The geology of the Strumble Head - Fishguard region, Dyfed, Wales

An investigation was undertaken on the volcanic and volcaniclastic rocks forming the Fishguard Volcanic Group of Lower Ordovician age, which crop out in the Strumble Head - Fishguard area of Southwest Dyfed (North Pembrokeshire). The nature of the lavas and volcaniclastic rocks suggests that this episode of vulcanicity was entirely subaqueous in aspect. It is shown that a variety of magma types were available for penecontemporaneous extrusion and intrusion at a high level. The form assumed by the extrusions depends primarily upon magma composition, which also largely determines magma viscosity. In addition these factors, plus the eruption depth, have also governed the development of volcaniclastic and pyroclastic debris. Basic magma was erupted quietly, and resulted in a thick lava pile with only a limited production of volcaniclastic and pyroclastic material. Similarily acidic magma also appears to have been erupted quietly in this area, although as a result of its viscosity it produced thick flows and domes, and abundant related autobreccias and collapse breccias. From an examination of whole-rock major and trace element analyses of a representative suite of rocks it is demonstrated that the intrusions and extrusions, which have tholeiitic characteristics, are comagmatic and that the majority of the igneous rocks examined are related to each other by a process of high-level crystal fractionation. The rocks of the area suffered low-grade regional metamorphism during the Caledonian Orogeny, indicated by the presence of pumpellyite and prehnite within the meta basites. In spite of this alteration, clinopyroxene remains as a metastable, relict phase within the meta-basites. From a microprobe study of the composition of these clinopyroxenes, it is clearly demonstrated that the rocks have tholeiitic affinities in addition to the fact that the composition of clinopyroxene within igneous systems is dependant upon cooling history

U–Pb zircon age constraints for the Ordovician Fishguard Volcanic Group and further evidence for the provenance of the Stonehenge bluestones

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

Composition of Pyromorphites from Broken Hill, New ·South Wales

Twenty eight specimens of pyromorphite from the oxidised zone of the Broken Hill orebody, New South Wales have been analysed using EPMA methods. Material was selected to represent all of the varieties of pyromorphite which have been described by earlier workers as occurring in the deposit. Aside from minor Ca2+ substitution for Pb2+ and occasionally YO/" for PO/", all specimens examined proved to be either pure end-member pyromorphite or arsenian pyromorphite with a maximum arsenate content corresponding to pyr2 . 1mim0 . 9 • In one specimen phosphate-arsenate zoning is evident; minor vanadate is present in the more arsenian material. Calcium-lead zoning has been detected in a pale grey-coloured specimen. These zoning patterns indicate chemical variations in aqueous solution during crystal growth. The arsenate contents appear to be directly related to the original distribution of the primary arsenides and arsenic-bearing sulfosalts. The compositions have been related to those of the solutions from which they crystallised. Apatite is a major accessory in the primary ore and this would provide the requisite phosphate ions

Linking derived debitage to the Stonehenge Altar Stone using portable X-ray fluorescence analysis

The Altar Stone at Stonehenge in Wiltshire, UK, is enigmatic in that it differs markedly from the other bluestones. It is a grey-green, micaceous sandstone and has been considered to be derived from the Old Red Sandstone sequences of South Wales. Previous studies, however, have been based on presumed derived fragments (debitage) that have been identified visually as coming from the Altar Stone. Portable X-ray fluorescence (pXRF) analyses were conducted on these fragments (ex situ) as well as on the Altar Stone (in situ). Light elements (Z<37) in the Altar Stone analyses, performed after a night of heavy rain, were affected by surface and pore water that attenuate low energy X-rays, however the dry analyses of debitage fragments produced data for a full suite of elements. High Z elements, including Zr, Nb, Sr, Pb, Th and U, all occupy the same compositional space in the Altar Stone and debitage fragments, and are statistically indistinguishable, indicating the fragments are derived from the Altar Stone. Barium compares very closely between the debitage and Altar Stone, with differences being related to variable baryte distribution in the Altar Stone, limited accessibility of its surface for analysis, and probably to surface weathering. A notable feature of the Altar Stone sandstone is the presence of baryte (up to 0.8 modal%), manifest as relatively high Ba in both the debitage and the Altar Stone. These high Ba contents are in marked contrast with those in a small set of Old Red Sandstone field samples, analysed alongside the Altar Stone and debitage fragments, raising the possibility that the Altar Stone may not have been sourced from the Old Red Sandstone sequences of Wales. This high Ba 'fingerprint', related to the presence of baryte, may provide a rapid test using pXRF in the search for the source of the Stonehenge Altar Stone

A re-appraisal of the petrogenesis and tectonic setting of the Ordovician Fishguard Volcanic Group, SW Wales

The Fishguard Volcanic Group represents an excellently preserved example of a volcanic sequence linked to the closure of the Iapetus Ocean. This study re-examines the petrogenesis and proposed tectonic setting for the Llanvirn (467–458 Ma) Fishguard Volcanic Group, South Wales, UK. New major and trace element geochemical data and petrographic observations are used to reevaluate the magma chamber processes, mantle melting and source region. The new data reveal that the Fishguard Volcanic Group represents a closely related series of basalts, basaltic andesites, dacites and rhyolites originating from a spinel lherzolite source which had been modified by subduction components. The rocks of the Fishguard Volcanic Group are co-genetic and the felsic members are related to the more primitive basalts mainly by low-pressure fractional crystallization. The geochemistry of the lavas was significantly influenced by subduction processes associated with a coeval arc, while significant amounts of assimilation of continental crust along with fractional crystallization appear to have contributed to the compositions of the most evolved lavas. The Fishguard Volcanic Group was erupted into a back-arc basin where extensive rifting but no true seafloor spreading had occurred