Strain geometry,microstructure and mineral chemistry in metagabbro shear zones: a study of softening mechanisms during progressive mylonitization |
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| Authors: | MJ Watts GD Williams |
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| Institution: | p/a Nam Assen, Schepersmaat 2, 9405 TA Assen, The Netherlands;Department of Geology, University College, P.O. Box 78, Cardiff CF1 1XL, U.K. |
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| Abstract: | The strain geometry, microstructure and metamorphism is described from two minor shear zones from the Chatelaudren metagabbro, N. Brittany. A serially slabbed shear zone reveals a strain geometry consistent with simple shear deformation. Strain calculations based on X trajectory angles coincide with those obtained from elliptical mineral clusters at high values of strain only. Strain profiles typically show a broad low-strain region with a narrow high-strain peak at the centre of the zones. Microstructures typically show distinct grain size reduction in both amphibole and feldspar towards the high strain region of shear zones, and this is discussed in terms of deformation mechanisms related to strain softening. A palaeostress estimate based on recrystallized feldspar grain sizes gives a differential stress of 32 MPa for the low strain region and 119 MPa in the shear zone centre. Electron probe analyses reveal chemical and mineralogical changes accompanying metamorphism within the shear zones. This suggests local conditions favourable for ionic diffusion and the activity of fluids is implied. |
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