denzer rhyolite; 16, Denzer diorite. Dotted pattern on lower map is Baraboo Quartzite. Relation- ship of Baraboo area to Fox River valley is shown in Figures 1 and 10. nonhydrated and hydrated glasses from the

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EXPLANATION ferred, dotted where buried

X Strike and dip of banding | Porphyritic Quartz-Alkali Feld- Vertical banding spar Rhyolite with interbedded X fine-grained rhyolite flow X Strike and dip of c an tact Unit G Normal fault, dashed where [ | Quartz-Plagioclase-Alkali ,'u approximately located, U- Feldspar Rhyolite upthrown side, D-downthrown Units F,D ana B side

tSw^Yl Plagioclase Rhyolite Anticline, showing trace of Units E and C axial plane and direction of plunge ¥¡•51$ Breccia Syncline, showing trace of | fwj Very fine-grained rhyolite, axial plane and direction coarsely porphyritic rhyolite of plunge

Overturned Anticline, showing Andesite (?) Dikes trace of axial plane and Fine-Grained Rhyolite Dike direction of plunge

jgC1 Overturned Syncline, showing trace of axial plane and direction of plunge

Figure 3. Geologic map of Marquette inlier.

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Na20 content, and as a result the Na20/ from zones of crystal accumulation within K20 ratio cannot be confidently used to the source chamber. The lower Rb/Sr ratios characterize this unit. Samples for units D in phenocryst- and feldspar-rich units (F, D, Marcellon Inlier and F also may have undergone small chem- and B) may be explained by a model where ical change, particularly in alkalis; there- Sr is concentrated into the crystallizing Another example of an inlier composed fore, the significance of the difference in feldspar and Rb is enriched in the liquid of texturally variable rhyolite is at Marcel- Na20/K20 ratios between the two units is phase. As a consequence, phenocryst- and lon (Fig. 2; Table 1). This inlier (Hobbs and doubtful (Fig. 4). Although unit B rhyolites feldspar-rich units would have lower Rb/Sr Leith, 1907; Smith, 1976) is formed by four may have undergone some chemical than the fine-grained, feldspar-poor vari- mineralogically and chemically distinct change, the changes of Na20 and K20 con- eties (units C and E) that formed from frac- ash-flow tuffs folded into a northeast- tents reported by Noble (1967, 1970) are tionated crystal- and feldspar-poor magma. striking asymmetric (and possibly over- not great enough to produce the difference Alternatively, fine-grained and phe- turned) antiform (Fig. 5). The western limb in Na20/K20 ratio observed between unit B nocryst-rich unit pairs may represent com- of the antiform strikes N50°E and dips 50° and the other quartz-bearing rhyolite units positionally zoned ash-flow sheets, with the to 85° to the northwest. The eastern limb (units D and F). Therefore, the higher phenocryst-rich unit at the top and the also strikes N50°E but dips steeply (80° to Na20/K20 ratio of unit B is probably pri- fine-grained unit at the base. Zoned ash- vertical) to the southeast. The rhyolite flows mary and can be used to characterize the flow tuffs commonly show upward in- are cut by a northeast-trending andesite unit. crease in MgO, CaO, A1203, Ti02, and dike and by an east-trending basalt dike Noteworthy is the cyclic change from Sr/Rb and may vary in composition from (Table 2). phenocryst-poor plagioclase rhyolite (units quartz latite at the top to rhyolite at the The structurally highest unit (unit A) at C and E) to phenocryst-rich three-mineral base (see, for example, Smith, 1960; Ratte Marcellon is a sparsely porphyritic plagio- rhyolite (units B, D, and F). This cyclic var- and Steven, 1964; Smith and Bailey, 1966; clase (1%), quartz (2%), alkali feldspar iation in mineralogy is also reflected in Noble and Hedge, 1969; Rhodes, 1976). (2%) rhyolite characterized by abundant trace-element chemistry (Table 5); for Phenocryst abundance and xenolith abun- large (as much as 15 cm in diameter) example, Rb/Sr ratios vary from 1.45 to dance and size usually increase upward, spherulites composed of radiating fibers of 1.23 for units E and C and from 0.71 to 1.0 with pumice commonly showing reverse alkali feldspar and quartz. On the eastern for units F, D, and B (Fig. 4). zonation (Sparks, 1976). Within one ash- flank of the fold, spherulites are less distinct Chemical and flow-direction data pre- flow sheet the transition from phenocryst- and smaller but still conspicuous. Structur- sented later (Figs. 9, 11) show that all of the poor to phenocryst-rich tuff can be abrupt ally, below unit A is a rhyolite (unit B) that Marquette units are comagmatic and that (Noble, 1970). Marquette units F and E contains sparse quartz (6%), alkali feldspar all flows erupted from a source to the may together represent a single compo- (4%), and plagioclase (1%) phenocrysts in northwest of the present outcrops. This evi- sitionally zone ash-flow sheet, with unit E a banded matrix with occasional faint dence suggests that cyclic variation in the differentiated fine-grained base and unit spherulitic growths. Several samples show chemistry, mineralogy, and texture reflect F the less differentiated phenocryst-rich top. perlitic cracks in the matrix. Unit C is eruption from a differentiating source. Grouping of units D and C is doubtful since characteristically well banded and contains Fine-grained units probably represent they are separated by a fault and by an plagioclase as the dominant phenocryst (14 to 18%). Banding is continuous and relatively consistent in orientation (N50°E), but locally broad westward-plunging folds are (,89 N«.|00, G '. • \ » exposed. Several lenses of spherulitic rhyo- M06 \ \ lite lie parallel to banding and have sharp I 1 contacts with nonspherulitic rock. Unit C Figure 4. Rb/Sr-Na20/K20 plot for 1^194 Marquette inlier samples. Tliis plot on the eastern flank of the fold is similar in demonstrates that mineralogically mineralogy to rock on the western flank, I?,., ,VF similar rhyolites can be distinguished but it lacks conspicuous banding. The core I 0 f- 1103 on basis of Rb/Sr and Na20/K20 /•92 of the antiform is formed by a rhyolite (unit ratios. Also note cyclic change in Rb/Sr D) that contains phenocrysts of quartz '.104, ¿98 ratio. Plagioclase-bearing rhyolites V* / 991 (2%), plagioclase (15%), and alkali feld- Rb/ r \ (units C and E) have Rb/Sr greater S than 1, whereas quartz-plagioclase- spar (2%) in a fine-grained devitrified alkali feldspar rhyolites (units B, D, groundmass with numerous shards, flat- and F) have Rb/Sr less than 1. Unit G is tened pumice, and perlitic fractures. All 191 (• \ "Andesite' quartz-alkali feldspar-plagioclase units at Marcellon are interpreted as ash- Rhyolite . \ \ dike rhyolite. dike \ flow tuffs. 190 Evidence for the northeast-striking an- '>»189 tiform at Marcellon includes (1) the sym- metric pattern of lithologies on the geologic 0 1.0 18 map (Fig. 5); (2) structural data (orienta-

