0000000000309180
AUTHOR
John S. Myers
Metamorphic History of Gneiss Terrains
The local metamorphic history is an essential topic in the study of high-grade gneiss terrains. This chapter only deals with those aspects of the metamorphic history that can be studied in the field. We will briefly outline terminology, the methods which can be applied, and the difficulties involved. For more information we refer to textbooks such as Miyashiro (1975), Winkler (1976), Mason (1981), Best (1982), Vernon (1983), Spry (1986), Yardley (1989) and to the references given in the text.
Geochemistry, Isotope Geochemistry and Geochronology: Application to Field Studies
Structural studies make it possible to reconstruct the metamorphic and deformation history of a rock body but do not provide information on the absolute timing of these processes. Likewise, field observations and structural work alone may not unambiguously identify the protoliths of many high-grade gneisses or the compositional changes associated with migmatisation, anatexis and the general action of fluid and/or vapour phases in a rock. Geochemical research can answer many of these problems but, as most of this research is laboratory-oriented, we limit ourselves to some general outlines of the possible lines of research, so that these can be borne in mind during fieldwork.
Interpretation of Structures and Fabrics Recognition and Interpretation of Fabric Patterns in Outcrop
High-grade gneisses with a long and complex history have a finite ‘memory’ of past events. This memory is formed by fabric elements such as foliations, lineations, folds, mineral assemblages, boudins and sequences of intrusion. One of the aims of a geologist is to tap this memory as effectively as possible. The memory of rocks, however, is rather inadequate in that it is partly destroyed by the same events which produce the fabric elements that are recorded (Williams, 1983). Strong deformation erases older fabric elements; intrusions, recrystallisation and partial melting do the same (Figs. 1.1; 4.1). One of the purposes of this manual is to help geologists working in high-grade gneiss terr…
Mapping in Gneiss Terrains
Whatever the purpose of working in gneiss terrains, a prerequisite for success is a thorough understanding of the geometry and relative age relations of rock units and the sequence of deformation and metamorphism. Because of the three-dimensional complexity of deformation and intrusion relations in gneiss terrains, it is in most cases impossible to understand relations by visiting a single outcrop, or even by means of a small number of transects through the area under consideration. If no detailed geological maps are present, or if the subject of study is insufficiently dealt with on available maps, it will be necessary to map an area in some detail.
Origin and Evolution of High-Grade Gneiss Terrains
In the previous chapters we have given an impression of the way in which the analysis of high-grade rocks can be approached in the field. Here we conclude with some current larger-scale ideas on the origin and evolution of such rock assemblages.
Fabric Development in Gneiss Terrains
This chapter describes some aspects of the development of fabrics in gneiss terrains. It provides essential background information that should be read before attempting to map a gneiss terrain. Inevitably, the interpretations are ‘state of the art’ and not necessarily the absolute answer. Additional information can be found in the cited literature and various specialised journals such as the Journal of Structural Geology, the Journal of Metamorphic Geology, Tectonophysics, Tectonics and Precambrian Research.