TECTONOTHERMAL SIGNATURES OF TACONIAN AND ALLEGHANIAN OROGENESIS IN THE CENTRAL VIRGINIA SEISMIC ZONE
A steeply dipping NE-striking schistosity (Sn) is the dominant foliation of the MRC. Sn 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 Sn 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 Sn. Eastward, across the regional Taconian biotite isograd, Sn WM yields ages of 405-425 Ma interpreted to reflect slow cooling from peak conditions. East of these samples a strong Sn+1 greenschist facies overprint is reflected in disturbed 40Ar/39Ar WM spectra that suggest that Sn+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.