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Comment-Reply Medaris-VanWyck Jour Geol 2005

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DISCUSSION AND REPLYDetrital Zircon Ages from Early Proterozoic Quartzites, Wisconsin,Support Rapid Weathering and Deposition of MatureQuartz Arenites: A DiscussionL. G. Medaris Jr., B. S. Singer, R. H. Dott Jr., and C. M. JohnsonDepartment of Geology and Geophysics, University of Wisconsin–Madison, Madison, Wisconsin 53706, U.S.A.(e-mail: medaris@geology.wisc.edu)IntroductionQuartzites of diverse ages and characteristics are is a member of the post-Penokean, Geon 17 graniteimportant features of the Precambrian basement in and rhyolite suite in the Lake Superior region,the Lake Superior region. Among these, the super- while the Baraboo Quartzite is nonconformable onmature quartzites of the Baraboo interval, which post-Penokean granite (1752 15 Ma) and rhyolitewere deposited between 1750 and 1630 Ma, record (1754 44 Ma; Van Wyck 1995; Medaris et al.a major episode of extensive chemical weathering 2003). Such relations clearly preclude correlationand siliciclastic sedimentation on a tectonically of the older Hamilton Mounds Quartzite with thestable plate margin (Medaris et al. 2003). Van Wyck younger Baraboo Quartzite and prohibit its inclu-and Norman (2004) have provided a large data set sion in the interval (1750–1630 Ma).on the ages of detrital zircons from two Proterozoic Further distinction between the two quartzitesquartzites in central Wisconsin: the Baraboo is provided by differences in their detrital zirconQuartzite, from which the Baraboo ...
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D I S C U S S I O NA N DR E P LY Detrital Zircon Ages from Early Proterozoic Quartzites, Wisconsin, Support Rapid Weathering and Deposition of Mature Quartz Arenites: A Discussion
L. G. Medaris Jr., B. S. Singer, R. H. Dott Jr., and C. M. Johnson
Department of Geology and Geophysics, University of Wisconsin–Madison, Madison, Wisconsin 53706, U.S.A. (email: medaris@geology.wisc.edu)
Introduction
Quartzites of diverse ages and characteristics are important features of the Precambrian basement in the Lake Superior region. Among these, the super mature quartzites of the Baraboo interval, which were deposited between 1750 and 1630 Ma, record a major episode of extensive chemical weathering and siliciclastic sedimentation on a tectonically stable plate margin (Medaris et al. 2003). Van Wyck and Norman (2004) have provided a large data set on the ages of detrital zircons from two Proterozoic quartzites in central Wisconsin: the Baraboo Quartzite, from which the Baraboo interval was named, and the Hamilton Mounds Quartzite. Al though the zircon age data provide useful insights into the provenance of the two quartzites, certain conclusions by Van Wyck and Norman regarding relations between Proterozoic magmatism and sed imentation do not agree with existing stratigraphic and geochronologic evidence. Their conclusions re quire discussion and clarification.
Quartzite Miscorrelation Regarding the Hamilton Mounds and Baraboo quartzites, Van Wyck and Norman (2004, p. 313) conclude that “The quartzites are, therefore, post Penokean in age, supporting their inclusion in the Baraboo interval,” thereby correlating the two quartzites and placing both in the Baraboo interval. However, a postPenokean age, that is, post1830 Ma, is insufficient by itself for assignment of the Hamilton Mounds Quartzite to the Baraboo inter val. The Hamilton Mounds Quartzite is intruded by 17637Ma granite (LaBerge et al. 1991), which
Manuscript received July 6, 2004.
