Shocked rocks and impact glasses from the El'gygytgyn impact structure, Russia

Abstract

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 Lake El’gygytgyn Scientific Drilling Project – Conquering Arctic Challenges through Continental Drilling

“…The shock-induced changes observed in porphyritic volcanic rocks from El´gygytgyn can be applied to a general classification of shock metamorphism of siliceous volcanic rocks. That El´gygytgyn is an impact crater was confirmed in the late 1970s by Gurov and co-workers (Gurov et al, 1978(Gurov et al, , 2007Gurov and Koeberl, 2004), who found shocked minerals and impact glasses in samples at the crater. However, the impact rocks on the surface have been almost totally removed by erosion, and so the El´gygytgyn Drilling Project provides the unique opportunity to study the crater-fill impactites in situ and determine their relations and succession.…”

Mass movement deposits in the 3.6 Ma sediment record of Lake El'gygytgyn, Far East Russian Arctic

“…The circular-shaped impact crater has a diameter of 18 km and was formed in Cretaceous volcanic rocks, such as rhyolitic and dacitic lavas, tuffs and ignimbrites as well as andesites and tuffs of andesitic composition (Gurov and Koeberl, 2004). The highlands at the crater rim today have heights between 50 and 430 m above lake level.…”

El'gygytgyn impact crater, Russia: Structure, tectonics, and morphology

“…Although the remnants of ejecta around the structure are completely eroded, El'gygytgyn crater is one of the better preserved impact structures on Earth (Dietz and McHone 1976;Layer 2000;Gurov and Koeberl 2004;Gebhardt et al 2006). Its surface expression in space images has been described as "excellent" (Garvin and Grieve 1986;Grieve et al 1988).…”

“…A classification of shock-metamorphosed siliceous volcanic rocks was proposed for shock pressures up to 55 GPa Gurova 1979, 1991;Gurov and Koeberl 2004;Gurov et al 2005). The high-pressure quartz polymorphs stishovite and coesite were detected in quartz from strongly shocked rhyolites and rhyolitic tuffs (Gurov and Gurova 1991;Gurov and Koeberl 2004).…”