NO20/K20 tion of banding within the rhyolite) that

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250 500 METERS

•'cfj? / /Wm. EXPLANATION • /•

Basalt Dike Andesite Dike, Coarse-grained Andesite Dike /ß* Quartz - Plagioclase - Alkali Feldspar Rhyolite Unit D

Plagioclase Rhyolite banded ond flow folded Unit C

Quartz-Plagioclase - Alkali Feldspar Rhyolite HZ3 Unit B

Spherulltlc Rhyolite Unit A Flow Folding Trace of axial plane of anticline, arrow indicates 4V Strike and Dip of Banding direction of plunge Vertical Banding Trace of axial plane of syncllne, arrow Indicates direction of plunge Contact, dashed where Inferred, dotted whsre 145 burled Sample Location

Anticline, showing trace of axial plane

Figure 5. Geologic map of Marcellon inlier. Area of complex folding within unit C consists of westward-plunging, tighdy spaced (30 to SO m) flow folds. Folding is localized in southern part of unit C exposure.

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indicates that the western part of the struc- unit C is different from the other rhyolites Rhyolite is sheared at Berlin (Weidman, ture strikes N50°E and dips to the north- in having higher Ba contents (Fig. 6). 1898) and slickensided surfaces are locally west; the eastern part also strikes northeast, found at Utley (Gram, 1947). Rhyolites at but dips steeply to the southeast; banding is OTHER MAJOR ROCK TYPES Utley and Observatory Hill are intruded by poorly developed in outcrop in the eastern both felsic and mafic dikes. part of the inlier; and (3) the chemical cor- Porphyritic Rhyolite relation of lithologically similar rhyolite Granophyric Granite from the western flank to the eastern flank Rhyolite porphyry with large (1 to 5 mm) of the structure. Similarities in the Rb/Sr pink and white feldspar and quartz pheno- Fine- to medium-grained granophyric ratios of lithologically similar units (Table crysts set in a black, dark-gray, or reddish granites (Table 1; Fig. 2) exposed at Mon- 3) suggest that they are stratigraphically brown matrix is exposed at Berlin, Utley, tello and at Redgranite are equivalent to the equivalent. For example, both spherulitic Endeavor, Marquette, Observatory Hill, Waushara Granite of Weidman (1898). The rhyolite exposures (unit A) show similar and Taylor Farm (Table 1; Fig. 2). Except granite is leucocratic and is pink on fresh Rb/Sr values. Similar groupings are appar- for minor color variation, faint banding, surfaces. Quartz and alkali feldspar com- ent for units B and C (Table 3). Unit D and variation in phenocryst density, this pose 90% to 98% of the rock; perthitic, (Quartz—plagioclase-alkali feldspar) can be rhyolite is texturally homogeneous. How- granophyric and myrmekitic textures are distinguished from the other quartz-bearing ever, common in the Utley Rhyolite are common. Subordinate minerals are biotite rhyolites by a lower Rb/Sr ratio and higher zones of spherulites and lithophysae; also, (partially or wholly altered to chlorite), Ba, CaO (Fig. 6), and FeO + Fe^ (2.82% disk-shaped coarse-grained inclusions may sphene, hornblende, muscovite, and zircon. as compared with 1.77% to 2.13%). Also, represent recrystallized collapsed pumice. The Redgranite inlier is intruded by thin (3

TABLE 2. CHEMICAL ANALYSES OF DIKE ROCK AND DENZER DIORITE

Marquette Marquette Granite porphyry Marcellon Marcellon Denzer diorite andesite dike rhyolite dike+ dike at andesite dike metabasalt dike Redgranite Sample no.: 105 190, 191 111 148 177 184 Major oxides (%)

