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1.
A variety of uncommon garnet-grade assemblages have been foundin rocks from three outcrops in the western part of centralNew Hampshire, and include the associations Grt+MrgCld, Grt+Bt+CldMrg,and Mrg+Cld+HblGrt (all rocks contain Ms, Chl, Ilm, and Qtz).These unusual rocks coexist with more typical Grt+Bt+Chl+Plmetapelites and amphibolites. Rim P–T conditions are {smalltilde}49035C and 5•751•25 kbar. Projection of the assemblages from Qtz, H2O, and Ilm into theCa–Al'–Na–(Fe+Mg) tetrahedron, and from Qtz,Ilm, H2O, and Chl into the Ca–Al'–Fe'–Mn tetrahedronindicates that Ca/(Ca+Na) and Mn differ among the assemblagesin a systematic fashion. Common Grt+Bt+Chl+Pl assemblages arerestricted to relatively high Mn and low Ca/(Ca+Na) values,whereas Cld+Bt+Mrg and Cld+Hbl+Mrg assemblages are stable atlow Mn and high Ca/(Ca+Na). These data suggest that at thisgrade Cld+Bt is more stable than Grt+Chl in the KFMASH system,whereas in the Ca—KFMASH system, Hbl+Cld assemblages arestable. Composition space analysis using the singular value decompositionmethod indicates that compositions of minerals from individualsamples are consistent with local equilibrium, but that differentoutcrops may not have all equilibrated at the same P–T–aH2Oconditions. Thermodynamic analysis suggests that a garnet-zoneprograde sequence of ferromagnesian associations for these bulkcompositions would be Hbl+Cld+Grt+ChlBt+Cld+Grt+ChlBt+Grt+Chl. Staurolite-grade rocks from the same stratigraphic units areexposed across strike, and contain the assemblage Grt+StBtPl(all rocks contain Ms, Qtz, Chl, and Ilm). Margarite is commonlypresent as inclusions in the cores of garnets, but is absentas inclusions near garnet rims and from the matrix; conversely,staurolite inclusions are present towards the rims of the garnets,but are absent from the cores. These inclusion relations suggestthat margarite may react to form staurolite and garnet withincreasing grade via a reaction such as chlorite+margarite=staurolite+garnet+H2O. Biotite is common in the matrix but is not typically abundant,and appears to have been the last phase to join the assemblage.Biotite is inferred to have joined the Grt+St+Chl assemblagesafter margarite breakdown through the reaction Grt+Chl+Ms=St+Bt+H2O. Thus, uncommon margarite assemblages may evolve into commonGrt+Bt+St+Chl assemblages. * Present address: Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706.  相似文献   

2.
The geochemistry of metamorphic quartz vein formation in Barrow’s index mineral zones north of Stonehaven, Scotland, was investigated in order to assess regional fluid flow and mass transfer. Metamorphic grade in the Dalradian metasediments increases to the north–northwest away from the Highland Boundary Fault (HBF) and associated ophiolitic rocks of the Highland Border Complex (HBC), passing through the Chlorite (Chl), Biotite (Bt), Garnet (Grt), Chloritoid (Cld), and Staurolite (St) zones. Syn-metamorphic fluid infiltration at 462±8.8 Ma (Breeding et al. in Am Mineral 89:1067, 2004) produced considerable quartz veining. Vein abundance varies from about 5 to 15 volume percent of the outcrops; veins tend to be more abundant in metapelitic layers than in metapsammitic ones. Metamorphic veins are surrounded by centimeter- to decimeter-wide zones of chemical and mineralogical alteration (selvages). Porphyroblasts, particularly Bt, Grt, Cld, and St, are typically larger in selvages than in wallrocks distal to veins. The altered selvages underwent fluid-driven addition of Na, Ca, and Sr, and loss of K, Rb, and Ba. Alteration is most intense within ∼750 m of the HBF, but is still very significant at the northern end of the field area some 2 km away. Mg/FeT (FeT=total iron) was either unchanged or increased due to alteration. Silica was added at some Chl and Bt zone localities near the HBF. Pb mass transfer was variable although Pb was added at a number of locations. Rare Earth elements (REE) were generally immobile, but light REE and possibly heavy REE were lost at one field site. The gain of Na and Ca and loss of K promoted the growth of plagioclase at the expense of micas (particularly muscovite) in selvages and wallrock inclusions throughout the field area and, probably, some calcite and/or dolomite growth directly adjacent to the HBF. The Ca gains were also critical for epidote production in the Bt zone. Gains of Ca and increases in Mg/FeT helped to stabilize Grt at the expense of Cld and St in some selvages. Hornblende and cummingtonite were discovered in strongly altered metapelitic rocks at one Cld zone locality. The metasomatism puts important constraints on the processes of mass transfer and suggests two models for regional fluid flow. In the first model, fluid flow in a direction of increasing temperature downward along the HBF added Na and Ca, and removed K from the Dalradian. In the second model, fluid flow upward from the HBC transported Na and Ca into the overlying Dalradian and, at the same time, stripped out K. The latter model is favored because it can most readily account for silica addition near the HBF, but neither model can be ruled out at present. In either case, the veins represent fractures that transmitted very large time-integrated fluid fluxes of at least ∼104 m3 (fluid)/m2 (rock). Consequently, the veins were conduits for regional fluid flow that caused considerable open-system chemical and mineralogical alteration during metamorphism. Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

