THE REDOX STATE OF THE FRANCISCAN TRENCH
Garnet-epidote oxygen barometry was also applied to each thin section using matrix epidote (Cz80-84 Ps16-20) texturally equilibrated with outermost garnet rims. These formed during exhumation and record fluid interactions in the subduction channel after blocks parted ways with the subducting slab. Garnet rims in these Franciscan blocks display complex and oscillatory zoning patterns in Ca, Fe and Mn that may record fluid or tectonic processes. Garnets in epidote-rich (Cz85-79 Ps14-21) patches of each thin section, exhibit less pronounced cation zoning and different rim composition (Alm63-58 Pyp9-2 Sps12-4 Grs27-23), which differ from garnets in areas richer in garnet (Alm60-54 Pyp7-5 Sps25-9 Grs26-12) with smaller amounts of epidote. However, calculations with matrix phases in both textures yield redox conditions at the hematite-magnetite buffer, even in the Ward Creek sample with more oxidized prograde conditions, suggesting similar redox conditions in the fluid-rich mélange of the subduction channel to the slab itself.
Oscillatory cation zoning in Franciscan garnets is not controlled exclusively by changes in fluid composition recorded by oxygen isotope ratios or oxygen fugacity. Oscilations in P and T such as those caused by yo-yo subduction may instead be responsible.