Evidence of Shock Metamorphism Effects in Allochthonous Breccia Deposits from the Colônia Crater, São Paulo, Brazil

The 3.6 km-diameter Colonia impact crater, centred at 2352'03"S and 4642'27"W, lies 40 km to the south-west of the S?o Paulo city. The structure was formed on the crystalline basement rocks and displays a bowl-shaped with steeper slope near the top that decreases gently toward the centre of the crater. Over recent years were drilled two boreholes inside the crater, which reached a maximum depth of 142 m and 197 m. Geological profile suggests four different lithological associations: 1) unshocked crystalline basement rocks (197 - 140 m); 2) fractured/brecciated basement rocks (140 - 110 m); 3) polymictic allochthonous breccia deposits (110 - 40 m); and 4) post-impact deposits (40 - 0 m). Petrographic characterisation of the polymictic allochthonous breccia reveals a series of distinctive shock-metamorphic features, including, among others, planar deformation features in quartz, feldspar and mica, ballen silica, granular texture in zircon and melt-bearing impact rocks. The occurrence of melt particles and very high-pressure phase transformation in suevite breccia suggest a shock pressure regime higher than 60 GPa.

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