Progress of actinolite-forming reactions in mafic schists during retrograde metamorphism: an example from the Sanbagawa metamorphic belt in central Shikoku, Japan

Hydration reactions are direct evidence of fluid–rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite → hornblende → actinolite in the high‐grade zone, and winchite → actinolite in the low‐grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass‐balance relations suggest that: (1) the actinolite‐forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H₂O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of actinolite to total amphibole, Y_act, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Y_act < 0.5), but it increases drastically in the high‐grade part of the garnet zone (Y_act > 0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Y_act > 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H₂O and the surface area of hornblende. In contrast to high‐grade rocks, the winchite breakdown reaction is well advanced in the low‐grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low‐grade rocks may be due to small size of winchite before the reaction.     

Water deficiency as a factor in metamorphism of rocks

Mineral equilibria in magnesial rocks undergoing metamorphosis in closed systems at different H₂O content but at constant pressures and temperatures may be interpreted as follows: a) if H₂O is excessive, as in pelitic rocks, the metamorphic zonation (an orderly sequence of dehydrations) correctly represents changes in the temperature during the progressive metamorphism; b) if H₂O is deficient, as in the ortho-rocks, the metamorphic zonation may not always be representative of the temperature; however, when H₂O itself is zoned in the rock and the metamorphism is isothermal, the metamorphic zonation may become entirely analogous to the zonation in the presence of excessive H₂O; c) presence of metastable associations and rapid alternations of mineral parageneses, as in certain magmatic rocks, represent variations in H₂O content of the rocks rather than different facies of the metamorphism.     

高温高压条件下实验变形石英闪长岩微观结构与熔体特征研究

本文以高温高压条件下石英闪长岩流变实验样品为研究对象,利用偏光显微镜进行微观结构观察,研究了样品在实验温度压力条件下的变形机制与斜长石结构对流变强度的影响;通过透射电镜能谱与电子探针,分析了熔体分布和成分特征,讨论了角闪石脱水熔融的影响因素与脱水熔融对岩石流变的影响。结果表明,随着温度升高,岩石从脆塑性过渡域逐渐向高温位错攀移和动态重结晶为主的塑性域转化。在高温条件下,角闪石出现了脱水与部分熔融,脱水熔融的熔体分布和成分体现出非均匀与非平衡熔融的特点,空间分布上,熔体主要出现在角闪石和黑云母矿物颗粒的边缘以及角闪石和长石颗粒之间的区域内;成分分布上,熔体的成分与参与熔融的矿物成分密切相关。角闪石边缘的熔体和黑云母边缘的熔体具有低硅铝、高铁镁特征,斜长石边缘的熔体具有高硅铝、低铁镁的特征,处于角闪石和斜长石颗粒中间的熔体,其成分间于斜长石与角闪石成分之间。实验中出现的非平衡非均匀部分熔融可以解释混合岩中的浅色体与暗色体的成因,富硅熔体可以形成富硅铝的花岗质岩石,而贫硅富铁镁的熔体可以形成基性岩。角闪石的脱水熔融程度依赖于样品的封闭条件,处于封闭环境的样品,角闪石不易脱水熔融,而处于开放环境时,角闪石脱水熔融显著。拆离断层带及其附近具备这样的开放环境,有利于角闪石发生脱水熔融。实验力学数据和微观结构显示,随机分布的斜长石对岩石强度影响并不明显,但斜长石的长轴方向与最大主应力方向呈大角度相交(近90°)会显著强化岩石的强度,这意味着岩石组构与主应力方向大角度相交或呈垂直方向时,不利于岩石变形和拆离断层的形成,反之,均匀岩石或岩石组构与最大主应力方向小角度相交,有利于岩石的变形,容易发育拆离断层。     

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 & 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 K_D=Δ (X_Ca,g) where K_D 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 a_O2. 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.     

