The King Ridge Road mélange is a unit of the Franciscan Complex, cropping out in an area of at least 50 km 2 around the town of Cazadero, coastal California. This unit is an olistostrome with a massive, unfoliated sandstone matrix, containing >232 large meta-igneous and chert blocks of greatly varying size, lithology, and metamorphic history within the study area. This sandstone matrix is litharenite or arkosic arenite and exhibits prograde prehnite-pumpellyite facies and retrograde zeolite facies metamorphism. It is devoid of megascopic textures except for rare simple bedding. No fossils have been found, and no Bouma units or other graded beds are present. Detrital zircon geochronology has established the maximum age of deposition of the sandstone matrix at 83 Ma, whereas apatite fission-track data indicate cooling of the olistostrome below 100 °C at ca. 35–38 Ma. The 232 exotic blocks sampled in the study area are dominantly low- to medium-grade greenstones and cherts, together with fewer high-grade blocks partly composed of blue amphibole and/or omphacitic pyroxene, and some amphibolites. Thus, many of the blocks have higher grade metamorphic assemblages than the matrix. All block types are well mixed together, so none greatly predominate anywhere. Blocks of oceanic-island-arc plutonic rocks, including granitoids and recemented breccias, are particularly distinctive for this mélange. One granitoid block has a zircon U-Pb age of 165 ± 1 Ma. The massive sandy matrix of the olistostrome formed by accumulation of hyperconcentrated sedimentary density flows (grain flows) sourced primarily from the Klamath-Sierra continental magmatic arc. Many of the blocks record a pre-mélange history of metamorphism and exhumation, followed by partial subduction and reburial with the matrix after 83 Ma. Cooling below 100 °C took place at 35–38 Ma, probably associated with partial exhumation of the unit, with subsequent removal of ~10 km of cover.

The Eastern belt of the Sierra Nevada comprises an Ordovician(?) to Devonian(?) succession of psammites and pelites belonging to the Shoo Fly Complex, and is overlain by three Paleozoic to Mesozoic arc volcanic sequences. The northern part of the belt, the subject of this chapter, is divided into a series of discrete blocks by steeply dipping faults, considered to be eastward-directed thrusts. The metamorphic history of this region has been little investigated previously. It has been argued that low-grade metamorphism of the Eastern belt is a Nevadan orogenic effect; in contrast, it has also been suggested that metamorphism of the arc volcanic rocks was a result of burial effects in the arc environment. In this study the metamorphic grade of the area has been established using mineral assemblages in metabasites and pelites, combined with illite crystallinity and b 0 data from pelitic rocks. The Shoo Fly Complex underwent epizonal metamorphism under Barrovian-type conditions prior to the earliest arc volcanism. Metamorphic grade in the overlying arc volcanic rocks ranges from pumpellyite-actinolite facies in the strongly foliated rocks of the (westernmost) Butt Valley and Hough blocks, through prehnite-pumpellyite facies in the Keddie Ridge and Genesee blocks, to low anchizone to diagenetic grade in Jurassic rocks of the (easternmost) Mt. Jura and Kettle Rock blocks. There is evidence for at least three discrete regional metamorphic events in these arc rocks; one is interpreted as being related to the burial of the arc volcanic rocks, which reached prehnite-pumpellyite facies; this event was followed by deformation and pumpellyite-actinolite facies metamorphism during the Nevadan orogeny; a final episode of static, low-grade metamorphism, possibly due to tectonic loading effects, probably also resulted in pumpellyite-actinolite facies. Subsequently, rocks exposed in the extreme east of the region were affected by contact metamorphism during the emplacement of Sierra Nevada batholith granitoids.

The western metamorphic belt is part of the Coast plutonic-metamorphic complex of western Canada and southeastern Alaska that developed during collision of the Alexander terrane and Gravina assemblage on the west against the Yukon Prong and Stikine terranes to the east. Deformation, metamorphism, and plutonism range from about 120 to 50 Ma. Subgreenschist to lower greenschist facies metabasalts exposed along the west end of the western metamorphic belt near Juneau, Alaska, record the earliest metamorphic event (M 1 ). The protolith of the M 1 , low-grade metamorphic mineral assemblages is mostly arc-affinity basaltic rocks of the Douglas Island Volcanics. The most common metamorphic mineral assemblages are chlorite-epidote-actinolite with or without pumpellyite and stilpnomelane. There is no systematic distribution of metamorphic mineral assemblages in the study area, and all assemblages are in the pumpellyite-actinolite facies near the transition to the lower greenschist facies. Different low variance assemblages can be attributed to minor differences in pressure ( P ), temperature ( T ), or X CO 2 . Mineral chemistry and phase equilibria suggest that thermal peak metamorphism of pumpellyite-bearing assemblages occurred at about 325 °C and 2 to 4.8 kbar. The geologic setting, the pumpellyite-actinolite to lower greenschist facies mineral assemblages, and the deduced P and T of peak metamorphism are all compatible with metamorphism of the Douglas Island Volcanics at a depth of 7 to 20 km. The low-grade rocks are contiguous with younger (M 5 ), higher grade assemblages that define an inverted metamorphic gradient. The discontinuity in pressure indicated by the M 1 mineral assemblages and M 5 geobarometry (9–11 kbar) suggests juxtaposition of the two metamorphic sequences by vertical uplift along the Coast Range megalineament.