Si02 63.92 69.33 72.10 60.59 48.94 56.21 TiÖ2 0.94 0.43 0.30 0.93 0.99 1.33 AI2O3 15.65 14.23 12.74 16.47 17.84 13.20 Fe203 1.92 1.20 1.06 1.61 2.21 1.95 FeO 4.38 2.73 2.21 4.50 6.56 6.66 MnO 0.20 0.18 0.12 0.18 0.19 0.14 MgO 1.40 0.48 0.09 1.77 6.57 5.10 CaO 1.58 1.66 1.08 3.86 9.59 6.01 NAJO 4.71 4.63 3.12 4.27 3.25 3.09 K2O 3.34 3.41 6.12 3.27 1.11 2.56 + H2O 1.51 1.09 0.73 1.50 2.97 2.34 H2O- 0.11 0.05 0.03 0.11 0.12 0.14 P2O5 0.31 0.11 0.04 0.48 0.31 0.98 Total 99.97 99.53 99.74 99.54 100.29 99.71

Elements (ppm) B 20 20 20 50 25 70 Ba 1,050 1,500 1,170 1,200 660 1,100 Co 3 5 3 4 24 37 Cr 12 29 26 16 42 150 Cu 65 80 16 70 50 80 La 40 39 88 68 10 95 Mo 10 20 10 10 10 10 Ni 10 5 10 6 24 51 V 15 5 5 92 240 260 Y 25 30 63 28 11 45 Zr 200 290 590 220 75 620 Pb 30 19 25 28 21 Rb 113 98 180 105 38 75 Sr 218 212 56 514 642 625 Zn 75 110 180 180 160 Sc 5 3 20 27 23 Note: Major-element analyses were made using conventional wet-chemical methods (K. Aoki, analyst). Trace-element analyses (Rb, Sr, Pb, and Zn) by atomic absorption spectrometery (O. Joensuu, analyst). All other trace elements by optical emission spectrography (O. Joensuu, analyst). The results for Zr are accurate to ±10%. Sr and Rb are accurate to ±5% of the amount present, except for low Sr (less than 20 ppm), which is accurate to ±10% of the amount present. See Figure 3 for location of samples 105 and 190; see Figure 5 for location of samples 148 and 177. Analysis is an average of two samples.

to 5 m) metabasalt and diabase dikes, and (21%), quartz (2%), and clots of chlorite, during intrusion. At Marcellon, the north-

to the south of the Flynn's Quarry County epidote, and clinozoisite (10%) set in a east dike contains 60% Si02 and is Park, it is cut by a 20-m-wide granite fine-grained devitrified matrix (21%). It is classified as an andesite (Irvine and Baragar, porphyry dike. Metabasalt also intrudes the separated from the Observatory Hill rhyo- 1971). granophric granite at Montello. Granite in- lite by a 0.5-m-wide fine-grained chill zone, liers in general lie to the northwest of which is black on fresh surfaces. The coarse Diorite and Rhyolite Water-laid Tuff rhyolite exposures (Hobbs and Leith, 1907; rhyolite can be distinguished from Obser- Van Schmus and others, 1975b). vatory Hill rhyolite by its color and lack of In addition to these six major rock types, quartz phenocrysts. Besides the minor there is a coarse-grained diorite and a Fine-grained Granite rhyolite dike at Marquette, this coarse- water-laid tuff (Stark, 1932) in separate ex- grained rhyolite and the Baxter Hollow posures near Denzer on the south limb of Fine-grained granite is exposed in Baxter granite are the only felsic rocks that intrude the Baraboo syncline (Fig. 2; Table 1). Hollow on the south limb of the Baraboo rhyolite. In addition, it will be shown in a syncline (Fig. 2; Table 1; Gates, 1942). This later section that the fine-grained granite at CHEMICAL RELATIONSHIPS fine-grained granite contains 66% feldspar Baxter Hollow and these coarse rhyolite BETWEEN INLIERS (orthoclase/plagioclase = 2/1), 29% quartz, dikes have similar chemical characteristics. and 4.6% biotite and chlorite. The rela- This stratigraphic and chemical evidence Because the rhyolite and granite outcrops tively high percentage of mafic minerals and suggests that these intrusions represent a are not continuous, structural and strati- the lack of distinct granophyric texture dis- discrete igneous event. graphic correlations between inliers are tinguish it from the granophyric granites. difficult. The only reliable means of relating The stratigraphic relationship between the Metabasalt Dikes the inliers to each other is by their chemis- Baxter Hollow granite and the Baraboo try. The major- and minor-element analyses Quartzite is still debated (Dalziel and Dott, Metabasalt dikes intrude all rock types presented here are the first modern analyses 1970). Gates (1942), however, suggested except coarse rhyolite at Observatory Hill published for these Fox River valley rocks. that the Baxter Hollow Granite intrudes the (Tables 1, 2). In general, the metabasalt is Earlier chemical work (Buckley, 1898; rhyolite that underlies the Baraboo fine-grained; plagioclase, epidote, and Weidman, 1904; Hobbs and Leith, 1907) is Quartzite. clinozoisite are common minerals. Coarse- grained varieties include a plagioclase- TABLE 3. CORRELATION OF Coarse-grained Rhyolite Dikes amphibole dike with diabasic texture which MARCELLON RHYOLITE UNITS intrudes the granite at Redgranite and a ON BASIS OF SIMILAR Rb/Sr RATIOS Coarse-grained rhyolite, originally coarse phase of the northeast-trending dike mapped by Hobbs and Leith (1907) as at Marcellon. Although the dikes are Anticline Sample Rb/Sr Unit area granite porphyry, occurs as two dikes less lithologically similar, they vary consid- than 30 m thick that intrude both the Ob- erably in chemical composition. For exam- West limb 173 1.82 A servatory Hill rhyolite and an east-trending ple, the dike at Marquette contains 63%, 174 2.70 B metabasalt dike (Fig. 2; Table 1). This Si02 and may be classified as an andesite 145 1.10 C rhyolite is coarsely crystalline and is red- (Irvine and Baragar, 1971). It contains East limb 176 1.77 A brown in color in outcrop. It contains pla- abundant glomeroporphyritic plagioclase, 161 2.82 B gioclase (46%), quartz and alkali feldspar similar to those in the rhyolite which it in- 175 0.63 C intergrown with a micropegmatitic texture trudes, and may have been contaminated Core 178 1.32 D

