Geological Society of America

T229 Exploring Subsurface Terranes and Buried Basins of Eastern and Central North America—Geology, Geophysics, and Geochronology

Wednesday, 12 October 2011
8:00 AM - 12:00 PM
Room 200A-C

ALLEGHANIAN THERMAL HISTORY OF BASEMENT-DERIVED TARGET ROCKS IN THE ICDP-USGS EYREVILLE-B CORE FROM THE CHESAPEAKE BAY IMPACT STRUCTURE

Paper number: T229-7
Program: Technical Sessions
Time: 9:45 AM – 10:00 AM

Authors

Michael J. Kunk Presenting Author
U.S. Geological Survey, Reston, VA
James Horton
U.S. Geol Survey, Reston, VA
Roger L. Gibson
University of the Witwatersrand, Johannesburg
Thomas Zack
Universitaet Mainz, Mainz, N/A
Ryan J. McAleer
Indiana Department of Geological Sciences, Bloomington, IN
Gabrielle N. Townsend
University of the Witwatersrand, Johannesburg, N/A

Abstract

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.