AGE OF MID-AMPHIBOLITE FACIES ALLEGHANIAN METAMORPHISM IN ROCKS FROM THE ICDP-USGS EYREVILLE-B CORE, CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA

The ICDP-USGS Eyreville-B drill core into the Chesapeake Bay impact structure intersected a 215 m section of basement-derived mica schists with subsidiary metapsammites, amphibolites, and calc-silicate rocks that is intruded by muscovite granite and granitic pegmatite. In this study, rutiles from both the mica schists and a tourmalinite were dated by in-situ U/Pb LA-ICP-MS geochronology. Additionally, four white-mica separates from the mica schists and two amphibole separates from an amphibolite were analyzed by the ⁴⁰Ar/³⁹Ar step-heating technique. A weighted mean ²⁰⁶Pb/²³⁸U concordia age of 259 ± 13 Ma (MSWD 0.054) was obtained for the rutile samples. Application of the Zr-in-rutile thermometer reveals a peak metamorphic temperature of 606 ± 24 °C, in good agreement with previous estimates based on pelitic mineral assemblages. The four white-mica separates from schists record cooling through white-mica closure between 242 Ma and 245 Ma, indistinguishable from a previously determined age of 244 ± 1 Ma from muscovite in a pegmatite vein. The amphibole separate from amphibolite has a ⁴⁰Ar/³⁹Ar plateau age of 249.0 ± 1.3 Ma. The rutile ages are consistent with a peak mid-amphibolite facies metamorphic event at ca. 250-260 Ma, towards the end of the Alleghanian orogeny. They overlap a published U/Pb SHRIMP zircon age of 254 ± 3 Ma from a granite megablock overlying gravelly sands above the impactites, and suggest that this megablock and rocks in the lower basement-derived section share the same Alleghanian tectonothermal history.