Deformation and recrystallisation mechanisms in naturally deformed cordierite |
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| Authors: | Herman L M van Roermund Robert J Konert |
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| Institution: | 1. Institute of Earth Sciences, State University Utrecht, Budapestlaan 4, P.O. Box 80021, NL-3508, TA Utrecht, The Netherlands
3. Institute of Earth Sciences, Free University Amsterdam, De Boelelaan 1085, P.O. Box 7161, NL-1007, MC Amsterdam, The Netherlands
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| Abstract: | Cordierite — (Mg,Fe)2Al4Si5O18 — occurs as porphyroclasts within metapelitic and metavolcanic rocks from the Kemiö-Orijärvi belt, SW Finland. After crystallisation the cordierites have been deformed at temperatures between 550–825° C and pressures of 3–5 kbar. Optical microscopy reveals the following deformation-induced microstructures: a bimodal size distribution between host, 0.3 to 4.0 mm, and recrystallised (new) grains, 0.1 to 0.5 mm; the intracrystalline defect-structures of host grains yield undulatory extinction, subgrains and some twinning. Recrystallised grains are optically strain free. Grain and subgrain boundaries are generally straight and parallel to crystallographic low-index planes. Orientation distribution diagrams for host and recrystallised grains yield similar fabric diagrams, i.e. 010] perpendicular to foliation -S-, 001] and 100] parallel to S and 001] parallel to lineation -L-. The fabric diagrams indicate that 001] (010) is the dominant slip system. Transmission electron microscopy reveals straight free dislocations, glide and climb loops, minor {130} and {110} microtwins, isolated nodal points and dislocation walls. Contrast analyses yield Burgers vector b = 001] being dominant and b = 100] subordinate. Climb loops consist of 〈c〉-dislocations that are dissociated in (001) planes, glide loops are defined by 100] 010] and 001] (100). The cordierite microstructures have been interpreted to be generated by dislocation creep. The dominant recrystallisation mechanism is thought to be subgrain rotation subsequently followed by minor grain or twin-band boundary migration. |
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