The Portscatho Formation, within the allochthonous unit of the Middle and Upper Devonian Gramscatho Group, is a thick sequence of deep-water sandstones and interbedded slates deposited by southerly-derived turbidity currents into the Gramscatho basin of south Cornwall. Throughout an approximately 3.5 km thick sequence, the Portscatho Formation is petrographically and chemically coherent, except that the upper section shows a higher proportion of metamorphic clasts, high, but variable Cr, and low, uniform Zr abundances. Complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition, similar to average upper continental crust, but with an admixture of minor intermediate/basic material. Flysch deposition took place in a fore-arc setting. The presence of an arc to the south of Cornwall during the Devonian implies that there was subduction at the margin of the Gramscatho basin, whose ultimate closure was accommodated by the northward stacking of flysch–ophiolite nappes.

The provenance of synorogenic greywackes from Devonian flysch successions occupying structurally similar positions at opposite ends of the Rhenohercynian zone in SW England and Germany is evaluated. Greywackes from both regions are petrographically and chemically similar, although minor differences are seen in the relative proportions of lithics; the Gramscatho group being richer in volcanic and metavolcanic clasts, whereas the Giessen group is richer in metasedimentary clasts. Absolute abundances of Ni-Cr-V and Zr-Hf-Y vary to a limited extent in each group and reflect variable mafic detritus and heavy mineral inputs respectively. Framework mode parameters and chemical data indicate the Rhenohercynian greywackes were derived mainly from a calc-alkali, acidic, dissected continental arc source, with minor MORB-like and argillaceous metasedimentary components. Upper continental crust-normalized multi-element patterns for the greywackes are characteristic of the continental arc/active margin tectonic environment. However, ‘Mid-Proterozoic’ model Nd ages for the Gramscatho greywackes suggest that an active Devonian arc source is unlikely. Instead the range of chemical and isotopic composition displayed mainly reflects mixing between acidic arc terranes of Proterozoic age and Devonian (Lizard-type) oceanic crust. Petrographic and geochemical discrimination diagrams alone cannot resolve the temporal decoupling between source and basin and may lead to an erroneous interpretation of tectonic setting.

The SW England Rhenohercynian passive margin initiated with rift-related non-marine RésuméHelp from editors required here please.2

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A stratigraphy for south Cornwall is combined with an analysis of the chronology of thrust nappe emplacement. The resulting model suggests that northward overthrusting of nappes of flysch and olistostrome, ophiolite, and continental basement is related to the closure of an oceanic basin in south Cornwall involving southward subduction throughout the Devonian. The subsequent continental collision between the Normannian High and part of Laurentia occurred in the Upper Devonian with deformatian continuing until late Carboniferous times.