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The 18-km-diameter El'gygytgyn crater is located on the Chukotka peninsula, northeastern Russia. It represents the only currently known impact structure formed in siliceous volcanics, including tuffs. The impact melt rocks and target rocks provide an excellent opportunity to study shock metamorphism of volcanic rocks. The shockinduced changes observed in porphyritic volcanic rocks from El'gygytgyn can be applied to a general classifi cation of shock metamorphism of siliceous volcanic rocks. Strongly shocked volcanic rocks with phenocrysts converted to diaplectic quartz glass and partially melted feldspars as well as cryptocrystalline matrices are widespread in the El'gygytgyn crater. In particular, the following different stages of shock metamorphism are observed: (i) weakly to moderately shocked lavas and tuffs with phenocrysts and clasts of quartz and feldspars; (ii) moderately shocked volcanic rocks and tuffs with diaplectic glasses of quartz and feldspars; (iii) strongly shocked lavas and tuffs with phenocrysts of diaplectic quartz glass and fused glasses of feldspars in melted matrixes; and (iv) impact melt rocks and impact glasses. In addition, thin glassy coatings of voids in impact melt rocks have been observed. While the shock-induced changes of clasts of framework silicates in these volcanic rocks do not differ from respective changes in other crystalline rocks, the fi negrained matrix of porphyritic rocks is converted into fused glass at the same shock pressures as feldspar minerals. No remnants of fi ne-grained quartz are preserved in matrix converted into fused glass by shock.

available online at Abstract-Isheyevo is a metal-rich carbonaceous chondrite that contains several lithologies with different abundances of Fe,Ni metal (7-90 vol%). The metal-rich lithologies with 50-60 vol% of Fe,Ni metal are dominant. The metal-rich and metal-poor lithologies are most similar to the CB b and CH carbonaceous chondrites, respectively, providing a potential link between these chondrite groups. All lithologies experienced shock metamorphism of shock stage S4. All consist of similar components-Fe,Ni metal, chondrules, refractory inclusions (Ca, Al-rich inclusions [CAIs] and amoeboid olivine aggregates [AOAs]), and heavily hydrated lithic clasts-but show differences in their modal abundances, chondrule sizes, and proportions of porphyritic versus non-porphyritic chondrules. Bulk chemical and oxygen isotopic compositions are in the range of CH and CB chondrites. Bulk nitrogen isotopic composition is highly enriched in 15 N (δ 15 N = 1122‰). The magnetic fraction is very similar to the bulk sample in terms of both nitrogen release pattern and isotopic profile; the non-magnetic fraction contains significantly less heavy N. Carbon released at high temperatures shows a relatively heavy isotope signature. Similarly to CB b chondrites, ~20% of Fe,Ni-metal grains in Isheyevo are chemically zoned. Similarly to CH chondrites, some metal grains are Ni-rich (>20 wt% Ni). In contrast to CB b and CH chondrites, most metal grains are thermally decomposed into Ni-rich and Ni-poor phases. Similar to CH chondrites, chondrules have porphyritic and non-porphyritic textures and ferromagnesian (type I and II), silica-rich, and aluminum-rich bulk compositions. Some of the layered ferromagnesian chondrules are surrounded by ferrous olivine or phyllosilicate rims. Phyllosilicates in chondrule rims are compositionally distinct from those in the hydrated lithic clasts. Similarly to CH chondrites, CAIs are dominated by the hibonite-, grossite-, and melilite-rich types; AOAs are very rare. We infer that Isheyevo is a complex mixture of materials formed by different processes and under different physico-chemical conditions. Chondrules and refractory inclusions of two populations, metal grains, and heavily hydrated clasts accreted together into the Isheyevo parent asteroid in a region of the protoplanetary disk depleted in fine-grained dust. Such a scenario is consistent with the presence of solar wind-implanted noble gases in Isheyevo and with its comparatively old K-Ar age. We cannot exclude that the K-Ar system was affected by a later collisional event. The cosmic-ray exposure (CRE) age of Isheyevo determined by cosmogenic 38 Ar is ~34 Ma, similar to that of the Bencubbin (CB a) meteorite.

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