The amphiboles anthophyllite, grunerite, and riebeckite appeared to be unstable in the presence of water vapor under pressure, and the absence of amphiboles in the granite pegmatites and their almost universal presence in the perthite-quartz granites indicate that the pegmatites were produced in a water-rich environment while the perthite-quartz granites crystallized in a water-deficient environment.
The rapakivi granite problem has been reviewed in the light of the experimental results, and it is pointed out that normal crystallization of magmas containing somewhat more potassium than the average granite can produce the rapakivi texture, providing water is concentrated during crystallization and the liquidus is depressed below the feldspar miscibility gap. The Tertiary granites of Skye are normal granites, chemically, and to some extent texturally; mineralogically, however, they are similar to rhyolitic rocks.
The quartz and feldspars resemble in many respects the corresponding phenocrysts of extrusive rocks.
It is suggested that these young granites represent quenched granites which, as a consequence, have some properties of both granites and rhyolites. Complete melting will take place if 9—10 per cent water is available. If the water content is 2 per cent, melting will still begin at the same depth; complete melting will not take place until some greater depth has been reached. It is proposed that this zone of melting, where temperatures are high enough to melt granite completely and more basic compositions at least partially, may offer a mechanism for producing large batholithic masses of granite.
A classification of salic rocks based on the nature of the alkali feldspar is proposed. The classification has two major divisions: In the hypersolvus rocks all the soda feldspar is or was in solid solution in the potash feldspar whereas in the sub-solvus rocks the plagioclase is present as discrete grains. The two major divisions are further subdivided according to the nature of the alkali feldspar modification. The suggestion that most granites finished crystallization with a single alkali feldspar precipitating has been questioned by some petrologists because rhyolites commonly carry phenocrysts of plagioclase and sanidine feldspar.
A study of the feldspars of extrusive rocks indicates that the plagioclase phenocrysts may react with the liquid during crystallization leaving a single alkali feldspar. If fractionation takes place, the tendency to complete crystallization with only a single feldspar crystallizing from the liquid is greatly enhanced. The proposition that two-feldspar granites may have gone through a one-feldspar stage has been examined in the light of experimental studies, and it was concluded that the required adjustments in mineral composition and texture are reasonable.
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The origin of granite has for long fascinated geologists though serious debate on the topic may be said to date from a famous meeting of the Geological Society. One might question the rationale and merits of a second edition of The Nature and Origin of Granite by W. S. Pitcher so close on the heels of the.
Geological Society of America. You must attribute the work in the manner specified by the author or licensor but not in any way that suggests that they endorse you or your use of the work. Noncommercial - you may not use this work for commercial purpose. No Derivative works - You may not alter, transform, or build upon this work. Citing Books via Google Scholar. In each of the subsequent chapters, Pitcher does a good job of blending the historical backdrop with contemporary research and its interpretations.
His prejudices include not only his own personal views of a subject, but whom else he thinks has made important contributions to the topic. Throughout the book, I was surprised by how frequently some authors were cited, and others whom I knew to have contributed every bit or more were scarcely or never mentioned. But the dominance of Pitcher's personal views and his selective treatment of other's work is precisely what the book is about—it is a vehicle for the author to present his own ideas, beliefs, and thoughts in a manner that is not consistent with or amenable to the journal or textbook format.
As in the first edition, Pitcher brings to the second edition his extensive familiarity of granites in the field, weighing the various hypotheses and conclusions from experimentation, isotope studies, numerical models, etc. He is fair in his evaluation of other's work though see my comments below , honest in denoting the limits of his expertise in certain issues, and steadfast when even widely held opinions or models remain inconsistent with his own observations.
Any reader of this and the previous edition of The Nature and Origin of Granite is likely to be vexed by the utilization and citation of references. The text mostly cites authors but not specific year references to their publications; nor is it even clear in some instances if the information that Pitcher is using originated in print or by word of mouth with colleagues. Hence, the hefty list of references in the bibliography entries in the second edition, if I counted correctly is somewhat misleading of the actual usage in the text. The lack of connection between the authors and the sources of data used by Pitcher leads to questions of accuracy, and I decided to evaluate accuracy in the context of my own specialized part of the granite system—pegmatites—for which the following three examples may be illustrative.
In Chapter 2, Pitcher states p.
Those who know my research are aware that I have made a career out of rejuvenating Jahns' original concepts of pegmatite formation, for which a vapor phase was not requisite and which emphasized the crystallization of melt far from equilibrium as the underlying process. In Chapter 17, which deals with pegmatites pp.
These illustrations give a hint to the strength of some of Pitcher's convictions and to the confusion inherent in his method of citation. The errors are no doubt accidental, but they are nevertheless travesties of accurate reporting. I do not claim that such inaccuracies are pervasive, but then I am not expert enough to assess all of the statements and conclusions that are attributed to others.
With these remarks, I may have spurred my colleagues to buy or at least read the second edition to see if and how their own work is dealt with. The Nature and Origin of Granite is very informative, appropriate for professionals in the discipline, which perhaps includes advanced graduate students. Beyond this audience, I would be cautious about recommending the book as a primer. Pitcher is correct to say that it is not a textbook, nor, I would add, a methods book; rather, it serves as a historical benchmark of Pitcher's view of granite and our studies of it.
The chapter titles of the second edition are: Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Close mobile search navigation Article navigation. The Nature and Origin of Granite.