Trace element and isotopic zoning of garnetite veins in amphibolitized eclogite, Franciscan Complex, California, USA

Here we present major element, trace element, and oxygen isotope data for garnet from an amphibolitized eclogite block from Ring Mountain, Franciscan Complex, California, USA. Garnetite veins 1–5 cm thick are laterally continuous up to 10 m within an Mg-rich blackwall zone of the eclogite block. Complex major and trace element zoning patterns reveal multiple stages of garnet growth in both the matrix and garnetite veins. Similarities in major and trace element zoning between matrix and vein garnet suggest that crystallization of the garnetite veins began toward the end of matrix garnet core growth, and continued throughout the garnet growth history of the rock. Oscillatory zoning in rare-earth elements suggests garnet growth in pulses, with matrix-diffusion-limited growth in between pulses. Oxygen isotope analyses of matrix and vein garnet have a range in δ¹⁸O values of 5.3–11.1 ‰. Differences in δ¹⁸O values of up to ~ 4 ‰ between garnet core and rim are observed in both the matrix and vein; garnet cores range from 9.8 to 11.1 ‰ (median 10.4‰), garnet mantles range from 8.3 to 10.0 ‰ (median 9.7 ‰), and garnet rims range from 5.8 to 7.8 ‰ (median 6.7 ‰). Late-stage vein crystallization appears as a garnet “cement” that fills in a network of small (typically 5–50 µm) garnet cores, and likely crystallized from an amorphous phase. The low δ¹⁸O values of this latest stage of garnet growth are consistent with interaction with serpentinites, and likely represent the physical incorporation of the eclogite block into the serpentinite matrix mélange.