PETROLOGY OF BLUESCHIST AND GRAPHITE-SCHIST BLOCKS IN THE FRANCISCAN MELANGE, SAN SIMEON, CALIFORNIA

Boudinaged blocks of graywacke, greenstone, chert, and rarer blueschist and graphite-schist are dispersed in the shale-matrix mélange terrane that is well exposed along 6 km of sea cliffs south of San Simeon. Block sizes range from 10 cm to 15 m. Field relations and petrographic characteristics of 34 blueschist (previously little studied) and 30 graphite-schist (unreported) blocks have been determined to deduce their origin, variability and extent of mixing during the tectonically driven flow that generated the mélange.

The discovery of at least three blocks with layers of mafic blueschist and clastic graphite-schist indicate these two lithologies are related. All mafic blueschists contain sodic amphibole and lawsonite. Varieties also include: chl, pump ± mus, mus + chl + epi ± pump, mus ± chl, and chl + mus + qtz ± pump. Boudinaged cm-thick layers rich in mica and/or quartz are present in 13 blocks. Aragonite is preserved in carbonate veins in 11 blocks. Most amphiboles have patchy compositional zoning. Some amphiboles are concentrically zoned with magnesio-riebeckite in the cores and glaucophane in the rims. Two of the blocks contain garnet and also have remnants of a tremolitic rind similar to that reported for many “high-grade” blocks present in the mélange east of the San Andreas fault. The protolith of the blueschists are basalts/gabbros with volumetrically trivial layers of metasediment.

The assemblage in the graphite-schist blocks is qtz + mus + plag + graph. Two types of graphite-schist blocks are recognized. One type (13 blocks) has relict sedimentary textures with a weak foliation defined by graphite and pressure solution seams. The other type (17 blocks) has a compositional layering with layers containing well recrystallized qtz. Nine of these blocks also contain lawsonite within the more graphitic layers.

Sediments were juxtaposed with mafic rock and subsequently metamorphosed under a range of pressure and/or temperature conditions, some of which attained conditions similar to those of the more voluminous mafic blueschists. Both were boudinaged with distortion accommodated by cataclastic flow along the margins and in necked regions. Metasomatic alteration to chl + pump along these zones is more extensive in the mafic blueschists because their composition makes them more reactive.