3.
Chemical relationships among four metapelites have been studied by investigation of mineral and bulk chemistry data and by singular value decomposition analysis of single and composite assemblage matrices. Bulk rock compositions cluster close together in an AFM diagram, all within the intersection space defined by the four sample assemblages. The similarity of bulk compositions normalized on a silica-free, anhydrous basis indicates that sample chemistries differ mainly as a result of inhomogeneous distribution of quartz layers. The existence of mass balance relationships among samples indicates that assemblages also overlap in the Si–Ti–Al–Fe–Mg–Mn–Ca–Na–K multisystem. Analysis of single and composite matrices helps in defining possible mass balances linking sample mineral facies to one another during progressive contact metamorphism. The assemblage in sample A can form as the result of the model reaction 5.000  Ky+0.269 Grt+0.965 Bt+0.314 Pl=0.049 Ilm+1.115 Ms+0.849 Chl+0.306 St and react to assemblage B via reaction 0.97 Chl+0.52 Grt+0.66 Ms+0.14 Ilm+1.26  Ky=0.42 St+0.63 Bt+0.22 Pl coupled with the Ky→And transition. Assemblage B can transform into C by initial progress of Ky+Ilm+Chl+Grt+Ms=And+St+Bt+Pl followed by 0.142 Ilm+0.175 St+1.089 Chl+1.533 Ms+0.003 Grt=5.000 And+1.266 Bt+0.551 Pl Matrix analysis cannot satisfactorily model the C–D transition, because it predicts a net production of staurolite, which is in disagreement with petrographic evidence. All mass balances in the C–D composite matrix indicate net consumption of muscovite; this is integrated with the contrasting evidence of prograde pseudomorphs of muscovite after staurolite, observed in the nodules of sample D, within a model involving the progress of ionic reaction cycles.  相似文献   

4.
Four assemblages from calcic pelitic schists from South Strafford,Vermont, have been studied in detail to determine the relationshipbetween reaction history and compositional zoning of minerals.The lowest-grade assemblage is garnet + biotite + chlorite +plagioclase + epidote + quartz + muscovite + graphite + fluid.Along a path of isobaric heating, the net reaction is Chl +Ms + Ep + Gr = Grt + Bt + Pl + fluid. Garnet grows with decreasingFe/(Fe + Mg) and XSpa, (from 0•2 to 0•05), XGra staysnearly constant between 0•20 and 0•25, and plagioclasegrows with XAn increasing from peristerite to 0•2–0•5. The subsequent evolution depends on whether chlorite or epidotereacts out first. If chlorite is removed from the assemblagefirst, the net reaction along an isobaric heating path becomesGrt + Ms + Ep + Qtz + Gr = Bt + Pl + fluid. XAn of plagioclaseincreases to 0•20–0•70, depending on the bulk-rockcomposition and changes in pressure and temperature. If epidoteis removed first, the assemblage becomes a simple pelite andthe net reaction becomes Chl + Pl + Ms + Qtz = Grt + Bt + H2O.Plagioclase is consumed to provide Ca for growing garnet, andXAn, Fe/(Fe + Mg) of garnet, XGra, and XSpa all decrease. Afterboth chlorite and epidote are removed, continued heating upto the metamorphic peak of {small tilde}600C produces littleprogress of the reaction Grt + Ms = Bt + Pl; and XAn increases. The four assemblages have been numerically modeled using theGibbs method starting with measured compositions. The modelssuccessfully predict the observed compositional zoning and trendsof mineral growth and consumption along the computed P–Tpaths. The models also predict the compositional mineral zoningthat would have resulted from other P–T paths. * Present address: Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487  相似文献   

