The blueschist-greenschist facies transition for a model basaltic system Na 2 O-CaO-MgO-Al2O 3 -SiO2-H 2 O is defined by a univariant reaction: 6 clinozoisite + 25 glaucophane + 7 quartz + 14 H 2 O = 6 tremolite + 9 chlorite + 50 albite; for the Fe 2 O 3 -saturated basaltic system, by a discontinuous one: 4 epidote + 5 Mg-riebeckite + chlorite + 7 quartz = 7 hematite (magnetite) + 4 tremolite + 10 albite + 7 H 2 O. These two reactions were experimentally investigated to determine the nature of the blueschist-greenschist transition. The results have located the first reaction at 350 ± 10°C, 7.8 ± 0.2 Kb and 450 ± 10°C, 8.2 ± 0.4 Kb. Reconnaissance experiments for the second reaction indicate that the minimum pressure for the occurrence of epidote + Mg-riebeckite + chlorite + quartz is about 4 Kb at 300°C for f O2 defined by the hematite-magnetite buffer. The presently determined P-T location for the blueschist-greenschist transition in the Fe-free basaltic system is about 3 Kb lower than the minimum pressure limit of glaucophane of Carman and Gilbert (1983), but is compatible with the revised stability field of jadeite + quartz determined by Holland (1980). Introduction of Fe 3+ into the model basaltic system significantly lowers the minimum pressure limit for occurrence of the buffered assemblage sodic amphibole + epidote + actinolite + chlorite + albite + quartz, and the participating phases gradually increase their Fe 3+ /Al ratio with decreasing pressure. Isopleths of sodic amphibole composition in the buffered assemblage in terms of X G1 are delineated and the effect of Fe 2+ and temperature on the isopleths are discussed. The Al 2 O 3 content of sodic amphibole coexisting with epidote + actinolite + chlorite + albite + quartz decreases systematically with decreasing pressure and hence can be used as a geobarometer. Pressure estimates for metabasites at Ward Creek of the Franciscan terrane, the Mikabu greenstones of the Sanbagawa belt, the Otago schists of Lake Wakitipu, New Zealand, and the blueschists at Ouegoa, New Caledonia, based on the proposed glaucophane geobarometry, are in agreement with those derived from sodic pyroxene geobarometry.

The Ellis and Green (1979) experimental calibration of the garnet-clinopyroxene K D (Fe, Mg) geothermometer generally agrees with information from experimental work in simple systems on major and minor phases of Class B and C eclogites and with simple thermodynamic extension of the phase equilibrium work. Correction for Fe 3+ must be made in microprobe analyses of natural assemblages in applying the garnet-clinopyroxene K D scale. Geobarometry of eclogites is more difficult. The absence of plagioclase sets quantitative lower-pressure limits on quartz eclogites. Additional information on equilibration pressures is given by experimental work on the stabilities of the minor eclogite phases lawsonite, zoisite, talc, paragonite, albite and tremolite. Many well-described natural occurrences have garnet and clinopyroxene compositions that demand minimum pressures of 12-13 kbar, based on thermodynamic analysis. Earlier estimates of 6-9 kbar based on long extrapolations of experimental work in dry mafic compositions at high temperatures are not valid. Eclogite could have formed at very high H 2 O pressures; P H2O much lower than P total may have occurred but was not generally necessary. Crustal eclogites can be roughly grouped in three categories: a low-temperature group (450°-500°C) with pseudomorphs after lawsonite, a high-temperature group (600°-750°C) with talc and kyanite or from terranes with this assemblage in co-metamorphic pelites, and an intermediate-temperature group (500°–600°C) which is neither lawsonite-related nor talc-kyanite-related. Pressures deduced for all of these groups vary from 12 to 18 kbar, and pressures may have been 20 kbar or more during the formation of coarse garnet and omphacite in some occurrences. Such very high pressures must have been the result of subduction at continental margins. An increasing T and P prograde subduction path is suggested but not proved by differences in composition between rims and cores of garnets and pyroxenes from Tasmania and southern Norway, and possibly, in the spread of eclogite mineral compositions from the Raspas Formation of Ecuador. Secondary greenschist assemblages in many eclogites may indicate uplift at relatively high-temperature conditions. Alternatively, isobaric cooling may have been followed by a discrete period of metamorphism at shallower depth and elevated temperatures prior to uplift to the surface. The lowest T/P-equilibration conditions were found for the younger (Late Cretaceous-Tertiary) occurrences, which include all of the lawsonite-related examples. Apparent average subduction gradients for these were about 11°C/km of burial or lower. The highest apparent geotherms, of about 16°/km, were found for the oldest (Late Precambrian-Early Cambrian) group, which includes the whiteschist-related samples. This spectrum of apparent subduction geotherms, although based on relatively few examples and subject to large errors in calculation, may indicate that conditions favorable for the formation of lower-temperature crustal eclogites were more widespread in later geologic history.