is a member of the postPenokean, Geon 17 granite and rhyolite suite in the Lake Superior region, while the Baraboo Quartzite is nonconformable on postPenokean granite (175215 Ma) and rhyolite (175444 Ma; Van Wyck 1995; Medaris et al. 2003). Such relations clearly preclude correlation of the older Hamilton Mounds Quartzite with the younger Baraboo Quartzite and prohibit its inclu sion in the Baraboo interval (1750–1630 Ma). Further distinction between the two quartzites is provided by differences in their detrital zircon age populations, as demonstrated by Van Wyck and Norman (2004). With respect to the Baraboo Quartzite, however, Van Wyck and Norman group their youngest zircon ages at 175429 Ma, al though the youngest five grains, for example, have 207 206 Pb Pbages of 1716, 1721, 1723, 1724, and 1735 Ma, which are 95.9% to 99.6% concordant (based 206 238207 206 on comparison ofPb Uand PbPb ages). 207 206 These results compare with thePb/ Pbages for the two youngest grains that have moderate to low U contents of 1712 and 1715 Ma, reported by us (Medaris et al. 2003), which are 89.8% and 91.8% concordant, respectively, based on comparison of 206 238207 206 the PbU andPb Pbages. It seems mis leading to us for Van Wyck and Norman to group the younger ages and ignore the ages of nearly con cordant individual grains; such an approach tends to obscure the difference in ages of the youngest detrital zircon grains between the Baraboo and Hamilton Mounds quartzites. Miscorrelation of the two quartzites led Van Wyck and Norman (2004, p. 313) to the conclusion that “The quartzites are deposited on and crosscut by postPenokean suite magmatic rocks, indicating that deposition and magmatism overlapped in
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time.” Although such a conclusion may be valid for the Hamilton Mounds Quartzite, which con tains a “young” detrital zircon suite of 177225 Ma, it is incorrect for the Baraboo Quartzite and other Baraboo interval sedimentary rocks, which were deposited after cessation of Geon 17 mag matism on mature paleosols (Medaris et al. 2003).
Definition and Significance of the Baraboo Interval The term “Baraboo interval” originally was intro duced by Dott (1983) to encompass the sequence of sedimentation, deformation, and metamorphism in the southern Lake Superior region between 1750 and 1450 Ma. However, we subsequently recog nized that it is preferable to restrict the term to the episode of weathering and sedimentation between 1750 and 1630 Ma in the Lake Superior region,
thereby redefining the Baraboo interval as a strat igraphic term and excluding disparate tectono metamorphic events (Medaris et al. 2003). It is now well established that the Baraboo and correlative Barron, Flambeau, McCaslin, Sioux, and Waterloo quartzites record a distinctive episode of advanced chemical weathering and deposition of superma ture siliciclastic sediments in a passive margin set ting. Timing of Baraboo interval sedimentation is well constrained between 1750 Ma, the age of the youngest nonconformably underlying basement, and 1630 Ma, the apparent time of quartzite folding and metamorphism (Holm et al. 1998). Other quartzites of diverse ages and characteristics occur in Wisconsin and adjoining states, and attempts to place them all in the Baraboo interval lead to se rious misinterpretations of the Proterozoic evolu tion of the Lake Superior region and failure to rec ognize the true significance of the Baraboo interval.
R E F E R E N C E SC I T E D
Dott, R. H., Jr. 1983. The Proterozoic red quartzite enigma in the northcentral U.S.: resolved by plate collision? Geol. Soc. Am. Mem. 160:129–141. Holm, D.; Schneider, D.; and Coath, C. D. 1998. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: implications for extent of foreland deformation during final assembly of Lau rentia. Geology 26:907–910. LaBerge, G. L.; Klasner, J. S.; and Myers, P. E. 1991. New observations on the age and structure of Proterozoic quartzites in Wisconsin. U.S. Geol. Surv. Bull. 1904B: B1–B18. Medaris, L. G., Jr.; Singer, B. S.; Dott, R. H., Jr.; Naymark, A.; Johnson, C. M.; and Schott, R. C. 2003. Late Pa leoproterozoic climate, tectonics, and metamorphism
in the southern Lake Superior region and protoNorth America: evidence from Baraboo interval quartzites. J. Geol. 111:243–257. Van Wyck, N. 1995. Oxygen and carbon isotopic con straints on the development of eclogites, Holsnoy, Norway and major and trace element, common Pb, SmNd, and zircon geochronology constraints on pet rogenesis and tectonic setting of pre and Early Pro terozoic rocks in Wisconsin. PhD thesis, University of Wisconsin–Madison, 280 p. Van Wyck, N., and Norman, M. 2004. Detrital zircon ages from Early Proterzoic quartzites, Wisconsin, sup port rapid weathering and deposition of mature quartz arenites. J. Geol. 112:305–315.
