Evidence for multiple stages of serpentinization from the mantle through the crust in the Redwood City Serpentinite mélange along the San Andreas Fault in California

Abstract

The active plate boundary of the California margin includes the active San Andreas Fault System and the active mountain building of the California Coast Ranges where ultramafic rocks are also commonly found. There are numerous hypotheses for the origin and the timing and emplacement mechanisms of such rocks that ultimately came from Earth's mantle, but typically the mineralogical, structural, and geochemical features of fresh serpentinite, and their reaction and deformation histories were unknown due to heavy weathering of most of the exposures. We investigated a block-and-matrix serpentinite mélange in Redwood City, California on the San Francisco Peninsula that is proximal to an active strand of the San Andreas Fault. The mélange is composed of 6 m- to cm-sized serpentinite blocks surrounded by fine-grained serpentine matrix. The blocks are polygonal in shape and show a remarkable internal mineralogical structure that has recorded at least three stages of serpentinization of a clinopyroxene-bearing harzburgite protolith. The largest, freshest polygonal serpentinite blocks contain a core comprised of surviving peridotite minerals plus lizardite (lz) ± antigorite (atg) + brucite (brc) + magnetite (mag) domains. The cores are surrounded by a ~10 cm thick peripheral rim of lizardite + magnetite. The reaction boundaries between rims and cores are parallel to the external polygonal surfaces of the blocks. These blocks are also sheathed by thin skins of sheared lizardite + chrysotile (ctl). The higher-temperature lz/atg + brc + mag serpentinization represented by the core mineralogy (Stage A) probably occurred on a large scale in the mantle and the hydration metamorphism producing the lz + mag mineralogy in the rims (Stage B) was likely triggered by the growth of a system of approximately planar fractures under crustal conditions that was enabled by the ingress of water at high pressure and that defined the polygonal block shapes as well as the Stage-A to Stage-B reaction boundaries. Stage-A serpentinization reactions are internally balanced except for addition of H2O, Cs and Rb as indicated by core mineral composition. The Stage-B serpentinization was open system with CaO, Cs and Rb migrating outside the blocks with addition of H2O, U, Pb ± LREE. The sheared lizardite skin probably formed during the ascent of the RCS through the crust. Several classes of mineral-filled veins were found in both cores and rims and indicate fluid pressures approaching lithostatic pressures that enabled these opening- mode fractures to occur. We posit that such high-pressure hydrothermal conditions were important in promoting the serpentinization reactions as well as enabling a weak rheology consistent with a cold-intrusion/diapiric emplacement mechanism. These factors give insights into where these mantle rocks came from, the sources of the water causing serpentinization, and the deformation mechanisms by which they come to Earth's surface. Similarities of the structures of serpentinite blocks reported in this paper with those in blocks from other serpentinite bodies in the San Francisco Bay Area (Lewis and Kirby, 2015 AGU Abstract T13H-05) suggests that the processes inferred from our investigation are regional in spatial extent.