A contemplation of time in plutonism

Abstract

Summary A general survey is first made of certain aspects of time in plutonism. These include the duration of plutonic times, their limits and subdivisions, the different dates concerned in them, and the unity or otherwise of plutonic processes. More specific inquiries begin with the consideration of time and crystallization. The criteria for the determination of time-sequences in the crystallization of the plutonic rocks are reviewed and examples of their application given. The more complex question of the time-relations of crystallization and deformation is next taken up, chief attention being given to the evidence for the rotation of porphyroblasts. The diagnostic requirements for pre-crystalline, para-crystalline and post-crystalline deformations are examined. The inquiry next moves to time-relationships of a different order, those relating plutonism and orogeny. The implications of folding of a metamorphic succession and the validity of inversion in the crystalline schists are discussed. The importance of metamorphic history, facies and convergence in geological interpretation is illustrated by examples from Scotland, central Europe and elsewhere. The problem of the subdivision of the metamorphic history of a rock and the evidence for monometamorphism or polymetamorphism are considered and examples from the Scottish Highlands are discussed. As the whole operation of plutonism depends upon the activity of migma-magma, it is necessary to deal with the time-relations of crystallization, deformation, granitization, migmatization, metamorphism, intrusion and orogeny. The general principles of the relation between magmatism and orogeny are explored. The micro tectonic and deep-tectonic studies of Demay are summarized and discussed— the discussion leading on to the consideration of the characters of synkinematic intrusion and of synkinematic permeation. Finally, the Granite Series is displayed and discussed. This attempts to relate the plutonic phenomena at the various levels of exposure, and to give a unity to the processes of granitization, migmatization and metamorphism at depth and at successively higher positions and later times. The series begins with the autochthonous granites associated with niigmatites and metamorphites produced in situ in the depths. The movement of the granitized material provides the successive terms of the series. The parting of envelope and core, the structural control of movement, straining off, selective mobilization, overtaking and chemical variation in the series are discussed. The Granite Series ends with the late high-level plutons emplaced as viscous masses. An example of the Granite Series from the Hercynian belt of north-western Europe is presented. The plutonic developments in the Massif Central, Armorica and South-West England are compared. In this belt, the,later members of the Granite Series appear at higher levels as we pass from south to north, and there is a progressive change in the style of contact in the same direction. The higher and younger members to the north have left their deeper and earlier roots in the Massif Central. At the three or four sample levels of the Granite Series provided by this example, fundamental differences of pattern reveal the relationships between place and time in the making of the patterns.