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Geology is the Way

What are the ‘dark balls’ in the granite of Sant’Andrea (Elba)?

If you remember my previous post, you will remember that I have left you with one question: what are the ‘dark balls’ in the granite?

Granites are spectacular objects deriving from the crystallization of magma at depth. When we can finally see them, exposed by erosion and other processes on the surface of our planet, we see them as solid aggregates of crystals and we may erroneously think that there is no way anymore to understand the processes acting in the magma when it was still molten.

Yet, you just need to look around to see many igneous structures formed in liquid magma, like the ‘dark balls’ or ‘blobs’, ranging in size from a few centimeters to several meters, that are visible everywhere in Sant’Andrea.

mafic enclave in monzogranite

mafic enclave in monzogranite

If you examine these blobs, they appear as masses included in the granite. Certainly, it is not the same stuff as the granite: they are dark, with rounded margins, and very fine-grained. As a first guess, one may think that perhaps they are fragments of other rocks encountered by the granitic melt and incorporated within it when it was molten. Fragments of exotic metamorphic rocks incorporated in the granite – called xenoliths – exist at Sant’Andrea. Here’s an example:

This one in particular is a xenolith precisely because we can recognize an internal structure, a compositional banding, that is truncated by the contact with the surrounding granite. Hence, we may reasonably conclude that this is a fragment of rock stripped away from somewhere by the magma. But our ‘dark balls’ have some extra feature that make them different.

mafic enclave in monzogranite

First, let’s learn to call them by their name. They are dark-colored, they are fine-grained with respect to the surrounding granite, and they are included in the granite… hence they are called – with no much imagination – mafic microgranular enclaves or simply mafic enclaves. Moreover, beyond the features listed above, mafic enclaves always show this rounded outline.

But how do they form? Do you remember last time when I told you that the granite here is also full of huge crystals of K-feldspar? And that, since they are so big and well formed, they liked crystallized earlier during the solidification history of the granite? A reminder:

alkali feldspar megacrysts and mafic enclaves

These crystals were floating in the granitic melt…. but what happened when they encountered the mafic enclaves? Here is a key observation:

After seeing this, it becomes more difficult to look at this stuff as solid objects and rather they start to appear pretty fluid! The outline of the mafic enclaves bends around objects in the granite, like crystals and xenoliths, and it is very common to find crystals ‘poking’ in the blobs.

Look again at the structures! Aren’t you reminded of droplets of oil in water? Or, even better, of lava lamps?

This is exactly how mafic enclaves are interpreted: masses of an immiscible mafic magma that mingled with a granitic melt in a magma chamber. In general, mafic (basaltic) magmas are much hotter than granitic melts and, consequently, when they mix with them, they are forced to cool down relatively rapidly developing fine-grained crystals. However, they remain liquid for a while and have enough time to interact with the surrounding melt and with other solid objects, like crystals, in the granite. And this is where things get interesting: what happens to the alkali feldspar crystals when they get in the enclave?

Here they are. They are still there, but they look quite unhappy compared to the perfect crystals in the granite. In the enclaves, the alkali feldspar crystals show a rounded or irregular outline, which means they have been partially reabsorbed by the mafic magma. This one of the ways magmas evolve: by mixing and reacting. For example, Gagnevin et al analyzed the granite and the enclaves and found that there is nearly no difference in chemical composition, as the two magmas chemically mixed as they mingled. This is a big difference with our lava lamp (where the two liquids don’t change in composition as they mix).

The structures that we see here witness the constant process of interaction and evolution of different magmas that constantly occurs beneath our feet. When lava comes out of volcanoes, these processes have already happened. Hence, studying exhumed intrusions is an excellent way to understand how magma migrates and evolves into different compositions before eruptions. Remember this the next time you will look at your granite countertop!

This post was possible thanks to the support of Sandra McLaren and Neal Landsberg (thank you!). If you like my geological posts and you wish to support me, you can do it by offering me a coffee at ko-fi!

References
Barboni, M., Annen, C., & Schoene, B. (2015). Earth and Planetary Science Letters432, 436-448.
Gagnevin, D., Daly, J. S., & Poli, G. (2004). Lithos78(1-2), 157-195.

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