Date Thesis Awarded
5-2025
Access Type
Honors Thesis -- Access Restricted On-Campus Only
Degree Name
Bachelors of Science (BS)
Department
Geology
Advisor
Clem Hamelin
Committee Members
Clem Hamelin
Chirstopher Bailey
Brent Owens
Louis Rossiter
Abstract
The Virginia Piedmont is largely comprised of terranes accreted onto Laurentia during three Paleozoic orogenies that built the Appalachian Mountains (Bailey & Owens, 2012). Key aspects of these terranes’ metamorphic history remain poorly understood, yet they provide valuable information about the architecture of the Appalachians and the processes involved in continental growth during Appalachian orogenesis (Fig. 1). The Chopawamsic terrane, an Ordovician volcanic island-arc metamorphosed to amphibolite facies conditions, contains kyanite quartzites with an unusual assemblage of nearly entirely quartz and kyanite (Owens & Pasek, 2007; Taber 1913). Detailed work on the P-T conditions recorded by these kyanite quartzites, from terrane to outcrop scale, is lacking, due in part to the challenging mineralogy of these samples for application of conventional geothermobarometry methods. In this study I investigate kyanite-crystallization textures using cathodoluminescence (CL) imaging, and petrogenesis of kyanite quartzites by analyzing the trace element abundances in kyanite and rutile. Furthermore, I constrain pressure-temperature (P-T) conditions recorded across the terrane using Ti-in-quartz and Zr-in-rutile trace element thermobarometry to spatially resolve metamorphic conditions recorded by these rocks.
Two distinct types of kyanite quartzites are found in the Chopawamsic terrane: blue-green kyanite and white to light grey-blue kyanite-bearing quartzites. CL imaging of kyanite reveals (1) core-rim zoning patterns with either patchy zoning or CL-dark cores, and CL-bright oscillatory zoning of kyanite rims, observed in both types of kyanite quartzites, and (2) kyanite with no core-rim structure. Kyanite CL-zoning is not correlated with optically identifiable features of kyanite such as kinking and twinning. It is likely that different kyanite CL domains (cores vs. rims) reflect differential availability of trace elements present during different stages of kyanite crystallization during metamorphism. Trace element analysis of kyanite grains reveals a strong correlation between CL zoning and the concentration of Cr and V, potentially recording a prograde P-T pathway. Trace element analysis of rutile and kyanite grains supports the interpretation of Owens & Dickerson (2001) that quartzites with blue-green kyanite had mafic (likely volcanic) protoliths, while those with white-grey kyanite had intermediate or felsic protoliths. Finally, Zr-in-Rt geothermometry records amphibolite facies P-T conditions, with rutile inclusions in kyanite recording lower temperatures than rutile inclusions in quartz. Thermobarometry using the crossed-isopleth method of Thomas et al. (2010) yields metamorphic conditions in the middle amphibolite facies, higher by 2-4 kbar and 0-50ºC compared to previous P-T estimated from schists adjacent to the kyanite quartzites (~6.5 kbar, ~600ºC, Cochrane, 1986). A trend of increasing equilibration temperature to the southwest is recorded. Ti-in-Qtz geothermometry records systematically lower temperatures, due to either partial equilibration of the rutile-quartz system, or depletion of Ti due to the activity of fluoride-bearing hydrothermal fluids.
Recommended Citation
Ingle, Nathaniel G., "Petrogenesis and Metamorphic History of Kyanite-Bearing Rocks in the Virginia Chopawamsic-Milton Terrane during Appalachian Orogenesis" (2025). Undergraduate Honors Theses. William & Mary. Paper 2254.
https://scholarworks.wm.edu/honorstheses/2254
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