A COMPARATIVE SIZE-COMPOSITION-DISTRIBUTION ANALYSIS OF XENOLITHS IN PLUTONS

Quantitative-size-composition distribution analysis of xenoliths in plutons may aid in a better understanding of block a) incorporation into magmas, b) deformation and displacement in magma chambers, c) preservation in magmas, and d) dissolution and melting in magmas. Such studies are all but absent in the literature and this set of findings aims to fill that void. Three plutons (Jackass Lakes, CA; Andalshatten and Vega, Norway) ranging in depth from subvolcanic to 25 km and which are granodiorite in composition have been the focus of this analysis. Establishing a relationship between pluton-xenolith distributions, can be applied to known observations to gain a better understanding of compositional evolution (in situ assimilation), rheology (near-solidus magma viscosity and crystallinity), and mechanics of host rock deformation and xenolith incorporation (fragmentation processes). Initial datasets compiled from plotting xenolith radii using this method results in slope values that range from -1.2 to -1.4. These numbers are near the predicted fractal dimension of -1.5 gained from previous calculations. The range of fractal dimensions above was obtained by measuring a cluster of adjacent points that excluded sub-horizontal and sub-vertical figures. The removal of these groupings limited the inclusion of binned sets with a small collection of blocks and those that held the same or similar log N-value as neighboring points. Despite the narrow range of slopes found when plotting each pluton individually, slope values obtained from combining the three groups proved insignificant. When joined into a single graph, the range of values provided a bi-fractal distribution of xenoliths binned into three distinct size categories. Only the grouping of the largest sizes (greater than 251 mm radius) found a fractal dimension in the limits of predicted values. Further research is needed to determine whether this bi-fractal array hints at a change in fracture mechanisms or is the product of right and left hand-truncation (assimilation of smallest blocks, blocks exceed size of outcrop, etc.).