QUESTIONING THE RELATIONSHIP BETWEEN HIGH-SILICA RHYOLITES AND APLITE DIKES

Geochemical data from 17 aplites cutting plutons in the Sierra Nevada batholith indicate that two distinct populations are present - those from titanite-bearing plutons and those from titanite-free plutons. Aplites preserved in titanite-bearing hosts are geochemically distinct from other aplites and erupted high-silica rhyolites (HSRs) in the western US and elsewhere, suggesting that there is no direct link between aplites from titanite-bearing host rocks and HSRs.

SEM analysis of heavy-mineral separates from aplite-bearing Sierran plutons shows that apatite and zircon are ubiquitous, allanite is prevalent only in high-silica plutons, and primary titanite is most abundant in granodiorites. Aplites from Sierran plutons lacking primary titanite display low Sr, high Y, and ”seagull” REE patterns characterized by enrichment of all REE except Eu, which is significantly depleted. These trace-element signatures are also characteristic of erupted HSRs. In contrast, aplite dikes in primary titanite-bearing host rocks display high and variable Sr, low Y, and U-shaped REE patterns with significant depletion of all middle REE. Modeling shows that titanite in the crystallizing assemblage plays a major role in determining these characteristics owing to its large partition coefficients for REE and Y. Other trace phases with high distribution coefficients for REE, including allanite, may also play a role; however, the direct correlation between presence or absence of titanite and aplite chemistry suggests that this phase has primary control. Modeling also shows that it is not possible to derive HSRs from aplites with U-shaped REE patterns, or vice-versa.

These data suggest that HSRs cannot be derived from granodiorites that were crystallizing titanite when the rhyolites were segregated. Instead, modeling suggests that 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. These possibilities could help explain the excellent correlation between aplite geochemistry and host rock mineralogy, and the lack of erupted equivalents to the aplites. If aplites represent low-F fluid-rich residual liquids or partial melts, they would likely never move far from their host.