Low-temperature mica schists of Anglesey, U.K: cold subduction on the margin of Avalonia Thesis submitted in accordance with the requirements of the University of Adelaide for an Honours Degree in Geology . Larissa Joy Collins November 2017 Larissa Joy Collins Cold subduction on the margin of Avalonia LOW-TEMPERATURE MICA SCHISTS OF ANGLESEY, UK: COLD SUBDUCTION ON THE MARGIN OF AVALONIA RUNNING TITLE: COLD SUBDUCTION ON THE MARGIN OF AVALONIA ABSTRACT Anglesey in North Wales is considered to contain the oldest exposures of lawsonite- glaucophane blueschist in the world, marking the first appearance of lawsonite in the geological record, and heralding the emergence of truly modern subduction thermal regimes. The blueschists formed in the late Neoproterozoic during subduction beneath Avalonia. Interlayered within the blueschist unit are rare lenses of garnet-bearing metapelite that form part of a lithological association with more voluminous garnet-free metapelites. Detrital U-Pb zircon geochronology of the metapelites indicates that deposition of the protolith occurred ~630-590 Ma ago. The dominant detrital zircon ages correspond to the age of arc magmatism along the inferred margin of Avalonia, suggesting that the bulk of the detritus was derived from erosion of the arc. The presence of less abundant older zircons that range in age up to 2Ga, suggest that the arc was built on an ancient continental margin. This suggestion is supported by Nd isotopic compositions of the metapelites, which indicate derivation from an evolved source. The rare garnet metapelites contain the metamorphic assemblage garnet-muscovite- chlorite-albite-quartz-titanite-rutile-pyrite in which coronas of rutile surround titanite. Phase equilibria forward modelling of the metapelites indicates a prograde burial path that culminated with conditions around 400-450°C, at pressures of 10-12kbar. These conditions give an average thermal gradient of around 40°C/kbar, which is comparatively warm for lawsonite-bearing rocks. This suggests the Anglesey i Larissa Joy Collins Cold subduction on the margin of Avalonia lawsonite-bearing blueschists record the onset of global subduction thermal regimes in the late Neoproterozoic that can stabilise lawsonite, rather than simply being a fortuitous preservation of widely developed refrigerated metamorphic rock systems. KEYWORDS Anglesey, lawsonite, blueschist facies metapelite, geochronology, phase equilibria forward modelling ii Larissa Joy Collins Cold subduction on the margin of Avalonia TABLE OF CONTENTS Running title: Cold subduction on the margin of Avalonia ............................................... i Abstract .............................................................................................................................. i Keywords .......................................................................................................................... ii List of figures and tables ................................................................................................... 3 Introduction ....................................................................................................................... 3 Geological setting and background ............................................................................... 7 Analytical methods ......................................................................................................... 12 Bulk rock geochemistry .............................................................................................. 12 Zircon U-Pb geochronology ....................................................................................... 12 U-Pb LA-ICP-MS ....................................................................................................... 13 Garnet !Nd isotopes .................................................................................................... 13 LA-ICP-MS maps of garnet ........................................................................................ 14 Electron probe microanalyses methods ...................................................................... 14 Phase equilibria forward modelling ............................................................................ 14 Pressure-temperature pseudosections ......................................................................... 14 Phase abundance contours .......................................................................................... 15 Results ............................................................................................................................. 16 Samples ....................................................................................................................... 16 Petrography ................................................................................................................. 16 Zircon U/Pb geochronology ........................................................................................ 19 εNd evolution .............................................................................................................. 28 Mineral chemistry ....................................................................................................... 29 Phase equilibria forward models ................................................................................. 34 Phase abundance contours .......................................................................................... 36 Discussion ....................................................................................................................... 39 Age of Penmynydd schists .......................................................................................... 39 Avalonian subduction and what this means for the emergence of cold pelites: ......... 45 Conclusions ..................................................................................................................... 47 Acknowledgments ........................................................................................................... 48 References: ...................................................................................................................... 49 Appendix A: U-Pb Geochronological Data .................................................................... 53 Appendix B: Representative electron probe microanalyses ........................................... 73 1 Larissa Joy Collins Cold subduction on the margin of Avalonia Appendix C: Extended laser trace element data ............................................................. 74 Appendix D: Whole rock geochemistry results .............................................................. 76 Appendix E: Geochemical plots ..................................................................................... 77 2 Larissa Joy Collins Cold subduction on the margin of Avalonia LIST OF FIGURES AND TABLES Figure 1: Location map of Anglesey, Wales. ................................................................... 6 Figure 2: Photomicrographs of thin sections ANG13b and ANG15a ............................. 18 Figure 3: U-Pb analyses of ANG8. .................................................................................. 20 Figure 4: U-Pb analyses of ANG12a <79"m. ................................................................ 21 Figure 5: U-Pb analyses of ANG12a >79"m .................................................................. 22 Figure 6: U/Pb analyese of ANG12b <79µm .................................................................. 23 Figure 7: U/Pb analyese of ANG12b >79µm .................................................................. 24 Figure 8: U-Pb analyses of ANG17 ................................................................................. 25 Figure 9: U-Pb analyses of ANG18 ................................................................................. 26 Figure 10: U-Pb analyses of ANG20. .............................................................................. 27 Figure 11: εNd isotope values of whole rock powders of ANG13a, 13b, 15a, &16 ..... 28 Figure 12: LA-ICP-MS garnet trace element maps of ANG13b 32 Figure 13: LA-ICP-MS maps of garnet trace elements from sample ANG15a. ............. 33 Figure 14: P-T Pseudosection of ANG13b ...................................................................... 35 Figure 15: P-T Pseudosection of ANG15a ...................................................................... 36 Figure 16: TCI outputs for ANG13b P-T. ....................................................................... 37 Figure 17: TCI outputs for ANG15a P-T ....................................................................... 38 Figure 18: The emergence of lawsonite blueschists during the period of deposition of the protolith. .................................................................................................................... 39 Figure 19: Detrital U-Pb zircon spectra of samples from this study (purple) and samples from Asanuma et al., (2017) ........................................................................................... 41 Figure 20: Map of Anglesey showing lithology and location of the Asanuma et al., (2017) samples with reference to the samples from this study (purple labels). .............. 42 Table 1: Summary of previous geochronological work across Anglesey Island and the neighbouring Lleyn Peninsula. ....................................................................................... 11 Table 2: Sample descriptions