内蒙古狼山地区早石炭世角闪辉长岩、花岗闪长岩的岩石成因及构造意义

内蒙古狼山山脉西侧分布有大面积的晚古生代岩浆岩,时代集中在早石炭世—晚二叠世,早石炭世角闪辉长岩、花岗闪长岩体出露于潮格温都尔镇西侧。角闪辉长岩体呈岩滴状产出,被花岗闪长岩体侵入,LA-ICP-MS锆石U-Pb年龄显示,角闪辉长岩的~(206)Pb/~(238)U加权平均年龄为329.0±2.3 Ma,花岗闪长岩的~(206)Pb/~(238)U加权平均年龄为331.1±0.9 Ma~330.0±4.2 Ma。花岗闪长岩暗色矿物以角闪石为主,富钠(Na2O=3.48%~4.46%),高钠钾比值(Na2O/K2O=1.03~2.39),钙碱性系列,P2O5-SiO_2之间存在较好的负相关性,岩石地球化学特征具Ⅰ型花岗岩的特点。Hf同位素及元素地球化学特征指示了角闪辉长岩及花岗闪长岩均来自于受地壳混染的亏损地幔,为同源岩浆演化的产物。角闪辉长岩及花岗闪长岩稀土元素配分型式一致,均为轻稀土元素富集,重稀土元素亏损,具弱的负Eu异常;角闪辉长岩富集Ba、Sr,亏损Nb、Ta、Zr、Hf;花岗闪长岩富集大离子亲石元素Rb、K、Pb、Sr,不同程度地亏损高场强元素Nb、Ta、P、Ti,总体反映了岩浆弧的地球化学特征。结合区域地质背景,早石炭世狼山地区侵入岩岩石组合为角闪辉长岩(闪长岩)+石英闪长岩+花岗闪长岩,认为狼山地区早石炭世处于大陆边缘弧构造背景。     

Coarsening kinetics of the exsolution microstructure in alkali feldspar

Actinolite, hornblende and biotite coexisting in greenschist mafic metagreywackes have been analysed with the electron microprobe to obtain information on their chemical relationship during metamorphism. As in some other parts of the world, the two calcic amphiboles coexist in the greenschist facies because of a miscibility gap between them which is observed under conditions of low-pressure regional metamorphism; it is thought that the two amphiboles are in equilibrium, or at least that the actinolite participated in hornblendeforming reactions. Contact metamorphism by granitic intrusives of these metagreywackes has converted them to hornblende hornfelses with the assemblage hornblende, andesine, quartz, biotite±cummingtonite; the hornblendes of the hornfelses are found to have compositions between actinolite and hornblende of the greenschists, and frequently show fine exsolution lamellae of cummingtonite as a result of oversaturation in this component. The distribution of Fe-Mg between hornblende and biotite changes from the greenschist to the hornblende hornfels facies, and the K _D is probably dependent on Al^VI in the hornblende.     

Phase Equilibria of Amphibolites from the Post Pond Volcanics, Mt. Cube Quadrangle, Vermont

