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1.
Approximate mixing properties of the end-member components of the quarternary garnet solid solution, (Fe,Mg,Ca,Mn)3Al2Si3O12, have been derived through theoretical analysis of observational data, combined with certain experimental results and crystal chemical considerations. The results suggest that the mixing of pyrope with grossularite, spessartite, and almandine would involve significant positive excess free energies of mixing leading to the critical mixing temperatures of 694±55, 535±140, and 479±63 °C respectively. Spessartite would mix with almandine nearly ideally, and with grossularite with small positive deviation from ideality. The quarternary solution reduces essentially to a ternary mixture of pyrope, grossularite, and almandine + spessartite. The solid solubility relation, and tie line coordinates in this ternary system has been calculated as a function of temperature; the solid solution is found to be intrinsically stable for practically all ternary compositions at 600 °C.  相似文献   

2.
Three garnet-biotite pairs and eleven garnet-cordierite-biotite triplets from the Steinach aureole (Oberpfalz, North-East Bavaria) were analyzed using an electron probe microanalyzer.The regional metamorphic muscovite-biotite schists contain garnets strongly zoned with Mn-Ca-rich centers and Fe-rich edges, the average composition being almandine 67 — spessartine 4 — pyrope 4 — grossular (+andradite) 25.The first contact garnet that is formed in mica schists of the outermost part of the aureole is small, virtually unzoned, and has an average composition of almandine 52 — spessartine 37 — pyrope 8 — grossular (+andradite) 3. With increasing metamorphic grade, there is a consistent trend to form garnets richer in Fe ending up with a composition almandine 84.5 — spessartine 5.5 — pyrope 7.5 — grossular (+andradite) 2.5. This trend is accompanied by a general increase in grain size and modal amount of garnet. Associated cordierites and biotites also become richer in Fe with increasing grade. While the garnets in the highest grade sillimanite hornfelses are poorly zoned, the transitional andalusite-sillimanite hornfelses contain garnets with distinct but variable zonation profiles.These facts can possibly be explained by the time-temperature relationships in the thermal aureole. In a phase diagram such as the Al-Fe-Mg-Mn tetrahedron, the limiting mineral compositions of a four-phase volume or a three-phase triangle are fixed by T and P (the latter remaining effectively constant within a thermal aureole). Thus, in garnet-cordierite-biotite assemblages, garnet zonation should be controlled by temperature variation rather than by a non-equilibrium depletion process. Taking into account the experimental data of Dahl (1968), a zoned garnet from a transitional andalusite-sillimanite hornfels would reflect a temperature increase of about 40° C during its growth. A hypothetical P-X diagram is proposed to show semi-quantitatively the compositional variation of garnet-cordierite pairs with varying pressures (T constant).  相似文献   

