Ss. Hughes et al., CHEMICAL DIVERSITY AND ORIGIN OF PRECAMBRIAN CHARNOCKITIC ROCKS OF THE CENTRAL PEDLAR MASSIF, GRENVILLIAN BLUE RIDGE TERRANE, VIRGINIA, Precambrian research, 84(1-2), 1997, pp. 37-62
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.