Mafic Schists Research Articles

Pre-Andean orogenesis in the Coast Ranges of central Chile

An intense structural-metamorphic event of probable Permian-Triassic age is inferred from petrologic and field observations of low-grade schists from the Coast Ranges of central Chile between latitudes 38°S and 43°S. Relict almandine-rich garnet, biotite, and aluminous amphibole in mafic schists are replaced by a pervasive assemblage consisting of albite + chlorite + epidote + actinolite + sphene ± quartz: ± stilpnomelane ± white mica ± iron oxide. Chemographic analysis of phase compatibilities based on electron microprobe analyses suggests a regional retrogradation from earlier upper greenschist and/or epidote amphibolite to lower greenschist facies. A regional northwest-southeast-trending fabric within the terrane was imposed during the lower greenschist metamorphism which involved widespread reintroduction of fluids into the system. This major orogenic event is thought to be the culminating episode in a subduction-related accretionary process along the late Paleozoic margin of South America. Convergence may have been markedly oblique to the present north-south-trending continental margin.

Structure, petrology, and metamorphic history of the Nome Group blueschist terrane, Salmon Lake area, Seward Peninsula, Alaska

Research Article| May 01, 1985 Structure, petrology, and metamorphic history of the Nome Group blueschist terrane, Salmon Lake area, Seward Peninsula, Alaska STEPHEN POLLOCK THURSTON STEPHEN POLLOCK THURSTON 1Department of Geological Sciences, University of Washington, Seattle, Washington 98195 and Chevron U.S.A. Inc., Concord, California 94524 Search for other works by this author on: GSW Google Scholar Author and Article Information STEPHEN POLLOCK THURSTON 1Department of Geological Sciences, University of Washington, Seattle, Washington 98195 and Chevron U.S.A. Inc., Concord, California 94524 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1985) 96 (5): 600–617. https://doi.org/10.1130/0016-7606(1985)96<600:SPAMHO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation STEPHEN POLLOCK THURSTON; Structure, petrology, and metamorphic history of the Nome Group blueschist terrane, Salmon Lake area, Seward Peninsula, Alaska. GSA Bulletin 1985;; 96 (5): 600–617. doi: https://doi.org/10.1130/0016-7606(1985)96<600:SPAMHO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract An 80-mi2 area adjacent to Salmon Lake, mapped at a scale of 1:36,680, includes a section of the Nome Group schist that is at least 2,000 m thick. It is relatively intact, with relict primary lithologic layering, and not mélange-like or highly disrupted by faults. The Nome Group metasediments include pelitic schist, graphitic quartzite, pure and impure marble, calcareous schist, chlorite-albite schist, chloritic quartz-albite, and mafic schists. Metaigneous rocks include Fe-Ti-rich metabasite and granitic to tonalitic orthogneiss. The protoliths are Precambrian(?) to Paleozoic in age, and radiometric ages indicate that the high-pressure metamorphism of the Nome Group is Late Jurassic. Blueschist-facies assemblages occur in Nome Group metabasites and in most metasedimentary schists. The blueschists are synkinematic; they formed during an episode of widely distributed penetrative deformation and mesoscopic intrafolial isoclinal folding of consistent orientation. The Nome Group of the Salmon Lake area underwent a prograde monocyclic polyfacial high-P/T metamorphism. Initial metamorphism was in the lower-T regime of the lawsonite-glaucophane field (300 °C, 7 kb?) and prograded with increasing T and P into the epidote-almandine-glaucophane field through an apparent P/T maximum, as indicated by the assemblage glaucophane-omphacite-garnet (420 °C, 9–10 kb). The later stages of the metamorphism were characterized by increased T/P, likely during initial stages of decompression and the return of the “normal” geothermal gradient, to the transition between the blueschist facies and the high-P albite-epiclote-barroisite field (450 to 525 °C, 7–8 kb?). This high-P/T cycle was followed by an incipient static retrogressive metamorphism at low greenschist-facies conditions during the later uplift of the Nome Group in the Late Cretaceous and early Tertiary. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis &amp; Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In KD=Δ (XCa,g) where KD is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn‐poor garnet and common hornblende of widely varying chemistry metamorphosed at low aO2.Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks.Quartzo‐feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.

