Brittle-plastic deformation in initially dry rocks at fluid-present conditions: transient behaviour of feldspar at mid-crustal levels |
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Authors: | Linus Brander Henrik Svahnberg Sandra Piazolo |
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Institution: | (1) Department of Earth Sciences, University of Gothenburg, Box 460, 405 30 Gothenburg, Sweden;(2) Department of Geological Sciences, Stockholm University, Stockholm, Sweden;(3) Department of Earth and Planetary Sciences, GEMOC National ARC Key Centre, Macquarie University, Sydney, NSW, Australia |
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Abstract: | We present detailed microstructural and chemical analyses from an initially dry anorthositic rock deformed during wet amphibolite
facies conditions. Three different domains representing the microstructural variation of the deformed samples are investigated
in detail in terms of fracture morphology and mode, grain characteristics and chemistry of present phases. Results show transient
deformational behaviour where a close interaction between brittle, plastic and fluid-assisted deformation mechanisms can be
observed. Our analysis allows us to describe the succession, interrelationships and effects of active mechanisms with progressively
increasing strain in three so-called stages. In Stage 1, initial fracturing along cleavage planes promoted fluid influx that caused fragmentation and chemical reactions, producing
fine-grained mineral assemblages in the fractures. Deformation twins and dislocations developed in clast pieces due to stress
relaxation. Passive rotation of conjugate fracture sets and interconnection of intracrystalline fractures formed micro-shear-zones,
constituting Stage 2. Microstructures and grain relationships indicate the activity and fluctuation between fracturing, dissolution-precipitation
creep, grain boundary sliding and locally dislocation creep, reflecting the transient behaviour of brittle and plastic deformation
mechanisms. Further rotation and widening of fractures into overall foliation parallel shear-bands (Stage 3) promoted strain partitioning into these areas through increased fluid influx, influence of fluid-assisted grain boundary
sliding, phase mixing and presence of weak phases such as white mica. We suggest that local differences in fluid availability,
volume fraction of weak phases produced by fluid present metamorphic reactions coupled with volume increase and local variations
in stress concentration induced transient brittle-plastic behaviour. The studied shear-zone represents an example of the transformation
of a rigid dry rock to a soft wet rock during deformation through syntectonic fracturing. |
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