FRACTAL ANALYSIS AND THERMAL-ELASTIC MODELING OF A SUBVOLCANIC MAGMATIC BRECCIA: THE SHATTER ZONE, MOUNT DESERT ISLAND, MAINE

The Shatter Zone of Mount Desert Island, Maine, is a 450m-1km thick magmatic breccia aureole 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 thought to have been emplaced at high temperature (~900°C). It was emplaced at shallow crustal depth (<5km), and fed numerous volcanic eruptions, the products of which are exposed on the nearby Cranberry Islands. The breccia is interpreted as having formed during a large subvolcanic explosion triggered by continued emplacement of mafic magma, resulting in explosive fragmentation of the hornfels wall rock and virtually instantaneous 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. Although field observations point to an explosive breccia mechanism, Particle Size Distribution (PSD) analysis yields fractal dimensions (D = 2) that are lower than those known to result from explosion (D = 2.5 or greater). Field and microstructural data and observations suggest that the clast sizes and shapes reflect post-brecciation modification by partial melting and thermal fracture. We employ numerical modeling to explore the possible effects of these processes on the size distribution of clasts. We assume instantaneous immersion of cold hornfels clasts (modeled range: 400-800°C) in a hot granitic matrix (900°C), and restrict our thermal analysis to conductive heat transfer. Latent heat production by the crystallizing magma is assumed to be 400kJ/kg over a temperature interval of 100°C ending with a solidus of 750°C. Our results define a cutoff size between clasts that are small enough to be modified and those that are too large. In addition, angular clasts are highly susceptible to corner breakoff owing to large tensile stresses associated with thermal shock. Considering the effects of these processes on clast size distribution, we conclude that the fractal dimension may have been 2.5 or greater at the time of breccia formation, and that subsequent modification through thermally-driven processes reduced the fractal dimension to 2.0.