Albitization of the granitic basement of the guilleries and roc de frausa massifs (ne spain) in relationship to the permian - triassic palaeosurface
- Autores: Carles Fàbrega Alsina
- Directores de la Tesis: David Parcerisa (dir. tes.), David Gómez-Gras (codir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2020
- Idioma: español
- Materias:
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- Resumen
The granitic basement below the Triassic unconformity in the Guilleries and Roc de Frausa massifs (NE Iberia), presents widespread red stained facies characterized by plagioclase albitization, biotite chlhoritization, K-feldspar microclinization and quartz precipitation. These main alterations are accompanied by precipitation of secondary hematite, maghemite, Cesynchisite, calcite and apatite and replacement of monazite by secondary monazite.
The alteration forms a 150-200 m thick vertical profile beneath the regional Triassic unconformity. The most intense and pervasive alteration occurs in the upper part, close to the Triassic unconformity, and progressively decreases towards depth and becomes restricted to the fracture walls in the lowermost part. The alteration presents a large lateral continuity and can be followed along tens of kilometers. This spatial arrangement below the Triassic unconformity indicates that the alteration was probably triggered by the interaction of the granites with stable and long-lasting groundwater tables. The oxygen isotope composition of the albitized plagioclase (11-13‰), the microclinized K-felds par (12-14‰) and s econdary quartz (˜ 12‰) systematically have higher values respect to the primary counterparts and suggest that albitization developed at temperatures around 55ºC, at a depth of several hundreds of meters below surface. The dating of the microclinized K-feldspar by K-Ar and the secondary monazite by U-Th-Pbtotal occurring in the red stained albitized rocks indicates that the alteration developed during Permian - Triassic times. The zone between the albitized profile and the true palaeosurface was probably occupied by a thick regolith eroded during Early Triassic times.
All these features strongly support that this red albitized profiles record a widespread Na-alteration of the crystalline basement formed in close relationship with the Permo- Triassic palaeosurface and the salty environtments of the Permo- Triassic landscape. Similar Permian - Triassic red albitization profiles have been reported in the French Central Massif and the Polish Sudetes, pointing to the hyphotesis that this kind of albitization was a common process that affected the Variscan basement of Western Europe situated relatively close to the surface during Late Permian to Early Triassic times.
