WIDESPREAD OXYGEN ISOTOPE DISEQUILIBRIUM IN FRANCISCAN HIGH-GRADE BLOCKS

Oxygen isotope ratios from blocks of grt hornblendite, blueschist, and eclogite can be a powerful indicator of ocean floor protolith alteration and/or metasomatism during exhumation of tectonic blocks. Due to slow O diffusion rates, grts are likely to preserve records of primary metamorphic formation conditions, while other minerals' O isotopes can be reset during exhumation. Grt δ18O may be mixtures of early and late-forming grt, however, thin retrograde rims suggest that measured values are closer to grt core compositions.

Mineral separates (grt, hbl, gl, omph) of 19 blocks from 9 localities in the Franciscan Complex, CA were analyzed by laser fluorination. The blocks can be categorized: grt hornblendite, eclogite, grt blueschist, and grt-bearing coherent metamorphic rocks of sedimentary and igneous origin.

If the minerals within the blocks are in equilibrium, their δ18O may record alteration before metamorphism; if not, fluids likely altered some minerals but others remained unchanged. Because fractionation between most minerals in eclogite and related lithologies is small, and grts typically began growth during subduction, grt values are likely close to WR protolith values. The overall high- δ18O (>6‰) of grts studied suggest a low-T altered MORB basalt protolith.

In grt-hornblendite blocks (n=3), grt and hbl are in disequilibrium with δ18O grt = 7.0-10.0‰ and hbl = 6.1-7.5‰. In eclogite blocks (n=3), two are in disequilibrium with grt δ18O values of 4.8-9.4‰ and omph of 5.8-8.9‰, but one glauc-bearing eclogite has mineral δ18O consistent with oxygen isotope equilibrium at 400ºC for both grt-omph and grt-glauc. The grts (7.4-10.7‰) in the blueschist blocks (n=5) are not in equilibrium with the glauc (8.8-10.6‰). Among blueschists with omph and grt in close association (n=3), grt and omph may preserve early high-T equilibrium. These blueschists may have once been eclogite, but underwent alteration which formed glauc. The coherent blocks (n=4) have δ18O grt = 6.0-9.7‰, omph = 6.9-9.8‰, and glauc = 7.1-10.3‰ and show no evidence of equilibrium. High-grade blocks of differing lithologies throughout the Franciscan commonly show O isotope disequilibrium between grt and other minerals, underlining the widespread nature of fluid metasomatism during exhumation.