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Origin and Tectonic Significance of the Albemarle-Nelson Soapstone Belt and Associated Metagabbros in Central Virginia
Jensen, Andrea
Jensen, Andrea
Abstract
The Albemarle-Nelson soapstone belt is part of a well-defined chain of ultramafic rock running from Newfoundland to Georgia through the Appalachians. The origin and tectonic significance of these rocks and associated mafics has been controversial since their discovery. Current debate focuses on whether these rocks are an ophiolitic mélange formed in a collisional setting, or intrusive and related to Iapetan rifting. This study focuses on petrography and geochemical analysis of these rocks with the goal of understanding their origin. We recognized at least three types of metamorphosed mafic to ultramafic rocks other than the Catoctin Formation. These include: 1) massive metagabbros, which chemically are quartz tholeitites (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% Al2,03) 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). Similarities in HFSE patterns and concentrations suggest the metagabbros may be deeper equivalents of the Catoctin greenstone. Major oxide (MgO, Al2O3, CaO and Mg#) and REE data (patterns, concentrations and Eu anomalies) suggest the ultramafics are not part of an ophiolite sequence and may instead be instrusives related to Iapetan rifting. The unusual mafic-talc bearing greenstones are interpreted as cumulates genetically related to the talcchlorite schists.
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2012-01-01
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Geology