3. Analysis of Greenstone Exposures in Cornwall

3.1. Potential sources based on greenstone petrography

The primary mineralogy of the majority of the Group I, Ia and III thin-sections is predominantly medium grained with, occasionally ophitic, Ca-poor clinopyroxenes (pigeonite and augite), plagioclase feldspar and titano-ferrous opaques. This, coupled with the rare presence of orthopyroxene and apatite and the lack of olivine and orthoclase feldspar, indicates a tholeiitic origin (Rollinson 1993; Mackenzie et al. 1982). Occasional diopside and higher Ca augite suggests a more evolved nature to the tholeiite in some cases. The lack of evidence for volcanic glass supports the observation that the medium grain size indicates an intrusive, possibly shallow level, emplacement of the original basic igneous rock. These observations suggest the greenstone may have been emplaced as sills or dykes within an N-MORB to E-MORB or within plate tectonic environment (Rollinson 1993, appendix 30).

Subsequent alteration of the primary mineralogy resulted in the assemblage: amphibole (tremolite-actinolite, hornblende), chlorite, sericite, epidote, sphene and albite which corresponds to metamorphic alteration within the pumpellyite-prehnite to greenschist facies (Floyd et al. 1993). Growth of subsequent biotite indicates the possible influence of K-rich fluids and could indicate proximity to or within a metamorphic aureole (Al Samman 1980).

N-MORB, E-MORB and within plate tholeiitic igneous activity is known to have occurred in Cornwall during the Devonian geological period (Floyd et al. 1993), along with low-grade regional metamorphism associated with the Variscan Orogeny and contact metamorphic aureoles due to the emplacement of the Cornubian granite batholith. This suggest an entirely reasonable hypothesis that IPG Group I, Ia, III and IIIa axe material originated from within Cornwall.

A total of 73 petrological thin-sections were prepared from samples collected from greenstones within Devon and Cornwall. Locations and thin-section references are listed in Table 3.

3.1.1 Lizard Peninsula: Lizard Dykes

Both Lizard Dyke thin-sections contain laths of plagioclase feldspar that are markedly less altered than the once ophitic, sub-mm, pyroxenes. Amphibole, especially acicular actinolite, is present throughout. Although the two thin-sections are mineralogically similar to Groups I-11 and GpIII-5, the texture found in these two sections, dominated by the fine laths of plagioclase feldspar, is not seen in any axe thin-section examined.


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