A geological oddity; orbicular granites of Tasmania
Intro: Tasmania sure does pack-a-punch when it comes to geological oddities. One of the strangest I would argue is the bizarre spotted granites on the west coast. Clustered within certain areas of the granite are orbs containing tourmaline, quartz and other minerals. The sight of them is spectacular, and their formation is still enigmatic! We ventured over to the west coast to sample and map these in more detail, as one of the PhD students in my department is doing his thesis attempting to unravel their mystery. I don’t have all the answers (maybe he will soon), but I will attempt explain a bit more about theses wonderful but odd features in this post.
Science Spiel: Odd Orbicules (Geology)
The northeast and west coast of Tasmania contain large granite batholiths around ~360 million years old. One of the largest ones is called the Heemskirk granite, and it is exposed along the shore at Trial and Granville Harbour, near the town of Zeehan in western Tasmania. This is where we went to see these strange features. It is also interesting to mention that another aspect to these granites is their common association with tin and tungsten ore deposits (Kitto, 2009).
Granites form from the solidification of felsic magma deep within the crust. Within the granite various processes take place, most of which earth scientist still don’t have a full understanding of. One of these processes is the generation of magmatic liquids and volatiles within the magma, which subsequently get released around the edges of the magma and result in hydrothermal veins and alteration. It is rare to see this transition from magmatic to hydrothermal conditions, but luckily there are some spectacular outcrops in Tasmania that exhibit features that are attributed to this enigmatic transition.
Unidirectional solidification textures (USTs), well-exposed at Granville Harbour, are formed by the downward growth of prismatic crystals, usually quartz or feldspar, at the roof of a crystallizing magma (Shannon et al., 1982; Kirkham and Sinclair, 1988). Is it thought that they form from the buoyant hydrothermal fluid that migrates to the top of the magma chamber. Eventually pressure builds up and the seal is breached and this results in melt and (possible metal-bearing) fluids to escape, and then rapid cooling and devolitisation of the magma, resulting in fine grained aplite layers between the UST layers. These fractures get sealed up soon after, and then another layer of UST forms, thus repeating the processes.
Other magmatic-hydrothermal transition features are giant pegmatite pods and veins, vein-dykes, miarolitic cavities and orbicules!
Orbicules are spectacularly exposed near Trial Harbour. They are very spherical, round and consist of dominantly tourmaline and quartz. They look like chickenpox within the granite, which is why I sometimes refer to this as a diseased rock! The orbicules aren’t the only odd round features with tourmaline and quartz, there are patches and spherical cavities with well-developed tourmaline crystals and quartz, and commonly potassium feldspar alteration rim at the edges.
How do they form? Again, this is still enigmatic! But a couple theories are that these represent bubbles of volatiles and/or melt coalescing near the roof of an intrusion, or by nucleation and outward growth of crystals within the melt.
Final Thoughts: As I mentioned at least half a dozen times, these really are odd rocks. Granites are a very common rock type in the geological record, but rarely do they exhibit these magmatic-hydrothermal transition features as we can see exposed within the Heemskirk granite on the west coast of Tasmania. Being able to see these beautifully USTs and orbicules allows a unique opportunity to think about and study what the processes are that take place within a crystallizing magma. The west coast of Tasmania is also a stunning place to explore, in case you needed another reason besides the rocks!
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