Questioning the relationship between aplite dikes and high-silica rhyolites (2008)

Undergraduate: Mark Stelten

Faculty Advisor: Drew Coleman
Department: Geology

---

Chemical data from 21 aplites cutting plutons in the Sierra Nevada batholith indicate that two distinct populations are present - those displaying seagull REE patterns and those displaying U-shaped REE patterns. SEM analysis of host rocks of eight aplites shows a strong correlation between the chemistry of aplite dikes and mineralogy of their host rocks. Aplites from Sierran plutons lacking primary titanite display seagull REE patterns characterized by enrichment of all REE except for a negative Eu anomaly. In contrast, aplites in primary titanite-bearing host rocks display U-shaped REE patterns with depleted middle and heavy REE. Modeling confirms crystallizing titanite plays the dominant role in determining REE patterns owing to its large partition coefficients for REE. Allanite, apatite, zircon, and hornblende also play a role in determining the REE pattern; however, when titanite is present in the crystallizing assemblage the effect of these minerals is negligible.
The strong correlation between aplite chemistry and host rock mineralogy suggests that aplites do not move far from their host rock and therefore are rarely (if ever) erupted. This supports previous claims by Glazner et al. (2008) that aplites are not the plutonic equivalents of high-silica rhyolite eruptions. Instead, modeling suggests a low-F equilibrium crystallization or melting of granodiorite plutons can produce a melt with trace element concentrations similar to those observed in the aplite dikes.