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1.
Detailed textural and chemical data for mineral assemblages on a regional scale are presented for the metaandesitic Eocene-Oligocene Taveyanne greywacke of the Glarus Alps, Eastern Switzerland. Presented data indicate an increase of metamorphic grade from zeolite facies to prehnite-pumpellyite and pumpellyite-actinolite facies. Low-grade outcrops contain laumontite, minor corrensite and pumpellyite (assemblage type 1), whereas outcrops of higher metamorphic grade contain prehnite and two populations of pumpellyite (type 2), prehnite—pumpellyite-(Al)—white mica (type 3), a single outcrop shows pumpellyite-actinolite facies (type 4). From the zeolite to prehnite-pumpellyite/pumpellyite-actinolite facies there are indications for an increase of the chemical equilibrium domain size for the critical paragenesis from a single detrital grain ≤1 mm) in type 1, to a few millimetres in type 2, and to a whole thin section in type 3. Metamorphic P - T conditions were determined by a combination of chlorite thermometry, fluid inclusion and vitrinite reflectance data. Peak temperatures range from 170-190 C for zeolite facies to 270-310 C for prehnite-pumpellyite and pumpellyite-actinolite facies. For the higher temperature range, pressures of 2-3 kbar are derived indicating a geothermal gradient of 24-32 C km-1. The well-constrained temperature estimations derived for the assemblages provide a useful test of the different empirical calibrations of chlorite thermometers recently proposed. The best correspondence to the temperatures determined here is for the Cathelineau calibration. In addition, in the lower grade samples differences in textures and calculated temperatures provide a mean to distinguish between detrital and newly formed chlorites.  相似文献   

2.
Metamorphism in the late Permian to early Cretaceous North Island basement greywackes has been investigated using petrography and clay mineral crystallinity. Several terranes are represented in the North Island greywackes and the study area includes Murihiku, Manaia Hill, Bay of Islands and Omahuta terranes and the Mélange Zone. Very low-grade metamorphic events in the greywackes have produced mineral assemblages of zeolite to pumpellyite-actinolite greywacke facies. Zeolite facies greywackes are characterized by the assemblage Zeo (Lmt, Anl, Hul)+Qtz±Ab±Cal± Chl±I±I/S* observed in the entire Murihiku terrane and in the eastern part of the Bay of Islands terrane and the Mélange Zone. The entire Manaia Hill, most of the Bay of Islands, the eastern area of the Omahuta terranes and the central part of the Mélange Zone are at prehnite-pumpellyite facies with mineral assemblages of Prh+Qtz+Chl+Pmp+Ab+± Ill±Cal±Lmt. Pumpellyite-actinolite facies with the mineral assemblage of Pmp±Act+Qtz+Ab+Chl±Ep±Ill±Cal±Chl occurs in the western part of the Mélange Zone and the Omahuta terrane.

Illite (IC) and chlorite (ChC) crystallinity values of greywackes are very similar and range from diagenetic zone to anchizone. Metamorphic conditions indicated by the IC and ChC and mineral facies are in excellent agreement and correlate as follows: crystallinity diagenetic-zone with the zeolite mineral facies, crystallinity lower anchizone with prehnite-pumpellyite mineral facies and crystallinity upper anchizone with pumpellyite-actinolite mineral facies. The general increase in the metamorphic grade from east to west, except in Murihiku terrane, is compatible with the sequence of accretion expected in a subduction environment.  相似文献   


3.
The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A12O3-Fe2O3-SiO2-H2O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the XFe3+ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe3+ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-XFe3+ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H2O, where the XFe3+ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-XFe3+ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant.  相似文献   

