METASOMATISM AT THE PEAK OF METAMORPHISM: USING COMPUTATIONAL THERMODYNAMIC MODELING TO UNDERSTAND DRAMATIC OXYGEN ISOTOPE ZONING IN CATALINA GARNET QUARTZITES
A meter-scale block of quartzite in the high-grade mélange consists of ~93% quartz, 6% garnet, < 0.2% chlorite, rutile, apatite, amphibole, zircon, sphene, potassium feldspar and white mica. Garnet occurs as both fine-grained (<200𝜇m) and larger crystals (1-3mm). The small garnets are mostly homogeneous with only limited Fe-Mg zoning. The large garnets display prograde cation zoning with decreasing Sps and steadily increasing Mg# from cores to rims, which are similar in composition to the small garnets. Despite differences in crystal size and cation zoning, both large and small garnets have similarly extreme δ18O zoning, with high cores (~25‰, VSMOW) and a sharp decrease near their rims (~10‰, Cameron et al., 2014, GSA Abstr).
We used the block’s bulk composition and Perple_X to generate equilibrium assemblage diagrams. Matching the large garnets’ compositional isopleths to the model shows increasing temperature from cores to rims (550-650°C). The small garnets and matrix assemblage are stable from 650-675°C, which is consistent with existing [Zr] in rutile thermometry (670-700°C, Hartley et al., 2016, GSA Abstr). We cannot constrain pressure due to low compositional variability and the mineral assemblage’s high thermodynamic variance.
These data show that large garnets nucleated at lower temperatures and continued to grow with Mg zoning as temperatures increased while the slab subducted. In contrast to cation zoning, the δ18O remained consistently high, indicating no external fluid infiltration until after a new generation of garnets nucleated at ~650°C with compositions similar to the large garnet rims. At this time, metasomatism occurred, causing δ18O to decrease dramatically. Pairing modeling with cation and δ18O zoning provides evidence that external fluid interaction did not occur continuously throughout the subduction process and that metasomatism instead began at depth.