Geochemistry of impactites and crystalline basement-derived lithologies from the ICDP-USGS Eyreville A and B drill cores, Chesapeake Bay impact structure, Virginia, USA

We investigated whole-rock chemical compositions of 318 samples of Exmore breccia (diamicton), impactite (suevite, impact melt rock, polymict lithic impact breccia), and crystalline basement-derived rocks from 444 to 1766 m depth in the International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville A and B drill cores (Chesapeake Bay impact structure, Virginia, USA). Here, we compare the average chemical compositions for the Exmore breccia (diamicton), the impactites and their subunits, sandstone, granite, granitic gneiss, and amphibolite of the lithic block section (1095.7-1397.2 m depth), cataclastic gneiss of the impact breccia section, and schist and pegmatite/granite of the basal crystalline section (1551.2-1766.3 m depth). The granite of the megablock (1097.7-1371.1 m depth) is of I-type and is seemingly related to a syncollisional setting. The amphibolite (1377.4-1387.5 m depth) of the lithic block section is of igneous origin and has a tholeiitic character. Based on chemical composition, the Exmore breccia (diamicton) can be subdivided into five units (444.9-450.7, 450.7-468, 468-518, 518-528, and 528-similar to 865 m depth). The units in the depth intervals of 450.7-468 and 518-528 m are enriched in TiO2, MgO, Sc, V, Cr, and Zn contents compared to the other Exmore breccia units. In some samples, especially at similar to 451-455 m depth, the Exmore breccia contains significant amounts of P2O5. The Exmore breccia is recognized as a mixture of all sedimentary and crystalline target components, and, when compared to the impactites, it contains a significant amount of a SiO2-rich target component of sedimentary origin. The chemical composition of the impactites overlaps the compositional range for the Exmore breccia. The impactites generally display a negative correlation of SiO2 and CaO, and a positive correlation of TiO2, Al2O3, Fe2O3, and MgO with depth. This is the result of an increasing basement schist component, and a decreasing sedimentary and/or granitic component with depth. Suevite units S2 and S3 display distinct enrichment of Na2O by a factor of similar to 2 compared to all other impactite units, which is interpreted to reflect a higher granitic component in these units.