Plagioclase relations in pelites, central Menderes Massif, Turkey. II. Perturbation of garnet-plagioclase geobarometers

Abstract Activity-composition relations in oligoclase near the peristerite gap are investigated in pelites from the Central Menderes Massif. The pressure of metamorphism is estimated independently, from garnet-rutile-ilmenite-kyanite-quartz, as being in the range 4–7 kbar. In the temperature range, 450–600°C approximately, both the Newton-Haselton calibration of the garnet-plagioclase-kyanite-quartz geobarometer and a related simple treatment of garnet-plagioclase-muscovite-biotite give a wide range of apparent pressures, correlated with plagioclase composition and ranging up to 11–12 kbar where the plagioclase is most sodic. This effect is attributed to failure of the activity model for plagioclase used in the Newton-Haselton treatment. It is inferred that, in the present area, γ^plag_An decreases with increasing X ^plag_An in the range An₁₅-An₂₅. The data can be interpreted in terms of high γ ^plag_An in the high-albite structure at these temperatures, modified to lower values by 'e'ordering in the more calcic oligoclases. The ordering appears to be independent of the peristerite gap, and the data do not support the interpretation of the gap as a solvus. Garnet-plagioclase assemblages are unreliable as geobarometers where the plagioclase is more sodic than approximately An₂₀ and T < 700°C, and should instead be used to investigate the γ -X behaviour of the plagioclase where independent geobarometry can be used as a constraint.     

A second natural occurrence of yoderite

A second example of yoderite has been discovered in whiteschists from the Southern Chewore Hills of northern Zimbabwe. The mineral is pale green in colour and occurs in an equilibrium assemblage with talc+chlorite+kyanite+dravite+hematite. There is no quartz present. Recalculated microprobe analyses give a structural formula of Mg₂Al_5.7Fe_0.3Si₄O₁₈(OH)₂, similar to that obtained for the type locality at Mautia Hill, Tanzania, i.e. Mg₂Al_5.6Fe_0.4Si₄O₁₈(OH)₂. Textural relationships and relative proportions of minerals suggest that the yoderite was formed by reaction between talc, chlorite, kyanite and hematite. Experimental evidence suggests high-water-pressure metamorphic conditions at temperatures exceeding a reaction curve that extends between 13  kbar at 590  °C and 21  kbar at 650  °C. The yoderite-bearing whiteschist is associated with a 1.4  Ga dismembered ophiolite. It is proposed that this yoderite occurrence is associated with a relict subduction/suture zone.     

Decompression-induced growth of albite porphyroblasts, Fleur de Lys Supergroup, western Newfoundland

Abstract Albite porphyroblasts are widely distributed in pelitic and semi-pelitic schists of the Fleur de Lys Supergroup, western Newfoundland. Textures and mineral assemblages indicate that albite grew during nearly isothermal decompression from P-T conditions of about 500° C, 9 kbar, to conditions of 550° C, 6.5 kbar. Three compositional varieties of albite-bearing schists, here termed PMAQ (paragonite-muscovite-albite-quartz), MMAQ (microcline-muscovite-albite-quartz), and PMMQ (paragonite-muscovite-margarite-quartz), can be distinguished on the basis of pre-porphyroblast mineral assemblages. Analysis of these assemblages in terms of the composition of the coexisting fluid [log a (Na⁺/H⁺) versus log a (K⁺/H⁺)] suggests that, as pressure and temperature changed, the stability field of albite expanded at the expense of coexisting matrix phyllosilicates. This promoted growth of albite on pre-existing or newly formed nuclei. Late oligoclase in PMAQ and PMMQ samples is associated with replacement of matrix garnet by plagioclase + mica ° Chlorite, particularly in strongly sheared samples.     

