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Serpentinite matrix mélange represents a significant, if less common, component of many accretionary complexes. There are two principal hypotheses for the origin of serpentinite mélange: (1) formation on the seafloor in a fracture zone–transform fault setting, and (2) formation within a subduction zone with mixing of rocks derived from both the upper and lower plates. The first hypothesis requires that the sheared serpentinite matrix be derived from hydrated abyssal peridotites and that the block assemblage consist exclusively of oceanic rocks (abyssal peridotites, oceanic basalts, and pelagic sediments). The second hypothesis implies that the sheared serpentinite matrix is derived from hydrated refractory peridotites with supra-subduction zone affinities, and that the block assemblage includes rocks derived from both the upper plate (forearc peridotites, arc volcanics, sediments) and the lower plate (abyssal peridotites, oceanic basalts, pelagic sediments). In either case, serpentinite mélange may include true mélange, with exotic blocks derived from other sources, and serpentinite broken formation, where the blocks are massive peridotite.

The Tehama-Colusa serpentinite mélange underlies the Coast Range ophiolite in northern California and separates it from high-pressure/temperature (P/T) metamorphic rocks of the Franciscan complex. It has been interpreted both as an accreted fracture zone terrane and as a subduction-derived mélange belt. Our data show that the mélange matrix represents hydrated refractory peridotites with forearc affinities, and that blocks within the mélange consist largely of upper plate lithologies (refractory forearc harzburgite, arc volcanics, arc-derived sediments, and chert with Coast Range ophiolite biostratigraphy). Lower plate blocks within the mélange include oceanic basalts and chert with rare blueschist and amphibolite. Hornblendes from three amphibolite blocks that crop out in serpentinite mélange and sedimentary serpentinite yield 40Ar/39Ar plateau ages of 165.6–167.5 Ma, similar to published ages of high-grade blocks within the Franciscan complex and to crystallization ages in the Coast Range ophiolite. Other blocks have uncertain provenance.

It has been shown that peridotite blocks within the mélange have low pyroxene equilibration temperatures that are consistent with formation in a fracture zone setting. However, the current mélange reflects largely upper-plate lithologies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large offset transform fault–fracture zone that evolved into a subduction zone mélange complex. Mélange matrix was formed by the hydration and volume expansion of refractory forearc peridotite, followed by subsequent shear deformation. Mélange blocks were formed largely by the breakup of upper plate crust and lithosphere, with minor offscraping and incorporation of lower plate crust. We propose that the methods discussed here can be applied to serpentinite matrix mélange worldwide in order to understand better the tectonic evolution of the orogens in which they occur.

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