Sunday, 19 May 2013
Back from another trip to Hospital for my daughter meant that we were ready to go out and enjoy the late Autumn sunshine. It was good timing too because Lismore Gemfest had started for the weekend.
Gemfest is a lovely combination of professional and amateur sellers of gems, minerals, fossils and jewelry. I found Frank, the fellow from Sydney that I'd bought some good mineral specimens off last year so I added some more to my collection. Frank is an interesting seller because he sells what he finds and what he finds is in Australia, so you know you are getting local examples. Mind you my collection is really just a selection of boring looking minerals with very little value. For instance this time I got hold of some muscovite, tourmaline, wolframite and epidote - quite valueless but interesting to me. The value is in the fun of collecting.
Again, this year Gemfest was full of great diversity of sellers and some good food stalls. I enjoyed my steak sandwich for lunch. I managed to take a few snaps of the end of Saturday but I forgot to take pictures inside the professional dealers area. Oh, well. Something to do next year. In the mean time here are a few snaps:
Thursday, 9 May 2013
Readers of this blog will probably notice I have an intense interest in volcanology. Volcanology has a wide variety of aspects some of which I’m comfortable, some less so. These aspects can be the chemistry of molten materials, the physics of earthquakes or the dynamic processes of pyroclastic flows. Volcanology and igneous rocks more generally seem to have their own weird language that can stop you and make you turn to a dictionary.
One of my favourite words in the ‘language’ of geology is the name of a large scale structure of lava flows. It is called aa. So, turning to a dictionary (this time the Omnificent English Dictionary in Limerick Form) you get the following possible definitions:
No consonants! Does this seem ominous?
It's with rough-surfaced lava synonymous.
Yet the thought it conveys
With two capital A's
Is, of course, Alcoholics Anonymous.
By Chris J. Strolin
I'm ascending a gentle volcano;
The climb's not the cause of my strain.
No, This lava is stressed,
Pretty jagged at best.
Cut my feet on sharp aa — the pain, Oh!
On Hawaii the lava's aflame
As observers, in awe, cry its name.
When that molten rock's oozing
Down paths of its choosing,
It's "A'a!" that tourists exclaim.
Probably one of the more interesting dictionary definitions I’ve seen. I Hope that helps with understanding? If these are a little bit obscure you can always visit my Glossary.
Wednesday, 1 May 2013
|Serpentinite at Port Macquarie|
The picture to the right shows the nature of one of the rock types at Port Macquarie. If I recall correctly this photo was taken at the southern end of Flynns Beach. It is a characteristic rock and given the odd shapes preserved in it implies quite an interesting history. The rock is serpentinite in two forms. The first being the banded appearing one which is called serpentinite schist. The second is a block of serpentinite which has not had the schistose fabric developed in it. I’ve discussed serpentinite occurring elsewhere such as at Baryulgil in previous posts but as far as Serpentinite goes the Port Macquarie area has heaps of it.
Serpentinite is a rock mainly comprised of the mineral group Serpentine. This is a very silica poor rock formed by the regional metamorphism of Olivine rich rocks such as Dunite or Peridotite. These parent rocks are from deep below the oceanic crust in the deepest parts of a layered sequence called Ophiolite and because of this it is rarely preserved on land. The metamorphism of the serpentinite is actually at the same time as large blocks of the Dunite and Peridotite rich oceanic crust are thrust and rotated during tectonic plate collision. Because serpentinite tends to be ‘slippery’ it is mostly present around major regional fault systems where it is ‘squeezed’ into place. However, its relationship to other nearby tectonic blocks is detailed and requires a separate blog post on its own.
At Port Macquarie the parent rock appears to have been a calcium rich variety of Peridotite called Harzburgite. There are also other rocks mixed in with the Serpentite, so much so that the area is often referred to as a melange. These other rocks are sometimes (but not always) part of the Ophiolite. For example slightly shallower ones such as gabbro which has been metamorphosed to rocks called Blue Schists. Also occurring are non Ophiolite rocks such as marble and other types of schist. Because of the complexity some 'inclusions' in the melange are from a different source than the Ophiolite, that is a story for another post.
As for the age of the Serpentinite unit, direct dating is impossible due to metamorphism re-setting the dating clock of the rock. The best that can be achieved is the last date of metamorphism. Even then the ultramafic (silica poor) nature of the rock means that minerals that can be used for dating (such as zircons) are uncommon or simply absent. Therefore the age of the Port Macquarie Serpentinite is only estimated from the surrounding rocks. However recent work by Nutman et al (2013) has narrowed the age of metamorphism and probable emplacement of the serpentinite to 251-220Ma which is the late Permian to early Triassic. How they found the date is quite interesting with adopting multiple techniques physical, nuclear and chemical.
*Aitchison, J.C. & Ireland, T.R. (1995). Age Profile of Ophiolitic Rocks across the Late Palaeozoic New England Orogen, New South Wales: Implications for tectonic models. Australian Journal of Earth Sciences. Vol.42.
*Nutman, A.P., Buckman, S., Hidaka, H., Kamiichi, T., Belousova, E., Aitchinson, J.C. 2013. Middle Carboniferous-Triassic eclogite-blueschist blocks within a serpentinite melange at Port Macquarie, eastern Australia: Implications for the evolution of Gondwana’s eastern margin. Gondwana Research.
*Och, D.J., Leitch, E.C. & Caprarelli, G. 2007. Geological Units of the Port Macquarie-Tacking Point tract, north-eastern Port Macquarie Block, Mid North Coast Region of New South Wales. Quarterly Notes of the Geological Survey of New South Wales. Vol.126.