DETRITAL ZIRCON DOUBLE DATING FROM THE SCOTTSVILLE BASIN, VIRGINIA: IMPLICATIONS FOR MESOZOIC TECTONICS ON THE ATLANTIC RIFT SHOULDER

Mesozoic rift basins in Eastern North America (ENAM) are the product of Pangaean rifting during the early Mesozoic which ultimately generated the Atlantic Ocean basin. The Scottsville basin is a small Mesozoic basin located between the Blue Ridge and Piedmont in central Virginia and forms one of the westernmost of the ENAM basins. We use U/Pb and (U-Th)/He double dating of zircons to determine sediment provenance and infer subsequent the low-temperature exhumation/erosion history of the Scottsville basin.

Three samples from sandstones and conglomerates in the Scottsville basin are characterized by unimodal populations of 1.0 to 1.15 Ga detrital zircons, sourced from Grenvillian Blue Ridge basement rock or its Neoproterozoic cover sequence in the football to the west of the basin. One sample of arkosic hornfels, at the contact of a ~80 m wide CAMP-related diabase dike, yielded detrital zircons with only Grenvillian ages and no ~200 Ma rims. The duration of the thermal pulse associated with dike emplacement was likely insufficient to produce overgrowths. Detrital zircons in a sandstone from the bottom of the Scottsville basin, and at its eastern margin, are dominated by Grenvillian ages, but contain a modest population of 450 to 470 Ma zircons likely derived from the Chopawamsic arc terrane to the east. These data are consistent with a well-developed footwall drainage network during Scottsville basin formation, but only modest detrital inputs from the hanging wall block.

(U-Th)/He zircon ages were obtained from zircons in the basin as well as along both a footwall and hanging wall traverse. Ages range from ~160 to 205 Ma, with a slight decrease of ages towards the basin in both the footwall and hanging wall. The absence of older (U-Th)/He zircon ages (Paleozoic to early Triassic) in the Scottsville basin rocks indicates that post-depositional burial was sufficient to reset the (U-Th)/He zircon system and/or a widespread post-Triassic thermal event affected rocks at the western margin of the Atlantic rift shoulder.