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The granite rocks of the Salamanca town of Juzbado were formed between 320 and 340 million years ago. They are deformed, precisely, because they are older than the fault in the ground on which they sit.
The data are consistent with other dating of the surroundings and reveal that the geological origin of the area dates back to the collision that led to the supercontinent Pangea, according to an international investigation in which the University of Salamanca and the Museo de la Falla, located in this municipality, have participated.
The results are published in the scientific journal Geogaceta.
The study is part of the dating work of igneous rocks in the northwest of the peninsula that Gabriel Gutiérrez-Alonso, scientist at the Geology department of the Salamanca academic institution, has been carrying out for years.
On this occasion, the material used was a few meters from the geological museum of Juzbado. "It comes from a rock that was almost at the door," says the researcher.
Scientists are able to determine the age of granite thanks to one of the minerals that compose it, zircon. "It is usually abundant, but this time we were not lucky, because from 25 kilos of rock we only obtained seven zircons of about 100 microns in size," he explains.
However, it was enough to achieve results that "reflect when the granite was formed and what rocks it came from."
The samples were analyzed in a laboratory in Dresden (Germany), where they were aged by laser ablation. This technique consists of firing the laser on the material and study the relationship of uranium and lead isotopes containing.
This elements change their isotopic ratio as a function of time elapsed since their formation, which allows dating them fairly accurately. "It is like a geological clock," says the researcher.
According to the results obtained, the youngest zircon would indicate that this granite is 340 million years old. However, as we have very few samples of this mineral, "we cannot be sure that there are no more recent zircons, from 320 million years ago", which, in principle, would be consistent with other dating of this area, known in geology as Domo del Tormes.
"The surrounding granites are 320 million years old and all contain 340 million-year-old zircons," says Gutiérrez-Alonso. However, quite nearby, in the Zamora town of Ricobayo, the granite is dated at 340 million, so either of the two possibilities may be true. "We need to crush more rocks to confirm this," he adds.
Deformed rocks from Juzbado to Penalva do Castelo
The researchers have another essential piece of information: the fault that runs from Juzbado to Penalva do Castelo (Portugal), a 160-kilometer-long fracture in the terrain that has 308 million years.
This lets you know that the granites of the place are older, because the fault changed their original shape and made them acquire their current shape, more elongated. Although it seems to us that the rocks cannot change because they are very hard, in reality they are deformed and these granites were already there when it happened ”, he explains.
The great geological event that marked the formation of these rocks and what is today the soil of western Castilla y León is known as Orogeno Varisco. “About 350 million years ago there was a continental collision that resulted in the supercontinent Pangea”, Comments the geologist from the University of Salamanca.
“Everything there is now has to do with what happened at that time, the geology that we observe and the conditions of the natural environment, why Juzbado is at a high point and why the river goes where it goes ”, he highlights.
More information for the Museum of the Falla in Juzbado
One of the objectives of these studies is increase the content the Museum of the Falla. "It is the only geology museum dedicated to a fault and it is nourished, precisely, by these scientific works," says its director, Jerónimo Jablonski.
With more than 2,000 visits in 2018, mostly from schoolchildren, its objective is to bring the world of geology closer to the population and for this “it is good that we closely follow scientific advances. For this reason, we are aware of what laboratories and universities do and we try to support it, for example, receiving interns every year ”, he highlights.
The Museum of the Falla It explains how geology has left its mark on the landscape and even on the facades of the town, where ornamental rocks are not lacking. Science and culture come together in the activities it organizes, such as geological routes. From now on, visitors will also know that a rock in the door was used to learn about the history of the ground they walk on.
«U-Pb geochronolgy of the deformed Juzbado Granite (Salamanca, NW Spain)«. Gabriel Gutiérrez-Alonso, Alicia López-Carmona, Javier Fernández-Suárez, Jerónimo Jablonski, Mandy Hofmann and Andreas Gärtner. Geogaceta, 64 (2018), 163-166.