Amphibolites of the Post Pond Volcanics, south-west corner ofthe Mt. Cube Quadrangle, Vermont, are characterized by a greatdiversity of bulk rock types that give rise to a wide varietyof low-variance mineral assemblges. Original rock types arebelieved to have been intrusive and extrusive volcanics, hydrothermallyaltered volcanics and volcanogenic sediments with or withoutadmixtures of sedimentary detritus. Metamorphism was of staurolite-kyanitegrade. Geothermometry yields a temperature of 535 ± 20°C at pressures of 5–6 kb. Partitioning of Fe and Mg between coexisting phases is systematic,indicating a close approach to chemical equilibrium was attained.Relative enrichment of Fe/Mg is garnet > staurolite >gedrite > anthophyllite cummingtonite hornblende > biotite> chlorite > wonesite > cordierite dolomite > talc;relative enrichment in Mn/Mg is garnet > dolomite > gedrite> staurolite cummingtonite > hornblende > anthophyllite> cordierite > biotite > wonesite > chlorite >talc. between coexisting amphiboles varies as a function ofbulk Fe/Mg, which is inconsistent with an ideal molecular solutionmodel for amphiboles. Mineral assemblages are conveniently divided into carbonate+ hornblende-bearing, hornblende-bearing (carbonate-absent)and hornblende-absent. The carbonate-bearing assemblages allcontain hornblende + dolomite+ calcite + plagioclase (andesineand/or anorthite) + quartz with the additional phases garnetand epidote (in Fe-rich rocks) and chlorite ± cummingtonite(in magnesian rocks). Carbonate-bearing assemblages are restrictedto the most calcic bulk compositions. Hornblende-bearing (carbonate absent) assemblages occur in rocksof lower CaO content than the carbonate-bearing assemblages.All of these assemblages contain hornblende + andesine ±quartz + Fe-Ti oxide (rutile in magnesian rocks and ilmenitein Fe-rich rocks). In rocks of low Al content, cummingtoniteand two orthoamphiboles (gedrite and anthophyllite) are common.In addition, garnet is found in Fe-rich rocks and chlorite isfound in Mg-rich rocks. Several samples were found that containhornblende + cummingtonite + gedrite + anthophyllite ±garnet +chlorite + andesine + quartz + Fe-Ti oxide ±biotite. Aluminous assemblages contain hornblende + staurolite+ garnet ± anorthite/bytownite (coexisting with andesine)± gedrite ± biotite ± chlorite ±andesine ± quartz ± ilmenite. Hornblende-absentassemblages are restricted to Mg-rich, Ca-poor bulk compositions.These rocks contain chlorite ± cordierite ± staurolite± talc ± gedrite ± anthophyllite ±cummingtonite ± garnet ± biotite ± rutile± quartz ± andesine. The actual assemblage observeddepends strongly on Fe/Mg, Ca/Na and Al/Al + Fe + Mg. The chemistry of these rocks can be represented, to a firstapproximation, by the model system SiO₂–Al₂O₃–MgO–FeO–CaO–Na₂O–H₂O–CO₂;graphical representation is thus achieved by projection fromquartz, andesine, H₂O and CO₂ into the tetrahedron Fe–Ca–Mg–Al.The volumes defined by compositions of coexisting phases filla large portion of this tetrahedron. In general, the distributionof these phase volumes is quite regular, although in detailthere are a large number of phase volumes that overlap otherphase volumes, especially with respect to Fe/Mg ratios. Algebraicand graphical analysis of numerous different assemblages indicatethat every one of the phase volumes should shift to more magnesiancompositions with decreasing µ_H2O. It is therefore suggestedthat the overlapping phase volumes are the result of differentassemblages having crystallized in equilibrium with differentvalues of µ_H2O or µ_CO2 and that the different valuesmay have been inherited from the original H₂O and CO₂ contentof the volcanic prototype. If true, this implies that eithera fluid phase was not present during metamorphism, or that fluidflow between rocks was very restricted.     

Feldspathic hornblende and garnet granulites and associated anorthosite pegmatites from Doubtful Sound,Fiordland, New Zealand

Feldspathic hornblende granulites from Doubtful Sound, New Zealand with the assemblage plagioclase+hornblende+clinopyroxene+orthopy-roxene +oxide+apatite are criss-crossed by a network of garnetiferous anorthosite veins and pegmatites. The feldspathic gneiss in contact with anorthosite has a reaction zone containing the assemblage plagioclase +garnet+clinopyroxene+quartz+rutile+apatite. The garnet forms distinctive coronas around clinopyroxene. The origin of these rocks is discussed in the light of mineral and whole rock chemical analyses and published experimental work.It is thought that under conditions leading up to 750 °C, 8 kb load pressure and 5 kb H₂O pressure, partial melting occured in feldspathic hornblende granulites. The melt migrated into extensional fractures and eventually crystallised as anorthosite pegmatites and veins. The gneisses adjacent to the pegmatites from which the melt was extracted changed composition slightly, by the loss of H₂O and Na₂O, so that plagioclase reacted simultaneously with hornblende, orthopyroxene, and oxide to form garnet, clinopyroxene, quartz and rutile.     

Granulite-Facies Metamorphism, Granite Falls-Montevideo Area, Minnesota

In the Granite Falls-Montevideo area, Minnesota, granulite faciesmineral assemblages were collected from three major lithologicunits—hornblende-pyroxene gneiss, garnet-biotite gneiss,and granitic gneiss. Mineral assemblages most commonly observedare: plagioclase-hornblende-orthopyroxene-clinopyroxene-magnetite-ilmenite-(biotite-quartz);quartz-plagioclase-biotite-garnet-orthopyroxene-(orthoclase);quartz-plagioclase-biotite-garnet-(orthoclase); quartz-plagioclase-microcline-(garnet-hematite);and quartz-plagio-clase-microcline-biotite-(garnet-hematite-rutile).Partial analyses of isomorphous phases from the hornblende-pyroxenegneiss and the garnet-biotite gneiss were determined with anelectron microprobe. Negligible compositional variation within single grains andof a particular mineral within a given specimen, regular distributionof Fe and Mg between coexisting hornblende, ortho-pyroxene,and clinopyroxene, obedience of the mineral assemblages to theGibbs Phase Rule, and lack of empirically determined incompatiblephases indicate a close approach to chemical equilibrium duringmetamorphism. Plots of coexisting biotite, garnet, and orthopyroxeneon an appropriate phase diagram result in some crossing tielines which cannot be adequately explained by temperature orpressure differences, but suggest that H₂O and/or O₂ were notperfectly mobile components during metamorphism. This is alsoindicated by interlayering of hornblende assemblages with pyroxeneassemblages and by different iron oxide phases in essentiallythe same mineral assemblage. Textural and chemical relationships of retrograde metamorphicassemblages suggest that some retrograde reactions are a resultof cooling following the granulite-facies metamorphism, butthat others may have resulted from recrystallization duringa stage of thermal metamorphism that is reflected in the potassium-argonand rubidium-strontium biotite ages of the metamorphic rocks.     

