摘要:

Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H2O, chlorite + quartz = talc + cordierite + H2O, and talc + kyanite + quartz = cordierite ± H2O by reversed runs in the system MgO‐Al2O3‐SiO2‐H2O (MASH). These reaction curves intersect at an invariant point (IP1) atPH2O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H2O at water pressures above 6.4 kbar shows a negativedP/dT, with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H2O at water pressures is clearly positive.The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg4.85Al1.15[Al1.15Si2.85O10](OH)8over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg2.94Al0.06[Al0.06Si3.94O10](OH)2atPH2O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit atP,H2O = 21 kbar.As a consequence of the new experimental data, the existing phase topologies of the MASH‐system and K2O‐MASH‐system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H2O = chlorite + cordierite are stable. For this reason, the development of medium‐ to high‐temperature metamorphic rocks compositionally approximating the MASH‐system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe2+into magnesian chlorite extends its stability field in the presence of quartz by 10°‐15°C in compari

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00619.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Sapphirine has been found in two types of magnesian, metabasic lenses from tectonite zones within the Central Gneiss Belt of the south‐west Grenville Province, Canada. The first type (association I) comes from a lenticular mafic lens within highly tectonized anorthosite, the second type (association II) comes from meta‐eclogitic pods with foliated amphibolite rims. In each case the sapphirine‐bearing assemblages record a wealth of reaction textures. The primary mineralogy in association II is represented by high alumina clinopyroxene, garnet and kyanite ± plagioclase and records pressures of around 14‐16 kbar; in association I the primary mineralogy is represented by plagioclase, two pyroxenes and possibly olivine but here the equilibrium pressure is unknown.The host gneisses equilibrated at approximately 8 to 10 kbar and 700‐750°C by continuous cation exchange reactions during and after the culmination of the Grenvillian orogeny at 1.16‐1.0 Ga. It is unlikely that the higher pressures recorded in the meta‐eclogitic pods represent an earlier high‐pressure metamorphism as the pods are restricted to shear zones. A tectonic mode of emplacement into a crust undergoing granulite facies metamorphism is more likely. Sapphirine formed by discontinuous decompression reactions; in association II this involved a reaction between garnet and kyanite and resulted in the formation of magnesian granulite facies assemblages. At the same time primary clinopyroxene became much less aluminous by evolving plagioclase. Pressures and temperatures from coexisting phases, that are believed to have equilibrated at the same time as sapphirine formation, are estimated as 11 to 12 kbar and 750°C. These probably represent the peak conditions for granulite facies metamorphism in the south‐wes

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00620.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

The Kanskaya formation in the Yenisey range, Eastern Siberia is a newly studied example of retrogression of granulite facies rocks. The formation consists of two stratigraphical units: the lower Kuzeevskaya group and the upper Atamanovskaya group. Rocks from both of these units show rare reaction textures such as replacement of cordierite by garnet, sillimanite and quartz, silimanite coronas around spinel and corundum, and garnet rims around plagioclase in metabasites, while plagioclase rims around garnet can be seen in associated metapelites. The paragenesis quartz + orthopyroxene + sillimanite is a feature of the Kuzeevskaya group. In many samples, chemical zoning of garnet and cordierite shows an increase in Mg from core to rim as well as the reverse.Biotite‐garnet‐cordierite‐sillimanite‐quartz as well as spinel±biotite‐garnet°Cordierite±sillimanite‐quartz assemblages were studied using geothermometers and geobarometers based on both exchange and net‐transfer reactions (Perchuk&Lavrent'eva, 1983; Aranovich&Podlesskii, 1983; Gerya&Perchuk, 1989). Detailed investigation of 10 samples including 1000 microprobe analyses revealed decompression (first stage) followed by the near isobaric cooling of the granulites. From geological studies, the 7 km total thickness of the sequence closely corresponds to the pressure difference (∼ 2.2kbar) measured by geobarometers in the samples taken from different levels in the sequence. Individual samples yieldP‐Tpaths ranging from 100°C/kbar to 140°C/kbar depending on their locations with respect to the large Tarakskiy granite pluton. In places the 100°C/kbar path changed to the 140°C/kbar due to the influence of the intrusion. In aP‐Tdiagram these trajectories are subparallel lines, whoseP‐Tmaxima define the Archaean geotherm between 3.1 and 2.7 Ga, determined isotopically. A petrological model forP‐Tevolution of the

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00621.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

A method is proposed for adjusting the mass balance to characterize quantitatively the behaviour of minerals in anatexis. The method is based on an unconstrained simple mixing model that can be expressed as:whereB,A0, andA1‐n, are compositional vectors of segregate, source rock and source minerals, respectively. The most important concepts are: (1) degree of partial fusion:FMM= 1/a0; (2) mineral fractionation index:and (3) plagioclase differentiation index:.For a given mineral, theMFIvalues have the following meaning: (a)MFIMFIa0: external contribution to the anatectic system defined bya0A0. To test the method and illustrate its use, it was applied to two real problems of partial melting in the Peña Negra Anatectic Complex (Central Spain). The first is a very simple case of segregation of a diktyonitic neosome from an orthogneiss through partial melting located in vertical shear zones. This process is characterized by: (1)FMM= 0.51; (2) active incorporation of K‐feldspar, plagioclase and biotite into the segregate; (3) disequilibrium melting of plagioclase; (4) residual behaviour of quartz and ilmenite. The second case concerns the formation of a cordierite‐bearing granite from granodioritoid diatexites through an anatectic process, whose most salient characteristics are: (1)FMM= 0.45; (2) incongruent melting of biotite; (3) residual behaviour of plagioclase, which melted with aPDIof 1.22; (4) preferential incorporation of quartz into the segregate; (5) total extraction of K‐feldspar from the

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00622.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

A reaction producing jadeitic pyroxene in metagreywackes of the northern Diablo Range has been identified on the basis of mineral distribution, isograd patterns and composition of coexisting minerals. The appearance of jadeitic pyroxene (∼Jd80) is closely followed by the disappearance of pumpellyite, which indicates that pumpellyite plays a major role in the pyroxene‐producing reaction. A new projection from hematite, lawsonite, chlorite, quartz and H2O on to the NaAlO2‐FeO‐MgO ternary confirms the role of pumpellyite in pyroxene production and suggests a reaction of the form: 1.00 pumpellyite + 0.31 chlorite + 8.71 albite + 0.70 hematite + 2.00 H2O = 8.54 jadeite + 0.57 glaucophane + 3.09 lawsonite + 5.26 quartz. Metagreywackes of the northern Diablo Range were metamorphosed under conditions ofPH2O=Ptotalat 200‐300 °C and 7.5‐10.0 kbar. Despite the low temperatures attained during metamorphism, the assumption of equilibrium yields results consistent with field observations and pha

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00623.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00624.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00625.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY