Klasyfikacja morfologiczna, terminologia i geneza foliacji

Zbigniew Cymerman


Scharakteryzowano parametry morfologiczne foliacji: odstęp, kształt i gęstość domen oraz stopień uporządkowania minerałów w mikrolitach. Przedstawiono wykres morfologiczny oraz zasady umiejscowienia na nim danej foliacji. Omówiono 4 główne typy morfologiczne foliacji (złupkowanie, kliważ krenulacyjny i spękaniowy) oraz zagadnienia terminologii. Przedstawiono poglądy o genezie foliacji w zależności od kierunków naprężeń i orientacji osi elipsoidy odkształcenia. Niektóre typy foliacji są równoległe do płaszczyzny XY elipsoidy odkształcenia, a inne - równoległe do płaszczyzny ab odkształcenia scinającego. W miarę wzrostu deformacji wszystkie foliacje dążą jednak do ułożenia równoległego do płaszczyzny XY końcowej elipsoidy odkształcenia. Omówiono foliację transpozycyjną, położenie foliacji względem płaszczyzny osiowej fałdu synchronicznego, refrakcję foliacji oraz niezgodność przebiegu foliacji płaszczyzny osiowej fałdu.





Foliation - a planal structure occurring in metamorphic rocks - is characterized by densely spaced anisotropy zones and expressed by either ordered orientation of mineral grains or a trend to subdivision into plates. It comprises all meso- and microscopic planar structures in deformed metamorphic rocks, except for bedding and fracture planes and joint.

Rocks with foliation are characterized by zonal structure. The structure comprises a number of minor foliation domains (zones), separated from one another by microlithotons (interfoliation zones).

Microlithotons differ from domains in composition and structure: the former are rich in quartz grains or quartz-feldspar aggregates, and the latter - in stratified silicates.

Morphological classification of foliation is based on the following parameters: spacing of foliation domains (in cm or mm), shape of domains (even, uneven, reticular, irregular), degree of ordering of minerals in microlithotons in relation to boundary planes of foliation domains (complete, very high, high, intermediate, weak, random), and frequency of foliation domains in rock (in %).

The enclosed morphological graph shows location of major types of foliations and their varieties.

There is also shown the method of plotting a given foliation in the graph.

Slaty cleavage represents the foliation type typical of deformed rocks (mainly pelitic ones), altered under conditions of weak metamorphism, and defined by planar ordering of orientation of mineral grains. Two extreme varieties of this type of foliation include lenticular and laminated slaty cleavage.

Schistosity is the foliation type characterized by ordered, parallel orientation of anisotropic minerals formed under conditions of intermediate to high degree regional metamorphism. Three extreme varieties of this type of foliation were differentiated: lenticular, laminate and dispersed schistosity.

True cleavage is found in rocks with old anisotropy planes, subjected to very small-scale folding.

It is subdivided into continuous (zonal) and discontinuous crenulation cleavage. The first of these varieties differs from the second in presence of gradual transitions between domains and microlithotons and continuity of domains of early crenulated foliation, connected with those of crenulation cleavage.

Fracture cleavage has been assigned to foliations taking into account characteristic geometric interrelations recorded in folded rocks and origin similar to that of foliations of other types.

It is proposed to use the term foliation in a wide sense, i.e. comprising the abovementioned 4 types of planar structures. The term cleavage should be used for structures of nonmetamorphic rocks, and those of fracture and crenulation cleavage - for structures of metamorphic ones. The term metamorphic lamination should not be used because of its ambiguous meaning.

Some authors explain origin of foliations in terms of compressional stresses acting at normal angle to the foliations, and others - in terms of effects of shearing stresses. A compromise solution would involve formation of foliations in planes normal to the axis of the maximum compressional stress and followed by formation of slips along these planes and due to shearing stresses, of course under conditions of heterogeneous deformation.

The origin of foliations appears related to various mechanisms. However, it should be noted that all of these mechanisms lead to ordering in orientation of mineral grains which define planar structure normal or roughly normal to the axis Z of-deformation ellipsoid. Some foliations, especially those defined by flattened fossils, ooids or xenoliths, are parallel to the plane XY whereas other may be parallel to the plane ab of shearing deformation. Along with increase in intensity of deformation the orientation of all the foliations will tend to be parallel to the plane XY of the final deformation ellipsoid.

The development of the ordered orientation of minerals may be due to mechanical rotation, recrystallization in the stress field, modification of shape of mineral grains, or a net result of processes of diffusion and solution under pressure. The development of a new orientation of mineral grains is additionally influenced by such features of rocks as shape of pores, permeability and previous orientation of mineral

Orientation of foliations in relation to axial plane of synchroneous fold may be parallel, oblique, normal or varying. Divergent and convergent fans, refraction of foliation along with its passage through layers varying in competence, and oblique orientation of foliation in relation to axial plane of fold (which may be due to either original irregularities on folded surface or heterogeneous deformation) are important for explanation of origin of both foliations and folds. It is necessary to make distinction between the phenomena of superposition and deviations from ideal interrelations of synchroneous folds and foliations. The degree of refraction and “fanning” of foliations are due to fold geometry. These features are well development in concentric folds but not those of the similar type.

Transpositional foliation is characterized by the lack of continuity of folded layers. Obliteration of fold kinks takes place in result of segmentation of individual layers, flattering of the kinks, and development of a new foliation in axial planes of folds. Migration of material is also of some importance here.

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