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Microstructural Stability and the Kinetics of Textural Evolution Microstructural Stability and The Kinetics of Textural Evolution Andrew Lind Submitted in accordance with the requirements for the degree of Doctor of Philosophy. University of Leeds Department of Earth Sciences May, 1996 The candidate confirms that the work submitted is his own and the appropriate credit has been given where reference has been made to the work of others. Abstract This thesis is concerned with constraining the processes by which a rock texture may be altered in the geological history after a rock has been deformed and the rates at which that microstructural modification may occur. The area which has been chosen to investigate microstructural change is the Ballachulish aureole in the NW Highlands of Scotland, using samples from the Appin Quartzite. Rocks in this region were deformed during the Caledonian orogeny. Post-dating the deformation was the intrusion of the Ballachulish granite. As a consequence of the thermal perturbation associated with the granite intrusion, deformation microstructures were gradually altered. This process is known as annealing. The Ballachulish aureole is essentially a ‘natural laboratory’ which preserves an annealing experiment over geological time; rocks are gradually annealed along the traverse towards the intrusive complex where higher temperatures were attained. Rocks have been sampled along a traverse from outside the igneous aureole towards the contact. A variety of microstructural elements have been characterised using image analysis and crystallographic techniques, some of which have been developed during this study. The changes in rock fabrics along the traverse are used as a basis for developing models of annealing for many microstructural features, including dislocations, subgrains, subgrain walls and grains. The relative stability of each of these microstructures is assessed to show that dislocations are the least stable, then subgrains and grains. An interesting ‘mesh’ microstructure is identified, preserving evidence of fracturing on a grain scale. These mesh-grains are very stable, surviving annealing in some cases altogether. The presence of impurities on all scales of observation are shown to greatly influence the mechanisms of microstructural change. The kinetics of microstructural modification are examined using theoretical models based on the processes of annealing which have been identified during this investigation. Again these indicate that impurities can have marked implications on the rates of microstructural change. Kinetic models for rocks undergoing regional prograde and retrograde metamorphosis are developed to indicate how this work may be applied on a more regional tectonic scale. Acknowledgements This project is the brainchild of Rob Knipe, Geoff Lloyd and Andy Bamicoat, all three of them are thanked for obtaining the grant, giving me the opportunity to carry out the research and their distinctive styles of supervision during my tenure of the studentship. NERC are acknowledged for their lavish funding of the project Special thanks for intellectual and technical support go to: Dave Prior, Graeme Potts and John Wheeler for their enthusiasm and good humour during my time at Liverpool; Andy Farmer, for some strange reason (only known to himself) has spent hours, if not days guiding me through the rigours of programming, microstructural geology and of late writing-up - cheers very much for putting yourself through it (I’m blaming you if it all goes wrong). Mark Jessell, Neil-Henrick Schmidt and Dave Mainprice are thanked for managing to show some interest in the work. Closer to home thanks to members of the struccy group some of whom have been a source of inner strength when things are going wrong (“It’s never too late......”) - Ned, Jock, Napalm, Slime, Neil, Edd, Lochlann, Will (you’re sorely missed), Wibbs, Yen, Louise, Dani, Rob, Helena, Shona, MOOOSE and Steve - no apologies to anybody for my bad temper at the computer. The Keiths did a grand job on the thin section front. Robert Marshall is thanked for his patience at the polisher. Eric Condliffe and Tony Nichols help in electron optics is appreciated. The Beer Police would like to offer long service medals to Ned, Napalm and Chopper and Sergeant Girth would like to thank Jock for his top tips (and his introduction to t*t cakes). Cheers to the “we’re only going for a gallon” crew - those I’ve already mentioned above know who they are, together with Klaus, Dave, Nigel, Chopper Sue and Clare. Other socialites include: G-Tech (all of them), Tim, uz&Edd, Kath, Alex H., Quebtin Fisher, Tim Buddin, Ken Baxter, A1 (Fat Knacker) Boulton, Slimbo, Karen B, Kathy, Natasha and Anne. Gold stars will be issued to Andy F, Sue, Ned, Napalm, Neil, Nigel, Wibbs and Clare; Andy and Sue for Otley trips, meals, hols etc.; Ned, Napalm and Neil for destroying my health; Nige, Wibbs (even if your socks are bleached round the toes) and Clare for being good for a laugh . The family have all been superstars throughout, cheers for your continued support (and money) - Mum, Michael, Stephen and Granddad etc.. In return you now receive a brand new door stop in the form of a thesis. Table of Contents Title Abstract........................................................................................................... i Acknowledgements........................................................................................ii Table of Contents.........................................................................................iv Table of Figures............................................................................................ xi CHAPTER 1 INTRODUCTION 1.1 The Geological Setting............................................................................ 2 1.2 Aims Of Thesis....................................................................................... 3 1.3 Thesis Layout...........................................................................................4 CHAPTER 2 DEFORMATION MECHANISMS AND MICROSTRUCTURAL MODIFICATION 2.1 Introduction............................................................................................ 6 2.1.1 Non-Crystal Plastic Mechanisms....................................................................................7 2.1.1.a Diffusive Mass Transfer.............................................................................................................7 2.1.1.b Cataclasis...................................................................................................................................7 2.1.2 Crystal Plasticity............................................................................................................... 8 2.2 Rock Rheologies 11 2.3 Processes Affecting Rock Rheologies...................................................12 2.3.1 Hardening......................................................................................................................... 12 2.3.2 Softening............................................................................................................................13 2.3.2.a Recovery by Dislocation Climb................................................................................................ 13 2.3.2.b Recovery by Cross Slip................................. ........................................................................... 13 2.3.2.C Annihilation...............................................................................................................................14 2.3.2.d Polygonization........................................................................................................................... 14 2.3.3 Cyclicity of Hardening/Softening................................................................................. 14 2.3.4 Dynamic Recrystallisation.............................................................................................. 15 2.3.4.a Subgrain Rotation......................................................................................................................15 2.3.4.b Grain Boundary Migration........................................................................................................16 2.3.5 Interaction of Deformation Processes......................................................................... 17 2.4 Static Recovery......................................................................................18 2.4.1 Static Recovery by Grain Growth................................................................................19 2.4.1 .a Primary Recrystallisation..........................................................................................................19 2.4.1.b Normal Grain Growth...............................................................................................................20 2.4.2 Kinetics of Normal Grain Growth................................................................................22 2.4.3 Abnormal Grain Growth................................................................................................24 2.5 Geological Implications of Grain Growth.......................................... 24 2.6 Criteria To Recognise Annealing.........................................................26 2.6.1 Microscopic Observations............................................................................................ 26 2.6.2 Image Analysis Criteria.................................................................................................. 27 2.6.3 Crystallographic Criteria................................................................................................27
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