Granites and granitic magmas: strange phenomena and new perspectives on some old problems

Clemens, J.D. (2005) Granites and granitic magmas: strange phenomena and new perspectives on some old problems. Proceedings of the Geologists' Association, 116(1), pp. 9-16. ISSN (print) 0016-7878


Granitic plutons generally afford pleasant, mostly unchallenging landscapes. However, on closer inspection of some of the outcrops, one may discover some strange, beautiful and baffling examples of patterns produced by geochemical and mineralogical self-organization. Fascinating as they are, these features probably reveal little about the origins of granitic magmas. It is the more usual features that one needs to study and understand, using geophysics, geochemistry, isotope petrology, textural analysis and various kinds of theoretical modelling. Many granites are products of high-temperature melting of continental crust and there is a relationship between upper granulite-facies metamorphism and the generation of granitic magmas. The heat needed for this process commonly comes from mafic magmas intruded into the deep crust and the melting reactions take place in the absence of free fluids. The relatively low viscosity and density of hydrous granitic magmas control the ascent and emplacement processes and the viscosity is not greatly changed during crystallization and cooling, at least for the first few tens of percent crystals. Modelling shows that granite diapirism is slow and inefficient and there is little evidence of it in the geological record. The granitic melt segregates into veins, shears and dykes, eventually forming larger feeder dykes that transport the magma rapidly upward to the emplacement sites. The potential speed of ascent and emplacement mean that even very large granitic plutons probably grow in thousands of years. Experimental data on crystallization rates suggest that many of the large crystals observed in granitic rocks could have grown in a matter of hours – certainly in no more than a few tens of years. Geological processes are commonly thought of as slow and continuous, but many are rapid and episodic. Granitic plutonism is of the latter kind.

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