Master Thesis Degree Project in Geology 60 hp A geochemical study of diamictite matrix in the Port Askaig Tillite Formation in western Argyll, Scotland Martin Dahlgren Stockholm 2020 Department of Geological Sciences Stockholm University SE-106 91 Stockholm Sweden May 2020 Master thesis Martin Dahlgren __________________________________________________________________________________ A geochemical study of diamictite matrix in the Port Askaig Tillite Formation in western Argyll, Scotland Abstract This study focuses on the diamictite matrix in the Port Askaig Tillite Formation which crops out in the Garvellachs and on Islay and consists of 47 glaciogenic diamictite beds interlayered with non-glacial rocks. Measurements using handheld XRF and rock sampling for further lab-based fusion-ICP and petrographic analysis were done in 134 stations. Geochemical logs were created containing three correlated high-resolution datasets with a total of 100 ICP, 736 handheld XRF and 101 thin sections entries, altogether 24 thousand datapoints. The data was analysed using statistical and analytical methods i.a. including variables correlation, linear regression, multivariate (PCA) analysis, Fourier frequency analysis and Mann-Whitney and Kruskal-Wallis rank-tests. Stratigraphic trend-plots, bivariate / ternary diagrams and box-and-whisker plots were produced showing variation patterns within the PAF. The major-, trace- and rare earth elements were analysed and geochemical proxies representing weathering and other environmental changes as well as provenance were analysed resulting in i.a. the following detailed interpretations: - The siliciclastic portion of the diamictite matrix has a granodioritic provenance originating from a continental volcanic arcs setting. - The diamictites have a similar composition to the Svecofennian bedrock and Swedish till derived therefrom but have a different composition than the Rhinns complex, which is thus not a source for the PAF material. - The limestone source for the diamictite matrix was abruptly switched-off directly after the Great Breccia. - The analysis supports Spencer’s (1971) original grouping of diamictite beds into members, but also open the possibility to further divide Member 1 into two sub-groups – M1a to cover the diamictite beds up to and including the Great Breccia and M1b starting from the Disrupted Beds up to and including the Upper Dolomites. - Member 5 clearly deviates from the other members in its geochemical footprint, implying a shift of provenance or different transport and sorting processes for the M5 material, which raise the question if this member really represents a true glacial deposit. Some general interpretations were also suggested: - The source of the extrabasinal part of the PAF diamictites is the Svecofennian domain in the Baltica crustal plate located palaeogeographically to the southeast of the PAF. - The PAF was deposited during 5 glacial cycles starting with gradual build-up of ice sheets followed by rapid meltdowns, and where the first cycle was unique in the sense of containing the multimillion years hiatus of the Snowball Earth event. The Great Breccia is interpreted as the final step in this first glacial build up. The exit from the Snowball Earth icehouse is interpreted to be through a series of possibly 5 major melting events of short durations, intermixed with somewhat longer periods when the ice managed to recover control. __________________________________________________________________________________ 1 May 2020 Master thesis Martin Dahlgren __________________________________________________________________________________ Table of contents Table of contents 1 Introduction and geological setting ………………………………………………………………… 6 1.1 Short description and aim of the study …………………………………………. 6 1.2 The Cryogenian Period …………………………………………………………………… 6 1.2.1 The Cryogenian ice ages …………………………………………... 6 1.2.2 The Snowball Earth hypothesis ………………………………… 7 1.2.3 Predictions and contradictory observations ………..…... 8 1.3 Geological setting …………………………………………………………………………… 9 1.3.1 The Dalradian Supergroup ……………………………………….. 9 1.3.2 The Port Askaig Tillite Formation (PAF) stratigraphy …. 10 1.3.3 The PAF tectonics, age and palaeogeography ……………. 11 1.3.4 Glaciogenic sedimentary regimes ……………………………… 12 1.3.5 Palaeoenvironment ………………………………………………….. 12 1.4 Review of proxies for climatic and environmental change ………………. 13 1.4.1 Chemical and physical weathering proxies ……………….. 13 1.4.2 Other proxies and methods ………………………………………. 14 2 Methodology …………………………………………………………………………………………………… 15 2.1 The methodological strategy …………………………………………………………… 15 2.2 Description of field measurements ………………………………………………….. 16 2.2.1 Sampling localities in the Garvellachs and Islay …………. 