Pyrometamorphic rocks in the Molinicos Basin (Betic Cordillera, SE Spain)

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Marco E. Ciriotti
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Pyrometamorphic rocks in the Molinicos Basin (Betic Cordillera, SE Spain)

Messaggio da Marco E. Ciriotti » sab 30 nov, 2019 16:49

Referenza:
▪ Abad, I., Sánchez-Gómez, M., Reolid, M., López Sánchez-Vizcaíno, V. (2019): Pyrometamorphic Rocks in the Molinicos Basin (Betic Cordillera, SE Spain): Insights into the Generation of Cordierite Paralavas. Minerals, 9, 748; https://doi.org/10.3390/min9120748

Abstract:
A singular thermal anomaly occurred in the Molinicos Miocene, lacustrine, intramontane basin (Betic Cordillera). This gave place to vitreous vesicular materials (paralavas) and baked rocks (clinker) inside of a sequence of marly diatomites and limestones. The chemical composition of the paralavas (SiO2 = 52–57, Al2O3 ≈ 20, Fe2O3 = 10–20, K2O + Na2O < 2.5, CaO < 4.5, and MgO < 1.5, % in weight), which is very different from typical igneous rocks, and their high-T mineralogy (cordierite, sillimanite, anorthite, mullite, and high-T silica polymorphs) suggest that they formed during a pyrometamorphic event. The occurrence of dry intervals in the lacustrine depositional system, the high Total Organic Carbon contents (>4% in weight) of dark clay layers and the existence of tectonic fractures give the right context for a combustion process. Short-term heating favoured the generation of paralavas, clinker and marbles. Thermodynamic modelling constrains the onset of melting at 870–920 °C for <10 MPa at equilibrium conditions. However, the presence of tridymite and/or cristobalite in clinker and paralavas and the compositional variation in both rock types suggests that the temperature at which first melting occurred ranged between 870 °C and 1260 °C due to melt fractionation processes.
Marco E. Ciriotti

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