Tectonothermal analysis of a major unit of the Cantabrian Zone (Variscan Belt, NW Spain) using conodont color alteration index

BASTIDA, F.; GARCÍA-LÓPEZ, S.; ALLER, J; VOLDMAN, G.G.

Fourth International Conodont Symposium. Cuadernos del Museo Geominero

Instituto Geológico y Minero de España

Año: 2017; p. 229 - 234

The conodont color alteration index (CAI) is a useful parameter to analyze the transition between diagenesis and metamorphism in orogenic belts. It is cheap and easy to apply, and covers a large temperature range (from 50 to >600ºC) (Rejebian et al., 1987). As with other thermal maturity indicators that follow the Arrhenius reactions, it is time-temperature dependent, progressive and irreversible. The correlation of the conodont CAI with the Kübler Index (KI) has allowed to distinguish three prograding zones (diacaizone,ancaizone, epicaizone) in the transition diagenesis/metamorphism, with the ancaizone ranging between CAI 4?5.5 (García-López et al., 2001). This zonation is very useful in areas where limestones are dominant and corresponds approximately to the KI zonation (diagenesis, anchizone, epizone).Conodont CAI and KI methods have been applied to the Cantabrian Zone, the foreland fold and thrust belt of the Variscan orogen in NW Spain (Fig. 1). The Cantabrian Zone presents a near complete stratigraphic succession from the Cambrian to the upper Carboniferous, with important gaps in some areas, particularly in the Ponga study area. The Cantabrian Zone underwent thin-skinned tectonics with large thrust units and associated folds whose curved cartographic trend gives place to the ?Asturian arc?. The Variscan deformation developed mostly under low pressure and low temperature conditions, although very low- or low-grade metamorphism is reached in some areas. Analysis of distribution of CAI and KI values in relation to stratigraphy and structure allowed to construct a preliminary view of the tectonothermal evolution of the Cantabrian Zone (García-López et al., 2007, and references therein). In this contribution, we analyze the distribution of conodont CAI values in the Ponga unit, one of the major of the Cantabrian Zone. This study completes the CAI framework of the Cantabrian Zone by connecting the eastern units (Picos de Europa and Pisuerga-Carrión units) with the western unit (Central Coal Basin) (Fig. 1).