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1.
Electron-probe microanalysis of a series of garnets in metapelitic rocks of the chloritoid staurolite, kyanite and sillimanite metamorphic zones, eastern area of the Sierra de Guadarrama, Sistema Central, Spain, manifest the well-known cryptozonation commonly observed in these minerals, with MgO and FeO increasing and MnO and CaO decreasing from the center to the outer rim of the crystals.The differences in composition of the garnets, from one metamorphic zone to another, is mainly a result of small differences in composition of the host-rock, since: (1) the amounts of MnO in the garnet are controlled by the amounts of SiO2, Al2O3 and FeO present in the host-rock; and (2) the percentages of MnO and MgO of the parent-rock influence in some way the concentration of CaO in the garnet, and those of MnO, Al2O3 and CaO influence the concentration of FeO. Nevertheless, the amount of FeO in the garnet is finally controlled, due to the diadochy, by the concentration of MnO + CaO in this mineral.  相似文献   

2.
Garnet-bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions. Modelling of thermodynamic data for minerals in the system Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O, however, predicts that garnet should be observed only in rocks of a narrow range of very high Fe/Mg bulk compositions. Traditionally, the nearly ubiquitous presence of garnet in medium- to high-grade pelitic schists is attributed qualitatively to the stabilizing effect of MnO, based on the observed strong partitioning of MnO into garnet relative to other minerals. In order to quantify the dependence of garnet stability on whole-rock MnO content, we have calculated mineral stabilities for pelitic rocks in the system MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O for a moderate range of MnO contents from a set of non-linear equations that specify mass balance and chemical equilibrium among minerals and fluid. The model pelitic system includes quartz, muscovite. albite, pyrophyllite, chlorite, chloritoid, biotite, garnet, staurolite, cordierite, andalusite, kyanite. sillimanite, K-feldspar and H2O fluid. In the MnO-free system, garnet is restricted to high Fe/Mg bulk compositions, and commonly observed mineral assemblages such as garnet–chlorite and garnet–kyanite are not predicted at any pressure and temperature. In bulk compositions with XMn= Mn/(Fe + Mg + Mn) > 0.01, however, the predicted garnet-bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes. Temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole-rock MnO content. The small MnO contents of normal pelitic schists (XMn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.  相似文献   

3.
Several petrographic studies have linked accessory monazite growth in pelitic schist to metamorphic reactions involving major rock‐forming minerals, but little attention has been paid to the control that bulk composition might have on these reactions. In this study we use chemographic projections and pseudosections to argue that discrepant monazite ages from the Mount Barren Group of the Albany–Fraser Orogen, Western Australia, reflect differing bulk compositions. A new Sensitive High‐mass Resolution Ion Microprobe (SHRIMP) U–Pb monazite age of 1027 ± 8 Ma for pelitic schist from the Mount Barren Group contrasts markedly with previously published SHRIMP U–Pb monazite and xenotime ages of c. 1200 Ma for the same area. All dated samples experienced identical metamorphic conditions, but preserve different mineral assemblages due to variable bulk composition. Monazite grains dated at c. 1200 Ma are from relatively magnesian rocks dominated by biotite, kyanite and/or staurolite, whilst c. 1027 Ma grains are from a ferroan rock dominated by garnet and staurolite. The latter monazite population is likely to have grown when staurolite was produced at the expense of garnet and chlorite, but this reaction was not intersected by more magnesian compositions, which are instead dominated by monazite that grew during an earlier, greenschist facies metamorphic event. These results imply that monazite ages from pelitic schist can vary depending on the bulk composition of the host rock. Samples containing both garnet and staurolite are the most likely to yield monazite ages that approximate the timing of peak metamorphism in amphibolite facies terranes. Samples too magnesian to ever grow garnet, or too iron‐rich to undergo garnet breakdown, are likely to yield older monazite, and the age difference can be significant in terranes with a polymetamorphic history.  相似文献   

4.
Amphibolite facies mafic rocks that consist mainly of hornblende, plagioclase and quartz may also contain combinations of chlorite, garnet, epidote, and, more unusually, staurolite, kyanite, sillimanite, cordierite and orthoamphiboles. Such assemblages can provide tighter constraints on the pressure and temperature evolution of metamorphic terranes than is usually possible from metabasites. Because of the high variance of most of the assemblages, the phase relationships in amphibolites depend on rock composition, in addition to pressure, temperature and fluid composition. The mineral equilibria in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O (NCFMASH) model system demonstrate that aluminium content is critical in controlling the occurrence of assemblages involving hornblende with aluminous minerals such as sillimanite, kyanite, staurolite and cordierite. Except in aluminous compositions, these assemblages are restricted to higher pressures. The iron to magnesium ratio (XFe), and to a lesser extent, sodium to calcium ratio, have important roles in determining which (if any) of the aluminous minerals occur under particular pressure–temperature conditions. Where aluminous minerals occur in amphibolites, the P–T–X dependence of their phase relationships is remarkably similar to that in metapelitic rocks. The mineral assemblages of Fe‐rich amphibolites are typically dominated by garnet‐ and staurolite‐bearing assemblages, whereas their more Mg‐rich counterparts contain chlorite and cordierite. Assemblages involving staurolite–hornblende can occur over a wide range of pressures (4–10 kbar) at temperatures of 560–650 °C; however, except in the more aluminous, iron‐rich compositions, they occupy a narrow pressure–temperature window. Thus, although their occurrence in ‘typical’ amphibolites may be indicative of relatively high pressure metamorphism, in more aluminous compositions their interpretation is less straightforward.  相似文献   

5.
Partial electron microprobe analyses of garnet, biotite and cordierite in sillimanite-K feldspar gneisses of the Brimfield Formation in south-central Massachusetts indicate that the compositions of these minerals are not constant in a thin section. The FeO/MgO mol ratio of biotite is sensitive to the nature of other FeO-MgO minerals occurring in close proximity. The most iron-rich biotites are those that do not contact either cordierite or garnet. The most iron-poor biotites occur as inclusions in garnet. Biotites in direct contact with either cordierite or garnet have intermediate FeO/MgO ratios. The bulk of a given grain of garnet or cordierite is homogeneous in composition. Chemical zoning is absent. All grains of garnet and cordierite in a thin section are constant in composition. However, where garnet and cordierite abut biotite, the FeO/MgO ratio of the garnet rim is increased and that of cordierite is decreased. The FeO/MgO ratios of garnet, cordierite and biotite bare a regular relation to each other indicating a possible equilibrium state. However the distribution coefficient defined by the compositions of minerals in direct contact are greater than those defined by the compositions of the interiors of garnet and cordierite matched with the compositions of biotites removed from these phases. This pattern is believed to be the result of two thermal events. The first event produced the mineral assemblages and widespread equilibrium was obtained. A subsequent retrograde event left the mineralogy intact but caused cation exchange reactions at immediate contacts between garnet, cordierite and biotite. The physical conditions of the first event are estimated at P=5–6 kb, T=700–750° C. The retrograde event occurred at lower temperatures and very low activities of H2O since no muscovite is developed at microcline-sillimanite contacts.  相似文献   

6.
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   

7.
Spinel-anthophyllite rocks that may be classified as ultrabasic low-Ca spinel amphibolites have been first discovered in the Kokchetav collision zone (northern Kazakhstan). They outcrop 2 km west of Enbek-Berlyk Village among schists and quartzites and are closely associated with spinel harzburgites and garnet pyroxenites. The main hosted minerals are spinel (hercynite) and anthophyllite. The rocks bear magnetite-hornblende-spinel-anthophyllite pseudomorphs with rounded and polygonal sections, which might have been resulted from the replacement of garnet grains. The prismatic anthophyllite crystals and scarce olivine relics contain elongate parallel spinel inclusions resembling spinel-olivine syntactic intergrowths in the Enbek-Berlyk spinel harzburgites. The spinel-anthophyllite rocks are similar to the associated spinel harzburgites in CaO, MnO, SiO2, and Al2O3 contents but are richer in FeO and poorer in MgO (F = FeO/(FeO + MgO) = 57% against 35% in the harzburgites). Geological, mineralogical, and geochemical data suggest that the spinel-anthophyllite rocks formed during the isochemical contact metamorphism of garnet-bearing spinel harzburgites, which contained more FeO and less MgO than garnet-free harzburgites of the same area. Variations in FeO and MgO contents in both types of harzburgites seem to be due to different chemical compositions of the chlorite protoliths of these rocks.  相似文献   