The new Mg-Al-Ti-silicate ellenbergerite occurs among abundant inclusions within decimetre-size garnets in the pyrope-quartzite layer of the Dora Maira massif, Western Alps, from which coesite relics have been reported. It is associated with pyrope (92 to 98 mole percent end-member), kyanite, talc, chlorite, rutile, zircon, and minor sodic amphibole, which all formed an apparently stable assemblage now exclusively preserved within large garnets. A petrologic analysis shows that the new mineral, which has the formula (Mg 1/3 , Ti 1/3 , □ 1/3 ) 2 Mg 6 Al 6 Si 8 O 28 (OH) 10 with extensive Ti ⇌ Zr substitution, is a high-pressure phase with a lower pressure stability limit above 20 kbar and an upper temperature limit near 800°C or less. Its stability field extends from the Mg-carpholite field to the pyrope field and broadly overlaps that of Mg-chloritoid. The high-pressure phase relationships in the magnesian pelitic system are thus considerably modified, in particular through the reactions chlorite + talc + kyanite + rutile + H 2 O = ellenbergerite and chlorite + kyanite + rutile + H 2 O = ellenbergerite + Mg-chloritoid, which provide new upper pressure limits for two important chlorite-bearing assemblages. The different assemblages preserved within the garnets and in the matrix record variations of pressure, temperature, and water activity along a prograde metamorphic path passing near 25 kbar, 700°C, and reaching the coesite field. This finding confirms the unusual depth reached by the enclosing continental unit along a low-temperature metamorphic gradient.

A serpentinite-matrix melange complex with blocks of greenschist, blueschist, metagabbro, orthogneiss, amphibolite, and eclogite is present within a small structural window beneath Upper Triassic ophiolite about 2 km north of Puerto Nuevo in the central Vizcaino Peninsula. The melange complex consists of three units: lower (more than 200 m thick) and upper (less than 100 m thick) serpentinite breccia units and the intervening exotic block unit (less than 100 m thick). The tectonic contact of the complex with the serpentinized (mainly chrysotile-bearing) basal harzburgite unit of the ophiolite is gradational and subparallel to the ophiolite stratigraphy; rocks below the melange complex are not exposed. The serpentinite breccia units are composed of pebble- to boulder-sized fragments of serpentinized harzburgite, dunite, pyroxenite, and minor rodingitized diabase in a friable foliated matrix of sheared chrysotile. The clasts were probably all derived from disruption of the lower part of the overlying ophiolite. The exotic block unit contains abundant metamorphosed blocks, 50 cm to 0.5 km in diameter, in an indurated and scaly antigorite matrix. The exotic blocks include metabasalt, metagabbro, metachert, meta-tuff, metaperidotite, and chromitite of ophiolitic character, and metagraywacke, metato-nalite, and metarhyolite blocks of possible arc affinity. Metabasite blocks of the former group have major and trace element characteristics of ocean-floor basalts and exhibit pumpellyite-actinolite (ab+ep+pm+act±chl±wt mica), greenschist (act+chl+ep+ab+wt mica), blueschist (crossite+ab+ep+wm±cc±lw), and epidote-amphibolite (barroisitic amph+ep+ab+wt mica±grnt) and eclogite (grnt+cpx) facies assemblages. Blueschists contain abundant epidote and are interlayered with greenschists. This moderate temperature/high pressure series is analogous to that displayed by the Sanbagawa belt in Japan. Blocks of arc affinity exhibit greenschist assemblages. Some blocks experienced calcium metasomatism (rodingization) prior to incorporation into the melange, whereas others display tremolitic amphibole+chlorite+fuchsite-bearing rinds which resulted from magnesium metasomatism (blackwall) after incorporation into the melange. The melange complex probably represents a thrust fault within the basal ultramafic part of the Sierra de San Andres ophiolite. The exotic metamorphic rocks are inferred to have been dragged up from one or more underlying metamorphosed ocean-floor and volcanic-arc terranes. Metamorphism, composition, lithology, tectonic position, and preliminary age data argue that the Puerto Nuevo melange should not be correlated with the melange complexes on Cedros and the San Benitos Islands nor with the melanges of the Franciscan Complex of Alta California. The Puerto Nuevo melange is instead interpreted as an older, more inboard, blueschist-bearing unit analogous to the blueschist units of the Klamath and Sierra Nevada Mountains. The multiplicity of subduction-zone complexes, ophiolites, and volcanic-arc sequences suggests that the Vizcaino Peninsula region is composed of a collage of tectonostratigraphic terranes that may be allochthonous relative to each other and the North American craton.

