Quartz ribbons in high grade granite gneiss: modifications of dynamically formed quartz c-axis preferred orientation by oriented grain growth |
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| Authors: | NG Culshaw WK Fyson |
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| Institution: | Department of Geology, University of Ottawa Ottawa, K1N 6N5, Canada |
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| Abstract: | Quartz ribbons form a well-defined LS fabric in granitic gneisses sheared and metamorphosed in the upper-amphibolite facies. Boundaries of quartz grains are smooth and sub-perpendicular to ribbon walls, suggesting post-deformational growth, whereas c-axes display asymmetric girdle patterns consistent with the sense of shear indicated by mesoscopic fabrics. Subdivision of the microfabrics according to the proportion of ribbon length occupied by individual grains indicates that the c-axes of relatively short grains exhibit across-girdle pattern similar to that of the whole sample. Longer grains reproduce elements of the pattern of the short grains, suggesting that oriented grain growth occurred. Grain boundary mobility, which clearly ended in a post-deformational static regime, probably began during dynamic recrystallization. Owing to space limitations imposed by the feldspathic matrix, grain growth in the ribbons ceased before the initial dynamic fabric was erased. In summary, this study shows that the c-axes pattern of longer grains within quartz ribbons reproduces part of the dynamically formed pattern of the shorter grains, reflecting an oriented grain growth which concluded in a static episode, although possibly initiated in the dynamic regime. |
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