TECTONIC EVOLUTION OF BLUESCHIST-FACIES BLOCKS IN THE FRANCISCAN MéLANGE AT SAN SIMEON, CALIFORNIA: INSIGHTS INTO SUBDUCTION ZONE EVOLUTION

The Franciscan mélange exposure near San Simeon contains a small volume (<1%) of mafic blueschist blocks, and an even smaller volume of graphite-schist blocks. Mafic blueschists are of low-T blueschists, and they are of two kinds: lawsonite-Na-amphibole, and epidote-lawsonite-Na-amphibole blueschists. They lack jadeitic pyroxene and, for the most part, also albite. The peak temperatures attained by these rocks are recorded by pre-M2 epidote, and/or actinolitic cores in sodic amphiboles. Actinolite-core-bearing, epidote-free blocks attained peak temperatures of ~300 ºC at 5 kbar, whereas peak temperatures for epidote-bearing blocks are estimated to be on the order of 325-350 ºC. The main epidote-free, blueschist-facies mineral assemblage (M2) preserved in most blocks in San Simeon was recrystallized at about 200-250 ºC and 5 kbar. These blocks record evidence of a counterclockwise P-T path.

Nine of the 34 studied blueschist blocks in San Simeon contain an actinolitic rind. This study is the first report of such rind in low-temperature Franciscan blueschist mélange blocks. As these blocks occur in pelitic-matrix mélange their separation from a terrane with ultramafic rock is evident. Remnants of the metasomatically altered ultramafic as well as mafic parts of the reaction zone developed along block margins are preserved in the San Simeon blueschists. The counterclockwise P-T path and presence of associated actinolitic rinds in low-T blueschist blocks from San Simeon indicates these rocks have a common metamorphic history with high-T garnet-epidote mafic blocks that are distinctive and better studied components of the mélanges from the Central Belt.

This is the first report of graphite-schist blocks in the Franciscan. Graphite-schists were likely derived from a layered shale/graywacke protolith. Lawsonite is present in about one-third of the 30 studied graphite-schist blocks, and several examples of interlayered mafic blueschist and graphite-schists were found, indicating at least some graphite-schist blocks were metamorphosed under similar high-P/low-T conditions as mafic blueschists.

The cessation of ocean floor spreading related to magmatism gave place to the initiation of Franciscan subduction along the margin between the old, cold Farallon plate and the newly-formed CRO-capped lithosphere at about 155-160 Ma. Graywacke sediments that bypassed the forearc basin and were transported to the trench axis along submarine canyons are the probable protolith of the graphite-schist blocks.

During the initial stages of Franciscan subduction in southern California, part of the top volcanic sequence and overlying sediments were underplated at the bottom of the overriding plate and were metamorphosed under high-P conditions soon after initiation of subduction. The age of epidote-bearing low-T blueschist blocks in San Simeon is similar to that of high-T garnet-bearing blocks in the northern mélange terrane (~150-156 Ma), which approximates that of the initiation of Franciscan subduction. Material underplated along the base of the hanging wall became refrigerated and variably retrograded. Actinolitic rinds formed soon after blueschist-facies metamorphism at the bottom of the overriding plate when mafic rock came into contact with mantle that became serpentinized under nearly static conditions. Mafic material continued to be underplated, and low-T dynamic blueschist-facies metamorphism continued to form until at least ~137 Ma.

A model is proposed in which the shallowing of the plate associated with the Laramide orogeny at about 80 Ma played an important role in the exhumation of high-P-blocks and formation of the Franciscan mélange. This shallowing triggered extensional thinning at the surface facilitating the exhumation of coherent metasedimentary terranes of the Eastern Belt as well. The underplated mafic blueschist terrane was brought up to depths of a few km by extension. Blocks were plucked from the bottom of the extended overriding plate and incorporated into the undercompacted shale-rich shear zone forming the plate interface, and brought near the surface during the upward flow of mélange driven by the movement of the downgoing plate. During upwelling extended blocks were boudinaged, and metasomatically altered in the cataclastic zones that developed along necks and margins of the blocks. Dewatering that led to final compaction occurred near the surface.