Geochemistry and provenance of Rhenohercynian synorogenic sandstones: implications for tectonic environment discrimination

Geological Society Special Publication - Tập 57 Số 1 - Trang 173-188 - 1991
P.A. Floyd1, RK Shail1, Brian E. Leveridge2, Wolfgang Franke3
1Department of Geology, University of Keele, Staffordshire, ST5 5BG, UK
2British Geological Survey, St Just, 30 Pennsylvania Road, Exeter EX4 6BX, UK
3Institut für Geowissenenschaften und Lithospharenforschung, Justus-Liebig Universität D-3600 Giessen, FRG

Tóm tắt

Abstract 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.

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Geological Society Special Publication

Tập 57 Số 1

173-188

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