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U-Pb apatite geochronology is increasingly recognised as a valuable tool for constraining the age of mid-crustal ductile shear zones. The crustal-scale Outer Hebrides Fault Zone (OHFZ) within the Laurentian foreland of the Scottish Caledonides has long been of uncertain age and tectonic significance. Earliest deformation within the OHFZ was associated with top-to-the-NW ductile thrusting that formed a belt of greenschist facies mylonites within host Archaean-Palaeoproterozoic basement gneisses. Previous estimates for the timing of thrusting vary between c. 1600 Ma and c. 430 Ma. The mylonitic fabrics are defined by a recrystallised assemblage of quartz + albite / oligoclase + sericite + actinolite + epidote + apatite ± calcite, consistent with deformation temperatures of 400-500 °C and within the range of reported closure temperatures for Pb diffusion in apatite. U-Pb (LA-ICP-MS) dating of two texturally distinct apatite grain types within the mylonites has yielded ages mostly in the range c. 1100 – 900 Ma. The OHFZ is therefore interpreted as a Grenville-Sveconorwegian structure that formed during the tripartite collision of Laurentia, Baltica, and Amazonia and the assembly of Rodinia. Supplementary material: https://doi.org/10.6084/m9.figshare.c.7084925
U-Pb apatite geochronology is increasingly recognised as a valuable tool for constraining the age of mid-crustal ductile shear zones. The crustal-scale Outer Hebrides Fault Zone (OHFZ) within the Laurentian foreland of the Scottish Caledonides has long been of uncertain age and tectonic significance. Earliest deformation within the OHFZ was associated with top-to-the-NW ductile thrusting that formed a belt of greenschist facies mylonites within host Archaean-Palaeoproterozoic basement gneisses. Previous estimates for the timing of thrusting vary between c. 1600 Ma and c. 430 Ma. The mylonitic fabrics are defined by a recrystallised assemblage of quartz + albite / oligoclase + sericite + actinolite + epidote + apatite ± calcite, consistent with deformation temperatures of 400-500 °C and within the range of reported closure temperatures for Pb diffusion in apatite. U-Pb (LA-ICP-MS) dating of two texturally distinct apatite grain types within the mylonites has yielded ages mostly in the range c. 1100 – 900 Ma. The OHFZ is therefore interpreted as a Grenville-Sveconorwegian structure that formed during the tripartite collision of Laurentia, Baltica, and Amazonia and the assembly of Rodinia. Supplementary material: https://doi.org/10.6084/m9.figshare.c.7084925
The Neoarchaean foreland basement of the Faroe-Shetland terrane (FST) displays abundant evidence for isotopic resetting of U-Pb systems in apatite between c. 1800 and 200 Ma, interpreted to result from episodic heating pulses associated with regional scale tectonic events. Major apparent age peaks of c. 1800-1600 Ma broadly correspond to the timing of Nagssugtoqidian-Laxfordian orogenesis >225 km further south. These are thought to reflect widespread heating during late- to post-orogenic delamination that affected a wide area of the orogenic foreland and resulted in a low-middle greenschist facies static overprint, affecting much of the FST basement. Late- to post-orogenic delamination might also account for major apparent age peaks at c. 1300-1100, 800, and 500-400 Ma, corresponding to, respectively, Grenvillian, Knoydartian and Caledonian orogenic events. However, east-dipping seismic reflectors in the basement west of Shetland may represent the northward extension of the Grenvillian Outer Hebrides Thrust (Zone) and/or Caledonian thrusts and so perturbation of isotherms during west-directed thrusting could therefore also account for these apparent age peaks. Minor apparent age peaks of c. 700, 600, 350 and 200 Ma are most easily interpreted as resulting from enhanced heat flux that accompanied periods of crustal extension prior to and following the Caledonian orogeny.
Graphite occurs in Neoproterozoic (probable Loch Ness Supergroup) marbles of The Aird, in the Northern Highland Terrane, Scotland. The graphite occurs particularly in association with phlogopite mica, and also with other micas and Mg-chlorite. Although the graphite-phlogopite association is recorded widely elsewhere in mantle-derived rocks, our data suggests graphite at The Aird does not have a mantle origin. The carbon isotopic composition of the graphite ( δ 13 C, 0.4 to -1.6 ‰) indicates graphitisation occurred from a CO 2 -rich fluid associated with decarbonation or devolatilisation reactions of a carbonate-silicate protolith. Graphite-phlogopite bearing marbles in the Aird underwent extensive brecciation and haematite deposition that preceded carbon-rich, mantle-derived (carbonatite) fluids. Pyrite in veins within The Aird marble has a sulphur isotope composition depleted in 34 S (-15.5 to -16.6 ‰), suggesting a biogenic origin. Elsewhere in The Aird and in surrounding fenitised rocks 34 S -enriched pyrite has sulphur isotope compositions between 7.7 to 6.1 ‰, outside the sulphur isotopic composition range of most carbonatite-hosted pyrite, suggesting pyrite veining was likely influenced by crustal fluid-rock interactions. The observations show that if the protolith has a carbonate-silicate composition, a graphite-phlogopite association can form without the need for mantle-derived fluids. Thematic collection: This article is part of the Early Career Research collection available at: https://www.lyellcollection.org/topic/collections/early-career-research
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