A LATE-STAGE ORIGIN FOR SCHLIEREN IN FELSIC PLUTONS

Schlieren are commonly observed in felsic plutons and are though to represent emplacement of the pluton, but may also be produced by anatectic melting of wallrock or prograde metamorphism. We present observations of schlieren from the Bernasconi Hills pluton (BHP) in the Peninsular Ranges of southern California that are suggestive of a late-stage igneous origin. We propose on the basis of field observations and modeling of residual felsic liquids with MELTS (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) that schlieren may be produced by disaggregation of mafic enclaves over a range of low melt fractions (F).

Schlieren in the BHP are primarily composed of biotite and amphibole and generally associated with fine-grained, biotite-rich rinds on mafic enclaves. In many cases the rinds appear to be eroding from enclaves and schlieren can be seen to derive directly from eroded rinds. Textural similarities between rinds and schlieren and absence of metasedimentary xenoliths within the BHP confirm this petrogenetic connection. Modeling late-stage felsic residual liquids with MELTS over a range of H₂O contents (0-4 wt%) demonstrates that temperature is buffered over a range of F at a given pressure. This eutectoid behavior is due to the delayed release of latent heat by H₂O and enables the melt to remain rheologcally active over a larger range of F as a result. Consequently, the stress field imposed on biotite-rich rinds persists for a longer period of time thereby enabling the formation of schlieren at low F. Based upon our observations and modeling we propose that schlieren, in some cases, may not necessarily be indicative of emplacement, anatectic, or metamorphic processes and may instead have a late-stage igneous origin.