PRE-IMPACT TECTONOTHERMAL EVOLUTION OF THE CRYSTALLINE TARGET IN THE ICDP-USGS EYREVILLE-B CORE, CHESAPEAKE BAY IMPACT STRUCTURE

Pre-impact crystalline rocks of the lower 221 m of the Eyreville-B drill core comprise a sequence of mica schists with subsidiary metapsammites, amphibolite and calc-silicate rock, intruded by muscovite (± biotite ± garnet) granite and granite pegmatite. The schists are commonly graphitic and contain up to 1 cm plagioclase porphyroblasts. Fibrolite knots, cordierite and rare garnet occur locally and indicate upper amphibolite facies peak metamorphic conditions. The schists display an intense shallowly-dipping S1 shear foliation with local micro- to dm-scale recumbent folds. Zones of chaotically-oriented foliation, resembling breccias but having the same peak metamorphic paragenesis, developed locally and are interpreted as shear-disrupted fold hinges. Mineral textural relations in the mica schists (plagioclase and garnet porphyroblast Si-Se relations, crenulation microfolds defined by fibrolite) indicate metamorphic peak was attained during D1. Fabric analysis indicates, however, that subhorizontal (present orientation) shear deformation continued during retrograde cooling, during which strain became partitioned into narrower zones leading to successive overprinting of high-T shear fabrics (S-C and S-C') by greenschist-facies fabrics. Carbonate-cemented breccias may reflect a final pulse of cataclastic deformation or be related to smaller, steeply inclined anastomosing fractures with chlorite and calcite infill (interpreted as D2). This event was accompanied by chlorite-sericite-calcite retrogression of the wall rocks. Granite and granite pegmatite display local discordance to the S1 foliation, but elsewhere are affected by high-T mylonitic shear deformation. The tectonothermal history of these rocks is compared to that of Appalachian terranes to the west.