Magmatic origin of low-T mafic blueschist and greenstone blocks from the Franciscan mélange, San Simeon, California

•Most blueschist and greenstone blocks analyzed are L-REE MORB and some OIB.•Linear trends of incompatible elements indicate fractionation from similar magma source.•Enrichment in K, Rb, and Na indicates rock–fluid interaction with interlayered sediment.•Blueschists were derived from underplated MORB-like uppermost oceanic crust containing seamounts.•Greenstones are fragments of detached seamounts from the same crust.

The Franciscan mélange exposure near San Simeon contains abundant greenstone and minor blueschist blocks that were tectonically boudinaged while encased in the shale-matrix. Tectonic deformation of the blueschists is evident from variable amounts of cataclastic flow along their margins and in pinched tails. Major, trace, and rare earth elemental analyses indicate that blueschist and greenstone blocks in this area of the mélange were derived from sources with MORB-like composition along with some having trace element and REE patterns similar to OIB compositions. Most blocks are low LREE basalts that probably formed in an open ocean ridge setting, but some blocks have high LREE contents similar to off-axis seamounts. Linear trends of incompatible elements for both blueschist and greenstone blocks indicate that both lithologies were probably derived from a similar, variably fractionated, tholeiitic magma.Blueschist blocks with sodic amphibole + lawsonite ± epidote were pervasively recrystallized at 300–350 °C and foliated during ductile deformation that included folding. Their protolith can only be identified as mafic. A few blocks contain very small amounts of metasedimentary materials indicating some were probably seafloor basalts, but some may have been diabase or even gabbro. Where interlayered sediment was present, the mafic protolith was enriched in K, Rb, and Na. Greenstones, on the other hand, contain abundant pseudomorphic evidence of magmatic textures. Alteration to albite, chlorite and pumpellyite at temperatures of 100–200 °C is intense, especially in cataclastic margins and pinched tails. Some of the basaltic greenstones have attached radiolarian chert, and a few have relict diabasic textures.The mafic blueschists and greenstones in the mélange near San Simeon are probably fragments from the uppermost part of the Farallon plate. The blueschists may be mostly mafic slabs uprooted from the subducting crust, underplated to the base of the North American plate and later detached and carried back towards the surface in upwelling mélange. The greenstones are most likely fragments detached from seamounts subducted during times of mélange upwelling.