Petrology of a portion of the Eastern Peninsular Ranges mylonite zone, Southern California

Peraluminous and metaluminous plutonic rocks of the Peninsular Ranges batholith near Borrego Springs in southern California were mylonitized in the large shear zone known as the eastern Peninsular Ranges mylonite zone (EPRMZ). Accompanying mylonitization in this portion of the EPRMZ was metamorphism at intermediate-low-pressure amphibolite-facies conditions. Deformation in the zone overlapped in time with Cretaceous intrusion of the batholith. In the San Ysidro Mountain — Pinyon Ridge area, four north-south trending zones of differing intensity of deformation have been defined; from east to west the degree and style of deformation gradually change from undeformed or weakly deformed rocks to strongly mylonitized rocks. Electron microprobe analysis shows that recrystallized hornblende, biotite, and plagioclase are variable in composition, probably reflecting a range of metamorphic conditions accompanying deformation. Comparison of mineral compositions with those from mafic schists of Vermont suggests conditions ranged from andalusite-staurolite through sillimanite-muscovite grades as defined for pelitic rocks. Stability of muscovite+quartz in mylonite assemblages and lack of remelting of granitic rocks indicate that temperature did not exceed about 650° C during mylonitization and lithostatic pressure did not exceed about 5 kbar. Over time, any given rock volume experienced a range of temperature, lithostatic pressure, and perhaps fluid pressure and differential stress. Mineral reactions in the zone involved hydration, requiring introduction of water. The possibility of large-scale migration of K and Fe is suggested by whole-rock chemical data. Brittle and ductile deformation features are closely associated in one part of the EPRMZ. The combined evidence suggests the presence of a pore fluid with fluid pressure close to lithostatic pressure. Short periods of low fluid pressure and possible high differential stress cannot be ruled out.

Geology of the Fenton Hill, New Mexico, hot dry rock site

The Phase I prototype hot dry rock (HDR) geothermal system was developed in Precambrian basement rocks at Fenton Hill, New Mexico. Core and cuttings samples from the four deep wells indicate that the reservoir of this Phase I HDR system lies within a homogeneous biotite granodiorite body of very low permeability. Natural fractures, although present, are typically sealed with secondary minerals. The biotite granodiorite is intrusive into a heterogeneous metamorphic complex comprised of gneisses of variable composition and minor mafic schist, amphibolite, and metavolcanic rocks. The deeper part of this metamorphic section is the site of a deeper Phase II system currently under development. Differences in geology between the phase I and underlying Phase II systems will provide an opportunity for further testing of HDR development techniques.

Geothermometry and geobarometry of plagioclase-hornblende bearing assemblages

The reaction Hb+Zo(Ep)+H2O+CO2⇌Pl+ Chl+Cc+Q was studied under hydrothermal conditions at P total=2, 4, 6 and 8 kb at $$X_{{\text{co}}_{\text{2}} } = 0.1$$ . The continious transition from tremolite (actinolite) to Al-rich hornblende was fixed along the equilibrium curve of the reaction, providing a complete solid solution in the calcic amphibole series. A dependence of Al content in Ca-amphiboles and coexisting plagioclases on PT-conditions of their crystallization, determined for a wide range of temperature (450–650° C) and pressure (2–8 kb), has been used for construction of the experimental geothermobarometer. This may be employed to deduce temperature and pressure conditions of metamorphism of the albite-epidote-amphibolite and the amphibolite facies metabasites, including zoisite (or epidote)-bearing assemblages. An application of the Hb-Pl geothermobarometer is illustrated on the Patom Highland amphibolites and also on the well-known mafic schists of Vermont.

