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1.
The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K2O content in muscovite, but in biotite it reflects an increase of not only the K2O content but also of the octahedral vacancy.
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water
and   andalusite=sillimanite.quartz=cordierite+K-feldspar+water
This implies that this zone probably has a P–T  path involving isobaric heating.  相似文献   

2.
Contact metamorphism has been recognized along a 4 km wide belt adjacent to the shallow‐dipping eastern margin of the Arthursleigh Tonalite, an Early Devonian pluton of the Marulan Batholith, eastern New South Wales. In Ordovician psammitic and pelitic rocks three zones of progressive contact metamorphism range from muscovite + biotite + chlorite assemblages in the outer zone to K‐feldspar + cordierite assemblages adjacent to the pluton and in metasedimentary xenoliths. Retrograde phenomena include extensive replacement of metamorphic minerals by ‘sericite’ and chlorite. Calcareous metasediments adjacent to the tonalite typically contain assemblages of quartz + calcic plagioclase + ferrosalite + sphene, or wollastonite + calcite + diopside with minor grossularite and vesuvianite. Thermal effects in volcanic rocks along the western margin of the pluton are confined to recrystallization of the groundmass.

The regional geology indicates confining pressures of approximately 1 kbar at the time of emplacement of the tonalite. Contact metamorphic temperatures were estimated from two‐feldspar geothermometry to attain a maximum of approximately 590°C for rocks in the innermost zone of the aureole and 700°C for the xenoliths. Fluid compositions attending progressive contact metamorphism were water‐rich (Xco2<0.2) and, during cooling, these fluids probably account for the extensive retrograde hydration observed in the aureole.  相似文献   

3.
In low- and middle-grade pelitic metamorphic rocks, the extentsof Tschermak substitution in muscovite, chlorite and biotitechange regularly with bulk-rock composition and external conditions.This paper gives a theoretical analysis of the changes. From equipotential lines for Al2O3 plotted on AFM diagrams,we have derived a series of Thompson-type muscovite compositiondiagrams, which show how the celadonite content of muscovitevaries with the associated ferromagnesian minerals or with thebulk-rock composition under constant external conditions. Thedistribution coefficient of the exchange reaction for Tschermaksubstitution between muscovite and chlorite varies greatly notonly with temperature but also with the extent of this substitutionin the two minerals because of their strong deviation from ideality.Muscovites with a high celadonite content (phengites) occurin low-temperature rocks in any of the high-, medium- and low-pressuretypes of metamorphism, probably because the exchange equilibriumfor Tschermak substitution between muscovite and chlorite isnot sensitive to pressure. When both Mg-Fe and Tschermak substitutionoccur in muscovite and some other silicates, a metapelite containingthree AFM phases together with muscovite and quartz has at leasttwo independent net-transfer reactions that take place sideby side with changing external conditions. The mass balancerequirement in the rock is imposed on a linear combination ofthe two reaction equations, leading to a constraint on the stoichiometricequations among phase components and the progressive compositionalchanges of muscovite, chlorite and biotite. From such a viewpoint,we examine reactions and progressive mineralogical changes inmetapelites, beginning with K-feldspar-bearing low-grade metapeliticrocks, in which biotite appears by reaction of K-feldspar withchlorite at a temperature lower than that of the biotite isograddefined for K-feldspar-free pelitic rocks. When both Mg-Fe and Tschermak substitution occur in muscoviteand some other silicates, a metapelite containing three AFMphases together with muscovite and quartz has at least two independentnet-transfer reactions that take place side by side with changingexternal conditions. The mass balance requirement in the rockis imposed on a linear combination of the two reaction equations,leading to a constraint on the stoichiometric equations amongphase components and the progressive compositional changes ofmuscovite, chlorite and biotite. From such a viewpoint, we examinereactions and progressive mineralogical changes in metapelites,beginning with K-feldspar-bearing low-grade metapelitic rocks,in which biotite appears by reaction of K-feldspar with chloriteat a temperature lower than that of the biotite isoerad definedfor K-feldsoar-free oelitic rocks. The equations for the reactions that produce biotite or biotite? garnet in K-feldspar-free metapelites have been derived. Combinedwith the composition relations of coexisting muscovite, chlorite,biotite and garnet, they lead to the inference that progressof these reactions with rising temperature causes a decreaseof the celadonite content of muscovite, the antigorite contentof chlorite and the phlogopite content of biotite in the metapelitesof the chlorite, biotite and lower garnet zones, and that thistrend may not continue into the staurolite zone. This inferencehas been verified by examination of the analytical data of mineralsin four progressive metamorphic terranes covering the high-,medium- and low-pressure types. The composition of chlorite is buffered by the garnet-producingreaction so that chlorites in garnet-bearing metamorphic rocksdevelop (Mg ? Fe)/Al2 ratios close to that of almandine at elevatedtemperatures. The maximum possible celadonite content in muscoviteunder given external conditions is realized in K-feldspar-bearingrocks, and decreases with rising temperature.  相似文献   

