The Guildhall Steps

Source: own photo

The Guildhall building in Derry is an interesting and impressive building.  When looking at the steps leading up to the entrance, we can observe the result of a rather interesting geological phenomena. However, before we take a closer look at the geological uniqueness that is evident in the stairs, let's determine what stone the steps are made of.

Diorite

The stone from which the stairs are made of is a plutonic rock. This is evidenced by crystals that are visible to the naked eye. The magma had quite a bit of time to solidify, and the crystals had plenty of time to grow as the magma cooled. And since we can observe and admire the crystals in the stones of the step, we can also determine what minerals this rock consists of.
At first glance, the rock of the stairs stands out is the mixture of darker and lighter colored minerals. The lighter minerals are felsic plagioclase feldspar minerals and the dark ones are mafic biotite and hornblende minerals. Such mineral composition tells us that the rock we are dealing with is diorite.

The mineral composition of diorite is due to the mixture of mafic and felsic minerals. This can tell us a lot about the areas where it forms. Its intermediate  composition between granite and gabbro is direct evidence that it forms in areas where there has been significant volcanic or tectonic activity, especially in the areas where oceanic and continental tectonic plates meet. In the areas where oceanic and continental plates meet also the process of magma mixing occurs. The diorite from the steps of the Guildhall has further evidence of mixing of felsic and mafic magma types. This evidence are mafic microgranular enclaves (MMEs)

What are Mafic Microgranular Enclaves (MMEs) and how are they formed?

Mafic Microgranular Enclaves (MMEs) are the result of mixing two types of magma. It is an example of magma differentiation (changes in the chemical composition of magma), which can take place in the form of various processes. In this case, however, we are dealing with mixing, which occurs when two magma chambers containing two different types of magma come together and the magma penetrates from one chamber to the other due to pressure and other factors.

Source: Own work

In the chamber into which the mafic type of magma penetrates, the chemical and mineral composition of the magma changes. If the magma solidifies before the mixing process is completely finished, we will be able to see enclaves of darker mafic magma that has penetrated the chamber. Such congealed enclaves that have not had time to dissolve and had not been completely absorbed are called mafia enclaves (MMEs).
Mafic enclaves are areas of rock that are surrounded by parent rock with different compositions and origin. It is simply the result of unfinishe process of magma mixing.

What are Veins in rocks and how they form?

Besides MMEs we can observe another interesting feature at the ground zero. We are talking here about hydrothermal veins. Hydrothermal veins are formed in all types of rock when mineral-containing fluids seep through cracks. This usually happens over thousands or even millions of years. Mineral veins are also younger than the rock in which they occur. Their dimensions vary in width and length from millimeters, centimeters to several meters.

Mineral veins form near fault zones where rocks have been fractured. The minerals that make up veins are typically deposited from hydrothermal fluids that travel under pressure through cracks and gaps in the rock. These liquids are rich in dissolved minerals, sometimes even rare metals.

Various factors play an important role in the formation of veins. These include fluid pressure, temperature and time. The greater these factors, the wider and longer veins are created. Veins form in cracks and sometimes can even intersect each other.

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Questions:

1. What is magma differentiation?
2. How are Mafic Microgranular Enclaves (MMEs) formed?
3. Take a closer look at the Guildhall steps. What size MME did you find? What is the largest MME?
4. Take a closer look at the Guildhall steps. How common is the occurance of the veins in the rock?
5. Were you able to find a vein which crosses MME as well as a diorite matrix rock at the same time? What does this tell you?
6. What is the evidence that veins can intersect each other?

Sources:
https://www.sciencedirect.com/science/article/abs/pii/S0024493717303067
https://opentextbooks.uregina.ca/geolmanual/chapter/overview-of-igneous-rocks/
http://hyperphysics.phy-astr.gsu.edu/hbase/Geophys/meltrock.html
Hugues Raimbourg, Kristijan Rajič, Benjamin Moris-Muttoni, Vincent Famin, Giulia Palazzin, Donald Fisher, Kristin Morell, Saskia Erdmann, Ida Di Carlo, Clément Montmartin, Quartz Vein Geochemistry Records Deformation Processes in Convergent Zones, Geochemistry, Geophysics, Geosystems, 05 March 2021
The Mechanics of Vein Formation, Stephen Taber, 1918