Metamorphism and deformation of blueschist belts and their tectonic implications, North Qilian Mountains, China

Abstract Two blueschist belts in the North Qilian Mountains occur in Middle Cambrian and Lower Ordovician strata and strike N30–35°W for about 500 km along the Caledonian fold belt on the south-west margin of the Sino-Korean plate. The styles of metamorphism and deformation are quite different in the two belts. The Middle Cambrian to Ordovician rocks in the high-grade belt are mainly blueschists and C-type eclogites in which six phases of lower and upper crustal deformation have been recognized. The rocks contain glaucophane, phengite, epidote, clinozoisite, chlorite, garnet, stilpnomelane, piedmontite, albite, titanite and quartz. The estimated P–T conditions of eclogites are 340 ± 10°C, 8 ± 1 kbar and, of blueschist, >380°C, 6–7 kbar. The Ordovician rocks in the low-grade belt are characterized by ductile to brittle deformation in the middle to upper crust. The low-grade blueschists contain glaucophane, lawsonite, pumpellyite, aragonite, albite and chlorite. The estimated P–T conditions are 150–250°C and 4–7 kbar.
K–Ar and ³⁹Ar/⁴⁰Ar geochronology on glaucophane and phengite from the high-grade blueschist belt suggest two stages of metamorphism at 460–440 and 400–380 Ma, which may represent the times of subduction and orogeny. The subduction metamorphism of the northern low-grade blueschist belt took place approximately at the end of the Ordovician.     

Continuous reactions between biotite, garnet, staurolite, kyanite-sillimanite-andalusite and P-T-time-deformation path in micaschists from the estuary of the river Vilaine, South Brittany, France

Abstract Microprobe analysis of the continuous chemical evolution of coexisting biotite-garnet and biotite-garnet-staurolite has been undertaken from interbedded micaschists of the volcanodetrital group of the Vilaine. A thermobarometric study using pertinent mineralogical equilibria reveals a complex P-T evolution, continuous throughout time, from high pressure, medium temperature (kyanite zone) to medium pressure, high temperature (sillimanite zone), then low pressure, medium temperature (andalusite zone). The T, P, f_H2o and X_H2o variations have been calculated from coexisting biotite-garnet pairs, and from the equilibria: paragonite (in white mica) + quartz ± albite (in plagioclase) + Al silicate + H₂O; and, 3 anorthite ± grossular + 2 Alsilicate + quartz. The P-T evolution is correlated with the continuous change in composition of minerals (using P–X_Mg and T–X_Mg diagrams) and with the evolution of assemblages. This continuous P-T-time evolution, correlated with the successive formation of S₁-S₂ foliations, allows us to propose a P-T-time-deformation path for the micaschists and to relate the growth of its mineral components to tectonic processes.     

Controls on the first appearance of jadeitic pyroxene, northern Diablo Range, California

A reaction producing jadeitic pyroxene in metagreywackes of the northern Diablo Range has been identified on the basis of mineral distribution, isograd patterns and composition of coexisting minerals. The appearance of jadeitic pyroxene (∼Jd₈₀) is closely followed by the disappearance of pumpellyite, which indicates that pumpellyite plays a major role in the pyroxene-producing reaction. A new projection from hematite, lawsonite, chlorite, quartz and H₂O on to the NaAlO₂-FeO-MgO ternary confirms the role of pumpellyite in pyroxene production and suggests a reaction of the form: 1.00 pumpellyite + 0.31 chlorite + 8.71 albite + 0.70 hematite + 2.00 H₂O = 8.54 jadeite + 0.57 glaucophane + 3.09 lawsonite + 5.26 quartz. Metagreywackes of the northern Diablo Range were metamorphosed under conditions of P _H2O= P _total at 200-300 °C and 7.5-10.0 kbar. Despite the low temperatures attained during metamorphism, the assumption of equilibrium yields results consistent with field observations and phase relations.     

