The provenance of Devonian sandstones from Shetland: a Sm-Nd and trace element study

Abstract

Synopsis A Sm-Nd isotope and trace element study has been performed on a selection of four whole rock samples of Devonian sediments, two impure/banded quartzite boulders and two unfoliated granite/granophyre boulders from a Devonian conglomerate. Included in the study are also three samples of Caledonian metasediments (from the Whiteness, Clift Hills and Yell Sound Divisions) and one Archaean tonalite gneiss (the Wilgi Geos Group) from Shetland. The Devonian sediments and the granite boulders have relatively low Nd model ages of 1.0 to 1.1 and 1.3 to 1.5 Ga, respectively, which suggests the presence of Caledonian juvenile material in both of these rock types. The Proterozoic metasediments have Nd model ages in the range 1.8 to 2.5 Ga, while the tonalitic gneiss has a high Nd model age of 2.9 Ga. The Caledonian metasediments form two compositionally contrasting groups where the Yell Sound and Clift Hills Divisions and the two impure quartzite boulders all have relatively low time-corrected 143 Nd/ 144 Nd ratios and low Nb-and Sr-contents relative to the Whiteness Division. These two groups of Caledonian metasediments together with representative Caledonian ophiolite rocks define a compositional field of time-corrected 143 Nd/ 144 Nd ratios and Nb and Sr contents, within which plot the compositions of the Devonian sediments and the granite/granophyre boulders. Cr is generally immobile at surface and low-grade metamorphic conditions, but is less suitable as a provenance indicator due to the wide variations in the Cr-content in Caledonian juvenile rocks.