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Element mobility and low-grade metamorphism of mudrocks in British Caledonian Basins

By R.J. Merriman, N. Breward, P. Stone, K. Green and S. Kemp


Changes in the abundance of ten major and trace elements in mudrock lithologies have been studied in relation to progressive low-grade metamorphism. Analysed mudrock samples represent metapelitic grades ranging from deep diagenetic to epizonal (low greenschist) found in three Lower Palaeozoic basins within the British Caledonides. Mudrocks in the Scottish Southern Uplands were deposited in a trench and deformed in an accretionary complex, and those from the southern Lake District were deposited in a foreland basin; in both basins early burial and metamorphism was characterized by low heat-flow. In contrast, mudrocks from the northern Lake District accumulated in an extensional basin setting characterised by early high heat-flow. When geochemical data from whole-rock and <2μm clay fractions are plotted against the Kübler index of illite ‘crystallinity’ they show patterns of element mobility generated by progressive metamorphism. Three types of mobility are identified: a) localized grain- or crystal-scale mobility related to fabric-forming reactions; b) basin-scale losses during prograde metamorphism; c) basin-scale mobility associated with hydrothermal processes. Elements Al, K, Cr, Ni and Rb show limited mobility and were conserved in reactions that generated slaty cleavage in the Southern Uplands and southern Lake District mudrocks; prograde reactions also caused 20-70% basin-scale losses of B, Cs and Li in these two terranes. These results show that the formation of slaty cleavage is an important geochemical process. In the northern Lake District the distribution of K, Ba, Rb, Sr and Ni indicate pre-cleavage mobility of these elements related to early, syn-sedimentary hydrothermal activity. Plots of Cr/Ni ratios against grade suggest that this is not a reliable provenance indicator for metapelitic rocks

Topics: Earth Sciences
Publisher: British Geological Survey
Year: 2009
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