MAGMA MIXING LIMITED BY REACTIVE PROCESSES

Mafic enclaves are common in many felsic plutons and are suggestive of magma mixing. However, compositional and rheological contrasts between enclaves and host magma indicate that mixing is inefficient. Additionally, enclaves are often mantled by fine-grained rims, which form during entrainment in the host magma. These have been interpreted as chill rinds, thereby implying that mixing occurs between solidified enclaves and host magma. Collectively this indicates that such mixing is a minor petrogenetic process. However, previous studies show mixing is an important means of generating intermediate rocks.

We present evidence from the Bernasconi Pluton in the Peninsular Ranges, southern California that mixing is, in part, driven by reaction between enclaves and host magma. Field observations show that enclave rinds are K-rich and biotite-rich, thus requiring reaction origin between enclaves and host magma. Geochemcial modeling with MELTS, JavaMELTS, and binary mixing equations show that measured rind compositions cannot be achieved by mixing between enclaves and a hypothetical interstitial felsic melt.. We propose that biotite-rich enclave rinds formed by reaction between K-rich aqueous fluids and enclave material and that such reaction enables mechanical mixing by reducing viscosity contrasts between enclave material and host magma. Strain accumulation in enclaves is accompanied by delamination of biotite rinds from enclaves, further supporting this notion. Mineralogical and textural similarities between biotite rinds and schlieren suggest that rinds develop into schlieren with additional strain accumulation.