FRAMING THE NEOPROTEROZOIC IAPETAN RIFT SEQUENCE: GEOLOGY OF THE ALBERENE 7.5' QUADRANGLE, EASTERN BLUE RIDGE, VIRGINIA
The overall structural geometry of the rocks in the Alberene quadrangle is a southeast-dipping sequence from the basement to the Candler Formation showing stratigraphic continuity and no evidence for major terrane boundaries. Rocks in the eastern Blue Ridge are dominated by three regional fracture sets, including subvertical WNW/ESE, NNW/SSE, and NE/SW sets that likely developed during Alleghanian contraction and later Mesozoic extension.
We have recognized at least three types of metamorphosed mafic to ultramafic rocks other than the Catoctin Formation. These are distinguished on the basis of field characteristics, mineralogy, and whole-rock chemical compositions and include: 1) massive metagabbros, which chemically are quartz tholeiites (Mg# = 0.40-0.53, 144-178 ppm Cr, 71-112 ppm Ni); 2) mafic, talc-bearing greenstones, which are olivine tholeiites (Mg# = 0.69-0.72, 732-1526 ppm Cr, 410-578 ppm Ni); and 3) somewhat aluminous (5-13 wt% Al2O3) ultramafic rocks, which contain variable proportions of chlorite, talc, tremolite and carbonate (Mg# = 0.74-0.85; 1006-3484 ppm Cr, 549-1389 ppm Ni). Their normative mineralogy is dominated by olivine and pyroxene, but the rocks have been variably metasomatized. We interpret these rocks as intrusive features not tectonically emplaced ophiolite slivers.