Detrital Zircon Support
Ages from Early Proterozoic Quartzites, Wisconsin, Rapid Weathering and Deposition of Mature Quartz Arenites: A Reply
1 Nicholas Van Wyck and Marc Norman
Sisyphus Consulting, Anchorage, Alaska 99503, U.S.A. (email: nvw@sisyphusconsulting.com)
Medaris et al. (2005) suggest that the Hamilton Mounds and Baraboo Quartzites are miscorrelated and should not be included together in the Baraboo interval. We fully recognize and stated in Van Wyck and Norman (2004) that the Hamilton Mounds are truncated by a 1763Ma intrusive rock, and the Bar aboo Quartzite contains detrital zircons with an age of 1754 Ma; consequently, the Baraboo is younger than the quartzite at Hamilton Mounds. We have never disputed this observation and in fact use this in our model of postPenokean sedimentation. Our point is that postPenokean intrusive rocks (what we refer to in our article as the PPCR suite) are found as both inclusions and crosscutting quartz ites, and this fact says something important about sedimentation models for the quartzites, although clearly it is at odds with earlier models by Medaris et al. (2003). Based on the original definition of the Baraboo interval (Dott 1983), our correlation of the Baraboo and Hamilton Mounds quartzites is completely ac ceptable. The original definition of the Baraboo in terval included those quartzites postdating the Pen okean orogeny, and it recognized that the quartzites were not strictly correlative. Although Van Wyck and Norman (2004) clearly demonstrated the post Penokean nature of both sandstones, Medaris et al. (2005) object to the inclusion of the Hamilton Mounds unit in the Baraboo interval. While they argue for strict temporal limits to correlating Ham ilton Mounds and Baraboo, they also correlate Bar aboo with quartzites at Barron, Flambeau, Mc Caslin, Sioux, and Waterloo, although they do not document the strict temporal equivalence of all of these quartzites.
Manuscript received August 9, 2004. 1 Research School of Earth Sciences, Australian National University, Canberra, Australian Capital Territory 0200, Australia.
The second point Medaris et al. (2005) raise is their discussion of the degree of discordance of zir con analyses. We recognize that measuring detrital zircon ages to constrain the maximum depositional age relies heavily on defining the youngest popu 207 206 lation. In Medaris et al. (2003)Pb/ Pbages rang ing from 1692 to 1715 Ma were presented. These ages were from discordant zircons and imply a sim ple leadloss model and that ages of 1692 to 1715 Ma are the age of the youngest zircon components. Van Wyck and Norman (2004) presented additional, less discordant data to support an alternative in terpretation, that the young discordant analyses represent scatter from a single population with an age identical to the Baxter Hollow Granite. We ac knowledge this is an important point, one that is in need of further study. The basis of our correlation of sediments such as Baraboo and Hamilton Mounds is not one of strict age equivalence but of tectonic equivalence. PostPenokean magmatism ranged in age from 1805 Ma to 1750 Ma (Van Schmus et al. 2001), and Van Wyck and Norman (2004) demonstrated that quartzite sedimentation was coeval with magmatism in the case of Ham ilton Mounds. We chose to place Baraboo deposi tion close to the timing of magmatism because we can demonstrate the same relation at Hamilton Mounds. Medaris et al. (2005) object to this because it does not fit with their model of deposition on a stable platform with weathering and paleosol for mation. Neither Medaris et al. (2003) nor Van Wyck and Norman (2004) have definitive evidence yet to prove their case. Medaris et al. (2003) did not ex plain where their zircons with ages from 1692 to 1715 Ma come from or what they represent. Their model requires sedimentation on a stable platform, so if these zircons do indeed represent primary crys tallization ages, exactly what is generating the mag matism to produce these zircons, and how did they