Heavy minerals in stream sediments,Southwest Norway

As part of a regional prospecting programme, 600 panned stream sediment concentrates from 1000 km² have been examined for scheelite (CaWO₄) using ultraviolet light. The mineralogical compositions of the heavy mineral concentrates were determined and related to bedrock geology. The results of the investigation are presented on maps showing the distribution of scheelite, magnetite, and ilmenite. Follow-up work resulted in the discovery of nine new tungsten occurrences. Magnetite dominates in areas of acid rocks, and ilmenite in areas of basic rocks. The areal distribution of hornblende and hypersthene reflects the metamorphic grades.     

Tremolite and H2O activity attending metamorphism of hornblende-plagioclase-garnet assemblages

Estimation of the activity of tremolite component (atr) in calcic amphiboles is an important problem in igneous and metamorphic petrology because equilibria involving tremolite are used in the estimation of the activity of H₂O attending crystallization of igneous and metamorphic rocks. Estimated values of atr from hornblende analyses using both ionic and coupled substitution crystalline solution models can be compared to values of atr calculated from vapor-absent mineral equilibria. In addition, these values of atr can be used in calculations of aH₂O for rocks for which there is an independent estimate of aH₂O. The values of atr calculated from vapor-absent equilibria are generally consistent with those estimated from the different crystalline solution models, but uncertainties in the calculations preclude choosing a preferred solution model. From computed mineral equilibria, it is clear that mineral assemblages with low values of atr can be in equilibrium with high values of aH₂O. Consequently, the low values of aH₂O estimated from hornblende-bearing high-grade rocks with low values of atr may be real. Rocks from Mica Creek, British Columbia, probably achieved extremely low values of aH₂O by vapor-absent metamorphism.     

Oxygen and hydrogen isotope studies of contact metamorphism in the Santa Rosa Range,Nevada and other areas

The O¹⁸/O¹⁶ and D/H ratios have been determined for rocks and coexisting minerals from several granitic plutons and their contact metamorphic aureoles in the Santa Rosa Range, Nevada, and the Eldora area, Colorado, with emphasis on pelitic rocks. A consistent order of O¹⁸/O¹⁶ and D/H enrichment in coexisting minerals, and a correlation between isotopic fractionations among coexisting mineral pairs are commonly observed, suggesting that mineral assemblages tend to approach isotopic equilibrium during contact metamorphism. In certain cases, a systematic decrease is observed in the oxygen isotopic fractionations of mineral pairs as one approaches the intrusive contacts. Isotopic temperatures generally show good agreement with heat flow considerations. Based on the experimentally determined quartz-muscovite O¹⁸/O¹⁶ fractionation calibration curve, temperatures are estimated to be 525 to 625° C at the contacts of the granitic stocks studied.Small-scale oxygen isotope exchange effects between intrusive and country rock are observed over distances of 0.5 to 3 feet on both sides of the contacts; the isotopic gradients are typically 2 to 3 per mil per foot. The degree of oxygen isotopic exchange is essentially identical for different coexisting minerals. This presumably occurred through a diffusion-controlled recrystallization process. The size of the oxygen isotope equilibrium system in the small-scale exchanged zones varies from about 1.5 to 30 cm. A xenolith and a re-entrant of country rock projecting into an intrusive have both undergone much more extensive isotopic exchange (to hundreds of feet); they also show higher isotopic temperatures than the rocks in the aureole. The marginal portions of most plutons have unusually high O¹⁸/O¹⁶ ratios compared to normal igneous rocks, presumably due to large-scale isotopic exchange with metasedimentary country rocks when the igneous rocks were essentially in a molten state. The isotopic data suggest that outward horizontal movement of H₂O into the contact metamorphic aureoles is very minor, but upward movement of H₂O is important. Also, direct influx and absorption of H₂O from the country rock appears to have occurred in certain intrusive stocks. The D/H ratios of biotites in the contact metamorphic rocks and their associated intrusions show a geographic correlation that is similar to that shown by the D/H ratios of meteoric surface waters, perhaps indicating that meteoric waters were present in the rocks during crystallization of the biotites.Except in the exchanged zones, the O¹⁸/O¹⁶ ratios of pelitic rocks do not change appreciably during contact metamorphism, even in the cordierite and sillimanite grades; this is in contrast to regional metamorphic rocks which commonly decrease in O¹⁸ with increasing grade. Thus, contact metamorphic rocks generally do not exchange with large quantities of igneous H₂O, but regional metamorphic rocks appear to have done so.Publications of the Division of Geological Sciences, California Institute of Technology, Contribution No. 1565.     

