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2.5 Summary of axe thin-section examination

The examination of the 149 axe thin-sections has shown that the primary mineralogy of IPG Groups I, Ia, III and IIIa are similar: clinopyroxene (plus rare orthopyroxene) + plagioclase feldspar + opaque (usually ilmenite), and that they all share similar, but varying degrees of, alteration to amphibole ± biotite ± epidote ± chlorite. The alteration seen in IPG Group III and IIIa is more complete than IPG Groups I and Ia, with very little primary mineralogy remaining. The original mineralogy of all four IPG groups appears to have been medium grained, indicating an intrusive, as opposed to extrusive, igneous environment of crystallisation.

Although compositionally similar, there are recognisable mineralogical and textural differences that have resulted in the recognition of 24 sub-groups. Petrographic similarity within the sub-groups suggests the members may have originated from a single exposure. However, the differences between the sub-groups may indicate that the sub-groups could have originated from a single exposure with distinguishable compositional variation or from several individual exposures.

If it can be assumed that the 119 IPG Group I thin-sections examined are representative of the 384 axes in the whole Group, then GpI-1 can be considered to represent the most common petrography within IPG Group I. Using the same argument, GpIa-1 represents approximately just less than one-third of IPG Group I. Therefore, the observed petrographic similarity between GpI-1 and GpIa-1 suggests about one-third of IPG Group Ia could be assigned to IPG Group I. If GpIa-2 (similar to GpI-6) and GpIa-3 (similar to GpI-7) are also considered, then it appears that the large majority of IPG Group Ia axes are potentially the same as those assigned to IPG Group I.

Sub-group GpIII-1 represents IPG Group III as it contains the founding member of the Group, axe Wi4/4. However, as the other 9 of the 11 axe thin-sections examined are assigned to other sub-groups, the mineralogical and textural homogeneity of the Group is called into doubt. This observation, coupled with those made in the previous paragraph, hint at the desirability of reclassifying the whole of IPG Groups I, Ia, III and IIIa.

During the re-examination it was noted that several thin-sections were markedly different from the majority of thin-sections examined, and especially different from GpI-1, GpIa-3 and GpIII-1, which are believed to represent the corresponding IPG group petrology. In these cases it must be questioned whether these axes, which represent 5% of the total axe thin-sections examined (listed in Table 1, below), actually do belong to the IPG group they have been assigned to.

In conclusion, it can be seen that individual IPG groups do not appear to be as petrographically homogeneous as the IPG concept behind establishment of petrological groups suggests. Petrographical similarities between IPG groups indicate that there is potential to redefine membership of IPG groups, if not the individual IPG groups themselves. Further, it appears that only a small number of axes are incorrectly categorised.

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