摘要:

AbstractThree major blastomylonitic synmetamorphic (epidote amphibolite to mid amphibolite facies) shear zones are seen on the NW coast of the Mullet Peninsula in NW Mayo. These shear zones occur at the contacts of major structural units and in an imbricated slice where rocks of the Erris Complex are deformed and chemically modified. Chemical changes associated with individual shear zones have been deduced by comparing the compositions of various gneisses both within and adjacent to the shear zones. Compositional changes are different in the constituent rock‐types within each unit and many elements normally considered immobile have been selectively mobilized within the shear zones. Little evidence of wholesale metasomatic introduction of components into these shear zones was found to accompany the selective mobilizatio

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1984.tb00281.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractA garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis&Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In KD=Δ (XCa,g) where KDis the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn‐poor garnet and common hornblende of widely varying chemistry metamorphosed at low aO2.Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks.Quartzo‐feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1984.tb00282.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractAn experimental study of the system CaCO3–MgCO3–FeCO3was undertaken in order to calibrate the iron correction to the calcite–dolomite geothermometer, which is based on the solubility of magnesium in calcite in the assemblage calcite + dolomite. The experiments, at 450°C and lower temperatures, resulted in products with a very small grain size and incomplete equilibration. However, application of a carefully‐devised automatic data processing algorithm to analyses of the phases in experimental charges, combined with a thermodynamic analysis, results in geothermometer diagrams which should be preferred to previous theoretical pre

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1984.tb00283.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractBlueschist‐facies rocks on the Seward Peninsula constitute a structurally coherent terrane measuring at least 100 × 150 km. Radiometric age data indicate that high‐pressure metamorphism probably occurred in Jurassic rather than in Palaeozoic or Precambrian time, as previously suggested. Protolith sediments (Nome Group) are of intracontinental basin or continental margin type, and of lower Palaeozoic and possibly late Precambrian age, thus predating the high pressure metamorphism by more than 200 m.y.Blueschist‐facies mineral assemblages were developed in almost all lithologies of the Nome Group, and are best preserved in FeTi‐rich metabasites (glaucophane + almandine + epidote) and pelites (glaucophane + chloritoid + phengite). A lawsonite–crossite subfacies was developed in possible Nome Group rocks on the east flank of the Darby Mountains. Albite–epidote–amphibolite facies assemblages characterize Nome Group rocks in the southwestern part of the Peninsula. Metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote–almandine–glaucophane schist subfacies with the local development (east of the Nome River) of eclogitic assemblages.The high pressure metamorphic minerals were synkinematic with the development of mesoscopic‐scale intrafolial isoclinal folds and a flattening foliation of consistent orientation. Initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles, and partial post‐kinematic greenschist facies replacements record later stages of decompression. Ophiolites and melange are not associated with the Seward Peninsula blueschists. The high‐pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin. The PT conditions of high pressure metamorphism were approximately 9–11 kbar, 400–450°C, thus falling between the PT paths of the Sh

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1984.tb00284.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractBiotite, garnet, staurolite and kyanite isograds in pelitic metasedimentary rocks are developed as a result of thermal metamorphism around syntectonic granitoids in Eastern Rouergue (France). Temperature estimates range between 400°C and 650°C at about 6.5 kbar. Geothermobarometry shows a steep isobaric T gradient which is consistent with the interpretation that the metamorphic highs are thermal aureoles. High grade rocks show evidence of two staurolite forming reactions in the presence of plagioclase and the absence of chlorite that have not been described previously in the literature. The reaction that occurs in the middle staurolite zone, alm‐rich ga + Ca‐rich pla + Na‐rich mu gro‐rich ga + Na‐rich pla + st + Na‐poor mu, is considered to be prograde, whereas the reaction that occurs in the kyanite zone, alm‐rich ga + Ca‐rich pla + w st + Ca‐rich ga + Na‐rich pla + qz, is retrograde. The topology of these reactions is illustrated in terms of end member compositions for the systems KNaFASH and

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1984.tb00285.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY