TECTONOTHERMAL SIGNATURES OF TACONIAN AND ALLEGHANIAN OROGENESIS IN THE CENTRAL VIRGINIA SEISMIC ZONE

New and published ⁴⁰Ar/³⁹Ar isotopic data from samples collected in concert with 1:24000 scale mapping contribute to our understanding of the bedrock framework in the Central Virginia Seismic Zone (CVSZ). This study delineates two distinct metamorphic domains: 1) a Taconian domain in the Mine Run Complex (MRC) west of the Chopawamsic Fault and 2) an Alleghanian domain to the east.

­A steeply dipping NE-striking schistosity (S_n) is the dominant foliation of the MRC. S_n is truncated by and therefore predates the ~441 Ma Ellisville Pluton and more mafic ~450 Ma Lahore and Green Springs plutons (GSP). Fine-grained white mica (WM) separated from regional chlorite zone phyllites directly adjacent to the GSP yield a ⁴⁰Ar/³⁹Ar plateau age of 445±2 Ma, indicating that pluton emplacement led to thermal resetting of the S_n WM. In contrast, WM from chlorite zone phyllites farther from the GSP yield climbing age spectra. Data from coarser-grained WM from 2 of these samples show that detrital WM grains retain Precambrian ages. This explains the climbing ages, but precludes determination of a precise age for the formation of S_n. Eastward, across the regional Taconian biotite isograd, S_n WM yields ages of 405-425­ Ma interpreted to reflect slow cooling from peak conditions. East of these samples a strong S_n+1 greenschist facies overprint is reflected in disturbed ⁴⁰Ar/³⁹Ar WM spectra that suggest that S_n+1 WM formed at <330 Ma. However, an amphibole age of ~450 Ma in this belt shows that peak metamorphism occurred in the Ordovician.

In contrast, 4­ km to the SE across the Chopawamsic and Long Branch faults multiple amphibole and WM samples yield plateau ages of ~310 and ~29­0 Ma, respectively. Similar plateau ages persist across the entire Chopawamsic terrane, and into the Elk Hill complex where amphibole and WM yield ages of 299 ± 1 Ma and 286 ± 1 Ma, respectively. These data suggest rapid exhumation of this entire belt of rocks against the MRC in the Early Permian.

At a regional scale the absence of an Acadian overprint in rocks west of the MRC is notable. In correlative rocks along the Potomac River, Acadian-aged fabrics and faults are prevalent. Perhaps the position of those rocks in the Pennsylvania recess left them exposed to the Acadian deformation while to the south more oblique collisions transferred little strain inboard to the rocks of the CVSZ.