The tenuous connection between high-silica rhyolites and granodiorite plutons

Abstract

The trace element compositions of aplite dikes in the Sierra Nevada batholith of California differ profoundly from high-silica rhyolites (HSRs) and contradict a genetic connection to them. The aplites are strongly depleted in all middle rare earth elements (REEs), whereas HSRs are strongly depleted only in Eu and enriched in other REEs; the aplites are strongly depleted in Y and variably enriched in Sr, whereas HSRs are enriched in Y and strongly depleted in Sr. Volcanic rocks with the trace element characteristics of these aplites are rare to absent in the geologic record. Aplite REE patterns are likely controlled by titanite, which has large distribution coefficients for REEs, whereas HSRs cannot have equilibrated with titanite. Titanite may crystallize late in dacitic magma and thus HSR may be extracted before titanite saturation is reached; aplites would form after titanite appears, but when the melt percentage is too low and the water content of the melt too high (at fluid saturation) for the magma to ascend without rapid crystallization, thus preventing eruption. HSRs could also form by low-degree partial melting of granodiorite plutons in which titanite melts out early. Alternatively, HSRs may be extracted from silica-rich plutons that lack titanite; leucogranite plutons with REE contents that could be complementary to HSRs are present but uncommon in the Sierra Nevada.