_L _L 1000 30 40 50 60 70 80 90 0 100 150 0.5 1.0 Ba La Sr Rb Rb/Sr CaO MgO Figure 6. Stratigraphic variation in element concentrations for Marcellon rhyolite units A, B, C, and D. Ba, La, Sr, and Rb are in ppm; CaO and MgO are in percent by weight. For comparison, element concentrations for Baraboo rhyolites (B at top) are also plotted.

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systematically high in weight percent A1203 new chemical study shows that most of the chemistry and are differentiated from the and CaO, low in Na20, and either high or l,765-m.y.-old rhyolites and granites fall other granites and rhyolites by higher Ti02, low in FeO. Analyses presented here super- into four distinct categories (Tables 4 CaO, Ba, V, and Sr and lower Si02 and sede the earlier chemical data. Even though through 7; Figs. 7, 8, and 9, B and C). The Rb/Sr ratio. most of the Fox River Valley rocks are re- four groups are chemically related and dif- The second group consists of both fine- markably fresh chemically for their age, fer only by their degree of differentiation. grained and porphyritic rhyolites at Mar- some have probably undergone chemical The chemical data also show that the gran- quette and the fine-grained plagioclase change, particularly in the alkalis. Samples ites may be the subvolcanic equivalents of rhyolites at Marcellon (unit C) (Table 5; 91,198,107,108,109,110, 111, 161,174, the rhyolites. These rocks show calc-alkalic Figs. 7, 8, and 9, B and C). These rocks are and 182 probably have lost Na20 and affinities and probably formed during the intermediate in most major- and trace- either lost or gained K20. The variety in waning stages of the Penokean orogeny. element concentrations between the fine- Na20/K20 ratios observed within each grained granites and coarse-grained rhyo- chemical group (see below and Tables 4 Chemical Groups lites of group 1 and the prophyritic rhyo- through 7) may be partially due to these lites and granophyric granites of group 3. secondary chemical changes. The mag- The first chemical group is formed by the However, they are lower in Zr than either nitude and nature of the gain and loss of fine-grained granite at Baxter Hollow and group 1 or 3 rocks. Compared to the alkalis and other elements from silicic rocks the coarse-grained rhyolite dikes at Obser- porphyritic rhyolites of group 3, they are was discussed by Noble (1967, 1970). This vatory Hill (Table 4; Figs. 7, 8, and 9 B and lower in Si02, K20/Na20, and Rb/Sr and C). These rocks are closely similar in higher in Ba, CaO, and A1203. Several