8.
The Jeori-Wangtu Gneissic Complex (JWGC) exposed as a tectonic window in the Lesser Himalayas represents one of the oldest Gneissic Complex of the Himalayas. Foliated granite and the metapelite constitute the dominant lithologies of the JWGC. The western margin of the JWGC is bounded by a brittle shear while in the east, the tectonic surface is a ductile shear zone.Kyanite schist, chloritoid schist, staurolite schist (St-1), garnet schist and staurolite schist (St-2) are present in a west to east sequence beginning from near to the Jhakhri thrust and up to the contact with the JWGC granite. Mica schist is intermittently present and is the dominant metapelite. Low to medium grade regional metamorphic conditions has been inferred for these rocks.Calc silicate enclaves within the JWGC granite preserve the contact metamorphic effects. These are reflected in development of narrow zones of disequilibrium assemblages of calcareous garnet (grs53), clinopyroxene, K feldspar, calcic plagioclase (An86), quartz, zoned sphene, zoned allanite, amphiboles, calcite and epidote.Recording of contact metamorphic assemblage of 1.80 Ga granite witin the enclave calc silicates and in the host metapelites over an earlier, relict low to medium grade assemblage indicates that the JWGC preserves palaeoproterozoic metamorphic imprints.  相似文献   

9.
Interpretations based on quantitative phase diagrams in the system CaO–Na2O–K2O–TiO2–MnO–FeO–MgO–Al2O3–SiO2–H2O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P–T path. That path displays a prograde P–T increase from 600 °C/7.0 kbar to 780 °C/9–10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P–T decrease to 600 °C/4 kbar. The data used to construct the P–T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al‐rich garnet–biotite–cordierite–sillimanite–K‐feldspar–plagioclase–quartz–graphite gneiss (2) a K‐poor, Al‐rich garnet–biotite–staurolite–cordierite–kyanite–sillimanite–plagioclase–quartz–rutile gneiss, and (3) a K, Al‐poor, Fe‐rich garnet–orthopyroxene–biotite–chlorite–plagioclase–quartz–rutile–ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro‐ and retrograde P–T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983 ).  相似文献   

10.
An automated method for the calculation of P–T paths based on garnet zoning is presented and used to interpret zoning in metapelitic schist from the southern Canadian Cordillera. The approach adopted to reconstruct the P–T path is to match garnet compositions along a radial transect with predictions from thermodynamic forward models, while iteratively modifying the composition to account for fractional crystallization. The method is applied to a representative sample of garnet‐ and staurolite‐bearing schist from an amphibolite facies Barrovian belt in the southern Canadian Omineca belt. Garnet zoning in these schists is concentric and largely continuous from core to rim. Three zones are present, the first two of which coincide with sector‐zoned cores of garnet crystals. Similar zoning is developed in rocks that contain or lack staurolite, respectively, suggesting garnet growth was restricted to the initial part of the prograde P–T path prior to the development of staurolite. Growth zoning in large garnet crystals has not been significantly modified by diffusion. This interpretation is based on zoning characteristics of garnet crystals and is further supported by results of a forward model incorporating the effects of simultaneous fractional crystallization and intracrystalline diffusion. The P–T path calculated for this rock includes an initial, linear stage with a high dP/dT, and a later stage dominated by heating. The approach adopted in this study may have application to other garnet‐bearing rocks in which growth zoning is preserved.  相似文献   

