Date of Award

2005

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Scott E. Johnson

Second Committee Member

Peter O. Koons

Third Committee Member

Charles V. Guidotti

Abstract

The porphyroblastic pelitic schists of western Maine are ideal for studying the deformation and metamorphism that accompanies pluton emplacement. A regional lowpressure, high-temperature metamorphic event affected the areally extensive Silurian sediments with the thermal peak occuning ca. 404 Ma. This metamorphism produced the widespread assemblage staurolite + andalusite + biotite +I- garnet, and occurred during the late stages of development of a northeast-trending, steeply-dipping axial-surface foliation. A period of extensive plutonism accompanied and followed this Acadian-aged deformation and metamorphism. This study focuses specifically on the contact aureole of the Mooselookmeguntic pluton emplaced ca. 370 Ma. Contact metamorphism accompanied the development of a gently northeast-dipping, crnplacement-related, foliation that varies from a crenulation cleavage in the middle and outer portions of the aureole, to an intense pervasive foliation proximal to the pluton margin. The deformation and metamorphic aureole of this pluton provides a rare opportunity to study the progressive development of crenulation cleavage, and to evaluate porphyroblast kinematics across a clearly defined deformation gradient. Staurolite porphyroblasts in the pelitic country rocks preserve spectacular inclusion trails, the three-dimensional orientations of which can be used to evaluate the geometric and kinematic development of the crenulation cleavage. Outside the pluton aureole, staurolite porphyroblasts overgrow the regional axial surface foliation, preserving straight inclusion trails that are continuous with the matrix foliation. This microstructure is consistent over a large area, and as such can be used as a "control" to evaluate the structural and metamorphic effects of crenulation cleavage development along transects across the pluton-related thermal gradient. Porphyroblast inclusion trails vary systematically from areas of the "control" microstructure outside the aureole through the deformation gradient towards the pluton. This variation provided an opportunity to evaluate the relations among porphyroblast kinematics, crenulation cleavage development and the dominant deformation mechanisms during progressive foliation development. The kinematic behavior of porphyroblasts during crenulation cleavage development has been examined in previous studies, but this thesis represents the first detailed examination across a clearly defined deformation gradient. This study provides new results that demonstrate conclusively that porphyroblasts rotate relative to one another during crenulation cleavage development, and that their kinematic behavior varies in relation to a change in the dominant deformation mechanism at specific stages of crenulation cleavage development. Rotation of porphyroblasts during crenulation cleavage development is intimately linked to the mechanisms of cleavagc formation. Strain in the early stages of crenulation cleavage development is accommodated by dissolution-precipitation creep of quartz and feldspar. Consequently, there is little to no relative rotation of porphyroblasts during the onset of crenulation cleavage. When the crenulation cleavage evolves to the stage where there are no longer significant amounts of quartz and feldspar to dissolve from the phyllosilicate-domains the principle deformation mechanism probably changes to a combination of dislocation creep and diffusion creep. This change in dominant deformation mechanism lead to an increased stress couple between the porphyroblast and the deforming matrix, resulting in the relative rotation of porphyroblasts during the later stages of fabric development.

Included in

Geology Commons

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