IPG Group I is described by Keiller as:
'Uralitised gabbro, epidiorite or greenstone in which the augite, sometimes slightly ophitic, occurs in colourless to pale brown plates which show no crystal outlines and exhibit a variable amount of alteration to fibrous green hornblende along cleavage cracks and outer edges. Occasionally, alteration from pyroxene to hornblende is complete. Feldspar, presumed albite, much altered and riddled with small needles of actinolite. (The) Main accessory mineral is ilmenite which is often altered to leucoxene' (Keiller et al. 1941, 60).All of the 119 IPG Group I thin-sections examined had been previously catalogued and placed in IPG Group I by experienced petrologists, hence it was anticipated that there would be little mineralogical variation between them. However, examination using a monocular polarising petrological microscope revealed that it was possible to differentiate between some thin-sections and recognise sub-groups within the main IPG group.
The criteria for establishing a sub-group was based on the petrographic distinctiveness of thin-section texture and mineralogy. Initially, one thin-section was selected at random and used as a reference against which other thin-sections could be compared. As this process progressed, it became possible to delineate several distinctive and individually recognisable sub-groups within IPG Group I that shared similar characteristics. After comparing every thin-section with the reference, the recognition of distinctive characteristics led to the establishment of thirteen sub-groups. Each of the thin-sections were once more examined to confirm that the members of each sub-group shared similar petrographic characteristics.
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Last updated: Wed Jul 29 2009