GETTING IN THE ZONE: OXYGEN ISOTOPE AND CATION ZONING IN MULTIPLE GENERATIONS OF FRANCISCAN GARNETS

Two compositionally- and texturally-distinct populations of garnet in a hornblende-eclogite block from the Ring Mountain provide a unique opportunity to investigate fluid flow in the mélange of the Franciscan Complex. In situ analysis of oxygen isotopes reveals zoning in both populations. We correlate δ18O and cation zoning to relate external fluid interactions for this rock to the nucleation of successive garnet populations.

The sample consists of garnet, hornblende, omphacite, phengite, zoisite, and rutile. Garnet occurs as matrix porphyroblasts (2-5 mm) and small crystals (5-50 μm) in 1-5 cm veins. Almandine (Alm) content decreases in matrix garnet from 60-48% core-to-mantle, whereupon it increases to 51% and plateaus through the rim. Grossular (Grs) content follows an equal and opposite trend (22-34% core-to-mantle, 27% rim). Pyrope (Prp) increases from 17-19% core-to-rim. Spessartine (Sps) is 5% at the center and increases to ~7% at the rim. Vein garnet is zoned in Sps (20% core, 8% rim), Grs (9% core, 26% rim), Alm (58% core, 48% rim), and Prp (13% core, 16% rim). Matrix and vein garnet have Mg#’s that increase from 0.17 to 0.26 core-to-rim. Vein garnet has highly elevated Sps content compared to matrix garnet in this sample and eclogitic garnet worldwide.

Oxygen isotope analyses were performed on the WiscSIMS CAMECA IMS 1280 ion microprobe with 10 µm spot size on ten garnet grains and corrected for compositional bias. Matrix and vein garnets have δ18O values that decrease from core (9.9±1.1‰, 2SD, VSMOW) to rim (6.8±1.0‰). This 3‰ drop is contemporaneous with Sps spikes. The δ18O values for garnet cores and rims found here are higher than those for zoned Ring Mountain garnets presented in earlier studies.

The correlation above suggests a timeline for fluid infiltration and garnet nucleation. Matrix garnets nucleated first and grew through prograde conditions, followed by the nucleation and growth of vein garnets. After a period of growth, a low δ18O fluid infiltrated the rock, causing a decrease in δ18O values throughout garnet populations. While this decrease in δ18O suggests a single late influx of external fluid, high Sps content in vein garnet points to more complexity. Further analysis of oxygen isotopes and trace elements throughout the vein will be necessary to parse the metasomatic timeline for this rock.