Geochronology of Archean and Proterozoic Rocks in ...

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CAN. J. EARTH SCI. VOL. 12, 1975

thesis. It may be-that the approach of Dkulynski (1965) where overloading causes reversed density gradients and subsequent readjustments including injection, should be explored further. Clearly the freezing concept has attractions in its simplicity as can be seen in the discussion of the Chaudikre example by Dionne and Shilts. But is it not significant that the freezing mechanism would probably not have been rejected without the strong regional data that sedimentation was related to glacio-lacustrine conditions? However, in conclusion, perhaps the judicious words of Smalley and Vita-Finzi (1969) should be recalled ". . . there are dangers involved in urging the abandonment of a concept; subsequently events may confer validity on the most outrageous theory" (p. 1593).

DIONNE,J. C. and SHILTS,W. W. 1974. A Pleistocene clastic dike, upper Chaudikre valley, Quebec. Can. J. Earth Sci. 11,pp. 1594-1605. DREIMANIS, A. Till wedges as indicators of direction of glacial movement. Geol. Soc. Am., Abstr. Prog. 7, pp. 52-53. D~ULYNSK S.I1965. , Experiments on clastic wedges. Bull. Acad. Pol. Sci., Ser. GOol. Geogr. 13, pp. 301-303. MORNER,N-A. 1972. The first report on till wedges in Europe and Late-Weichselian ice flows over southern Sweden. Geol. Foren. Forh. 94, pp. 581-587. 1973. New find of till wedges in Nova Scotia, Canada. Geol. Foren. Forh. 95, pp. 272-273. I. J. and VITA-FINZI,C. 1969. The concept of SMALLEY, "system" in the earth sciences, particularly geomorphology. Bull. Geol. Soc. Am. 80, pp. 1591-1594. WORSLEY, P. 1973. The first report on "till wedges" in Europe. A discussion. Geol. Foren. Forh. 95, pp. 152-155. 1974. Further discussion of the 'till wedge' concept in the light of replies from Professor Dreimanis and Dr. Momer. Geol. Foren. Forh. 96, pp. 279-282.

Geochronology of Archean and Proterozoic Rocks in the Southern District of Keewatin: Discussion GRANTM. YOUNG Department of Geology, University of Western Ontario, London, Ontario N6A 3K7 Received March 3, 1975 Accepted for publication April 9, 1975

In their recent paper on the geochronology of Precambrian rocks in the southern District of Keewatin, Wanless and Eade (1975) concluded that the supracrustal rocks of the Hurwitz Group were deposited about 1800 m.y. ago. As a corollary to this conclusion they stated that their results showed a proposed correlation between the Hurwitz and part of the Huronian succession (Young 1973) to be untenable. Pyritic oligomictic quartz pebble conglomerates occur in the Montgomery Lake Group, which underlies the Hurwitz Group. Roscoe (1969, 1973) suggested that such deposits might reflect an essentially anoxygenic atmosphere and might be useful as a time marker designating Precambrian rocks older than about 2300 m.y. Based on this concept, together with data from Rb-Sr isotopic analyses from quartz latite of the Hurwitz Group and K-Ar data from an altered gabbro, Wanless and Eade concluded that there was a 300 to 400 million year gap between Can. J. EarthSci., 12, 1250-1254(1975)

deposition of the Montgomery Lake and Hurwitz Groups. The purposes of this note are to comment on the validity of the interpretation by Wanless and Eade of their isotopic data, to present stratigraphic evidence opposed to the idea of a large hiatus between the deposition of Montgomery Lake and Hurwitz Groups, and to reaffirm the geological evidence in favor of a HurwitzHuronian correlation. The radiometric age determinations reported by Wanless and Eade were derived from rocks that have undergone folding and metamorphism. Metamorphic andalusite and biotite are common in sedimentary rocks of the Hurwitz fold belt. The volcanic rocks of the Hurwitz were described by Wanless and Eade (p. 105) as having "plagioclase that is highly altered". An analyzed gabbro was described (p. 106) as consisting of "amphibole and chlorite derived from pyroxene that is present as rare relicts, and


