Evolution of the Archean crust: insights from the geochronology of the Lewisian of Scotland

Overview

The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane and the North Atlantic Craton. The oldest protoliths of the Lewisian Gneiss Complex may span the Paleo-, Meso- and Neo-Archean and as such, provide evidence for geological processes operating during early Earth history, e.g., plate tectonics.

By 3,100 million years ago, it is thought that the processes of plate tectonics had begun to operate, and so continents moved slowly across the surface of the globe, colliding with each other to form mountain ranges or splitting apart to produce new oceans. The surface of the Earth would have looked very different to today; the only life-forms were simple clusters of cells floating in the oceans, and there was little or no oxygen in the atmosphere. The Lewisian Gneiss Complex is composed of Archean gneisses which have been variably reworked during the Proterozoic and has proven to be a key region in the study of crustal growth and continental tectonics.

Although the Lewisian Gneiss Complex has been the subject of geological research for over a century, beginning with Peach et al. (1907), many aspects of its geological evolution remain controversial. Indeed, some of these outstanding controversies are likely a legacy of the long history of research in this area in that many isotope studies were under-taken when techniques were still in their infancy and have since either been improved upon or disregarded as obsolete (e.g.Giletti et al., 1961;Pidgeon and Aftalion, 1972). Among the key points still disputed are: (1) protolith ages of the gneisses, (2) age of granulite metamorphismin the Assynt block and (3) relative affinities of different fault bounded blocks.

The refractory nature of zircon means it often records and preserves relic age signatures, even when the original zircon has been subjected to burial deep in the Earth’s crust and has experienced extreme metamorphic conditions. Although such information is useful for re-constructing geological histories of complex terranes, age determinations on zircon displaying multi-age domains are not without their analytical challenges. Zircon from the Lewisian Gneiss Complex often display extremely complex zonation in CL images (e.g. Corfu et al., 1998; Goodenough et al., 2013) and it is difficult to further our understanding of evolution of the Lewisian Gneiss Complex, and some aspects of global Archean and Proterozoic tectonic processes, using conventional TIMS or micro-beam dating methods. A technique integrating some form of spatial resolution with the high analytical precision of TIMS may resolve outstanding issues for this enigmatic and classic area of geology.

Methodology

By dating the same zircons using (1) ion probe (µm’s), (2) laser ablations ICPMS (10’s µm’s) a Mattinson (2005) Chemical Abrasion TIMS approach with sequential dissolution will yield useful insights on the true protolith age and timing of metamorphism in these difficult rocks. This project will build on a recent study by Crowley et al. 2015. Once we have analysed zircon U-Pb using the microbam techniques, our aim is to exploit the ability of high-temperature annealing to reduce zircon susceptibility to decoupling of U and Pb from areas suffering varying degrees of alpha-recoil damage and then to use partial dissolution steps, rather han the more conventional single high temperature dissolution step, to map out different degrees of zircon discordance. Following the monitored progressive removal of the bulk of zircon affected by Pb-loss, subsequent dissolution steps will be employed to attempt analysis of structurally ± temporally differing zircon domains by exploiting small differences of high temperature acid solubility. As the resultant concordant data might reflect differing proportions of primary and metamorphic zircon, there is the potential for generating data arrays along concordia that could then be interpreted without the complication of superimposed Pb-loss.

Samples will be obtained from a series of extensive field campaigns conducted across the Outer Hebrides and mainland Scotland. Sample will be prepared using selective fragmentation (SELFRAG) at SUERC and characterised using electron microprobe, SEM and LA-ICPMS at St Andrews. High precision TIMS dating will be conducted at SUERC. Ion microprobe dating will be conducted at the NERC Ion Microprobe dating facility at Edinburgh University. During the project the student will spend 3 months working directly with Thermo Scientific (CASE Partners) in their Bremen factory to push the limits of thermal ionisation mass spectrometry.

Project Timeline

Year 1

– Literature review
– Hands on experience learning isotope dating techniques, including clean room approaches and mass spectrometry
– Fieldwork to sample Lewisian Gneiss
– Characterisation of collected materials (SEM, microprobe, geochemistry)

Year 2

– Ionmicroprobe U-Pb dating of zircon at the NERC Ion Microprobe Facility
– LA-ICPMS U-Pb geochronology
– Development of stepwise dating approach for high precision TIMS analyses
– Start TIMS dating of zircons, this will a significant analytical phase of the project that extends into Yr3
– Attendance at a national conference

Year 3

– Complete TIMS dating of zircons
– Data interpretation and development of publications
– Attendance at international conference
– Start to draft PhD thesis

Year 3.5

– Completion of Phd thesis and submission.

Training
& Skills

– Generic skills training through the University of Glasgow Graduate School and IAPETUS DTP
– In depth knowledge and expertise in mass spectrometry and clean room geochemistry
– Wide knowledge of isotope geochronology
– Intensive field training with supervisors in analysis of ancient geological terranes
– Opportunties to support lectures and hands on practical classes

References & further reading

https://reader.elsevier.com/reader/sd/pii/S1342937X14001014?token=5D709AE3EE3A7308BD761842B973C178D6DE576535FC9F3E4FF7E2E1FB4EE0C755C52E933F377AC83B3684C5FF8A971E&originRegion=eu-west-1&originCreation=20211114135525https://www.sciencedirect.com/science/article/pii/S0301926815000224https://link.springer.com/article/10.1007/s004100100283

Further Information

Contact darren.mark@glasgow.ac.uk for further information.

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