Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

A geological oddity; orbicular granites of Tasmania

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Orbicular granite near Trial Harbour – Tasmania, Australia

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.

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Heemskirk granite on the coast near Trial Harbour – Tasmania, Australia

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Orbicular granite near Trial Harbour – Tasmania, Australia

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).

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Location and Trial and Granville Harbour, and geological map of the Heemskirk granite in western Tasmania, Australia

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.

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Folded UST band with parallel smaller UST band, near Granville Harbour – Tasmania, Australia

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science, formation, earth science

Formation of unidirectional solidification textures

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.

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Quartz USTs near Granville Harbour – Tasmania, Australia

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Myself by a planar layer of quartz USTs near Granville Harbour – Tasmania, Australia

Other magmatic-hydrothermal transition features are giant pegmatite pods and veins, vein-dykes, miarolitic cavities and orbicules!

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Tourmaline and quartz crystals within a cavity in the Heemskirk granite near Granville Harbour – Tasmania, Australia

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Orbicular granite near Trial Harbour – Tasmania, Australia

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Tourmaline cavity

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.

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Orbicules with K-feldspar alteration rims, in the Heemskirk granite near Trial Harbour – Tasmania, Australia

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.

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Orbicular granite near Trial Harbour – Tasmania, Australia

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!

-Stephanie

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Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science, sunset

Sunset near Granville Harbour – Tasmania, Australia

Tasmania, Australia, West Coast, Travel, Geology, Adventure, Blog, Granite, UST, Orbicules, unidirectional solidification textures, tourmaline, quartz, aplite, feldspar, cavities, slushy texture, magmatic, hydrothermal, textures, rocks, explore, outdoors, Trial Harbour, Granville Harbour, Zeehan, Western Tassie, Tassie, nature, science

Folded quartz UST, near Granville Harbour – Tasmania, Australia

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13 Comments

  1. Morgaine

    Hi Stephanie, I stopped by while googling for something completely different, because of the terms “granite” “orbicules” and “quartz” together.
    Those Tasmanian rocks bear some similarities with an aplite outcroup I saw this summer at Elba Island, in Italy.
    The outcrop is called Capobianco aplite and, in its upper part, contains black-bluish orbicules of microcrystalline quartz, blue elbaite and schorl, ranging from some mm to over 10-15 cm in size. The aplite itself is very microcrystalline, unlike the Tasmanian granite.
    Those orbicules are thought originated from bubbles where lighter elements like boron were trapped. Hydrothermal processes have been excluded in Capobianco.
    If that sounds interesting to you, here you can find a link to an article about Capobianco aplite (some geology with some history as well):
    https://www.researchgate.net/publication/233886110_Sooty_sweat_stains_or_tourmaline_spots_The_Argonauts_at_Elba_Island_Tuscany_and_the_spread_of_Greek_trading_in_the_Mediterranean_Sea

    Thank you very much for a very interesting blog post! I had looked previously for some other geological localities with similar characteristics. Funny to have found them on the other side of planet 🙂

    • Hey, thanks for the comment! Glad I was able to provide something useful on your Google search 😉 Yeah the student who was doing the PhD on these also found high boron content in the orbicules I believe. And in the field you also see similar things as mentioned in the Dini et al 2007 paper of a “quartz-tourmaline vein network connecting the orbicules” (though this feature is not ubiquitous). I also have seen similar tourmaline-magnetite-quartz orbicules before when I did an Ore Deposits of South America field trip in 2015. It was near Caldera, Chile. I made a short video on the trip (https://youtu.be/olYvJhps514) and at the very end you can see the orbicules. I didn’t have an explanation for them then either, haha. Anyways, thanks for stopping by and happy you liked the blog post and found it interesting. Cheers! -Stephanie

  2. gary23902003

    COOL! Chocolate chip cliffs. Amazing. I notice most striations I see seem to form upwards at an angle, rarely ever flat. Geology has it’s mysteries. Great post, BTW.

    • Stephanie

      Chocolate chip cliffs, what a great name! Thanks for reading and I’m glad you like the post!

  3. They’re pretty tourmaline-rich! Do they ever contain beryl or other? The fact there are Sn-W deposits in the are testifies to all the volatiles in the magma plus hydrothermal alteration. Are there actual pegmatites, or is it just these inclusions and other smaller features? Are the orbicles concentrated near the roof? That folded quartz is obviously near the roof. Do the orbicles only occur in the granite, or at contact with the country rock? Good post!

    • Hey, thanks for reading and great questions! From what I know they don’t contain beryl. Yes there are actual pegmatites, usually consisting of large K-feldspar and muscovite crystals. The orbicules appear to be near the roof of the granite, however, to my knowledge the granite’s 3D distribution has not been fully mapped as it is tough to tell where the roof is and the way-up within granites… The orbicules occur just within the granite, and at the contact between different phases of the granite (i.e. finer and coarser-grained phases). Once my college is done his thesis I’ll have a read and hopefully can answer these questions better for you. Cheers!
      Stephanie

  4. Marina Sacht

    I saw the folded Quartz on rocks st Kelsey bay just north of Campbell river . I thought they were so cool!

    • Cool, I’m glad you thoughts so as well. You should send me a picture of them!

  5. Eyup Can

    I found this blog 2 years ago when was surfing. Actually geology just hobby for me. İt’s so far from my job. But people can find so interesting and amazing things in here. Nature is a mysterious journey.
    Success at your studies!

    • Two years and you are still sticking with me, thanks so much!! I’m glad you find nature and geology interesting and amazing as well 🙂 Cheers!