4.
Almost pure andradite and intermediate members of the andradite-grossular series (gros40–49, and 47–54, py0–3, alm0–3, spess0–2, hydrogarnet0–3), often framboidal in habit, are widespread in metabasites including lavas, minor intrusions, and volcanic sandstones and breccias metamorphosed under prehnite-pumpellyite and pumpellyite-actinolite facies conditions, possibly extending into the zeolite facies. Coexisting phases include iron-rich epidotes (100 Fe*/Fe*+Al=22–34), pumpellyite, prehnite, actinolite, and chlorite, electron microprobe analyses of which are given, as well as quartz, albite, and calcite. Zoisite (100 Fe*/Fe*+Al=1–5) and iron-poor epidote (100 Fe*/Fe*+Al=11–18) occur in 2 rocks in pseudomorphs after plagioclase together with more iron-rich epidote, but not in close association with the garnets. Coexisting pumpellyite is iron-rich (FeO* 9–14%) in the prehnite-pumpellyite facies and iron-poor (FeO* 5%) in the pumpellyiteactinolite facies. Chlorites and actinolites vary widely and sympathetically in FeO/MgO+FeO ratio. Andradite is also described from a stilpnomelane-actinolite-hematite-bearing andradite quartzite of the pumpellyite-actinolite facies. Conditions of formation involved temperatures of 300 to 400 ° or less, at pressures up to a few kilobars. A wide range of oxygen fugacities is possible, but in the fluid phase was low. Grandite and chlorite are incompatible in the pumpellyite-actinolite and greenschist facies in the presence of quartz but the 2 minerals occur together in some pumpellyite-actinolite facies assemblages as a result of incomplete reaction and/or local deficiency in silica. In the greenschist facies the association is replaced by epidote-actinolite±hematite and sodic amphibole. Whereas at medium to high grades of metamorphism andradite and grandite are characteristic of skarns irrespective of , at very low grades they are found in mafic volcanic rocks and volcanogenic sediments as well as in certain cherty rocks of unusual composition, rodingites, and serpentinites, where was very low.  相似文献   

5.
Metamorphic assemblages and mineral compositions in basic to intermediate volcanic rocks of two Precambrian units (the sub-Jotnian and Jotnian) in the Dala region, central Sweden, reveal a history comprising two metamorphic episodes. The sub-Jotnian rocks were metamorphosed during a first episode: the metamorphic grade ranges from prehnite-pumpellyite facies (in a displaced block with sub-Jotnian (?) lava occurring in the border zone of a major impact structure), to pumpellyite-actinolite facies, and to greenschist facies in the lower part of the unit. The unconformably overlying Jotnian rocks were metamorphosed to prehnite-pumpellyite facies during a subsequent episode, which also left its imprint in the sub-Jotnian basement. Two other Jotnian sequences in central Sweden are in the same facies. There are systematic chemical differences in some of the secondary minerals. Sphenes contain more Al, and epidotes and pumpellyites are more Fe-rich in Jotnian (prehnite-pumpellyite facies) rocks compared with sub-Jotnian rocks of pumpellyite-actinolite facies. Most epidotes in Jotnian lavas are remarkably rich in Fe, with a replacement of Al by Fe3+ up to 1.5 atoms per formula unit (Ps50). Compositions of epidote and pumpellyite in pervasively altered rocks vary sympathetically. Pumpellyite composition is partly controlled by the extent of alteration in its host rock. There is a trend for the compositional field of pumpellyite to shrink and shift away from the Al corner of the Al-Fe*-Mg triangle with increasing extent of host rock alteration. This trend is most evident in Jotnian rocks. Since the fields in the Al-Fe*-Mg triangle given in the literature as representative of pumpellyites from different metamorphic fades are partly based on analyses of samples containing relicts, these fields should be adjusted if future investigations show that the trend is of general validity.  相似文献   

6.
伊利石结晶度及其在低温变质研究中若干问题的讨论   总被引：4，自引：0，他引：4  
王河锦  陶晓风  M.Rahn 《地学前缘》2007,14(1):151-156
文中介绍与伊利石结晶度相关的伊利石矿物新的定名方案;讨论了伊利石结晶度的结晶学基础与Scherrer方程式的关系;讨论了三个重要的伊利石结晶度指数Weaver、K櫣bler和Weber指数的关系,并介绍推导的Weaver、K櫣bler和Weber指数的理论关系式;评价了衍射仪测量条件对结晶度数值的影响和三套伊利石结晶度标样的作用以及成岩带、变质带与葡萄石-绿纤石相、沸石相、绿纤石-阳起石亚相对比的依据。建议(1)停用绢云母一词;(2)建立可供各实验室使用的具有晶体结构和晶体化学参数和多型参数的伊利石结晶度标样;(3)建议多开展规范的可进行国际对比的低温变质研究,划分出与国际上一致的成岩带和近变质带,不仅利于低温变质作用理论的研究,而且由于成岩带到近变质转变带是重要的煤气油的生成带,也利于中国煤气油资源的勘探与开发,利于国民经济的长远持续发展。  相似文献   

