Our understanding of low-grade, mafic, metamorphic rocks is relatively primitive compared to higher grade equivalents, in large part because of the abundance of relict minerals, the difficulties of studying fine-grained rocks, and the difficulties of establishing the existence and scale of chemical equilibrium. However, work carried out during the past decade has identified the systematic correspondence between effective bulk compositions and mineral assemblage that is required by the metamorphic facies concept, and a large body of data defines the mineral pangeneses of coexisting pumpellyite (Pmp), prehnite (Prh), amphibole (Ab), and epidote (Ep). At pressures and temperatures typical of many regional metamorphic terrains, rocks containing Ep + Ab + quartz (Qtz) may have, in addition, chlorite (Chl) + actinolite (Act) in magnesian bulk compositions, Chl + Pmp in more ferroan compositions, and Prh in compositions relatively lower in aluminum and ferric iron. Act + Pmp ± Chl ± Prh may coexist in a narrow range of compositions with intermediate values of Mg/(Mg + Fe 2+ ). Improvements in thermodynamics data bases now permit calculation of pressures and temperatures for low variance assemblages. The results of systematic examination of mineral assemblages and calculations of petrogenetic grids for epidote-bearing rocks suggest that the stability field of the Prh-Pmp facies is entirely contained within the overlapping stability fields of the Pmp-Act and Prh-Act facies. At pressures typical of the most common metamorphic field gradients, the stability fields of the three facies cannot be distinguished. The predictability of mineral assemblages and the recent success in calculating meaningful pressure and temperature for low-grade rocks suggest that, under favorable circumstances, mineralogical approaches rooted in the assumptions of equilibrium thermodynamics are likely to prove as useful for determining the physical conditions of low-grade metamorphism as they have been for high-grade rocks.

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.

Epidosites (epidote- and quartz-rich rock with granoblastic texture) from the Solea graben are believed to form from upflowing hydrothermal fluid that deposited massive sulfide ores at the seawater-sea-floor interface of the Cretaceous age oceanic crust. Epidosite within the graben occurs as massive epidosite (epidosite concentration greater than 80%) and zones of incipient epidotization (between 5% and 80%). Regions of incipient epidotizations are characterized by the juxtaposition of greenschist facies alteration and epidosite; the fluids responsible for each of these alteration styles had similar oxygen isotope compositions and temperatures. Petrographic observations suggest that the epidosite protolith was a previously altered rock. In addition, evidence of secondary porosity is found in the areas of incipient epidotization, suggesting that the reactions creating epidosite lead to the creation of porosity. Computer simulated fluid-rock interaction at 350 °C and 500 bar between a Ca-rich, seawater-derived hydrothermal fluid and a greenschist facies mineral assemblage (36% albite, 36% quartz, 18% chlorite, 6% magnetite, 3% clinopyroxene, and 1% epidote) produced an epidosite assemblage (epidote + quartz + chlorite + hematite/magnetite) at fluid/rock mass ratios of 1250:1 to 50:1. The results suggest that reaction of this hydrothermal solution with albite and chlorite, as opposed to fresh diabase, controls epidosite formation. The calculations also indicate initial reduction in rock volume, which correlates with the observed porosity in incipiently epidotized rocks of the Solea graben. We suggest that hydrothermal circulation related to the localized intrusion of high-level gabbro in highly attenuated crust resulted in the transformation of green-schist facies rocks to epidosite. This is consistent with evidence from the Mid-Atlantic Ridge at the Kane Fracture Zone for reaction between upflowing hydrothermal solutions and previously altered basalt.

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.

