5. Greenstones from Wales

During the Ordovician, Wales was part of a large back-arc basin and characterised by extensive bimodal volcanism, much of it expressed as large intrusive dolerite sills and sheets (Fitton and Hughes 1970; Bevins et al. 1984; Bevins and Roach 1979). Like the dolerites of SW England, the Welsh dolerites also exhibit secondary low-grade assemblages generally typical of the pumpellyite facies (Bevins and Rowbotham 1983). Although the main volcanic event took place in the Caradocian (Snowdonia and Conway valley), the intrusions further south are probably Arenigian-Llanvirian in age (e.g. St David's Head, Preseli Hills).

Figure 7

Figure 7: Geochemical comparison of Welsh metadolerites and Stonehenge bluestones

However, what is important is that dolerites from the two different age groups can be chemically distinguished. This is significant in differentiating the source of the 'bluestones' of Stonehenge, which, on the basis of mineralogical and geochemical comparisons, clearly indicate a Preseli source (Thorpe et al. 1991) as suggested by earlier workers (e.g. Thomas 1923). The chemical comparison can be illustrated by the plots in Figure 7 (Floyd et al. 1976, and unpublished data by P.A. Floyd), and using the data of Thorpe et al. (1991). Although the bluestone data form a tight cluster, only the Zr/Y ratio provides a good discrimination, with values of around Zr/Y = 4 suggesting affinities with South Wales locations (Preseli Hills); ratios of Zr/Nb and V/Ti are not definitive, in part owing to insufficient data.


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