5.
The objective of this study is to provide insights into the REE and Y behavior during garnet porphyroblast formation in staurolite-bearing schists as a constituent of Late Paleoproterozoic metapelites of the Ladoga Complex. The MnNCKFMASH P–T pseudosection for a single sample and Grt–Bt thermometry indicate that the garnet core grew at 520°C and under 7.0–7.2 kbar in the Grt–Bt–Pl–Chl–Ms–Zo field, whereas the garnet rim was equilibrated at 590–600°C and under 3.5–4.0 kbar. The measured zoning profiles are strongly depleted in REE + Y in the garnet core containing high Mn and Ca concentrations. The intermediate zone of garnet is enriched in La, Ce, Pr, and Nd (inner LREE + Nd annulus), as well as in Dy, Er, Yb, Lu, and Y (outer HREE + Y + Dy annulus). According to pseudosection analysis, these peaks were probably produced owing to breakdown of epidote-group minerals (allanite, REE-rich epidote) at T < 535°C and P > 6.5 kbar. Towards the rim, the HREE + Y contents gradually decrease, whereas MREE (Sm, Eu, Gd) display an inverse trend. The rim also exhibits a negative Eu anomaly. The former tendency reflects an increase in temperature during garnet crystallization and partitioning of elements between garnet and monazite. It is thought that the latter is linked to oppositely directed change in garnet-monazite partition coefficients for HREE and MREE with increasing temperature.  相似文献   

6.
The Southern Brittany Migmatite Belt (SBMB), which evolved through the metamorphic peak between c. 400 Ma and c. . 370 Ma ago, consists of a heterogeneous suite of high-grade gneisses and anatectic migmatites, both metatexites and diatexites. Rare garnet-cordierite gneiss layers record evidence of an early prograde P-T path. In these rocks, growth-zoned garnet cores and a sequence of included mineral assemblages in garnet, from core to rim, of Qtz + Ilm + Ky, Pl + Ky + St + Rt + Bt and Pl + Sil + St + Rt + Bt constrain a prograde evolution during which the reactions Ilm + Ky + Qtz→ Aim + Rt, Ms + Chl→ St + Bt + Qtz + V and St + Qtz→ Grt + Sil + V were crossed. Parts of this prograde evolution are preserved as inclusion assemblages in garnet in all other rock types. In all rock types, garnet has reverse zoned rims, and garnet replacement by cordierite and/or biotite and plagioclase suggests the following reactions have occurred: Grt + Sil + Qtz→ Crd → Hc ± Ilm, Bt + Sil + Qtz → Crd ± Hc → Ilm → Kfs + V and (Na + Ca + K + Ti) + Grt → Bt + Pl + Qtz. Microstructural analysis of reaction textures in conjunction with a petrogenetic grid has enabled the construction of a tightly constrained 'clockwise' P–T path for the SBMB. The high-temperature part of the path has a steep dT/dP slope characteristic of near isothermal decompression. It is proposed that the P-T path followed by the SBMB is the result of the inversion, by overthrusting, of a back-arc basin and that such a tectonic setting may be applicable to other high-temperature migmatite terranes. The near isothermal decompression is at least partly driven by the upward (diapiric) movement of the diatexite/anatectic granite core of the SBMB.  相似文献   

7.
The prograde evolution of minerals in metapelites of a Barrovian sequence from the tri-state area (Massachusetts, Connecticut, New York) of the Taconic Range involves assemblages with garnet (Ga), chlorite (Ch), chloritoid (Ct), biotite (Bi) and staurolite (St). Detailed petrologic observations, mineral compositions and chemical zoning in garnet show: (1) garnet high in Mn and Fe but low in Mg is stable with chlorite at grades below those where chloritoid+biotite is found; (2) early formed garnet reacted partially to form Ct+Bi at intermediate grades; (3) at higher grades garnet (with low Mn)+chlorite is again produced, at the expense of chloritoid+biotite, suggesting a reversal in the continuous reaction involving the phases Ga, Ch, Ct and Bi. Thermodynamic modeling of the assemblage Ga+Ch+Ct+Bi±St in the MnKFMASH system reveals: (1) in the MnKFASH system the prograde reaction is Ga+Ch=Ct+Bi whereas in the KFMASH system the prograde reaction is the opposite: Ct+Bi=Ga+Ch; (2) the Ga–Ch–Ct–Bi–St invariant point in the KFMASH system occurs twice, at approximately 6.5 kbar, 545° C and 14.8 kbar, 580° C (although one of them may be metastable in a complex phase system); the appearance of the petrogenetic grid is markedly different from that of Albee, but similar to that of Spear and Cheney; (3) as a consequence, in the KFMASH system, chloritoid+biotite are stable over a wide range of P-T conditions whereas garnet+chlorite assemblages are restricted to a narrow band of P-T conditions; (4) MnO increases the stability field of Ga+Ch relative to both Ct+Bi at low temperatures, and St+Bi at high temperatures; (5) in natural samples the occurrence of Ct+Bi is controlled more by bulk Mg–Fe(-Mn) composition than P-T conditions. Specifically, Ct+Bi is restricted to bulk compositions with Fe/(Mg+Fe+Mn)>0.6. Rocks with Fe/(Mg+Fe+Mn)<0.5 are likely to display only chlorite+biotite at low grade. These observations are consistent with Wang and Spear and Spear and Cheney.  相似文献   