Geothermometry, Geobarometry and T-X(Fe-Mg) Relations in Metapelites, Snow Peak, Northern Idaho

The most recent of two metamorphic events (M2) in the Snow Peakarea caused progressive changes in mineral parageneses in peliticrocks ranging from chlorite-biotite to kyanite grade. Systematicpartitioning of elements between coexisting phases indicatesa close approach to equilibrium during M2. Temperature estimatesfor M2 range from 440 ?C in the chlorite-biotite zone to 565?C in the kyanite zone. Coexistence of kyanite, garnet, ilmenite,and quartz places an upper pressure limit of approximately 60kb, and an upper temperature limit at the kyanite-sillimaniteboundary. Equilibrium of garnet, kyanite, plagioclase, and quartzindicates that total pressure of equilibration of kyanite-bearingassemblages was approximately 6 kb. Pressure estimates basedon equilibrium of garnet, muscovite, biotite, and plagioclaseindicate a pressure gradient between garnet and lower staurolitezone samples, which equilibrated at approximately 3? 5 kb, andupper staurolite to kyanite zone samples, which equilibratedat 5? 5 kb. Equilibrium of paragonite component of muscovitewith plagioclase, kyanite and quartz, distribution of speciesin C-O-H fluids in equilibrium with graphite, and the presenceof zoisite in adjacent calc-silicate rocks indicate that themetamorphic fluid in kyanite-bearing assemblages contained 65-90mole per cent H₂O. However, the experimentally calibrated equilibriumof staurolite, quartz, garnet, and kyanite can be reconciledwith estimated temperature only if X_H2O in the fluid was verylow ( 33 mole per cent). T-X(Fe-Mg) relations among chlorite, biotite, garnet, staurolite,kyanite, muscovite and quartz are calculated at 6 kb on thebasis of 3 independent Fe-Mg exchange equilibria: garnet-biotite,chlorite-biotite (empirical, this study), garnet-staurolite(empirical, this study), and three independent net transferequilibria. Alternative sets of data for Mg-chlorite and Fe-stauroliteare evaluated by comparing observed and calculated changes inmineral paragenesis and mineral composition with grade. Chloritedata from Helgeson et al., 1978 give T-X(Fe-Mg) relations consistentwith trends observed in these rocks, whereas data derived frombreakdown of clinochlore and clinochlore + dolomite do not.Calculation of T-X(Fe-Mg) relations consistent with observationsrequires lower values of and than those consistent with experiments on the breakdown of staurolite+quartz.     

辽东半岛古元古代北辽河群浪子山组巴罗式变质带分布、变质特征及其构造意义

巴罗式递增变质带能为地壳增厚及造山作用带来全新的认识.为了解辽东半岛北辽河群浪子山组内巴罗式变质带的野外分布和变质特征,对本溪地区连山关-祁家堡巴罗式变质带进行了详细的野外调查和室内研究工作.野外填图表明,浪子山组巴罗式变质带由南向北可以分为黑云母带、石榴石带、十字石带和蓝晶石带.岩相学研究表明,蓝晶石带的蓝晶十字石榴云母片岩保存了3阶段的矿物组合:（1）进变质阶段（M₁）矿物组合为Pl+Qz+Ms+Bt+Ctd+Chl±Grt;（2）峰期变质阶段（M₂）以Ky+St+Grt+Bt+Ms+Qz+Pl+Ilm为特征;（3）退变质阶段以毛发状的矽线石（M_3-1）和变斑晶边缘或裂隙处生长的绿泥石和绢云母为特征（M_3-2）.相平衡模拟表明,蓝晶十字石榴云母片岩所经历的进变质和峰期变质温压条件分别为~440℃/~3.7 kbar,~670℃/~7.9 kbar,具有典型的顺时针P-T轨迹.变质锆石U-Pb定年结果得到蓝晶十字石榴云母片岩经历了~1.96 Ga的峰期变质作用;碎屑锆石U-Pb年龄分布于2 631~2 020 Ma,浪子山组蓝晶十字石榴云母片岩碎屑物源可能来自于太古宙基底（~2.5 Ga）和2.2~2.1 Ga的岩浆岩.结合前人研究资料表明,浪子山组巴罗式变质带所记录的P-T-t轨迹以及大量的逆冲推覆构造揭示了胶-辽-吉带在~1.96 Ga经历了与造山作用相关的地壳增厚过程.      

