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Abstract-El'gygytgyn in northeast Chukotka (Russia) is a 3.6 Ma, 18-km-diameter impact structure. The impact crater was recently drilled in the framework of a project sponsored by the International Continental Scientific Drilling Program (ICDP). Target rocks at the El'gygytgyn area are dominated by the felsic members of the Late Cretaceous OkhotskChukotka Volcanic Belt (OCVB). Such a target lithology is unique among terrestrial impact craters, thereby providing the opportunity to study shock metamorphism in siliceous volcanic rocks. Here, we present a petrographic, geochemical, and isotopic study of the section of the drill core underneath the lacustrine sediments, extending from $316 m to 517 m below the lake bottom (blb). The drill core stratigraphy includes $80 m of suevite and a cross section through a volcanic suite, which consists of (1) a middle section ( $390-423 mblb) with dominant felsic tuffs and a few mafic members, and (2) a welded rhyoliticdacitic ignimbrite ( $423-517 mblb). The melt fragments embedded in the suevite are interpreted as being impact-related by comparison with impact glasses from the crater and in opposition to the target rock, which does not include similar melts. A suevitic dyke crosscuts the lower section of the core at the depth 471.40 mblb. Evidence for shock metamorphism is concentrated in the upper 10 m of the drill core and almost limited to the suevitic breccia. The geochemical and isotope (Nd and Hf) composition of samples from the target and the drill core reveals relationships to the "Berlozhya magmatic assemblage" (BMA) arguing for similar source magmas. The primitive upper mantle (PUM)-normalized trace element plot of rocks investigated here confirms a subduction-related signature, as previously proposed for rocks from both OCVB and BMA.

Abstract-In 2008, the International Continental Scientific Drilling Program (ICDP) obtained drill cores from the El'gygytgyn impact structure located on the Chukotka Peninsula (Russia). These cores provide the most complete geological section ever obtained from an impact structure in siliceous volcanic rock. The lithostratigraphy comprises a thick sequence of lacustrine sediments overlying impact breccias and deformed target rock. The interval from 316 m (below lake floor-blf) to the end of the core at 517 m depth can be subdivided into four lithological sequences. At 316 m depth, the first mesoscopic clasts of shocked target rock occur in lacustrine sediments. The growing abundance of target rock clasts with increasing depth and corresponding decrease of lacustrine sediment components indicate the extent of this transition zone to 328 m depth. It constitutes a zone of mixed reworked impact breccia and lacustrine sediments. Volcanic clasts in this reworked suevite section show all stages of shock metamorphism, up to melting. The underlying unit (328-390 m depth) represents a suevite package, a polymict impact breccia, with considerable evidence of shock deformation in a wide variety of volcanic clasts. This includes fragments with quartz that exhibit planar fractures and planar deformation features (PDF). In addition, at three depths, several centimeter-sized clasts with shatter cones were detected. Due to microanalytical identification of relatively rare, microscopic impact melt particles in the matrix of this breccia, this material can be confidently labeled a suevite. Also in this sequence, three unshocked, <1 m thick intersections of volcanic blocks occur at 333.83, 351.52, and 383.00 m depths. The upper bedrock unit begins at 390.74 m depth, has a thickness of 30.15 m, and represents a sequence of different volcanic rocks-an upper part with basaltic composition from 390.74 to 391.79 m depth overlying a lower, rhyodacitic part from 391.79 to 420.27 m depth. This (parautochthonous) basement unit is only very weakly affected by the impact: only one shocked quartz grain with two sets of PDF was recorded at 391.33 m depth. The lower bedrock unit (420.89-517.09 m depth [end of core]) is a brittly deformed, rather homogeneous welded ignimbrite that in part can be considered a cataclasite. The top three meters of this section are sheared, which could represent pre-impact tectonic deformation. A 54 cm thick injection of polymict impact breccia occurs at 471.42-471.96 m depth.

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