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

By A.M. Hopgood

This publication has been written according to whereas huge attempt has been made to requests from a couple of colleagues within the make sure that the insurance is complete, it really is earth sciences in several components of the realm. now not unavoidably exhaustive. fashionable between those are geologists whose the significance connected to the structural pursuits lie within the fields of isotopic and research of migmatites (and different advanced financial geology and who've a particu­ structural institutions) stems from the very fact larly willing appreciation of the significance to that answer in their structural complexity their paintings of an intensive knowing of the has the sort of major referring to many elements structural relationships in rocks, specially of geology, either educational and fiscal. the place such rocks, like migmatites, have a protracted Examples of a few of those purposes are and infrequently complicated developmental heritage. indexed in bankruptcy 14. fairly impor­ What those geologists requested for used to be a consultant tant facets are that the structural characteris­ to the equipment hired in resolving the struc­ tics so pointed out can be utilized (1) to correlate tural complexity of time and again deformed rocks among separated rock devices with similar (i. e. these suffering from 'polyphase' or a number of deformational histories, together with segments deformation), specially in Precambrian base­ of fragmented supercontinents, and (2) to ment terranes.

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Archaean Pre-Ketilidian migmatite, West Greenland. 17 (a) Photograph of a palaeosome block in neosome containing traces of relic palaeosome structure, a nebulite. (b) Sketch of (a) showing the trace of the later, post-agmatization foliation in the neosome (dashed lines) discordant to the early foliation and four smaller sketches showing the stages (1-4) in the development of the present structure. The neosome encloses, at (1), a clearly-defined enclave (palaeosome) containing relict folds formed in early foliation prior to the neosome and at (2), faint 'ghost' remnants of early folds not completely replaced by the neosome.

Vein 5. Vein emplacement 3. Shear surface 4. Offset 2. Fold 3. Folding 1. Foliation 2. Metamorphism (? + deformation) 1. Sedimentation? 2 IMPORTANCE OF THE STRUCTURAL SUCCESSION IN MIGMATITE TERRANE INTERPRETATION In determining the relative chronology of successively-formed structures one is also establishing the basis for determining the relative chronology of metamorphic and igneous episodes. This in turn forms a basis for isotopic studies enabling time spans to be established for tectonothermal episodes as well as allowing one to determine the timing of the intervals between the events responsible for structure sets within these episodes.

8b) or pervasive. 9) further complicates the structure and, together with repeated deformation and emplacement of new rock bodies, the early-formed structures can be modified so drastically as to leave very few clues to their existence and almost none concerning their relationships. 6 Migmatites showing absence of clear, continuous reference layers. Pre-Ketilidian Complex, West Greenland. 7 Feldspar megacrysts in anorthosite. Pre-Ketilidian migmatite, West Greenland. 10). In such cases even the suspicion that some folds belong to different sets should form the basis for separating them provisionally from others.

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