DIKE BRECCIAS IN THE DEEP BASEMENT-DERIVED SECTION OF THE EYREVILLE B CORE, CHESAPEAKE BAY IMPACT STRUCTURE

Dike breccias, including monomict and polymict lithic breccia, suevite, impact melt, and locally produced pseudotachylite (= friction melt), are known from many impact structures. Their origin is strongly debated. The Eyreville B core intersected a number of dike and fault breccia occurrences in the deep, basement-derived section (unshocked mica schists and granite/granite pegmatite) below 1551m depth. Mylonites occur in this section as well but are thought to predate the impact event like some of the brittle fault zones. Twenty samples from 15 dike breccia occurrences (< 1 cm - > 1 m in thickness) are available for petrographic and chemical analysis. Polymict lithic breccia (without melt particles) and suevite (with impact melt clasts), both with ample shocked clastic material, monomict lithic breccia (after quartz-rich muscovite schist, with shocked clasts), melt-bearing suevitic breccia (melt particles but without shocked clasts), and cataclasite (generally mica schist-granite mixtures, unshocked clasts) could be distinguished. In addition, one sample from 1554 m depth requires confirmation as polymict lithic breccia or suevite, and a sample from 1691.85 m depth could represent a pseudotachylite. Micropetrographic analysis is in progress. Only polymict lithic breccia and suevite dikes from the uppermost part of the section contain significant amounts of sedimentary target material. A large proportion of locally derived clastic material is everywhere observed. Seven suevitic dike breccias analysed so far have similar mica schist-like major element compositions, even if the immediate host rock is granitic. The dike materials have lower SiO2 and MgO contents and higher Al2O3, Fe2O3, Na2O and K2O contents than the main suevite package (67 samples analysed). It seems likely that the less competent schists facilitated stoping by impact breccias in comparison with granitic rocks.