3.
Reaction of zoning of garnet   总被引：1，自引：0，他引：1  
Compositional zoning of garnet in metamorphic or igneous rocks preserves evidence of the equilibration history of the sample and can be interpreted in terms of a growth-fractionation, diffusion-exchange, or diffusion-reaction model. Diffusion zoning is usually assumed to result from exchange reactions between garnet and other phases as the partitioning coefficient varies in response to changing environmental conditions, primarily temperature. However, in many natural environments where garnet grew originally in divariant equilibrium with other phases, changing conditions can promote continuous or “divariant” reactions and consequent compositional shifts of phases that can be much greater in some systems showing these reactions than those related to the small changes of partitioning. Diffusional zoning related to overstepping of these continuous reactions must be related to incongruent reaction and necessitates formulation of a kinetic diffusion-reaction model involving moving phase boundaries as well as solid-state diffusion. Three samples containing zoned garnets from the metamorphic aureole around the Ronda ultramafic intrusion in southern Spain are used to illustrate two possible models of diffusion-reaction processes. The examples are particularly informative because the reactions are demonstrably irreversible and evidence of the reaction system is preserved. Partitioning data indicates that compositions of product phases are not in equilibrium with the original garnet and do not vary with extent of reaction; therefore, exchange reactions with garnet were not possible and garnet changed composition only by incongruent reaction. After a small amount of reaction, Mg/Fe of the rim composition approaches a value apparently in equilibrium with product phases, but the garnets are zoned inward to the original garnet composition preserved in the interior. Grossularite content is approximately constant and spessartite content variable but small, thus, the rim composition of pyrope or almandine is assumed to be fixed by the external reaction process and is taken as a boundary condition in the following models. The zoning profile of pyrope or almandine component between the fixed rim and core compositions (assumed to extend to ∞) is described in semiinfinite, half-space models appropriate for large garnets with narrow rims. The first model corresponds to a reaction system in which all garnet compositions are metastable (case 1) and zoning depends on the independent variables of the diffusion constant, velocity of the interface between garnet and matrix, and time. The second model, corresponding to systems in which the initial garnet composition is metastable but an equilibrium composition is stable (case 2), depends on the independent variables diffusion constant, time, and a function of reaction compositions. In case 1 the consumption velocity is assumed constant and a steady state zoning profile is reached at large time, whereas, in case 2, the velocity decreases with the concentration gradient and steady state is not possible. The models were tested using a reaction time estimated from cooling models of the aureole, mass of garnet consumed, determined petrographically, and phase compositions. The two cases are somewhat independent in that different parameters are independent variables. The estimate of the diffusion constant of 10?18±2 cm2/sec (assumed to be a mutual or binary coefficient for almandine and pyrope) is considered reasonable for the temperature range of reaction (probably 600–900° C), and the two models are consistent considering the probable error and possible real temperature differences. It is obvious that details of the metamorphic reaction system must be known to successfully apply diffusion models. Kinetic models, involving consumption or growth of the phase as well as diffusion are probably necessary when dealing with natural rocks. Several possible and interesting complications, such as cross coupling between components, can be investigated if more data were available. Experimental determination of diffusion constants allow natural reaction rates to be estimated by this method. Diffusion zoning is an important consideration that could increase the efficiency of experimentation with chemically recalcitrant phases.  相似文献   

4.
湖北省宜昌市彭家河石榴石矿床地质特征及成因分析   总被引：1，自引：1，他引：0  
运用矿物学、岩石学的方法对宜昌黄陵地区彭家河石榴石矿床中含矿层的矿物组合、化学成分、变质相及原岩特征进行分析与研究,结果表明彭家河石榴石矿床的含矿层位为水月寺群的周家河组,石榴石主要赋存于含(石墨)矽线石榴黑云斜长片麻岩及石榴黑云斜长片麻岩中,前者石榴石含量较高10%～50%,后者含量稍低12%～36%,X衍射及电子探针分析矿床中石榴石以铁铝榴石为主,其次是镁铝榴石,少量钙铝榴石.运用变质反应平衡温压方程计算出成矿温压条件为:P=600 MPa～800 MPa,T=650℃～750℃;变质相为高角闪岩相,高角闪岩相的富铁铝质沉积变质岩系--孔兹岩系是寻找此类石榴石矿床的重要找矿标志.  相似文献   