4.
Micas in 17 pelitic (K-feldspar-free) and 8 psammitic (K-feldspar-bearing) rocks from the Wazuka and the Asemi-gawa areas in the Ryoke and the Sanbagawa metamorphic terrains, respectively, were analyzed on an electron-probe microanalyzer. The deficiency of alkali cations in the low- to middle-grade metamorphic micas is ascribed to the illite substitution, KXII+AlIV=XII (vacancy)+SiIV.At the same metamorphic grade, the deficiency of interlayer cations in micas from the pelitic rocks is greater than that from the psammitic rocks. However, it decreases with rising temperature in both rock-types, irrespective of the pressure of metamorphism.K-feldspar and biotite buffer the illite substitution. Two reactions are proposed to explain the decrease of the alkali-cation deficiency in both muscovite and biotite.  相似文献   

5.
攀西中元古结晶片岩系遭受了前进区域变质作用。盐边和米易的砂屑岩和泥质岩分别可划出:绿泥石、黑云母、铁铝榴石、十字石和夕线石带以及绿泥石、黑云母-石榴子石、红柱石和夕线石带。在中、低级泥砂质岩石中,白云母的Na/(Na+K)比值随变质级增高。白云母、绿泥石和黑云母中的契尔马克替换范围大体上随变质级增高而降低。白云母和绿泥石之间契尔马克替换交换反应的分配系数,大致是白云母的绿鳞石含量的函数,并随变质温度升高而降低,在夕线石带,该分配系数变得很小。黑云母和白云母契尔马克替换交换反应的分配系数有类似的趋势。  相似文献   

6.
The basement of the central Qilian fold belt exposed along the Minhe-Ledu highway consists of psammitic schists, metabasitic rocks, and crystalline limestone. Migmatitic rocks occur sporadically among psammitic schist and metabasitic rocks. The mineral assemblage of psammitic schist is muscovite + biotite + feldspar + quartz ± tourmaline ± titanite ± sillimanite and that of metabasitic rocks is amphibole + plagioclase + biotite ± apatite ± magnetite ± pyroxene ± garnet ± quartz. The migmatitic rock consists of leucosome and restite of various volume proportions; the former consists of muscovite + alkaline feldspar + quartz ± garnet ± plagioclase while the latter is either fragments of psammitic schist or those of metabasitic rock. The crystalline limestone consists of calcite that has been partly replaced by olivine. The olivine was subsequently altered to serpentine. Weak deformations as indicated by cleavages and fractures were imposed prominently on the psammitic schists, occasionally on me  相似文献   

7.
The oligoclase-biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (XCa= 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 = Psolid 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:  相似文献   

8.
The Kelly's Mountain gneiss complex of Cape Breton Island, Nova Scotia, is a migmatitic paragneiss dominated by biotite- and cordierite-bearing assemblages. Metamorphic grade throughout the complex is in the upper amphibolite facies, with garnet absent and only retrograde muscovite present. In the high grade core of the complex the reaction biotite+andalusite+quartz=cordierite+K-feldspar+sillimanite+ilmenite+H2O is preserved. The pelitic migmatites contain cordierite- and K-feldspar-rich leucosomes and biotite-rich melanosomes. Minor clinopyroxene-bearing amphibolite in the complex does not show migmatitic textures. The migmatites are interpreted as in situ peraluminous partial melts on the basis of phase relations and textural criteria. Retrograde metamorphism under conditions of high fluid pressure locally produced muscovite after K-feldspar and muscovite+green biotite+chlorite after cordierite in paragneiss, and sphene after ilmenite in amphibolite. Peak metamorphic conditions of 1–3.5 kb and 580–700° C are estimated. The high geothermal gradient inferred from these conditions was probably caused by the intrusion of diorites associated with the gneiss complex. The Kelly's Mountain complex represents a rare example of migmatites formed in the low-pressure facies series, and illustrates some of the reactions involving melting in high grade pelitic rocks.  相似文献   

