Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

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AGE AND UNCERTAIN STRATIGRAPHIC CONTEXT A sample of “quartz--phyric felsic tuff” (Davis 1990, p.3) exposed along the north shore of Pickerel Arm (Davis 1990, Figure 2) was considered to be from the Minnitaki Group by Davis (1990) but these rocks are described herein as part of the Central Volcanic belt. The problem is that these rocks are located along a zone that may be between the Central Volcanic belt and the Minnitaki Group, and their stratigraphic affinity is thus uncertain: they may be part of the Central Volcanic belt, the Minnitaki Group, or they may be part of a small, thin younger volcanic unit. The sampled tuff’s age of 2701–2707 Ma is based on a single zircon grain, and this unpublished but repeatedly cited age was quoted with certainty by Blackburn et al. (1991, Figure 9.40). This age might be flawed (Devaney 1999a, p.161; i.e., about 10 Ma too young for Minnitaki Group deposition?), but other volcanic rocks (which may include reworked tuffs?) of this ca. 2704 Ma age have recently been found elsewhere in the western Wabigoon subprovince (e.g., Sanborn--Barrie and Skulski 1999) and other parts of Superior Province, so the 2704 Ma age may be valid for the Pickerel Arm area pyroclastic rocks. (Because postarc orogenic activity in western Wabigoon subprovince has previously been thought to begin at ca. 2710 Ma (Davis et al. 1988, Davis 1990, Blackburn et al. 1991), it is not clear whether these apparently ca. 2704 Ma volcanic rocks represent late arc volcanism or synorogenic volcanism. Perhaps late arc volcanism overlapped with early orogenesis during the 2700–2710 Ma period?) Pickerel Arm Area Porphyry Bodies Slightly mineralized (gold--copper) felsic porphyry bodies, which may represent subvolcanic intrusions, were previously described by Johnston (1969), Sutherland and Colvine (1979) and McMullan (1980). These porphyry bodies occur at or near the contact between the basaltic part of the Central Volcanic belt (to the north; thought to be the lower part of the Central Volcanic belt) and the Minnitaki Group (Johnston 1969, 1972; Devaney et al. 1995a,b), but the nature and amount of any structural repetition or disturbance of the original stratigraphic section is unknown. At the largest porphyry body (east end of Pickerel Arm), compositional variability within the porphyry and the presence of dikes suggests either border phase variability within the intrusion, or a composite intrusion. The adjacent mafic volcanic rocks contain felsic porphyry dikes (some quartz porphyry, with notably round quartz “eyes” up to 1 cm, and some feldspar porphyry). Minor pyrite, malachite stain, and rusty weathering were observed at the surface exposures. (Most of the old trenches and pits shown on the map of Johnston (1969) could not be located.) INTERPRETED STRATIGRAPHIC CONTEXT The age of the porphyry bodies is unknown. The geochemical composition of the similar porphyry at Burnthut Island (see above) suggests that the calc--alkalic porphyry (see Geochemistry section) is related to “Stage 4” volcanism (the youngest stage of volcanism in the <strong>Sioux</strong> <strong>Lookout</strong> orogenic belt, and broadly correlative with the Minnitaki Group; see below). If the felsic porphyry bodies are indeed Stage 4 subvolcanic intrusions, they may represent parts of the volcanic centres which were sources of clastic detritus (e.g., felsic volcanic and porphyry clasts, volcanic sand--size rock fragments) and minor tuffaceous interbeds during Minnitaki Group deposition. Trowell et al. (1983, p. 68) made a similar suggestion regarding the provenance of the Minnitaki Group. (Stratigraphic evidence of this type of proximal volcanic source area appears to be preserved in the southeastern--most part of the Minnitaki Group; see below). Stratigraphic Interpretation The stratigraphic context of the rocks exposed in the above four areas is not reliably known, and potential structural complexity precludes simple stratigraphic correlations. However, the rocks in these areas have characteristics of facies and rock bodies commonly considered to be proximal to “felsic” (rhyolitic to dacitic) volcanic centres (e.g., coarse pyroclastic facies, potentially subvolcanic porphyry bodies, exhalative 30