In the Catalina Schist terrane, a nappe of blueschist facies melange is overlain by one of isoclinally-folded greenschist facies rocks. The blueschist unit contains meta-graywacke, metashale, metaconglomerate, greenstone, quartz schist, blueschist, and eclogite blocks in a fine-grained, schistose matrix which probably recrystallized from a mixture of ultramafic and clay-rich, quartzose detritus. The greenschist unit consists of metabasites, graywacke-composition grayschist, and quartz schist. Some metabasites are glaucophanic greenschists which initially re-crystallized in the blueschist facies; others originally were epidote amphibolites. Both are overprinted by greenschist facies minerals. Most metabasites from both tectonic units have major, minor, and trace element characteristics of ocean-floor tholeiites. The white micas of the blueschist unit probably recrystallized under substantially higher P fluid /T conditions than those of the greenschist unit. Amphiboles from metabasites of the greenschist unit are crossites, barroisites, and sodic actinolites in glaucophanic greenschists, and hornblendes rimmed by actinolites in epidote amphibolites. Crossites from the glaucophanic greenschists probably formed at somewhat higher T/P conditions than glaucophanes and crossites from the lawsonite-bearing metabasites of the blueschist unit. The mineral assemblages and metamorphic histories of the two units evidently represent two distinct, relatively high P/T trajectories.

High-pressure/low-temperature metamorphic rocks in the Franciscan Complex of western California are primarily found as small blocks in mud-matrix melange and as extensive coherent, bedded sheets or slabs. The highest-pressure rocks, the high-grade sodic and/or calcic amphibole-epidote-garnet-omphacitic pyroxene-bearing blueschists, amphibolites, or eclogites, are found as blocks (typically meters to a few tens of meters across) in the melanges. Low-grade schistose blueschists are found as small blocks in melanges and in extensive coherent belts. Many Franciscan greenstones and metagray-wackes in both the melanges and coherent tracts locally contain sodic pyroxene + quartz, lawsonite, and/or aragonite but are neither blue nor strongly schistose. Petrotectonic constraints on models for uplift and preservation of Franciscan high-pressure/low-temperature metamorphic rocks include: (1) high-grade blueschist and eclogite blocks found in mud-matrix melange terranes underwent cooling under high-pressure conditions and were once immersed in serpentinite; whereas lower-temperature/high-pressure rocks found both as blocks in melanges and extensive coherent tracts are typically only incipiently recrystallized and show no evidence of former immersion in serpentinite; (2) extensive tracts of schistose blueschists are largely in fault contact with the base of the overriding plate in northern California, whereas both the high-grade blocks and the jadeitic pyroxene + quartz-bearing coherent units in northern California are not juxtaposed against the overriding plate; (3) uplift of both blueschist blocks in melange and extensive coherent tracts typically occurred without thoroughly penetrative strain; (4) high-grade blocks in mud-matrix melanges must have been displaced oceanward and upwards from beneath the base of the overriding plate; (5) synsubduction uplift of Franciscan blueschists to depths less than 10 kilometers or so is indicated by the lack of retrograde greenschist facies alterations, the widespread preservation of aragonite, and the present exposure of blueschists where subduction continues off northernmost California and southern Oregon; and (6) the scarcity of high-pressure metamorphic detritus in Franciscan sediments indicates that large tracts of blueschists were not exposed in California by synsubduction erosion.

