Abstract

Dozens of mafic blueschist blocks are found in the Franciscan mélange, which is well exposed along 6+ km of nearly continuous sea cliffs and wave-cut benches near San Simeon, California. Thirty-four blocks were studied to discover all the varieties in this classic locality of mélange. The Na-amphibole and lawsonite ± epidote–rich schists were dynamically deformed, folded, and veined (including lawsonite and aragonite) before becoming incorporated into the shale-matrix mélange. Tectonically driven flowage of the matrix caused all lithologic components to pinch and swell, forming boudins that separated to become isolated ellipsoidal blocks. This megascopically ductile style of deformation was accommodated in mafic blocks by cataclasis concurrent with alteration to chlorite and pumpellyite. Shearing and alteration were most intense along block margins and within block faults. Petrographic analyses, aided by backscattered electron imaging, microprobe mineral analyses, bulk chemistry, and thermodynamic calculations, reveal these blueschists traveled along a counterclockwise pressure-temperature (P-T) path. Na-amphiboles have actinolitic cores that indicate an early greenschist-facies stage. Peak T and P conditions for the epidote-bearing blueschists were ∼350 °C at pressures of 5–9 kbar. Many blocks have textural evidence of lawsonite replacing epidote and Na-amphibole with Fe3+-rich rims. This indicates epidote replacement occurred as the rocks cooled below ∼250 °C at ∼5 kbar. It is evident that the phase of dynamic epidote-blueschist-facies recrystallization that nearly obliterated evidence of an earlier greenschist-facies stage was followed by retrograde recrystallization under much less dynamic to nearly static, but still high-P, conditions. Cooling and much of the observed veining must have happened after these rocks were deeply underplated along the bottom of the ophiolitic leading edge of the North American plate. Pieces of the underplated blueschist terrane were probably detached from the hanging wall as slabs that boudinaged and reboudinaged while entrained in shale-matrix mélange upwelling from depths of at least 15 km.

You do not currently have access to this article.