BIF units, sub--exhalite alteration), and may collectively form a suite/assemblage of proximal units compared with more distal volcanic and sedimentary deposits of the Big Vermilion–Daredevil unit and Minnitaki Group (i.e., it can be interpreted that they are all “Stage 4” deposits of a mixed volcanic--sedimentary basin; see Devaney 1999a, and below). The presence of “felsic” volcanic or porphyritic clastic detritus, “felsic” pyroclastic units or interbeds, and exhalative BIF units in both the Big Vermilion–Daredevil unit and the Minnitaki Group supports this interpretation. Previous workers considered the predominantly volcanic and felsic rocks in the above Pickerel Arm area sites to be part of the Central Volcanic belt, but these Pickerel Arm area rocks could alternatively be assigned to the Minnitaki Group, or could be considered to represent the stratigraphic interfingering of the Central Volcanic belt and the Minnitaki Group. If the Central Volcanic belt and Minnitaki Group in this area (Pickerel Arm–central Minnitaki Lake) are lumped together there is, in a very broad sense, an overall north to south transition from felsic pyroclastic facies to clastic sedimentary facies (wacke--dominated) with detritus likely sourced from the felsic pyroclastic facies. The mafic flow facies in the northwesternmost part of the Central Volcanic belt could also be considered as part of the generalized north to south transition. This is why the gross sequence in this area was previously described as “transitional,” and it was suggested that the Minnitaki Group could be considered as the uppermost part of the Central Volcanic belt (Devaney et al. 1995a, p. 27). Geochronological data suggest that there is likely a major time gap between the basaltic part of the Central Volcanic belt (Stage 2) and the younger rocks to the south (e.g., Stage 4 Minnitaki Group, presumed Stage 4 Pickerel Arm area felsic and volcanic rocks, and the Pickerel Arm area pyroclastic rocks dated at ca. 2704 Ma and discussed above), and more geochronological work would be required to refine these interpreted stratigraphic scenarios (for interpretations, including Stages 2 and 4, see below). MINNITAKI GROUP Following very early work by Pettijohn (1936), the deep--water sedimentary deposits of the Minnitaki Group, including turbidites, were described in detail and interpreted by Walker and Pettijohn (1971), and are further discussed below. Two main types of facies assemblages are present: thinly bedded facies (e.g., typical sandy turbidites) and thickly bedded facies (coarser; includes “resedimented” conglomerate, possible channel facies). The preservation of sedimentary structures and sedimentological relationships (e.g., bed thickness trends) is commonly excellent in this area. Important proximal--to--distal stratigraphic trends appear to be resolvable too. Lithofacies THINLY BEDDED FACIES The thinly bedded facies assemblage consists of laminated to thinly bedded sandstone (wacke) and siltstone. The typical greenish grey colour of the wacke indicates that it is a muddier sandstone than the “clean” whitish arenities seen elsewhere in the Minnitaki Group. Graded bedding is very common (e.g., BC turbidites). Cross--lamination, which may form the C layer in Bouma sequences, outlines well defined asymmetric ripples, micro--ripples, ripple trains, starved ripples, and climbing ripples. Convolute lamination (including slightly folded to loaded ripples), load structures and small scours are locally present. Interestingly, the approximate direction down paleoslope suggested by flame structure asymmetry (“flames” being upward pointing cusps between downwardly convex lobate load structures; flames point downslope where soft sediment--shearing during gravity--induced flowage has rotated the flames toward the downslope direction) agrees with that indicated by local ripple paleocurrents. Anomalous green (chloritic?), coarse--grained interbeds in one outcrop are interpreted to be tuff, and thus constitute a rare small--scale example of interfingering of volcanic facies (synchroneity of volcanism 31
Page 1: Ontario Geological Survey Open File
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Page 11: Large--Scale Structures ...........
