Caracterización de los Centros de Maestros en el marco de los SEFCSP

First discovered during the construction of the Canadian Pacific Railway in the 1880’s (Pye et al., 1984), the 1.85 Ga Sudbury impact structure has provoked much discussion about how it was formed. The structure (Fig. 2) was originally interpreted to be a product of igneous activity (e.g., Burrows and Rickaby, 1930; Fairbairn and Robson, 1941, 1942; Speers, 1957; Thomson, 1957, Williams, 1957) but was eventually discovered to contain evidence for a very

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large bolide impact (e.g. Dietz and Butler, 1964; French, 1967; Dence, 1972; Peredery, 1972 a,b; Gibbins 1994; Ames, 1999). When the entire basin is considered, including the thick basin fill, the combination of impact, magmatism and hydrothermal alteration processes best

describe the end product (e.g. Ames et al., 2009; Grieve et al., 2010; Ubide et al., 2017). It is important to remind readers that the basin has undergone multiple deformation events and is preserved as a 60 x 30 km eroded synformal ellipsoid representing only a small portion of the original structure that was at least 200 km in diameter (e.g. Deutsch et al., 1995; Therriault et al., 2002; Spray et al., 2004; Riller, 2005; O’Sullivan et al., 2016).

The impact structure is made up of three main features, defining the stratigraphy of the basin (Fig 2). The stratigraphically lowest element comprises the brecciated and shock- metamorphosed footwall rocks. Because the basin straddles an ESE-WSW trending

Neoarchaean terrane boundary, a large number of rock types with very different radiogenic isotope character were encountered by the bolide. To the north of the boundary lies the Southernmost Neoarchaean Abitibi Subprovince, a granitoid-greenstone and high grade tonalitic gneiss terrane at the southernmost edge of the vast Superior Province. The majority of these rocks were formed in a series of nine magmatic-sedimentary pulses between 2.75 and 2.675 Ga (Ayer et al., 2002). To the south of the terrane boundary lies the Archaean Southern Province. No basement rocks are exposed in outcrop but the extreme antiquity of the basement can be inferred from the detrital zircons in quartzites and quartz-arenites that cover the

basement (Rainbird et al., 2006; Petrus et al., 2015). The terrane boundary is of late

Neoarchaean age and produced highly potassic granites emplaced during accretion (e.g. Petrus et al., 2016). Huronian metasediments (mostly quartz-rich meta-sandstones) were deposited during a Palaeoproterozoic rifting event and covered the Archaean basement either side of the terrane boundary. Importantly, at the time of impact the target area is interpreted to have been in a subaqueous foreland basin setting (Shanks and Schwerdtner, 1991), thus suggesting a relatively shallow submarine impact site.

The second element of the structure is the 2.5-3 km thick Sudbury Igneous Complex (SIC). This is widely interpreted to be the crystallised differentiated impact melt sheet and

stratigraphically overlies (Fig. 2) the footwall rocks (Therriault et al., 2002). The lowest SIC lithology is the contact or sublayer, the host rock of the majority of the world-class Ni, Cu and platinum group element deposits. The main mass of the SIC is divided into norite, quartz gabbro and granophyre. Some workers (e.g. Avermann and Brockmeyer, 1992; Anders et al., 2015) also attribute a suite of intrusive bodies in the roof of the SIC (the Onaping intrusions) to the meltsheet. The coherent stratigraphy around the preserved basin (Fig. 2) does not do justice to the fact that in detail, the North and South Ranges of the SIC have discernibly different

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petrology (Naldrett et al., 1970). Regardless of this complexity, the main mass of the SIC geochemically resembles average upper continental crust (UCC; Taylor and McClennan, 1985) and is very similar in composition to the assemblage of target rocks exposed at the present surface (Stöffler et al., 1994).

The third element of the structure is the stratigraphically youngest Whitewater Group that filled the basin (Fig. 2). It is divided into three Fm; the Onaping Fm., the Onwatin Fm. and the Chelmsford Fm. For the purpose of this study, the Onwatin (black shales) and Chelmsford (turbiditic sands) Fms. are too young to be of importance. By contrast, the immediate basin fill, the Onaping Fm. has assumed a central role in the interpretation of the entire structure.