Metamorphic P–T Paths from pelitic schists and greenstones from the south-west Tauern Window, Eastern Alps

Abstract Petrological data from intercalated pelitic schists and greenstones are used to construct a pressure–temperature path followed by the Upper Schieferhülle (USH) series during progressive metamorphism and uplift in the south-west Tauern Window, Italy. Pseudomorphs of Al–epidote + Fe-epidote + albite + oligoclase + chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite-albite-chlorite subfacies of the blueschist facies at P ± 7–8 kbar. Geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475±25°C, 5–6 kbar; calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of ∼ 7.5 kbar. Garnet zoning patterns are complex and reversals in zoning can be correlated between samples. Thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity. Fluid inclusion data from coexisting immiscible H₂O–CO₂–NaCl fluids constrain the uplift path to have passed through temperatures of 380 + 30°C at 1.3 + 0.2 kbar.
There is no evidence for metamorphism of USH at pressures greater than ∼ 7.5 kbar in this area of the Tauern Window. This is in contrast to pressures of ± 10 kbar recorded in the Lower Schieferhülle only 2–3 km across strike. A history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the P–T data obtained from these adjacent tectonic units.     

Petrology of pelitic schists in the oligoclase-biotite zone of the Sanbagawa metamorphic terrain, Japan: phase equilibria in the highest grade zone of a high-pressure intermediate type of metamorphic belt

The oligoclase-biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (X_Ca= 0.10–0.28)-bearing assemblages in pelitic schists, and represents the highest-grade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet + biotite + chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming PH₂O = P_solid suggests that metamorphic conditions of this zone are about 610 ± 25°C and 10 ± 1 kbar.
Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase-biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high-pressure type of metamorphic belt and with those in a generalized'upper staurolite zone'as an example of a medium-pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as:     

A ternary solid solution model for omphacite and its application to geothermobarometry of eclogites from the Middle Adula nappe (Central Alps, Switzerland)

A ternary solid solution model for omphacite with the end-members jadeite (NaAlSi₂O₆), diopside (CaMgSi₂O₆) and hedenbergite (CaFeSi₂O₆) was derived from experimental data from the literature. The subregular solution model, fitted by linear programming, is best suited to omphacites with very little aegirine component in common eclogites. Applying this solution model to the calculation of equilibrium phase diagrams of eclogites from the Adula nappe (Central Alps, Switzerland) results in large stability fields for common eclogite assemblages (garnet+omphacite+quartz+H₂O±kyanite). Within this field the compositions of garnet and omphacite show very little variation. A precise determination of the peak-pressure and temperature is not possible. The occurrence of amphibole, overgrowing the peak-pressure assemblage in fresh eclogite, suggests retrograde re-equilibration, still under eclogite facies conditions. The computation of isopleths for garnet and pyroxene end-members allows the estimation of the pressure and temperature conditions of this re-equilibration event (19–21  kbar, c .  700 °C).     

Regional distribution of Al-silicates and metamorphic zonation in the low-grade Verrucano metasediments from the Northern Apennines, Italy

Abstract Mineralogical and petrological studies of Triassic Verrucano metasediments of the Northern Apennines are reported. The widespread occurrence of Al-silicates allows the delineation of four metamorphic zones with increasing metamorphic grade: (1) kaolinite zone (well Perugia 2, Umbria); (2) kaolinite-pyro-phyllite zone (Monte Argentario and part of the Verrucano of the Monticiano-Roccastrada area and Monti Leoni); (3) pyrophyllite zone (Monti Pisani, Iano, Monti Leoni, the Monticiano-Roccastrada area and some wells in the Larderello region); (4) kyanite zone (Massa area and some wells in the Larderello area).
The four metamorphic zones correspond to temperatures ranging from 300°C to about 450°C. On the basis of the Si content of muscovite and geological arguments, pressures of between 3 and 5 kbar are estimated. The metamorphic zones are located more or less parallel to the bent north-west-south-east trending structural zonation of the Northern Apennines, with the concave side towards the Tyhrrenian Sea.
During the Alpine orogeny, the Verrucano metasediments underwent three folding phases each of which has produced an axial plane schistosity (S₁, S₂, S₃). During the first folding phase the Verrucano sediments were buried increasingly deeply within the crust from east to west. The climax of Alpine metamorphism was attained prior to the second folding phase with crystallization of porphyroblasts of kyanite and chloritoid in a central area located between Massa and Larderello. The inferred paleo-temperature distribution pattern resembles an asymmetric thermal high defined by the kyanite zone, and surrounded by the pyrophyllite zone. A similar pattern is still present in the Tuscan crust, as indicated by a series of geothermal anomalies passing through the Northern Apennines.     

