METAMORPHOSED MANTLE XENOLITHS IN THE MAFIC TO ULTRAMAFIC DIANA MILLS PLUTON, PIEDMONT PROVINCE, VIRGINIA

The Silurian Diana Mills Pluton (~21 km²) is a metamorphosed mafic-ultramafic body in the Piedmont Province of central Virginia. Major rock types in the pluton include metadiorite and Ca-amphibole + chlorite rock (CACR), with lesser amounts of cpx-bearing hornblendite and hornblende pegmatite. However, the most visually striking rocks, found at four separate locations, consist of tan-weathering nodules (up to ~12 cm across) in a dark green matrix. The purpose of this study was to investigate the origin of these unusual nodular rocks, and to evaluate possible relationships to other rocks in the pluton. All rocks show evidence of greenshist facies metamorphism, but relict igneous textures are preserved locally. Apart from the nodules, Ca-amphibole is ubiquitous in all rock types, and typically displays patchy zoning with compositions ranging broadly from tremolite to magnesio-hastingsite. Nodules are dominated by serpentine or talc (+ magnetite ± chromite), and many show a thin (<5 mm) radially oriented shell of serpentine against adjacent matrix. Whole-rock analyses of six nodules show their ultramafic character (Mg# ~0.8-0.9, up to7563 ppm Cr, up to 2038 ppm Ni, typically flat REE patterns at 1-5 x chondrites); their normative mineralogy is dominated by olivine and orthopyroxene. A straightforward interpretation is that they represent metamorphosed harzburgites. Matrix minerals are dominantly chlorite + tremolite, with minor talc + magnetite. Some matrix samples are chlorite-rich, reflecting metasomatic reaction with nodules, i.e., they are small-scale “blackwalls.” Matrix samples are also ultramafic (Mg# ~0.8-0.9, up to 1971 ppm Cr, up to 1192 ppm Ni). A key finding of this study is that the dark green matrix of the nodule-bearing outcrops is essentially the same as other CACR elsewhere in the pluton (which lack nodules). We consider the CACR to represent original hornblende peridotites based on several lines of evidence: 1) abundance of Ca-amphibole in all rocks of the pluton; 2) relict poikilitic textures in which large hornblendes enclose either “olivine” (now pseudomorphed by talc + serpentine) or cpx; and 3) additional ultramafic characteristics (high Mg#, Cr, and Ni). We suggest that the hornblende peridotites were emplaced as a crystal mush, which locally carried harzburgite xenoliths from the mantle.