Pressure, temperature, and time indicators in mafic schist; their application to reconstructing the polymetamorphic history of Vermont

Mineral chemistry and overgrowth relationships in mafic schist from Vermont are shown to be sensitive indicators of pressure, temperature, and relative time and to be good chronicles of the Paleozoic history of this polymetamorphic terrane. Within the common assemblage, amphibole + chlorite + epidote + plagioclase +quartz+ Ti-phase ± carbonate ± K-mica ± Fe^(3+)-oxide, electron microprobe analyses show that increasing metamorphic grade (as defined by intercalated pelitic schist) is recorded by an increase in the edenite, glaucophane, and tschermakite contents of amphibole, in the anorthite content of plagioclase, and in the substitution of (Al^(VI),F^(2+), Ti), Al^(IV) for (Fe^(2+), Mg, Mn), Si in biotite, chlorite, and muscovite. With increasing pressure the glaucophane component of amphibole increases. For medium-pressure metamorphism the albite-oligoclase gap is in the garnet zone, where amphibole has between 1.2 and 1.8 formula proportion Al^(IV) and (Al^(VI) + Fe^(3+) + Ti + Cr). This gap is below the garnet isograd in low-pressure mafic schist where Al^(IV) and (Al^(VI) + Fe^(3+) + Ti + Cr) in the amphibole are both less than 0.6. Mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two Ordovician (Taconic) and two Devonian (Acadian) events. Silurian-Devonian mafic schist in northeastern Vermont records two periods of low-pressure, Devonian metamorphism. These events are expressed by medium-pressure and low-pressure metamorphism in the Cambrian and Ordovician rocks to the west and south. In the pre-Silurian rocks high-pressure and medium-pressure metamorphism is assigned to the Ordovician. The observed high-pressure metamorphism is confined to a 110 by 40 km area along the Green Mountain anticlinorium axis in north-central Vermont.

High-pressure metamorphism in mafic schist from northern Vermont

The only confirmed occurrence of glaucophane and omphacite in the Appalachian Mountains of New England is in mafic schist from Tillotson Peak (44°48', 72°33'), north-central Vermont. Mineral assemblages observed, all with epidote + sphene ± magnetite ± phengitic muscovite ± carbonate (calcite or dolomite) ± apatite ± sulfide, are: (1) actinolite + glaucophane + chlorite + garnet + quartz, (2) actinolite + glaucophane + chlorite + albite + garnet, (3) actinolite + glaucophane + omphacite + garnet, (4) glaucophane + chlorite + garnet+ quartz, (5) glaucophane + omphacite + quartz, (6) albite + glaucophane + chlorite + quartz, and (7) albite + paragonite + chlorite + quartz. Electron microprobe investigations show that plagioclase, chlorite, sphene, white mica, carbonate, and magnetite grains are relatively homogeneous. However, amphibole, pyroxene, epidote, and garnet grains are zoned, indicating incomplete equilibration and changing physical conditions during metamorphism. Glaucophane in assemblage (6) has the general formula (rim composition): □ (Na_(1.9^(M4)Ca_(0.1)) (Mg_(1.8Fe_(1.2)^(2+)Al_(1.0)^(VI)Fe_(0.3)^(3+)) (Si_(7.9)Al_(0.1)^(IV)) O_(22) (OH)_(2•) Glaucophane in assemblages (1) to (5) is somewhat poorer in the glaucophane endmember and richer in actinolite. Ca-rich amphibole grains are zoned extensively with barroisite cores and actinolite rims. Omphacite grains in assemblages (3) and (5) have the general formula (rim composition) (Na_(0.5)Ca_(0.5)) (Mg_(0.4)Fe_(0.1)^(2+)Al_(0.4)^(VI)Fe_(0.1)^(3+)) Si_2O_6. In spite of the significant zoning, the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition. Correlation of mineral compositions and assemblages with mineral stability data suggests high-pressure facies series metamorphism at about 9 ± 2 kb and 450 ± 100°C with P_(h2O) ≈ P_(Total). It is proposed that the metamorphism occurred during the Ordovician.

Metamorphic zones and physical conditions of metamorphism in Leros Island, Greece

On the basis of the systematic variation and the appearance and disappearance of some metamorphic minerals in metapelitic assemblages, the metamorphic terrain of Leros can be divided into chlorite, biotite, garnet and staurolite-kyanite zones of progressive regional metamorphism. The matapelites are interbedded with blueschists containing magnesioriebeckite in Fe3+-rich mafic assemblages in the chlorite zone and “more normal” greenschist and amphibolite facies in higher grade zones. Combining the observed mineral assemblages in pelitic and mafic schists with the available experimental or calculated relevant phase equilibria, one can deduce temperature conditions of metamorphism ranging from about 350° C up to about 700° C and pressures ranging between a minimum value defined by the pressure of the triple point of the Al2SiO5 polymorphs and a possible minimum around 7 kb.

