Fertile abyssal peridotites within the Franciscan subduction complex, central California: Possible origin as detached remnants of oceanic fracture zones located close to a slow-spreading ridge

This paper describes lherzolite and harzburgite within shale-matrix mélanges of the late Mesozoic Franciscan Complex at the western margin of central California. The Cr# ( = Cr/(Cr + Al) atomic ratio) of spinels in the lherzolite is extremely low (0.11 to 0.13), plotting at the low Cr# end of the abyssal peridotite field, although the moderate to high Cr# of spinels in the harzburgite leaves two possible origins as forearc or abyssal peridotite. The relationships between the Fo and NiO contents of olivines, plot along the mantle olivine array and indicate the residual character of the lherzolite. A high Mg# ( = Mg/(Mg + Fe2+) atomic ratio) for clinopyroxenes is one of the characteristics of the lherzolite, reflecting low-temperature Mg–Fe redistribution with olivine beneath the ocean floor at around 700 °C. The rare earth element patterns of clinopyroxenes in sampled lherzolite show simple light rare earth element-depleted patterns with flat middle and heavy rare earth elements patterns, resembling those obtained for peridotites dredged from the Vulcan Fracture Zone at the American–Antarctica Ridge. This mineral chemistry, in combination with a lack of association with gabbros and sheet–dike complexes, implies that the lherzolite and harzburgite represent fertile abyssal peridotites along oceanic fracture zones near a slow-spreading ridge; the peridotites experienced a low degree of melting during their history. This origin is in contrast to that of the Coast Range ophiolite, the most extensive ophiolitic belt in California, which formed in a supra-subduction zone setting.