JSSIONS

highly altered plagioclase ...." On page 108 they state that the Hudsonian orogeny in this part of the Churchill Structural Province is characterized by pervasive deformation, accompanied by some dynamic metamorphism and low grade regional metamorphism. In this pervasively deformed region amphibolites and altered quartz latites have provided dates ranging from 1815 f 116 m.y. to 1675 111 m.y. It is well known that K-Ar ages are typically much younger than the 'real' ages of the rocks and may commonly indicate a time of late cooling in areas that have undergone metamorphism (Harper 1967), so that even the 1815 m.y. K-Ar date obtained by Wanless and Eade from a gabbro may be a minimum age. Age determinations based on Rb-Sr isotopes are commonly considered to be more 'reliable' than K-Ar dates. However, there are numerous cases where Rb-Sr isochrons have yielded ages demonstrably younger than the time of formation of the rocks. For example, Goldich (1973, p. 1131) plotted Rb-Sr ages for Middle Precambrian rocks of parts of the Canadian Shield and pointed out that of fifteen whole-rock isochrons only two could be safely considered as original ages. Rb-Sr isochrons for Huronian rocks from areas of metamorphic grade comparable to the Hurwitz rocks gave ages similar to those considered by .Wanless and Eade as indicative of the time of formation, and are equally unreliable. Goldich (1973, p. 1132) also brought up the interesting possibility (previously suggested by Young (1966) and Church and Young (1970)) that not only the Huronian, but also the Animikie of the Lake Superior region may be older than 2.0 b.y. despite the younger ages typically found by isotopic analysis, including Rb-Sr isochrons. Studies by Jackson and Taylor (1972) led them to conclude that in the northern part of the Canadian Shield Aphebian supracrustal successions, considered by them to be correlative with the Hurwitz Group, underwent a midAphebian orogeny some 2.0 to 2.2 b.y. ago. If this interpretation is correct it would place the Hurwitz Group in the same age bracket (early Aphebian) as the Huronian. These data conflict with those reported by Wanless and Eade yet appear to have been ignored by them. The presence of hematite in the Hurwitz Group sediments in contrast to pyritic oligomic-

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tic quartz pebble conglomerates of the Montgomery Lake Group was suggested by Wanless and Eade to be one of the lines of evidence favoring a significant (300 to 400 m.y.) break between sedimentation of the two groups. However, precisely the same contrast in sediment types is present in the Huronian sequence, and indeed the idea of a change in atmospheric conditions was conceived by Roscoe (1969, 1973) as being evidenced within the Huronian succession. A significant hiatus has not been found necessary to explain this association in the Huronian. Why is it necessary in the case of the Montgomery Lake and Hurwitz Goups where essentially the same rock types are involved? The association of these unusual rock types does not provide grounds for separation of the Montgomery Lake and Hurwitz Groups, but rather strengthens comparison and correlation with the Huronian. The nature of the boundary between the Montgomery Lake and Hurwitz Groups is uncertain. Even if the Montgomery Lake Group should prove to be significantly older than the Hurwitz (there is no theoretical reason why such pyriterich conglomerates should not have been formed very early in Proterozoic, or even in late Archean time) this in no way affects the Hurwitz - upper Huronian correlation suggested by Young (1973). However, the following points are presented tentatively as evidence against a long time period between sedimentation of the two groups: 1. In many places there is no significant discordance. 2. The two groups are generally found together, suggesting deposition in the same basin(s ?). 3. Indurated clasts of Montgomery Lake Group rocks have not been positively identified in Hurwitz rocks. 4. Where the boundary between the two groups is exposed east of Henik Lakes the writer has observed 'clasts' of Montgomery Lake Group quartzite with diffuse boundaries, producing a mixed rock type in the basal part of the Padlei Formation. These relationships are very similar to those between the Mississagi and Bruce Formations of the northwestern part of the Huronian outcrop belt and may be explained by incorporation of lumps of frozen(?) sand into a till. In one instance clasts of quartz in a mixtite of the Padlei Formation are pressed into the

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WYOMIU


FRENCH SLATE

TOWNER GNSTDNE

HURONIAN

NASH FORK

GORDON LAKE

L , GOWGANDA

HEADQUARTERS

MONTGOMERY L A

-------- --________

St

STROMATOLlTlC DOLOSTONE

Fe

IRON

f~ A1 ti

FUCHSITE-RICH QUARTZITE

FM. AND HEMATITIC

KAOLINITE,ANDALUSITE.

QTZITE.

KYANITE Q T Z I T E .