8.
This study performed equilibrium experiments in order to evaluate previously proposed formulations of the garnet (Grt)–clinopyroxene (Cpx) thermometer as applied to eclogites. The starting material is fine-grained powder of natural eclogite (<10 μm), whose main constituents are Grt (Fe:Mg:Ca∼44:28:28), Cpx (Na pfu∼0.55–0.60), phengite, quartz and rutile. Experimental conditions are 1,100–1,250°C at 2.5 GPa, and the run duration is 193–334 h. The experimental run products mainly consist of Grt, Cpx, and glass. In a preliminary experiment at 1,000°C for 144 h, Cpx grains are clearly zoned and most Grt grains maintain primary compositions. In the higher T (≥1,100°C) and longer run (≥193 h) experiments, Cpx in the run products becomes poorer in Na and higher in Fe/Mg compared with the starting material, and each grain does not show clear chemical zoning. Garnet compositions become poorer in Ca [Ca/(Fe+Mn+Mg+Ca)∼0.2–0.25] and lower in Fe/Mg compared with the starting material. The average composition of Cpx and the average of Ca-poor Grt compositions in each run product were used to evaluate previously proposed formulations of the Grt–Cpx thermometer. Temperatures calculated with formulations by Pattison and Newton (1989) and Berman et al. (1995) are much lower than the experimental temperatures, even though these formulations are based on the compositional bracketing-type experiment. One of the reasons for this discrepancy might be uncertainty of solid-solution properties of Al in Cpx, because the value of the excess interaction parameter for Al in the generally low-Al Cpx modeled by Berman et al. (1995) is much higher than those proposed by independent experiments, resulting in the estimated temperatures being significantly lower than the experimental temperatures.  相似文献   

9.
Mineral assemblages in metapelites of the contact aureole of the Tono granodiorite mass, northeast Japan, change systematically during progressive metamorphism along an isobaric path at 2-3 kbar. The bulk rock compositions of metapelites are aluminous with A' values on an AFM projection larger than that of the chlorite join. The metapelites commonly contain paragonite in the low-grade zone. With increasing temperatures, andalusite is formed by the breakdown of paragonite. The importance of pyrophyllite as a source of Al2SiO5 polymorphs is limited in typical pelitic rocks.
The most common type of metapelite in the study area has FeO/(FeO + MgO) = 0.5–0.6, and develops assemblages involving chlorite, andalusite, biotite, cordierite, K-feldspar, sillimanite and almandine, with paragenetic changes similar to other andalusite-sillimanite type aureoles. Rocks with FeO/(FeO + MgO) > 0.8 progressively develop chloritoid-bearing assemblages from Bt-Chl-Cld, And-Bt-Cld, to And-Bt at temperatures between the breakdown of paragonite and the appearance of cordierite in the more common pelitic rocks in the aureole. The paragenetic relations are explained by a KFMASH univariant reaction of Chl + Cld = And + Bt located to the low-temperature side of the formation of cordierite by the terminal equilibrium of chlorite. A P-T model depicting the relative stability of chloritoid and staurolite at low- and medium-pressure conditions, respectively, is proposed, based on the derived location of the Chl + Cld = And + Bt reaction combined with the theoretical phase relations among biotite, chlorite, chloritoid, garnet and staurolite.  相似文献   

10.
Abstract Muscovite-poor pelitic schists in the wallrocks of the Proterozoic Annex sulphide deposit, near Prieska, South Africa, contain peak metamorphic assemblages including Crd + Bt + Sil, St + Sil + Bt, Crd + St + Bt and, rarely, Ky + St ° Crd. All rocks include oligoclase, quartz and commonly Fe–Mn garnet, with or without muscovite. Peak assemblages, assigned to M2 regional metamorphism in the Gordonia Belt (Namaqua Province), are syn- to post-kinematic with respect to the main S2 fabric although larger staurolite grains contain S1 inclusion trails. Garnet–biotite thermometry, utilizing corrections for Fe3+, Mn, AlVI and Ti, yields peak temperatures of 571–624°C at pressures of 4.5–6.0 kbar. Consideration of the sympathetic variation of XMn in garnet with XMg in biotite and the preserved zoning patterns in prograde garnets, together with the inferred prograde transition from kyanite to sillimanite, indicates that heating occurred during mild decompression to the M2 metamorphic peak. Sillimanite and cordierite grew last in the prograde sequence, possibly related to a pulse of thermal metamorphism (M3) that is found along the margin of the Keimoes Suite batholith to the north. Retrograde assemblages, including Ms + Ky + Chl + Qtz (after Crd + Bt), Ky + Ms (after Sil) and Chl + Ms (after St) indicate a period of isobaric cooling (M4a) terminated by rehydration in the kyanite stability field at about 500°C. The size difference between prograde (1–2-mm) and retrograde (0.05–0.1-mm) mineral grains indicates substantial undercooling below equilibrium positions of relevant retrograde reactions prior to rehydration, and explains why cordierite that grew during M2 is almost completely destroyed. Post-M4a regrowth of staurolite and garnet (M4b) is spatially linked to sites of M4a rehydration. It reached temperatures of 510–530°C, remaining within the stability field of kyanite. A best fit of the observed textural history to the Namaqua orogenic cycle involves collision and heating (M2/D2) followed by granite intrusion (M3), rifting (M4a) and renewed heating due to crustal loading during volcanism (M4b). The P–T path for the Annex region is consistent with those derived from elsewhere in the Gordonia Belt and, with modification, to that published already for the nearby Prieska Copper Mines.  相似文献   

