Appendix History of the Study of Metamorphism

Appendix History of the Study of Metamorphism 1. VIEWS ON METAMORPHIC ROCKS IN THE LATE EIGHTEENTH AND THE EARLY NINETEENTH CENTURY Fragments of geological knowledge began in Classical Greece or even at an older time. The science of geology as a field of systematic knowledge was initiated in the middle eighteenth century by J. E. Guettard and Nicolas Desmarest in France. Toward the end of the century, two great theorists appeared: Werner and Hutton. A. G. Werner (1749-1817) was an influential professor of geology and mineralogy (collectively called geognosy by him) at the School of Mines at Freiberg, Saxony. This school was founded in 1765 as one of the oldest institutions for education of professional mining engineers and geologists. Werner and his followers, who were called Neptunists, explained all the rocks as being of sedimentary origin (chemical and clastic). They had no ideas of igneous and metamorphic rocks and of tectonic movements. James Hutton (1726-97) was a leisured gentleman and typical eighteenth century intellectual living in Edinburgh, who had active interest in all the branches of human knowledge including chemistry, meteorology, agriculture and philosophy. He spend much time in geological studies, and regarded basaltic and granitic rocks as being formed by consolidation of molten materials (magmas). The advocates of this idea were calledPlutonists. The concept of metamorphism is said to have begun with Hutton's theory, a systematic description of which was given in his book, Theory of the Earth (1795). In Hutton's view, some sedimentary rocks were brought to depths of the earth where high temperature and high pressure caused metamorphism of them. The schists and gneisses of the Scottish Highlands, for instance, were regarded as being metamorphic, though he did not introduce such a technical term. According to Johannsen (1931, p. 185), the term 'metamorphism' was introduced by A. Boue in 1820, while the term 'metamorphic rocks' became popular through the first edition of Charles Lyell's Principles of Geology (vol. 2, 1833). As regards the origin of basalt and granite, the Piutonists had secured a victory by 1820, that is, immediately after the death of Werner. However, the problem as to whether schists and gneisses are metamorphic rocks, primordial igneous rocks or chemical sediments that were deposited in primordial oceans, 430/HISTORY OF THE STUDY OF METAMORPHISM was not settled until the middle of the nineteenth century. According to the common view at that time, gneisses were formed in the oldest geologic age, and then schists were formed, being followed by phyllites. Fossiliferous sedimentary rocks were considered to have been formed at a still later time. In the meanwhile, transitional relationships between unmetamorphosed sediments and schists were found at places. Thus, a greater number of geologists gradually came to think that some or all of the phyllites and schists are metamorphic in origin. 2. VIEWS ON REGIONAL METAMORPHISM IN THE SECOND HALF OF THE NINETEENTH CENTURY The distinction between contact and regional metamorphism is said to have been first noticed in the middle of the nineteenth century by Elie de Beaumont and A. Daubre. The term regional metamorphism was proposed by the latter author. The geosynclinal theory for mountain building was formulated in the period 1859-1910 by James Hall, J. D. Dana, and E. Haug. The widespread occurrence of regional metamorphic rocks in orogenic belts attracted the attention of all these authors. The high temperature, high pressure, and deformational movement in the depths of geosynclinal piles were considered to be the cause of metamorphism. Though I am not familiar with the publications of this period, it appears that there were two contrasting views on the cause of regional metamorphism. One school of geologists stressed the importance of high temperature in the depths of the earth and effects of plutonic masses as the agents of regional metamorphism, while the other school emphasized the effects of pressure (hydrostatic or non-hydrostatic) and deformational movement. The former school included many geologists in Britain and France, some of whom used the name of 'plutoniC metamorphism' for regional metamorphism. The effect of water and other materials emanating from the associated plutonic masses was especially emphasized by French authors. The latter school who accentuated the effects of pressure and deformational movements included many German and Swiss geolOgists. They used the name of 'dynamic' or 'dislocation metamorphism' for regional metamorphism. The term 'dynamic metamorphism' (or dynamometamorphism) was proposed in 1886 by H. Rosenbusch, who was a great master in descriptive petrography. This term, and the idea attached to it, were propagated by his students and pervaded generations of geologists all over the world. Furthermore, some later authors began to use the names of dynamic and dislocation metamorphism to denote cataclastic metamorphism (i.e. intense mechanical deformation of rocks). Many other names were introduced to 3. GOLDEN AGE OF MICROSCOPIC PETROGRAPHY /431 represent the supposedly dominant agents such ~ thermal and dynamothermal metamorphism. This increased the confusion of nomenclature. Metamorphic rocks closely associated with granitic rocks are widely exposed in Britain and France, whereas the Alpine metamorphic rocks are rarely accompanied by granitic ones. This difference may have been a part of the factual basis underlying the contrasting views on regional metlmlorphism. 