11.
The Priest pluton contact aureole in the Manzano Mountains, central New Mexico preserves evidence for upper amphibolite contact metamorphism and localized retrograde hydrothermal alteration associated with intrusion of the 1.42 Ga Priest pluton. Quartz–garnet and quartz–sillimanite oxygen isotope fractionations in pelitic schist document an increase in the temperatures of metamorphism from 540 °C, at a distance of 1 km from the pluton, to 690 °C at the contact with the pluton. Comparison of calculated temperature estimates with one‐dimensional thermal modelling suggests that background temperatures between 300 and 350 °C existed at the time of intrusion of the Priest pluton. Fibrolite is found within 300 m of the Priest pluton in pelitic and aluminous schist metamorphosed at temperatures >580 °C. Coexisting fibrolite and garnet in pelitic schist are in oxygen isotope equilibrium, suggesting these minerals were stable reaction products during peak metamorphism. The fibrolite‐in isograd is coincident with the staurolite‐out isograd in pelitic schist, and K‐feldspar is not observed with the first occurrence of fibrolite. This suggests that the breakdown of staurolite and not the second sillimanite reaction controls fibrolite growth in staurolite‐bearing pelitic schist. Muscovite‐rich aluminous schist locally preserves the Al2SiO5 polymorph triple‐point assemblage – kyanite, andalusite and fibrolite. Andalusite and fibrolite, but not kyanite, are in isotopic equilibrium in the aluminous schist. Co‐nucleation of fibrolite and andalusite at 580 °C in the presence of muscovite and absence of K‐feldspar suggests that univariant growth of andalusite and fibrolite occurred. Kyanite growth occurred during an earlier regional metamorphic event at a temperature nearly 80 °C lower than andalusite and fibrolite growth. Quartz–muscovite fractionations in hydrothermally altered pelitic schist and quartzite are small or negative, suggesting that late isotopic exchange between externally derived fluids and muscovite, but not quartz, occurred after peak contact metamorphism and that hydrothermal alteration in pelitic schist and quartzite occurred below the closure temperature of oxygen self diffusion in quartz (<500 °C).  相似文献   

12.
Using a previously published, internally consistent thermodynamic dataset and updated models of activity–composition relations for solid solutions, petrogenetic grids in the model system KFMASH (K2O–FeO–MgO–Al2O3–SiO2–H2O) and the subsystems KMASH and KFASH have been calculated with the software THERMOCALC 3.1 in the P–T range 5–36 kbar and 400–810 °C, involving garnet, chloritoid, biotite, carpholite, talc, chlorite, staurolite and kyanite/sillimanite with phengite, quartz/coesite and H2O in excess. These grids, together with calculated AFM compatibility diagrams and pseudosections, are shown to be powerful tools for delineating the phase equilibria and P–T conditions of pelitic high-P assemblages for a variety of bulk compositions. The calculated equilibria and mineral compositions are in good agreement with petrological observation. The calculation indicates that the typical whiteschist assemblage kyanite–talc is restricted to the rocks with extremely high XMg values, decreasing XMg in a bulk composition favoring the stability of chloritoid and garnet. Also, the chloritoid–talc paragenesis is stable over 19–20 kbar in a temperature range of ca. 520–620 °C, being more petrologically important than the previously highlighted assemblage talc–phengite. Moreover, contours of the calculated Si isopleths in phengite in P–T and P–X pseudosections for different bulk compositions extend the experimentally derived phengite geobarometers to various KFMASH assemblages.  相似文献   

