46th Lunar and Planetary Science Conference (2015) 1819.pdf GEOCHEMISTRY OF SUDBURY BRECCIA IN THE NORTH RANGE OF THE SUDBURY IMPACT STRUCTURE, CANADA. J. R. Weirich1 and G. R. Osinski1,2, A. Pentek3, J. Bailey3 1Dept. of Earth Sciences & Centre for Planetary Science and Exploration, University of Western Ontario, London, ON, N6A 5B7, Canada, 2 Dept. of Physics and Astronomy, University of Western Ontario, London, ON, N6A 5B7, Canada, 3Wallbridge Mining Company Limited, Lively, ON, P3Y 1L7, Canada. ([email protected]) Introduction: The 1.85 Ga Sudbury impact structure straddles the Superior Province to the north, and the Southern Province to the south, and is thought to be a multi-ring impact structure >200 km in diameter , . The Superior Province is an older unit (>2.50 Ga) and consists of granitic and gneissic rocks, while the Southern Province is younger (2.45-2.22 Ga) and contains mostly sedimentary rocks . Some of these sedimentary units are still found in the Superior Province and are referred to as Huronian outliers. Sudbury Breccia (SDBX) is a unique rock type that surrounds the impact melt sheet, known as the Sudbury Igneous Complex (SIC). SDBX has angular or rounded clasts in a black aphanitic matrix, and individual examples can be 1 cm to 100’s of m wide. SDBX typically contains clasts of the surrounding country rock, and is a breccia at all scales. It is similar to pseudotachylite found at the Vredefort Dome in South Africa . In this work we present geochemistry and petrographic observations of SDBX in the Superior Province to help elucidate its formation conditions. Here we present preliminary data from the Halfway Lake outcrop found 30 km north of the SIC in the Superior Province; data from outcrops along a transect towards the SIC will be forthcoming. Sudbury Breccia formation: The most popular scenarios of SDBX formation are 1) frictional melting/cataclasis during crater collapse , 2) shock induced melting or cataclasis , or 3) injection of SIC into the surrounding footwall . Cataclasis (i.e. mechanical grinding of rock into powder) and frictional melting are typically viewed as two end-members of a continuous progression from initial shearing (when the rock is cold) to late stage shearing (when the rock is hot). Scenario 1) is similar to tectonic pseudotachylite found in earthquake fault zones, albeit on a much larger scale. Scenarios 1) and 2) require the composition of the breccia to be a mix of the composition of the surrounding country rock, while scenario 3) requires the composition of the breccia to have a component from the SIC. Lafrance and Kamber  has shown that a contact between sandstone and diabase near the SIC in the Southern Province has no contribution from the SIC, casting doubt on scenario 3). However, that study was only at one location and may not be applicable to the entire basin. Scenario 2) works well for small (~1 cm) SDBX occurrences, but it is unlikely that large fluctuations in shock pressure would be present at the scale of 100’s of meters. A criticism of formation due to frictional melting is that once melting begins it will lubricate the sliding surface, which would inhibit further production of melt. Melosh  suggests that melt could extrude into adjacent low pressure zones in the country rock, thus keeping the sliding surfaces unlubricated, or that the melt was viscous enough to sustain shear, though both of these processes require strict conditions which may not be matched by SDBX formation conditions. Samples and Methods: Since SDBX has a large clast content we attempted to improve upon the quality of SDBX data presented in other studies by picking out these clasts. Hand samples were cut into ~1 cm cubes on a circular saw and 40–60 g of the most clast-poor material was selected for picking and geochemistry. Samples were sieved, and the 1–2 mm size fraction was picked of all clasts >1 mm to obtain “matrix enriched” material. Typically about half of this size fraction had clasts >1 mm. We found that attempting to pick all clasts >0.5 mm was slow (~1g/hr) and required a larger initial mass to obtain enough for geochemical analysis. Smaller size fractions of the crushed samples were analyzed to provide a “whole rock” comparison to the “matrix enriched” portion. To test the variability of SDBX and country rock across the