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Classification of Metamorphic Rocks

temperature, metamorphism, stress, pressure, zone, low, zones and minerals

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CLASSIFICATION OF METAMORPHIC ROCKS Metamorphism being the response to change of condition in respect of temperature and pressure, the diversity in mineral composition of metamorphic rocks can be regarded as the resultant of two independent variables, viz., (a) the ultimate composition of the rocks, and (b) the physical conditions operating during metamorphism. These two variables may well form the basis of a classification of the products of metamorphism.

In Grubenmann's well known classification of the crystalline schists these two variables occupy the dominant positions, and are the foundation upon which the classification is erected. The composition variable is expressed in the 12 groups recognized, six of which correspond to sedimentary types, these being the shales, sandstones, limestones, marls and the weathering residuals represented by laterites and bauxites, and six corresponding to igneous rocks, granites, diorites, gabbros, ultrabasic and alkali rocks, the latter including the syenites, nepheline syenites, thera lites, lamprophyres, etc. The physical factors of pressure and temperature are recognized as a threefold division into zones, epi, meso and kata. In a general way the factors temperature and pressure are a function of depth and the divisions epi, mesa and kata thus correspond to bathymetric zones, each with its distinc tive types of mineral paragenesis.

In the uppermost or epi zone, temperatures are low, hydro static pressure is low, while shearing stress may be high. The mineral products of this zone are those rich in OH or H, and are characteristically those of low density, such as chlorite, zoisite, sericite, hornblende, chloritoid, etc.

The rock products are those of mechanical deformation with little recrystallization, cataclastic rocks of all types and the rocks of a low grade metamorphism such as phyllites, chlorite and talc schist, epidote schists, etc. In the second or meso zone the tem peratures are higher, while stress probably reaches the maximum value. Owing to the increase of temperature the imposition of shearing stress results in recrystallization without mechanical rupture. Hydroxyl-containing minerals are now less prominent. The type minerals are kyanite, staurolite, almandine, antho phyllite, muscovite, biotite, etc., while the assemblages appear in such rocks as biotite, garnet, staurolite, kyanite and actinolite schists, marbles and quartzites. In the deepest, or kata zone, the high stress of the meso zone under the influence of the very high temperature, is replaced by high uniform pressure. Reactions

take place pre-eminently in response to the volume law leading to the generation of anhydrous minerals of high density. Pyrox enes, olivine, pyrope, sillimanite, spinel, anorthite, jadeite, potash felspar, etc., are the type minerals, while the resulting rocks are gneisses, granulites, eclogites, etc., characterized by these minerals. It is not to be concluded that natural assemblages can be fitted into such a simple scheme without serious difficulties. The factors of temperature and pressure are not dependent simply on depth below the surface, but may be subject to wide variation locally. These physical factors include among themselves at least two independent variables : the intrusion of igneous magma brings into play the factor of high temperature without the incidence of pressure and local intense orogenic movement, the factor of high stress without necessarily the incidence of temperature. Thus in this system we find no well-defined place for assemblages of contact metamorphic origin. The range of possibilities thus opened up is, however, to a large extent diminished by the fact that the maximum value of shearing stress is a function of the temperature, and the possible range of stress is therefore decreased as the tem perature rises. Where metamorphism is of the regional type, it may be expected that stresses are maintained close to their maximum value, and to this degree the changes effected are in response to a single variable. The ease with which the rock assem blages originating under higher temperatures can be fitted into the meso and kata zones, finds its explanation in this dependent relationship between maximum stress and temperature. In rocks of the epi zone, the great variety of rock products is a direct reflection of the widely variable stress maxima of different ma terials. The peripheral areas of a mountain tract that has under gone metamorphism are usually of a low grade of metamorphism, and as zones of weakness are prone to be cut off from observation by subsequent dislocation, or covered by the overlap of later sediments. Frequently, therefore, the metamorphism over a wide tract may be of nearly uniform (but high) intensity. In some areas of regional metamorphism, however, these marginal zones are preserved for observation, and a continuous passage from unaltered sediments to schists and gneisses of high meta morphic grade can be traced.

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