Page 15: 25. Stage 5 sedimentation and tecto
Page 19: A large volume of basaltic strata (
Page 22 and 23: Many of the Sioux Lookout orogenic
Page 24 and 25: pillowed basalt flows) are not give
Page 26 and 27: Units of chert-magnetite (BIF), up
Page 28 and 29: SANDIER FACIES At central Vermilion
Page 30 and 31: wrench--induced verticalization of
Page 32 and 33: alteration). Low--intensity mafic a
Page 34 and 35: few clues regarding the specific se
Page 36 and 37: seems to be the case here; see disc
Page 38 and 39: 1969, 1972; Trowell et al. 1977, 19
Page 40 and 41: Page and Clifford (1977) interprete
Page 42 and 43: upward. Examples of both westward--
Page 44 and 45: shapes, suggesting intrusion into s
Page 46 and 47: The abundance of very coarse and an
Page 48 and 49: One small island outcrop contains t
Page 52 and 53: and sedimentation). Other good exam
Page 54 and 55: (illustrating how an increased clas
Page 56 and 57: For the present study, attention wa
Page 58 and 59: Structural Geology Because the focu
Page 60 and 61: LARGE--SCALE STRUCTURES Description
Page 62 and 63: As, Pb, Zr, Y); inductively coupled
Page 64 and 65: DISCRIMINATION PLOTS Data from the
Page 66 and 67: It is notable that the Northeast In
Page 68 and 69: (ca. 2734 Ma) 60 km to the east at
Page 70 and 71: The Stage 5 cartoon (Figure 22) is
Page 72 and 73: also during later phases of Stage 5
Page 74 and 75: aquabasinal (lacustrine?) mudstones
Page 76 and 77: Note that similar felsic volcanic r
Page 78 and 79: 22) At Southeast Bay of Minnitaki L
Page 80 and 81: Given that the adjacent Sturgeon La
Page 82 and 83: structurally--controlled Archean me
Page 84 and 85: This andesitic sequence and its com
Page 86 and 87: young as 2701 Ma. However, a few mi
Page 88 and 89: volcanism (interpreted extensional
Page 90 and 91: eyond the resolution of surface bed
Page 92 and 93: asin would be adjacent to an orogen
Page 94 and 95: The moderately to steeply dipping,
Page 96 and 97: References Allen, R.L. 1988. False
Page 98 and 99: Ciceri, D.L., Cruden, A.R. and Robi
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Dupuy, C. and Dostal, J. 1984. Trac
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Horwood, H.C. 1938. Geology of the
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Nilsen, T.H. and McLaughlin, R.J. 1
Page 106 and 107:
Sage, R.P., Breaks, F.W., Stott, G.
Page 108 and 109:
Trowell, N.F., Blackburn, C.E. and
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Figure 1b. Location of Sioux Lookou
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Figure 3. Roadside cross--section o
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Figure 5a. Geochemical sample locat
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Figure 6. Discrimination plots show
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Figure 8. Plots of selected high fi
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Figure 9. Plots of selected oxides
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Figure 10. Plots of selected trace
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Figure 11. Continued. --77 to --81)
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Figure 12. N--MORB normalized exten
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Figure 12. Continued. m) Northeast
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Figure 14. For all Sioux Lookout or
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Figure 15. Zr/Yb versus Nb/Yb discr
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Figure 17. Discrimination diagrams
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Figure 19. REE plots for representa
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Figure 21. Plot of N--MORB normaliz
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Figure 22b. Corresponding stages of
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Figure 24. Schematic cross--section
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Figure 26. Stage 6 Sioux Lookout or
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Figure 28. Cartoon views of potenti
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Figure 30. Theory of small--scale k
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Figure 32. Riedel flake structure;
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Figure 34. Theoretical shear fractu
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Table 1. Continued. Sample 94JRD--0
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Table 1. Continued Sample 94JRD--00
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Table 1. Continued. Sample 94JRD--0
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Table 1. Continued. Sample 94JRD--B
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Table 1. Continued Sample 94JRD--B1
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Table 1. Continued. Sample 95JRD--B
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Table 2. Continued. Northeast Bay P
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Table 4. Altered Sioux Lookout orog
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Metric Conversion Table Conversion
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Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

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