A40/K40 age of gneiss from McMurdo Sound, Antarctica
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Abstract:
Biotite from paragneiss of Gneiss Point, McMurdo Sound, Antarctica is dated at 520 m y by the A 40 /K 40 method. Metamorphism of Precambrian sediments at the close of Precambrian or in Cambrian time is indicated, but additional work is needed.Massif
Pseudomorph
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1958)
The regional metamorphism which produces glaucophanites can be regarded as metamorphism under low temperature and high pressure. It has been expressed by many authors including the present writer that the glaucophanites have no special characteristics in the chemical composition. However, it is evident from the statistical study of the available chemical data that the metamorphic rocks, especially the green schists in the glaucophanitic regional metamorphic terrains have generally higher content of Na2O and higher ratio of Fe2O3: FeO than the metamorphic rocks subjected to other types of regional metamorphism. These features are considered to be due to the soda-metasomatism and rather high water vapour pressure prevailing in some glaucophanitic regional metamorphic terrains.
Metasomatism
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Pelite
Isograd
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For many metamorphic petrologists, the study of low-grade metamorphic rocks is something of a black art. Because low-grade rocks are frequently fine-grained, contain a wide variety of low-temperature minerals with limited stability fields, and have high porosities and fluid contents they are not as readily understood as higher grade metamorphic rocks. For these rocks, few, if any, of the standard thermobarometers or thermobarometric datasets are applicable and the derivation of fluid evolution histories is complicated by the high concentration of, frequently reduced, fluids. Low-grade metamorphism, at T <400°C and P <4–5 kbar, spans the P – T interval between what may be erroneously termed ‘proper’ metamorphism and the field of diagenesis, although in reality there is no clear or sharp break between the diagenetic and metamorphic fields. As such, techniques relevant to the study of low-grade metamorphism span those of both the sedimentary and metamorphic fields. This …
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Isograd
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Protolith
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Diachronous
Foliation (geology)
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