13.
The Darreh Sary metapelitic rocks are located in the northeast of Zagros orogenic belt and Sanandaj-Sirjan structural zone. The lithological composition of these rocks includes slate, phyllite, muscovitebiotite schist, garnet schist, staurolite-garnet schist and staurolite schist. The shale is the protolith of these metamorphic rocks, which was originated from the continental island arc tectonic setting and has been subjected to processes of Zagros orogeny. The deformation mechanisms in these rocks include bulging recrystallization (BLG), subgrain rotation recrystallization (SGR) and grain boundary migration recrystallization (GBM), which are considered as the key to estimate the deformation temperature of the rocks. The estimated ranges of deformation temperature and depth in these rocks show the temperatures of 275–375, 375–500, and >500°C and the depths of 10 to 17 km. The observed structures in these rocks such as faults, fractures and folds, often with the NW-SE direction coordinate with the structural trends of Zagros orogenic belt structures. The S-C mylonite fabrics is observed in these rocks with other microstructures such as mica fish, σ fabric and garnet deformation indicate the dextral shear deformation movements of study area. Based on the obtained results of this research, the stages of tectonic evolution of Darreh Sary area were developed.  相似文献   

14.
Partial chemical analyses of eighteen biotite samples along with complete analyses of the respective host rocks were carried out on some metamorphic and granitic rocks of the area around Richughuta in the Palamau district of Bihar. Chemical changes in the biotites show an increase of Ti, Fe, Mn and alkali contents with decrease of Al, Mg and MgO/FeO ratio from schist to granite. The ratio MgO/FeO increases with increase in the oxidation ratio of the host rock.  相似文献   

15.
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   

16.
During prograde metamorphism garnet and, in some higher grade samples, staurolite were produced in a chlorite-chloritoid schist, part of the Precambrian Z to Cambrian Hoosac Formation near Jamaica, VT. Garnet grew during two prograde events separated by a retrogression. This sequence resulted in distinctive inclusion textures and zoning anomalies in garnet produced by diffusive alteration. Textures, reaction space analysis, and mineral compositional variations constrain the possible sequence of reactions in these rocks. Below the staurolite isograd, and to some unknown extent above it, garnet grew by the reaction chloritoid+chlorite+quartz→garnet+H2O. With increasing grade the mineral compositions are displaced towards lower Mn/Fe and higher Mg/Fe ratios. The data are compatible with equilibrium with respect to exchange reactions for the matrix assemblages on a thin section scale and with minerals having closely followed equilibrium paths during reaction. The staurolite isograd coincides with the reaction chloritoid+quartz→garnet+staurolite+chlorite+H2O. This reaction is continuous and trivariant with ZnO becoming an additional component concentrated in staurolite. During this reaction both the Mn/Fe and Mg/Fe ratios of the phases appear to have decreased. This new chemical trend is recorded by garnet zoning profiles and is compatible with trends predicted from phase diagrams. Thus there are two distinct types of garnet zoning reversals in these samples. One is near the textural unconformity and is best explained by diffusive alteration during partial resorption of first stage garnet. The other occurs near the outer rim of garnet in staurolite zone samples and marks the onset of a new prograde garnet producing reaction.  相似文献   

17.
相山地区变质基底新认识及其原岩归属的对比研究   总被引：5，自引：0，他引：5  
在相山北部首次发现十字石片岩及堇青石片岩。根据随变质作用增强而出现的新变质矿物，将相山变质岩基底划分为绢云母千枚岩带、黑云母片岩带、铁铝榴石片岩带和十字石片岩带。获得相山地区黑云母片岩－十字石片岩的Ｒｂ－Ｓｒ等时线年龄值为７１９Ｍａ，斜长角闪片岩的Ｒｂ－Ｓｒ等时线年龄值为７２６．６Ｍａ，表明相山基底变质岩属新元古期变质岩而不是加里东期变质岩。提出并采用微量元素地球化学比值聚类分析方法，确认相山地区变质岩原岩不属震旦系，而与华夏地块（古陆）的陈蔡群相当。  相似文献   