A thermodynamic model for Ca–Na clinoamphiboles in Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–O for petrological calculations

A calibration is presented for an activity–composition model for amphiboles in the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–O (NCFMASHO), formulated in terms of an independent set of six end‐members: tremolite, tschermakite, pargasite, glaucophane, ferroactinolite and ferritschermakite. The model uses mixing‐on‐sites for the ideal‐mixing activities, and for the activity coefficients, a macroscopic multicomponent van Laar model. This formulation involves 15 pairwise interaction energies and six asymmetry parameters. Calibration of the model is based on the geometrical constraints imposed by the size and shape of amphibole solvi inherent in a data set of 71 coexisting amphibole pairs from rocks, formed over 400–600 °C and 2–18 kbar. The model parameters are calibrated by combining these geometric constraints with qualitative consideration of parameter relationships, given that the data are insufficient to allow all the model parameters to be determined from a regression of the data. Use of coexisting amphiboles means that amphibole activity–composition relationships are calibrated independently of the thermodynamic properties of the end‐members. For practical applications, in geothermobarometry and the calculation of phase diagrams, the amphibole activity–composition relationships are placed in the context of the stability of other minerals by evaluating the properties of the end‐members in the independent set that are in internally consistent data sets. This has been performed using an extended natural data set for hornblende–garnet–plagioclase–quartz, giving the small adjustments necessary to the enthalpies of formation of tschermakite, pargasite and glaucophane for working with the Holland and Powell data set.     

Anthophyllit und Hornblende in einigen metamorphen Reaktionen

Reactions which occur at the lower boundary of the hornblende-hornfels facies and in the so-called pyroxene-hornfels facies were experimentally investigated for an ultrabasic rock at 500, 1000 and 2000 bars H₂O pressure.The starting material used was a mixture of natural chlorite, talc, tremolite and quartz such that its composition, except for surplus quartz, corresponded to that of an ultrabasic rock. The atomic ratio Fe²⁺+Fe²⁺/Mg+Fe³⁺+Fe³⁺ in the system was 0.16.The lower boundary of the hornblende-hornfels facies was defined by the formation of the orthorhombic amphibole anthophyllite and hornblende according to the following idealized reaction: chlorite+talc+tremolite+quartz hornblende+anthophyllite+H₂O In effect, this reaction consists of the two bivariant reactions: chlorite+tremolite+quartz hornblende+anthophyllite+H₂O talc+chlorite anthophyllite+quartz+H₂OThe equilibrium temperatures obtained for the two reactions in the given system are practically the same and are as follows: 535±10°C at 500 bars H₂O pressure 550±20°C at 1000 bars H₂O pressure 560±10°C at 2000 bars H₂O pressure 580±10°C at 4000 bars H₂O pressureAt 2000 bars and higher temperatures within the hornblende-hornfels facies, anorthite is formed in addition to hornblende and anthophyllite, probably according to the following reaction: hornblende₁+quartz hornblende₂+anthophyllite+anorthite+H₂O; because of the formation of anorthite it is to be expected that the hornblende in this case is poorer in aluminium than the hornblende at 500 and 1000 bars. Winkler (1967) suggests renaming the pyroxene-hornfels facies as K-feldspar-cordierite-hornfels facies which, in turn, is subdivided into a lower-temperature orthoamphibole subfacies without orthopyroxene and a higher-temperature orthopyroxene subfacies without orthoamphibole. The orthopyroxene subfacies itself may in its lower temperature part still carry hornblende which finally disappears in the higher temperature part.The appearance of orthopyroxene characterizes the transition from the orthoamphibole to the orthopyroxene subfacies of the K-feldspar-cordierite hornfels facies. The following reaction takes place at pressures lower than 2000 bars: hornblende₁+anthophyllite hornblende₂+enstatite+anorthite+H₂OSince at 2000 bars an Al-poor hornblende already exists in the hornblende-hornfels facies, it is very likely that here only anthophyllite breaks down to give enstatite+quartz+H₂O.The equilibrium temperatures for these reactions which give rise to enstatite are: 650±10°C at 250 bars H₂O pressure 690±10°C at 500 bars H₂O pressure 715±10°C at 1000 bars H₂O pressure 770±10°C at 2000 bars H₂O pressureOnly after an increase in temperature to about 710°C at 500 bars and about 770°C at 1000 bars does hornblende in the system investigated here break down completely according to the reaction: hornblende = enstatite+anorthite+diopside+H₂OExcept at very small H₂O-pressures (see Fig. 3), there exists, therefore, a region within the orthopyroxene subfacies where hornblende, enstatite and anorthite coexist. As a result we have, as mentioned above, a lower-temperature and a higher-temperature part of the orthopyroxene subfacies, and it is only in the latter part that the parageneses correspond to the pyroxene-hornfels facies as stated by Eskola (1939).Summing up, the starting material consisting of chlorite, talc, tremolite plus quartz remains unchanged in the albite-epidote-hornfels facies; this gives rise in the hornblende-hornfels facies to the paragenesis hornblende+anthophyllite, or — at higher pressures — to hornblende+anthophyllite+anorthite. For the particular composition of the starting material, however, no reactions take place at the transition of the hornblende-hornfels facies to the orthoamphibole subfacies of the K-feldspar-cordierite-hornfels facies as this transition is typified by the breakdown of muscovite in the presence of quartz. However, at the end of the orthoamphibole subfacies the breakdown of anthophyllite, by which orthopyroxene is formed, heralds the onset of the orthopyroxene subfacies. In this subfacies — at greater than about 300 bars — hornblende is still present and coexists with enstatite and anorthite, but with rising temperature hornblende breaks down to give way to the paragenesis enstatite+anorthite+diopside. The experimentally determined parageneses confirm known petrographic occurrences.

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Für die Förderung dieser Arbeit danken wir der Deutschen Forschungsgemeinschaft vielmals. Der Dank von Choudhuri gilt dem Akademischen Auslandsamt der Universität Göttingen für ein Stipendium, das ihm den Abschluß seiner Studien an der Universität Göttingen ermöglichte.     

Symplectite breakdown of Ca-rich almandines in upper amphibolite-facies Skagit Gneiss,North Cascades,Washington

In upper amphibolite-facies Skagit Gneiss, certain rocks, usually carrying hornblende, display post-kinematic breakdown of almandine-rich garnets to symplectite consisting of plagioclase plus biotite and/or hornblende. Other almandine-rich garnets, widespread in hornblende-free schists and gneisses, remained stable to the end of the metamorphic cycle. Analysed garnets with symplectite coronas have 14 to 30 mol.% grandite. A set of garnets free of such coronas have 6 to 9% grandite and contain relatively more pyrope.In all symplectite-bearing rocks selected for mineral analyses, symplectite plagioclase is more calcic than the earlier main-fabric plagioclase. Fe and Mn are higher, and Mg and Ti lower, in symplectite than in main-fabric biotite and hornblende, except where late re-equilibration has been locally accomplished. Main-fabric biotite, hornblende, and cummingtonite partly preserve a record of original element partitioning between these phases and garnet, indicating that equilibrium was approached during the essentially syn-kinematic main stage of the metamorphism. Between the main-stage phases and their post-kinematic symplectite counterparts, equilibrium has rarely been attained, despite high T and presence of H₂O. Instead, there are sequences of arrested chemical exchanges and of highly incomplete attainments of successive equilibria. The principles revealed by this study are thought to be more broadly applicable to questions of equilibrium vs. disequilibrium in regional-metamorphic sequences.Combined with petrographic-petrologic data, the mineral analyses permit one to calculate model equations that quantitatively describe symplectite-forming reactions, including the amounts of materials added to and removed from garnets transformed into symplectites. Na, K, minor Ba, H₂O, and part of the Ti present were added; major portions of the garnets' Fe and of their subordinate Mn were removed. Transfers of Mg, Al, and Si during symplectite formation were minor, if any. Source and disposal of added and removed substances are discussed. The mass balance of symplectite formation ranges from significant losses in all hornblende-bearing reactions examined to minor gains in some only-biotite-bearing reactions. V is positive for all reactions examined and ranges from a few per cent for subordinate only-hornblende-bearing symplectites to somewhat over 50% for certain only-biotite-bearing symplectites; intermediate values are obtained for symplectites carrying both hornblende and biotite.     

