CHEMICAL DIVERGENCE BETWEEN VOLCANIC AND PLUTONIC ROCKS PRODUCED BY INFILTRATION AND RECRYSTALLIZATION OF APLITIC LIQUIDS
We studied multiple generations of aplite dikes in the Half Dome Granodiorite in Yosemite National Park, California. Exquisite glaciated exposures reveal multiple generations of aplite dikes cutting granodiorite. The youngest set is sharply defined and fine-grained, but older sets are progressively coarser-grained, with more diffuse margins, and thus are harder to recognize. The oldest recognizable dikes are leucogranite with textures comparable to the host. All generations have aplite geochemical signatures (e.g., low Y) that distinguish them from erupted rhyolites and from leucogranite plutons. The host pluton shows linear negative correlations between many incompatible elements (e.g., Y, Zr, Ba) oriented at a high angle to the positive correlations exhibited by comparable volcanic suites such as Mt. Lassen. The correlations are difficult to explain by crystal fractionation but are well-explained by late-stage extraction of aplite from part of the pluton and infiltration and recrystallization of that melt in other parts, thus differentiating a single magma composition into one with a wide compositional range. Extraction and transport of aplite melt in dikes cannot occur until a magma body reaches a crystal content high enough (e.g., 70 vol%) for brittle fracture. Such a mush is too crystal-rich to erupt and thus this process, although effective in plutons, cannot occur in erupted rocks.