TABLE 4. CHEMICAL ANALYSES FOR GROUP 1 GRANITES AND TABLE 5. CHEMICAL ANALYSES FOR GROUP 2 RHYOLITES RHYOLITES Marcellon Observatory Hill Baxter Hollow Marquette Rhyolite units unit rhyolite dike granite sample 189 sample 183 Gf F E* Cf Bf C+ Sample no.:* 89 101 100 103 91 92 145 Major oxides (%) 102 194 104 198 98, 99 175 106 197 Si02 69.28 70.26 Ti0 0.42 0.30 2 Major oxides (%) AI2O3 13.90 14.10 Fe203 1.37 0.86 Si02 72.18 71.11 72.14 72.30 71.71 71.64 71.90 FeO 2.89 2.66 Ti02 0.26 0.28 0.26 0.26 0.24 0.28 0.29 MnO 0.14 0.10 AI2O3 14.17 14.68 14.58 14.07 14.18 14.00 13.66 MgO 0.90 1.02 Fe203 1.55 1.67 1.60 0.91 1.53 1.76 1.62 CaO 1.68 1.48 FeO 0.83 0.84 0.69 1.46 0.70 0.80 130 Na20 4.17 4.29 MnO 0.06 0.06 0.09 0.10 0.15 0.12 0.11 K20 4.33 3.69 MgO 0.25 0.26 0.22 0.30 0.19 0.33 0.48 + H2O 0.92 1.32 CaO 1.13 1.11 1.02 1.20 1.64 1.46 0.98 H2O- 0.07 0.04 Na20 3.83 4.00 4.02 3.73 3.37 4.37 4.39 P2O5 0.18 0.17 K2O 4.54 4.35 4.31 4.43 4.57 3.89 4.70 H O+ 1.02 1.21 0.78 0.87 1.94 1.39 0.90 Total 100.25 100.29 2 H2O- 0.06 0.06 0.06 0.06 0.05 0.07 0.06 P O 0.07 0.09 0.05 0.09 0.07 0.10 Elements (ppm) 2 5 0.08 Total 99.95 99.72 99.82 99.78 100.34 100.21 100.47 B 20 25 Ba 1,300 1,200 Elements (ppm) Co 3 3.5 Cr 32 25 B 27 20 24 21 25 24 25 Cu 30 50 Ba 643 650 690 675 730 677 1,010 La 49 31 Co 3 3 3 3 3 3 4 Mo 10 10 Cr 9 10 16 43 11 10 37 Ni 12 10 Cu 78 45 68 60 63 72 73 V 7 6 La 37 45 43 43 42 38 71 Y 32 15 Mo 10 10 10 10 10 10 10 Zr 320 320 Ni 10 10 10 10 10 10 5 Pb 28 22 V 8 10 5 6 6 11 7 Rb 165 120 Y 27 30 29 26 26 27 48 Sr 210 324 Zr 213 240 210 213 205 228 185 Zn 210 110 Pb 38 30 40 40 50 38 20 Se 5 5 Rb 138 134 154 129 152 98 106 Sr 100 137 133 141 129 124 136 Note: Major-element analyses were made Zn 115 105 105 93 120 107 119 using conventional wet-chemical methods (K. Se 7 Aoki, analyst). Trace-element analyses (Rb, Sr, Pb, and Zn) by atomic absorption spectrometry Note: Major-element analyses were made using conventional wet-chemical methods (K. Aoki, (O. Joensuu, analyst). All other trace elements by analyst). Trace-element analyses (Rb, Sr, Pb, and Zn) by atomic absorption spectrometry (O. Joensuu, optical emission spectrography (O. Joensuu, ana- analyst). All other trace elements by optical emission spectrography (O. Joensuu, analyst). Results lyst). Results for Zr are accurate to ±10%.. Sr for Zr are accurate to ±10%. Sr and Rb are accurate to ±5% of amount present, except for low Sr and Rb are accurate to ±5% of amount present, (less than 20 ppm), which is accurate to ±10% of amount present. except for low Sr (less than 20 ppm), which is ac- * See Figure 3 for locations of samples. curate to ±10% of amount present. Analysis is an average, from samples listed.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/6/875/3429517/i0016-7606-89-6-875.pdf by guest on 26 September 2021 TABLE 7. CHEMICAL ANALYSES FOR GROUP 4 RHYOLITES TABLE 6. CHEMICAL ANALYSES FOR GROUP 3 RHYOLITES AND GRANITES Marcellon units Baraboo units A B D Caledonia Lower Montello Observatory Endeavor Granite at Berlin Utley granite Hill rhyolite rhyolite Redgranite rhyolite rhyolite Church Narrows Sample no.: 107 108 109 110 112 114 Sample no.:* 173 176 174 161 145 175 178 182 147 Major oxides (%) Major oxides (%) «

Si02 75.68 78.25 75.55 76.63 71.99 71.80 73.77 73.83 72.76 Si02 75.83 75.09 73.73 76.14 75.30 75.60 Ti02 0.12 0.11 0.14 0.12 0.26 0.32 0.14 0.16 0.22 Ti02 0.21 0.21 0.29 0.24 0.19 0.17 AI2O3 12.46 11.05 12.21 11.38 13.57 13.74 12.35 13.33 13.34 AI2O3 11.92 12.33 12.09 11.79 12.04 12.59 Fe203 0.68 0.95 0.42 0.97 1.93 1.30 1.01 0.96 1.77 Fe203 1.08 1.08 1.83 1.10 1.04 0.99 FeO 1.09 0.84 1.71 1.13 0.88 1.72 1.81 1.21 0.72 FeO 0.98 1.12 1.34 0.88 1.04 0.71 MnO 0.13 0.07 0.17 0.12 0.11 0.11 0.13 0.07 0.12 MnO 0.07 0.08 0.11 0.02 0.06 0.03 MgO 0.12 0.16 0.29 0.13 0.37 0.58 0.39 0.31 0.18 MgO 0.06 0.04 0.04 0.09 0.04 0.04 CaO 0.36 0.29 0.45 0.62 1.10 0.85 0.94 0.43 0.45 CaO 0.53 0.39 0.36 0.45 0.33 0.13 Na20 3.66 3.66 2.82 3.16 4.34 4.43 3.26 2.51 4.39 Na20 3.43 3.76 3.50 3.16 4.65 4.46 K2O 5.00 3.72 5.16 4.66 4.91 4.49 4.92 5.28 5.25 K2O 5.45 5.58 6.03 5.65 4.63 4.95 + + H2O 0.57 0.70 0.85 0.59 1.20 0.60 1.25 1.43 0.77 H2O 0.54 0.49 0.36 0.58 0.33 0.47 H2O- 0.04 0.09 0.04 0.01 0.08 0.04 0.04 0.05 0.08 H2O- 0.01 0.00 0.01 0.01 0.03 0.02 P2O5 0.07 0.09 0.06 0.06 0.03 0.13 0.07 0.08 0.01 P2O5 0.01 0.01 0.00 0.01 0.00 0.00 Total 99.98 99.98 99.87 99.58 100.77 100.11 100.08 99.65 100.06 Total 100.12 100.18 99.69 100.12 99.68 100.16 lements (ppm) Elements (ppm) B 20 27 B 25 20 20 20 22 22 40 25 35 30 20 22 35 Ba 240 160 180 240 970 1,050 410 1,100 950 Ba 440 410 390 390 115 545 Co 3 3 3 3 3 3 3 5 Co 3 3 3 3 3 3 5 Cr 54 44 62 41 58 Cr 38 23 27 8 13 16 15 58 25 22 Cu 95 160 80 Cu 23 22 26 9 22 25 115 40 105 110 42 45 La 47 50 52 42 La 78 90 120 95 72 65 35 45 90 49 80 Mo 10 10 10 10 10 10 10 10 10 Mo 10 10 10 10 10 10 Ni 5 11 5 10 Ni 10 10 10 10 10 10 5 5 5 5 5 V V 5 5 5 5 5 5 5 5 5 5 8 5 5 3 5 Y 50 42 52 50 60 35 38 31 Y 70 71 85 65 49 49 33 Zr 200 180 200 190 Zr 420 590 550 590 450 480 150 220 160 240 140 Pb 22 19 180 14 22 18 Pb 22 23 22 28 19 28 19 5 25 Rb 131 108 165 130 102 110 Rb 190 152 152 202 117 120 108 205 115 Sr 72 61 61 46 95 176 82 110 62 Sr 25 31 11 21 5 28 Zn 100 130 105 110 102 135 110 35 115 Zn 55 125 115 65 105 95 Se 5 5 5 5 7 7.5 5 5 7 Se 3 3 3 3 3 3 Note: Major-element analyses were made using conventional wet-chemical methods (K. Aoki, Note: Major-element analyses were made using conventional wet-chemical methods (K. Aoki, analyst). Trace-element analyses (Rb, Sr, Pb, and Zn) by atomic absorption spectrometry (O. Joensuu, analyst). Trace-element analyses (Rb, Sr, Pb, and Zn) by atomic absorption spectrometry (O. Joensuu, analyst). All other trace elements by optical emission spectrography (O. Joensuu, analyst). Results for analyst). All other trace elements by optical emission spectrography (O. Joensuu, analyst). Results for Zr are accurate to ±10%. Sr and Rb are accurate to ±5% of the amount present, except for low Sr Zr are accurate to ±10%. Sr and Rb are accurate to ±5% of amount present, except for low Sr (less (less than 20 ppm), which is accurate to ± 10% of amount present. than 20 ppm), which is accurate to ±10% of amount present. * See Figure 5 for locations of samples. Unit C is a group 2 rhyolite but is listed here for comparison.

