Li and δ7Li in mudrocks from the British Caledonides: Metamorphism and source influences

Mudrocks from three lower Paleozoic basins in the British Caledonides (southern Lake District, northern Lake District and Southern Uplands) were investigated to determine the influence of sub-greenschist facies metamorphism on Li and the factors that control Li in fine-grained terrigenous sedimentary rocks. Metamorphic grade, as determined by K1 (Kubler index) does not correlate with Li content ([Li]) and delta Li-7, indicating that sub-greenschist facies metamorphism has negligible effect on Li in these rocks. Collectively, the data for all three basins show it negative correlation between [Li] and delta Li-7 and a positive correlation between [Li] and the Chemical Index of Alteration (CIA), suggesting that provenance exerts the greatest control on Li in mudrocks. Samples from the northern Lake District, which were deposited in an extensional basin, have homogeneous REE patterns, similar to shale composites (PAAS), the highest CIA, Th/U and [Li] and the lowest delta Li-7 and epsilon(Nd), consistent with their derivation from a highly weathered, ancient continental source. By contrast, mudrocks from the Southern Uplands range to the lowest CIA, Th/U and [Li] and have the highest delta Li-7 and epsilon(Nd). These samples were deposited in a forearc basin on the southern margin of the Laurentian craton and contain volcanic detritus. Their REE patterns are the most variable, ranging from average shale-like patterns to less LREE-enriched patterns. The compositional heterogeneity within the Southern Uplands mudrocks is consistent with a mixed provenance that includes juvenile crustal materials (lower [Li], epsilon(Nd) and Th/U, higher delta Li-7), likely derived from the arc, as well as more highly weathered continental detritus. Mudrocks from the southern Lake District were deposited in a foreland basin, and exhibit geochemical characteristics intermediate between the northern Lake District and the Southern Uplands mudrocks, indicating their derivation from a mixed source. Our study shows that Li concentrations and delta Li-7 can provide additional information on the degree of weathering of the provenance of mudrocks. Published by Elsevier Ltd.