11.
Summary The Karimnagar granulite terrain is an integral part of the Eastern Dharwar Craton (EDC). It has received much interest because of the only reported granulite facies rocks in the EDC. These granulites contain quartz-free sapphirine-spinel-bearing granulites, kornerupine – bearing granulites, mafic granulites, orthopyroxene-cordierite gneisses, charnockites, amphibolites, dolerite dykes, granite gneisses, quartzites and banded magnetite quartzite. The orthopyroxene-cordierite gneisses occur as enclaves within granite-gneiss in association with banded magnetite quartzites, charnockites and amphibolites. The observed reaction textures, spectacular as they are, have an extraordinary information content within a tiny domain. Coronas, symplectites and resorption textures are of particular interest as they reflect discontinuous or continuous reactions under changing physical conditions. The main mineral assemblages encountered in these gneisses are orthopyroxene – cordierite – biotite – plagioclase – perthite – quartz and garnet – orthopyroxene – cordierite – biotite – quartz – plagioclase – perthite ± sillimanite. Multiphase reaction textures in conjunction with mineral chemical data in the KFMASH system indicate the following reactions: Based on chemographic relationships and petrogenetic grids in the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system, a sequence of prograde (early stage), isothermal decompression (middle stage) and retrograde (late stage) reactions (‘back reactions’ and hydration reactions) are inferred. Relatively lower P–T estimates (0.35 GPa/550–750 °C) obtained from the different geothermobarometers are attributed to late Fe–Mg re-equilibration during cooling. Therefore, the convergence method has been applied to retrieve simultaneously the P–T conditions of the thermal peak of metamorphism. The near thermal peak condition of metamorphism estimated by the convergence method are 850 °C/0.62 GPa. The P–T estimates define a retrograde trajectory with substantial decompression.  相似文献   

12.
文飞  田忠华  刘平华  向华 《地球科学》2020,45(9):3403-3419
巴罗式递增变质带能为地壳增厚及造山作用带来全新的认识.为了解辽东半岛北辽河群浪子山组内巴罗式变质带的野外分布和变质特征,对本溪地区连山关-祁家堡巴罗式变质带进行了详细的野外调查和室内研究工作.野外填图表明,浪子山组巴罗式变质带由南向北可以分为黑云母带、石榴石带、十字石带和蓝晶石带.岩相学研究表明,蓝晶石带的蓝晶十字石榴云母片岩保存了3阶段的矿物组合:（1）进变质阶段（M1）矿物组合为Pl+Qz+Ms+Bt+Ctd+Chl±Grt;（2）峰期变质阶段（M2）以Ky+St+Grt+Bt+Ms+Qz+Pl+Ilm为特征;（3）退变质阶段以毛发状的矽线石（M3-1）和变斑晶边缘或裂隙处生长的绿泥石和绢云母为特征（M3-2）.相平衡模拟表明,蓝晶十字石榴云母片岩所经历的进变质和峰期变质温压条件分别为~440℃/~3.7 kbar,~670℃/~7.9 kbar,具有典型的顺时针P-T轨迹.变质锆石U-Pb定年结果得到蓝晶十字石榴云母片岩经历了~1.96 Ga的峰期变质作用;碎屑锆石U-Pb年龄分布于2 631~2 020 Ma,浪子山组蓝晶十字石榴云母片岩碎屑物源可能来自于太古宙基底（~2.5 Ga）和2.2~2.1 Ga的岩浆岩.结合前人研究资料表明,浪子山组巴罗式变质带所记录的P-T-t轨迹以及大量的逆冲推覆构造揭示了胶-辽-吉带在~1.96 Ga经历了与造山作用相关的地壳增厚过程.   相似文献   