7.
In the Ligurian Apennines and the Brianzonese area of the Maritime Alps (Northern Italy), and in the Lucanian Apennines and Calabria (Southern Italy), pumpellyite formed during the Alpine metamorphism is widespread in terranes of Late Paleozoic to Miocene age, particularly in mafic ophiolites (Tethyan ophiolites of Jurassic-Early Cretaceous age).Pumpellyite developed under variable metamorphic conditions, which include zeolite, prehnite-pumpellyite, pumpellyite-actinolite, lawsonite-albite and blueschist facies. Pumpellyite from rocks belonging to all these facies was studied in 30 specimens of different chemistry, derived mostly from ophiolitic basalt and gabbro, but also from gneiss, amphibolite, and greywacke protoliths.Microprobe data give evidence of strong compositional variations of pumpellyite. The ranges are extreme for Al and Fetot (Al2O3=18.74–30.91; FeOtot=0.46–12.71), to a lesser extent for Mg (MgO=0.58–4.00), with a reciprocal variation of Al and Fetot which suggests that most Fe is Fe3+.Pumpellyite compositions can be related with the metamorphic conditions, particularly an increase of the Al/Fetot ratio with increasing pressure, but larger compositional variation for each facies in the Al-Fetot-Mg diagram than those previously described was recognized. Particularly the compositions of pumpellyite from the pumpellyite-actinolite and blueschist fades assemblages extend towards higher Fetot contents. Moreover, pumpellyites of the lawsonite-albite facies rocks result to be more aluminous than in blueschist facies; such relations appear to be inconsistent with that expected by the effects of pressure on the Al content.As the rocks are generally incompletely re-equilibrated, their bulk chemistry is not an important factor of pumpellyite composition; in fact the correlations of Al, Fetot and Mg are low. The local environment of crystallization, more evidently the composition of the precursor minerals, seems to be a major controlling factor. The observed topologic relations indicate that either the nature of the precursor mineral, or the presence of relict phases have a great influence on the pumpellyite composition. The observed variations within individual specimens, as well as the interspecimen ones, can be explained by small-scale inhomogeneities and in some cases by differences in the activity of O2.It can be therefore concluded that the composition of pumpellyite often reflects disequilibrium crystallization and cannot be generally used as an indicator of metamorphic conditions.  相似文献   

8.
Pumpellyite from four-phase assemblages (pumpellyite + epidote + prehnite + chlorite; pumpellyite + epidote + actinolite + chlorite; pumpellyite + epidote + Na-amphibole + chlorite, together with common excess phases), considered to be low variance in a CaO-(MgO + FeO)-Al2O3-Fe2O3 (+Na2O + SiO2+ H2O) system, have been examined in areas which underwent metamorphism in the prehnite-pumpellyite, pumpellyite-actinolite and low-temperature blueschist facies respectively. The analysed mineral assemblages are compared for nearly constant (basaltic) chemical composition at varying metamorphic grade and for varying chemical composition (basic, intermediate, acidic) at constant metamorphic conditions (low-temperature blueschist facies). In the studied mineral assemblages, coexisting phases approached near chemical equilibrium. At constant (basaltic) bulk rock composition the MgO content of pumpellyite increases, and the XFe3+ of both pumpellyite and epidote decreases with increasing metamorphic grade, the Fe3+ being preferentially concentrated in epidote. Both pumpellyite and epidote compositions vary with the bulk rock composition at isofacial conditions; pumpellyite becomes progressively enriched in Fe and depleted in Mg from basic to intermediate and acidic bulk rock compositions. The compositional comparison of pumpellyites from high-variance (1–3 phases) assemblages in various bulk rock compositions (basic, intermediate, acidic rocks, greywackes, gabbros) shows that the compositional fields of both pumpellyite and epidote are wide and variable, broadly overlapping the compositional effects observed at varying metamorphic grade in low-variance assemblages. The intrinsic stability of both Fe- and Al-rich pumpellyites extends across the complete range of the considered metamorphic conditions. Element partitioning between coexisting phases is the main control on the mineral composition at different P-T conditions.  相似文献   

9.
Sandstones of the juxtaposed and partially coeval quartzofeldspathic Torlesse terrane and volcanogenic Waipapa terrane of North Island, New Zealand, are generally described as having been derived from silicic continental arc and evolved intermediate volcano-plutonic arc sources, respectively. Modal and chemical compositions of the two terranes differ slightly as a result. From textural considerations, their single-grain (unitary) detrital mineral populations are inferred to have been derived largely from the plutonic components in their sources. Intensive microscopic and electron microprobe study of two representative samples shows that the unitary detrital mineral assemblages in the two terranes are virtually identical, comprising quartz, plagioclase, K-feldspar, white mica, epidote, titanite, pumpellyite, ilmenite, rutile, tourmaline, zircon, and apatite. Detrital chlorite, garnet, and graphite also occur in the Torlesse sample, whereas amphibole, clinopyroxene, and prehnite occur in the Waipapa sample. Authigenic mineral assemblages are also similar, consisting of quartz, albite, chlorite, phengitic mica, epidote, titanite, pumpellyite, pyrite, and calcite. Stilpnomelane and pyrrhotite also occur in the Torlesse sample, and prehnite in the Waipapa specimen. These assemblages define upper prehnite-pumpellyite to lower pumpellyite-actinolite facies conditions (Torlesse) and lower prehnite-pumpellyite facies metamorphism (Waipapa). By comparison with published compositional data for minerals from plutonic, metamorphic, and volcanic rocks, electron microprobe analyses of individual minerals confirm that the unitary detrital grains in both terranes were largely derived from calc-alkaline S-type granitoid plutonic rocks. Contrasts in mineral compositions between the two terranes show that the Torlesse unitary mineral detritus was derived almost entirely from granodiorite, whereas the Waipapa grains originated from a mixed diorite, monzonite, and granodiorite plutonic component in their source. In neither terrane was detritus derived from granite in the strict sense. Although the plutonic components in their sources are lithologically similar, the compositional contrasts seen indicate that they were not coeval or spatial components of the same terrane. Detailed electron microprobe analysis of unitary detrital phases in low-grade metasedimentary rocks thus enables identification of specific source terrane lithotypes, and hence is a valuable complement to existing petrographic, modal, and chemical approaches that define more generalized provenances.  相似文献   

10.
Abstract Fairly strong (r= 0.75–0.85) positive linear correlations were found between crystallinity indices (peak widths) measured on the first two basal reflections of chlorite and those of illite–muscovite in <2-μm fractions of a representative shale–slate–phyllite series from Palaeozoic and Mesozoic formations of northeast Hungary. The metamorphic grade ranges from late or deep diagenesis through anchizone to epizone conditions. Chlorite crystallinity values measured on air-dried and ethylene-glycol-solvated samples suggest that the effects of expandable interlayers are negligable, especially in the higher grade (～temperature) part of the series. However, the greater scattering of crystallinity values for the chlorite 001 reflection compared to those of the 002 reflection may be related to the effects of minor amounts of interlayered and/or discrete smectite and/or vermiculite. With increasing metamorphic grade and advancing equilibrium recrystallization, the chlorite compositions in different samples become more homogenous. No correlation exists between crystallinity and changes in chlorite composition as estimated from the intensity ratios of basal reflections. Hence an increase of domain size and a decrease of lattice distortion with increasing grade (～temperature) may be decisive factors affecting chlorite crystallinity. Chlorite crystallinity can be applied as a reliable regional, statistical technique complementary with, or instead of, the illite crystallinity method. The illite and chlorite crystallinity scales used here are related to Kübler's epi-, anchi- and diagenetic zones and correlated with coal rank, conodont colour alteration and mineral facies data. As the effects of the detrital white mica can be observed even in the <2-μm fractions of anchizonal metapelites, the anchizone boundaries determined solely on the base of ‘fixed’illite crystallinity values may vary with amounts of detrital and newly formed muscovite–illite. Hence a complex approach utilizing more than one method for determination of grade is preferred for petrogenetic purposes, even if relationships between crystallinity scales, coal rank and mineral facies also vary strongly in different tectonic settings and lithologies.  相似文献   

