The hiding peak at Glenugie forest

Some time ago Mark left a comment where he asked whether the basalt at Glenugie Peak (once known as Mount Elaine) was part of the Ebor Volcano. I didn't think it was likely but at that stage I did not know much about this peak, in fact I'd only glimpsed it through the trees while driving along the Pacific Highway to Grafton. Since then I've been trying to find out more about the peak, although I still have not had the chance to actually get there, staff from the New South Wales Geological Survey recently have reviewed the mapping of the area including the peak. What they observed reinforces my understanding that it is not related to the Ebor Volcano but the visit found out some very unusual things.

Glenugie Peak is hidden quite well by the forest all around as well as the lack of other hills to see it from. This means that it often goes unnoticed but if you have a look at a topographic map you will see that it is a very significant feature in the landscape. Before I learned what the rocks were here I thought it was likely to be an old flow of basalt from a period of volcanism that occurred before the chain of volcanoes from the East Australian Hot Spot. This is because there are many outliers of basalt that occur in the region that are too early for the hot spot volcanism. In addition, the old geological mapping of the area has Glenugie Peak being comprised of Tertiary aged extrusive Basalt. This contrasts with the surrounding rock which is the Grafton Formation of the Clarence Moreton basin.

I  came across Jopin (1968) who described a sample of the Glenugie Peak obtained from another authors petrographic analysis as Limbugite. I have heard of Limburgite before but I could not remember exactly what it was or the implications of such a rock type. I don't think I have ever even seen such a rock before. So, I had to look it up! Limburgite is essentially looks a like a basalt in hand specimen but contains no quartz and is so silica poor that not even feldspar is present in the rock.  Instead of feldspar (the most common rock forming mineral) other minerals called feldspathoids are present. This is termed silica under-saturation or ultramafic.

The NSW Geological Survey have now identified that the Glenugie Peak is intrusive and is a dyke, volcanic plug or similar. It has been intruded through the underlying sedimentary rocks of the Clarence Moreton Basin. Additionally, a review of mapping of the region that is being undertaken includes investigation of the rock composition at Glenugie Peak. The Investigation includes analysis of samples which identified two types of rock: Teschenite and Meltiegite. Teschenite and Meltiegite is quite consistent with the Limburgite classification by Joplin 1968. These two are also silica under-saturated rocks. The feldspathoid mineral in this rock is called nepheline.

So what, what does that mean? Well, these rocks are actually very unusual in the coastal region. Phonolite, a related but still not as silica-undersaturated (it is also higher in the elements sodium and potassium) as the rock found at Glenugie, occurs in the New England tablelands but this seems to be quite old in comparison to Glenugie Peak. These silica under-saturated rocks form where there is a significant thickness of continental crust allowing the bottom of the crust to partially melt (but not melt too much). The melted component then migrates and is emplaced either in more shallow crust or erupted to the surface. It is comparatively rare and unfortunately these rocks tend to weather easily making accurate chemical dating hard.

It seems that Glenugie Peak is made from a weird rock. I was very surprised (and excited) to see the unusual classifications that have been made. As far as I am aware this rock does not occur anywhere else nearby and even on an Australian scale is rare. When a fresh piece of rock is obtained the general appearance resembles basalt and therefore may be quickly passed over and forgotten. Luckily, the peak continues to be looked at and although nothing has been published yet it is exciting that more is being learnt about the geology of the region. Without the Geological Survey and university geology departments knowledge of our land would be so much less.

Knowing what I now do, the next time I'm spending some time in the Grafton area I'm going on a bushwalk to Glenugie Peak! Apparently it is within a flora reserve and is particularly good for bird spotting too.

Note: since writing the above post I have come across another early reference to Limbugite and Teschenite by Vallance et al (1969) who also refer to a 1919 description but unfortunately little extra information is given.

2nd note: since wrinting the above note I came accross a record from 1915 which includes analysis of apparently of one of the two types of rocks found at Mount Elaine. The geo-chemical classification of this rock (according to the TAS method) is a picro-basalt (essentiall a very low silica and very low sodium and potassium basalt).

References/bibliography:

Joplin, G. A., 1968, A Petrography of Australian Igneous Rocks, Angus and Robertson.
Valance, T.G., Wilkinson, J.F.G., Abbott, M.J., Faulks, I.G., Stewart, J.R., Bean, J.M., 1969, IX Mesozoic and Cainozoic Igneous Rocks, Journal of the Geological Society of Australia.V16.