Document Type
Thesis
Abstract
The Maidens gneiss forms the largest component of the Goochland Terrane in the central Piedmont Province of Virginia. Many aspects of the geologic history of the Maidens gneiss remain poorly understood, including the timing of high-grade metamorphism, the nature of protolith rock-types, and the history of deformation. The Maidens gneiss locally preserves evidence for a granulite-facies event that has long been assumed to have occurred in the Mesoproterozoic. However, ages for monazite crystallization obtained from four high-grade samples (pelites and granulites) of Maidens using the Th-U-total Pb chemical dating method on an EMP are no older than ~420 Ma. Results for two samples are similar, with core ages of 387 + 4 Ma and 377 + 4 Ma, and thin, younger (293 + 4 Ma and 289 + 20 Ma) rims. The core ages seem to reflect highgrade Acadian, not Grenvillian, metamorphism, with rims recording recrystallization and overgrowth during the Alleghanian. Bulk compositions of nine samples of biotite gneiss (one of the most common Maidens lithologies) suggest intermediate igneous protoliths of high-K, calc-alkaline affinity, and MORB-normalized multi-element diagrams are virtually identical to those of high-K, calc-alkaline rocks elsewhere. Such protoliths probably reflect deep-seated emplacement of magmas coincident with high-grade metamorphism in associated rocks (to account for a similarity in zircon and monazite ages). Structural analyses of 3 exposures of Maidens show consistent geometry, including meter-scale folds that trend NE-SW and verge NW. Strain estimates on deformed pegmatites suggest ~200-500% material elongation (NE-SW), and shear sense indicators are consistently dextral. In general, the Maidens experienced transpressional deformation under a flattening strain during the Alleghanian Orogeny.
Date Awarded
2004
Department
Geology
Advisor 1
Brent E. Owens
Recommended Citation
Shirvell, Catherine, "The Maidens Gneiss, Goochland Terrane, Virginia: An investigation of the timing of high-grade metamorphism, protolith rock-type, and history of deformation" (2004). Geology Senior Theses. William & Mary. Paper 142.
https://scholarworks.wm.edu/geologyseniors/142
