Determination of Structural Successions in Migmatites and by A.M. Hopgood

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By A.M. Hopgood

This booklet has been written according to whereas enormous attempt has been made to requests from a couple of colleagues within the make sure that the insurance is accomplished, it's earth sciences in several elements of the area. now not unavoidably exhaustive. in demand between those are geologists whose the significance hooked up to the structural pursuits lie within the fields of isotopic and research of migmatites (and different advanced fiscal geology and who've a particu­ structural institutions) stems from the very fact larly willing appreciation of the significance to that solution in their structural complexity their paintings of an intensive knowing of the has this kind of major pertaining to many features structural relationships in rocks, specifically of geology, either educational and monetary. the place such rocks, like migmatites, have an extended Examples of a few of those functions are and sometimes advanced developmental historical past. indexed in bankruptcy 14. rather impor­ What those geologists requested for used to be a advisor tant elements are that the structural characteris­ to the tools hired in resolving the struc­ tics so pointed out can be utilized (1) to correlate tural complexity of again and again deformed rocks among separated rock devices with related (i. e. these laid low with 'polyphase' or a number of deformational histories, together with segments deformation), in particular in Precambrian base­ of fragmented supercontinents, and (2) to ment terranes.

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This demonstrates the relationship between distribution on the plot and degree of reorientation and shows that the distribution of the earlier sets is more widespread as a result of having been subjected to a greater degree of reorientation by later folding, whereas the latest are restricted to a narrow zone trending, in this case, NNE. Lewisian complex, Outer Hebrides, Scotland. From Figure 6, Hopgood, 1980. Reproduced with the permission of the Royal Society of Edinburgh. 10. 12 Effect of fold scale on grouping and dispersion of points on a scatter diagram (stereo plot) of refold data.

A clear definition of the word 'migmatite' in modern petrological terms appears to present some difficulty, as pointed out by Ashworth, 1985, pp. 1), whereas Sederholm's original definition neatly encapsulates the characteristics of migmatites and is appropriate in the context of the study of structural relationships. 16) and in migmatites this (by definition) is especially the case, because of the diversity of relationships between the different rock units comprising them, so they are one of the most difficult groups of rocks to deal with structurally.

Earlier open folds have usually been tightened by continued or repeated deformation with reduction of their inter-limb angle, in some cases to zero. In evolutionary terms this commonly means that folds first formed as open structures on upright axial planes, gradually became tighter, inclined, recumbent, overturned and ultimately isoclinal as deformation continued. In this way, later (more open) structures were 31 imposed on a complex made relatively rigid in consequence of having already been highly folded during earlier deformational phases so that progressively more contorted structures resulted.

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