Moine─Dalradian relationships and their palaeotectonic significance

Abstract

Regional geochemical and lithogeochemical data, when reviewed in relation to the development of the Caledonian orogen in Scotland, indicate that the Moine-Dalradian boundary coincides with a long-lived crustal discontinuity. It is suggested that this boundary follows the original southeastern margin of an old continental slab in which the Lewisian basement was overlain by a thick layer of ‘Old Moine’ metasediments affected by pre-Caledonian (750 Ma or over) deformation and metamorphism. Early Caledonian ‘Young Moine’ sediments on this slab resemble the underlying metasediments in lithofacies but are somewhat poorer in Zr and Y. To the southeast of the boundary, a thick Dalradian succession accumulated in a marine ensialic basin on a thinned basement of granulites and gneisses. In the Dalradian, elements of basic-ultrabasic association are high, especially in the upper Argyll and Southern Highland Groups where they are associated with products of basic volcanicity. Stratabound Ba, Pb and Zn mineral deposits occur widely not far below the volcanic horizon, and magmatism and mineralization are attributed to the opening of a palaeo-oceanic rift within the Dalradian basin. The geochemistry of the Torridonian, Old Moine and Young Moine detrital sediments suggests derivation from a common source dominated by intermediate-acid calc-alkaline rocks. Low to moderate large-ion litho-phile (l. i. l.) element levels suggest variable degrees of depletion caused by deep metamorphism of the source rocks. Isotopic data show that the sourceland was Archaean and early Proterozoic, and suggest that it may have resembled the Ketilidian and pre-Ketilidian of southern Greenland. The geochemical influence of this ancient western sourceland can be recognized throughout the Dalradian succession, constraining models that involve the availability of young island arc or exotic tectonic materials during the filling of the Dalradian basin. The mechanical strength, relatively low density and high heat production of the Old Moine rocks enhanced the contrast between the crustal slab incorporating a thick Moine layer and the main Dalradian basin beneath which this layer is absent. Tectonic and metamorphic develop­ments differed in the adjacent regions during orogeny, and deep discon­tinuities at the Moine-Dalradian boundary acted as conduits for Late Caledonian appinitic and metalliferous granites rising from sub-crustal sources.