|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.