Phase Equilibria in Mafic Schist from Vermont

Electron microprobe analyses of coexisting minerals in mafic schist from Vermont define systematic compositional variations which are shown by least-squares, mass balance calculations to be due to differences in metamorphic grade and facies series as is indicated by intercalated pelitic schist. The metamorphism of basaltic bulk rock compositions from greenschist to low-grade amphi-bolite facies and from greenschist to blueschist facies primarily involves changes in the mineral composition and modal abundance of five phases (amphibole + chlorite + epidote + plagioclase + quartz) in the common assemblage (which also includes a Ti-phase ± Fe3+ oxide ± carbonate ± K-mica). Progressive metamorphism of common mafic schist at medium pressure results in dehydration and the following continuous changes: amphibole grows at the expense of chlorite, epidote, and plagioclase; the edenite, glaucophane, and Tschermak substitutions in amphibole increases; the Tschermak substitution in chlorite increases; and the anorthite content of plagioclase increases (discontinuously at first from albite to oligoclase and then continuously to andesine). Along with these continuous changes in the common assemblage, stilpnomelane disappears and garnet appears during the medium-pressure, progressive metamorphism of Fe-rich samples. Low-pressure facies series metamorphism results in the same continuous changes as are observed for medium-pressure metamorphism except that plagioclase increases rather than decreases in modal abundance; epidote disappears from basaltic compositions at a lower grade; and the glaucophane substitution in amphibole is less. High-pressure metamorphism culminating in the greenschist-blueschist transition is described by the discontinuous reaction: Actinolite + Chlorite + Albite → Glaucophane + Epidote.

Geology of the older Precambrian rocks of the Grand Canyon

The amphibolites and mafic schists in the Precambrian metamorphic rocks of the Grand Canyon are divided into five major groups on the basis of field occurrence and mineral composition: (1) anthophyllite- and cordierite-anthophyllite-bearing rocks, (2) early (plagioclase-hornblende) amphibolites, (3) Granite Park mafic complex, (4) hornblende-bearing dikes, (5) tremolite-bearing dikes. Cordierite-anthophyllite rocks and associated schists represent sedimentary types which attained their bulk composition prior to metamorphism. They were derived from basic igneous materials which had suffered intense alteration and loss of lime and alkalis, probably by reaction with sea water. The early amphibolites were originally a series of lavas and tuffs of approximately basaltic composition. These meta-volcanic rocks have a tholeiitic character, and trace element abundances suggest that they may have formed in an island arc environment. Field and chemical data for the Granite Park mafic complex strongly indicate that it originated as a layered series of plagioclase-rich cumulates. Rocks of the complex show calc-alkaline chemical trends but with some tholeiitic affinities. Hornblende-bearing dikes range from ultramafic types composed only of hornblende plus opaques, to more normal plagioclase-hornblende amphibolites. They are strongly tholeiitic and may represent either ocean floor or island arc intrusives according to trace element data. Tremolite-bearing dikes are highly magnesian in composition, plot as early differentiates, and belong to either the calc-alkali or the tholeiitic series.

Archaean precious-metal hydrothermal systems, Dome Mine, Abitibi Greenstone Belt. I. Patterns of alteration and metal distribution