11.
Electron microprobe analyses are presented for new-formed mineralsfrom a small exposure of semi-schistose Taveyanne Formationof the pumpellyite-actinolite facies near Lo?che, Valais. Comparisonsare drawn with minerals of other low-grade metamorphic areas,especially in southern New Zealand. Sphene shows considerablesubstitution of Ca(Al,Fe)SiO4(OH) for CaTiSiO5. Epidotes aresharply divided into early pistacitic (Ps = 0.28–0.37)and later clinozoisitic varieties (Ps = 0.11–0.19). Pumpellyitesrange from pumpellyite-(Fe) to pumpellyite-(Al) and are generallyless Fe-rich than those of zeolite and prehnite-pumpellyitefacies. Pumpellyite inclusions in albitized plagioclase areparticularly low in Mg. Actinolites are low in A12O3, TiO2,and Na2O, essentially identical compositions being nucleatedon detrital augite, hornblende, and in the matrix. Phengitesare also extremely low in Na2O and TiO2. Chlorites are ripidolites.Albitized clastic plagioclase has the composition An0.7–1.6and albite in clinozoisite-calcite-albite-phengite-chloriteveins An2.1–2.3. Calcites carry minor Mn > Fe ? Mg.New-formed iron oxides are absent, whereas pyrrhotite and minorpyrite occur in one rock, buffering fs2 and indicating low fo2. Ratios Mg: Fe* (Fe* = total Fe) in coexisting chlorites andA1, Na-poor actinolites vary sympathetically both in the Lo?cheand southern New Zealand rocks here considered, giving KD =(Mg/Fe*) actlnolIte/(Mg/Fe*)chlorle = 1.72. Mg/Fe* ratios inpumpellyites tend to vary sympathetically with those of coexistingchlorites and actinolites but are more variable. Substitutionof (Fe, Mg)Si for A12 in phengitic micas and chlorites variessympathetically in the same suites between mafic volcanic andmore pelitic extremes. Various minor elements also behave ina consistent fashion, indicating an encouraging tendency towardsequilibrium. Variable (though small) A12O3 contents of actinolite,Fe: Al ratios in epidotes and pumpellyites, and Mg: Fe* ratiosin phengites, even within a single grain, are evidence of short-rangedisequilibrium; metamorphic equilibration is evidently easierbetween some crystal structures and structural sites than betweenothers. In phase rule analysis of assemblages in such rocks it is commonlynecessary to treat Fe2O3, FeO, and MgO as separate componentsand it may also be necessary to regard CO2 as an inert componentand/or to interpret observed assemblages as of low variance.The presence of the Ca-Al silicates and sphene indicates verylow Xco2 in the metamorphic fluids in all rocks examined exceptan albite-chlorite-calcite-quartz-anatase assemblage. But higherAn in albites than in isofacial and in greenschist facies rocksof southern New Zealand can be ascribed to significantly higherXco2 at Lo?che, especially in the veins, than in New Zealand. Pumpellyite and epidotes of the pumpellyite-actinolite faciestend to be lower in Fe and richer in Al than those of lowergrade facies. Important reactions include those of the formpumpellyite-(Fe3+)+chlorite+quartz+H2=pumpellyite-(Al)+actinolite,and pumpellyite+chlorite+quartz- ‘epidote’+actinolite+water.Careful selection of pumpellyite and chlorite compositions isrequired for experimental and chemographic analysis of pumpellyitestability. In the absence of critical data, temperatures ofabout 250–350? and pressures of several kilobars are provisionallysuggested for the Lo?che metamorphism.  相似文献   

12.
A new petrogenetic grid for low-grade metabasites   总被引：7，自引：0，他引：7  
Abstract We have used internally-consistent thermodynamic data to present calculated phase equilibria for the system Na2O-CaO-MgO-Al2O3-SiO2-H2O (NCMASH), in the range 0–500° C and 0.1–10 kbar, involving the phases anorthite, glaucophane, grossular, heulandite, jadeite, laumontite, lawsonite, paragonite, prehnite, pumpellyite, stilbite, tremolite, wairakite, zoisite with excess albite, clinochlore, quartz and pure water. Average activity terms derived from published mineral chemical data were included for clinochlore, glaucophane, prehnite, pumpellyite, tremolite, and zoisite. The new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite, prehniteactinolite, prehnite-pumpellyite, pumpellyite-actinolite, blueschist and greenschist facies metabasites. The stability fields of mineral assemblages containing prehnite, pumpellyite, epidote, actinolite (+ albite + chlorite + quartz) were analysed in some detail, using activity data calculated from five specific samples. For example, the prehnite-actinolite facies covers a P-T field ranging from about 220 to 320° C at pressures below 4.5 kbar. The transition from the prehnite-actinolite and pumpellyite-actinolite to greenschist facies occurs at about 250–300° C at 1–3 kbar and at about 250–350° C at 3–8 kbar. P-T fields of individual facies overlap considerably due to variations in chemical composition.  相似文献   