Agreement exists that it can be subdivided into three distinct Members (Fig. 2). The Garson Member (up to 100m thick) is only found on the south eastern edge of the basin and could represent the fallback material of the primary impact plume. It is largely composed of a quartzite mega-breccia. Where the Garson Member is not developed, it is considered to have been consumed by the roof of the meltsheet. On account of its patchy preservation and its relatively high degree of deformation the Garson Member was not included in this study.

By contrast, the much thicker Sandcherry (300-500 m thick) and Dowling Members (1000 m thick) are uniformly found around the basin (Ames et al., 2002). Both members are

composed of breccias and finer grained (sand and mud-sized) equivalents, rich in devitrified glass products. Rocks of both members also contain abundant shock-deformed lithic clasts (mainly quartz with planar deformation features). Grieve et al. (2010) made a convincing case that the vitric products show evidence for fuel-coolant interaction and because of its uniform composition similar to the SIC, these authors argued that the Onaping Fm. formed due to violent explosive interaction between the original meltsheet and water that entered the basin. However, more recently, Petrus et al. (2016) showed that only the Sandcherry Member has U/Pb zircon age distributions matching the collage of target rocks whereas the thicker Dowling Member had zircon sourced from the local footwall below the SIC. This difference as well as clear chemostratigraphic trends across the Onaping Fm. led O'Sullivan et al. (2016) to propose that the Sandcherry Member alone was the result of explosive interaction between the

meltsheet and seawater flooding the basin butthat the Dowling Member was longer-lived and contained products of an additional magma source. Ubide et al. (2017) studied vitric products from the Onaping Fm. and argued that the green-weathering volcanic shards in the Dowling Member were not related to the SIC. Regardless of whether the entire Onaping Fm. was caused by re-working of the original meltsheet or represents two distinctive deposits, there is wide agreement that it is largely not the product of the collapse of the primary impact plume but

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Fig. 2: Geological map of the Sudbury impact basin and surrounding country rock lithologies, modified from McNamara et al. (2017). Sampling locations from McNamara et al. (2017) and O’Sullivan et al. (2016) are indicated with red circles and dark blue circles respectively. The locations of the felsic norite sample (PR17A) and the vitric rim sample (07AV-02-2A) are noted by light blue and green circle, respectively. The stratigraphic units of the Sudbury basin, from the brecciated and shock-metamorphosed footwall rocks to the Chemlsford Fm. at the top of the Whitewater Group are shown on in a log. A separate stratigraphic log shows the detail and complexity of the Onaping Fm. after O’Sullivan et al. (2016). Samples used in this study describe a complete transect from the norite in the Sudbury Igneous Complex through to the lower Onwatin Fm.

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instead at least in part formed as a result of a secondary plume caused by fuel- coolant interaction (similar to the findings at the Ries impact structure; Stöffler et al., 2013). The existence of secondary plumes will be shown to be a possibly important phenomenon of subaqueous impact basins for volatile element loss.

Nearly all the samples analysed in this study were obtained and petrographically studied as part of a larger re-investigation of the Sudbury impact basin (e.g. Petrus et al. 2015, 2016; O’Sullivan et al. 2016; Kenny et al. 2016; McNamara et al., 2017; Ubide et al., 2017; Kenny et al., 2017). Collectively, these samples cover the full transect from the base of the SIC, through the SIC, into its roof, across the entire Onaping Fm., ending in the base of the Onwatin Fm. (Fig 2).

Samples from the SIC were collected from a transect along the NW Bypass of route 144 in 2014 (Fig. 2). This transect was one of several documented in a larger study into the Pb isotope and trace element geochemistry of feldspars in the SIC (McNamara et al., 2017), the data of which were available for the present study. One additional norite sample (PR17A) was collected from the North Range on a roadside outcrop at the junction of Route 144 and Elks Club road in 2011 (location 467206.12, 5162502.32).

The samples from the Onaping Fm. were collected in three subsequent field seasons in 2012, 2013 and 2014 and documented in Petrus et al. (2015), O’Sullivan et al. (2016), and Ubide et al. (2017). Exact sample locations are found in Table 1 of O’Sullivan et al. (2016). Geochemical data for a large collection of additional samples from the Onaping Fm., from many transects along the North Range, were also available for this study from Ames et al. (2002). One of these samples, 07AV-02-2A, (from NW Bypass of Route 144, location 470670.54, 5159701.35) contains well-developed vitric rims around lapilli and was re- investigated in this study.