These pics are from the NE flank of Carningli, close to the entrance passage leading into the hill fort, and the rocks look to me like banded dolerites. Not my area of expertise, but I don't think they are andesites, or dacites, or rhyolites. The GS map for the area shows dolerites and gabbros -- but this doesn't look like a gabbro either. Over to the experts? Suggestions, anybody? The little strips that stand proud seem to be very narrow quartz layers.....
PS. I stand corrected -- this post referred originally to "banded dolerites" but it appears that there are no such things. So "banded" will probably be OK, since that's simply a descriptive term that makes no assumptions as to origins.
Do not know. Knock a piece off and post.
ReplyDeleteFlow banding in the sense you are using it is not possible in dolerites, they are by definition (high level) intrusives. Need to see it in the flesh.
I think were they 'flow-banded dolerites' they would be in the geological literature and I do not recall any such writing.
M
Thanks Myris -- I thought I saw the term somewhere. Maybe just for rhyolites? Anyway, next time I go up I'll try to bash a bit off for you to look at....
ReplyDeleteThese "bands" are genuinely confusing -- the bands stand proud of the weathered surface, and they are regularly spaced. They cannot really be referred to as foliations, because that implies crystal alignments at a micro-scale -- and why should they be aligned in discrete layers with non-aligned crystals in the spaces between the bands? Are we looking at a cooling phenomenon? Or are the bands metamorphic features created some time after the initial cooling of the dolerite? I have assumed that the prominent bands are made of quartz crystals, but I found an image from St Kilda of some similar looking dolerite with light-coloured and prominent bands, and those were referred to as being bands of feldspar crystals. The mystery deepens.....
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