A quantitative analysis of xenolith fragmentation and incorporation in magma



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A comparative analysis of xenolith size and shape versus frequency was conducted on datasets collected from four granodioritic plutons: 1) Jackass Lakes pluton, CA (n=104 metavolcanic/metasedimentary xenoliths, 0.001-1.0 km2); 2) Andalshatten batholith, Norway (n=103 meta-calcareous xenoliths, 0.001-1.0 km2); 3) Vega intrusive complex, Norway (n=1069 metasedimentary xenoliths, 0.001-1.0 m2); and 4) English Peak pluton, CA (n=31 metasedimentary xenoliths, 0.01-50 m2). The initial hypothesis is that host rock xenolith size-populations follow a predicted fractal size-distribution relating host rock xenolith size-populations to the fragmentation mechanism of stoping while non-fractal distributions are the result of diking. This hypothesis assumes that stoping, like many other fragmentation processes of Earth materials, yields size-frequency populations related by a power-law function, following the theory of fractal fragmentation. Results of size-frequency distributions from the four locations display a general non-fractal trend over the entire population, with tails defined by a change in slope in the smallest and sometimes largest size categories. Assuming the theory of fractal fragmentation holds true for xenoliths fragmented by stoping and that temperature conditions in the early stages of pluton emplacement will favor stoping, there is an obvious size problem in measured xenolith suites. A size problem is a product of a lack of certain sizes, particularly small sizes, which prohibit a linear trend from defining a population within a log-log plot of cumulative frequency versus size. We postulate this size problem may be due to: 1) observational bias, 2) assimilation, 3) size effect, 4) a change in the mechanism of fragmentation, 5) a change in the physical property of the host rock, 6) composite fragmentation episodes calculated as a single population, or any combination of the above. Results of shape-frequency analysis revealed the most common and average axial ratio was ~0.50 across all four populations, which corresponds to an elongate shape. Since axial ratio measurements are generally consistent across locations, shape characteristics are shown to be independent of size and potentially independent of fragmentation history. Defining these size and shape-population trends through correlation with specific processes of xenolith fragmentation and incorporation may add a quantifiable parameter further illuminating the physical and chemical affects host rock xenoliths may have on the physical and chemical evolution of magma chambers in the crust.



Xenolith, Pluton Emplacement