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    Varying Trends in the Metamorphism of Dolerites
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    Abstract:
    Abstract The metamorphism of Pre-Cambrian dolerites in the Northwest Highlands is described and compared with that of basic rocks in the Southwest Highlands, Banffshire and other regions. The first products of metamorphism are not the same in every area. It is suggested that the trend of regional metamorphism differs according to the environment in which it took place, and that a sequence of changes established in one area cannot be used as a standard by which to judge the changes in other areas. In particular, rocks of high metamorphic grade have not invariably passed through stages characterized by low-grade minerals.
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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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