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:     

Paragenesis of sodic pyroxene-bearing quartz schists: implications for the P-T history of the Sanbagawa belt

Sodic pyroxene (jadeite content X _jd=0.1–0.3) occurs locally as small inclusions within, albite porphyroblasts and in the matrix of hematite-bearing quartz schists in the Sanbagawa (Sambagawa) metamorphic belt, central Shikoku, Japan. The sodic, pyroxene-bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene+subcalcic (or sodic) amphibole+phengitic mica+albite+quartz+hematite+titanite±epidote. Spessartine-rich garnet occurs in Mn-rich samples. Sodic pyroxene in epidote-bearing samples tends to be poorer in acmite content (average X _Acm=0.26–0.50) than that in the epidote-free samples (X _Acm=0.45–0.47). X _Jd shows no systematic relationship to metamorphic grade, and is different among the three sampling regions [Saruta-gawa, Asemi-gawa and Bessi (Besshi)]. The average X _Jd of the Saruta-gawa samples (0.21–0.29) is higher than that of the Asemi-gawa (0.13–0.17) and Bessi (0.14–0.23). The P-T conditions of the Asemi-gawa and Bessi regions are estimated at 5.5–6.5 kbar, >360°C in the chlorite zone, 7–8.5 kbar, 440±15°C in the garnet zone and 8–9.5 kbar, 520±25°C in the albite-biotite zone. Metamorphic pressure of the Saruta-gawa region is systematically 1–1.5 kbar higher than that of the Asemi-gawa and Bessi regions, and materials of the Saruta-gawa region have been subducted to a level 3–5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the Asemi-gawa and Bessi regions. Pressure slightly increases toward the north (structurally high levels) through the Sanbagawa belt of central shikoku. Two types of zonal structure were observed in relatively coarse-grained sodic pyroxenes in the matrix. One type is characterized by increasing X _Jd from core to rim, the other type by decreasing X _Jd from core to rim. Both types of zoned pyroxenes show an increase in X _{Fe 2+}[=Fe²⁺/(Fe²⁺+Mg)] from core to rim. The first type of zoning was observed in a sample from the chlorite zone of lowest grade, whereas the latter occurs in the garnet and albite-biotite zones of higher grade. The contrast in zonal structure implies that dP/dT during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher-grade zones. The estimated dP/dT of the prograde stage of the chlorite zone is 3.2 kbar/100°C, and that of the garnet and albite-biotite zones is -1.8 to 0.9 kbar/100°C. The variation of dP/dT at shallow and deep levels of a subduction system probably reflects the difference of heating duration and/or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle.     

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.     

Amphibole composition as an indicator of subtle grade variation in epidote-glaucophane schists

Abstract The paragenetic relations of epidote-glaucophane schists are described in terms of the system Al₂O₃-Fe₂O₃-Fe₂O₃-MgO-CaO with excess of quartz, albite and epidote. If alkali-amphibole is free from Ca and Al^IV, its composition when associated with epidote is invariant, univariant or divariant at a given pressure and temperature on Miyashiro's (1957) diagram of alkali-amphibole solid solution if it is also associated, respectively, with three, two or one additional minerals in the system.
Using a group of epidote-glaucophane schists from the Kotu area of the Sanbagawa metamor-phic belt in Shlkoku, Japan (isophysical compositional),univariant boundary lines were determined for the assemblages that, in addition to the ubiquitous quartz + albite + phengitic mica, contain hematite + chlorite, garnet + chlorite and actinolite + chlorite, respectively. The slopes of the univariant boundary lines obtained from petrographical data are in good agreement with those calculated in a model system.
The positions of isophysical univariant boundary lines on the amphibole compositional diagram serve to distinguish the grade of metamorphism among the rocks of the same mineral facies. The hematite-chlorite univariant boundary line can be used to divide the zone of epidote-glaucophane schists of the Sanbagawa metamorphic belt into three, and the garnet-chlorite-paragonite invariant equilibrium can be used to divide the epidote zone of New Caledonia into three.     