9.
Pressure-temperature conditions of metamorphism in the Yanai district, Ryoke belt, SW Japan, have been determined using garnet-biotite thermometry in combination with an empirically calibrated barometer in the assemblage common in pelitic and siliceous rocks, garnet + biotite + plagioclase + quartz. The barometer estimates pressure difference between a well-established sample and unknown samples based on the reaction,
Pressure and pressure gradient increased with increasing temperature such that pressures of high-grade areas exceeded that of the triple point of aluminosilicates. The thermobaric structure of the study area shows that pressure increased up to 5 kbar with southward increase in metamorphic temperature up to the highest-grade area, the garnet-cordierite zone. Further south, pressure was almost the same as that of the garnet-cordierite zone and temperature decreased. This asymmetric distribution of metamorphic conditions on both sides of the garnet-cordierite zone can explain the asymmetric distribution of metamorphic zones; the K-feldspar-cordierite zone and sillimanite-K-feldspar zone on the north and south sides of the garnet-cordierite zone, respectively. The breakdown reaction of muscovite and quartz defines the beginning of both the K-feldspar-cordierite zone and sillimanite-K-feldspar zone, which took place under low and high pressures, respectively. These thermobaric structures suggest that temperature varied laterally at mid-crustal level during the peak of metamorphism.Editorial Responsibility: T.L. Grove  相似文献   

10.
Abstract. Many granitic plutons of Early Cretaceous age are intruded on various scales in the Kitakami Mountains. The stock‐type Ganidake pluton accompanies enormous Fe‐Cu mineralization of the Kamaishi deposits, whereas the Kurihashi pluton accompanies less mineralization. To elucidate the cause of these differences, the metamorphic conditions and redox state of the contact metamorphic aureole around the Kurihashi pluton have been examined by the petrochemical study and gas analysis of the metamorphic rocks. A typical mineral assemblage in the pelitic rocks in the lowest‐grade part is biotite‐muscovite‐chlorite‐quartz‐plagio‐clase‐graphite, which occur more than 2 km away from the contact point with the Kurihashi pluton. Graphite disappears at the 1550 m point, and cordierite and garnet appear in the middle and highest‐grade parts, respectively. A typical mineral assemblage in the tuffaceous rocks in the lower‐grade part is chlorite‐actinolite‐biotite‐quartz‐plagioclase. Actinolite changes into hornblende near to the pluton. The CO2/CH4 ratios obtained in measurements by gas chromatography exceeds 100 in the pelitic rocks at the contact point with the pluton. The ratios decrease and become less than 0.1 with distance from the pluton. Equilibrium temperatures calculated from a garnet‐biotite pair in the pelitic rock and a hornblende‐plagioclase pair in the tuffaceous rock are 640d? and 681 d?C at the contact point, respectively. The log?o2 values among these metamorphic aureoles estimated from the CO2/CH4 ratios are slightly lower than the FMQ‐buffer. Redox states of the contact metamorphic aureole are kept in an intermediate condition between oxidized magma of the Kurihashi pluton and graphite‐bearing pelitic country rocks. Judging from these metamorphic conditions around the Kurihashi pluton and from the re‐evaluation of the previous knowledge about contact metasomatism around the Ganidake pluton, the Kurihashi metamorphism has occurred at higher temperatures and dry conditions than the Ganidake metasomatism. These differences in the metamorphic conditions and presence or absence of a large limestone mass around the pluton might be the principal reasons why the Kurihashi pluton accompanies less mineralization and the Ganidake pluton accompanies gigantic Kamaishi skarn mineralization.  相似文献   

11.
The regional distribution of metamorphic mineral assemblages in Mesozoic carbonate rocks of the Western Hohe Tauern allows the mapping of isograds based on the appearance of biotite+calcite and biotite+zoisite+calcite. The latter isograd corresponds approximately to the thermal maximum of the alpidic metamorphism in the central part of this area. An estimate of P, T, X fluid conditions can be obtained from phase relations among muscovite, biotite, chlorite, margarite, tremolite, zoisite, anorthite, quartz, calcite, and dolomite in the system K2O-CaO-MgO-Al2O3-SiO2-H2O-CO2 which approximates the composition of marls. Calculations based on various experimental and thermodynamic data have been made with emphasis on phase relations pertinent to a group of carbonate rocks with very low Fe and Na contents in non-opaque minerals. Significant and opposite deviations from the phase relations for stochiometric end member mineral compositions are due to the substitutions F-OH and Mg+Si-2Al. Consistency of observed and calculated phase relations is favoured by high F-contents. For the majority of carbonate rocks in the high metamorphic zone, maximum temperatures around 550° C, minimum pressures of 4–6 kb, and relatively low XCO2 values within the stability field of zoisite and of biotite+calcite+quartz are indicated.  相似文献   

