SIGNIFICANCE OF MUSCOVITE + QUARTZ SYMPLECTITES IN ANATECTIC WALLROCKS FOR EVALUATING BACKGROUND GEOTHERMAL GRADIENTS IN MAGMATIC ARCS

Symplectic intergrowths are an important marker of incomplete retrograde metamorphism. Because they represent arrested reactions that were limited by diffusion, these textures can provide some constraints on how pressure and temperature conditions varied following peak-grade metamorphism. For example, symplectic intergrowths of muscovite + quartz in upper amphibolite facies, sillimanite + K-feldspar wallrocks of continental margin batholiths permit assessment of the pressure-temperature trajectory during cooling from peak-grade, pluton-related heating back to the ambient geotherm in magmatic arcs.

The voluminous 98–92 Ma tonalite-trondhjemite-granodiorite (TTG) plutons of the Peninsular Ranges batholith represent a flux in excess of 100km³/km strike length/m.y. and constitute over 50% of the total intrusive mass of the batholith between 30–34°N in southern California and Baja California. Thermochronologic data indicate that temperatures above ca. 450°C persisted from 94–86 Ma and above 400°C until 78 Ma. These conditions—near-isobaric, slow-cooling—represent an ideal environment for retrograde muscovite growth. Peak-grade (4–6 kbar) anatectic wallrocks reached 675 ± 25°C during intrusion of the TTG plutons, significantly above the stability of muscovite for graphitic pelitic bulk compositions with H₂O activities ca. 0.7. While pervasive, coarse, unoriented muscovite commonly overgrew fibrolitic sillimanite in these rocks, finer grained muscovite + quartz symplectites grew with myrmekite at the expense of K-feldspar in sillimanite + K-feldspar assemblages. Preliminary Ti-in-quartz measurements reveal that symplectic quartz grew at 60-80°C lower than matrix quartz. These measurements constrain the background geotherm to conditions near the sillimanite-kyanite equilibrium boundary. We tentatively interpret that grain size variation among symplectic intergrowths reflects reaction progress as temperatures relaxed toward the ambient geotherm (i.e., the largest symplectites most closely approximate the ambient geotherm).