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Saturday, 12 February 2011

New technique confirms Pont Saeson source for some bluestones

I have been reminded that a paper due out in March has now been placed on the web.  It confirms, through the use of zircon chemistry, that some of the rhyolites in the Stonehenge area have come from the Pont Saeson area on the northern flank of Mynydd Preseli.  This had earlier been suggested by Rob Ixer and Richard Bevins on the basis of other methods of analysis.

Stonehenge rhyolitic bluestone sources and the application of zircon chemistry as a new tool for provenancing rhyolitic lithics
Richard E. BevinsNick J.G. Pearce and Rob A. Ixer


The source of the bluestones at Stonehenge has long been debated, and while there is general consensus that the so-called spotted dolerites are derived from a relatively small number of outcrops exposed in the highest parts of Mynydd Preseli, in southwest Wales the source of the rhyolitic component has attracted relatively little detailed attention. This is largely because unlike the uniqueness of the spots in the coarser grained doleritic rocks, the rhyolites are fine-grained in character and lack any obvious distinctiveness, especially in hand specimen. This makes their provenancing difficult. A recent study, however, suggested that there was a close lithological similarity between the informally-termed ‘rhyolite with fabric’ bluestone component and rhyolitic rocks from the Ordovician Fishguard Volcanic Group exposed in the Pont Saeson area of north Pembrokeshire. This study aims to see if the chemistry of zircons, which are present in both sets of samples, could be used to support the petrographical association. Analyses for certain high field strength elements (including the rare earth elements) obtained by LA-ICP-MS showed that indeed the analyses were nearly identical when compared using a range of statistical approaches, including similarity coefficients, statistical distance, and principal component analysis, while showing clear differences to sample sets which had no reason to be correlated with the Pont Saeson samples. There are two important conclusions arising from this study. Firstly, the identification of the Pont Saeson source of the ‘rhyolite with fabric’ bluestone from outcrops in low ground to the north of the Mynydd Preseli will without doubt lead to fresh debates about the mechanisms of transport of this component of the bluestones to the Stonehenge site. Secondly, the chemistry of zircons may well prove to have a wider application in the provenancing of fine-grained rhyolitic rocks which have an archaeological context.

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