3. GOLDEN AGE OF MICROSCOPIC PETROGRAPHY The period of 1870-1900 was the golden age of microscopic petrography. Microscopic observation of thin sections was a powerful new technique in geology at that time, before which rocks had been examined ]:>y means of the magnifying glass and chemical analysis. Reliable determination of rock-forming minerals and their textural relations became possible for the tlISt time by the introduction of the microscope. It was then a new finding that plagioclases form a continuous series of solid solution. Thus, the nature and the extent of diverSity of the rocks were first clarified. A great number of rock names were coined and systems of rock classifications were proposed. F. Zirkel (1838-1912) of Leipzig and H. Rosenbusch (1836-1914) of Heidelberg were the two greatest masters in this field who attracted students from all over the world. Since their contributions were so remarkable, micro scopic petrography became the major field in the study of rocks. The threefold classification of rocks into igneo~s, sedimentary and metamorphic was proposed by B. von Cotta in 1862, and was accepted by Zirkel and Rosenbusch. They published voluminous books, among which Rosenbusch's Elemente der Gesteins lehre (3rd ed., 19~0) is relatively concise and readable. The last phase of the pre-eminence of microscopic petrography was represent ed by the great work Die Kristallinen Schiefer (1904-6) written by U. Grubenmann (1850-1924) of Zurich. In this book, h~ classified all the regional metamorphic rocks into twelve groups according to their chemical compositions. The rocks of each group were divided into three categories according to the depth-zones, named epi-, meso- and kata-zones iIi order of presumably increasing depth of their metamorphism. The epi-zone (Le. shallow zone) was assumed to be characterized by low temperature, low pressure and strong deformation, the kata-zone by high temperature, high pressure and weak deformations. The meso-zone was assumed to be intermediate. This idea of depth zones had been originally formulated by Becke (1903), but became popullp" through Gruben mann's book. All the observed regional metamorphic rocks were assigned to one of the three zones. For example, phyllite, chlorite schist, and glaucophane schist were assigned to the epi-zone, mica schist and amphibolite to the meso-zone, and some of the gneisses, eclogite and jadeite rock to the kata-zone. The ass~gnment 432/HISTORY OF THE STUDY OF METAMORPHISM was made on the basis of general impressions about the grain sizes and mineral compositions, since the mineral zonation of progressive metamorphic terranes was not known at that time. In Zurich, Grubenmann was succeeded by Paul Niggli, who continued to advocate the basic idea and terminology of his predecessor. Hence, Grubenmann's doctrine dominated a large part of con tinental Europe almost to the present (e.g. Grubenmann and Niggli, 1924). In about 1920-30, advocates of new approaches of metamorphic geology had to justify their existence by criticizing the doctrine of Grubenmann and Niggli. For example, Harker criticized Grubenmann's static attitude (as was already mentioned in § 3-1). Eskola also repeatedly criticized Grubenmann's depth zones in order to show the superiority of his facies classification. 4. DISCOVERY AND SIGNIFICANCE OF PROGRESSIVE METAMORPHIC ZONES Progressive Metamorphic Zones Prior to the beginning of study of the progressive mineral changes, the progressive textural changes were investigated by Rosenbusch (l877) in the contact aureole of the Barr-Andlau area in Alsace (§ 10-1). He distinguished two textural zones on a geologic sketch map. Progressive textural zones were later mapped in regional metamorphic terranes of New Zealand and other countries. Progressive mineral zones were mapped for the first time in 1893 by George Barrow in a part of the regional metamorphic terrane of the Scottish Highlands (fig. 3-1). He was a self-taught mapping geolOgist (1853-1932) of the British Geological Survey. His work was completed in 1912, and was a most epoch making contribution to metamorphic petrology. However, it did not attract much attention until the confirmation of his work by Tilley (1925) and Harker (l932). Without being aware of Barrow's work, V. M. Goldschmidt (1915) investi gated the progressive regional metamorphism in the Trondheim area (then called Trondhjem) of Norway (fig. 1-2). The investigation of mineral zoning did not become popular until the publication of studies by Tilley (1925) and Harker (l932) in the Scottish Highlands, by Vogt (l927) in the Sulitelma area of Norway, and by Barth (1936) in Dutchess County, New York State.

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