ARGON DATING CONSTRAINTS ON THE AGE AND COOLING HISTORY OF PRE-CRETACEOUS BASEMENT ROCKS BENEATH THE MID-ATLANTIC COASTAL PLAIN OF MARYLAND AND VIRGINIA

Hornblende, muscovite, biotite and K-feldspar mineral separates and slaty whole rock samples from drill cores and cuttings have been analyzed using ⁴⁰Ar/³⁹Ar dating techniques to constrain the timing of cooling from emplacement and metamorphism and subsequent below closure temperature growth of white mica of pre-Cretaceous basement rocks beneath the Mid-Atlantic Coastal Plain in Maryland and Virginia.

Near the Fall Zone in the western portion of the study area muscovite data from the Potomac composite terrane (~380 Ma) and the Wilmington Complex (365 Ma) ~50 km northeast of Baltimore indicate cooling through argon closure temperatures (~350^oC) from the Acadian orogeny in the Late Devonian and later below closure temperature growth of white mica. Muscovite from an extension of the Goochland terrane ~30 km south of Washington DC (WDC) yielded an Alleghanian cooling age of ~239 Ma. Hornblende from rocks beneath the Taylorsville rift basin ~20 km northeast of Richmond, VA yielded a cooling age (~500^oC) of ~330 Ma.

In the central part of our study area, biotite from rocks overlaying the Salisbury gravity anomaly ~50 km east of WDC yielded a cooling age (~300^oC) of ~252 Ma. Whole rock slaty tuffaceous metasiltstones from Crisfield, MD yielded age spectra suggesting white mica growth 442 Ma and partial thermal resetting at 280 Ma in the Alleghanian. Hornblende, muscovite, and K-feldspar (~250^oC) from metagranodiorite of Isle of Wight ~40 km west of Norfolk, VA yielded cooling and partial resetting ages ranging from ~ 635 Ma (Hb) to <220 Ma (Ksp), indicating a Neoproterozoic to Mesozoic thermal history. Muscovite, biotite, and K-feldspar cooling ages from the granitoid Cambridge, Portsmouth and Dort plutons ranged from 280 Ma to 243 Ma and likely represent cooling from emplacement. Hornblende and muscovite samples from near the tip of the Delmarva Peninsula yielded ages of 249 Ma and 244 Ma respectively suggesting fairly rapid cooling after metamorphism.

Finally, an amphibole sample from a metagabbro near Berlin, MD in the eastern part of the study area yielded an age spectrum suggestive of the inclusion of excess argon with a minimum age of 524 Ma.

Together these data from beneath the coastal plain of VA and MD indicate a complex tectonothermal environment that spans from the late Neoproterozoic into the Late Triassic.