The higher grade metamorphic zonation of the Sambagawa metamorphic belt in central Shikoku, Japan

The higher grade metamorphic zonation of the Sambagawa (= Sanbagawa) belt is established for the first time for the whole area of central Shikoku. As discontinuous reactions to define the isograd are absent, the metamorphic grade is primarily determined by the Mg-Fe partitioning between garnet and chlorite along representative traverses. However, for regional mapping, mineralogical features of the pelitic schists, such as using mineral assemblages of more than divariant equilibrium, the modal garnet to chlorite ratio, and the optical properties of chlorite, are employed as auxiliary criteria.
The presence of the highest grade mineral zone in the middle of the structural level is confirmed, but its spatial distribution is far more complex than hitherto accepted. Thermal axes are now confirmed at three different structural levels. A model is presented in which the stacking of thrust sheets of different grade took place while metamorphic reactions were in progress. Thermal readjustment brought a continuous metamorphic temperature gradient across and within the thrust sheets. Tectonic blocks of metagabbro and ultramafic rock were emplaced synchronously with thinning and subsequently also re-equilibrated. Local anomalies of metamorphic grade, represented by mixing of schists of different metamorphic grade, exist, but they are due to a later stage event.     

Metamorphism of metapelites: calculations of equilibrium assemblages and numerical simulations of the crystallization of garnet

Abstract This work uses a simplified model of equilibrium to predict the assemblage sequence and compositional zoning in garnet that should result from prograde metamorphism of common bulk compositions of pelitic rocks. An internally-consistent set of model thermodynamic data are derived for natural mineral compositions from natural assemblages. Equilibrium assemblages can be calculated for pelitic compositions with excess quartz and either muscovite or K-feldspar at any pressure and water pressure. The compositions and abundances of phases in equilibrium assemblages can be calculated where the elements Mg, Fe and Mn are exchanged among phases. The prograde metamorphic assemblage sequences and the effects of pressure on assemblages, predicted by the simulation method presented here, are similar enough to natural observations to suggest that the simulations can be used to analyse natural equilibrium and growth processes. The calculated phase diagrams at moderate and high crustal pressures explain the mineral assemblage sequence produced by prograde metamorphism in common pelitic compositions. Garnet appears by continuous reaction of biotite and chlorite as the garnet-biotite-chlorite divariant field migrates toward higher Mg/Fe ratios over the bulk composition. Staurolite appears in common bulk compositions when garnet and chlorite become incompatible. An aluminum silicate phase can appear when staurolite and chlorite react. Staurolite breaks down at an extremum point to produce garnet. Continuous reaction of biotite and sillimanite causes growth of abundant garnet. The reaction sequence involving garnet, staurolite and aluminum silicates is probably different at low pressure, but the main reason that staurolite and garnet are rare is the restricted compositional range over which their assemblages exist. Andalusite appears by the divariant reaction of chlorite and cordierite appears at low temperature in low pressure assemblages for common bulk compositions by the extremumpoint breakdown reaction of chlorite. Compositional zoning of garnet and the systematic variation of biotite composition in metamorphic sequences indicate that garnet is probably fractionated during growth. Fractionation of garnet causes garnet-consuming, univariant reactions to become multivariant. The metastable persistence of garnet should reduce the abundance and stability range of staurolite. Fractionation of even small quantities of garnet should deplete the equilibrating bulk composition of Mn, but have little effect otherwise. The simulations show that the prograde assemblage sequence in pelitic rocks can be complex in detail, with some assemblages lasting over temperature intervals of only a few degrees. The major prograde reactions that release water are the breakdown of chlorite to form garnet at low grade and the breakdown of muscovite at high grade. The volume of water released by formation of garnet at high grade is also important. These reactions have the capacity to buffer water pressure. The density of anhydrous pelitic rock increases markedly when chlorite breaks down and by the continuous reaction forming garnet at high grade. The heat content is controlled principally by heat capacity and continuous reactions. Discontinuous reactions have little thermal buffering capacity. Simulations of garnet fractionation show that commonly-observed garnet zoning profiles can be formed by garnet growth in the assemblage garnet-biotite-chlorite in common bulk compositions. A reversal of Fe-zoning in garnet can occur when garnet resumes growth above staurolite grade in the assemblage garnetbiotite-sillimanite. Discontinuities in zoning profiles can be caused only by disequilibrium. The disequilibrium can be due to either metastable persistence during a hiatus in growth or to growth by irreversible reaction. Because the appearance of garnet is controlled by a continuous rather than a discontinuous reaction, the appearance of garnet is very sensitive to bulk composition. The early development of garnet is also sensitive to the pressure and water pressure of metamorphism. As a consequence the first garnet isograd is of limited thermometric value. Metastable persistence of kyanite and manite at high grades could reduce the abundance of garnet and allow biotite to persist. Metastable persistence would also limit the of cordierite formation.     

An Exotic Eclogite/Blueschist Slice in a Barrovian Style Metamorphic Terrain, Alanya Nappes, Southern Turkey

A 600 m thick, more than 40 km long slice of eclogite faciesrocks, called the Sug?z? Nappe, occurs in the Alanya area, southernTurkey, sandwiched between two other crystalline nappes whichdo not show HP/LT metamorphism. All three nappcs were affectedby a later Barrovian-type metamorphism and penetrative deformationwhich welded the nappes into a single tectonic unit. The SugOzuNappe consists predominantly of garnet-mica schists with lensesof eclogite and blueschist metabasites. The mineral assemblagein the eclogites, garnet+omphacite+glaucophane+paragonite+quartz+phengite+rutile, is estimated to have formed at 13?5?1?5 kb and510?25?C. The eclogite assemblage is variably overprinted bya later Barrovian metamorphism with the development of barroisite,chlorite, and albite. The extent of the Barrovian overprintwas controlled by the supply of fluid to the rocks. The Barrovian metamorphism increases in grade downwards in thestructural sequence; biotite and garnet isograds are mappedin the lowermost Mahmutlar Nappe, made up mostly of metapelitesand metapsammites. The metapelites in the garnet zone consistof garnet+biotite+chlorite+muscovite+albite+oligoclase+quartz+ilmenite;metamorphic conditions are estimated as 6?5?1?0 kb and 469?13?C. The HP/LT rocks of the Sug?z? Nappe underwent a cooling of about100?C during a dry uplift from a depth of about 48 km to 21km where they were intercalated with the other nappes, and affectedby a Barrovian metamorphism caused by the incoming fluids. Thecase of the Alanya Nappcs illustrates that the Barrovian overprintobserved in many eclogites and blueschists may not be due toincreasing temperature during uplift, but simply due to theintroduction of a fluid phase during part of the uplift P-Tpath.     

内蒙古头道桥地区蓝闪石片岩和相关变质岩石岩石学特征及其构造意义

内蒙古头道桥地区出露了一套经高压变质形成的岩石组合。本次研究通过岩相学和矿物化学分析,根据矿物组合的不同,识别出蓝片岩、绿片岩两种不同类型的岩石类型。其中,蓝片岩的矿物组合为角闪石（蓝闪石、蓝透闪石）+绿帘石+钠长石+绿泥石+石英+赤铁矿±多硅白云母±方解石±榍石;绿片岩的矿物组合为绿泥石+钠长石+石英±绿帘石±角闪石（阳起石、镁角闪石、蓝透闪石、冻蓝闪石等）±多硅白云母±赤铁矿。确定了蓝片岩的峰期变质级别为绿帘-蓝闪片岩相,峰期变质温度为400~600℃,压力为1.2~1.4 GPa。绿片岩的峰期变质级别为绿帘-角闪岩相。结合前人研究成果,认为蓝片岩和绿片岩的形成与额尔古纳地块和兴安地块的碰撞拼合有关。     

Phase equilibria and metamorphic evolution of glaucophane‐bearing UHP eclogites from the Western Dabieshan Terrane,Central China

