CHEMICAL DIVERSITY AND ORIGIN OF PRECAMBRIAN CHARNOCKITIC ROCKS OF THE CENTRAL PEDLAR MASSIF, GRENVILLIAN BLUE RIDGE TERRANE, VIRGINIA

Middle Proterozoic rocks of the Pedlar massif are comprised of varied charnockitic and granulitic lithologies representing similar to 1150-1 000 Ma Grenvillian metamorphic and igneous events in the central Appal achians of Virginia. Chemically and mineralogically diverse units incl ude metaluminous to peraluminous charnockites, enderbites, jotunites a nd granulite gneisses ranging in SiO2 from 47 to 79 wt.%. Relative to bulk continental crust, typical charnockites are enriched by similar t o 3-6 x in Rb, K, Ba, LREE, Zr and Hf, less enriched in Ta, Sr, P and HREE, and depleted in Cs, Th, Ti and Sc, although low-SiO2 members hav e relatively high Ti and P and some high-SiO2 units are depleted in K, Rb and Pa. The Pedlar River Charnockite Suite (PRCS), Vesuvius megacr ystic charnockite (VMC), Nellysford Granulite Gneiss (NGG) and the Lad y Slipper Granulite Gneiss (LSGG) exhibit common chemical signatures a nd trends, notably evident in uniform LREE-enriched patterns with vari able Eu anomalies, which reflect their derivation from a mixed protoli th of volcanics and reworked volcaniclastic sediments dated at similar to 1130-1150 Ma. Pedlar River charnockites represent portions of deep -seated granulite gneisses that were mobilized by granulite facies deh ydration melting and emplaced en masse as plutons into overlying and s urrounding gneisses during the Grenville episode. Divergent REE patter ns in the PRCS are attributed to crystallization of quartz, feldspar /- garnet, or HREE-compatible mafic and accessory phases leading to co mplementary chemical signatures in mafic and felsic layers. Depletion of Cs throughout the Pedlar massif is attributed to its incompatibilit y with major mineral phases during either protolith evolution or granu lite facies dehydration, while dispersions in K, Rb, Sr, Pa and Eu are due to the relative proportions of the major phases quartz, K-feldspa r, plagioclase and biotite. Mobility of insoluble trace elements Zr, H f, Ta and Th require partial melting and crystallization of accessory mineral phases, whereas Sc, Cr and REE mobility depends on the proport ions of pyroxene and garnet. Several leucocharnockites and other felsi c rocks of the PRCS show markedly elevated Th values ranging from simi lar to 10 to 83 ppm which complement depleted Th in typical PRCS units . These charnockitic rocks represent low-degree partial melts (similar to 1-5%) from normal PRCS or NGG protoliths to produce high-silica ma gmas which separated from the main charnockitic body. (C) 1997 Elsevie r Science B.V.