4.4 Quantitative arithmetical comparison between selected axe sub-groups and exposures

Residues from comparisons between axe sub-group and selected (cf. Markham 2000, section 4.7, table 9) exposure thin-section scores are reported in Table 5. At this stage of the investigation it is not known what score constitutes a 'good' match, with more work being required to establish the relationship between the magnitude of the residue and the associated petrographic similarity. But, based on the assumption that a perfect match results in zero residue, then values close to zero must indicate a marked similarity in profile (which is supported by findings discussed above), hence mineralogy and texture. Figures reported in Table 5 below indicate that the lowest residuals (hence 'best' match) between axe sub-groups and greenstone rocks are between GpI-1 and GpIa and MM3, GpI-6 and GpIII-2 and MM4 (note that MM3 and 4 were not collected from actual greenstone exposures). The lowest residual between an axe sub-group and greenstone exposure is between GpIa and GUR001/2 (2.70) followed by GpIa-1 and ZEN002 (5.40) GpI-8 and PEN004 (5.40), GpIa-1 and PEN004 (5.80), and. GpI-1 and PEN004 (5.86)

Table 5: Summarised residues of comparisons between selected axe sub-group and exposure thin sections. The smallest residues, equivalent to the closest matches, are indicated in bold
 GpI-1GpI-6 GpIa-1 GpIII-2MM4PEN003PEN004ZEN002GUR001/2
MM3 3.17 12.38 2.80 12.00 11.25 14.00 7.00 6.60 5.50
MM4 10.18 2.67 10.45 3.25 - 7.25 8.25 8.65 10.75
PEN003 14.93 9.46 14.00 8.00   - 13.00 10.60 14.50
PEN004 5.86 10.92 5.80 7.80     - 2.40 4.50
ZEN002 6.30 11.32 5.40 8.60     - 4.10
GUR001/2 4.04 9.88 2.70 8.70       -

As stated above, this method demonstrated that the closest petrographic match between an axe sub-group and greenstone exposure is between GpIa-1 and the greenstone found at Gurnard's Head. However, any assumptions about provenance must be treated with caution, as the two Gurnard's Head thin-sections shared petrographic similarities with GpI-7 and GpIa-3, which is not seen here. Also, the Gurnard's Head samples contain secondary biotite, not often seen in axe thin-sections and which is not covered in any of the calculations/categories above. Additional evidence for caution in interpreting the residues is given by one of the 'best' (i.e. the lowest residual) matches being between two exposure specimens, PEN004 and ZEN002 (2.40), which are geographically separated by >30km.

4.4.1 Remarks on the usefulness of quantitative arithmetical comparison

The most significant aspect of this method of comparison is that the single numerical value produced is related to the observed petrographic similarities, even after noting the difficulties discussed in the paragraph above. However, the relationship between the magnitude of the residual and the degree of petrographic similarity has not been ascertained, other than the lowest residuals represent the 'best' petrographic matches. This means that the magnitude of residues alone cannot be used to indicate petrographic matches with certainty, but does provide an indication of similarity. The addition of more scoring criteria, possibly related to the volume of minerals present, could improve the situation.


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