5.
A Cretaceous to low-Tertiary sequence of interbedded pelites, cherts, basic and acidic volcanics and calcareous lenses has been metamorphosed by an Oligocene event. A complete intergradational metamorphic sequence is exposed in the Ouégoa destrict. The following metamorphic zones have been recognised: — (1) lowest-grade rocks consisting of quartz-sericite phyllites and pumpellyite metabasalts (2) lawsonite zone, characterized by the association of lawsonite and albite (3) epidote zone, characterised by epidote-omphacite-sodic hornblendealmandine bearing metabasalts and epidote-albite-almandine-glaucophane bearing metasediments; calcareous metasediments may also carry omphacite. The epidote and lawsonite zones are separated by a narrow belt of transitional rocks. Garnets occur in metasediments throughout the lawsonite zone as rare tiny crystals (<0.03 mm diam.). Garnets first appear in metabasalts in lawsonite-epidote transitional rocks. Garnets are widespread and abundant in epidote-zone metasediments and metabasalts. 45 garnets from rocks representative of all lithologies and metamorphic grades have been analysed with an electron-probe microanalyser. The garnets were consistently zoned. Garnets in lawsonite and low-grade epidote zones show a “bell-type” zoning with cores enriched in Mn relative to Fe and rims enriched in Fe, Mg and frequently Ca. Garnets from high-grade epidote-zone metapelites and metabasalts show, in addition, a shallow oscillatory zoning with complimentary variations in Mn and Fe equivalent to 5 mole- % spessartine and almandine. The Fe-for-Mn substitutional zoning, believed to be caused by a diffusion/saturation effect similar to that of the Rayleigh fractional model (Hollister, 1966), appears to have had superimposed on it the effects of parent-rock chemistry and metamorphic grade which control in a complex manner the composition of the cores and the rims of garnets. Garnets from different rock types and metamorphic grade are compositionally distinct. Garnets from lawsonite-zone rocks, irrespective of parent-rock chemistry appear to be spessartine. Garnets from epidote-zone metaigneous rocks and most metasediments are almandine. Garnets from epidote-zone metasediments with bulk-rock compositions which are manganiferous, or have high oxidation ratios, or both, may be spessartine-rich. Garnets from metabasalts are consistently more pyropic in both core and rim compositions than garnets from pelitic metasediments; the pyrope content of cores and rims of garnets from equivalent rock types and mineral assemblages increases with increasing metamorphic grade. Cores of garnets from epidote-zone pelites are richer in grossular than garnets from lower-grade pelites. The reaction which brings almandine garnet into Ouégoa district blueschist assemblages simultaneously with the replacement of lawsonite by epidote involves components of chlorites and sodic amphiboles and can be represented by the following simplified equation: ferroglaucophane+Fe-rich chlorite+lawsonite → glaucophane+Mg-rich chlorite+epidote+almandine.  相似文献   

6.
Textural evidence for the partial breakdown of staurolite-biotite and andalusite-biotite assemblages to cordierite-orthoamphibole implies high temperature metasomatic depletion of K2O in semi-pelitic rocks from Springton, South Australia. The origin of the reaction textures is discussed with reference to K2O-T diagrams derived from the topologically equivalent K2O–(-H2O) diagram showing both discontinuous and Fe–Mg continuous reactions. The involvement of fluids in the metasomatic process is implied by the scale of K2O removal and suggests that the outcrop pattern of cordierite-gedrite rocks reflects, at least in part, a heterogeneous distribution of advecting fluids in the metamorphic pile at high temperatures.Mineral abbreviations used in text and figures ab albite - alm almandine - als aluminosilicate - and andalusite - anth anthophyllite - bt biotite - cd cordierite - fe-bt Fe-rich biotite - fe-cd Fe-rich cordierite - fe-oa Fe-rich orthoamphibole - fe-st Fe-staurolite - gt garnet - ksp potassium feldspar - ky kyanite - mg-cd Mg-rich cordierite - mg-oa Mg-rich orthoamphibole - mg-st Mg-rich staurolite - mu muscovite - oa orthoamphibole - phl phlogopite - plag plagioclase - py pyrope - sill sillimanite - st staurolite - v vapour  相似文献   

7.
《Chemical Geology》2006,225(3-4):336-346
We present results of high temperature, high pressure atomistic simulations aimed at determining the thermodynamic mixing properties of key binary garnet solid solutions. Computations cover the pressure range 0–15 GPa and the temperature range 0–2000 K. Through a combination of Monte-Carlo and lattice-dynamics calculations, we derive thermodynamic mixing properties for garnets with compositions along the pyrope–almandine and pyrope–grossular joins, and compare these with existing experimental data. Across the pressure–temperature range considered, simulations show virtually ideal mixing behaviour in garnet on the pyrope–almandine join, while large excess volumes and enthalpies of mixing are predicted for garnet along the pyrope–grossular join. Excess heat capacities and entropies are also examined. These simulations shed additional light on the link between the behaviour at the atomic level and macroscopic thermodynamic properties: we illustrate the importance of certain atomistic Ca–Mg contacts in the pyrope–grossular solid solutions. For simulation techniques of this type to become sufficiently accurate for direct use in geological applications such as geothermobarometry, there is an urgent need for improved experimental determinations of several key quantities, such as the enthalpies of mixing along both joins.  相似文献   