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get into the Baraboo depocenter? We believe that application of their model would logically interpret these zircons as we do, namely as weathered and discordant zircons derived from the youngest PPCR magmatism at 1750 Ma. Much of the discussion in Medaris et al. (2003) revolves around the extreme degree of weathering of the Baraboo Quartzite characterized by their high chemical alteration index values. They interpret these values as indicating a long period of weath ering and that this may represent microbial soil stabilization (Dott 2003), yet nothing in their data precludes the interpretation that the high chemical alteration index values are due to postdeposition alteration. Nor indeed do the data require that this took a long period of time. There has been no com pelling study documenting when the Baraboo Quartzite was leached of its alkalis; Medaris et al. (2003) suggest some alkalis were added after 1.4 Ga but present no data to prove they were removed prior to 1.4 Ga. Could the difficulty of producing 40 39 Rb/Sr and Ar/Ar datesolder than 1.45 Ga from Baraboo be related to this? Quartzites containing aluminumrich phyllosilicates appear elsewhere in the geologic record; consider the following quote from Phillips and Law (1997, p. 28) on the com
position of Witwatersrand sediments: “This areally extensive alteration has generated unusual alumi nous bulk compositions within several rock types that show element correlations (uniform ratios be tween Al, Ti, Nb, and rare earth elements) and min eralogical and whole rock variations all incompat ible with the type and scale of depositional changes expected in finegrained clastic rocks.” Interest ingly, earlier workers in the Witwatersrand inter preted these too as paleosols, but later study showed paleosols truncating stratigraphy (Philips et al. 2001). In summary, the issues of chemical maturity and quartzite composition raised by Medaris et al. (2005) are arguable, and we hope they will be the subject of further scrutiny. Possible topics include examination of how quickly an immature quartzite can be converted into a quartz arenite via postde positional removal of feldspar and lithic compo nents and whether this process is essentially a dia genetic (Cox et al. 2002) or a tectonic/metamorphic event. Finally, we hope future research will shed light on the source and means of transport of the enigmatic 1.9 to 2.4Ga detrital zircons found in the Baraboo Quartzite (Van Wyck and Norman 2004).
R E F E R E N C E SC I T E D
Cox, R.; Martin, M. W.; Comstock, J. C.; Dickerson, L. S.; Ekstrom, I. L.; and Sammons, J. H. 2002. Sedimen tology, stratigraphy, and geochronology of the Prote rozoic Mazatzal Group, central Arizona. Bull. Geol. Soc. Am. 114:1535–1549. Dott, R. H., Jr. 1983. The Proterozoic red quartzite enigma in the northcentral U.S. resolved by plate col lision? Geol. Soc. Am. Mem. 160:129–141. ———. 2003. The importance of eolian abrasion in su permature quartz sandstones and the paradox of weathering on vegetationfree landscapes. J. Geol. 111: 387–405. Medaris, L. G., Jr.; Singer, B. S.; Dott, R. H., Jr.; and John son, C. M. 2005. Detrital zircon ages from Early Pro terozoic quartzites, Wisconsin, support rapid weath ering and deposition of mature quartz arenites: a discussion. J. Geol. 113:233–234. Medaris, L. G., Jr.; Singer, B. S.; Dott, R. H., Jr.; Naymark, A.; Johnson, C. M.; and Schott, R. C. 2003. Late Pa leoproterozoic climate, tectonics, and metamorphism
in the southern Lake Superior region and protoNorth America: evidence from Baraboo interval quartzites. J. Geol. 111:243–257. Phillips, G. N., and Law, J. D. M. 1997. Hydrothermal origin for Witwatersrand gold. Soc. Econ. Geol. Newsl. 31:26–33. Phillips, G. N.; Law, J. D. M.; and Myers, R. E. 2001. Is the redox state of the Archean atmosphere con strained? Soc. Econ. Geol. Newsl. 47:1–18. Van Schmus, W. R.; MacNeill, L. C.; Holm, D. K.; and Boerboom, T. J. 2001. New UPb ages from Minnesota, Michigan and Wisconsin: implications for Late Paleo proterozoic crustal stabilization.InInstitute on Lake th Superior Proceedings, 47annual meeting (Madison, Wis.), pt. 1, p. 100–101. Van Wyck, N., and Norman, M. 2004. Detrital zircon ages from Early Proterzoic quartzites, Wisconsin, sup port rapid weathering and deposition of mature quartz arenites. J. Geol. 112:305–315.
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