18.
A kilometre-scale shear zone is recognized in the Cambro–Ordovician schist of the Bossòst dome, a Variscan metamorphic and structural dome in the Axial Zone of the central Pyrenees. Non-coaxial deformation is recorded by rotated garnet and staurolite porphyroblasts following regional metamorphism M1, while coaxial conditions prevailed during later contact metamorphic M2 growth of andalusite and cordierite. Mineral compositions and bulk rock analyses show that garnet–staurolite–andalusite–cordierite assemblages are significantly enriched in Mg and Mn over the garnet–staurolite assemblage, which lacks sufficient Mg for cordierite to form. The garnet–staurolite assemblage preserves conditions during M1, estimated by AFM diagrams and P–T pseudosections to be 5.5 kbar and 580 °C, respectively. Pseudosections also indicate that staurolite is not a stable phase in cordierite–andalusite assemblages of M2, suggesting polyphase metamorphism and decompression along a clockwise P–T path for the staurolite–cordierite–andalusite assemblages. This concurs with proposed extensional tectonics along the regional shear zone. To cite this article: J.E. Mezger et al., C. R. Geoscience 336 (2004).  相似文献   

19.
The textural and chemical evolution of allanite and monazite along a well‐constrained prograde metamorphic suite in the High Himalayan Crystalline of Zanskar was investigated to determine the P–T conditions for the crystallization of these two REE accessory phases. The results of this study reveals that: (i) allanite is the stable REE accessory phase in the biotite and garnet zone and (ii) allanite disappears at the staurolite‐in isograd, simultaneously with the occurrence of the first metamorphic monazite. Both monazite and allanite occur as inclusions in staurolite, indicating that the breakdown of allanite and the formation of monazite proceeded during staurolite crystallization. Staurolite growth modelling indicates that staurolite crystallized between 580 and 610 °C, thus setting the lower temperature limit for the monazite‐forming reaction at ~600 °C. Preservation of allanite and monazite inclusions in garnet (core and rim) constrains the garnet molar composition when the first monazite was overgrown and subsequently encompassed by the garnet crystallization front. Garnet growth modelling and the intersection of isopleths reveal that the monazite closest to the garnet core was overgrown by the garnet advancing crystallization front at 590 °C, which establishes an upper temperature limit for monazite crystallization. Significantly, the substitution of allanite by monazite occurs in close spatial proximity, i.e. at similar P–T conditions, in all rock types investigated, from Al‐rich metapelites to more psammitic metasedimentary rocks. This indicates that major silicate phases, such as staurolite and garnet, do not play a significant role in the monazite‐forming reaction. Our data show that the occurrence of the first metamorphic monazite in these rocks was mainly determined by the P–T conditions, not by bulk chemical composition. In Barrovian terranes, dating prograde monazite in metapelites thus means constraining the time when these rocks reached the 600 °C isotherm.  相似文献   

20.
Schreyer and Chinner (1966) described staurolite-quartzite bands of unusual bulk chemical composition associated with the Big Rock kyanite deposit in northern New Mexico. They discussed two possible hypotheses-syngenetic or metasomatic-for the origin of the bands. This paper discusses additional evidence in support of the metasomatic hypothesis.Quartz-muscovite schist and quartz-kyanite rock are believed to be metarhyolite that was altered metasomatically by hydrothermal activity during metamorphism. Experimentally demonstrable reactions that involve ionic exchange equilibria between pore fluid and solid phases best explain the observed alteration. Staurolite-quartzite bands are postulated to be basalts or amphibolites that were hydrothermally altered in a manner analogous to alteration of the enclosing rocks. The process would deplete a basaltic parent rock in alkali and alkaline earth elements and produce a chemical composition similar to the bulk chemical composition of staurolite-quartzite.Schreyer and Chinner used textural evidence to prove that staurolite formed at the expense of chloritoid. Based on the chemical compositions of staurolite and relict chloritoid reported by them, several chemical equations were constructed, using the principle of ionic equilibria between pore fluid and solid phases, in an attempt to realistically portray the reaction. It is suggested that, under open-system metasomatic conditions, the ratio aMg+2/aH+ in the supercritical aqueous fluid is an important parameter that controls the stability of chloritoid vs. staurolite.  相似文献   

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