THE COMPLEXITY OF SHOCK FEATURES IN A VOLCANIC LITHOLOGY: THE EL'GYGYTGYN “SUEVITE”

The 2009 ICDP drilling project in the 3.6 Ma, 18-km-diameter El’gygytgyn impact structure, excavated in Late Cretaceous siliceous volcanic rocks of central Chukotka, northeastern Russia, has provided a drill core through the impact lithic breccia, underlying the lake sediments, and the fractured bedrock. The stratigraphy of the core is by now widely accepted and it mainly consists of: (i) lithic impact breccia, including possibly volcanic and impact melt clasts, locally with shocked minerals, in a fine-grained clastic matrix (316 - 385 m below lake bottom); (ii) a transition zone with a reddish breccia crosscut by basalt-like veins (385 - 427 mblb); (iii) greenish ignimbrite-like rock, crosscut by conjugate sets of fractures locally filled by carbonate, silica, or clay, and with abundant melt particles of intermediate composition, elongated parallel to the flattening direction (427 - 517 mblb).

Here we focus on the shock features in the upper part of the core. The shock features vary in abundance and are localized in some rock fragments, especially in the preserved phenocrysts. The minerals mostly affected are quartz, locally with abundant PDFs, with [10-13] as the main orientation, and feldspars that contain a few sets of PDFs and show local amorphization. Some high pressure silica phases, such as coesite, occur locally. The breccia is also rich in melt drops and glasses that are almost completely altered. Nevertheless, as the target consists mainly of ignimbrite, tuff, and lava of rhyolitic composition, we do not yet have clear criteria for the distinction between impact and volcanic melts; thus the classification of this breccia as a suevite, despite the presence of shocked minerals, is not straightforward. Furthermore, the lithic impact breccia might be considered monomict because it includes exclusively volcanic lithologies, but polymict for the variety in composition and provenance of the rock fragments.

The geochemistry of the lithic impact breccia is consistent with the bulk composition of the unshocked to slightly shocked lower domain, which is of trachy-rhyolitic composition. No chemical anomalies occur in the impact lithic breccia, except in the dark veins in the transition zone, which seem to have an ultramafic composition consistent with late Neogene volcanism in the area.