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The Effects of Overprinting on Isotopic Ages and Rock Fabrics: The Lewisian Gneiss Complex as a Case Study Thesis submitted in accordance with the requirements of the University of Liverpool for the degree of Doctor in Philosophy by John Murdoch MacDonald September 2012 2 Abstract Zircon is a key mineral in understanding tectonothermal overprinting in metagranitoid rocks of the continental crust, principally through radiometric dating. Exposed examples of such rocks are often Precambrian in age and have undergone multiple tectonothermal events. This thesis aims to understand the overprinting effects of deformation and metamorphism in multiple tectonothermal events on zircon, using the Precambrian Lewisian Gneiss Complex (LGC) of Northwest Scotland as a case study. Deformation of zircon at the grain-scale, in the form of distortion of the crystal lattice, was found to heterogeneously affect Ti, Rare Earth Elements (REEs) and U-Pb systematics. Ti and REE abundances increased or decreased as dislocation planes caused by distortion enabled diffusion of elements in or out of the crystal lattice. Pb isotopes were also lost, resulting in discordant young ages relative to undistorted zircon. Zircons with lattice distortion may therefore record information about tectonothermal events not recorded in undistorted zircon. Investigating the effects of multiple metamorphic episodes on zircons from the LGC revealed that different metamorphic events could not be reliably distinguished due to the occurrence of volume diffusion of Pb. A ~600Ma concordant age spread was interpreted to have been caused by volume diffusion; all other possible explanations were eliminated and age patterns from individual zircon crystals supported this hypothesis. Volume diffusion would have required ~3-500Myr at temperatures of 875-975°C. As well as these more generic findings about the effects of overprinting on zircon, zircon U-Th-Pb, REE and Ti data were analysed to investigate meta-igneous gneisses and metasediments of the Assynt Terrane, a key part of the LGC. The oldest cores yield a mean age of 2958±7Ma, a possible magmatic protolith crystallisation age but volume diffusion of Pb in zircon may have affected these zircon core ages. The period of volume diffusion of Pb in zircon is interpreted to have ended at ~2482Ma with the peak of granulite-facies metamorphism shortly before this. Zircons in the metasediments have relatively flat chondrite-normalised heavy REE profiles (low Yb/Gd ratios) which suggest they equilibrated with granulite-facies metamorphic garnet. Ti-in-zircon thermometry records average crystallisation temperatures of 790°C for zircons from the magmatic protolith to the meta- igneous gneisses and 823°C for zircons from the metasediment. The zircons in the metasediments are interpreted to be detrital and the calculated temperatures are interpreted to record zircon crystallisation in a currently unknown protolith. 3 4 Acknowledgements Firstly thanks to my many supervisors: Simon Harley, University of Edinburgh for help and advice with the ion microprobe and various geochemical and geochronological discussion; Quentin Crowley, Trinity College, Dublin for lots of discussion on geochronological data; Kathryn Goodenough, British Geological Survey for help on the field side as well as her ability to tease out the main points from my confused writing!; Elisabetta Mariani, University of Liverpool for microstructural advice and latterly to Dan Tatham who stepped in; and of course to John Wheeler, University of Liverpool who was the main man and whose desire to understand the Lewisian was the starting point for this project. Thanks also to Eddie, Carmel, Dave P and Dan T for help with those troublesome SEMs while Mark, Dave M and many others helped me to settle in and to get me going with the work. It’s not just been work though. I’ve played sport with lots of people in Liverpool: Willem and Chris for tennis; Andy, Katrien and Ludo for badminton; Ceri’s Soccer School, Armo, Oshaine and all the other football lads; and latterly Paddy and Rhodri for squash. Thanks to all those who came hillwalking: Rhodri, Goz, Jutley, Paddy, Gemma, Megan & James, Kirstie and Tash. Liverpool also saw a pick-up in my bagpiping and may thanks to all those in the Wirral Pipe Band who I had a great time with, whether competing, doing a gig or just at Wednesday practice. A big thanks too to all those in the department who were around for tea break (Kirstie), Boswell lunch and pub on a Friday (and other days!). Particularly to Tash, Paddy, Gem & Gem, Vic and Jutley, the original RCA. And to my parents whose love of the outdoors got me into geology in the first place, without which this work would never have been done. 5 6 Contents List of Figures ........................................................................................................................... 9 List of Tables .......................................................................................................................... 15 1. Introduction ................................................................................................................... 19 Definitions and Importance ............................................................................................... 19 The Problem ....................................................................................................................... 20 The Aims ............................................................................................................................. 21 The Location ....................................................................................................................... 22 Thesis Structure ................................................................................................................. 23 Manuscript Status .............................................................................................................. 25 2. Chapter 2 – Reviews ....................................................................................................... 29 2a: Zircon ........................................................................................................................... 29 What is Zircon and Why is it Useful? ............................................................................. 29 Formation of Zircon ....................................................................................................... 29 Fig. 2.1 Zircon crystallographic structure. ..................................................................... 30 What Geological Information Can Be Extracted From Trace Elements in Zircon? ........ 34 2b: The Lewisian Gneiss Complex ................................................................................... 43 Early Work .................................................................................................................... 43 Geological Summary ..................................................................................................... 44 Structure ....................................................................................................................... 50 Geochemistry and Metamorphism .............................................................................. 54 Geochronology .............................................................................................................. 64 3. Methodology ................................................................................................................. 81 Mapping............................................................................................................................. 81 Sampling ............................................................................................................................ 82 Sample Preparation .......................................................................................................... 82 Petrography ...................................................................................................................... 83 Mechanical zircon separation .......................................................................................... 83 Scanning Electron Microscope Analysis ......................................................................... 84 BSE Imaging .................................................................................................................. 85 Cathodoluminescence Imaging ................................................................................... 85 Energy Dispersive X-Ray Spectroscopy ...................................................................... 86 EBSD .............................................................................................................................. 87 X-Ray Fluorescence Analysis ........................................................................................... 88 7 EMPA.................................................................................................................................. 90 Ion Microprobe Analysis .................................................................................................. 93 4. Sample Characterisation .............................................................................................. 101 4a: Field Relationships and Sample Petrography ............................................................. 101 Badcall Point ................................................................................................................
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