FRACTAL FRAGMENTATION OF XENOLITHS IN SILICIC MAGMAS

Many types of geologic materials undergo fractal fragmentation (e.g. sea ice, asteroids and fault gouge). Frequency size distributions of host rock xenoliths in silicic magmatic systems indicate that they also undergo fractal fragmentation. This may be an important mechanism by which plutonic systems process host rock and ascend through the crust.

We measured xenolith frequency size distributions in multiple systems. On Kodiak Island, AK, xenolith populations were examined in two locations: The Kodiak batholith and the Shaft Peak pluton. The Kodiak batholith contains abundant schistose and gneissic xenoliths of moderate size (1.0-500 cm radius), whereas in the Shaft Peak pluton a smaller size (.1-10 cm) xenolith population composed of meta-graywacke and argillite was measured. In comparison to the above pelitic xenolith systems, the Sierra Nevada (CA) Mitchell Peak Intrusive suite, contains much larger granitic xenoliths that range from 5-200 m in radius.

In all three systems, the xenolith frequency size distribution can be modeled by fractal fragmentation with D-values between 2-3. However, the two larger systems, (Kodiak and Mitchell Peak) are bi-fractal with D-values of 2-3 for the larger size fraction and 1.0 for smaller xenoliths. Bi-fractal particle distributions exist for other types of fragmented material (e.g. fragmented basalt). Also, the change in D-value occurs at different absolute block sizes (100 m in the Mitchell Peak and 10 cm in the Kodiak batholith).

A comprehensive explanation of bi-fractal fragmented particle distributions does not yet exist. However, D-values between 2-3 have been interpreted to indicate catastrophic fragmentation, whereas lower D-values indicate a decreased degree of fragmentation. This idea can be applied to the three observed magmatic systems. The Shaft Peak pluton is small and cooled quickly at a shallow crustal level. Therefore, a single episode of catastrophic xenolith fragmentation was preserved. However, the Kodiak batholith and Mitchell Peak are large and longer lived systems. In both, the larger blocks underwent catastrophic fragmentation, whereas the smaller blocks did not. Therefore upon reaching a smaller size, blocks must have attained some type of thermal, chemical or mechanical equilibrium that inhibited further fragmentation.