- Lecturer: Arnaud Villaros, Researcher, Institute des Sciences de la Terra, CNRS, Orleans, France
- Date: Sep, 18, 2018 12:00 am
- Place: Sala d'Actes de l'Institut de Ciències de la Terra Jaume Almera (ICTJA)
The formation and growth of the continental crust is an important question in understanding the evolution of the Earth. Presently, we are still unable to assess whether the volume of continental crust increases, is stable or decreases. Estimating the rates of such processes remains a challenge for Earth scientists. Crustal growth can be summarized by the transformation of mantle material into a granitic rock. This can be easily achieved by the crystallisation of a mantle derived magma that leads to the formation of a granitic residual melt. Consequently, studies focusing on crustal growth estimate growth rates by dating granitic material (or derived from granites, e.g. zircons). Unfortunately, not all granites results from differentiation. Melting of the continental crust also forms granites. On the field, granitic rocks commonly have a wide compositional range. In order to determine the rate of crustal growth it is necessary to determine the volume of magma that participate to crustal growth and the volume of magma that is the expression of crustal rejuvenation (i.e. crustal melting). The evolution of magmas sources prior to melting or the mingling and mixing of magmas alter the geochemical record of crustal growth. Other processes may limit the expression of crustal growth. For instance, magma underplating, commonly considered as a trigger for crustal melting, may as well buffer crustal melting by releasing low a(H2O) fluids (e.g. CO2-rich fluids). Consequently, it lowers crustal rejuvenation compare to crustal growth. Similarly, a network of crustal melt within the crust acts as a barrier that limits the rise of mantle magma. As a result, the expression of crustal growth is reduced compare to crustal rejuvenation. Deciphering crustal growth and crustal rejuvenation is not an easy task. Depending on the context, crustal growth favours or inhibits the expression of crustal rejuvenation. In the same way, crustal rejuvenation may reduce the expression of crustal growth. Because magmas interaction is complex and that entire crust sections are not always outcropping, it can not be done by simply studying rock compositions or zircons. On the contrary, it necessitates a detail petrological study to unravel crustal evolution.