The U-Pb isochron method is a promising new approach to the geochronology of high-pressure-low-temperature metabasites. In samples with favorable U/Pb ratios, metamorphic minerals such as sphene, apatite, lawsonite, glaucophane, garnet, and hornblende partition U and Pb in such a way as to provide a range of U/Pb ratios suitable for isochron dating. In a manner analogous to Rb-Sr isochron dating, these U-Pb isochrons provide not only ages, but also information on the initial isotopic composition of Pb at the time of metamorphism, a significant petrogenetic tracer. Sphene is the key mineral for dating. Its relatively high-U/Pb ratio results in the evolution of moderately radiogenic Pb, and it is highly resistant to resetting. Some metabasites have U/Pb ratios that are extremely low, perhaps owing to severe U depletion at some stage of metamorphism. These samples are not useful for dating, but still provide valuable data on the initial isotopic composition of Pb. Analysis of a Type III metabasalt (blueschist) from the Taliaferro complex near Leech Lake Mountain yields a U-Pb isochron age of 162 ± 3 Ma., slightly older than the widely quoted 150–155 Ma. K-Ar ages for high-grade Franciscan tectonic blocks. Two garnet amphibolite blocks from the Catalina Schist terrane yield identical ages, with an isochron for both samples giving an age of 112.5 ± 1.1 Ma. These samples, plus three more Franciscan metabasites, have isotopic compositions of initial Pb ( 206 Pb/ 204 Pb = 18.40–18.85; 207 Pb/ 204 Pb = 15.55–15.66) that plot distinctly above the field for modern MORB; instead they plot in the fields for some island arcs and granodiorites from the Sierra Nevada.

Rocks of the Eastern Franciscan belt, northern California, are divided into two tectonostratigraphic terranes metamorphosed to the blueschist facies, both with a distinct lithologic association and deformational history. The easternmost terrane, the Pickett Peak terrane of Early Cretaceous isotopic age, consists of crenulated mica schist and gneissic to schistose metagraywacke, with lesser alkalic mafic metaigneous rocks and scarce metachert. The Pickett Peak terrane retains evidence of three periods of penetrative deformation, the first of which is characterized by segregation layering, and the second and third by crenulation cleavages. Blueschist-facies conditions persisted during the first two deformations. The Yolla Body terrane of Late Jurassic and Early Cretaceous paleontologic age lies structurally below and to the west of the Pickett Peak terrane. It is characterized by voluminous metagraywacke and lesser argillite, coherent interbedded radiolarian chert, and alkalic gabbroic dikes and sills. The Yolla Bolly terrane retains evidence for two phases of penetrative deformation that were coaxial with the second and third phases of deformation in the Pickett Peak terrane. The first phase of deformation (parallel to the second phase in the Pickett Peak terrane) was also accompanied by blueschist-facies metamorphism.

The Condrey Mountain Schist occupies a window through Late Triassic(?) amphibolite facies mélange of the Western Paleozoic and Triassic Belt in the north central Klamath Mountains. Along the western margin of the window, the schist comprises a sequence of multiply deformed, greenschist facies metavolcanic and fine-grained meta-sedimentary rocks, which are in thrust or high-angle fault contact with graphite-quartz-mica schist exposed in the window interior. Transitional blueschist-greenschist facies parageneses are developed in metabasites, metacherts, and metalliferous metasedimentary rocks in the graphitic schist in the central part of the window. All units record progressive, polyphase, deformational and metamorphic histories. The earliest stages of deformation in the blueschists generated isoclinal intrafolial folds (F 1 ), a layer-parallel transposition foliation, and a strong crossite lineation. The foliation is folded by two later sets of coaxial, isoclinal to tight folds (F 2 and F 3 ) that produce kilometer-scale, N-S trending, recumbent folds in the central part of the window. The same deformational sequence is recorded in the graphitic schist by continued regeneration of the transposition foliation through at least the second set of folds, and tight to isoclinal folding of the resulting surfaces. Both lithologies have been further deformed by extension parallel to F 3 axes (boudinage and fracturing) and a late folding (F 4 ) that produced kink bands, box folds, and chevron folds with E-W trending axes. Fold styles and asymmetries suggest that the early stages of progressive deformation (F 1 folding, transposition) resulted from noncoaxial deformation. Shear strains diminish during F 2 and F 3 folding and are replaced by irrotational flattening strains by the time of boudinage. The mineral assemblages formed prior to F 2 folding indicate greenschist-blueschist facies conditions during transposition. F 2 and F 3 folding were accompanied by the growth of deerite that lies in the axial planes of minor folds in meta-ironstones, indicating P-T conditions similar to those existent during transposition. Boudinage is concurrent with and followed by the static growth of chlorite, actinolite, albite, stilpnomelane, spessartine, and the Ba-silicate, cymrite. Ferroglaucophane rims on crossite may also have grown at this time. Pressure and temperature estimates, the relative time framework of deformational events, and the noncoaxial geometry of ductile strain are all consistent with, but not restricted to, a subduction zone environment. High shear strains may reflect descent and burial, whereas flattening and static mineral growth occur during uplift. Regional relationships favor an interpretation that relates metamorphism and ductile deformation to a Middle Jurassic subduction event, but do not preclude a Late Jurassic age for deformation and metamorphism.