Glaucophane‐bearing ultrahigh pressure (UHP) eclogites from the western Dabieshan terrane consist of garnet, omphacite, glaucophane, kyanite, epidote, phengite, quartz/coesite and rutile with or without talc and paragonite. Some garnet porphyroblasts exhibit a core–mantle zoning profile with slight increase in pyrope content and minor or slight decrease in grossular and a mantle–rim zoning profile characterized by a pronounced increase in pyrope and rapid decrease in grossular. Omphacite is usually zoned with a core–rim decrease in j(o) [=Na/(Ca + Na)]. Glaucophane occurs as porphyroblasts in some samples and contains inclusions of garnet, omphacite and epidote. Pseudosections calculated in the NCKMnFMASHO system for five representative samples, combined with petrographic observations suggest that the UHP eclogites record four stages of metamorphism. (i) The prograde stage, on the basis of modelling of garnet zoning and inclusions in garnet, involves P–T vectors dominated by heating with a slight increase in pressure, suggesting an early slow subduction process, and P–T vectors dominated by a pronounced increase in pressure and slight heating, pointing to a late fast subduction process. The prograde metamorphism is predominated by dehydration of glaucophane and, to a lesser extent, chlorite, epidote and paragonite, releasing ～27 wt% water that was bound in the hydrous minerals. (ii) The peak stage is represented by garnet rim compositions with maximum pyrope and minimum grossular contents, and P–T conditions of 28.2–31.8 kbar and 605–613 °C, with the modelled peak‐stage mineral assemblage mostly involving garnet + omphacite + lawsonite + talc + phengite + coesite ± glaucophane ± kyanite. (iii) The early decompression stage is characterized by dehydration of lawsonite, releasing ～70–90 wt% water bound in the peak mineral assemblages, which results in the growth of glaucophane, j(o) decrease in omphacite and formation of epidote. And, (iv) The late retrograde stage is characterized by the mineral assemblage of hornblendic amphibole + epidote + albite/oligoclase + quartz developed in the margins or strongly foliated domains of eclogite blocks due to fluid infiltration at P–T conditions of 5–10 kbar and 500–580 °C. The proposed metamorphic stages for the UHP eclogites are consistent with the petrological observations, but considerably different from those presented in the previous studies.     

Prehnite–pumpellyite to greenschist facies transition, Smartville Complex, near Auburn, California

The Smartville Complex is a late Jurassic, rifted volcanic arc in the northern Sierra Nevada, California. Near Auburn, California, it consists of a lower volcanic unit, dominated by basaltic flows, and an upper volcanic unit of andesitic volcaniclastic rocks, both of which have been intruded by dykes and irregular bodies of diabase. These rocks contain relict igneous minerals, and the metamorphic minerals albite, chlorite, quartz, pumpellyite, prehnite, epidote, amphibole, titanite, garnet, biotite, K-feldspar, white mica, calcite, and sulphide and oxide minerals.
Prehnite–pumpellyite (PrP), prehnite–actinolite (PrA), and greenschist (GS) zones have been identified. The pumpellyite-out isograd separates the PrP and PrA zones, and the prehnite-out isograd separates the PrA and GS zones. The minerals Ab + Qtz + Mt + Tn are common to most assemblages in all three zones. The MgO/(MgO + FeO) ratio of the effective bulk composition has an important and systematic effect on the observed mineral assemblages in the PrP zone. Prehnite-bearing assemblages contain the additional minerals, Pmp + Amp + Ep + Chl in MgO-rich rocks, and either Pmp + Ep + Chl or Amp + Ep + Chl in less magnesian rocks. Subcalcic to calcic amphibole is common in the PrP zone. The mineral assemblage Prh + Act + Ep + Chl, without Pmp, characterizes the PrA zone, and the mineral assemblage Act + Ep + Chl, without Prh or Pmp, characterizes the GS zone. The disappearance of pumpellyite and prehnite occurred by continuous reactions.
The sequence of mineral assemblages was produced by burial metamorphism at P–T conditions of 300° 50°C at approximately 2.5 ± 0.5 kbar. During metamorphism, the composition of the fluid phase was nearly 100% H₂O and the oxygen fugacity was between the hematite–magnetite and quartz–fayalite–magnetite buffers.     