12.
A regional petrographic reconnaissance of psammitic and pelitic rocks in the Otago Schist, New Zealand, has revealed the presence of garnet (“grossalspite” with typical rim composition almandine41, spessartine25, grossular33, pyrope1) and biotite in 37 new samples, more than doubling the previously known number. A new garnet–biotite–albite zone can now be defined in the greenschist facies Otago Schist that is distinct from the better-known biotite, garnet and oligoclase zones in the along-strike Alpine Schist. The garnet–biotite–albite zone is in part metamorphically discontinuous with adjacent schists and does not support models of simple, continuous, progressive Jurassic regional metamorphism in Otago. The structurally higher (lower grade) boundary of the zone coincides in at least three places with previously mapped regional shear zones. The structurally lower (expected higher grade) boundary of the zone appears to be obliterated by a chlorite zone overprint which can be spatially related to Alpine Schist recrystallisation of ?Cretaceous age. The Otago situation serves as an example of the subtle metamorphic discontinuities that probably pervade many orogenic belts.  相似文献   

13.
A suite of pelitic rocks around Kandra, Singhbhum District, Bihar, displays a metamorphic gradient registered by the index minerals chlorite, biotite, garnet, staurolite and sillimanite in a Barrovian sequence. Metamorphism was by and large coeval with folding movements, and correlating the internal fabric of minerals and deformational characters, a regular sequence of the index minerals is derived. It is argued that the chronological order by itself is not sufficient to prove that metamorphism was progressive in time.Among the index minerals, garnet appears to have formed by the reaction chlorite+biotitea+quartz garnet+biotiteb+H2O. For the origin of sillimanite, a new reaction, 3 staurolite+muscovite+quartz=7 sillimanite+biotite+3H2O, is suggested on the basis of significant textural features. Textural and petrological indications regarding the formation of staurolite are in discordance. Staurolite was either derived from the biotite zone phases, or should be taken to have formed, against textural evidences, from chloritoids of the garnet zone.Graphical analysis of the assemblages by Thompson's AFM projection reveals that chlorite and staurolite are excess phases owing to retrogression and incomplete reaction. Shifting of apices of triangular fields and intersection of garnet-biotite tie lines within a zone can be satisfactorily explained in terms of extra components CaO and MnO or their ratios. It is pointed out that if MgO/(MgO + FeO) between two phases show a linear relation, their tie lines will be concurrent on the AF side of the projection, the point of concurrence reflecting equilibrium and temperature of recrystallisation.  相似文献   

14.
In the middle part of the Cévennes, situated at the south-eastern border of the Massif Central, there are outcrops of micaschists, gneisses and granites in which the increase of metamorphic grade was investigated along the river Beaume. This paper presents the results of the petrographic-geochemical analysis of the micaschists of the greenschist facies.Chemical analyses of the micaschist-samples examined petrographically show their nearly constant composition, a fact which e.g., is expressed by similar CaO/Na2O- and MgO/FeO-proportions. As the average values of the determined oxides approach the respective average values of analyses of graywackes recorded by Pettijohn (1957) the original sediment is likely to have been an argillaceous graywacke deficient in calcite.In enrichments of the phyllosilicates pyrophyllite was detected by X-ray diffraction; its amount is about 2 to 5 weight percent of the rocks.With increasing metamorphic grade the following parageneses were found in the metamorphic zones of the greenschist facies: Zone Ia: Quartz+chlorite+pyrophyllite+muscovite+clinozoisite. Zone Ib: Quartz+chlorite+pyrophyllite+muscovite+biotite+clinozoisite.The absence of biotite in Zone Ia, though the chemism of the rocks is practically the same, is obviously due to the different Al2O3-content of the chlorites of Zones Ia and Ib. The chlorite of Zone Ia is more deficient in Al2O3 than the one of Zone Ib. With passage from Zone Ia to Zone Ib the position of the tie line between chlorite and muscovite in the ACF-A'FK-diagram changes in such a way that in Zone Ia, because of purely chemical reasons, biotite cannot occur as coexisting mineral.The beginning of Zone II is characterized by the occurence of almandine, rich in spessartine. The following paragenesis is typical of this zone: Quartz+chlorite+pyrophyllite +muscovite+biotite+almandine+clinozoisite.Additionally the micaschists of these three zones display albite, the greater part of which is concentrated in mm-thin layers with associated minor amounts of quartz and micas. Paragonite whose formation by reaction between albite and pyrophyllite is to be expected based on experimental results (Winkler, 1967, p. 95) could not be proved by X-ray diffraction.In Zone III andalusite occurs instead of pyrophyllite. Furthermore, as chlorite and clinozoisite are absent and oligoclase occurs for the first time this zone is regarded as the first subfacies of the almandine-amphibolite-facies. The chemism and the observed mineral parageneses of the subfacies of the almandine-amphibolite-facies will be treated in a separate publication.  相似文献   

