CHEMICAL DIVERGENCE BETWEEN VOLCANIC AND PLUTONIC ROCKS PRODUCED BY INFILTRATION AND RECRYSTALLIZATION OF APLITIC LIQUIDS

Examination of global geochemical databases shows that volcanic and plutonic rocks produced by subduction zones are remarkably similar in their compositions. Although mafic rocks are far more common among erupted rocks than felsic ones, the patterns of geochemical variation are comparable. Overall, the analysis shows little evidence for a complementary relationship between typical volcanic and plutonic rocks in which plutons represent unerupted cumulate residues from differentiating magma systems; rather, pluton compositions are dominantly equivalent to those of volcanic rocks. However, subtle trace-element distinctions point to late-stage processes that distinguish the two suites. Here we show that late-stage separation and infiltration of aplite liquid can produce significant geochemical variations in an otherwise homogeneous granodiorite pluton—a process that cannot operate in volcanic systems.

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.