14 MILLION YEARS OF SERPENTINITE DIAPIR ACTIVITY IN CENTRAL CALIFORNIA: COMPARATIVE WHOLE-ROCK CHEMISTRY OF THE MIOCENE BIG BLUE FORMATION AND MODERN NEW IDRIA MASSIF

The New Idria serpentinite massif in central California is believed to represent hydrated mantle wedge emplaced diapirically in a forearc tectonic setting. Field evidence supporting this interpretation includes a dome structure in the surrounding sedimentary layers, inclusions of high-grade eclogitic and jadeititic tectonic blocks, and frequent landslides putatively caused by continuous localized uplift. Recently published whole-rock geochemical data are also consistent with a deep origin, revealing enrichments in fluid-mobile elements characteristic of slab fluids, and signatures of highly depleted peridotitic protoliths associated with extensive melt flux in the mantle wedge. About 10 km to the east lies the Big Blue formation, a sedimentary serpentinite containing a prominent protrusive unit texturally and petrologically similar to the New Idria mélange. The Big Blue formation has been dated to around 12-14 Ma, suggesting serpentinite diapirism in the area since the Miocene, and several authors have proposed that Big Blue is an earlier exposure of the New Idria diapir. However, the geochemical relationship between New Idria and Big Blue has never been investigated. Here, we present whole-rock major, minor, and trace elemental data to assess the degree to which New Idria compositions overlap with those of the Big Blue formation. Our results will help constrain whether the rocks have identical tectonic origins, whether they are part of the same diapiric system, and whether the nature of this system has changed over time. More broadly, our study may aid in understanding the geochemical dynamics and evolution of forearc serpentinite diapir systems.