Polyphase and anticlockwise P‐T evolution for Franciscan eclogites and blueschists from Jenner, California, USA

Abstract

High-grade exotic blocks in the Franciscan Complex at Jenner, California, show evidence for polydeformation/metamorphism, with eight distinct 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 crossite + omphacite + epidote + ilmenite + titanite + quartz (E), and glaucophane + actinolite + epidote + titanite (BS) in the central parts of zoned garnets, represents the epidote blueschist facies. The onset 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 foliation of orientated barroisite + epidote + titanite (E), or subcalcic actinolite + epidote + titanite (BS) at c. 90" to S1, with syntectonic growth of garnet, defining the (albite-)epidote-amphibolite facies. A third stage, with aligned inclusions of glaucophane + (subcalcic) actinolite + phengite parallel to S2 in the outermost rims of large garnet grains, is assigned to the transitional (albite-)epidote-amphibolite/(garnet-bearing) epidote blueschist facies. The fourth stage represents the peak metamorphism, and was identified by unorientated matrix minerals in the least retrograded samples. In this stage the mineral assemblages garnet + omphacite + glaucophane + phengite (E) and garnet + winchite + phengite + epidote (BS) both represent the eclogite facies. Stage 5 is represented by the retrogression of eclogite facies assemblages to the epidote blueschist facies assemblages crossite/glaucophane + garnet + omphacite + epidote + phengite (E), and glaucophane + actinolite + epidote + phengite (BS), with the development of an S5 foliation 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 is further weakly crenulated during a D7 event. Post-D7 (stage 8) is recorded by the growth of lawsonite + chlorite f actinolite replacing garnet, and by veins of lawsonite + pumpellyite + aragonite and phengite + apatite. The different, yet coeval, mineral parageneses observed in rock types (E) and (BS) are probably due to differences in bulk chemistry.The metamorphic evolution from stage 1 to stage 8 seems to have been broadly continuous, following an anticlockwise P-T path: (1) epidote 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 + pumpellyite + aragonite-bearing assemblages. This anticlockwise P-T path may have resulted from a decreasing geothermal gradient with time in the Mesozoic subduction zone of California at early or pre-Franciscan metamorphism.

Review of Late Jurassic-early Miocene sedimentation and plate-tectonic evolution of northern California: illuminating example of an accretionary margin

“…Most high-grade tectonic blocks were transformed at *10-12 kbar, and *400-600°C (Fig. 5), but some evidently formed at even higher P-T ranges (e.g., Krogh et al 1994;Tsujimori et al 2006).…”

Petrology and P–T path of high-pressure granulite from the Dulan area, North Qaidam Mountains, northwestern China

“…For example, the prograde metamorphic history is mainly recorded in the growth of zoned garnets preserving inclusions (e.g. Krogh, 1980Krogh, , 1982Krogh et al, 1994;Liati and Seidel, 1994;Möller, 1998). In contrast, many characteristic decompressional textures, such as corona, kelyphite, symplectite, reaction rims and compositional zonings are vital for defining the retrograde P-T path (Elvevold and Gilotti, 2000).…”

Low-grade metamorphic rocks from the Pulur complex, NE Turkey: implications for the pre-Liassic evolution of the Eastern Pontides

“…Approximate boundaries between metamorphic facies are shown as dotted lines: GS greenschist facies; LBS lawsonite-blueschist facies; EBS epidoteblueschist facies; EA epidote-amphibolite facies; AM amphibolite facies (Krogh et al 1994 and references therein); Si isopleths for phengites (dashed lines) are from Parra et al (2002). b P-T estimates deduced for metabasites in the albite-epidote amphibolite-facies domain.…”

“…11). The interbedded garnet-and biotitebearing phyllite (sample 228A) contains muscovite/phengite with an average Si content of 3.25 cpfu, corresponding to a minimum pressure (no K-feldspar) of c. 0.4 GPa at 400 C. A temperature-dependent upper pressure limit is given by the general absence of sodic or calcic-sodic amphibole which defines the transition to the epidoteblueschist facies according to Krogh et al (1994). Summarizing, we can state that the rocks within the albite-epidote amphibolite-facies domain were equilibrated at pressures between of 0.6 and 1.1 GPa and temperatures between 400 and 470 C (stage II; Fig.…”