RECORD OF LATE GRENVILLIAN MAGMATISM AND DEFORMATION IN THE NORTHERN BLUE RIDGE ANTICLINORIUM, VA AND MD, AND COMPARISON TO OTHER MASSIFS

Detailed geologic mapping in the core of the Blue Ridge anticlinorium in northern Virginia and Maryland has subdivided the Grenvillian basement into 14 units, the majority of which are metaplutonic rocks of granitic composition. Layered gneisses of metasedimentary and metavolcanic origin are volumetrically minor. All have a Late Mesoproterozoic tectonic foliation formed under hornblende-granulite facies metamorphic conditions. TIMS and SHRIMP U-Pb dating of zircon in the granitic rocks have delineated three age groups that define magmatic pulses corresponding to the late Shawinigan and Ottawan phases of the Grenville orogeny: (1) ~1140-1153 Ma; (2) ~1111 Ma; and (3) ~1055-1077 Ma. Rocks of Group 1 contain a strong NW-trending foliation, whereas rocks of Groups 2 and 3 contain a variably oriented weaker foliation with associated downdip lineations. Group 3 rocks locally crosscut Group 1 rocks. The fabrics preserved in these rocks, combined with U-Pb ages, suggest the following deformational sequence: D1, between 1140 and 1111 Ma, producing NW-trending foliation in Group 1 rocks; D2, post-1055 Ma, producing variably oriented NE- to NW-trending foliation synchronous with the development of steeply plunging sheathlike folds and coaxial mineral lineations; and D3, post-1055 Ma and post-D2, development of local, tight, gently N- or S-plunging folds and broader arching of D2 foliation. Monazite obtained from some of these rocks has complex growth zonation patterns and U-Pb ages ranging from 1130 Ma to 1033 Ma. These ages indicate that the younger Ottawan dynamothermal events in the region did not reset older monazite ages and therefore did not produce regional temperatures in excess of about 730^oC.

The three groups of Grenvillian plutonic ages determined for the northern Blue Ridge have approximate correlatives in the Adirondack Highlands. In addition, Group 1-age plutonic rocks occur in the Hudson Highlands and Green Mountains massif but are volumetrically minor; most of the intrusive rocks in these massifs are 1170 Ma or older. Indeed, it is the dominance of Ottawan fabrics and absence of pre-Shawinigan age rocks that most distinguishes the northern Blue Ridge Grenvillian rocks from massifs to the north.