Provenancing the rhyolitic and dacitic components of the Stonehenge landscapebluestone lithology: new petrographical and geochemical evidence

摘要

The source of the bluestone component found in the Stonehenge landscape has long been the subject ofgreat interest and considerable debate. The bluestones are a mix of lithologies, the standing orthostatsbeing predominantly dolerites, variably ‘spotted’, with only four of them being of dacitic and rhyoliticcomposition and the Altar Stone being sandstone. However in the 1920s the spotted dolerites weresourced to outcrops which comprise tors in the summit regions of the Mynydd Preseli in north Pembrokeshire,west Wales. There were also speculations about the possible sources of the dacitic andrhyolitic components, ideas which were elaborated on in the early 1990s when the original petrologicalprovenancing was supplemented by whole-rock geochemical analysis. Most recently, new petrographicalinvestigations have been combined with zircon geochemical data to determine the possible source of onetype of rhyolite, the so-called ‘rhyolite with fabric’, found abundantly as débitage in the Stonehengelandscape (but not composing the four orthostats) to outcrops in the vicinity of Pont Saeson, especiallya large craggy outcrop called Craig Rhos-y-felin, located in low ground to the north of the MynyddPreseli. In order to test this provenance whole-rock geochemical analysis has been undertaken onsamples of débitage from the Stonehenge landscape and from the Pont Saeson area, including CraigRhos-y-felin. These data are then compared with other new and existing geochemical data for dacitic andrhyolitic lithologies recovered from the Stonehenge landscape, including the four orthostats, as well asgeochemical data from outcrops of the same lithologies from the two main volcanic horizons exposedacross north Pembrokeshire, namely the Fishguard Volcanic Group and the Sealyham Volcanic Formation,both of Ordovician age. This study concludes that previous, 20th century, attributions of provenanceto a number of dacitic and rhyolitic outcrops in the north Pembrokeshire have been in error whilst thenew data for the Pont Saeson rhyolite accords well with elemental contents recorded in the ‘rhyolite withfabric’ lithology from the Stonehenge landscape débitage. This study therefore endorses the proposal thatthe Pont Saeson area is indeed the source of the ‘rhyolite with fabric’ lithology recovered from numeroussites in the Stonehenge landscape, and is the only reliable provenance for any of the dacitic and rhyoliticbluestone material collected to date. It also serves to endorse the use of zircon chemistry as a provenancingtool in archaeopetrological investigations.