POLYPHASE AND ANTICLOCKWISE P-T EVOLUTION FOR FRANCISCAN ECLOGITES AND BLUESCHISTS FROM JENNER, CALIFORNIA, USA

High-grade exotic blocks in the Franciscan Complex at Jenner, Californ ia, show evidence for polydeformation/metamorphism, with eight distinc t stages. Two parallel sets of mineral assemblages [(E) eclogite, and (BS) laminated blueschist] representing different bulk chemistry were identified. Stage 1, recorded by parallel aligned inclusions (Sl) of c rossite + omphacite + epidote + ilmenite + titanite + quartz (E), and glaucophane + actinolite + epidote + titanite (BS) in the central part s of zoned garnets, represents the epidote blueschist facies. The onse t of a second stage (stage 2) is represented by a weak crenulation of S1 and growth of garnet. This stage develops a well-defined S2 foliati on of orientated barroisite + epidote + titanite (E), or subcalcic act inolite + epidote + titanite (BS) at c. 90' to Sl, with syntectonic gr owth of garnet, defining the (albite-)epidote-amphibolite facies. A th ird stage, with aligned inclusions of glaucophane + (subcalcic) actino lite + phengite parallel to S2 in the outermost rims of large garnet g rains, is assigned to the transitional (albite-)epidote-amphibolite/(g arnet-bearing) epidote blueschist facies. The fourth stage represents the peak metamorphism, and was identified by unorientated matrix miner als in the least retrograded samples. In this stage the mineral assemb lages garnet + omphacite + glaucophane + phengite (E) and garnet + win chite + phengite + epidote (BS) both represent the eclogite facies. St age 5 is represented by the retrogression of eclogite facies assemblag es to the epidote blueschist facies assemblages crossite/glaucophane garnet + omphacite + epidote + phengite (E), and glaucophane + actino lite + epidote + phengite (BS), with the development of an S5 foliatio n subparallel to S2. Stage 6 represents a crenulation of S5, with the development of a well-defined S6 crenulation cleavage wrapping around relics of the eclogite facies assemblages. This crenulation cleavage i s further weakly crenulated during a D7 event. Post-D7 (stage 8) is re corded by the growth of lawsonite + chlorite +/- actinolite replacing garnet, and by veins of lawsonite + pumpellyite + aragonite and phengi te + apatite. The different, yet coeval, mineral parageneses observed in rock types (E) and (BS) are probably due to differences in bulk che mistry. The metamorphic evolution from stage 1 to stage 8 seems to hav e been broadly continuous, following an anticlockwise P-T path: (1) ep idote blueschist (garnet-free) to (2) (albite-)epidote-amphibolite to (3) transitional epidote blueschist (garnet-bearing)/(albite-)epidote- amphibolite to (4) eclogite to (5) epidote blueschist (garnet-bearing) to (6-7) epidote blueschist (garnet-free) facies to (8) lawsonite + p umpellyite + aragonite-bearing assemblages. This anticlockwise P-T pat h may have resulted from a decreasing geothermal gradient with time in the Mesozoic subduction zone of California at early or pre-Franciscan metamorphism.