8.
Meta-graywacke and meta-argillite of Archean age near Yellowknife contain biotite, cordierite, gedrite and sillimanite isograds towards the Sparrow Lake granite pluton. The chemistry of biotite, cordierite, gedrite and garnet in rocks that up-grade from the cordierite isograd indicate a small range of chemical composition, particularly with reference to Mg, Fe and Mn. The analyses show further that among the coexisting ferromagnesian minerals Fe/Fe+ Mg ratio decreases in the sequence: garnet, gedrite, biotite, cordierite while Mn/Fe+Mg+Mn ratio decreases in the sequence garnet, gedrite, cordierite, biotite. The same order is also observed in the distribution diagrams. The regular distribution of Mg, Fe and Mn among the coexisting phases demonstrate that chemical equilibrium was attained and preserved in these Archean rocks. Mg-Fe distribution between cordierite and biotite appears to be dependent on the temperature of crystallization or metamorphic grade.  相似文献   

9.
The chemistry and phase relations of calcic and sodic amphiboles in the Ouégoa blueschists are investigated. The first appearance of sodic amphiboles is controlled by bulkrock chemistry. Sodic amphibole appears first in weakly-metamorphosed pumpellyite metabasalts prior to the crystallization of lawsonite but does not crystallize in pelitic schists until the middle of the lawsonite zone; sodic amphibole continues as an apparently stable phase in rocks of all bulk compositions into, and throughout, the highest-grade rocks in the district. Calcic amphibole is widespread in metabasalts of the lawsonite and epidote zones and also occurs in metasediments of appropriate composition. Coexisting pairs of calcic and sodic amphiboles are common in metabasalts but they have also been found in some metasediments. A grunerite-riebeckite pair is described.Electron-probe analyses of 120 amphiboles from representative rock-types are presented in graphical form. Sodic amphiboles show an increased Mg/(Mg+Fe) ratio with increasing metamorphic grade. Sodic amphiboles in pelitic schists are ferroglaucophane in the lawsonite zone and crossite and glaucophane in the epidote zone. Sodic amphiboles in metabasalts are iron-rich crossites in weakly-metamorphosed rocks and more-magnesian crossites and glaucophanes in the lawsonite and epidote zones. The abrupt increase in Mg/(Mg+Fe) ratio in sodic amphiboles at the epidote isograd is attributed to the crystallization of epidote and almandine which take the place of lawsonite and spessartine of the lawsonite zone. Calcic amphiboles are fibrous actinolites in the lawsonite zone and grade with increasing Al and Na/Ca ratio into prismatic blue-green hornblendes (barroisites) in the upper epidote zone. In calcic amphiboles, increasing metamorphic grade effects the coupled substitution of (Na+Al) for (Ca+Mg) and a small increases in Fe/Mg ratio; octahedrally and tetrahedrally coordinated Al increases in an approximately 11 ratio. Both the calcic and the sodic amphiboles show an increase in A-site occupancy with increasing metamorphic grade. In two-amphibole assemblages Ti, Mn and K are concentrated in the calcic amphibole.The textural and chemical relations between coexisting calcic and sodic amphiboles are discussed. If the calcic and sodic amphiboles are an equilibrium pair then the data collected from the Ouégoa amphiboles gives a picture of a very asymmetric solvus in the system glaucophane-actinolite-hornblende, i.e. steep-sided to glaucophane and with a gentle slope to the calcic amphibole field; there is no indication of any termination of the solvus under the pressure-temperature conditions of crystallization of the Ouégoa schists.  相似文献   

10.
In contrast to Ferry (1980) (X Ca)-values in garnet even lower than 0.1 have a significant effect on the calculated equilibrium temperature using the experimental calibration of the Fe and Mg paritioning between garnet and biotite. Garnet compositions and Mg/Fe — distribution coefficients from samples of the Eoalpine staurolite — in zone in the southern Ötztal are related by the quadratic regression equation: InK D= -1.7500 (±0.0226) + 2.978 (±0.5317)X Ca Gt -5.906(±2.359)(X Ca Gt )2 Temperatures derived by the Ferry and Spear (1978) calibration using chemistry — correctedK D values are petrologically realistic.Analysis of our data supports non ideal mixing of grossular with almandine — pyrope solid solution. The derived excess mixing energies are quite small for the almandine — pyrope solution (W FeMg= –133 cal/mole) and about +2775 cal/mole for the difference between pyrope-grossular and almandine-grossular solutions (W MgCa —W FeCa) at metamorphic conditions of 570° C and 5,000 bar. The mixing parameters proposed by Ganguly and Saxena (1984) are not confirmed by our data as they would result in significantly lower temperatures.  相似文献   