15.
The Motuo area is located in the east of the Eastern Himalayan Syntaxis. There outcrops a sequence of high-grade metamorphic rocks, such as metapelites. Petrology and mineralogy data suggest that these rocks have experienced three stages of metamorphism. The prograde metamorphic mineral assemblages(M1) are mineral inclusions(biotite + plagioclase + quartz ± sillimanite ± Fe-Ti oxides) preserved in garnet porphyroblasts, and the peak metamorphic assemblages(M2) are represented by garnet with the lowest XSps values and the lowest XFe# ratios and the matrix minerals(plagioclase + quartz ± Kfeldspar + biotite + muscovite + kyanite ± sillimanite), whereas the retrograde assemblages(M3) are composed of biotite + plagioclase + quartz symplectites rimming the garnet porphyroblasts. Thermobarometric computation shows that the metamorphic conditions are 562–714°C at 7.3–7.4 kbar for the M1 stage, 661–800°C at 9.4–11.6 kbar for the M2 stage, and 579–713°C at 5.5–6.6 kbar for the M3 stage. These rocks are deciphered to have undergone metamorphism characterized by clockwise P-T paths involving nearly isothermal decompression(ITD) segments, which is inferred to be related to the collision of the India and Eurasia plates.  相似文献   

16.
A quantitative petrogenetic grid for pelitic schists in the system KFMASH that includes the phases garnet, chlorite, biotite, chloritoid, cordierite, staurolite, talc, kyanite, andalusite, sillimanite, and pyrophyllite (with quartz, H2O and muscovite or K-feldspar in excess) is presented. The grid is based on thermodynamic data of Berman et al. (1985) and Berman (1988) for endmember KFASH and KMASH equilibria and natural Fe-Mg partitioning for the KFMASH system. Calculation of P-T slopes and the change in Fe/(Fe+Mg) along reactions in the KFMASH system were made using the Gibbs method. In addition, the effect on the grid of MnO and CaO is evaluated quantitatively. The resulting grid is consistent with typical Buchan and Barrovian parageneses at medium to high grades. At low grades, the grid predicts an extensive stability field for the paragenesis chloritoid+biotite which arises because of the unusual facing of the reaction chloritoid+biotite + quartz+H2O = garnet+chlorite+muscovite, which proceeds to the right with increasing T in the KFMASH system. However, the reaction proceeds to the left with increasing T in the MnKFASH system so the assemblage chloritoid + biotite is restricted to bulk compositions with high Fe/(Fe+Mg+Mn). Typical metapelites will therefore contain garnet+chlorite at low grades rather than chloritoid + biotite.  相似文献   

17.
The biotite isograd in pelitic schists of the Waterville Formationinvolved reaction of muscovite + ankerite + rutile + pyrite+graphite + siderite or calcite to form biotite + plagioclase+ ilmenite. There was no single reaction in all pelites; eachrock experienced a unique reaction depending on the mineralogyand proportions of minerals in the chlorite-zone equivalentfrom which it evolved. Quartz, chlorite, and pyrrhotite werereactants in some rocks and products in others. All inferredbiotite-forming reactions involved decarbonation and desulfidation;some were dehydration reactions and others were hydration reactions.P-T conditions at the biotite isograd were near 3500 bars and400 °C. C-O-H-S fluids in equilibrium with the pelitic rockswere close to binary CO2-H2O mixtures with XCO2 = 0.02–0.04.During the biotite-forming reaction, pelitic rocks (a) decreasedby 2–5 percent in volume, (b) performed – (4–11)kcal/liter P-V work on their surroundings, (c) absorbed 38–85kcal/liter heat from their surroundings, and (d) were infiltratedby at least 0.9–2.2 rock volumes H2O fluid. The biotite isograd sharply marks the limit of a decarbonationfront that passed through the terrane during regional metamorphism.Decarbonation converted meta-shales with 6–10 per centcarbonate to carbonate-free pelitic schists. One essential causeof the decarbonation event was pervasive infiltration of theterrane by at least 1–2 rock volumes H2O fluid early inthe metamorphic event under P-T conditions of the biotite isograd.Average shale contains 4–13 per cent siderite, ankerite,and/or calcite, but average pelitic schist is devoid of carbonateminerals. If the Waterville Formation serves as a general modelfor the metamorphism of pelitic rocks, it is likely that worldwidemany pelitic schists developed by decarbonation of shale caused,in part, by pervasive infiltration of metamorphic terranes byseveral rock volumes of aqueous fluid during an early stageof the metamorphic event.  相似文献   