Pressure–temperature conditions and retrograde paths of eclogites, garnet–glaucophane rocks and schists from South Sulawesi, Indonesia

High-pressure metamorphic rocks exposed in the Bantimala area, c . 40  km north-east of Ujung Pandang, were formed as a Cretaceous subduction complex with fault-bounded slices of melange, chert, basalt, turbidite, shallow-marine sedimentary rocks and ultrabasic rocks. Eclogites, garnet–glaucophane rocks and schists of the Bantimala complex have estimated peak temperatures of T  =580–630 °C at 18  kbar and T  =590–640 °C at 24  kbar, using the garnet–clinopyroxene geothermometer. The garnet–omphacite–phengite equilibrium is used to estimate pressures. The distribution coefficient K _D1=[( X _pyr)³( X _grs)⁶/( X _di)⁶]/[(Al/Mg)_M2,wm (Al/Si)_T2,wm]³ among omphacite, garnet and phengite is a good index for metamorphic pressures. The K _D1values of the Bantimala eclogites were compared with those of eclogites with reliable P–T  estimates. This comparison suggests that peak pressures of the Bantimala eclogites were P =18–24  kbar at T  =580–640 °C. These results are consistent with the P–T  range calculated using garnet–rutile–epidote–quartz and lawsonite–omphacite–glaucophane–epidote equilibria.     

Petrology and geothermometry of retrograded Hercynian charnockites and host gneisses, Agly Massif, French Pyrenees

Abstract A Hercynian charnockite occurs within high-grade gneisses in the Agly Massif, French Pyrenees. Its thermal history has been evaluated using the Fe-Mg distribution coefticient ( K _D) between garnet and biotite. These minerals have different origins but similar compositions in the charnockites and host gneisses. In the charnockite, the Bi–Ga pairs are the retrograde products of Opx alteration. This Opx reaction with feldspar can be written. Opx + PI + Fluid 1(H₂O + Al + K + Fe + Ti) = Bi + Ga + Q + Fluid 2(H₂O + Na). The garnets are relatively Ca poor (4–2.5% grossular); they are automorphic and zoned in the gneisses and poikiloblastic in the charnockites. Both types show a retrograde rim (of few hundred microns'width) across which Fe and Mn increase as Mg decreases. The biotites show a good correlation between the octahedral cations (Ti⁴⁺+ Fe²⁺) and (Mg²⁺+ Al³⁺_VI); Ti and Fe both increase, whereas Mg and Al_VI decrease. There is an inverse linear correlation between Fe²⁺ and Mg²⁺ and the Fe/Mg ratio increases as Ti increases. The relation between Ti and K ^Ga-Bi_DFe-Mg is less clear: it seems that K _D slightly decreases as Ti increases. The equilibration temperatures of Ga–Bi pairs are discussed: the charnockite Ga-Bi pairs have equilibrated between 550°C and 600°C; whereas those of the gneisses have equilibrated between 550°C and 650°C. Two main thermal steps appear: one in the gneisses between 600-650°C and a second one in both the gneisses and the charnockites between 550°C and 600°C.     

Peristerite compositions in quartzofeldspathic schists, Franz Josef-Fox Glacier Area, New Zealand

