FRAMING THE NEOPROTEROZOIC IAPETAN RIFT SEQUENCE: GEOLOGY OF THE ALBERENE 7.5' QUADRANGLE, EASTERN BLUE RIDGE, VIRGINIA

The Alberene 7.5’ quadrangle straddles the eastern limb of the Blue Ridge anticlinorium and western Piedmont in central Virginia. Mesoproterozoic basement rocks are overlain by a Neoproterozoic to Cambrian cover sequence. The basement complex is primarily a coarse-grained biotite granitoid gneiss intruded by Neoproterozoic granite, metagabbro, and lamprophyre bodies. The Neoproterozoic Lynchburg Group unconformably overlies the basement and includes metamorphosed pebble conglomerate, arkose, greywacke, graphitic mudstone, and siltstone. Metabasalt and minor metasedimentary rocks of Catoctin Formation overlie the Lynchburg Group. Phyllite of the Cambrian Candler Formation overlies the Catoctin and crops out in the southeast. Metagabbro dikes and sills intrude both the basement and Lynchburg Group. Several ultramafic dikes, confined to the Lynchburg Group, strike parallel to the NE/SW regional trend. NNE/SSW-striking en echelon Jurassic diabase dikes cross cut older units.

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.