Document Type
Thesis
Abstract
A previously uninvestigated lens of metagabbro crops out approximately 4 km to the southwest of the metapyroxenite studied by Murray (2002) with a similar appearance in hand sample and similar orientation. The exposure is dominantly float in an area ~1.25 km long and ~0.25 km wide that is oriented southwest to northeast. Textures are: 1) gneissic with medium amphibole and plagioclase grains of subequal size (1.0 to 5.0 mm) and; 2) massive, dominated by large, dark green, blocky amphibole clusters (1.0 to 3.0 cm). In thin section the rocks consist of amphibole, plagioclase, clinopyroxene, titanite, apatite, zeolite, epidote and rare biotite. Symplectites are common in rocks with gneissic texture. Amphibole compositions are dominantly edenite with lesser amounts of pargasite, magnesiohornblende and tschermakite. Amphiboles are all enriched in K which indicates that they are not pseudomorphs after pyroxene. All samples are olivine normative. Normative plagioclase (29 to 43%) and diopside (21.6-44.9%) are the most abundant phases with significant amounts of normative olivine (9.36 and 15.4%) and orthoclase (4.84-15.5%). Trace transition element compositions are high with significant enrichment in Ba (249 to 1321 ppm), Sr (306 to 699 ppm), and Cr (78 to 990 ppm). MORB-normalized patterns show peaks in Ba and Ce and troughs in Nb, Ti and Y typical of arc basalts. Geochemical data indicate minor olivine and pyroxene accumulation in the protolith. Because of their proximity, similar appearance and orientation I compared the metagabbro to the metapyroxenite to examine the possibility of a genetic relationship. None of the evidence discovered provides a definite link although the possibility was not entirely disproven. Important differences in mineralogy and petrology are: 1) high K amphiboles that are not pseudomorphic after pyroxene; 2) the protolith had only minor olivine and pyroxene accumulation and; 3) clinopyroxene and plagioclase are much more abundant.
Date Awarded
2009
Department
Geology
Advisor 1
Brent E. Owens
Recommended Citation
Kneiser, Brian J., "A Field and Petrological Study of a Metagabbro Occurrence in the Chopawamsic Terrane, Piedmont Province, Virginia" (2009). Geology Senior Theses. William & Mary. Paper 213.
https://scholarworks.wm.edu/geologyseniors/213
