Topics addressed include: greenstone belt externalities boundaries rock terranes synthesis and destiny tectonic evolution rock components and structure sedimentology stratigraphy volcanism metamorphism and geophysics. Workshop on Techtonic Evolution of Greenstone Beltsĭewit, M. Belts of ultramafic rocks warrant investigatijon as possible ophiolites. That basement contains older Archean belts with pillow basalts and komatiites. The Archean Carajas belt in the southeastern craton probably formed in an extensional basin on older continental basement. Early proterozoic belts in the northern region probably originated as ensimatic island arc complexes.
Greenstone belts exposed amid gneisses, granitoid rocks, and less abundant granulites along the northern and eastern margins of the Amazonian Craton yield Trans-Amazonican metamorphic ages of 2.0-2.1 Ga. Tectonics of some Amazonian greenstone belts The effects of tectonic setting, structural geometry and evolution, associated plutonic activity and sedimentation are discussed. Surrounding terrains provide information on the context of greenstone belts. Most belts developed rapidly, in less than 100 Ma, leaving large gaps in the geological record. Greenstone belts are an important part of the fragmented record of crustal evolution, representing samples of the magmatic activity that formed much of the Earth's crust. Greenstone belts: Their boundaries, surrounding rock terrains and interrelationships Four particular stratigraphic problems that afflict studies of Archean greenstone belts are considered: determination of facing directions, correlation of lithologic units, identification of primary lithologies and discrimination of stratigraphic versus structural contacts. Many current controversies of greenstone belt petrogenesis, sedimentology, tectonics and evolution arise more from an inability to develop a clear stratigraphic picture of the belts than from ambiguities in interpretation. Unfortunately, early Precambrian greenstone terrains present particular problems to stratigraphic analysis. Ability to resolve the petrologic, sedimentological and structural histories of greenstone belts, however, hinges first on an ability to apply the concepts and procedures of classical stratigraphy. Knowledge of the character and evolution of the Earth's early crust is derived from the studies of the rocks and structures in Archean greenstone belts. The rock components and structures of Archean greenstone belts: An overview Large stratigraphy thicknesses are explained. The effects of contact with granitoids, geophysical properties, recumbent folds and late formation structures upon greenstones are examined. Also discussed are the continental environments of greenstone successions. The chemical composition of greenstones is described. Greenstone sucessions are defined as the nongranitoid component of granitoid- greenstone terrain and are linear to irregular in shape and where linear are termed belts. Greenstone belts: Their components and structure
As part of an on-going investigation aimed at answering these and other questions, the extremely well-exposed Cameron River Greenstone Belt and the Sleepy Dragon Metamorphic Complex in the vicinity of Webb Lake and Sleepy Dragon Lake was mapped. Where unconformities between gneisses and overlying sediments are indisputable, such as at Point Lake, the significance of faults which occur below the base of the volcanic succession also needs to be evaluated. What remains unclear in these examples is the significance of the so-called later faulting of the greenstone - gneiss contacts. A key question relating to the tectonics of greenstone belts is therefore the original spatial relationship between the volcanic assemblages and presumed-basement gneisses, and how this relationship has been modified by subsequent deformation. Other models require originally large separations between gneissic terranes and greenstone belts. Many tectonic models for the Slave Province, N.W.T., Canada, and for Archean granite - greenstone terranes in general, are implicitly dependent on the assumption that greenstone belt lithologies rest unconformably upon older gneissic basement. Is the Cameron River greenstone belt allochthonous?
0 kommentar(er)
