The effects of overprinting on isotopic ages and rock fabrics : the Lewisian Gneiss Complex as a case study

• Main Author: MacDonald, John
• Other Authors: Wheeler, John
• Published: University of Liverpool 2012
• Subjects: 549; Q Science (General)
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.579285

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.