Subvolcanic Thermodynamics

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Fracture mechanics and melt in a subvolcanic magmatic breccia

Sam G. Roy, Scott E. Johnson, Peter O. Koons, Zhihe Jin

We examine the development of a subvolcanic magmatic breccia located along the contact of a granitic intrusion using fractal analysis and thermal-elastic modeling. The breccia grades from clastsupported, angular clasts adjacent to unfractured host rock to isolated, rounded clasts supported by the granitic matrix adjacent to the intrusion. Field observations point to an explosive breccia mechanism, and clast size distribution analysis yields fractal dimensions (Ds > 3) that exceed the minimum value known to result from explosion (Ds > 2.5). Field observations, clast size distribution data, clast circularity data, and boundary roughness fractal dimension data suggest that the clast sizes and shapes reflect post-brecciation modification by partial melting and thermal fracture. Numerical modeling is employed to explore the possible thermal-elastic effects on the size distribution of clasts. Instantaneous immersion is assumed for metasedimentary clasts of a fractal size distribution in a superheated granitic matrix for different matrix volume percentages. Thermal analysis is restricted to conductive heat transfer corrected for latent heat. Partial melting of metasedimentary clasts is an effective secondary modification process that was probably responsible for markedly altering the clast size distribution for clast populations adjacent to the intrusion. Diabase clasts experienced late-stage fracture due to the instantaneous thermal pulse in which angular clasts with high surface area to volume ratios were preferentially fractured, although this process does not appear to have markedly influenced the clast size distribution.