13.
The 6km-thick Karmutsen metabasites, exposed over much of Vancouver Island, were thermally metamorphosed by intrusions of Jurassic granodiorite and granite. Observation of about 800 thin sections shows that the metabasites provide a complete succession of mineral assemblages ranging from the zeolite to pyroxene hornfels facies around the intrusion. The reaction leading to the appearance of actinolite, which is the facies boundary between prehnite-pumpellyite and prehnite-actinolite facies, was examined using calcite-free Karmutsen metabasites collected from the route along the Elk river. In the prehnite-pumpellyite facies, X Fe3+[Fe3+/(Fe3++Al)] in prehnite, pumpellyite and epidote buffered by the four-phase assemblage prehnite+pumpellyite+epidote+chlorite systematically decreases with increasing metamorphic grade. Such a trend is the reverse of that proposed by Cho et al. (1986); this may be related to the higher in the Mt. Menzies area. The actinolite-forming reaction depends on the value of X Fe3+ in pumpellyite. If using a low value of Fe3+, 3.89 Pr(0.06)+0.48 Ep(0.26)+0.60 Chl+H2O=2.10 Pm (0.08)+0.17 Act+0.88 Qz is delineated. The number in parentheses stands for the X Fe3+value in Ca-Al silicates. On the other hand, replacing the X Fe3+ of 0.08 in pumpellyite with a higher X Fe3+ value (0.24) changes the reaction to 0.41 Pm+0.02 Chl+0.42 Qz=0.11 Pr+0.62 Ep+0.10 Act+H2O. The first (hydration) reaction forms pumpellyite and actinolite on the high-temperature side, whereas the second (dehydration) reaction consumes pumpellyite to form prehnite, epidote and actinolite. The former reaction seems to explain the textural relationship of Ca-Al silicates in the study area. However, actinolite-forming reaction changes to a different reaction depending on the compositions of the participating minerals, although in the other area even physical conditions may be similar to those in the study area. Chemographic analysis of phase relations in the PrA facies indicates that the appearance of prehnite depends strongly on the bulk FeO/MgO ratio: this may explain the rarity of prehnite in common metabasites in spite of the expected dominant occurrence in the conventional pseudo-quaternary (Ca-Al-Fe3+-FM) system. An increasing FeO/MgO ratio stabilizes the Pr+Act assemblage and reduces the stability of the Pm+Act one. Therefore, the definition of pumpellyite-actinolite facies should include not only Pm+Act but also the absence of Pr+Act assemblages. In addition to the possible role of high (Cho and Liou 1987) and/or high to mask the appearance of prehnite, the effect of the FeO/MgO ratio is emphasized.  相似文献   

14.
The Ordovician volcanic rocks in the Mayaxueshan area have been pervasively altered or metamorphosed and contain abundant secondary minerals such as albite, chlorite, epidote, prehnite, pumpellyite, actinolite, titanite, quartz, and/or calcite. They were denoted as spilites or spilitic rocks in terms of their petrographic features and mineral assemblages. The metamorphic grades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. This indicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area have formed as a result of Caledonian regional metamorphism. We suggest that the previously denoted spilitic rocks or altered volcanic rocks should be re-denoted as metabasalts or metabasaltic rocks. The metamorphic grade of the volcanic rocks increases with their age: prehnite-pumpellyite facies for the upper part of the Middle Ordovician volcanic rocks, prehnite-pumpeilyite to lower greenschist facies for the lower part of the Middle Ordovician vol  相似文献   