Occurrence of calcite in Sanbagawa pelitic schists: implications for the formation of garnet, rutile, oligoclase, biotite and hornblende

The frequency of occurrence of minerals in 1876 samples of Sanbagawa pelitic schist in central Shikoku is summarized on the basis of microscopic observation accompanied, in part, by use of an electron microprobe. All samples contain quartz, plagioclase, phengite, chlorite and graphite. More than 90% of samples contain clinozoisite, titanite and apatite. Garnet is present in 95% of samples from the garnet zone, and biotite is present in 64% of samples from the albite‐biotite zone. Calcite is found in about 40% of samples of the pelitic schist collected from outcrop, but occurs in 95% of the pelitic schist from drill cores. Calcite was apparently ubiquitous in the pelitic schist during the Sanbagawa metamorphism, but must have been dissolved recently by the action of surface or ground water. The mineral assemblages of the Sanbagawa pelitic schist have to be analyzed in the system with excess calcite, quartz, albite (or oligoclase), clinozoisite, graphite and fluid that is composed mainly of H₂O, CO₂ and CH₄. In the presence of calcite, reactions that produce garnet, rutile, oligoclase, biotite and hornblende, some of which define isograds of the metamorphic belt, should be written as mixed volatile equilibria that tend to take place at lower temperature than the dehydration reactions that have been proposed. The presence of calcite in pelitic schist suggests that fluid composition is a variable as important in determining mineral assemblages as pressure and temperature. Thus Ca‐bearing phases must be taken into account to analyze the phase relations of calcite‐bearing pelitic schist, even if CaO content of Sanbagawa pelitic schist is low. As calcite is a common phase, the mineral assemblages of the biotite zone pelitic schist may contravene the mineralogical phase rule and warrant further study.     

Garnet and staurolite producing reactions in a chlorite-chloritoid schist

During prograde metamorphism garnet and, in some higher grade samples, staurolite were produced in a chlorite-chloritoid schist, part of the Precambrian Z to Cambrian Hoosac Formation near Jamaica, VT. Garnet grew during two prograde events separated by a retrogression. This sequence resulted in distinctive inclusion textures and zoning anomalies in garnet produced by diffusive alteration. Textures, reaction space analysis, and mineral compositional variations constrain the possible sequence of reactions in these rocks. Below the staurolite isograd, and to some unknown extent above it, garnet grew by the reaction chloritoid+chlorite+quartz→garnet+H₂O. With increasing grade the mineral compositions are displaced towards lower Mn/Fe and higher Mg/Fe ratios. The data are compatible with equilibrium with respect to exchange reactions for the matrix assemblages on a thin section scale and with minerals having closely followed equilibrium paths during reaction. The staurolite isograd coincides with the reaction chloritoid+quartz→garnet+staurolite+chlorite+H₂O. This reaction is continuous and trivariant with ZnO becoming an additional component concentrated in staurolite. During this reaction both the Mn/Fe and Mg/Fe ratios of the phases appear to have decreased. This new chemical trend is recorded by garnet zoning profiles and is compatible with trends predicted from phase diagrams. Thus there are two distinct types of garnet zoning reversals in these samples. One is near the textural unconformity and is best explained by diffusive alteration during partial resorption of first stage garnet. The other occurs near the outer rim of garnet in staurolite zone samples and marks the onset of a new prograde garnet producing reaction.     

Petrology of coesite-bearing eclogite from Habutengsu Valley, western Tianshan, NW China and its tectonometamorphic implication

