FIELD, MINERALOGICAL, AND GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF A METAPYROXENITE DIKE(?), CENTRAL PIEDMONT PROVINCE, VIRGINIA

One of the most prominent mafic to ultramafic dike-like bodies in the Piedmont Province of Virginia occurs in the Chopawamsic Terrane, western Goochland County. It was originally mapped as pyroxenite (Brown, 1937, VA Geol. Soc. Bull. 48), later as hornblende gabbro (1963 state geologic map), but omitted from the 1993 state geologic map. We investigated this body to understand better its field setting, settle lingering uncertainties of mineralogy, and further constrain its petrogenesis using whole-rock geochemical data. The body is well-exposed in outcrop or float for ~5 km, ranges up to ~200 m wide, and is oriented ~N45E. All exposures are dominated by dark green, medium- to coarse-grained (<1 cm), blocky amphibole, and textures range from massive to slightly foliated. In thin section, the rocks typically consist of ~80-95% actinolitic amphibole, minor amounts of ep and qtz, and rare tlc. Some samples contain relict cpx (up to 10%). Eight samples from along the length of the body show similar SiO₂ (52-55 wt%), Al₂O₃ (3-7 wt%), and Mg# (73-82), but a range in MgO (14-23 wt%) and CaO (5-14 wt%). Normative mineralogy of typical samples is dominated by subequal amounts of cpx and opx (70-87% combined), with minor feldspar (9-23%; An_62-77); all are qtz-normative. All samples contain relatively high concentrations of trace transition elements, including Sc (50-80 ppm), V (140-210 ppm), Cr (530-1860 ppm), Co (50-85 ppm), and Ni (80-280 ppm). Primitive mantle-normalized patterns of transition elements are uniform, displaying troughs at Ti, Cr, and Ni, and peaks at Sc and V. Concentrations of Sr are low (<50 ppm), but correlate positively with Eu and Al. Typical samples contain similar low levels of REE (La_N=1-7; Lu_N=1-2), display slightly LREE-enriched, and flat HREE patterns with no Eu-anomalies. Collectively, our mineralogical observations and geochemical results provide strong evidence that this body is a metamorphosed pyroxenite, and the protolith must have contained two pyroxenes + plag. No contacts have been observed, but one sample from near the inferred margin contains higher SiO₂ (58 wt%), REE, Zr, U, and Th, possibly reflecting slight contamination during emplacement. This interpretation, coupled with the overall shape of the body, lead us to tentatively conclude that it was emplaced as a dike (perhaps as a crystal mush).