Rhyolites and Rhyolites

Although phase equilibria drive the compositions of residual liquids in igneous rocks toward high-silica rhyolite (HSR), not all rhyolites are created equal. Residual high-silica liquids (HSLs) in many granodiorite plutons are rhyolitic but have trace-element patterns that differ greatly from those of erupted high-silica rhyolites (HSRs). This observation conflicts with the commonly held view that granodiorite plutons are crystal cumulates left behind after eruption of HSR. Where then are the crystalline residues of HSR eruptions?

Aplite dikes from Mesozoic and Tertiary plutonic rocks in the western US have consistent HSR compositions, but their trace-element patterns vary widely and correlate closely with the presence or absence of primary titanite in the host rock. Aplites in titanite-free hosts are characterized by Eu depletion (yielding seagull-shaped REE patterns), whereas aplites in titanite-bearing hosts are depleted in Y and all MREEs, yielding deep U-shaped REE patterns. Similarly, residual HSLs in the titanite-bearing Half Dome Granodiorite of the Tuolumne Intrusive Suite have distinctly U-shaped REE patterns that are indistinguishable from the REE pattern of glass from the titanite-bearing Fish Canyon Tuff. U-shaped REE patterns are essentially unknown among erupted HSRs. Because HSRs all have seagull REE patterns and aplite geochemistry closely correlates with host-rock mineralogy, we conclude that HSLs in titanite-bearing granodiorite bodies do not travel far from their hosts. HSRs thus appear to have a different origin from HSLs developed in long-lived magma bodies.

High-silica granites with negative Eu anomalies like those in HSRs are sparse, but known. However, plutonic rocks with positive Eu anomalies that could be the solid residues of fractionated HSR liquids are equally rare. These observations suggest that the plutons commonly represent unerupted equivalents of volcanic rocks rather than residua from extraction of erupted magma.