ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00316.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractIn pelites of the central Menderes Massif, albite and oligoclase with only slight chemical zoning coexist in apparent textural equilibrium in the garnet zone, staurolite zone, and staurolite + kyanite transition zone. The metamorphic temperature range is estimated as approximately 440–550°C (from the Hodges‐Spear calibration of the garnet‐biotite geo‐thermometer), or 440–500°C (Ganguly‐Saxena calibration). While oligoclase composition at the peristerite gap changes from An22to An14, albite also becomes more sodic (An1,5–An0.6). The slope of the albite limb is thus the reverse of that reported in other areas, and may not be a true equilibrium feature. Occurrence of kyanite, at temperatures below the crest of the gap, is due to low water activity in the presence of graphite:aH2ois estimated at approximately 0.1–0.2 from the Na content of muscovite coexisting with albite +

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00317.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractActivity‐composition relations in oligoclase near the peristerite gap are investigated in pelites from the Central Menderes Massif. The pressure of metamorphism is estimated independently, from garnet‐rutile‐ilmenite‐kyanite‐quartz, as being in the range 4–7 kbar. In the temperature range, 450–600°C approximately, both the Newton‐Haselton calibration of the garnet‐plagioclase‐kyanite‐quartz geobarometer and a related simple treatment of garnet‐plagioclase‐muscovite‐biotite give a wide range of apparent pressures, correlated with plagioclase composition and ranging up to 11–12 kbar where the plagioclase is most sodic. This effect is attributed to failure of the activity model for plagioclase used in the Newton‐Haselton treatment. It is inferred that, in the present area, γplagAndecreases with increasingXplagAnin the range An15‐An25. The data can be interpreted in terms of highγplagAnin the high‐albite structure at these temperatures, modified to lower values by ‘e’ordering in the more calcic oligoclases. The ordering appears to be independent of the peristerite gap, and the data do not support the interpretation of the gap as a solvus. Garnet‐plagioclase assemblages are unreliable as geobarometers where the plagioclase is more sodic than approximately An20andT<700°C, and should instead be used to investigate theγ‐X behaviour of the plagioclase where inde

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00318.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractThe calibration of geothermometers and geobarometers should involve not only the determination of the parameters in the equation used, but also the uncertainties on, and the correlations between, these parameters. This necessitates the use of a technique such as least squares. Given the poor performance of least squares in the presence of outliers in the data, techniques for identifying outliers for exclusion—regression diagnostics, and techniques for handling data which include outliers—robust regression and jackknifing, are essential. These techniques are summarized and their importance is emphasized, and they are applied to the calibration of the garnet‐clinopyroxene Fe‐Mg exchange geothermometer.The experimental data of Raheim&Green (1974) and Ellis&Green (1979) are explored using regression diagnostics to discover outliers in the data. After exclusion of the two influential outliers found, a new geothermometer equation for garnet‐clinopyroxene Fe‐Mg exchange is derived using robust regression and based on all the data: thus,T(K) = 2790 + 10P+ 3140xca,g/1.735 + InKDwhereTis in Kelvin andPis in kbar. This equation, as might be hoped, is essentially identical to that of Ellis&Green (1979). Equations for calculating the uncertainty in a calculated temperature, contributed by uncertainties in the calibration, are a

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00319.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractThe Kvamsøy pyroxenite complex consists of olivine websterite, olivine gabbro and leucogabbro‐norite which have been subjected to regional highP‐T(HPT) metamorphism. The metamorphism has resulted in a range of disequilibrium textures with the development of coronas and pseudomorphism of the igneous phases. Reactions between felsic and mafic mineral domains have been controlled by variable and selective diffusion of elements, resulting in a variety of local plagioclase‐breakdown reactions and in significant compositional variations for the product garnet. Restricted diffusion favours transient reaction stages with garnet ± spinel ± corundum ± zoisite after calcic plagioclase in olivine gabbro and olivine websterite and garnet ± spinel ± kyanite ± quartz + sodic plagioclase in leucogabbro‐norite. Complete HPT reaction has produced garnet pyroxenite which consists of garnet + diopside + hornblende + zoisite in gabbroic rocks, while amphibolitization continued during the cooling and uplift history. Grt + Ky + Pl + Qtz geobarometry suggests pressures in the range 13‐16 kbar forT= 750°C, comparable with the regional eclogite‐f

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00320.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractPrecambrian granulites of the Aldan shield in southern Yakutia, USSR, form a massif of 200,000 km2bounded by younger fold‐belts to the south, west and east. The massif consists of several blocks that reflect a primary heterogeneity of composition and differences in structural and thermodynamic evolution of different parts of the area. According to structural and petrological data the massif can be divided into two megablocks: eastern Aldan and western Aldan. They are separated by a narrow meridional fold‐belt. Structural evolution of this central zone was determined by the geodynamics of the mega‐blocks and was completed in the late Archaean. Towards the south, this central zone is ‘transformed’into the relatively small Sutam block adjoining the Stanovoy fold‐belt that bounds the Aldan shield on the south. The Sutam block is separated from the other structural units of the Aldan shield by a system of north trending grabens filled by post‐Archaean sediments.The Aldan shield is composed of Archaean high‐grade granulites, while the Stanovoy fold‐belt, to the south, consists of highly foliated Proterozoic rocks metamorphosed under relatively lower‐grade conditions. However, relics of the granulites are mapped within the fold‐belt. They contain high‐grade assemblages (e.g. Opx + Sil + Qz, Sap + Qz, Opx + Gr + Sil, etc.). One of the relics, the Tokskii block, which is only slightly touched by diaphthoresis, is located in the southeastern part of the Stanovoy fold‐belt. Metamorphic conditions of the Tokskii block are compared with those of the Sutam block and a similar evolution of the units is revealed.Mineral assemblages and mineral compositions do not vary within each unit, but they change in a north‐south direction. The Opx + Sil + Qz assemblage has been found only in Sutam and Tok, but not in eastern Aldan and western Aldan. The Sap + Qz assemblage has been found in the Tokskii block but has not yet been found in the Sutam block. The pyrope content in garnets, from metapelites of both blocks, is significantly higher than that from the Aldan (eastern and western blocks) rocks to the north. The most important assemblages from different units of the Aldan shield have been studied using the electron microprobe in order to unravel the metamorphic evolution of the granulites and thus to deduce the thermodynamic regime of this evolution. A geodynamic model for the Aldan shield is discussed in terms of Arch

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00321.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractZircons have been studied from different layers of migmatites (from Arvika, western Sweden and Nelaug, southern Norway) and from a paragneiss (from Arvika) associated with one of the migmatites. The main purpose of the investigation is to establish whether or not information about zircons can help in the elucidation of the parentage and rock‐forming processes of migmatites.The elongation ratio of zircons from all layers is small and characteristic of sedimentary zircons. Further, the absence of characteristic colours and the growth trends of the zircons (indicated by the reduced major axes) observed in the various samples both support a sedimentary parentage for these rocks. The zircons of all layers exhibit secondary growth (overgrowth, outgrowth and multiple growth) due to metamorphism. Compared with the zircons from the paragneiss, those of the migmatite layers are more clouded and less rounded, some of them becoming opaque or even skeletal; this is especially true of the zircons from the leucosomes. These observations indicate an alteration of the original sedimentary zircons in the migmatite, especially in the leucosomes, in response to the migmatization process, previously interpreted as partial meltin

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00322.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00323.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY