ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1983.tb00260.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

An occurrence of quartz‐eclogite is described from the Inner Schieferhülle unit of the Pennine Basement Complex in the SE Tauern Window, Austria.Field relations strongly suggest a pre‐Alpine age for the primary eclogitic mineral assemblage (garnet + omphacite + quartz + rutile). This implies that there was no connection between the formation of these eclogites and the late Cretaceous and Tertiary tectonic evolution of the Eastern Alps. The quartz‐eclogite mineral assemblage crystallized under conditions of 620 ± 100°C and at pressures in excess of 12 kbar, and suffered amphibolitic overprinting of Alpine and possibly Hercynian age.A four‐stage polymetamorphic history is proposed for the Inner Sch

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1983.tb00261.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

An assemblage consisting of corundum, sapphirine, spinel, cordierite, garnet, biotite and bronzite is described from the Messina area of the Limpopo Mobile Belt, and consideration given to its petrogenesis. Various geothermometers and geobarometers have been applied in an attempt to determine the temperatures and pressures of metamorphism.A former coexistence of garnet and corundum is suggested to have developed during the earliest high pressure phase of the metamorphism, where temperatures exceeded 800°C and pressures as high as 10kbar may have been experienced. Subsequently, continuous retrograding reactions from medium pressure granulite facies at about 800°C and 8kbar towards amphibolite facies generated spinel, cordierite, sapphirine and possibly also bronzite. The most notable reaction was probably of the form: garnet + corundum = cordierite + sapphirine + spine

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1983.tb00262.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Mafic and ultramafic xenoliths in a basaltic cone at The Anakies in south‐eastern Australia are geochemically equivalent to continental basaltic magmas and cumulates. The xenolith microstructures range from recognizably meta‐igneous for intrusive rocks to granoblastic for garnet pyroxenites. Contact relationships between different rock types within some xenoliths suggest a complex petrogenesis of multiple intrusive, metamorphic and metasomatic events at the crust/mantle boundary during the evolution of south‐eastern Australia. Unaltered spinel lher‐zolite, typical of the uppermost eastern Australian mantle, is interleaved with or veined by the metamorphosed intrusive rocks of basaltic composition.Geothermobarometry calculations by a variety of methods show a concordance of equilibration temperatures ranging from 880°C to 980°C and pressures of 12 to 18 kbar (1200‐1800 mPa). These physical conditions span the gabbro to granulite to eclogite transition boundaries. The water‐vapour pressure during equilibration is estimated to be about 0.5% of the load pressure, using amphibole breakdown data. Large fluid inclusions of pure CO2are abundant in the mineral phases in the xenoliths, and it is suggested that flux of CO2from the mantle has been an important heat source and fluid medium during metamorphism of the mafic and ultramafic protoliths at the lower crust/upper mantle boundary.The calculated pressures and temperatures suggest that the south‐eastern Australian crust has sustained a high geothermal gradient. In addition, the nature of the mineral assemblages and the contact relationships of granulitic rock with spinel lherzolite, characteristic of mantle material, suggest that the Moho is not a discrete feature in this region, but is represented by a transition zone approximately 20 km thick. These inferences are in agreement with geophysical data (including seismic, heat‐flow and electrical resistivity data) determined for south‐eastern Australia.Underplating at the crust/mantle boundary by continental basaltic magmas may be an important alternative or additional mechanism to the conventional andesite model fo

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1983.tb00263.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Plagioclase compositions vary from An0.1–2.5to An32with increasing grade in chlorite zone to oligoclase zone quartzofeldspathic schists, Franz Josef‐Fox Glacier area, Southern Alps, New Zealand. This change is interrupted by the peristerite composition gap in rocks transitional between greenschist and amphibolite facies grade. Oligoclase (An20‐24) and albite (An0.1–0.5) are found in biotite zone schists below the garnet isograd. With increasing grade, the plagioclase compositions outline the peristerite gap, which is asymmetric and narrows to compositions of An12and An6near the top of the garnet zone. In any one sample, oligoclase is the stable mineral in mica‐rich layers above the garnet isograd, whereas albite and oligoclase exist in apparent textural equilibrium in adjacent quartz‐plagioclase layers. The initial appearance of oligoclase in both layers results from the breakdown of epidote and possibly sphene. Carbonate is restricted to the quartz‐plagioclase rich layers and probably accounts for the more sodic composition of oligoclase in these layers. The formation of more Ca‐rich albite and more Na‐rich oligoclase near the upper limit of the garnet zone coincides with the disappearance of carbonate and closure of the peristerite gap. Garnet appears to have only a localized effect on Ca‐enrichment of plagioclase in mica‐rich layers within the garnet zone. The Na‐content of white mica increases sympathetically with increasing Ca‐content of oligoclase and metamorphic grade.Comparison of the peristerite gap in the Franz Josef‐Fox Glacier schists and schists of the same bulk composition in the Haast River area, 80 km to the S, indicates that oligoclase appears and epidote disappears at lower temperatures, and that the composition gap between coexisting albite and oligoclase is narrower in the Franz Josef‐Fox Glacier area. It is suggested that a higher thermal gradient (38‐40°C/km) and variations in Si/Al ordering during growth of the plagioclases between the two areas may account for these differences. In the Alpine schists the peristerite gap exists over a temperature and pressure interval of about 370‐515°

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1983.tb00264.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

A review of currently available information relevant to the Basal Gneiss Complex (BGC) of Western South Norway, combined with the authors’own observations, leads to the following conclusions.1. Most of the BGC consists of Proterozoic crystalline rocks and probably subordinate Lower Palaeozoic cover.2. The last major deformation of these rocks was during the Caledonian orogeny and involved large‐scale thrusting, recumbent folding and doming. The structural development of the BGC is closely tied in with that of the Caledonian allochthon.3. The whole eclogite‐bearing part of the BGC has suffered a high pressure metamorphism with conditions of between 550°C, 12.5 kbar (Sunnfjord) and about 750°C, 20 kbar (Møre og Romsdal) at the metamorphic climax.4. This metamorphism was of Caledonian age, probably rather early in the Caledonian tectonic history of the BGC and is considered to have been a rather transient event.By setting these conclusions in a framework provided by geophysical evidence for the deep structure of the crust in southern Norway we have constructed a geotectonic model to explain the recorded metamorphic history of the BGC. It is suggested that considerable crustal thickening was caused by imbrication of the Baltic plate margin during continental collision with the Greenland plate. This resulted in high pressure metamorphism in the resulting nappe stack. Progradation of the suture caused underthrusting of the Baltic foreland below the eclogite‐bearing terrain causing it to emerge at the Earth's surface, aided by tectonic stripping and erosion.Application of isostacy equations to the model shows that eclogites can be formed byin‐situmetamorphism in crustal rocks and reappear at the land surface above a normal thickness of crust in a single orogenic episode of approximately 65‐

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1983.tb00265.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY