SIERRA CITY MÉLANGE: UPPER PLATE OPHIOLITE OR INTRA SUBDUCTION COMPLEX MÉLANGE?

Ophiolitic rocks are crucial to understanding large-scale orogenic assembly processes, but their significance depends on their specific structural-tectonic affinity that is commonly difficult to discern from field and supporting studies. Ophiolitic rocks can occur as offscraped scraps in subduction complexes, both as coherent units and as sedimentary mélanges, as well as making up largely intact sheets representing the upper plate of a paleosubduction zone. The Sierra City mélange (SCM) has been considered an ultramafic unit occupying the highest structural horizon of the Shoo Fly Complex, a subduction complex representing the oldest components of the Cordilleran orogen in the northern Sierra Nevada of California. The SCM ranges from about 1 to 5 km in thickness and is exposed over a strike length of at least 30 km. Near the town of Quincy the SCM consists of a ~1 km mélange unit with a phyllite matrix derived from clastic rocks ranging from shales to sandstones. Blocks include serpentinite to 65 m, sandstone to 20 m, basalt to 10 m, gabbro to 8 m, and amphibolite to about 5 m. The matrix has a neoblastic white mica, chlorite, and epidote that suggests low greenschist facies (or lower) conditions of metamorphism; all blocks except the amphibolites appear isofacial. A sedimentary origin for the mélange is suggested by the presence of amphibolite/metagabbro and rare serpentinite clasts in sandstone. Chrome spinel from a serpentinite clast has a high Cr# suggesting a possible forearc mantle source for the serpentinite clast. The SCM appears to be a subduction complex mélange rich in ophiolitic debris, possibly shed from the upper plate.