11.
大别山北缘地区发育厚达近万米的中新生代碎屑岩，它们记录着大别山造山带和在株罗纪以来的演化历史。在安徽省六安地区毛坦厂组中，发现多块榴辉岩砾石。砾石新鲜，质地坚硬，表明属于第一旋回砾石。榴辉岩由石榴子石、多硅白云母、绿帘石、石英、金红石等组成。具有明显的退变质作用，发育以钠长石和闪石类组成的后合成晶、以及石榴子石周边的次为边。石榴子石以铁铝榴石和钙铝榴石为主，恪地C类榴辉岩。根据其特征应该属于大别山南部和北部超高压榴辉岩。毛坦厂组层位确切，古生物化石和同位素年龄都表明以晚株罗世为主，这一发现表明，在晚株罗世，以榴辉岩为代表的造山带根部物质，即：超高压变质岩已经出露地表，并作为毛坦厂组的物源。因此，大别造山带超高压为质岩折返到地表，最迟在晚株罗世。  相似文献   

12.
Garnet and plagioclase pairs from fourteen selected samples, from garnet to sillimanite zones, collected along a NS traverse throughout the metamorphic basement of NE Sardinia, have been analyzed by microprobe.Beyond the garnet isograd, plagioclase has albitic composition and the garnet (a Ca-rich almandine) shows Ca/ Ca+Mg+Fe ratios of about 0.35–0.30, fairly constant from core to rim.Towards the North, still in the garnet zone, when on the large albitic core of plagioclase a thin and discontinuous oligoclasic rim (An22–An18) formed, we observe in the garnet edge an abrupt decrease of the Ca/Ca+Mg+Fe ratio (0.27–0.16).In the staurolite and sillimanite zones garnet does not show significant Ca-zoning and it is characterized by low Ca content (Ca/Ca+Mg+Fe<0.1); the coexisting plagioclase has oligoclasic (An16–An27) composition.The chemical data and the microstructural evidence on growth time indicate that the garnet and plagioclase had a strong mutual interference in determining the relative Ca distribution.The most relevant reactions are discussed and, in particular, the antipathetical Ca-zoning, recorded by garnet and plagioclase in the garnet zone, is considered as the evidence of temperature increase during growth of the two minerals. It is also suggested that the sharp variation of Ca content at the garnet edge was controlled by the discontinuous nature of plagioclase solid solution in the peristeritic range.The order of appearance of garnet and oligoclase in the basement of NE Sardinia is also discussed in comparison with other well known metamorphic sequence (Vermont, New Zealand and Dalradian). It is concluded that the different order of appearance is controlled other than the different nature of the calcic phases in the lower grade zones also by the in the fluid phase.  相似文献   

13.
This is the third in a series of papers on glaucophane schistsfrom the Franciscan Formation near Cazadero, California. Previouspapers describe three distinct types of glaucophane-bearingFranciscan metamorphic rocks near Cazadero. The purpose of this study is to investigate the garnets presentin metamorphic types III (bedrock schists) and IV (tectonicblocks) as defined by Coleman & Lee (1963). Twenty-fourgarnet analyses are presented. Sixteen of these are from (aragonite-bearing)type III glaucophane schists, and eight are from type IV glaucophaneschists. Type IV rocks include California eclogites. Type III rocks include metabasalt, metachert, metashale, meta-ironstone,and metacarbonate that were formed under high pressure and relativelylow temperature. These rocks contain garnets that display awide range of composition, but the dominant molecules representedare consistently almandine, spessartine, and grossular. Type IV rocks are mainly metabasalts that were probably formedunder higher temperatures and pressures than type III rocks.There is a distinct difference between garnets from type IIIrocks and those from type IV (including eclogites); the lattercontainless spessartine and more pyrope, and the dominant moleculesare almandine and grossular. The four analyses of garnets fromCalifornia eclogites have an average pyrope content of aboutten molecular per cent, and they extend the range of compositionreported for eclogite garnets. Quantitative spectrographic determinations of minor elementsare listed for each of the garnets described. The values determinedfor some of the minor elements have a wide range and a capriciousdistribution over a few feet of outcrop area. As a group, both the garnets from type III rocks and those fromtype IV are pyralspites with large contents (as much as 35 molecularper cent) of ugrandite. This unusual admixture of the pyralspiteand ugrandite garnet series may have resulted in part from theconditions (high pressures and relatively low temperatures)under which the enclosing rocks were recrystallized.  相似文献   

