A QUANTITATIVE ANALYSIS OF XENOLITH FRAGMENTATION AND INCORPORATION IN MAGMA

We report results of statistical analyses that aim to quantify the effect of composition and the resultant size-distribution of blocks on xenolith incorporation and fragmentation in magma. Datasets comprise xenoliths from three granodioritic plutons: 1) Jackass Lakes, CA, (n=104 metavolcanic and metasedimentary xenoliths, 0.001-1.0 km2); 2) Andalshatten batholith, Norway (n=103 meta-calcareous xenoliths, 0.001-1.0 km2); and 3) Vega intrusive complex (VIC), Norway (n=1159 metasedimentary xenoliths, 0.001-1.0 m2). Many natural fragmentation processes produce a linear size-distribution when the normalized radius of each block is compared to the number of blocks greater than that radius on a logarithmic scale. Results of measured xenoliths produce a non-linear or log-normal relationship between sizes when plotted in this manner. This deviation from size-distribution studies of naturally fragmented materials is due to the scarcity of small block sizes.

Analysis of block shape was accomplished by comparing axes measurements (short/long) of each block as a function of lithology. Detailed study of xenolith populations from the VIC indicate no dominant block shape, with values ranging from <0.1 (pencil-like) to 1.0 (equidimensional); ~18% have an axial ratio of 0.5. Additionally, sorting xenoliths from the VIC into compositional classes found no strong preference to either block shape or size. A more significant role in block fragmentation and incorporation in magma may then be anisotropy, thermal disequilibrium, and fluid pressure rather than host rock lithology.

The departure of our size-distributions from those of presumably similar fragmentation processes that yield power-law and fractal dimensions suggests that processes other than catastrophic fragmentation might control the formation and subsequent incorporation of xenoliths in plutons. Preferential removal of small sizes by assimilation or change in fracture mechanism (or sampling bias) could also skew studies of size and shape distributions and minimize the potential for observable population trends in xenolith shape and size. Therefore, resultant log-normal trends and quantitative conclusions based on these trends may not offer a definitive means of evaluating mechanisms of xenolith fragmentation and incorporation in magma.