PETROGRAPHIC AND CHEMICAL ANALYSIS OF THE LOWER CRYSTALLINE BASEMENT-DERIVED SECTION, AND THE UPPER GRANITE AND AMPHIBOLITE MEGABLOCKS, EYREVILLE-B CORE, CHESAPEAKE BAY IMPACT STRUCTURE

Crystalline rocks intersected in the ICDP-USGS Eyreville-B drillcore Chesapeake Bay impact structure were studied in two zones: a 218.76-m-thick basement-derived section located between 1547.46 and 1766.22 m (beneath suevitic breccias), and a granitic megablock and an amphibolitic megablock located between 1095.74 and 1389.71 m (above the suevitic breccias). The basement-derived section comprises roughly equal proportions of strongly foliated, largely graphitic, upper amphibolite facies mica schists (muscovite + quartz + plagioclase + graphite + biotite + pyrite ± fibrolite ± cordierite ± garnet) and granites/granite pegmatites (also containing muscovite ± biotite ± garnet). Subsidiary rock types include amphibolite, meta-psammite, calc-silicate and tourmalinite. The amphibolite “megablock” (amphibole + quartz + plagioclase + biotite ± K-feldspar) contrasts with the amphibolite below the suevitic breccias as it contains biotite. The granite megablock contains biotite as the main mica, and K-feldspar is more abundant than in the granite pegmatites below the suevitic breccias as reflected in the bulk rock geochemistry. Rare xenoliths of biotite-amphibole gneiss (biotite + plagioclase + quartz + epidote + amphibole) in the granite have strong foliation as in the deeper basement-derived section. All rocks are variably altered. Shock features are lacking in the upper megablocks, where deformation is manifested by mica foliation. Initial petrography of the lower basement-derived samples did not indicate any confirmed shock features. First chemical analyses of the granites in the upper megablock and from the lower basement-derived section indicate a peraluminous ASI index of 1.04 to 1.12, consistent with syn-collisional origin. Our observations suggest that the upper megablocks and the lower basement-derived section could have formed from one or more middle to upper amphibolite-facies metamorphic suites within the Chesapeake Bay target that differ from other known greenschist-facies target rocks.