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Petrography and geochemistry of target rocks and impactites from the Ilyinets Crater, Ukraine
Author(s) -
GUROV Eugene P.,
KOEBERL Christian,
REIMOLD Wolf Uwe
Publication year - 1998
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1998.tb01316.x
Subject(s) - breccia , geology , impact structure , petrography , geochemistry , impact crater , plagioclase , clastic rock , country rock , basement , feldspar , lithic fragment , gneiss , quartz , petrology , mineralogy , sedimentary rock , metamorphic rock , paleontology , astrobiology , physics , civil engineering , engineering
— The ∼400 Ma old Ilyinets impact structure was formed in the Precambrian basement of the Ukrainian Shield and is now mostly covered by Quaternary sediments. Various impact breccias and melts are exposed in its southern section. The crater is a complex structure with a central uplift that is surrounded by an annular deposit of breccias and melt rocks. In the annulus, brecciated basement rocks are overlain by up to 80 m of glass‐poor suevitic breccia, which is overlain (and partly intercalated) by glass‐rich suevite with a thickness of up to 130 m. Impact‐melt rocks occur within and on top of the suevites—in some cases in the form of devitrified bomb‐shaped impact‐glass fragments. We have studied the petrographic and geochemical characteristics of 31, mostly shocked, target rock samples (granites, gneisses, and one amphibolite) obtained from drill cores within the structure, and impact breccias and melt rock samples from drill cores and surface exposures. Multiple sets of planar deformation features (PDFs) are common in quartz, potassium feldspar, and plagioclase of the shocked target rocks. The breccias comprise more or less devitrified impact melt with shocked clasts. The impact‐melt rocks (“bombs”) show abundant vesicles and, in some cases, glass is still present as brownish patches and schlieren. All impact breccias (including the melt rocks) are strongly altered and have significantly elevated K contents and lower Na contents than the target rocks. The alteration could have occurred in an impact‐induced hydrothermal system. The bomb‐shaped melt rocks have lower Mg and Ca contents than other rock types at the crater. Compared to target rocks, only minor enrichments of siderophile element contents ( e.g. , Ni, Co, Ir) in impact‐melt rocks were found.