13.
Amphibolites of the Post Pond Volcanics, south-west corner ofthe Mt. Cube Quadrangle, Vermont, are characterized by a greatdiversity of bulk rock types that give rise to a wide varietyof low-variance mineral assemblges. Original rock types arebelieved to have been intrusive and extrusive volcanics, hydrothermallyaltered volcanics and volcanogenic sediments with or withoutadmixtures of sedimentary detritus. Metamorphism was of staurolite-kyanitegrade. Geothermometry yields a temperature of 535 ± 20°C at pressures of 5–6 kb. Partitioning of Fe and Mg between coexisting phases is systematic,indicating a close approach to chemical equilibrium was attained.Relative enrichment of Fe/Mg is garnet > staurolite >gedrite > anthophyllite cummingtonite hornblende > biotite> chlorite > wonesite > cordierite dolomite > talc;relative enrichment in Mn/Mg is garnet > dolomite > gedrite> staurolite cummingtonite > hornblende > anthophyllite> cordierite > biotite > wonesite > chlorite >talc. between coexisting amphiboles varies as a function ofbulk Fe/Mg, which is inconsistent with an ideal molecular solutionmodel for amphiboles. Mineral assemblages are conveniently divided into carbonate+ hornblende-bearing, hornblende-bearing (carbonate-absent)and hornblende-absent. The carbonate-bearing assemblages allcontain hornblende + dolomite+ calcite + plagioclase (andesineand/or anorthite) + quartz with the additional phases garnetand epidote (in Fe-rich rocks) and chlorite ± cummingtonite(in magnesian rocks). Carbonate-bearing assemblages are restrictedto the most calcic bulk compositions. Hornblende-bearing (carbonate absent) assemblages occur in rocksof lower CaO content than the carbonate-bearing assemblages.All of these assemblages contain hornblende + andesine ±quartz + Fe-Ti oxide (rutile in magnesian rocks and ilmenitein Fe-rich rocks). In rocks of low Al content, cummingtoniteand two orthoamphiboles (gedrite and anthophyllite) are common.In addition, garnet is found in Fe-rich rocks and chlorite isfound in Mg-rich rocks. Several samples were found that containhornblende + cummingtonite + gedrite + anthophyllite ±garnet +chlorite + andesine + quartz + Fe-Ti oxide ±biotite. Aluminous assemblages contain hornblende + staurolite+ garnet ± anorthite/bytownite (coexisting with andesine)± gedrite ± biotite ± chlorite ±andesine ± quartz ± ilmenite. Hornblende-absentassemblages are restricted to Mg-rich, Ca-poor bulk compositions.These rocks contain chlorite ± cordierite ± staurolite± talc ± gedrite ± anthophyllite ±cummingtonite ± garnet ± biotite ± rutile± quartz ± andesine. The actual assemblage observeddepends strongly on Fe/Mg, Ca/Na and Al/Al + Fe + Mg. The chemistry of these rocks can be represented, to a firstapproximation, by the model system SiO2–Al2O3–MgO–FeO–CaO–Na2O–H2O–CO2;graphical representation is thus achieved by projection fromquartz, andesine, H2O and CO2 into the tetrahedron Fe–Ca–Mg–Al.The volumes defined by compositions of coexisting phases filla large portion of this tetrahedron. In general, the distributionof these phase volumes is quite regular, although in detailthere are a large number of phase volumes that overlap otherphase volumes, especially with respect to Fe/Mg ratios. Algebraicand graphical analysis of numerous different assemblages indicatethat every one of the phase volumes should shift to more magnesiancompositions with decreasing µH2O. It is therefore suggestedthat the overlapping phase volumes are the result of differentassemblages having crystallized in equilibrium with differentvalues of µH2O or µCO2 and that the different valuesmay have been inherited from the original H2O and CO2 contentof the volcanic prototype. If true, this implies that eithera fluid phase was not present during metamorphism, or that fluidflow between rocks was very restricted.  相似文献   

14.
ZINGG  A. J. 《Journal of Petrology》1996,37(3):471-496
The literature data of nine different occurrences of coexistingmineral pairs of Ca-amphibole have been studied and the bulkvectors, spanning the miscibility gap, derived. The additivecomponent is always impure Mg-tremolite accompanied by someglaucophane and cummingtonite component. The four major exchangecomponents required to describe the compositional variationin coexisting mineral pairs are the edenite (ED), tschermak's(TS), FeMg–1 and Fe3+-tschermak's (FeTs) vector. Trivalentiron is postulated on the basis of excess charges in the bulkvector the size of which coincides with residuals in Altet,–Si, Fe and –Mg. The four cations have equal sizes,forming the vector Fe3+ Altet Mg–1Si–1. This distributionscheme is consistent for all the different occurrences and setsthe basis for a comparison. Deviations from the scheme wouldradically complicate the proposed exchange pattern. The ratioTS:ED in most mineral samples fluctuates between one and two.Projection of the data points in the vector space TS–EDonto the line 1ED: 2TS (Tr–Hbl) or 1ED:1TS (Tr–Prg)provides the projected tremolite content (= 1–XHbl or = 1–Xprg). This parameter,applied to coexisting pairs, and plotted against the ratio Mg/(Mg+ Fe) shows some characteristic features about the miscibilitygap. In the Mg-pure system the solvus is almost symmetric andlocated in the temperature range between 800 and 870C. Smallamounts (0.10 pfu) of Fe2+ in the M(4) -sites and replacingCa have a dramatic effect, forcing the solvus to much lowertemperatures of 650C. An increase in the ratio Fe/(Fe + Mg)causes a shift of the solvus towards more tremolitic compositionswith temperatures 500–650C. The maximum asymmetry ofthe solvus is reached where the Al-poor member (tremolite) hasa composition of =1.0 and Mg/ (Mg + Fe) 0.6. The corresponding Al-rich member has =0.5 and Mg/ (Mg + Fe) 0.4. An anomalyof the solous is observed at Mg/ (Mg + Fe)=0.8. It manifestsas a kind of highly asymmetric ‘sub-gap’ in thetremolite-rich composition range. This is explained by the partitioningof Fe2+ into the single M(3) -site and is characterized by athermal hump to 650–700C. KEY WORDS: tremolite; hornblende; pargasite; immiscibility; solous  相似文献   

