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Although there are numerous publications on the geology of
high-grade gneiss terrains, few descriptions exist of how to map
and carry out structural analysis in these terrains. Textbooks on
structural geology concentrate on technIques appli cable to
low-grade terrains. Geologists who have no experience of mapping
high-grade gneisses are often at a loss as to how to apply
techniques to high grade rocks that were developed for low to
medium grade metamorphic terrains. Any study of deep crustal
processes and their development through time should begin with
examination of the primary data source - outcrops of high grade
metamorphic terrains. We feel that the urge to apply advanced
techniques of fabric analysis, petrology, geochemistry, isotope
geochemistry and age deter mination to these rocks often results in
brief sampling trips in which there is little, if any analysis of
the structural and metamorphic history revealed by outcrop
patterns. Many studies of the metamorphic petrology and
geochemistry of high-grade gneiss terrains make ineffective use of
available field data, often because the authors are unaware of
structural complexities and of the ways to recognise and use them.
This is unfortunate, because much data can be collected in the
field at minimal cost that cannot easily, if at all, be obtained
from material in the laboratory. The primary igneous or sedimentary
nature of a rock, the relative age of intrusive veins, and the
sequence of deformation that they under went, can usually best be
determined by straightforward observation in the field.
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