Metamorphic mineral assemblages in metamorphosed mafic volcanic rocks located on and straddling the prehnite (Prh)-pumpellyite (Pmp) to greenschist facies transition in the Flin Flon, Manitoba, area were studied using singular value decomposition. Mass balances obtained for a sample from the transition zone are identical to previously deduced equilibria in the system Na 2 O-CaO-MgO-Al 2 O 3 -SiO 2 -H 2 O and we cannot reject the hypothesis that they equilibrated under conditions of invariant (in this model system) equilibrium. Mass balances obtained between two samples straddling the isograd are in accord with petrological observations across the transition and suggest that it can be modeled by three coincident isograds approximated by: Prh - out , Act - in : Prh + Chl + Ab = Act + Ep + H 2 O , and Pmp - out , Act - in : Prh + Chl + Ab = Act + Ep + H 2 O , where Act is actinolite; Chl is chlorite, Ab is albite, and Ep is epidote. Thermochemical data from various sources cited herein were used to estimate metamorphic conditions at the transition at 2.8–3.4 kbar, 280–290 °C.

Pumpellyite and prehnite are associated closely with epidosite in two well-exposed sections of the Josephine ophiolite and are interpreted to have formed during hydrothermal metamorphism beneath a spreading axis. In the upper 75 m of the extrusive sequence, epidosite grades upward into “pumpellyosite” (granoblastic pumpellyite + quartz + chlorite ± epidote rock) and, in interpillow hyaloclastite, into “prehnitite” (granoblastic prehnite + quartz + epidote ± chlorite rock). Probable hydrothermal pumpellyite also occurs in the lower hematitic pillow lavas as amygdules that contain pumpellyite + chlorite ± epidote ± chalcopyrite. The second occurrence of pumpellyosite and prehnitite is in the basal sheeted dike complex, where these minerals formed during the late stages of retrograde hydrothermal metamorphism. The bulk-rock composition of pumpellyosite and prehnitite shows extensive Ca metasomatism very similar to that of epidosite. Like epidosites, these rocks are inferred to have formed by interaction with large volumes of upwelling, highly reacted hydrothermal fluids, similar to those at modern high-temperature hot springs on mid-ocean ridges. The change in the upper 75 m of the pillow lavas from epidosite to pumpellyosite and prehnitite may reflect cooling of upwelling fluids to less than ~315 °C. The presence of interpillow prehnitite immediately below sediments overlying the ophiolite implies that these fluids leaked directly onto the sea floor. The Josephine pumpellyosites and prehnitites formed at temperatures between 200 and 315 °C, within the overlapping stability fields of epidote, prehnite, and pumpellyite. The influence of fluid composition on mineral equilibria is evaluated at 250 °C and 500 bar using an a Ca 2 + / a H + 2 versus a Fe 3 + / a H + 3 diagram modified from Rose and Bird (1987). The topology of the pumpellyite-epidote and pumpellyite-prehnite phase boundaries was derived using compositions of coexisting Ca-Al silicates in the Josephine samples. The pumpellyite and prehnite stability fields are generally at lower a Fe 3 + / a H + 3 than epidote, whereas the pumpellyite stability field is generally at lower a Ca 2 + / a H + 3 than prehnite.

Metavolcanic rocks of the Winterville Formation from the prehnite-analcime subfacies of the prehnite-pumpellyite facies in north-central Aroostook County, Maine, contain an alteration assemblage including chlorite, chlorite/smectite (C/S), analcime, prehnite, and calcite. Field and laboratory study has identified areas where hydrothermal alteration has been pervasive in and around pillows. Compositional, crystal chemical, and structural variations in chlorite appear to be related to distance from this hydrothermal alteration. Samples were studied by whole-rock chemical analysis, electron microprobe analysis of individual mineral grains, X-ray powder diffraction of the clay fraction, and by computer modeling of diffraction patterns to determine the percentage of chlorite in interstratified C/S and to estimate the distribution of Fe and the size of coherent diffracting domains in pure chlorites. Whole-rock and pyroxene compositions suggest that the rocks have undergone Mg metasomatism. Modeling of X-ray diffraction data indicates that the percentage of chlorite in C/S increases to 100%, that Fe atoms become more equally distributed between octahedral sites in chlorite as it becomes more Fe-rich, and that diffracting domains grow larger with proximity to areas of more intense hydrothermal alteration. Analcime also increases near areas of hydrothermal alteration. The areal distribution of hydrothermal effects suggests that the alteration occurred as two separate events, or that two different thermal regimes were active concurrently.