The Porcupine District, Abitibi Greenstone Belt is one of the most extensive areas of Archaean auriferous mineralization. At least two stages of lode gold emplacement may be recognized. The first involves gold-bearing ferroan dolomite layers with subordinate chert, sulphides, and graphite deposited as laterally extensive chemical sediments at interflow horizons within the mafic volcanic sequence. The second stage is represented by major gold-bearing hydrothermal quartz–albite–dravite veins which transect diverse host rocks including the carbonate chemical sediments. Differences between gold-bearing chemical sediment and auriferous hydrothermal veins, in terms of texture, mineralogy, and nature of inclusions, together with considerations of chemistry are not compatible with local derivation of veins from enveloping chemical sediments or adjacent host rocks. The chemical sediments display slump structures and predate all tectonic deformation. In general, auriferous hydrothermal quartz veins transect bedding and/or schistosity, and are at a low state of internal strain. They appear to have been emplaced late during the second regional fold episode.Au, Ag, and Pd average 10, 2, and 0.1 ppm respectively in ore types at the Dome mine; representing concentration factors of 10 000,40, and 10 times background values in unmineralized metabasalt, and primary igneous rocks worldwide. Au and Ag are inhomogeneously distributed.Mineralized metabasic rocks adjacent to vein stockworks have Ti/Zr and Ti/Al2O3 ratios comparable to tholeiitic basalts, but display variable enrichment or depletion of silica, systematic depletion in Na2O, and where intensely altered significant extraction of calcium. The low Ni and Cr contents of the carbonate layers, together with low Ti/Zr ratios (43–78) of the carbonates and their enveloping mafic schists, are not consistent with the hypothesis that these auriferous rocks are carbonated ultramafics. Massive banded quartz–fuchsite–dravite veins have Cr and Ni abundances averaging 350 ppm, implying hydrothermal transport of these elements. Ti/Zr ratios of 120, together with high Mg, Cr, and Ni abundances in magnesite–dolomite–quartz– chlorite schists which host the banded veins are compatible with a primary komatiitic composition. Mineralized metabasic rocks are reduced (Fe2+/ΣFe = 0.9) relative to rocks with primary background abundances of precious metals (Fe2+/ΣFe = 0.7). This change of oxidation state implies that large volumes of reducing hydrothermal solutions were involved in vein mineralization.

Amphibolitic rocks from the Precambrian of Grand Canyon: mineral chemistry and phase petrology

SummaryThe amphibolites and mafic schists that occur in the Precambrian metamorphic rocks of the Grand Canyon are divided into five major groups: anthophyllite and cordierite-anthophyllite rocks, early amphibolites, Granite Park mafic complex, hornblende-beating dykes, and tremolite-bearing dykes. Many types of amphibole occur in these rocks. Microprobe analyses identify gedrite, anthophyllite, cummingtonite, and grunerite, as well as three groups of calcic amphiboles. These last range in composition from colourless tremo-litic or actinolitic amphiboles, through pale-green horn-blende, to strongly pleochroic green-brown hornblende, which contains a significant proportion of the tscherma-kite and pargasite endmembers. Phase relationships between the coexisting amphiboles and other minerals are presented for two regional metamorphic events. Assem-blages containing chlorite, garnet, and hornblende were formed during the first event; from the absence of staurolite, but the presence of almandine garnet and oligoclase-andesine, it is concluded that the metamorphic grade was between the upper greenschist and the lower amphibolite facies. The second period of metamorphism produced rocks of the staurolite and sillimanite zones, within which three main ‘subfacies’ can be distinguished on the basis of phase relationships in the mafic schists.

Feather River Ultramafic Body, Northern Sierra Nevada, California

The Feather River ultramafic body is an elongate mass of metamorphosed peridotite and dunite with subordinate lenses of mafic schist. Classification of the body as an alpine-type peridotite is indicated by its mineral chemistry (olivine, Fo 88–92 ; enstatite, En 89–92 ; chromium-rich spinel) and the presence of “flow-layering” and kink bands in olivine and enstatite. The body is located along the northernmost part of the Melones fault zone and is faulted against surrounding slate units derived from thick mudstone-chert and volcaniclastic-carbonate sequences of Paleozoic and Triassic age. The ultramafic rock underwent an early episode of high-temperature flowage, probably as a crystal mush, and later was metamorphosed in the greenschist facies, at least in part contemporaneously with emplacement. Mafic schist lenses within the ultramafic body were metamorphosed under amphibolite- and then greenschist-facies conditions. The western contact of the northern part of the body is a zone of intense shearing and is bordered by a layer 100 to 600 m thick of complexly deformed schist which is intermediate in metamorphic grade between amphibolite and greenschist facies. Evidence is inconclusive as to whether the ultramafic body was intruded into the Sierran environment at high temperatures or was tectonically emplaced as a cool slab of oceanic upper mantle. The latter origin, however, is supported by the tabular form of the body and the discordance of high-temperature flow features with the northwest trend of the body.