14.
The Ubende terrane is one of the eight structural blocks constituting the Palaeoproterozoic Ubendian Belt of southwest Tanzania. The Ubende terrane is made up of high-pressure granulite-facies rocks, which occur in association with amphibolites and hornblende gneisses. Preliminary mineralogical studies of the granulite-facies metagabbros indicate that they were derived from rocks, which originally consisted of clinopyroxene and intergranular plagioclase. The mineralogy of these metagabbros consists principally of sodian augite and garnets (50% almandine, 30% pyrope and 20% grossular). Plagioclase, hornblende, quartz, rutile and haematite occur as accessory components. The sodian augite is slightly zoned with Mg content increasing towards the margin and Fe, Al and Na concentrations decrease towards its rim. Thermobarometric calculations indicate that the metagabbros were formed at metamorphic conditions of about 840–900 °C and pressures of 12.2–13.7 kbar. Presence of supersiliceous clinopyroxene (quartz lamellae in clinopyroxene) and decompositional textures suggest that these granulite-facies metagabbros might be retrogressed eclogites.  相似文献   

15.
The i.r. spectrum of 13 analyzed garnets of the pyralspite group has been investigated in the 1400-200 cm–1 region, and correlations have been found between the spectrum and the chemical composition. The results include: typical features in the spectrum of the end-members pyrope, almandine and spessartine; relationships between the spectrum and the pyrope percentage in pyrope-almandine solid solutions; and the influence of the CaO (grossularite) amount on the shape of the low-frequency absorption bands. These data allow a semiquantitative determination of the pyrope percentage in pyrope-almandine solid solutions.  相似文献   

16.
The Raman spectra of the natural end members of the garnet-group minerals,which include pyrope, almandine and spessarite of Fe-Al garnet series and grossularite ,andradite and uvarovite of Ca-Fe garnet series, have been strdied.Measured Raman spectra of these minerals are reasonably and qualitatively assigned to the internal modes, translational and rotatory modes of SiO4 tetrahedra, as well as the translational motion of bivalent cations in the X site.The stretch and rotatory A1g modes for the Fe-Al garnet series show obvious Raman shifts as compared with those for the Ca-Fe garnet series ,owing to the cations residing in the Xsite connected with SiO4 tetrahedra by sharing the two edges.The Raman shifts of all members within either of the series are attributed mainly to the properties of cations in the X site for the Fe-Al garnet series andin the Y site for the Ca-Fe garnet series.  相似文献   

17.
喜马拉雅结晶岩系中的石榴子石   总被引：1，自引：0，他引：1       下载免费PDF全文
高喜马拉雅地区广泛分布着一套结晶岩系,它是由前寒武系铁铝榴石-闪岩相的巴罗型区域变质岩系、混合岩和第三纪花岗岩组成。在我国境内聂拉木一带出露宽度达50公里。应思淮(1973)把这套岩系命名为珠穆朗玛群,张旗(1979)把它叫做聂拉木群。对于其中区域变质岩的变质带划分,存在不同的看法。  相似文献   

18.
The mineralogy and petrochemistry of the garnet-amphibolites from the highgrade part of the Abukuma metamorphic belt have been studied, using five analyses of rocks, five of hornblendes, three of garnets and one analysis of cummingtonite, Garnetiferous amphibolites are rich in Fe, whereas non-garnetiferous ones are rich in Mg, especially in cummingtonite-amphibolite. The chemical composition of hornblendes associated with garnet is pargasitic and rich in FeO and poor in CaO, but that of non-garnetiferous rocks is rich in MgO. The garnets are rich in almandine molecule. Mg/Mg + Fe2+ ratios of both hornblendes and garnets correspond with those of the host rocks. The development of garnet in the Adirondack metabasites belonging to the upper almandine-amphibolite and granulite facies is observed in Mg-rich rocks as well as in Fe-rich rocks, in which both garnet and hornblende are rich in Mg respectively. However, under the conditions of the andalusite-sillimanite type metamorphism as shown in the Abukuma Plateau, Fe-rich garnet occurs in Fe-rich basic rocks, but cummingtonite occurs in Mg-rich ones instead of Mg-rich garnet. Finally, the problem of polymetamorphism is discussed. The cummingtonite-amphibolite may be the product of polymetamorphism, and Mg-rich garnet which had been present previously was decomposed to cummingtonite and plagioclase by the subsequent regional metamorphism of andalusite-sillimanite type.  相似文献   

