New U–Pb zircon ages from rhyolite samples of the Fishguard Volcanic Group, SW Wales, confirm a Middle Ordovician (Darriwilian) age for the group. One of the samples is from Craig Rhos-y-felin, which has recently been identified on petrological and geochemical grounds as the source of much of the debitage (struck flakes) at Stonehenge. Analysis of a Stonehenge rhyolite fragment yields an age comparable with that of the Craig Rhos-y-felin sample. Another Stonehenge fragment, thought to come from orthostat (standing stone) 48 and on petrographical grounds to be derived from the Fishguard Volcanic Group (but not Craig Rhos-y-felin), yields an age also consistent with a Fishguard Volcanic Group source. Supplementary material: Details of analytical methods and a table of data are available at https://doi.org/10.6084/m9.figshare.c.3518175 .
The North Coast Schist of Tobago is part of the leading edge of the Caribbean Plate, which has been in oblique collision with northern South America for much of the Cenozoic. The North Coast Schist is dominated by two volcanic "formations" metamorphosed under greenschist-facies conditions during later deformation. The Parlatuvier Formation mostly consists of mafic to intermediate tuffs and tuff breccias with a U-Pb zircon ID-TIMS age of Ma. Trace element data and radiogenic isotopes reveal that the Parlatuvier Formation is derived from a heterogeneous subduction-modified, locally incompatible trace element–enriched, mantle source with some rocks containing the highest 176Hf/177Hf ratios found in the offshore Caribbean. The Mount Dillon Formation comprises silicified tuffs and tuff breccias that are derived from a more isotopically enriched mantle source with a significant slab fluid-related component. A thin belt of amphibolite-facies dynamothermally metamorphosed metavolcanic rocks lies in contact with a younger island arc pluton. Some of these amphibolites have an isotopically similar source to the Parlatuvier Formation but lack a clear subduction-related component. The age, geochemical heterogeneity, and proximal nature of eruption confirm that the North Coast Schist lay within an east-dipping proto–Greater Antilles arc. We propose that the arc system at the time of North Coast Schist magmatism was actively rifting, possibly during development of a back-arc basin. This arc system shut down during the Cretaceous, making way for southwest-dipping Greater Antilles subduction and relative eastward motion of the Caribbean Plate.
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