Buried Precambrian Rocks of South Dakota

Early Precambrian granitic gneiss, mafic schist, and granite, outlined by prominent east-northeast-trending magnetic and gravity anomalies, are extensive in the buried basement of northeastern South Dakota, the southwestern-most extension of the Superior geological province. These rocks were involved in widespread orogenesis about 2500 m.y. ago. In central and western South Dakota equally prominent geophysical anomalies define the trend of middle Precambrian rocks. A middle Precambrian sedimentary succession in the Black Hills was deformed and intruded by granite approximately 1700 m.y. ago. Four distinct metamorphic belts which possibly date mainly from middle Precambrian time are recognized. A large granite terrane in southern South Dakota is tentatively interpreted as having formed 1430 to 1460 m.y. ago. The basement complex in southeastern and central South Dakota is overlain by a sheet of Sioux Quartzite. A new Rb-Sr age determination on a rhyolite suggests that the Sioux Quartzite is at least 1470 m.y. old.

Petrologic comparison of the franciscan and sanbagawa metamorphic terranes

Mafic schists in California carry the typical blueschist facies assemblage of glaucophane—crossite + lawsonite 4 sphene + chlorite ± aragonite or calcite ± pumpellyite. Albite, epidote and actinolite are not common in “in situ” metabasalts. In contrast, typical Sanbagawa schists contain crossite or actinolite + albite + epidote + chlorite ± calcite; the highest-grade rocks generally contain garnet and blue—green hornblende, lower grade rocks may contain pumpellyite and/or stilpnomelane, rarely lawsonite. Franciscan metaclastic rocks show three principle assemblages, all of which contain the phases quartz 4 white mica + chlorite + sphene ±lawsonite ± stilpnomelane: albite 4 calcite, albite 4 aragonite; and jadeitic pyroxene ± aragonite ± glaucophane. Quartzose Sanbagawa metaclastic rocks represent the albite ± calcite type compatibilities; lawsonite, aragonite and jadeitic pyroxene are either absent or are very rare. Many Franciscan siliceous metasediments contain, in addition to quartz, the phases riebeckite, stilpnomelane and/or spessartine. In contrast, many Sanbagawa siliceous schists carry abundant piemontite ± magnesioriebeckite in addition to spessartine. Correlation of the above mineral assemblages with experimental data suggests that blueschist-facies metamorphism took place in the Franciscan terrane at high fluid pressures, and involved temperatures of 200–300°C and lithostatic pressures of 6.5–8 kbar. Apparently Sanbagawa metamorphism reflected physical conditions transitional between the blueschist, greenschist and albite—epidote—amphibolite facies, approximating 250–400°C at 5–7 kbar lithostatic pressure.

Basement Rocks in Continental Interior of United States

Basement Rocks in Continental Interior of United States

Behavior of carbon-dioxide in low-grade regional metamorphism

Some detailed statistical analysis of mineral assemblages of mafic schists in low-grade regional metamorphic terrains of Japan show that (1) Partial pressure of chemical potential of CO2 must have clearly controlled the mineral associations formed in low-grade regional metamorphic rocks, (2) CO2 pressre varies with areas as well as with grade of regional metamorphism.

Low-grade metamorphic rocks in the Yaguki district, Abukuma plateau

A mass chiefly composed of politic, psammitic and mafic metamorphic rocks in which pumpellyite is stably associated with chlorite, epidote, actinolite and sodic plagioclase are found in the Yaguki district, NE Japan. Glaucophane is also stably found in some mafic schists. Serpentinized peridotite and serpentinite are intruded into these metamorphic rocks.Western part of this metamorphic and igneous complex was also contact metamorphosed by the intrusion of granodioritic rocks.Geological and petrological features indicate that the Yaguki metamorphic rocks may be an extension in NE Japan of Sanbagawa metamorphic terrain.