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group 2 rocks show minor differences in chemistry to group 3 rocks on the basis of Redgranite, and the Denzer Diorite. These chemistry when compared with the major- low CaO, and high K20/Na20, but their rocks complement the data from the three ity of group 2 samples. Marcellon unit C trace-element chemistry (high Ba and Sr, groups and better define the trend. For sample 145 has higher Y (60 ppm) than low Zr) and their mineralogy (high percent- comparison, Nockolds' (1954) average other group 2 rocks (22 to 30 ppm) and is age of plagioclase) show affinities with calc-alkalic basalt, andesite, dacite, and not in the group 2 field in Figure 9, part C. group 2. Also, units A and B at Marcellon rhyolite from the Cascade province and an The other unit C sample (175) has lower are more closely related to group 3 rocks in alkalic trend for rocks of the Scottish High- Rb/Sr (0.63) than group 2 rocks (0.71 to terms of their low CaO and Ba and high lands (Nockolds and Allen, 1954) are also 1.45) and therefore is out of the group 2 K20/Na20 and Y, but they differ from plotted. The trend of the central Wisconsin field in Figure 7. Lastly, Marquette unit C group 3 by having low La and Zr and inliers closely parallels the calc-alkalic Cas- sample 91 and unit B sample 197 are higher higher Sr. Unit D at Marcellon is similar to cade trend. in CaO (2.03% and 1.60%, respectively) group 3 rhyolites in terms of mineralogy, The consanguinity of the l,765-m.y.-old than most group 2 rocks (Fig. 7). Sample high K20/Na20, and low Ba, but it is lower rocks is evidenced by a smooth trend on a 197 also has a lower Rb/Sr (0.64) than most in Ti02 and higher in CaO, MgO, and Sr Rb, Sr, Y plot and by U, Th, and K values. group 2 samples. than group 3. All Marcellon rhyolites ex- Rb, Sr, Y data (Fig. 9 C) show a trend from Granophyric granites at Montello and at cept for unit C belong to chemical group 4. Sr-rich Denzer Diorite (625 ppm) to Sr- Redgranite and porphyritic rhyolite at Ob- It is noteworthy that the Marcellon inlier is depleted (as low as 5 ppm) granites and servatory Hill, Berlin, Utley, and Endeavor the only rhyolite outcrop where flows of porphyritic rhyolites. This trend is similar are remarkably similar in chemistry and more than one chemical group are inter- to plots of other well-documented co- compose the third group (Table 6; Figs. 7, layered. magmatic calc-alkalic suites (Nockolds, 8, and 9 B and C). They are characterized 1954). Each calc-alkalic suite is mutually by high Si02, K20/Na20, La, Zr, Y, and Chemical Relationships Between Groups exclusive, but all show Rb and Y enrich- Rb/Sr and low CaO, A1203, and Ba. Sr con- ment with Sr depletion down the liquid line

centrations as low as 5 ppm are found in On AMF and Na20-K20-Mg0 + CaO of descent. The Fox River valley rhyolites rhyolite from Berlin. plots (Fig. 9 A and B), the four chemical are somewhat distinct from other suites be- The fourth chemical group contains rock groups described above fall along a trend cause of their Sr depletion and degree of Y types that are transitional in chemistry be- typical of a rock series showing calc-alkalic enrichment. tween groups 2 and 3 (Table 7; Figs. 7, 8, affinities. Also plotted in parts A and B of Th/U and K/U ratios for porphyritic and and 9 B and C). For example, rhyolite ex- Figure 9 are dike rocks at Marquette and fine-grained rhyolites and for granophyric posed on the flanks of the Baraboo syncline, at the Lower Narrows, and at the Cal- edonia Church locality (Fig. 2), is similar in 4r