The effects of primary porosity on fluid flow during contact metamorphism were studied in basalts from central East Greenland. The gabbroic Skaergaard magma intruded interbedded massive and aa basalts with mean macroscopic primary porosities of 4% and 11%, respectively. Heat transport from the cooling gabbros led to three metamorphic mineral zones within 1 km of the contact: the actinolite + chlorite zone beyond 250 m, where the mineral assemblage records peak temperatures (7) of ≤550 °C; the pyroxene zone ( T = 700–850 °C); and the olivine zone, within 10 m ( T > 850 °C). In the actinolite + chlorite zone, aa clasts record more extensive mineralogic alteration of igneous minerals than do massive samples. Extents of prograde recrystallization in the olivine and pyroxene zones are 100% in both flow morphologies, but modal volumes of retrograde minerals in the pyroxene and olivine zones are higher in aa units. Extents of prograde reactions do not correlate with primary porosity because they were solid-solid reactions that occurred at high temperatures, whereas retrograde alteration involved low-temperature hydration reactions in which the availability of H 2 O as a reactant, as controlled by porosity, probably influenced reaction extent. In the pyroxene zone, where mineralogic and textural evidence suggests oxygen isotope exchange equilibrium, whole-rock δ 18 O compositions are 1.7‰ to 3.0‰ and are similar or lower in aa units than in massive units at any given distance from the contact. The isotopic ratios suggest average time-integrated fluid fluxes of 3.6 and 4.0 × 10 3 mol cm −2 in massive and aa units, respectively, if fluid infiltration occurred during prograde metamorphism. Similar values were computed assuming that part of the isotopic exchange was retrograde. These differences imply that time-averaged matrix permeability was ~10% higher in aa flow breccias.

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.

The interaction between rocks and water during low-grade metamorphism leads to the growth of a predominantly hydrated secondary mineral assemblage and results in the elevation of whole-rock oxygen isotope values. Rb-Sr whole-rock isotope systems are also disturbed during low-grade metamorphism and give reset metamorphic ages. The process of isotope disturbance during low-grade metamorphism is examined by comparing the behavior of the oxygen isotope systems in rocks of similar composition that have undergone differing levels of metamorphic recrystallization. Two intrusions from the Ordovician volcanic province of North Wales represent metamorphically undisturbed systems. They give low oxygen isotope whole-rock values of δ 18 O SMOW (standard mean ocean water) = 7.3‰ ± 0.3‰, their Rb-Sr whole-rock systems are undisturbed and they show minimal metamorphic recrystallization. By comparison, other Ordovician igneous rocks have elevated oxygen isotope values enriched between 1‰ and 7‰ over undisturbed rocks and give metamorphic Rb-Sr whole-rock regression ages. Interaction with water is shown to be the main influence on the resetting of Rb-Sr systems during low-grade metamorphism. The metamorphic pressure-temperature conditions are not a controlling factor.

Pumpellyite associated with actinolite, epidote, prehnite, chlorite, albite, white mica, titanite, smectite, and calcite is found in tholeiitic dolerites of Late Triassic (Liassic?) age, where it fills veinlets, replaces primary plagioclase and orthopyroxene, and appears in the groundmass. The dolerites, metamorphosed during the Eo-Alpine event, crop out as tectonic blocks several hundred meters in size, covering an approximate area of 0.1 km 2 next to the town of Archidona, Málaga province, southern Spain. A 4.0-mm-thick vein filled with compact bundles of needlelike pumpellyite was selected for analysis. Very minor amounts of calcite and smectite appear as a thin wall coating in this vein. Separates of pure pumpellyite were analyzed by X-ray diffraction, transmission electron microscopy–analytical electron microscopy, inductively coupled plasma, and electron probe microanalysis (XRD, TEM-AEM, ICP, and EPMA). The chemical analyses show a close agreement between the ICP analysis and the mean value of 60 probe measurements. The average of six semiquantitative AEM analyses plot close to the points above. The X Fe 3+ values (= 100Fe 3+ /Fe 3+ + Al tot ) are 20.7% for the EPMA mean (all Fe as Fe 3+ ) and 18% for the ICP analysis (with Fe 3+ and Fe 2+ analyzed separately). Cation distribution in the Z, Y, X, and W positions agree closely with the ideal stoichiometry (6, 4, 2, and 4, respectively), whereas the cation totals are 15.91 for the EPMA mean and 15.99 for the ICP analysis. The Archidona pumpellyite is strongly impoverished in rare earth elements (REE) as a whole and shows a smooth U-shaped REE pattern. A maximum enrichment of about four to five times chondrites is shown by the heaviest rare earth elements Yb and Lu, respectively. The unit-cell parameters of the pumpellyite were determined as follows: a = 8.814 ± 0.002, b = 5.925 ± 0.001, c = 19.125 ± 0.003 A, V = 990.307 ± 0.228 A 3 , and β = 97° ± 0.8′. Pumpellyite needles in the vein were also examined by selected-area electron diffraction (SAED) and TEM imaging. From [010] zone axis electron diffraction patterns with no streaks along c * and images showing only some twin faults parallel to (001), it was concluded that pumpellyite was not significantly intergrown with epidote, lawsonite or sursassite. This absence of microdefects indicates a remarkable structural homogeneity of this pumpellyite sample.