Plagioclase compositions vary from An_0.1–2.5 to An₃₂ with increasing grade in chlorite zone to oligoclase zone quartzofeldspathic schists, Franz Josef-Fox Glacier area, Southern Alps, New Zealand. This change is interrupted by the peristerite composition gap in rocks transitional between greenschist and amphibolite facies grade. Oligoclase (An_20-24) and albite (An_0.1–0.5) are found in biotite zone schists below the garnet isograd. With increasing grade, the plagioclase compositions outline the peristerite gap, which is asymmetric and narrows to compositions of An₁₂ and An₆ near the top of the garnet zone. In any one sample, oligoclase is the stable mineral in mica-rich layers above the garnet isograd, whereas albite and oligoclase exist in apparent textural equilibrium in adjacent quartz-plagioclase layers. The initial appearance of oligoclase in both layers results from the breakdown of epidote and possibly sphene. Carbonate is restricted to the quartz-plagioclase rich layers and probably accounts for the more sodic composition of oligoclase in these layers. The formation of more Ca-rich albite and more Na-rich oligoclase near the upper limit of the garnet zone coincides with the disappearance of carbonate and closure of the peristerite gap. Garnet appears to have only a localized effect on Ca-enrichment of plagioclase in mica-rich layers within the garnet zone. The Na-content of white mica increases sympathetically with increasing Ca-content of oligoclase and metamorphic grade. Comparison of the peristerite gap in the Franz Josef-Fox Glacier schists and schists of the same bulk composition in the Haast River area, 80 km to the S, indicates that oligoclase appears and epidote disappears at lower temperatures, and that the composition gap between coexisting albite and oligoclase is narrower in the Franz Josef-Fox Glacier area. It is suggested that a higher thermal gradient (38-40°C/km) and variations in Si/Al ordering during growth of the plagioclases between the two areas may account for these differences. In the Alpine schists the peristerite gap exists over a temperature and pressure interval of about 370-515°C and 5.5-7 kbar (550-700 MPa) P_H2O.     

Petrological characterization and tectonic significance of retrogressed garnet peridotites, Raobazhai area, North Dabie Complex, east-central China

The Raobazhai ultramafic body of the North Dabie Complex is re-interpreted as a mantle-derived peridotitic slice enclosed in, and isofacially metamorphosed with, surrounding granulite-to-amphibolite facies gneisses. The ultramafic sheet consists mainly of metaharzburgite, but includes subunits of metadunite and mylonitic lherzolite. The rocks contain spinel but neither garnet nor plagioclase. However, in the mylonitic lherzolite, fine-grained intergrowths of spinel, orthopyroxene and clinopyroxene outline domains resembling the habit of garnet in two dimensions; broad-beam microprobe analyses imply pseudomorphs after a pyropic garnet precursor. The mineral assemblage of the metadunite and metaharzburgite is: olivine (Fo₉₂)+orthopyroxene (En₉₂)+tremolitic-to-magnesiohornblende+Mg–Al-chromite, indicating amphibolite facies recrystallization. The mineral assemblage of the mylonitic lherzolite is: olivine (Fo₉₀)+orthopyroxene (En₉₀)+clinopyroxene+Cr-bearing spinel+pargasitic amphibole, indicative of granulite-to-amphibolite facies metamorphism. Phase equilibria and geothermometric estimations show that the Raobazhai meta-ultramafics have undergone at least three stages of recrystallization: (I) 950–990 °C, (II) 750–860 °C, and (III) 670–720 °C, assuming equilibrium in the spinel peridotite stability field ( c. 6–15 kbar), although an early, high-pressure stage (≥18 kbar) is probable, based on the inferred garnet pseudomorphs. Petrochemical and geothermobarometric data suggest that the ultramafic slice represents a fragment of the mantle wedge, tectonically incorporated into subducted continental crust and re-equilibrated at granulite-to-amphibolite facies conditions while being exhumed to shallow levels.     