\ / \ CL 1 a \ o 1 w \ O 3 •1 1« • I I I

100 200 300 400 500 600 30 100 150 30 40 100 150 200 250 Zr Cr Cu Pb Zn /

1 * AO I • V \ 42

Q. a o w O 2.4 CaO Figure 7. Rhyolites and granites in Fox River

Valley and Baraboo area can be divided into four 100 200 150 200 50 70 groups on basis of this CaO-Rb/Sr plot. Letters A Ba La Sr Rb Y through D represent Marcellon rhyolite units. BB is Baraboo rhyolite. Unlabeled data point at (ppm) about 2% CaO is Marquette unit C sample 91 Figure 8. Some element concentrations for the four chemical groups. These elements as well as (discussed in text). total iron (see text) can be used to chemically fingerprint each Marcellon unit.

granites are relatively constant (Table 8). TABLE 8. U, TH, AND K CONCENTRATIONS FOR FOX RIVER VALLEY However, one sample of group 1 granite AND BARABOO RHYOLITES AND GRANITES from Baxter Hollow shows lower K/U and Sample no. U Th K K/U Th/U higher Th/U than the Fox River valley (ppm) (ppm) (%) (xlO4) rhyolites and granites. Relatively constant K/U and Th/U ratios are commonly charac- Utley rhyolite GL-1 6.8 14.6 4.2 0.62 2.15 0.46 teristic of a comagmatic sequence, even Montello granite GL-2 10.6 24.5 4.9 2.31 Montello granite GL-4 9.4 24.9 4.7 0.50 2.64 though K, U, and Th content and Th/U Baxter Hollow granite GL-3 3.5 6.5 3.2 0.91 1.86 ratios may differ in wet and dry phases of Marquette rhyolite GL-5 6.7 14.7 3.8 0.57 2.19 the same rock (Smithson and Decker, Note: Analyses by R F. Roy and James Lee by gamma ray spectrometry. 1973). The Sr-depletion characteristic of some of the porphyritic rhyolites has also been ob- CaO • MgO

K20 Na2CK

Figure 9. A. A(Na20 + K20), M(MgO), F(FeO + 0.9 Fe203) plot, showing calc-alkalic affinities for l,765-m.y.-old volcanic and plutonic rocks. Solid circles = rhyolites, solid triangles = dike rocks, solid squares = granites and diorite. For comparison, also plotted are Nockolds' (1954) average calc-alkalic suite (open circles), and alkalic trend for Scottish Highlands (Nockolds and Allen, 1954) (open triangles). B. CaO + MgO, Na20, K20 plot showing that the four chemical groups fall along trend characteristic of rock suite showing calc-alkalic affinities. Dashed line = alkalic trend (Nockolds and Allen, 1954). Dot and dash line = average calc-alkalic trend (Nockolds, 1954). Solid line = trend of central Wisconsin rocks. Lettered data points are described in caption of Figure 6. C. Rb, Sr, Y plot showing that the four chemical groups fall along a smooth trend, suggesting that they are comagmatic. Note that chemical groups can be easily distinguished in this diagram. For comparison, calc-alkalic suites from Lassen Volcanic National Park (dashed line) and Medicine Lake Highlands (dot and dash line) are plotted (Nockolds, 1954). Lettered data points are described in caption of Figure 6. D. Composition of Baraboo and Fox River valley rocks in terms of Niggli normative quartz, albite, and orthoclase. Boundary curve for pH20 = 1,000 b is plotted for comparison.

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served in rhyolites from Mono Glass Moun- below the thermal minimum at pH20 = roughly N50°E and lies parallel to major tain, California, and is explained by 1,000, and extend along the thermal trough structural trends in exposures close to the feldspar fractionation (Noble and others, toward the Ab-Or side (Fig. 9, D). In con- contact. Southeast of the porphyritic rhyo- 1972). Since the porphyritic rhyolites (with trast, mesonorms of specimens from the lite are the texturally variable rhyolites of the exception of unit G at Marquette) and anorogenic Wolf River batholith are sig- groups 2 and 4. The projected contact be- the granophyric granites are depleted in nificantly displaced toward the Or apex tween the porphyritic and texturally vari- plagioclase (<1% of total phenocrysts) (Van Schmus and others, 1975a). able rhyolites trends N50°E, then curves to relative to the fine-grained rhyolites and the southeast between the Utley porphyritic granites (20% to 100% of total pheno- Geographic Significance of rhyolite (group 3) and the Marquette expo- crysts), plagioclase fractionation may be a Chemical Trends sure. Again, structural trends in the expo- plausible agent of differentiation for these sures near the contact reflect the trend of rocks. The distribution of the chemical groups the contact between chemical groups. At To display the normative composition of shows well-defined geographic trends (Fig. Marcellon and Marquette, fold axes strike the rhyolites and granites, Niggli molecular 10). Group 3 granites lie to the northwest of to the northeast, and at Utley, the dominant norms were calculated. Molecular norms group 3 porphyritic rhyolites. The projected orientation is N60°W (Fig. 10). for the rhyolites and granites cluster just contact between the two rock types strikes The chemical trends indicated by relating