TILLITE

Ur,py URANIFEROUS,PYRITIFEROUS

QUARTZ PEBBLE CG-

ARCHEAN

FIG.1. Schematic representation of maximum stratigraphic thicknesses from three widely separated areas of Aphebian supracrustal rocks. In all three areas the succession beginning with tillite (Headquarters Schist, lower Padlei, and lower Gowganda) is more extensive than that below. The dotted lines between the columns show the proposed correlations.


DISCUSSIONS

underlying quartzites of the Montgomery Lake Group, suggesting that the latter were unconsolidated at the time of deposition of the Padlei. Lack of consolidation does not prove that there was no significant gap, but is suggestive. 5. In some places quartzites of the Montgomery Lake Group are contorted close to the contact with the Padlei Formation. These contortions die out downwards such that the relationships suggest 'soft sediment' deformation in response to the depositional process (ice transport?) involved in the formation of the Padlei. 6. Stratigraphic relationships between the Montgomery Lake Group and Padlei Formation are similar to those between the Padlei and Kinga Formations of the Hurwitz Group in that the underlying unit in each case is overstepped by the younger, which commonly rests on the Archean basement. In the latter case no significant hiatus has been found necessary. 7. The rock types and stratigraphic relationships of the Montgomery Lake and Hurwitz Groups are similar to those between the lower part of the Huronian sequence and the Gowganda Formation in more northerly parts of the Huronian outcrop belt. In the case of the Huronian this has not prompted suggestions of a 300 to 400 m.y. hiatus. 8. Occurrence together of extensive polymictic conglomerates (mixtites) and laminated argillites with rafted clasts is fairly good evidence of frigid conditions. Such evidence is present in the Padlei Formation. Polymictic conglomerates of similar aspect occur in the Montgomery Lake Group. If these mixtites should also prove to be tillites then it would lend support to the idea that there is no significant time break between the Montgomery Lake and Hurwitz Groups glaciations are relatively uncommon in the Precambrian stratigraphic record. This would also provide another point of similarity between the Montgomery Lake Group and the Lower Huronian where two pre-Gowganda mixtites (Ramsay Lake and Bruce Formations) are considered to be glacial deposits. Perhaps the most compelling argument for correlation of the Huronian and the HurwitzMontgomery Lake sequences is the strikingly similar successions of unusual rock types in these widely separated regions. Some of these similarities are schematically shown in Fig. 1, which illustrates not only the close comparison possible

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between the Huronian and Hurwitz-Montgomery Lake, but also the surprisingly similar Aphebian succession of southeastern Wyoming. While it is realized that lithostratigraphic correlation has led to many wrong conclusions, the unusual homotaxial succession, involving uraniferous pyritic quartz pebble conglomerates overlain by a more widespread succession that begins with tillites followed by aluminous quartzites and rocks that are red and hematitic, is considered a reasonable basis for approximate time correlation of these Aphebian sedimentary sequences. Wanless and Eade (1975) suggested that isotopic data obtained from metamorphosed rocks of the Hurwitz Group and intrusions therein in the central part of the Churchill Structural Province rendered untenable correlation of these rocks with the early Aphebian Huronian succession of the Great Lakes region. By the same token Rb-Sr isochrons obtained from the more folded and metamorphosed part of the Huronian outcrop belt (Fairbairn et al. 1969) yielded "Hudsonian" ages and could be considered to make untenable correlation between these rocks and that part of the Huronian to the north where the Aphebian supracrustal rocks have yielded older dates. Fortunately in this case the geological sequence is so similar that the stratigraphic correlation is unequivocal. In a more positive vein it might be argued that because the Huronian rocks of the Southern Structural Province have Rb-Sr ages similar to those of the Hurwitz Group that they are correlative (and as a corollary that the northern parts of the Huronian belt are not correlative because they are too old!). Such arguments are obviously spurious, but are precisely the arguments presented by Wanless and Eade. The Huronian example clearly shows that calculation of 'ages' of rocks on the basis of isotopic content, particularly from metamorphic rocks, does not necessarily provide a basis for stratigraphic correlation or its refutation.

