Regional Sampling of Mantle Peridotites in Serpentinite Blocks Collected from Serpentinite Bodies in the San Francisco Bay Area, California: Petrological Trends

Abstract:

We have collected and investigated 278 ultramafic and related rock samples from 18 polygonal-block -and- sheared-matrix-type serpentinite bodies in the Coast Ranges in the San Francisco Bay Area. These sites include Ring Mountain in the north to Silver Creek near San Jose in the south, spanning nearly 100 km of Coast Range geology. These bodies show extensive variation in volume ratios of serpentinite blocks (some including peridotite minerals) to sheared matrix, in peridotite mineral modes, in the degrees of weathering, in the proportions of peridotite minerals versus alteration minerals, and in the degree of late-stage brittle deformation. However, we found remarkable coherence in the serpentinite alteration mineralogy and our samples bear a strong resemblance to those in the Redwood City serpentinite body studied recently by Uno and Kirby (GRL submitted). In particular, we see mineralogical and geochemical evidence for multiple stages of alteration of the original peridotite minerals that reflect partial peridotite alteration, likely in the mantle, and then a later reaction to lizardite + magnetite in the crust. These reactions are followed by localized late-stage partial alteration of serpentinite to silica minerals and magnesite by carbonated water. Our findings suggest that the mantle sources of this type of partially-serpentinized peridotite in this section of the Coast Ranges are remarkably similar and that the processes leading to later-stage alteration reactions have operated repeatedly over the area that we sampled. Internal deformation in these bodies during later stages of alteration probably occurred during ascent through the crust, as reflected by sheared lizardite skins on the serpentinite blocks that we collected. We put forward several working hypotheses that provide insights into the origins and geologic histories of these rocks.