19.
The garnet-cordierite zone, the highest-grade zone of the Ryoke metamorphic rocks in the Yanai district, SW Japan, is defined by the coexistence of garnet and cordierite in pelitic rocks. Three assemblages in this zone are studied in detail, i.e. spinel + cordierite + biotite, garnet + cordierite + biotite and garnet + biotite, all of which contain quartz, K-feldspar and plagioclase. The Mg/(Fe + Mg) in the coexisting minerals decreases in the following order: cordierite, biotite, garnet and spinel. Two facts described below are inconsistent with the paragenetic relation in the K2OFeOMgOAl2O3SiO2H2O (KFMASH) system in terms of an isophysical variation. First, garnet and biotite in the last assemblage have Mg/(Fe + Mg) higher than those in the second. Second, the first two assemblages are described by the reaction,
while they occur in a single outcrop. The addition of MnO, ZnO and TiO2 to the system can resolve the inconsistencies as follows. The assemblage garnet + biotite can consist of garnet and biotite higher in Mg/(Fe + Mg) than those in garnet + cordierite + biotite as long as they are enriched in spessartine and depleted in Al, respectively. The assemblage garnet + cordierite + biotite becomes stable relative to spinel + cordierite + biotite with increasing spessartine content or decreasing gahnite content and the Ti content of biotite. The constituent minerals of the assemblages, spinel + cordierite + biotite and garnet + cordierite + biotite, preserve several reaction microstructures indicative of prograde reactions,
and
together with retrograde reactions,
and
This suggests that the pressure-temperature path of the rocks includes an isobaric heating and an isobaric or decompressional cooling. The high-grade areas consisting of the K-feldspar-cordierite zone, sillimanite-K-feldspar zone and garnet-cordierite zone have prograde paths involving isobaric heating and show a southwards increase in pressure with a thermal maximum in the middle. These high-grade zones are closely associated with the gneissose granitic rocks, suggesting that the Ryoke metamorphism, one of the typical low-pressure type, is caused by the heat supply from the syn-tectonic granitic rocks that emplaced at the middle level of the crust. Received: 22 August 1997 / Accepted: 11 May 1998  相似文献   

20.
Summary Garnet occurs as a significant mineral constituent of felsic garnet-biotite granite in the southern edge of the Třebíč pluton. Two textural groups of garnet were recognized on the basis of their shape and relationship to biotite. Group I garnets are 1.5–2.5 mm, euhedral grains which have no reaction relationship with biotite. They are zoned having high XMn at the rims and are considered as magmatic. Group II garnets form grain aggregates up to 2.5 cm in size, with anhedral shape of individual grains. The individual garnet II grains are usually rimmed by biotite and have no compositional zoning. The core of group I garnets and group II garnets contains 67–80 mol% of almandine, 5–19 mol% of pyrope, 7–17 mol% of spessartine and 2–4 mol% of grossular. Biotite occurs in two generations; both are magnesian siderophyllites with Fe/(Fe + Mg) = 0.50–0.69. The matrix biotite in granites (biotite I) has high Ti content (0.09–0.31 apfu) and Fe/(Fe + Mg) ratio between 0.50 and 0.59. Biotite II forms reaction rims around garnet, is poor in Ti (0.00–0.06 apfu) and has a Fe/(Fe + Mg) ratio between 0.61 and 0.69. The textural relationship between biotite and garnet indicates that garnet reacted with granitic melt to form Ti-poor biotite and a new granitic melt, depleted in Ti and Mg and enriched in Fe and Al. In contrast to the host durbachites (hornblende-biotite melagranites), which originated by mixing of crustal melts and upper mantle melts, the origin of garnet-bearing granites is related to partial melting of the aluminium-rich metamorphic series of the Moldanubian Zone.  相似文献   

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