Mapping a hidden terrane boundary in the mantle lithosphere with lamprophyres

Abstract

Lamprophyres represent hydrous alkaline mantle melts that are a unique source of information about the composition of continental lithosphere. Throughout southwest Britain, post-Variscan lamprophyres are (ultra)potassic with strong incompatible element enrichments. Here we show that they form two distinct groups in terms of their Sr and Nd isotopic compositions, occurring on either side of a postulated, hitherto unrecognized terrane boundary. Lamprophyres emplaced north of the boundary fall on the mantle array with εNd −1 to +1.6. Those south of the boundary are enriched in radiogenic Sr, have initial εNd values of −0.3 to −3.5, and are isotopically indistinguishable from similar-aged lamprophyres in Armorican massifs in Europe. We conclude that an Armorican terrane was juxtaposed against Avalonia well before the closure of the Variscan oceans and the formation of Pangea. The giant Cornubian Tin-Tungsten Ore Province and associated batholith can be accounted for by the fertility of Armorican lower crust and mantle lithosphere.

Late Paleozoic extensional reactivation of the Rheic–Rhenohercynian suture zone in SW England, the English Channel and Western Approaches

“…However, there are several indirect lines of evidence that indicate, whilst SW England basement is of peri-Gondwanan provenance (e.g. Cocks & Torsvik 2006), it might not be exclusively Avalonian: (1) εNd values for SW England granites are rather more negative than those for typical Avalonian basement, overlapping with values for Armorica/Saxothuringia (Shail and Leveridge 2009); (2) εNd isotopic signatures of Devonian sedimentary rocks in southern SW England, the South Portuguese Zone and the isotopically more juvenile plutons they host, are similar to those for the Meguma terrane in Nova Scotia (Nance et al 2015); (3) Sr and Nd isotopic compositions of Early Permian lamprophyres and basalts change between northern and southern SW England, approximately corresponding to the southern margin of the Culm Basin; this has been interpreted as the trace of a pre-Devonian terrane boundary, between Avalonia and Armorica, in the lithospheric mantle (Dijkstra & Hatch 2018). In addition, northerly dipping reflectors, tentatively classified as 'Caledonian', have been recognized in the lower crust and mantle below SW England (Bois et al 1990;.…”

“…Seismic lines and the wells mentioned in the text are also highlighted. The dashed lines correspond to the Northern Segment Rheic Suture (Dijkstra & Hatch 2018), Central Segment Rheic-Rhenohercynian Sutures and Southern Segment Rheic Suture from north to south as described in the text. Fig.…”

“…Preferred plate tectonic synthesis following some modifications from Shail & Leveridge (2009) from Silurian to Permian. This model involves an early closure of the northern (Dijkstra & Hatch 2018) and central Rheic Ocean segments in the latest Silurian and subsequent opening of Rhenohercynian Ocean as a successor basin (Early Devonian) in a similar location perhaps reactivating the former suture. An alternative model allows the Rheic Ocean to remain open while the Rhenohercynian Ocean forms on SW England basement.…”

“…A northern segment of the Rheic Ocean, developed during Cambrian-Ordovician rifting of the Gondwana margin, separated Avalonia from SW England (Armorican basement affinities), and closed by the latest Silurian by N-dipping subduction beneath Avalonia (e.g. Woodcock et al 2007;Shail & Leveridge 2009;Dijkstra & Hatch 2018) (Fig 9). This corresponds to the northern dashed line overlain on the magnetic and gravity compilations (Figs.…”

British Neolithic Axehead Distributions and Their Implications

“…These also represent geologically distinct zones on either side of the channel with the hard-stone resources and landscapes of the Armorican massif (of Hercynian orogeny) Table 3 Simpson diversity indices for each cluster, as calculated for the observed data and for each of the three simulation models (see the "Methods" section for details of the three models). Also provided here is a measure of the median radiocarbon intensity within each region K-means region linked to similar ones in the English south-west, while the flint-rich Upper Cretaceous chalk deposits of south-eastern England are much closer to the landscapes and flint- mining areas of neighbouring Normandy and southern Belgium (Mortimore 2012;Collin 2016;Dijkstra and Hatch 2018). Settler communities therefore could have moved to nearby locations with similar geological and ecological profiles, as well as similar axehead raw materials, with the western half exploiting hard-stone sources in Cornwall, Wales, and Cumbria and the eastern half exploiting flint from Sussex and Norfolk (cf.…”