CHURCH, W. R. and YOUNG, G. M. 1970. Discussionof the progress report of the Federal-Provincial Committee on Huronian stratigraphy. Can. J. Earth Sci., 7, pp. 912-918. H . W . , HURLEY,P. M., CARD,K. D., and FAIRBAIRN, KNIGHT, C. J. 1969. Correlation of radiometric ages of Nipissing diabase and Huronian metasediments with


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Proterozoic orogenic events in Ontario. Can. J. Earth Sci., 6, pp. 489-497. GOLDICH, S. S. 1973. Ages of Precambrian banded ironformations. Econ. Geol., 68, pp. 1126-1 134. HARPER,C. T. 1967. On the interpretation of potassiumargon ages from Precambrian Shield and Phanerozoic orogens. Earth Planet. Sci. Lett., 3, pp. 128-132. F. C. 1972. Correlation of JACKSON, G. D. and TAYLOR, major Aphebian rock units in the northeastern Canadian Shield. Can. J. Earth Sci., 9, pp. 1650-1669. ROSCOE, S. M. 1969. Huronian rocks and uraniferous conglomerates in the Canadian Shield. Geol. Surv. Can., Pap. 68,205 p. 1973. The Huronian Supergroup, a Paleoaphebian succession showing evidence of atmospheric evolu-

tion. In : Huronian stratigraphy and sedimentation (G. M. Young (Ed.)). Geol. Assoc. Can., Spec. Pap. No. 12, pp. 3 1 4 7 . YOUNG,G. M. 1966. Huronian stratigraphy of the McGregor Bay area, Ontario: relevance to the paleogeography of the Lake Superior region. Can. J. Earth Sci., 3, pp. 203-210. 1973. Tillites and aluminous quartzites as possible time markers for Middle Precambrian (Aphebian) rocks of North America. In Huronian stratigraphy and sedimentation (G. M. Young (Ed.)). Geol. Assoc. Can., Spec. Pap. No. 12, pp. 97-127. WANLESS, R. K. and EADE,K. E. 1975. Geochronology of Archean and Proterozoic rocks in the Southern District of Keewatin. Can. J. Earth Sci., 12, pp. 95-114.

Geochronology of Archean and Proterozoic Rocks in the Southern District of Keewatin: Reply R. K. WANLESSAND K. E. EADE Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K I A OE8 Received April 1, 1975 Accepted for publication April 9, 1975

Professor Young's thoughtful criticism of our paper is appreciated as it presents an alternative interpretation of the Proterozoic geology of southern District of Keewatin. We welcome the opportunity to briefly reply to some of his comments. For the greater part of the Hurwitz Group regional metamorphism is greenschist facies to lower amphibolite facies. Kyanite and andalusite are rare in these rocks. K-Ar ages may be affected by such metamorphism, but a Rb-Sr whole-rock system is less likely to have been disturbed. With respect to correlation of Hurwitz Group rocks with supracrustal rocks in northern District of Keewatin, Jackson and Taylor (1972, p. 1666) quote Heywood (1967) as tentatively correlating the Penrhyn Group with the Hurwitz Group. The suggestion was based solely on similar lithologies, i.e., a quartzitecarbonate-pelitic rock succession (W. W. Heywood, personal communication, 1975). During recent more detailed mapping of the Penrhyn Group additional material has been collected for age determinations, but it has not yet been processed. At the present time the age of the Penrhyn Group, other than being Aphebian, is unknown (J. E. Reesor, personal communication, 1975). Can. J. Earth Sci.. 12,1254-1255 (1975)

We agree that the presence of hematite in Hurwitz Group rocks contrasted with pyrite in the Montgomery Lake Group does not necessarily indicate a significant break between the two groups. However, field evidence at several localities indicates angular discordance between the two groups of rocks. In addition, Bell (1971) noted that 760 m of the Montgomery Lake Group section is cut out in about 3 miles (-5 km) in the Padlei area. Further, the upper thinly bedded limy siltstone member present at the type locality has not been observed elsewhere. This is indicative of erosion of Montgomery Lake Group rocks prior to deposition of the Hurwitz Group rather than simple overstepping of the Montgomery Lake Group by the Hurwitz Group. Regarding the comment that rocks of the Hurwitz Group and the Montgomery Lake Group are generally found together, suggesting deposition in the same basin(s), we note that regionally the Montgomery Lake Group is very restricted in occurrence, and more striking is the common association of Hurwitz Group rocks with Archean volcanic and sedimentary rocks. This suggests that in southern District of Keewatin there are 'basins', little affected by orogeny and metamorphism, in which both