Coesite inclusions in garnet have been found in eclogite boudins enclosed in coesite‐bearing garnet micaschist in the Habutengsu Valley, Chinese western Tianshan, which are distinguished from their retrograde quartz by means of optical characteristics, CL imaging and Raman spectrum. The coesite‐bearing eclogite is mainly composed of porphyroblastic garnet, omphacite, paragonite, glaucophane and barroisite, minor amounts of rutile and dotted (or banded) graphite. In addition to coesite and quartz, the zoned porphyroblastic garnet contains inclusions of omphacite, Na‐Ca amphibole, calcite, albite, chlorite, rutile, ilmenite and graphite. Multi‐phase inclusions (e.g. Czo + Pg ± Qtz, Grt II + Qtz and Chl + Pg) can be interpreted as breakdown products of former lawsonite and possibly chloritoid. Coesite occurs scattered within a compositionally homogenous but narrow domain of garnet (outer core), indicative of equilibrium at the UHP stage. The estimate by garnet‐clinopyroxene thermometry yields peak temperatures of 420–520 °C at 2.7 GPa. Phase equilibrium calculations further constrain the P–T conditions for the UHP mineral assemblage Grt + Omp + Lws + Gln + Coe to 2.4–2.7 GPa and 470–510 °C. Modelled modal abundances of major minerals along a 5 °C km^?1 geothermal gradient suggests two critical dehydration processes at ～430 and ～510 °C respectively. Computed garnet composition patterns are in good agreement with measured core‐rim profiles. The petrological study of coesite‐bearing eclogite in this paper provides insight into the metamorphic evolution in a cold subduction zone. Together with other reported localities of UHP rocks from the entire orogen of Chinese western Tianshan, it is concluded that the regional extent of UHP‐LT metamorphism in Chinese western Tianshan is extensive and considerably larger than previously thought, although intensive retrogression has erased UHP‐LT assemblages at most localities.     

Pumpellyite–actinolite facies of the Sanbagawa metamorphism

The pumpellyite–actinolite facies proposed by Hashimoto is defined by the common occurrence of the pumpellyite–actinolite assemblage in basic schists. It can help characterize the paragenesis of basic and intermediate bulk compositions, which are common constituents of various low-grade metamorphic areas. The dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite–actinolite and prehnite–pumpellyite facies in P–T space. In the Sanbagawa belt in Japan, the mineral parageneses of hematite-bearing and -free basic schists, as well as pelitic schists have been well documented. The higher temperature limit of this facies is defined by the disappearance of the pumpellyite+epidote+actinolite+chlorite assemblage in hematite-free basic schists with X_Fe3+ of epidote around 0.20–0.25 and the appearance of epidote+actinolite+chlorite assemblage with X^Ep_Fe3+≤0.20. In hematite-bearing basic schists, there is a continuous change of paragenesis to higher grade, epidote–glaucophane or epidote–blueschist facies. In pelitic schists, the albite+lawsonite+chlorite assemblage does occur but only rarely, and its assemblage cannot be used to determine the regional thermal structure. The lower temperature equivalence of the pumpellyite–actinolite assemblage is not observed in the field. The Mikabu Greenstone complex and the northern margin of the Chichibu complex, which are located to the south of the Sanbagawa belt, are characterized by clinopyroxene+chlorite or lawsonite+actinolite assemblages, which are lower temperature assemblages than the pumpellyite+actinolite assemblage. These three metamorphic complexes belong to the same subduction-metamorphic complex. The pumpellyite–actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes     

甘肃北山榴辉岩矿物特征及温压条件

甘肃北山榴辉岩中矿物组合形成于四个不同阶段：（１）以柯石英为代表的早期超高压阶段,（２）以绿辉石＋石榴石平衡共生组合为代表的主期高压阶段,（３）以单斜辉石＋角门石＋斜长石交生组合为代表的晚期较高压退变阶段,（４）以绿泥石＋绿帘石＋科长石为代表的后期叠加阶段。前三个阶段反映了相辉岩相变质作用的主要过程。后一阶段,反映了榴辉岩在地壳浅部层次绿片岩相环境下遭受轻微的叠加改造。     

Petrology of the Strathgordon area,Western Tasmania: Si4+‐content of phengite mica as monitor of metamorphic grade

Important mineral assemblages of metapelite and quartzite of the Strathgordon area are phengite + chlorite + tourmaline + quartz and phengite + garnet + chlorite + tourmaline + quartz. Over a limited area the Si⁴⁺‐content of phengite does not vary significantly and is considered to depend only on the PT regime. The Si⁴⁺‐content of phengite coexisting with almandine‐grossular‐spessartine garnet and tourmaline indicate that the maximum metamorphism of the area occurred at 400 ± 50°C and 3 ± 1 kb.     