Halfway Lake outcrop, multiple samples of each were collected. Given that clasts in SDBX can be 10s of meters in size, it is sometimes hard to distinguish between host rock and large clasts. Here we adopt the term “country rock” since we are attempting to sample host rock, but acknowledge this may not be the case. Five samples of SDBX were collected from a 50 m trek along an outcrop, and three of these samples (the two northern most and the southern most) were used for clast picking. We also collected two samples of diabase, three samples of granite/gneiss, and one sample of a rock type that is similar to siltstone found in a Huronian outlier 4 km to the south. Further investigation of this “unknown” rock is underway. Geochemistry: Major and trace elements, including the Rare Earth Elements (REE) were determined for each sample. Picking had little affect on the silica and mafic element concentrations; all other elements had similar small differences. Hence, the “matrix enriched” samples will not be discussed further, and are not included in any of the figures. 46th Lunar and Planetary Science Conference (2015) Figures 1 and 2 show the mafic vs. silica content and REE signature, respectively, of the matrix and country rock. SDBX appears to be intermediate to the country rocks in both major and trace elements, though the REE have a higher concentration than both the diabase and granite/gneiss. Since the REE are relatively immobile, they indicate that a contribution from the “unknown” sample is probable. To test this hypothesis, we performed a component decomposition on Si for each SDBX sample using the average of the two diabases, the average of the three granite/gneisses, and the “unknown” as three end members. We then selected the combination that gave the lowest Chi2 for the REE, plus the immobile ele- Figure 1. Harker diagram showing SDBX is intermediate to country rocks. Note legend. Figure 2. Spider diagram showing SDBX intermediate to country rocks. Symbols same as Fig. 1. ments Ti, Zr, Y, Al, Th, and Nb. We then calculated the P-value (the probability the Chi2 would be that poor by chance) for that combination of end members. Th and Nb have a higher concentration in the SDBX than any of the three end members and give large errors, resulting in a P-value <0.001 which indicates a poor model. However, if we don’t include Th and Nb when choosing the lowest Chi2, the P-values are >0.01 (>0.1 in four out of five cases) which indicate a good fit. While the large Th and Nb errors are concerning, it is remarkable how the majority of the elements are reproduced using a combination of the three country rocks, as can be seen in Figures 3 and 4. In these models granite/gneiss is 38–55%, diabase is 22–42%, and 1819.pdf Figure 3. Harker diagram of SDBX (blue) and three component model values (green). Figure 4. Spider diagram of SDBX (blue) and three component model values (green). “unknown” is 17–23% of the breccia. Replacing “unknown” with SIC as the third end member does give reasonable P-values, however the large distance (30 km) to the SIC makes it an unlikely contributor. Future work will explore the SIC contribution in more detail. Conclusions: As found by other authors, diabase is a larger component of the breccia than one would expect given the diabase abundance at the outcrop; perhaps the result of preferential melting of mafic material during shear . We have found an unknown rock type, perhaps sediment from a Huronian outlier, which can explain the REE abundance in the breccia. The abundance of this rock type at the outcrop has not been determined, but the nearest known Huronian outlier is 4 km away. Our working hypothesis is that breccia melt has been transported large distances, either via intrusion of melt into the country rock  or due to faulting over many kilometers . References:  Krogh T. et al. (1984) The geology and ore deposits of the Sudbury structure, 431-446.  Lafrance B. and Kamber B. (2010) Precambrian Res., 180, 237–250.  Rousell D. (2003) EarthScience Rev., 60, 147-174.  Reimold. W. (1995) Earth-Science Rev., 39, 247-265.  Spray J. and Thompson L. (1995) Nature, 373, 130-132.  Riller U. et al. (2010) Geology, 38, 619-622.  Melosh J. (2005) Impact Tectonics, 55-80.  Mungall J. and Hanley J. (2004) J. Geol., 112, 59-70.
© Copyright 2017 ExploreDoc