4. Greenstones from SW England

Details of the petrological and geochemical variation of extrusive and intrusive basic rocks from this region have been published previously (e.g. Floyd 1976; 1984; 1995; Floyd et al. 1993). Note that a distinction is made between the extrusives, which are mainly basaltic pillow lavas, whereas the intrusives are greenstones (strictly meta-dolerites), although both groups in any one area are probably temporally penecontemporaneous. The older literature of this region (e.g. Phillips 1876) also draws attention to the division between greenstones and spilites (now recognised as altered basalts).

Ranging in age from Devonian to Lower Carboniferous, the greenstones from different-aged strata and locations have distinctive mineralogical and chemical characteristics that can be used to identify each major occurrence. All have suffered variable low-grade metamorphism in the early Upper Carboniferous, generally within the actinolite-pumpellyite or greenschist facies (e.g. Robinson and Read 1981; Floyd and Rowbotham 1982).

Figure 5: Geochemical data for SW England metadolerites intruded into Devonian and Carboniferous strata

Owing to the random effects of secondary alteration, the geochemical variation exhibited by the greenstones are more definitive in discriminating between different locations of outcrop, and thus identifying possible axe sources (Markham and Floyd 1998). As seen in Figure 5, a number of chemical diagrams using stable incompatible elements have been constructed to illustrate the variation within the greenstones. This variation is most clearly defined from ratios of the elements concerned. The Zr/Nb ratio best discriminates between the Devonian anhydrous greenstones (Zr/Nb 10-30) on the one hand and the Devonian hydrous group and all greenstones of Carboniferous age on the other (Zr/Nb<10). Primary mineralogy can be used to differentiate between the Devonian hydrous group (with brown amphibole and biotite) and the Carboniferous greenstones (with large apatites). In a similar manner, the Zr/Y ratio differentiates between the Devonian anhydrous group (Zr/Y < or =4) and the rest, whereas the Ti/V ratio provides no discrimination. Differences in the patterns of chondrite-normalised plots of the rare earth elements (REE) allow a distinction to be made between alkaline and tholeiitic dolerites (Fig. 6).

Figure 6: Normalised REE distribution patterns for tholeiites and alkali basalts/dolerites

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