浙西南八都杂岩早中生代泥质麻粒岩变质作用及构造意义

遂昌-大柘泥质麻粒岩出露于华夏地块东北部的浙西南八都杂岩中,该岩石保留了典型的减压反应结构.但其变质演化特点、变质作用时代及构造意义目前尚不明确.通过系统的岩相学、矿物化学和同位素年代学分析,结果表明遂昌-大柘泥质麻粒岩记录了4个阶段的变质矿物组合,其中早期进变质阶段M₁的矿物组合为石榴石+黑云母+石英;压力峰期变质阶段M₂的矿物组合为石榴石+铝绿泥石+金红石+蓝晶石+刚玉+黑云母+石英±十字石,该矿物组合可能预示着岩石曾经历了超高压变质作用过程;峰期变质阶段M₃的矿物组合为石榴石+黑云母+夕线石+石英±钾长石±斜长石±钛铁矿;峰后近等温降压M_4-1阶段的矿物组合为石榴石+黑云母+夕线石+堇青石+石英+钛铁矿±尖晶石±斜长石±钾长石;M_4-2阶段的矿物组合为石榴石+堇青石+夕线石+斜长石+黑云母+石英±钾长石.相平衡模拟结合传统地质温压计限定其峰期变质阶段的温压条件为T=780~810 ℃、P=8.0~9.2 kbar;峰期后近等温降压的M_4-1阶段的温压条件为T=780~860 ℃和P=5.7~6.0 kbar,M_4-2阶段的温压条件为T=~700 ℃和P=~4.4 kbar,具有典型的顺时针近等温减压型P-T轨迹特征.LA-ICP-MS U-Pb定年结果表明其麻粒岩相变质作用时代为233.5~238.9 Ma.变质作用历史说明浙西南地体可能卷入了古特提斯洋域内印支-华南-华北板块之间的俯冲-碰撞过程,并经历了早中生代的麻粒岩相变质作用后快速折返至地表.      

Prograde pressure–temperature paths in the pelitic schists of the Sambagawa metamorphic belt, SW Japan

Prograde P–T paths recorded by the chemistry of minerals of subduction‐related metamorphic rocks allow inference of tectonic processes at convergent margins. This paper elucidates the changing P–T conditions during garnet growth in pelitic schists of the Sambagawa metamorphic belt, which is a subduction related metamorphic belt in the south‐western part of Japan. Three types of chemical zoning patterns were observed in garnet: Ca‐rich normal zoning, Ca‐poor normal zoning and intrasectoral zoning. Petrological studies indicate that normally‐zoned garnet grains grew keeping surface chemical equilibrium with the matrix, in the stable mineral assemblage of garnet + muscovite + chlorite + plagioclase + paragonite + epidote + quartz ± biotite. Pressure and temperature histories were inversely calculated from the normally‐zoned garnet in this assemblage, applying the differential thermodynamic method (Gibbs' method) with the latest available thermodynamic data set for minerals. The deduced P–T paths indicate slight increase of temperature with increasing pressure throughout garnet growth, having an average dP/dT of 0.4–0.5 GPa/100 °C. Garnet started growing at around 470 °C and 0.6 GPa to achieve the thermal and baric peak condition near the rim (520 °C, 0.9 GPa). The high‐temperature condition at relatively low pressure (for subduction related metamorphism) suggests that heating occurred before or simultaneously with subduction.     