Amphibolites with staurolite and other aluminous minerals: calculated mineral equilibria in NCFMASH

Amphibolite facies mafic rocks that consist mainly of hornblende, plagioclase and quartz may also contain combinations of chlorite, garnet, epidote, and, more unusually, staurolite, kyanite, sillimanite, cordierite and orthoamphiboles. Such assemblages can provide tighter constraints on the pressure and temperature evolution of metamorphic terranes than is usually possible from metabasites. Because of the high variance of most of the assemblages, the phase relationships in amphibolites depend on rock composition, in addition to pressure, temperature and fluid composition. The mineral equilibria in the Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCFMASH) model system demonstrate that aluminium content is critical in controlling the occurrence of assemblages involving hornblende with aluminous minerals such as sillimanite, kyanite, staurolite and cordierite. Except in aluminous compositions, these assemblages are restricted to higher pressures. The iron to magnesium ratio (X_Fe), and to a lesser extent, sodium to calcium ratio, have important roles in determining which (if any) of the aluminous minerals occur under particular pressure–temperature conditions. Where aluminous minerals occur in amphibolites, the P–T–X dependence of their phase relationships is remarkably similar to that in metapelitic rocks. The mineral assemblages of Fe‐rich amphibolites are typically dominated by garnet‐ and staurolite‐bearing assemblages, whereas their more Mg‐rich counterparts contain chlorite and cordierite. Assemblages involving staurolite–hornblende can occur over a wide range of pressures (4–10 kbar) at temperatures of 560–650 °C; however, except in the more aluminous, iron‐rich compositions, they occupy a narrow pressure–temperature window. Thus, although their occurrence in ‘typical’ amphibolites may be indicative of relatively high pressure metamorphism, in more aluminous compositions their interpretation is less straightforward.     

Distribution of elements in biotite-hornblende pairs and in an orthopyroxene-clinopyroxene pair from zoned plutons,northern Sierra Nevada,California

Distribution of major and minor elements has been determined for five hornblende-biotite pairs from hornblende-biotite quartz diorite and monzotonalite and for a clinopyroxene-orthopyroxene pair from pyroxene diorite collected from the border zones and centers of zoned plutons in the northern Sierra Nevada, California. The distribution coefficients K _d [Mg/Fe] for biotite/hornblende are of the same magnitude (0.61–0.67) for both the mafic border zone and the silicic center.For comparison, K _D [Mg/Fe] values for biotite/hornblende from plutonic rocks of the central Sierra Nevada and the southern California batholith were calculated from data published by others. Rocks of the oldest age group (ca. 150 m.y.) in the central Sierra Nevada have an average distribution coefficient, K _D , of 0.64, close to the average K _D in the study area, where K-Ar dates are 143 to 129 m.y. The intermediate age group has an average K _D =0.81, and the youngest group has K _D =0.77. K _D [Mg/Fe] for biotite/hornblende from the southern California batholith is 0.83, close to the average of the intermediate age group in the central Sierra Nevada. The calculated difference in pressure of crystallization between rocks of the Feather River area and the southern California batholith is 1 kb; the rocks of the Feather River area being crystallized at a higher pressure. This is in good agreement with the low-pressure contact metamorphism in the south (pyroxene hornfels facies), as compared with a medium-pressure metamorphism around the northern plutons, where andalusitesillimanite-cordierite and andalusite-staurolite subfacies of the amphibolite facies indicate pressures of about 4 kb.Trace elements Cr, V, Ni, Co, Ga are distributed equally between biotite and hornblende, whereas Ba and possibly Cu are concentrated in biotite and Sr and Sc and possibly Zr in hornblende.Publication authorized by the Director, U.S. Geological Survey.     

