PETROGRAPHY, GEOCHRONOLOGY, AND SIGNIFICANCE OF CRYSTALLINE BASEMENT ROCKS AND IMPACT-DERIVED CLASTS IN THE CHESAPEAKE BAY IMPACT STRUCTURE, SOUTHEASTERN VIRGINIA

Crystalline basement rocks, including impact ejecta, were recovered from the NASA Langley (City of Hampton) and North and Bayside (Matthews Co.) coreholes in the late Eocene Chesapeake Bay impact structure, southeastern Va.  Each core has a complete section of the Exmore diamicton overlying impact-disturbed Cretaceous sediments.  Crystalline basement below the Cretaceous section was recovered at Langley (626.3 m to 635.1 m depth) and Bayside (708.9 m to 728.5 m depth).  Basement rocks in both cores are massive, pale red, medium-grained monzogranites that are highly chloritized.  The tops of the granites are weathered but not saprolitized.  Features diagnostic of shock metamorphism were not found in these granites.  Most crystalline clasts in the diamicton and underlying sediments are rounded and assumed to be detrital.  However, some angular clasts from all three cores have cataclastic fabrics in which shock-metamorphosed quartz is an integral part, and shocked plagioclase is present in a clast from the Bayside core.  Clasts interpreted to be ejecta include felsic to mafic plutonic rocks and felsite.  Preliminary SHRIMP U-Pb zircon ages (± 2s) for the basement granites indicate Neoproterozoic crystallization (625 ± 11 Ma at Bayside and 612 ± 10 Ma at Langley).  White mica in a cataclastic leucogranite clast from the Exmore diamicton (Bayside core) has ⁴⁰Ar/³⁹Ar age-spectrum data consistent with Neoproterozoic U-Pb ages of the other granites and shows no evidence for Paleozoic metamorphism >350^oC.  A preliminary U-Pb age of 87 Ma reported at AGU (2002) for this clast is retracted because the zircon separate was contaminated.  K-feldspars from the clast and from the Langley granite have ⁴⁰Ar/³⁹Ar age spectra indicating temperatures >250^oC until about 346 Ma and 324 Ma, respectively, and final cooling through feldspar closure (~150^oC) at about 255 Ma and 245 Ma, respectively.  Fission-track ages (± 2s) of zircon (362 ± 49 Ma and 332 ± 44 Ma at Bayside, 375 ± 44 Ma at Langley) and apatite (239 ± 79 Ma and 198 ± 40 Ma at Bayside, 184 ± 32 Ma at Langley) from the basement granites show no impact-related thermal disturbance.  The granites differ from Mesoproterozoic rocks beneath the N.J. Coastal Plain, but are similar to Neoproterozoic rocks beneath the N.C. Coastal Plain.  This similarity raises doubts about an intervening Chesapeake Bay suture.