F Vh'i.'tii.'!.ÒREO GRANITE

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Figure 10. Map of Fox River valley-Baraboo area, showing geographic distribution of chemical groups. Dashed lines = inferred contacts between chemical groups. Since these contacts parallel structures in exposures, patterns on this map probably reflect geology of buried Precambrian basement in this area. Foliation symbol indicates trend of banding in rhyolites; fold symbols indicate mean direction of axial plane traces; and shaded areas are outcrops of Baraboo Quartzite.

areas with similar chemistry and the struc- FLOW-DIRECTION STUDIES Field relationships, geochronology, and tural data provide insight into die unex- chemistry suggest the following history for posed geology of this area. These data Flow-direction determination using mi- the l,765-m.y.-old felsic rocks. suggest that the rock types exposed within croscopic fluidal textures (Elston and 1. Rhyolite flows and ash-flow tuffs the inliers continue for a lateral distance of Smith, 1971; Smith and Rhodes, 1972) erupted from a source to the northwest of at least 100 km and indicate in a general were made for rhyolite flows and ash-flow the present location of the inliers. Three way locations of subsurface contacts be- tuffs in each inlier. Because primary tex- chemical groups of flows were erupted dur- tween rock types. tures are well preserved in most of the ing this episode. At Marcellon, group 2 flows, both lineation and azimuth can be flows (unit C) lie between units that belong Relation to Penokean Orogeny and to successfully determined after correcting for to group 4. Granophyric granites probably Post-Penokean Intrusions tilt by rotating the bed about a surface- represent subvolcanic equivalents of the parallel axis. No additional corrections rhyolites. Sims (1976) stated that granite in- Volcanic and plutonic rocks of the cen- were attempted. Flow-direction determina- trudes porphyritic rhyolite in the Fox River tral Wisconsin complex (Fig. 1) probably tions show common northwest to southeast Valley. However, the relative age of the two formed during the major phase of the Peno- flow for all of the rhyolite inliers (Fig. 11), rock types cannot be directly determined kean orogeny and are dated at 1,850 to and, in detail, each of the Marquette units because critical contacts are not exposed. 1,900 m.y. B.P. (Van Schmus, 1976; Banks shows the same sense of flow. These flow- 2. Basalt and andesite dikes were in- and Cain, 1969). These rocks show strong direction data suggest that the eruptive cen- truded into the rhyolites and granophyric calc-alkalic affinities (Anderson and others, ter for these flows lies to the northwest at an granites. Some of these dikes may have been 1975). The anorogenic Wolf River unknown distance. Common flow-direction contaminated during emplacement. The batholith (1,500 m.y. old) in northeastern measurements support the conclusion, de- granite at Redgranite is intruded by a thick Wisconsin shows alkalic affinities and was rived from chemical data, that the rhyolites granite porphyry dike. intruded during regional extension (Ander- of the inliers erupted from a common 3. Fine-grained granites and rhyolite son and others, 1975). As shown above, the source. dikes belonging to chemical group I were Fox River Valley rocks are calc-alkalic in intruded into rhyolites at, respectively, Bax- nature. Because the calc-alkalic series is SUMMARY AND ter Hollow and Observatory Hill. characteristic of orogenic belts (Miyashiro, GEOLOGIC HISTORY 4. In the Baraboo area, and perhaps re- 1975) and because the peak of the Peno- gionally, extensive sedimentation occurred, kean orogeny occurred at about 1,850 to The rhyolites and granites of the central forming the Baraboo and Waterloo sand- 1,900 m.y. ago (Van Schmus and others, Wisconsin inliers are divided into six im- stones and overlying Precambrian sedimen- 1975b), the l,765-m.y.-old rocks probably portant lithological types and into four tary rocks (Dalziel and Dott, 1972). formed during the waning stages of the chemical groups. Rocks of these groups are 5. Extensive folding of the area fol- Penokean orogeny. This conclusion is con- comagmatic and show calc-alkalic affini- lowed. During this event, the rhyolites of sistent with the suggestion of Anderson and ties; these rhyolites and granites probably the inliers and overlying sedimentary rocks others (1975) that these granites and rhyo- formed during the waning stages of the were folded and metamorphosed. Rhyolites lites represent melting of a tectonically Penokean orogeny. Geochemical correla- were locally sheared, and local low-grade thickened crust during the last stages of the tion was an important tool for relating metamorphism occurred, but the rhyolites Penokean orogeny, but, as yet, isotopic data flows within inliers and for identifying re- remained largely unaffected mineralogically are insufficient to confirm this hypothesis. lated exposures. by this event. Van Schmus and others (1975b) reported an event at 1,650 m.y. B.P. that reset Rb-Sr dates and may be re- <9 lated to this deformation. KILOMETERS 10 20 30

t ACKNOWLEDGMENTS Montello Field work and chemical studies were partially funded by two University of Wis- Figure 11. Flow-di- consin — Parkside summer research grants rection azimuths and liid- and a Faculty Fund grant. Earlier versions eations for l,765-m.y.-old rhyolites. Source for these of this paper were read by M. J. Mudrey, rocks lies to northwest of Jr., W. R. Van Schmus, C. E. Dutton, and Fox River valley. G. L. LaBerge; their comments helped considerably in improving the manuscript. EXPLANATION D. M. Pyper made many helpful editorial ^ Flow Azimuth corrections. I thank R. F. Roy, for allowing Beaver ^ Flow Lineation Dam the use of unpublished U and Th data. I also Direction thank the following students for assistance: Precambrian Barbara Burke, Russell Gifford, James Inliers Grimes, David Hartlaub, Glen Kilberg, Jill Ann Hartnell, and Raymond Spanjers.

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