16 2.2.2 Stratigraphic level of samples ……………………………………. 21 2.2.3 Sampling and field measurement methodology ………… 24 2.3 Description of lab analysis ………………………………………………………………. 24 2.3.1 Sample preparation and ICP chemical elements analysis 24 2.3.2 Thin sections microscopy ………………………………………….. 25 2.4 Data handling …………………………………………………………………………………. 27 2.4.1 Preparation and presentation of data ……………………….. 27 2.4.2 Statistical analyses applied ……………………………………….. 28 2.4.3 Normalization of data ……………………………………………….. 28 2.4.4 Data precision and reliability …………………………………….. 28 2.4.5 Correlation between datasets ……………………………………. 31 3 Results …………………………………………………………………………………………………………….. 32 3.1 Major elements geochemistry …………………………………………………………. 32 3.2 Rare earth elements geochemistry ………………………………………………….. 49 3.3 Trace elements geochemistry ………………………………………………………….. 56 3.4 Sandstone beds geochemistry …………………………………………………………. 68 3.5 Lateral and transverse trends within diamictite beds ……::……………….. 72 3.6 Weathering indexes ………………………………………………………………………... 73 3.6.1 CIA ……………………………………………………………………………. 73 3.6.2 Other weathering indexes …………………………………………. 75 3.7 Other palaeoclimate proxies ……………………………………………………………. 77 3.8 Provenance proxies …………………………………………………………………………. 78 4 Discussion ……………………………………………………………………………………………………….. 80 4.1 Early authigenesis and later post-depositional change …………………….. 80 4.1.1 Present composition of main and accessory minerals … 81 __________________________________________________________________________________ 2 May 2020 Master thesis Martin Dahlgren __________________________________________________________________________________ 4.1.2 Diagenesis ………………………………………………………………… 84 4.1.3 Metamorphism ………………………………………………………… 85 4.1.4 Pre-depositional alterations …………………………………….. 86 4.2 Provenance of the PAF sediments and clasts ………..………………………... 88 4.3 The PAF Members – how to interpret them ……………………………………. 95 4.4 Depositional and palaeoenvironmental interpretation of the PAF … 105 5 Conclusions ……………………………………………………………………………………………………… 111 6 Acknowledgements …………………………………………………………………………………………. 112 7 Recommendations for future studies ………………………………………………………………. 112 8 Appendix with supplementary data …………………………………………………………………. 112 9 References ………………………………………………………………………………………………………. 113 __________________________________________________________________________________ 3 May 2020 Master thesis Martin Dahlgren __________________________________________________________________________________ List of Figures Figure Chapter Page 1. Simplified Snowball Earth energy balance model …........................................................................................................................................1.2.2 8 2. Garvellachs site measurement stations, June 2019 …...............................................................................................................................2.2.1 19 3. North Islay site measurement stations, October 2019 …...................................................................................................................... 2.2.1 20 4. South Islay site measurement stations, October 2019 …........................................................................................................................ 2.2.1 21 5. ICP samples and corresponding thin sections stratigraphic positions within the PAF …........................................................................ 2.2.2 22 6. XRF measurements stratigraphic positions within the PAF …............................................................................................................... 2.2.2 23 7. Microscope photos showing microcline …............................................................................................................................................. 2.3.2 25 8. Microscope photos showing recrystallization resulting from metasomatic fluids …........................................................................... 2.3.2 26 9. Microscope photos showing crystal foliation pattern …....................................................................................................................... 2.3.2 26 10. Microscope photos showing pyrite surrounded by hematite with intergrown chalcopyrite …........................................................ 2.3.2 26 11. Box-and-whisker plot of major elements showing the interquartile range, median, “whiskers” and outliers …................................2.4.1 27 12. UCC normalized spider diagram covering major elements from the PAF diamictite matrix ICP dataset …...................................... 3.1 32 13. UCC normalized spider diagram covering major elements on a carbonate free basis …..................................................................