BEDROCK GEOLOGY OF THE MADISON 7.5’ QUADRANGLE, BLUE RIDGE PROVINCE VIRGINIA

Rocks exposed in the Madison 7.5' quadrangle, north-central Virginia, lie in the core of the Blue Ridge anticlinorium. The oldest rocks include banded gneisses that occur as screens within Mesoproterozoic plutons. The banded gneisses, are intruded by leucocratic granitic gneisses and younger charnockites, biotite granitoids, and alkali-feldspar granites associated with the Grenvillian event. Based on field observations the charnockite-biotite granitoid contact is not intrusive, rather it is a 1-km wide zone of rocks transitional in composition. Two distinct foliations commonly occur in the basement complex: a fabric defined by recrystallized feldspar and quartz aggregates that is crosscut and overprinted by a younger fabric defined with greenschist facies minerals. The older fabric is interpreted to have formed at the upper amphibolite to granulite facies and be related to the Grenvillian event, whereas the younger fabric is Paleozoic.

A-type granitoid plutons of the 730-700 Ma Robertson River Igneous Suite intrude the basement complex. Metasedimentary rocks of the Neoproterozoic Mechum River Formation crop out in a narrow (0.4-1.5 km wide) NE-SW trending belt. On its western edge, the Mechum River Formation unconformably overlies the basement. To the east, the belt is bounded by steeply dipping reverse faults. Tabular bodies of hornblende gabbro cut all Proterozoic units in the map area.

The Robertson River granitoids are cut by the NE-SW striking White Oak Run high-strain zone. Mylonites in this zone dip steeply to the NW and record dip-slip extensional movement under amphibolite facies conditions. This zone may record deformation associated with Neoproterozoic rifting. The Quaker Run high-strain zone (QRHSZ), in the NW part of the Madison quadrangle, is a belt of greenschist facies mylonitic rocks that cuts the basement complex. Foliations in the QRHSZ strike NE-SW and dip moderately to the SE with down dip elongation lineations. The QRHSZ is a Paleozoic reverse dip-slip fault zone that accommodated approximately 1.5 km of displacement, but does not separate two distinct basement massifs.