SUPRA-SUBDUCTION AND ABYSSAL MANTLE PERIDOTITES IN SERPENTINITE MELANGE, COAST RANGE OPHIOLITE, CALIFORNIA: IMPLICATIONS FOR MELANGE FORMATION

The Coast Range ophiolite (CRO) of California is one of the most extensive tracts of oceanic crust preserved in the Cordillera, however, its origin remains controversial. Most studies support a supra-subduction zone (SSZ) origin, but others infer origin at a mid-oceanic ridge. In northern California, the ophiolite is underlain by sheared serpentinite-matrix mélange containing kilometer-scale blocks of unsheared harzburgite, dunite, and lherzolite, along with smaller blocks of pillow basalt, chert, volcanogenic sandstone, low-grade metasediments, and high-grade metamorphic blocks of amphibolite and garnet amphibolite. Proposed origins for the mélange mirror those for the ophiolite as a whole: 1) the hydration of upper plate peridotites followed by shear deformation and mixing with crustal blocks from both the upper plate and lower, subducting plate, or 2) formation within an oceanic transform fault/fracture zone, followed by emplacement into a subduction complex.

We present here new data on mineral compositions in refractory mantle peridotites that underlie crustal sections of ophiolite and show that these are dominantly related to hydrous melting in a supra-subduction zone setting, most likely in the fore-arc of an intra-oceanic arc complex. Abyssal peridotite occurs at one location where it is associated with supra-subduction zone (SSZ) mantle peridotite and volcanic rocks with both oceanic and arc affinities. In all cases fore-arc SSZ mantle peridotites are associated with crustal sections containing arc-related volcanics, including boninites. These data strongly support mélange origin within the upper plate of a subduction zone by hydration of upper plate peridotites and mixing with crustal lithologies. This convergence between conclusions based on crustal lithologies and their underlying mantle sections confirms previous proposals that link the CRO to SSZ processes, and seriously undermines hypotheses that call on formation of the ophiolite at a mid-ocean ridge spreading center.