sgr ms thesis - University of Maine

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 (D s > 3) that agree with those
known to result from explosion (D s > 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