The Shuksan Metamorphic Suite (Misch 1966) and correlative Easton Schist (Smith 1903) occur on the western flank of the North Cascades of Washington as large fault-hounded fragments (10s of kms long) imbricated with other rock units in a north-south belt extending more than 180 km. The Shuksan Suite is dominated by greenschist, blueschist, quartzose carbonaceous phyllite, and quartzofeldspathic semi-schist. A Jurassic, oceanic, near-arc tectonic site of deposition is hypothesized. Metamorphism began with production of Late Jurassic high pressure amphibolites in a contact aureole localized near peridotite, and was followed by regional blueschist facies metamorphism that lasted into the Early Cretaceous. Phase assemblages of the regional metamorphism include (in addition to quartz, chlorite, phengite, and sphene) lawsonite + albite in pelitic and psammitic schists, actinolite + albite + epidote + pumpellyite in greenschists, and crossite + albite + epidote + iron oxide in blueschists. The protolith rock type markedly controlled the development of the metamorphic index minerals. Temperatures of regional metamorphism ranged from approximately 330 to 400°C; pressures, not easily estimated, were perhaps 7 to 9 kilobars. Deformation during the blueschist metamorphism was directed approximately normal to the continental margin, as evidenced by the regional pattern of shear lineations. Uplift and imbrication of the Shuksan Suite with other units occurred by fault motion parallel to the continental margin. The metamorphic and uplift events are correlated with periods of high angle and low angle convergence respectively between the Farallon and North American plates.

The eclogitic terrane near Fairbanks, Alaska, consists of interlayered basic, calcmagnesian, quartzose, and pelitic schists, providing an opportunity to evaluate mineral parageneses in a diverse suite of high pressure metamorphic rocks. The terrane is interpreted to have equilibrated at 600 ± 25°C and 15 ± 2 kbars based on: 1) the jadeite content of omphacite coexisting with quartz + albite and 2) the pelitic assemblage garnet + chloritoid + staurolite + kyanite + quartz. The Ellis and Green (1979) calibration of K D -Fe/Mg of garnet/pyroxene also gives a 600°C temperature for basic schists where X Ca Ga = .30. However, the application of this geothermometer to calc-magnesian schists, where X Ca Ga ranges up to .47, gives temperatures that are much too high > (700°C). The K D values show some scatter, but do not systematically vary with the jadeite content of pyroxene in the range from 8 to 45%, or with the grossularite content of garnet from 25 to 47%. Temperatures derived from biotite-garnet K DS with the calibration of Ferry and Spear (1978), are also approximately 600°C. At such temperatures, glaucophane in the Fairbanks eclogites exceeds the maximum stability limit defined by the experimental studies of Maresch (1977). Comparison of the phase relations and inferred P-T of formation of the Fairbanks rocks with those from other high pressure terranes leads to the derivation of pressure-dependent equilibria that express the transition from amphibolites of the Sanbagawa belt to the substantially higher pressure kyanite eclogites of the Tauern Window. The Fair-banks eclogitic terrane is intermediate in this progression.

The glaucophane-bearing Raspberry Schist occurs as discontinuous, fault-bounded slivers along the northwest side of the Kodiak Islands in southern Alaska. The lithologies, predominantly metabasites and quartzites, experienced minor pre-metamorphic stratal disruption, extensive syntectonic deformation, and pervasive post-metamorphic disruption by faults. A major change in metamorphic assemblages and degree of recrystallization occurs across a fault zone, dividing the Raspberry Schist into units Js1 to the southeast and Js2 to the northwest. Js1 is characterized by relict igneous textures and phases and contains assemblages compatible with prehnite-pumpellyite, pumpellyite-actinolite, lawsonite-albite-chlorite, and lower greenschist transitional to blueschist facies. Rocks composing Js2 have been completely, dynamically recrystallized and contain assemblages described as transitional blueschist facies or high-temperature blueschist facies. Rb-Sr and K-Ar dates from the Raspberry Schist indicate that it was metamorphosed in Early Jurassic time. Lower Jurassic calc-alkaline mafic volcanics and associated plutons lie immediately northwest of the schist, juxtaposed with the schist by a wide brittle shear zone. Another calc-alkaline suite composed of Lower to Middle Jurassic plutonic and volcanic rocks lies 100 km to the northwest on the Alaska Peninsula. The metamorphism of the Raspberry Schist may be related to one of these calc-alkaline suites, but more evidence of the type and timing of fault movements in the region is needed to prove this conclusion.