15.
Moderately manganiferous siliceous pelagites near Meyers Pass, Torlesse Terrane, South Canterbury, New Zealand, have been metamorphosed in the prehnite–pumpellyite facies. A conodont colour index measurement suggests T max in the range 190–300 °C. Porphyroblastic manganaxinite, manganoan pumpellyite, manganoan chlorite and trace spessartine-rich garnet and sphalerite have formed in an extremely fine-grained quartz–albite–berthierine–phengite–titanite groundmass. Porphyroblastic manganaxinite semischists and schists are distinctive rocks in prehnite–pumpellyite to lower-grade greenschist and blueschist facies of New Zealand and Japan. Mn in the manganoan pumpellyites substitutes for Ca in W sites. Total Fe/(Fe+Mg) ratios in chlorite are dependent on oxidation state, being ≤0.22 in red hematitic hemipelagites, and ≥0.61 in low-f O2 grey metapelagites. In the low-f O2 metapelagites, manganoan berthierine with little or no chlorite is inferred in the groundmass and iron-rich chlorite occurs as porphyroblasts and veinlets, whereas in the red rocks, Mg-rich chlorite occurs both in groundmasses and veinlets. Variably high Si in the manganoan chlorites correlates with evidence for contaminant phases. The Mn content of chlorite contributing to garnet growth is dependent on metamorphic grade; incipient spessartine indicates a saturation value of 6–8% MnO in chlorite in low-f O2 rocks at Meyers Pass. Lower MnO contents are recorded for otherwise analogous rocks with increasing metamorphic grade, but at a given grade coexisting chlorite and garnet are richer in Mn where f O2 is high. Manganaxinite and manganoan pumpellyite also contributed to reactions forming grossular–spessartine solid solutions. Formation of garnet in siliceous pelagites is dependent on both Mn and Ca content. The spessartine component increases with grade into the greenschist facies. Partial recrystallization of berthierine to chlorite and the growth of porphyroblastic patches of other minerals was facilitated by brittle fracture and access of fluids to an otherwise impermeable matrix; to this extent the very low-grade metamorphism was episodic.  相似文献   

16.
A sequence of regional metamorphic isograds indicating a range from prehnite-pumpellyite to lower amphibolite facies was mapped in metabasites near Flin Flon, Manitoba. The lowest grade rocks contain prehnite + pumpellyite and are cut by younger brittle faults containing epidote + chlorite + calcite. Isobaric temperature- X CO2 and pressure-temperature (constant X CO2) diagrams were calculated to quantify the effects of CO2 in the metamorphic fluid on the stability of prehnite-pumpellyite facies minerals in metabasites containing excess quartz and chlorite. Prehnite and, to a lesser extent, pumpellyite are stable only in fluids with X co2 <0.002. For X co2>0.002, epidote + chlorite + calcite assemblages are stable. Our calculated phase relations are consistent with regional metamorphism in the Flin Flon area in the presence of an H2O-rich fluid and a more CO2-rich fluid in the later fault zones. We believe that the potential effects of small amounts of CO2 in the metamorphic fluid should be assessed when considering the pressure-temperature implications of mineral assemblages in low-grade metabasites.  相似文献   

17.
40Ar/39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/39Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite 40Ar/39Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite 40Ar/39Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.  相似文献   

18.
The pumpellyite–actinolite facies proposed by Hashimoto is defined by the common occurrence of the pumpellyite–actinolite assemblage in basic schists. It can help characterize the paragenesis of basic and intermediate bulk compositions, which are common constituents of various low-grade metamorphic areas. The dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite–actinolite and prehnite–pumpellyite facies in P–T space. In the Sanbagawa belt in Japan, the mineral parageneses of hematite-bearing and -free basic schists, as well as pelitic schists have been well documented. The higher temperature limit of this facies is defined by the disappearance of the pumpellyite+epidote+actinolite+chlorite assemblage in hematite-free basic schists with XFe3+ of epidote around 0.20–0.25 and the appearance of epidote+actinolite+chlorite assemblage with XEpFe3+≤0.20. In hematite-bearing basic schists, there is a continuous change of paragenesis to higher grade, epidote–glaucophane or epidote–blueschist facies. In pelitic schists, the albite+lawsonite+chlorite assemblage does occur but only rarely, and its assemblage cannot be used to determine the regional thermal structure. The lower temperature equivalence of the pumpellyite–actinolite assemblage is not observed in the field. The Mikabu Greenstone complex and the northern margin of the Chichibu complex, which are located to the south of the Sanbagawa belt, are characterized by clinopyroxene+chlorite or lawsonite+actinolite assemblages, which are lower temperature assemblages than the pumpellyite+actinolite assemblage. These three metamorphic complexes belong to the same subduction-metamorphic complex. The pumpellyite–actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes  相似文献   

