Paper No. 7
Presentation Time: 9:45 AM
ALLEGHANIAN THERMAL HISTORY OF BASEMENT-DERIVED TARGET ROCKS IN THE ICDP-USGS EYREVILLE-B CORE FROM THE CHESAPEAKE BAY IMPACT STRUCTURE
KUNK, Michael J., U.S. Geological Survey, 926A National Center, Reston, VA 20192, USA USA, HORTON, J. Wright, U.S. Geol Survey, 926A National Center, Reston, VA 20192, GIBSON, Roger L., School of Geosciences, University of the Witwatersrand, PO WITS, Johannesburg, 2050, South Africa, ZACK, Thomas, Institut fuer Geowissenschaften, Universitaet Mainz, Becherweg 21, Mainz, 55128, Germany, MCALEER, Ryan J., Indiana Department of Geological Sciences, 1001 East 10th Street Bloomington, IN 47405, Bloomington, IN 47405 and TOWNSEND, Gabrielle N., School of Geosciences, University of the Witwatersrand, Private Bag 3, PO Wits, Johannesburg, 2050, South Africa, mkunk@usgs.gov
40Ar/
39Ar age-spectrum data from amphibole, muscovite, and biotite, together with a weighted mean
206Pb/
238U concordia age from rutile, collected from target rocks in the ICDP-USGS Eyreville-B core are consistent with simple cooling from peak Alleghanian metamorphism for target rocks prior to the time of the ~ 35.5 Ma impact event. From the base upward, the core consists of basement-derived schist, pegmatite, and granite (1766.3- 1551.2 m depth), melt-poor polymict impact breccias (1551.2-1474.1 m), melt-rich suevites (1474.1-1397.2 m), sand with lithic boulders (1397.2-1371.1 m), and a granite slab (1371.1-1095.7 m). Although the rocks are not in their original emplacement positions due to the disturbance of impact, they can be used to model the cooling history of coastal-plain basement target rocks of the Chesapeake Bay impact structure prior to the impact event.
Rutile from a mica schist in the lower basement-derived section yields an apparent age of 259 ± 13 Ma. The Zr-in-rutile thermometer suggests a peak metamorphic temperature of ~606ºC for this sample. 40Ar/39Ar dating results from amphibole and biotite from the lowest portion of the core give ages of 249 ± 1.3 Ma and 240 ± 1.2 Ma, and indicate cooling through the 500ºC and 300ºC isotherms respectively. Dating results from two muscovite samples from a pegmatite in the lowest portion of the core gives reproducible ages of 244.1 ± 1.3 Ma, indicating cooling through the 350ºC isotherm. These apparent age results suggest fairly rapid cooling of the basement rocks from a maximum temperature of ~ 600ºC at about 259 Ma to ~300ºC at about 240 Ma, an average cooling rate of about 16ºC per million years, although the rate may not have been constant through time.