The Garnet+Hornblende Isograd in Calcic Schists from an Andalusite-Type Regional Metamorphic Terrain, Panamint Mountains, California

Calcic schists in the andalusite-type regional metamorphic terrainin the Panamint Mountains, California, contain the low-varianceassemblage quartz+epidote+muscovite+biotite+calcic amphibole+chlorite+plagioclase+spheneat low grade. Near the sillimanite isograd, chlorite in thisassemblage is replaced by garnet. The low variance in many calcicschists allows the determination of the nature of the reactionthat resulted in the coexistence of garnet+hornblende. A graphicalanalysis of the mineral assemblages shows that the reactioncan not be of the form biotite+epidote+chlorite+plagioclase+quartz=garnet+hornblende+muscovite+sphene+H₂Obecause garnet+chlorite never coexisted during metamorphismand the chlorite-bearing and garnet-bearing phase volumes donot overlap. The compositions of the minerals show that withincreasing grade amphibole changed from actinolite to pargasitichornblende with no apparent miscibility gap, the partitioningof Fe and Mg between chlorite and hornblende changed from K_D(Mg/Fe, chl&amp) < 1 to K_D > 1, the partitioning betweenbiotite and hornblende changed from K_D (Mg/Fe, bio/amp) <1 in chlorite-zone samples to K_D > 1 in garnet + hornblende-zonesamples, and the transition to the garnet-bearing assemblageoccurred when the composition of plagioclase was between An₅₅and An₈₀. Both the graphical analysis and an analytical analysisof the compositions of the minerals using simplified componentsderived from the natural mineral compositions indicate thatat the garnet+hornblende isograd the composition of hornblendewas colinear with that of plagioclase and biotite, as projectedfrom quartz, epidote, muscovite, and H₂O. During progressivemetamorphism, chlorite+biotite+epidote+quartz continuously brokedown to form hornblende+muscovite+sphene until the degeneracywas reached. At that point, tie lines from hornblende couldextend to garnet without allowing garnet to coexist with chlorite.Thus, the garnet+hornblende isograd was established throughcontinuous reactions within the chlorite-grade assemblage ratherthan through a discontinuous reaction. In this type of isograd,the low-grade diagnostic assemblage occurs only in Mg-rich rocks;whereas the high-grade assemblage occurs only in Fe-rich rocks.This relation accounts for the restricted occurrence of garnet+hornblendeassemblage in low-pressure terrains. In Barrovian terrains,garnet+chlorite commonly occurs, and the first appearana ofgarnet+hornblende can simply result from the continuous shiftof the garnet+chlorite tie line to Mg-rich compositions.     

High-and Ultrahigh-pressure Metamorphism and Retrogressive Textures of Gneiss in the Donghai Area--Evidence from gneisses in drillhole ZK2304

In the gneisses from the drillhole ZK2304 of the Donghai area, there have been preserved high- and ultrahigh-pressure metamorphic mineral assemblages, a series of complicated retrogressive textures and relevant metamorphic reactions. In addition to garnet, jadeititic-clinopyroxene and rutile, other peak stage (M2) minerals in some gneisses include phengite, aragonite and coesite or quartz pseudomorphs after coesite. The typical peak-stage mineral assemblages in gneisses are characterized by garnet + jadeitic-clinopyroxene + rutile + coesite, garnet + jadeitic-clinopyroxene + phengite + rutile ± coesite and garnet + jadeitic-clinopyroxene + aragonite + rutile ± coesite. The grossular content (Gro) in garnet is high and may reach 50.1 mol%. The SiO2 content of phengite ranges from 54.37% to 54.84% with 3.54- 3.57 p.f.u. Quartz pseudomorphs after coesite occur as inclusions in garnet.The gneisses of the Donghai area have been subjected to multistage recrystallization and exhibit a closewise P-T evolutional path characterized by the near-isothermal decompression. The inclusion assemblage (Hb+Ep+Bi+Pl+Qz) within garnet and other minerals has recorded a pre-peak stage (M1) epidote amphibole facies metamorphic event. High- and ultrahigh-pressure peak metamorphism (M2) took place at T=750- 860° C and P>2.7 GPa. The symplectitic assemblages after garnet, jadeitic-clinopyroxene and rutile imply a near-isothermal decompression metamorphism (M3, M4) during the rapid exhumation. Several lines of evidence of petrography and metamorphic reactions indicate that both gneisses and eclogites have experienced ultrahigh-pressure metamorphism in the Donghai area. This research may be of great significance for an in-depth study of the metamorphism and tectonic evolution in the Su-Lu ultrahigh-pressure metamorphic belt.