A new petrogenetic grid for low-grade metabasites

Abstract We have used internally-consistent thermodynamic data to present calculated phase equilibria for the system Na₂O-CaO-MgO-Al₂O₃-SiO₂-H₂O (NCMASH), in the range 0–500° C and 0.1–10 kbar, involving the phases anorthite, glaucophane, grossular, heulandite, jadeite, laumontite, lawsonite, paragonite, prehnite, pumpellyite, stilbite, tremolite, wairakite, zoisite with excess albite, clinochlore, quartz and pure water. Average activity terms derived from published mineral chemical data were included for clinochlore, glaucophane, prehnite, pumpellyite, tremolite, and zoisite. The new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite, prehniteactinolite, prehnite-pumpellyite, pumpellyite-actinolite, blueschist and greenschist facies metabasites. The stability fields of mineral assemblages containing prehnite, pumpellyite, epidote, actinolite (+ albite + chlorite + quartz) were analysed in some detail, using activity data calculated from five specific samples. For example, the prehnite-actinolite facies covers a P-T field ranging from about 220 to 320° C at pressures below 4.5 kbar. The transition from the prehnite-actinolite and pumpellyite-actinolite to greenschist facies occurs at about 250–300° C at 1–3 kbar and at about 250–350° C at 3–8 kbar. P-T fields of individual facies overlap considerably due to variations in chemical composition.     

Regression Modelling of Metamorphic Reactions in Metapelites, Snow Peak, Northern Idaho

The second of two periods of regional metamorphism that affectedpelitic rocks near Snow Peak caused complete re-equilibrationof mineral assemblages and resulted in a consistent set of metamorphicisograds. Metamorphic chlorite and biotite occur in the lowestgrade rocks. With increasing grade, garnet, staurolite, andkyanite join the assemblage, resulting in a transition zonecontaining all the above phases. At higher grade, chlorite,and finally staurolite disappear. Mass balance relations at isograds and among minerals of low-varianceassemblages have been modelled by a non-linear least-squaresregression technique. The progressive sequence can be describedin terms of schematic T-X_H2O relations among chlorite, biotite,garnet, staurolite, and kyanite at P_total above the KFMASH invariantpoint involving those phases. The first appearance of garnetwas the result of an Fe-Mg-Mn continuous reaction. As temperaturerose, the garnet zone assemblage encountered the stauroliteisograd reaction, approximated by the model reaction: 3?0 chlorite + 1?5 garnet + 3?3 muscovite + 05 ilmenite = 1?0staurolite + 3?1 biotite + 1?5 plagioclase + 3?3 quartz + 10?3H₂O. The staurolite zone corresponds to buffering along this reactionto the intersection where chlorite, biotite, garnet, staurolite,and kyanite coexist. The transition zone assemblage formed byreaction at this T–X _H2O intersection which migrates towardmore H2O-rich fluid composition with progressive reaction. Thenet reaction at the intersection is approximated by the transitionzone reaction: 1?0 chlorite +1?1 muscovite + 0?2 ilmenite = 2?7 kyanite + 1?0biotite + 0?4 albite + 4?2 H₂O. Chlorite was commonly the first phase to have been exhaustedand the remaining assemblage was buffered along a staurolite-outreaction, represented by the model reaction: 1?0 staurolite + 3?4 quartz + 0?4 anorthite + 1?4 garnet + 0?1ilmenite + 7?9 kyanite + 2?0 H₂O. Consumption of staurolite by this reaction resulted in the highestgrade assemblage, which contains kyanite, garnet, biotite, muscovite,quartz, plagioclase, ilmenite, and graphite.     

Metamorphic and structural evolution of the Early Proterozoic Puolankajärvi Formation, Finland – II. The pressure–temperature–deformation–composition path

The paragenetic relationships between sillimanite, andalusite, kyanite, chlorite, cordierite, biotite, garnet and staurolite in the Early Proterozoic Puolankajärvi Formation (PjF), together with mineral compositions, are used to construct a partial petrogenetic grid for metapelites with significant Mn content (MnO = 0.1–0.5%) by adding a six-phase invariant point over the garnet-absent invariant point for Mn-free AMF-phases.
The grid and textural relations of the PjF are used to construct part of the P–T –deformation path for the PjF. Relatively short deformation pulses and associated flow of oxidizing fluid along shear zones were responsible for the paragenetic and compositional changes during cooling and decompression at 600–500°C and 6.0–2.5 kbar. Oxidation led to decreased Fe²⁺ and further stressed the importance of Mn (increased Mn/divalent cations).
A tectonothermal evolution of the Kainuu Schist Belt is presented which includes crustal thinning and steepening of a previously established thermal gradient. This was followed by thrusting and folding of the isotherms into a thermal antiform on the western side of the belt.