Magnetic fabrics in Archean granitoids, Northwestern Ontario: Isolation of accessory and matrix contributions by inspection of AMS data
Introduction
On the north shore of Lake Superior, in the region of Thunder Bay, the Archean bedrock of the Superior Province yields a bimodal distribution of U–Pb zircon ages (Davis, 2003) with prominent modes at 2.73 Ga and 2.69 Ga (Card, 1990, Corfu and Stott, 1986, Davis et al., 2005). The former derive from notably strained Archean greenstone with a mostly vertical, single schistosity trending ENE–WSW with L–S fabrics having L inclined mostly down to the West at a small plunge angle. The younger ages are associated with meta-sedimentary conglomerate and volcanic units in pull-apart basins; the so-called Timiskaming-type rocks. Primary schistosity in the younger meta-sedimentary rocks post-dates the deformation of the underlying greenstone belts since schistose pebbles of greenstone are included in conglomerates of the younger meta-sedimentary rock.
Schistosity in the older Archean greenstone is notably deflected by some granite–adamellite K-feldspar megacryst intrusions and contact metamorphism is detectable in thin aureoles, 25 to 100 m wide. We sampled two of these younger plutons, at Trout Lake and Barnum Lake where they show concentric orientation distributions of K-feldspar megacrysts (Kehlenbeck, 1976, Kehlenbeck, 1977) (Fig. 1). Inflation of plutons may be responsible for the diagnostic domal and concentric patterns of foliations of penetrative L–S fabrics as noted elsewhere in gneiss domes of the region (e.g., Borradaile and Gauthier, 2003). However, it is worth noting that the feldspar megacrysts are found also to have grown inside mafic xenoliths, albeit sparsely. We conclude that these grew in the solid state by diffusion. Therefore, syn-crystallization stress rather than strain-rotation controls the megacrysts' orientations.
In the Trout and Barnum Lake plutons, microscopic textures indicate modest strain; undulatory extinction, moderate sub-grain development, kinking of micas etc. Chloritization is extensive and epidote and calcite partially replace feldspar. Whereas these intrusions appear to be post-tectonic from traditional large-scale field criteria, Borradaile and Kehlenbeck (1996) discovered that the Trout and Barnum Lake plutons possessed a consistently oriented cryptic magnetic fabric using anisotropy of magnetic susceptibility (AMS). Since the susceptibility of those plutons is very high, the AMS is mostly attributable to magnetite (also shown below) and records late stage magnetic-domain-realignment in the magnetite. Thus, they reported what is essentially a late-stage tectonic AMS-overprint; its foliation trends ENE–WSW, similar to the regional schistosity outside the plutons. The sensitivity of magnetite domains to late tectonic stresses is so great that one would not expect any other manifestation of this event. Although the matrix is mildly cataclastically deformed, there is no consistent fracture pattern or embryonic foliation of the silicate matrix.
The non-deformed granites have a pronounced aeromagnetic signature giving positive anomalies (e.g., Trout, Barnum, White Lily and Greenstone Lake plutons: Fig. 2a; T, B, WL, G). The magnetic signature of the Greenstone Lake pluton cuts the MacKenzie granite and is a clue to their relative ages (Fig. 2a). Large susceptibilities (> 20,000 μSI) are due to high magnetite concentrations providing measurable remanent magnetizations albeit of doubtful paleomagnetic significance. U–Pb dating of their zircon was unsuccessful (D.W. Davis, University of Toronto, pers. comm., 2009) and Kehlenbeck, 1976, Kehlenbeck, 1977 described their general geology and petrofabrics. Aeromagnetic anomalies show the well-defined discrete nature of these cylindrical intrusions (Fig. 2a). Gravity lows are more diffuse and may favor their subterranean connection (Fig. 2b). Overall, they show the characteristics of “I” type granites (Chappell and White, 1974).
Covering a greater areal extent, other K-megacryst granitoids crop out in the form of irregularly bounded gneisses with L < S fabrics and nearly horizontal L-components. In general, their boundaries conform to the regional structure. The Mackenzie granite-gneiss is the chief outcrop in the south-east of the area mapped (Fig. 1, Fig. 2). The penetratively strained petrofabric comprises aligned chlorite, mica and hornblende, and extensive subgrain formation with dynamic recrystallization of quartz. Although field relations and exposure are insufficient to determine the precise margins of this body, gravity and aeromagnetic anomalies (Fig. 2a, b) define an outline similar to the inferred outcrop distribution. The Mackenzie granitoid is also gray-shaded in Fig. 2a; its contacts with the country rock are poorly exposed or diffuse. Unlike the Trout-Lake series of plutons, these granitoids have low susceptibility, very low remanent magnetization and sparse zircon could not be dated (D. Davis, pers. comm., 2009). Mafic xenoliths are present and K-feldspar megacrysts are abundant in the matrix but smaller and less common in the mafic xenoliths. The xenoliths are primarily of amphibolites but Rogers (1979, p. 41) recorded oolitic chalcedony within an arenite specimen, which may have been a xenolith.
Section snippets
Magnetic fabrics and rock magnetism
Rock magnetism and in particular magnetic fabrics provide ways of determining the petrofabric of a rock in a non-destructive fashion, quickly and precisely using very weak laboratory fields that do not impart a permanent magnetism to the specimen. Methods involving large fields can change the anisotropy of magnetite (“field-impressed” anisotropy, Potter and Stephenson, 1990). Hrouda (1982) thoroughly reviewed the subject supported by numerous subsequent publications of his group (especially
The less strained (younger) Trout and Barnum Lake granites
These plutons do not bear a visible tectonic petrofabric in the field and show an inconsistently oriented cataclastic fabric under the microscope. Nevertheless, both plutons bear a consistently oriented AMS foliation oriented NE–SW and dipping very steeply to the north-west (Fig. 1, Fig. 4). This is very similar in orientation to the mean schistosity of the Archean country rock and the principal lithological boundaries shown.
Confidence regions around the mean axes of the tensor are quite small
The intensely strained (older) Mackenzie granitoid gneiss
The Mackenzie granitoid is visibly schistose in the field with a NW–SE schistosity that dips steeply to the NW. This fabric is parallel to the S1 schistosity in the adjacent Archean greenstones and the granitoid is therefore pre-tectonic or syn-tectonic with respect to D1. From the histogram of k (Fig. 3c), two broad groups of specimens may be differentiated into low susceptibility (k < 2000 μSI) and high susceptibility (k > 5000 μSI) and then compared with the stacked data from all specimens (Fig. 3
Discussion
The overall regional trend to schistosity in the Archean schist in the area is approximately 070°. The older Mackenzie plutonic granitoid is syntectonic and shows this well. However, its mean AMS foliation dips more gently to the NE or NNE than the regional schistosity. We are aware that high susceptibility, accessory magnetite dominates over the matrix AMS foliation so it is logical to suppress it by normalizing each specimen's susceptibility magnitudes by their mean value. This suppresses the
Acknowledgments
Borradaile acknowledges the continuous financial support of NSERC (Ottawa) (1979–present). We are indebted to anonymous reviewers for improving this manuscript.
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