UNDERSTANDING FRANCISCAN COMPLEX SUBDUCTION ZONE RECYCLING VIA MICRO-SAMPLED GARNET GEOCHRONOLOGY FROM JENNER BEACH, CALIFORNIA

The Franciscan Complex, a subduction complex preserved in California, contains mélanges which accreted between 120 Ma and 32 Ma. These melanges include high-grade mafic lithologies yielding older Lu-Hf ages from ~115 Ma to ~170 Ma. Blueschist and eclogite blocks on Jenner beach (Sonoma County) are some of the highest grade blocks in the Franciscan complex. These rocks display evidence of polyphase metamorphism, as evidenced by mineral inclusions within garnet interpreted to reflect at least eight stages of garnet growth. Additionally, the garnets exhibit significant Mn growth zoning, with cores and rims containing ~12% and ~1% spessartine, respectively. Previous work by Anczkiewicz et al. (2004) reported a Sm-Nd clinopyroxene-garnet age of 178 ±11 Ma and a Lu-Hf clinopyroxene-garnet age of 157 ±9 Ma from this locality. However, there have been no attempts to microsample individual garnet zones in eclogites from Jenner Beach. Garnet cores and rims were separated from a single sample of eclogite, identified via chemical zonation and mineral inclusions, by microdrill and dated with Sm-Nd and Lu-Hf geochronology. Lu-Hf dating resulted in an age of 158.35±0.46 Ma, with no significant difference in age between garnet core and rim fractions. This age is within the error of the Lu-Hf garnet-clinopyroxene date from Anczkiewicz et al. (2004). The garnet core and rim age equivalence indicates that this sample of eclogite likely experienced a single episode of subduction instead of multiple cycles of burial and exhumation. It also indicates that this episode of subduction occurred relatively quickly.