PETROLOGY OF A GREENSCHIST FACIES METATONALITE BLOCK IN THE FRANCISCAN COMPLEX, JENNER, CA

The metatonalite block is one meter in diameter and is located in Jenner, CA at the mouth of the Russian River (UTM 10 S 0488719E 4255854N NAD 27). The block is only accessible during low sand and water conditions, and lies among many other blueschist blocks.

The protolith was probably an equigranular medium-grained biotite tonalite. Original geographic location of the pluton is unknown. Albite displays relict twinning and crystal shape ranges from euhedral to anhedral. In some samples, myrmekite is extensively developed and surrounds euhedral albite crystals with a thick rind. The quartz crystals are subhedral to anhedral with a mottled texture. A Pearce diagram (1984) geochemical signature suggests a volcanic arc protolith. A REE plot shows La/Sm=1.35, Eu/ Eu*=0.78.

Biotite has been completely converted to chlorite and plagioclase to albite+epidote, displaying a greenschist facies overprint. The rock now is 61% albite, 30% quartz and 9% chlorite, with trace amounts of epidote, apatite, zircon, ilmenite, sphene, and rutile(?). Small bodies of intergrown titanium minerals (sphene, ilmenite and rutile(?)) are closely associated with the chlorite. No foliation or lineation has developed and the rock is best described as a metatonalite fels.

The surface of the block is irregularly pitted, unlike the smoothly rounded blueschist blocks it lies among. Abundant patchy red hematite stains are present on the surface of the block.

The rock is cut by many microbreccia veins up to 1 mm wide, striking in all directions in an intricate network. The microbreccia is due to hydrofracturing after the formation of the metamorphic phases. It then recrystallized.

The block was originally metamorphosed in the subsurface in a subduction zone or contact metamorphic environment. A small segment of it was moved to the surface, transported to the trench, and incorporated in an olistostrome. The olistostrome was itself moderately subducted as shown by local burial metamorphism of the matrix, and finally slowly uplifted and the block exposed.