The Central and Feather River peridotite belts of the northern Sierra Nevada metamorphic belt constitute a major suture zone between Paleozoic–early Mesozoic continental-margin rocks (Shoo Fly Complex and superjacent strata) of the Eastern belt and Jurassic arc and ophiolitic rocks (Smartville Complex) of the Western belt. This suture zone is structurally complex and has previously been described as mélange. Our data suggest that six major fault-bounded rock assemblages are present across this zone. The faults are isoclinally folded and transposed along steep hinge planes, but have shallowly dipping enveloping surfaces. Rocks of the Eastern belt occupy the highest of five east-dipping thrust sheets which are technically overlain by a sixth, west-dipping thrust sheet. The Western belt rocks are built into a basement composed of the last thrust sheet and postdate the thrust faults. All these rocks and structures are cut by faults of the Late Jurassic Foothills fault system which bound the lithotectonic “belts” (Eastern belt, etc.). The Feather River peridotite belt consists of the Feather River peridotite and the Red Ant Schist, and is extended to include the newly recognized Devils Gate ophiolite. The Feather River peridotite is correlative with or intruded by the Devils Gate ophiolite, and together they constitute a nearly complete cogenetic or polygenetic Paleozoic ophiolite. The Red Ant Schist contains metasedimentary and metavolcanic rocks with local blueschist parageneses; blueschist facies metamorphism is early Mesozoic or older. Several small outliers of Shoo Fly (Eastern belt) sandstone are present in the Feather River peridotite belt. The Central belt consists of the Calaveras Complex, the Fiddle Creek Complex, and the Slate Creek Complex. The late Paleozoic–early Mesozoic Calaveras Complex is an assemblage of phyllite-diamictite, chert, and minor volcanic rocks and marble in the eastern part of the Central belt. The Fiddle Creek Complex lies west of the Calaveras Complex and contains an intact stratigraphic succession, which includes, in ascending stratigraphic order, late Paleozoic ophiolitic mélange, pillow basalt with minor felsic tuff, Middle Triassic–Early Jurassic volcaniclastic and hemipelagic sedimentary rocks, and Middle–Late Jurassic(?) quartzose clastic rocks. The Slate Creek Complex is an Early Jurassic pseudostratigraphic sequence that contains a basal serpentinite-matrix mélange overlain by plutonic and volcanic rocks. The Western belt consists of the Middle–Late Jurassic (160 Ma) volcanic and intrusive rocks of the Smartville Complex and, at one locality, older tonalitic basement equivalent to the Slate Creek Complex. The Slate Creek Complex was juxtaposed against the other rock assemblages before formation of the Smartville Complex, so the Smartville Complex formed in situ. Crosscutting relations define three generations of macroscopic structures. The earliest structures include major mapped and cryptic, west-vergent (east-dipping) thrust faults that juxtapose, in descending structural order, the Shoo Fly Complex (Eastern belt), Feather River–Devils Gate Ophiolite, Red Ant Schist, Calaveras Complex, and Fiddle Creek Complex. These faults predate the east-vergent “Slate Creek thrust,” which carries the Slate Creek Complex over the Fiddle Creek Complex, Calaveras Complex, and Red Ant Schist. The Slate Creek thrust is cut by a pluton dated at about 165 Ma, which places an upper age constraint on assembly of the nappe pile. This nappe pile is cut and overprinted by the steep throughgoing faults (Foothills fault system), folds, and cleavages that dominate the structure of the area; this youngest set of structures makes up the Late Jurassic Nevadan orogeny. Eastward overthrusting of the Slate Complex along the Slate Creek thrust occurred after amalgamation of the other units. Eastward overthrusting was followed by rifting and arc magmatism represented by the Smartville Complex, then by Nevadan faulting, folding, and penetrative deformation. These data preclude the existence of a Late Jurassic suture or collision, but the Fiddle Creek and Slate Creek Complexes, both interpreted as remnants of Early Jurassic arcs, could have collided in the late Early Jurassic or early Middle Jurassic.