18.
The terrane in the Panamint Mountains, California, was regionallymetamorphosed under low-pressure conditions and subsequentlyunderwent retrograde metamorphism. Prograde metamorphic isogradsthat mark the stability of tremolite + calcite, diopside, andsillimanite indicate a westward increase in grade. The studywas undertaken to determine the effects of the addition of Caon the types of assemblages that may occur in pelitic schists,to contribute to the understanding of the stability limits inP – T – aH2O – XFe of the pelitic assemblagechlorite + muscovite + quartz, and to estimate the change inenvironment from prograde to retrograde metamorphism. Peliticassemblages are characterized by andalusite + biotite + stauroliteand andalusite + biotite + cordierite. Within a small changein grade, chlorite breaks down over nearly the entire rangein Mg/(Mg + Fe) to biotite + aluminous mineral. Chlorite withMg/(Mg + Fe) = 0.55 is stable to the highest grade, and thegeneralized terminal reaction is chlorite + muscovite + quartz= andalusite + biotite + cordierite + H2O. Calcic schists arecharacterized by the assemblage epidote + muscovite + quartz+ chlorite + actinolite + biotite + calcite + plagioclase atlow grades and by epidote + muscovite + quartz + garnet + hornblende+ biotite + calcite + plagioclase at high grades. Epidote doesnot coexist with any AFM phase that is more aluminous than garnetor chlorite. Lithostatic pressure ranged from 2.3 kb to 3.0kb. During prograde-metamorphism temperatures ranged from lessthan 400° to nearly 700°C, and XH2O (assuming PH2O +PCO3 = Ptotal) is estimated to be 0.25 in siliceous dolomite,0.8 in pelitic schist, and 1.0 in calcic schist. Temperatureduring retrograde metamorphism was 450° ± 50°C,and all fluid were H2O-rich. A flux of H2O-rich fluid duringfolding is believed to have caused retrograde metamorphism.The petrogenetic grid of Albee (1965b) is modified to positionthe (A, Cd) invariant point relative to the aluminosilicatetriple point, which allows the comparison of facies series thatinvolve different chloritoid-reactions.  相似文献   

19.
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-XH2O relations among chlorite, biotite,garnet, staurolite, and kyanite at Ptotal 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.  相似文献   

20.
Mitsuhiro Toriumi 《Lithos》1979,12(4):325-333
The process of shape-transformation of quartz inclusions from polyhedral to spherical grains in albite single crystals during metamorphism is mainly controlled by the grain boundary diffusion of oxygen along the quartz/albite interface to reduce the interfacial free energy. The rate of the process, which is represented by the growth rate of the curvature of the edge surface of the grain, depends significantly on temperature and on the grain size of the quartz inclusion. The relations between temperature, T, the time, tr, and the critical radius, Rc, which is equal to the radius of maximum spherical grains, are given by log Rc = −0.11Eb/RT + 0.25log tr + C, in which Eb is the activation energy of the grain boundary diffusion of oxygen along the quartz/albite interface and C is a material constant.

The mean critical radius of spherical quartz inclusions in albite is 5 μm for the upper chlorite zone and garnet zone, 10 μm for the lower biotite zone, and 20 μm for the upper biotite zone in the Sambagawa metamorphic terrain. The mean values of the critical radii of spherical quartz inclusions in oligoclase of the Ryoke metamorphic rocks is about 5 μm for the chlorite zone and about 10–20 μm for the sillimanite zone.

Assuming temperatures of about 350°C for the upper chlorite and garnet zones, 400°C for the lower biotite zone, 550°C for the upper biotite zone, and 700°C for the sillimanite zone, the activation energy for the grain boundary diffusion of oxygen along the quartz/plagioclase interfase is estimated to be about 30 kcal/mol.  相似文献   


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