15.
The basement of the North China craton (NCC) can be divided into eastern and western blocks separating the Trans-North China orogen on the basis of petrologic associations, structures, metamorphic processes, and isotopic ages. Aluminous gneiss khondalites occur in the western block, and record a clockwise metamorphic P–T history characterized by nearly isothermal decompression following peak metamorphism at ca. 1.3 GPa and 825°C. Four metamorphic stages are recognized based on mineral assemblages. The early prograde metamorphic assemblage contains Ky+Bt+Ms+Grt+Pl+Qtz. The peak metamorphic mineral assemblage is characterized by Grt+Sil+Bt+Kfs+Pl+Qtz and the formation of cordierite after garnet, leading to a retrograde assemblage of Grt+Sil+Crd+Pl+Kfs+Qtz. Garnet retrogrades to biotite and the formation of pervasive matrix muscovite define a final metamorphic stage, inferred at ca. < 0.6 GPa and 700°C. Quantified metamorphic stages and a related clockwise P–T path derived from pseudosection analysis in the KMASH system suggest collision of the north Yinshan block with the South Ordos block at 1.92 Ga, before final suturing of the entire NCC basement.  相似文献   

16.
The dominant foliation (S2) in the metapelites of the Southalpine basement, near the western side of the Tertiary Adamello intrusive stock, is a Variscan greenschist facies planar fabric, slightly reworked during thick-skin Alpine tectonics. S2 is defined by muscovite and chlorite and was formed by decrenulation of pre-existing foliations, which are confined to metre-size, less-deformed domains and defined by biotite and white mica. The pre-S2 fabric is composite (D1a & D1b) and defined by contrasting amphibolite facies metamorphic assemblages in different residual sites. Cld+BtI+Grt+MsI+Pl+Qtz and St+BtII+Grt+MsII+Pl+Qtz assemblages mark D1a and D1b fabrics respectively; these developed during successive steps of a single, temperature-prograde polyphase event, rather than during separate tectonometamorphic imprints affecting different tectonic units, later coupled during a D2 greenschist facies stage. Thermobarometric estimates of assemblages formed during D1a, D1b and D2 show a transition from T  =480–540  °C (during D1a) to T  =570–660  °C (during D1b), corresponding to a slight pressure-increase from 0.75–0.95  GPa to 0.85–1.15  GPa. D2 greenschist retrogression corresponds to a pressure and temperature decrease ( T  <400–550  °C and P <0.3–0.4  GPa). This P–T– deformation–time path is inferred to be the result of uplift from a depth of c. 35  km, after Palaeozoic subduction and continental collision; it is consistent with models postulated for other metamorphic units of the Variscan Belt in Europe. This is the first documented example in the Southern Alps of temperature-prograde metamorphism before Palaeozoic collision.  相似文献   

17.
Metapelites containing muscovite, cordierite, staurolite and biotite (Ms+Crd+St+Bt) are relatively rare but have been reported from a number of low-pressure (andalusite–sillimanite) regional metamorphic terranes. Paradoxically, they do not occur in contact aureoles formed at the same low pressures, raising the question as to whether they represent a stable association. A stable Ms+Crd+St+Bt assemblage implies a stable Ms+Bt+Qtz+Crd+St+Al2SiO5+Chl+H2O invariant point (IP1), the latter which has precluded construction of a petrogenetic grid for metapelites that reconciles natural phase relations at high and low pressure. Petrogenetic grids calculated from internally consistent thermodynamic databases do not provide a reliable means to evaluate the problem because the grid topology is sensitive to small changes in the thermodynamic data. Topological analysis of invariant point IP1 places strict limits on possible phase equilibria and mineral compositions for metamorphic field gradients at higher and lower pressure than the invariant point. These constraints are then compared with natural data from contact aureoles and reported Ms+Crd+St+Bt occurrences. We find that there are numerous topological, textural and compositional incongruities in reported natural assemblages that lead us to argue that Ms+Crd+St+Bt is either not a stable association or is restricted to such low pressures and Fe-rich compositions that it is rarely if ever developed in natural rocks. Instead, we argue that reported Ms+Crd+St+Bt assemblages are products of polymetamorphism, and, from their textures, are useful indicators of P–T  paths and tectonothermal processes at low pressure. A number of well-known Ms+Crd+St+Bt occurrences are discussed within this framework, including south-central Maine, the Pyrenees and especially SW Nova Scotia.  相似文献   

