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Geology is the Way

When two different magmas meet (Neves, Sud Tirol)

The glacier of Neves, located on the Italian side of the Eastern Alps at the border with Austria, close to the Neves Lake (Neves-Stausee), is an open air free geology exposition that is famous worldwide for its superbly exposed granitoids, stunning shear zones, and mylonitic structures. With this post I am starting a new series on Neves that will culminate with the photographs of the shear zone. Today, we will have a look at the granitoids and the beautiful magmatic structures preserved in this amazing outcrop!

The Neves glacier, on the background, used to cover these outcrops in the past. Its retreat, after the last glacial maximum, left behind an open air geology museum! The geologists discussing at the center of the photograph are Neil Mancktelow and Giorgio Pennacchioni, who studied this area in great detail.

As you can see above, there is a reason that make this place so special: its beautiful exposures, characterized by polished glaciated outcrops that allow to follow structures for hundreds of meters in all directions. The outcropping rocks are granitoids, with prevailing granodiorite composition, that emplaced at the end of the Variscan Orogeny, 300 million years ago. These granodiorites were then affected by alpine metamorphism up to 500 – 600 °C, during the formation of the Alps, and are now exposed at the surface as part of the Zentralgneise unit, one of the deep units cropping out in the Tauern Window.

Neves granite with dykeNeves panorama interpreted

Now pay attention to the figure above. As you can see we have a situation where granodiorite is surrounded by orthogneiss and mylonite. The surrounding orthogneiss is the result of the deformation of Variscan granitoids, whose structure has been changed by alpine metamorphism and transformed into a metamorphic rock. The granodiorite of Neves is very special because, despite experiencing the same metamorphism of the surrounding rocks, its resistant lithology allowed to largely preserve its original igneous structures (a different situation compared to the previous Joshua Tree example, where the contact between granites and gneiss is magmatic). You might be surprised by how good the preservation of igneous structures is:

The well preserved phaneritic igneous texture of the Neves granodiorite, containing biotite (black), quartz (grey, transparent), and feldspars (white). Most of the feldspars consist of plagioclase but there is also K-feldspar, recognizable thanks to its pale pink colors.

Coming back to the slider image above, you can see that the granodiorite is crosscut by tabular intrusions (dykes) of darker magmatic rocks: lamprophyres. A lamprophyre is a relatively rare magmatic rock where mafic minerals – in this case biotite and hornblende – constitute large crystals and feldspars are confined to the groundmass. Here in Neves, lamprophyres crosscut the granitoids often as magmatic dykes.

Large lamprophyre intrusion (dark) crosscutting the granodiorite. The sharp contact between the dyke and the surrounding granitoids is well visible.

Detail of the contact between dyke and granite. The striated surfaces are glacial striations, produced by the glacier which once covered these outcrops.

Close up of a small lamprophyre dyke, crosscutting the surrounding granodiorite. Well visible within the dyke, shiny black biotite grains constitute abundant large crystals.

A sharp cut of the granitoid structures, as the one represented by the dykes, indicates that the granodiorite was already solid when the lamprophyric magma arrived, filling the dykes, but there are situations in Neves where two magmas – a mafic magma and the surrounding granodiorite – appear to have interacted at a partially molten state, like in the case of the dyke over here:

synplutonic mafic dyke

This situation is completely different from the dykes we have seen so far. First of all, the contact between the dyke and the surrounding granodiorite is really wavy to lobate and there are veins of granodiorite injected in the mafic dyke separating it in several segments. There are lobate blobs (enclaves) of granodiorite within the mafic dyke, and finally, the granodiorite is really fine-grained and darker at the contact with the dyke. Here is a detail.

Injection veinlets of granodiorite separating segments (boudins) of mafic magma. Note the fine-grained granodiorite rim at the contact between the two rocks.

These features have been interpreted as an evidence of mingling/mixing between two different magmas when they were still in part liquid. When the mafic dyke intruded the granodiorite, likely found a still partially molten ‘dense hot soup’ of crystals and melt and the two magmas started to mingle. The dyke was dismembered in several segments (boudins) while granodioritic magma filled the spaces and invaded the mafic melt. The fine-grained, darker layers of granodiorite at the contact are likely domains where the two melts started to mix. This interaction likely occurred at the latest stages of cooling of the granodioritic intrusion, allowing to preserve a more-or-less elongated mafic dyke. But what if magma mingling occurs when the granitoids are hotter and contains a higher fraction of melt?

These are dark-colored blobs of mafic magma that are surrounded by the granodiorite. When the mafic magma arrived and found a hot granodioritic ‘soup’, the two magmas started to slowly mingle, forming rounded to lobate mafic enclaves, which are surrounded and injected by granodioritic magma. Injection structures are particularly spectacular in Neves: in many cases the injection veins are really lobate, wavy, or wriggly, indicating that the two melts where relatively fluid when this structure formed. In other cases, granodioritic veins may be even straight and very sharp.

And in this case they likely formed when the mafic blobs were nearly solid. This may happen because mafic melts usually become solid at higher temperature than granitic melts. Indeed, have you noticed how fine-grained these mafic blobs are? The rapid cooling to the temerature of the surrounding granitoids hinders, indeed, the growth of large grains.

There is no study -here in Neves – on these specific structures beyond the geological analysis and you have to bear in mind that what I describe here is based on my analysis and speculations. In any case, when you have magma mingling, usually the composition of the two magmas change because they incorporate elements coming from the other magma. In the mafic blobs, you may find structures like this one:

Do you see the white feldspars surrounding darker material? These are coronitic structures (corona = crown), in this case of plagioclase around amphibole or pyroxene. They are absolutely not investigated and therefore I am not sure if they are related to the re-equilibration of magma to more ‘granodioritic’ compositions due to mingling or if they are linked to later alpine metamorphism but… they are nice and worth showing! What’s your opinion about them? Write it in the comment section!

Acknowledgements
I have had the possibility to visit these outcrops during the EGU Summer School on Structural Analysis of crystalline rocks. I would like to thank Neil Mancktelow and Giorgio Pennacchioni for bringing me there and explaining me all the nice structures exposed in the area… and obviously the EGU for all its support to the organization of beautiful geology field trips like this one!

References and Further Reading
Cesare, B., Rubatto, D., Hermann, J., & Barzi, L. (2002). Evidence for Late Carboniferous subduction-type magmatism in mafic-ultramafic cumulates of the SW Tauern window (Eastern Alps). Contributions to Mineralogy and Petrology142(4), 449-464.

Finger, F., Frasl, G., Haunschmid, B., Lettner, H., von Quadt, A., Schermaier, A., … & Steyrer, H. P. (1993). The Zentralgneise of the Tauern Window (eastern Alps): insight into an intra-Alpine Variscan batholith. In Pre-Mesozoic geology in the Alps (pp. 375-391). Springer, Berlin, Heidelberg.

Frisch, W., Vavra, G., & Winkler, M. (1993). Evolution of the Penninic basement of the Eastern Alps. In Pre-Mesozoic geology in the Alps (pp. 349-360). Springer, Berlin, Heidelberg.

Mancktelow, N. S., & Pennacchioni, G. (2005). The control of precursor brittle fracture and fluid–rock interaction on the development of single and paired ductile shear zones. Journal of Structural Geology27(4), 645-661.

Pennacchioni, G., & Mancktelow, N. S. (2007). Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions. Journal of Structural Geology29(11), 1757-1780.

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