Mineralogical and Structural Evolution of the Metamorphic Rocks

Abstract

Classic views on the scope and subdivision of the field of rock metamorphism are reviewed and summarized, and the general basis for classification of metamorphic rocks is outlined.The zonal distribution of rocks showing different degrees of metamorphism in those metamorphic terranes that have been studied in detail is attributed to progressive variation in physical conditions controlling development of a state of internal equilibrium in rocks of varying composition in different chemical environments. The resulting equilibrium assemblages of minerals can be interpreted broadly in terms of the phase rule. This leads to the concept of metamorphic facies as a basis for classifying and interpreting metamorphic rocks. Eskola's scheme of facies is modified and subdivided insofar as this is at present justified by the data of metamorphic petrography. While metamorphism in some cases involves only insignificant change in total chemical composition of the rocks affected, in others important change results either from metasomatism or from metamorphic differentiation.The fabric of metamorphic rocks has developed by growth and/or deformation of crystalline minerals in a solid medium. Its characters therefore differ strikingly from those of typical igneous and sedimentary fabrics. The essential features of metamorphic fabrics are reviewed, and interpretation of the fabric of deformed rocks is considered in some detail.The memoir concludes with a discussion of some of the problems specially connected with regional metamorphism, namely, the causes of regional metamorphism, the phenomenon of retrogressive metamorphism, the development of migmatites, and the relation of regional metamorphism to plutonic igneous intrusion. on June 21, 2015 memoirs.gsapubs.org Downloaded from on June 21, 2015 memoirs.gsapubs.org Downloaded from CHAPTER I. SCOPE OF METAMORPHISM DEFINITIONAccording to current general usage of the term, metamorphism is the mineralogical and structural adjustment of solid rocks to physical or chemical conditions that have been imposed at depths below the surface zones of weathering and cementation, and which differ from the conditions under which the rocks in question originated. Van Hise's (1904) extension of the scope of metamorphism to include weathering and cementation has now been generally abandoned, since it so enlarged the field of metamorphism that the term became virtually synonymous with "rock alteration" and therefore had little practical value in petrology. Other writers have narrowed the field of metamorphism as defined above by excluding such rock transformations as involve addition or removal of material, i.e., changes into which metasomatism enters. It is true that in many cases of metamorphism the composition of the rock remains a,lmost unchanged throughout, and to such Eskola (1939, p. 264) has applied the term isochemical metamorphism. On the other hand addition or removal of substances of high vapor pressure, notably water and carbon dioxide, may play an essential part in rock alterations universall...

Metamorphism of the Belt series in the Elk River-Clarkia area, Idaho

“…The principle of this presentation is the same as used by Eskola ( 1915Eskola ( , 1939 and later by Turner ( 1948), Barth {1936, 1952, Thompson {1957), Fyfe, Turner and V erhoogen ( 1958), and many others, and the same theoretical considerations concerning the equilibrium assemblages are valid. The 4-phase assemblages in this tetrahedral projection form from any bulk composition that plots within subtetrahedra determined by the index minerals at the corners.…”

Porphyroblast rotation: eppur si muove*?

“…2a) have traditionally been interpreted along these lines as rigid objects which rotated with respect to a foliation fixed to the flow plane of simple shear (Fig. 2b;Clough, 1897;Flett, 1912;Turner, 1948;Ramsay & Huber, 1987, p. 633). However, if the foliation is not fixed with respect to the kinematic frame, both the foliation and the porphyroblast can rotate and porphyroblasts with S, oblique to S, can form in other ways.…”

The chronological analysis of crystallization and deformation of some Tasmanian Precambrian rocks

“…The most characteristic feature of post-tectonic crystals (particularly albite) is helicitic structure (Teall, 1897;Cunningham-Craig, 1909;Niggli, 1912;Bailey, 1923;Turner, 1933;Hutton, 1940;Turner and Hutton, 1941;Cloos and Hietanen, 1941;Reynolds, 1942;Rast, 1958;Zwart, 1960aZwart, , 1960bJones, 1961). A porphyroblast may not expel all of the unwanted material in the space it ultimately occupies.…”