19.
The major mineral assemblages of the metabasites of the Omoiji-Nagasawaarea in central Shikoku are hematite+epidote+chlorite+actinolite,riebeckitic actinolite+epidote+chlorite, epidote+chlorite+actinolite,and pumpellyite+epidote+chlorite+actinolite. The constituentminerals are often heterogeneous and assemblages in the fieldof a thin section sometimes do not obey the phase rule, butif grains apparently in non-equilibrium with others are excludedand domains of chemical equilibrium are appropriately chosenthe assemblages approximately obey the phase rule. The stability of hematite, pumpellyite, and epidote associatedwith chlorite and actinolite can be dealt with in terms of aternary system with appropriate excess phases. By fixing theFe2+/(Fe2+ +Mg) ratio of chlorite, it is dealt with in termsof stability relations in the system Ca2Al3Si3O12(OH)–Ca2AlFe2Si3O12(OH)with excess chlorite, actinolite, quartz, and controlled PH2O.The maximum and minimum Fe3+ contents of epidote in this modelsystem are determined by hematite+epidote+chlorite+actinoliteand pumpellyite+epidote+chlorite+actinolite assemblages. Themaximum Fe3+ of the three phase assemblage epidote+chlorite+actinoliteis insensitive to temperature, but the minimum Fe3+ contentof epidote is sensitive to temperature and can be used to definethe metamorphic grade by a continuous quantity related to temperature.The phase relations expected for the model system are in goodagreement with the parageneses of the Sanbagawa terrain in centralShikoku and offer an explanation to the rule of Miyashiro &Seki (1958a) that the compositional range of epidote enlargeswith increasing temperature. The model also makes it possibleto estimate semi-quantitatively the temperature range in whichthe assemblage pumpellyite+epidote+chlorite+actinolite is stable.The possible maximum range is about 120 ?C, but the assemblageis stable in metabasite only for about 90 ?C. The higher temperaturelimit of the pumpellyite-actinolite facies defined by the disappearanceof pumpellyite in metabasite corresponds to the temperatureat which epidote with Fe3+/(Fe3+ +Al) = 0.10 0.15 coexistswith pumpellyite, actinolite, and chlorite. The compositions of epidotes in the metabasites of the Omoiji-Nagasawaarea cluster around Fe3+/(Fe3+ +Al) = 0.33. The grade of thisarea is close to the lower temperature stability limit of thepumpellyite+epidote+chlorite+actinolite assemblage.  相似文献   

20.
This study uses clay mineral assemblages, illite ??crystallinity?? (IC), chlorite ??crystallinity?? (CC), illite polytypes, the b cell-dimension of K-white mica, mineral geothermo-geobarometers and homogenization temperatures of fluid inclusions to investigate the transition from diagenesis to metamorphism in a 7?km thick Triassic flysch sequence in the well Hongcan 1, eastern Tibetan plateau. The 7,012.8?m deep borehole penetrated flysch of Upper to the Middle Triassic age and represents a unique chance to characterize low temperature metamorphic processes in an unusually thick sedimentary sequence developed on thickened continental crust. Mineral assemblage analysis reveals a burial metamorphic pattern with kaolinite and chlorite/smectite mix-layer phases present in the upper 1,500?m, and illite/smectite mixed-layer phases extending to a depth of 3,000?m. The metamorphic index mineral, graphite, was detected in sedimentary rock below 5,000?m using Raman spectroscopy. There exists a good correlation between IC and CC within the prograde burial sequence; with CC anchizonal boundaries of 0.242 and 0.314°2?? (upper and lower boundaries, respectively) corresponding to Kübler??s IC limits at 0.25 and 0.42°2??. Illite polytypism also shows an increase in the 2M 1 polytype with increasing depth, with ca. 60?% 2M 1 abundance compared to the 1M type at the surface, to 100?% 2M 1 at the bottom of the borehole. Fluid inclusion analysis show HHC-rich bearing fluids correspond to the diagenetic zone, CH4-rich bearing fluids appear at transitional zone from diagenetic to low anchizone and H2O-rich bearing fluids mark the high anchizone to epizone. Based on chlorite chemical geothermometer, calcite?Cdolomite geothermo-barometers as well as homogenization temperature of fluid inclusions, a paleotemperature range of 118?C348?°C is estimated for the well with a pressure facies of low to intermediate type.  相似文献   

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