18.
Constraints from P-T pseudosections (MnNCKFMASH system), foliation intersection/inflection axes preserved in porphyroblasts (FIAs), mineral assemblages and textural relationships for rocks containing all three Al2SiO5 polymorphs indicate a kyanite→ andalusite→ sillimanite sequential formation at different times rather than stable coexistence at the Al2SiO5 triple point. All three Al2SiO5 polymorphs grew in the Chl, Bt, Ms, Grt, St, Pl and Crd bearing Ordovician Clayhole Schist in Balcooma, northeastern Australia separately along a looped P-T-t-D path that swaps from clockwise to anticlockwise in the tectono-metamorphic history of the region. Kyanite grew during crustal thickening in an Early Silurian Orogenic event followed by decompression/heating, andalusite and fibrolitic sillimanite growth during Early Devonian exhumation.  相似文献   

19.
Rare centimeter-sized superzoned garnets (SZGs) were discovered in two coesite-bearing whiteschists of the Brossasco-Isasca Unit (BIU), southern Dora-Maira massif (DMM), Western Alps. The superzoned garnet consists of a reddish-brown almandine core crowded with inclusions of staurolite, chloritoid, kyanite, chlorite and paragonite, and of a pinkish pyrope rim with sporadic inclusions of kyanite, and magnesian chlorite. The core–rim contact is relatively sharp and marks the termination of the inclusion-rich portion. The core composition of the superzoned garnet is almost identical to, or slightly richer in Mg, than that of the rim of porphyroblastic garnet in metapelites from the same unit. In the rim of the superzoned garnet, Mg–Fe ratio increases abruptly towards the outermost rim, whose composition is identical to that of the common pyrope in the whiteschist. At the core–rim boundary, there is no chemical gap. Chloritoid and staurolite are common inclusions in the core of the superzoned garnet in the whiteschist and in the porphyroblastic garnet in the metapelite. The staurolite composition (Si=2.00 and total R2+<2.0 for O=23 basis) and its reverse Fe–Mg distribution with respect to garnet suggest a HP origin. The Fe–Mg distribution between chloritoid and garnet is reverse in the superzoned garnet, but normal in the garnet of metapelite. Because normal Fe–Mg distribution was reported from other eclogite-facies metapelites, a model petrogenetic grid was constructed in the FMASH model system considering St, Cld, Ky, Chl, Grt, and assuming the following Fe–Mg partitioning of St>Grt>Cld>Chl. The resulting petrogenetic grid suggests that the core of the superzoned garnet contains incompatible assemblages, such as St–Cld–Chl vs. Cld–Chl–Ky. New and literature data and results of experiments in the KFASH system suggest that: (1) the superzoned garnet was formed under a single prograde high-pressure/ultra high-pressure (HP/UHP) Alpine metamorphism, (2) the almandine inclusion-rich core of the superzoned garnet crystallized at disequilibrium in a pelitic composition system at around 600°C and less than 16 kbar, probably from a former metapelite xenolith included in a Variscan granitoid, and (3) the chemical environment of the host rock suddenly changed from the normal pelite to the whiteschist composition by a metasomatic process during the rim growth, i.e., at a stage close to the UHP climax.  相似文献   

20.
Quartz Al–Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual high-grade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxene–sillimanite–garnet–quartz, sapphirine–quartz and spinel–quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+Qtz→Opx+Sil, (2) Opx+Sil→Grt+Spr+Qtz, (3) Grt+Sil+Qtz→Crd, (4) Grt+Sil→Crd+Spr, (5) Grt+Sil+Spr→Crd+Spl, (6) Grt+Sil→Crd+Spl, (7) Grt+Crd+Sil→Spl+Qtz and (8) Grt+Sil→Spl+Qtz. Minerals in quartz Al–Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe2+) range of the main minerals is as follows: cordierite (0.81–0.97), sapphirine (0.77–0.88), orthopyroxene (0.65–0.81), garnet (0.33–0.64) and spinel (0.23–0.56). The reaction textures and the evolution of the mineral assemblages in the quartz Al–Mg granulites indicate a clockwise P–T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.  相似文献   

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