Date of Award

2011

Level of Access

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor

Scott E. Johnson

Second Committee Member

Christopher C. Gerbi

Third Committee Member

Zhihe Jin

Abstract

The Shatter Zone of Mount Desert Island, Maine, is a 450-1000m thick magmatic breccia that defines the perimeter of the Cadillac Mountain Intrusive Complex. The 400 Ma complex consists of gabbro-diorite sheets overlain by three different granites, the largest of which is an A-type granite emplaced at high temperature (~900oC) and at shallow crustal depth (<5km), having fed numerous volcanic eruptions, the products of which are exposed on the nearby Cranberry Islands. Results from a contact metamorphism numerical model show that development of the intrusive complex occurred incrementally, with multiple sequences of magma emplacement with bimodal composition. Metamorphism continued prior to wall rock fragmentation and injection of granitic magma. The Shatter Zone formed during a large volcanic eruption triggered by continued emplacement of mafic magma, resulting in elastic failure of the metasedimentary and diorite wall rock and virtually instantaneous fragmentation and entrainment of the clasts in hot granitic magma. The degree of brecciation is gradational, with clast supported breccias at the outer margin of the zone grading inward to granitic-matrix supported breccia, and finally into clast-free Cadillac Mountain Granite. Field observations point to an explosive breccia mechanism, and clast size distribution analysis yields fractal dimensions (Ds > 3) that agree with those known to result from explosion (Ds > 2.5). Field and microstructural data and observations suggest that the clast sizes and shapes of the metasedimentary host rocks reflect post-brecciation modification by partial melting and thermal fracture, while diorite dike fragments experienced little modification after the original brecciation event. Clast circularity increases with proximity to the magma reservoir, whereas clast boundary shape decreases; this implies thermal wear on clast surfaces. Numerical modeling is employed to explore the possible thermalmechanical effects on the size distribution of clasts. Instantaneous immersion is assumed for metasedimentary clasts (650°C) in a hot granitic matrix (800°C - 900°C), and our thermal analysis is restricted to conductive heat transfer corrected for latent heat. The amount of clast melt is primarily dependent on the melt temperature of the clast, the matrix to clast volume ratio, and the initial magma intrusion and clast temperatures. Results show that thermal fracture and clast melt were viable secondary modification processes, and magma flow was necessary for disaggregation of melted clasts to occur. Angular clasts are highly susceptible to corner break-off owing to large tensile stresses associated with thermal shock. Considering the effects of these processes on clast size distribution, we conclude that the Shatter Zone formed from explosion, and latestage magma emplacement effectively altered the size and shape for many of the metasedimentary clasts.

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