Experimental investigations of the role of H2O in calc-alkaline differentiation and subduction zone magmatism

Phase relations of natural aphyric high-alumina basalts and their intrusive equivalents were determined through rock-melting experiments at 2 kb, H₂O-saturated with fO₂ buffered at NNO. Experimental liquids are low-MgO high-alumina basalt or basaltic andesite, and most are saturated with olivine, calcic plagioclase, and either high-calcium pyroxene or hornblende (±magnetite). Cr-spinel or magnetite appear near the liquidus of wet high-alumina basalts because H₂O lowers the appearance temperature of crystalline silicates but has a lesser effect on spinel. As a consequence, experimental liquids follow calcalkaline differentiation trends. Hornblende stability is sensitive to the Na₂O content of the bulk composition as well as to H₂O content, with the result that hornblende can form as a near liquidus mineral in wet sodic basalts, but does not appear until liquids reach andesitic compositions in moderate Na₂O basalts. Therefore, the absence of hornblende in basalts with low-to-moderate Na₂O contents is not evidence that those basalts are nearly dry. Very calcic plagioclase (>An₉₀) forms from basaltic melts with high H₂O contents but cannot form from dry melts with normal are Na₂O and CaO abundances. The presence of anorthite-rich plagioclase in high-alumina basalts indicates high magmatic H₂O contents. In sum, moderate pressure H₂O-saturated phase relations show that magmatic H₂O leads to the early crystallization of spinel, produces calcic plagioclase, and reduces the total proportion of plagioclase in the crystallizing assemblage, thereby promoting the development of the calc-alkaline differentiation trend.     

Apatite and scapolite as petrogenetic indicators in granolites of Milford Sound,New Zealand

A network of 5 cm wide subplanar zones of garnet-granolite with accessory apatite as the dominant hydrous mineral, is overprinted on basic hornblende-granolites in an area of present and past tectonic uplift. Fracturing and the garnet forming reactions appear to be caused by destabilisation of hornblende, as a hydrous phase, through a critical drop in the PT ratio. Whereas the apatites from the hornblende-granolite have normal compositions, apatites from the garnet-granolite zones are among the most chlorine enriched hydroxyfluorapatites known. A later amphibolite facies event has depleted hydrous minerals of fluorine and chlorine, affecting hornblende more strongly than apatite. Based on literature data on F and Cl in coexisting minerals, original hornblende compositions are tentatively restored. It is then possible to derive all the chlorine of the garnet zone apatites from the original rock, with differential loss of H₂O and HF over chlorine during the dehydration reaction. Diffusion coefficients would have been larger for H₂O and HF than for the large chloride ion, and if the gas phase was in contact with even minor anatectic melts, activity gradients would also have been relatively small for chlorine. Low-Cl scapolite is present in quartz-free pegmatoid veins. Field evidence for a genetic tie between the garnet zones and these veins is inconclusive, but liberation of H₂O and HF under granolite facies conditions is likely to have caused limited fusion of the plagioclase. Since many garnet zones do not contain even small pegmatoid veins, melts related to their formation could have collected in veins only after initial diffusion or infiltration over a distance, and without leaving segregated mafic residues.     

Metamorphic Characteristics of Granulite Facies Rocks in the Laixi-Pingdu Area in Eastern Shandong Province

Abstract Granulite in eastern Shandong is mainly exposed in Laixi, Pingdu, Changyi and Anqiu, and the diagnostic mineral assemblage is Opx+Cpx+Hb+Pl ± Q ± Sea. The appearance of orthopyroxene and its coexistence with hornblende indicate that the reaction Hb+Q = Opx+Cpx+Pl+H₂O did not proceed completely and therefore these rocks belong to the amphibolite- granulite transition facies, i.e., belonging to hornblende-granulite subfacies. According to the data obtained from such geothermometers and geobarometers as Opx- Cpx, Opx- Hb, Cpx- Hb, Hb- PI, Sca- Pl and Fe- Ti oxides, it has been determined that the temperature of the main metamorphic stage was 720° – 810°C, the pressure 0.5 GPa and fo₂10^?15.5, showing a geothermal gradient of 41–46°C / km, and thus the rocks belong to “low-temperature” and low-pressure granulite facies.