Impactite Generation in the El'gygytgyn Depression, Northeast Russia, as a Volcanic Phenomenon.2. On the Petrography and Geochemistry of the Impactites

摘要

A detailed petrographic study of the impactites found in the El'gygytgyn depression suggested a division of those into the dacite—rhyolite (dominant) and the andesite—basalt (about 1% of the total impactite volume) groups. The protolith for the dacite—rhyolite group was the Cretaceous ignimbrite rock association of the Okhotsk—Chukchi volcanic belt (OChVB), from which the depth of their origin is estimated as 1-1.5 km. The impactites of the andesite—basalt group, which are generally older, formed from protoliths of apparently metamorphic and magmatic complexes in the OChVB basement occurring at depths of 6.5-8.5 km. The hypothesis that the depths of origin for these groups are different is supported by certain features of clinopyroxene composition. Geochemical relationships are found to exist between deep and shallow impactites, as shown in the distribution of Ni, Cr, and Co. The succession of development for the shallow impactite generation forms the following series: impact ignimbrite→ pumice→ cinder→ massive glass. The glasses have the most homogeneous overall chemical composition, but these too are characterized by a high inhomogeneity in the composition of the vitric phases, indicating a rapid and discrete impactite generation that impeded melt homogenization. The glasses are the final product of impactite generation; they show a dramatic drop in fluid content and the associated reducing components.