Document Type
Thesis
Abstract
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. I investigated this body to improve the understanding of its field setting, settle lingering uncertainties of mineralogy, and further constrain its petrogenesis using wholerock geochemical data. The body is well exposed in outcrop or float for ~7 km, is up to ~200 m wide, and is oriented ~N40E. All exposures are dominated by dark green, medium- to coarse-grained (<1 cm), blocky calcic amphibole, and textures range from massive to slightly foliated. In thin section, the rocks typically consist of ~80-95% calcic amphibole, minor amounts of epidote and quartz, and rare talc. Some samples contain clinopyroxene (up to 10%). Amphibole compositions range from actinolite to actinolitic hornblende to magnesio-hormblende. Eight samples from along the length of the body show similar whole-rock SiO, (52-55 wt. %), Al,O3 (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 clinopyroxene and orthopyroxene (70-87% combined), with minor feldspar (9-23%; Ang2-77); all are quartz-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 (Lay=1-7; Luy=1-2), display slightly LREE-enriched, and flat HREE patterns with no Eu-anomalies. Collectively, these mineralogical observations and geochemical results provide strong evidence that this body is a metamorphosed pyroxenite, and the protolith was a cumulate containing two pyroxenes and plagioclase. Traditional evidence for intrusive emplacement is lacking. Its proximity to the extrapolation of the Little Fork Church Fault may indicate a tectonic emplacement, but there is not clear evidence either in support of or in opposition to this interpretation.
Date Awarded
2002
Department
Geology
Advisor 1
Brent E. Owens
Recommended Citation
Murray, Jeffery D., "Field, Mineralogical, and Geochemical Constraints on the Origin of an Amphibole-rich Metapyroxenite, Central Piedmont Province, Virginia" (2002). Geology Senior Theses. William & Mary. Paper 101.
https://scholarworks.wm.edu/geologyseniors/101
