A SUBDUCTION FLUID RECORD FROM CORRELATED OXYGEN ISOTOPE AND TRACE ELEMENT ANALYSIS OF RUTILE AND SPHENE FROM GARNET-BEARING TECTONIC BLOCKS IN THE FRANCISCAN COMPLEX, CALIFORNIA, USA

Eclogite and related high-grade metamorphic tectonic blocks hosted by the Franciscan Formation of California have provided our community with ever-evolving tectonic and geochemical conundrums since first described over a century ago. From petrographic observations of mineral overgrowths to in situ measurements of trace element and isotopic zoning, these enigmatic rocks record a wealth of information on the nature and timing of fluid flow, both within the subducting slab and during exhumation. Recent work on mineral-scale oxygen isotope disequilibrium and δ¹⁸O zoning in garnet shows that the oxygen isotope ratios of most minerals in high-grade blocks were reset by late fluid infiltration, with only garnet and zircon cores preserving prograde δ¹⁸O. To explore timing of oxygen metasomatism relative to the formation of peak and retrograde phases, we analyzed oxygen isotope ratios in rutile and sphene by ion microprobe and correlated trace element compositions by LA-ICPMS in rutile from 9 samples of eclogite, garnet blueschist, and garnet hornblendite from 5 localities (Junction School, JS; Panoche Pass, PNP; Tiburon, TIB; Jenner, J; Ward Creek, WC) in the Franciscan. In 6 samples of eclogite (JS, WC) and hornblende eclogite (TIB) containing rutile rimmed with sphene, the δ¹⁸O of sphene rims (4.6-5.9‰) is equal to (or less than) that of rutile cores (4.2-7.3‰). These disequilibrium values are consistent with textural observations of replacement, but could represent equilibrium with the same oxygen reservoir at ~600˚C for rutile and <400˚C for sphene. Rutile grains are unzoned and generally poor in trace elements, but differ among localities particularly in their Nb, V, and Cr content. Zr concentrations are similar for all samples (60-110ppm) with the exception of a garnet hornblendeite block (PNP, 428ppm), yielding temperatures of 580-620˚C for eclogite and hornblende eclogite and 705˚ for the PNP hornblendite. The average δ¹⁸O of rutile in all samples is in equilibrium with garnet rims, but not garnet cores, requiring that the matrix rutile recrystallized or was diffusively reset after late metasomatism. Domains within some JS rutile grains preserve low δ¹⁸O (1-3‰), the lowest of which are in equilibrium with 4‰ garnet cores, suggesting limited preservation of an earlier rutile generation.