A commentary on place in plutonism
Authors: Read Herbert Harold 
Published: October 2009

A general examination is conducted into the proposals concerning place and time in the making of the plutonic rocks—these constituting the metamorphic, migmatitic and granitic classes. Such notions as zones, levels, fronts, facies and assemblages, and the whole modern apparatus of metamorphism, are dealt with mainly from the space aspect.

Space variations of metamorphic significance are zones and facies. The classic upward succession of gneiss, schist and phyllite has become developed into the depth zones of Becke and Grubenmann and the ectinite zones of stratigraphical style of modern French workers. The association of metamorphism and depth is found to colour all this work. The classificatory facies-concept, in that it deals with rocks considered to be equilibrium assemblages, is not of value in the determination of metamorphic history. A correlation of metamorphic zones of various kinds and metamorphic facies is possible.

After this presentation of zones and facies, a series of comments, criticisms and discussions upon them is advanced. The depth involved in depth-zoning may be of various kinds and validities. True or geosynclinal depth can be, by itself, of little account; tectonic depth depends upon a thickening of cover and damming back of the heat supply, and is taken to be likewise of restricted application; it is proposed that magmatic-migmatic depth, controlled by the height reached by magma or migma, is of the greatest significance in plutonism.

A consideration of the relation between depth and pressure shows that the Volume' Law has little application in metamorphism, an opinion strengthened by a study of the packing indexes of metamorphic minerals and rocks. Again, the celebrated division of minerals into stress and antistress groups is of restricted significance since all stress minerals can be formed under no-stress conditions.

The application of the zones and facies concepts by the field-worker is a difficult. matter, since complications are introduced by the original composition effect, by polymetamorphism, and by the time-dimension in general. Non-equilibrium assemblages are much more important in that they allow the metamorphic history to be read.

Various considerations indicate that metamorphic systems are open. The preliminary results of Lapadu-Hargues favour this view and the distribution of certain minerals (for example, tourmaline) support it. The movement of material may be carried out by diffusion in a pore-liquid or in the solid, a fundamental control being exercised by the size of opening. Privileged paths or privileged bands, both textural or compositional, also control diffusion and influence mimetic crystallization on the one hand and so-called lit-par-lit injection on the other. Related to these factors is the common coincidence of schistosity and bedding. It may be suggested that tangential drag due to convection currents is of more significance in metamorphism than orogenic compression.

In most metamorphic rocks banding is bedding, but it must be conceded that in the deep Archean a mechanical sorting into bands of constituent minerals might take place. In the study of foliation, metamorphic differentiation and granitization, the plutonic geologist may be faced with problems involving both homogenization and heterogenization and the controls of these processes require consideration.

Involving both place and time, the proposals of migmatic or metamorphic fronts deserve the closest scrutiny. The regionally metamorphosed rocks most likely result from the passage of waves or fronts of metasomatizing solutions out from the central granitization core about which arise the zones of metamorphism. Divergent from this view is the modern French proposal that the migmatite front may be independent both in place and time of the metamorphic stratigraphy. This proposal raises profound difficulties connected with transfer of material in metamorphism and the interpretation of field data. In that its full discussion is concerned with time, it may be left for a later occasion when, too, the whole question of the Granite Series—a time-place series relating the plutonic phenomena at various levels of exposure—can be profitably examined. These present remarks draw attention to the geometry of the plutonic rocks so that, later, time can be applied and the history of great parts of the crust become revealed.