Abstract-The El'gygytgyn impact structure is about 18 km in diameter and is located in the central part of Chukotka, arctic Russia. The crater was formed in volcanic rock strata of Cretaceous age, which include lava and tuffs of rhyolites, dacites, and andesites. A mid-Pliocene age of the crater was previously determined by fission track (3.45 ± 0.15 Ma) and 40 Ar/ 39 Ar dating (3.58 ± 0.04 Ma). The ejecta layer around the crater is completely eroded. Shock-metamorphosed volcanic rocks, impact melt rocks, and bomb-shaped impact glasses occur in lacustrine terraces but have been redeposited after the impact event.Clasts of volcanic rocks, which range in composition from rhyolite to dacite, represent all stages of shock metamorphism, including selective melting and formation of homogeneous impact melt. Four stages of shocked volcanic rocks were identified: stage I (≤35 GPa; lava and tuff contain weakly to strongly shocked quartz and feldspar clasts with abundant PFs and PDFs; coesite and stishovite occur as well), stage II (35-45 GPa; quartz and feldspar are converted to diaplectic glass; coesite but no stishovite), stage III (45-55 GPa; partly melted volcanic rocks; common diaplectic quartz glass; feldspar is melted), and stage IV (>55 GPa; melt rocks and glasses). Two main types of impact melt rocks occur in the crater: 1) impact melt rocks and impact melt breccias (containing abundant fragments of shocked volcanic rocks) that were probably derived from (now eroded) impact melt flows on the crater walls, and 2) aerodynamically shaped impact melt glass "bombs" composed of homogeneous glass. The composition of the glasses is almost identical to that of rhyolites from the uppermost part of the target. Cobalt, Ni, and Ir abundances in the impact glasses and melt rocks are not or only slightly enriched compared to the volcanic target rocks; only the Cr abundances show a distinct enrichment, which points toward an achondritic projectile. However, the present data do not allow one to unambiguously identify a meteoritic component in the El'gygytgyn impact melt rocks.

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.

Abstract-The 3.6 Myr old El'gygytgyn impact crater is located in central Chukotka, northeastern Russia. The crater is a well-preserved impact structure with an inner basin about 15 km in diameter, surrounded by an uplifted rim about 18 km in diameter. The flat floor of the crater is in part occupied by Lake El'gygytgyn, 12 km in diameter, and surrounding terraces. The average profile of the rim is asymmetric, with a steep inner wall and a gentle outer flank. The rim height is about 180 m above the lake level and 140 m above the surrounding area. An outer ring feature, on average 14 m high, occurs at about 1.75 crater radii from the center of the structure.El'gygytgyn crater is surrounded by a complex network of faults. The density of the faults decreases from the bottom of the rim to the rim crest and outside the crater to a distance of about 2.7 crater radii. Lake El'gygytgyn is surrounded by a number of lacustrine terraces. Only minor remnants are preserved of the highest terraces, 80 and 60 m above the present-day lake level. The widest of the terraces is 40 m above the current lake level and surrounds the lake on the west and northwest sides. The only outlet of the lake is the Enmivaam River, which cuts through the crater rim in the southeast. In terms of structure, El'gygytgyn is well preserved and displays some interesting, but not well understood, features (e.g., an outer ring), similar to those observed at a few other impact structures.