Susana García-López

Transition from Diagenesis to Metamorphism Near the Front of the Variscan Regional Metamorphism (Cantabrian Zone, Northwestern Spain)

Distribution of paleotemperatures has been studied in Paleozoic rocks of the Variscan fold and thrust belt of the NW Cantabrian Zone (Spain) by a combined application of illite crystallinity (IC) and conodont color alteration index (CAI). Results show that the diagenetic‐metamorphic grade increases with the increase in rock age, as marked by decreasing IC values and increasing CAI values. Moreover, the CAI isogrades roughly reflect the structural traits of the area, indicating that maximum temperatures were attained during sedimentary burial previous to folding. The geothermal paleogradient obtained suggests conditions near the boundary between low and middle P/T conditions. There is evidence of at least two thermal events during the tectonothermal development of the belt. The main stage is interpreted as the result of the burial of the rocks characterized by late diagenesis conditions that occurred under a geothermal gradient of ∼35°C km−1. The narrow zone with low anchizonal conditions in the western part of the area could represent the front of the Variscan orogenic metamorphism. This stage was followed by thrusting that juxtaposed higher‐grade rocks of the internal zone (Westasturian‐Leonese Zone) over lower‐grade rocks of the external one (Cantabrian Zone), giving rise to an inverted metamorphic pattern. Subsequently, rocks in the southern part of the area were affected by contact metamorphism associated with emplacement of granitoid bodies. This late‐Variscan thermal event led to local enhancement of CAI values and to changes in the clay mineralogy by metasomatism.

Transition from diagenesis to metamorphism in a calcareous tectonic unit of the Iberian Variscan belt (central massif of the Picos de Europa, NW Spain)

Transition from diagenesis to metamorphism has been characterized in the central part of the Picos de Europa unit (Cantabrian Zone) by conodont colour alteration index (CAI), complemented with Kubler index (KI) data. This unit essentially comprises Carboniferous limestone. The study leads us to deduce two successive tectonothermal events. The first event generated a pattern with palaeotemperatures increasing towards the Pisuerga–Carrion unit, located south of the Picos de Europa unit. The present pattern is the result of an original dip to the north of the isothermal surfaces and the subsequent rising of the Picos de Europa unit along the frontal ramp of a deep Alpine thrust. This episode is interpreted as related to an extensional tectonic regime that occurred close to the Carboniferous–Permian boundary. The second event, which gave rise to thermal anomalies in the pattern of the first episode, was the result of hydrothermal processes in which fluid movement was facilitated by a dense network of fractures in the area close to the eastern section of the studied region. These processes gave rise to numerous mineralizations and have been related to a Permian extensional tectonic regime.

Conodont alteration and tectonothermal evolution of a diagenetic unit in the Iberian Variscan belt (Ponga-Cuera unit, NW Spain)

Tectonothermal analysis of a mainly carbonate unit located in the external part of the Variscan orogen in NW Spain is dealt with using the conodont colour alteration index (CAI) and the study of textures of these microfossils. The Kubler index of the illite (KI) is used as a complementary method. The area is characterized by a great uniformity in the CAI values, which in most cases are

Tectonothermal evolution of a major thrust system: the Esla–Valsurbio unit (Cantabrian Zone, NW Spain)

The tectonothermal evolution of a unit in the foreland fold-and-thrust belt of the Iberian massif is established using the conodont colour alteration index (CAI). The unit consists of two parts with different tectonothermal histories – the Esla nappe region and the Valsurbio region – separated by a synorogenic Carboniferous basin (Guardo–Valderrueda basin). The Esla nappe region evolved in diacaizonal conditions (corresponding to the diagenetic conditions of the pelites) whose palaeotemperatures were controlled by rock burial. Maximum values were reached before the emplacement of the thrust nappes, so tectonic superimposition is not registered by the CAI. Overburial due to the emplacement of the thrust units was prevented by simultaneous intense erosion. The geothermal gradient obtained for burial was c . 35 °C km −1 and the temperature reached by the older Cambrian rocks was c . 210–230 °C. The Valsurbio region was affected by an extensional tectonothermal post-orogenic event that gave rise to metamorphism with ancaizonal or epicaizonal conditions (corresponding to anchizone or epizone of the pelites). The most common maximum palaeotemperatures reached in this event fall within the range 305–415 °C, although higher palaeotemperatures could be reached locally as a consequence of contact metamorphism. This event gave rise to subhorizontal cleavage that cuts the main Variscan folds. Coal rank data indicate an increase in maximum palaeotemperatures eastwards from the Esla nappe region to the Valsurbio region through the Guardo–Valderrueda basin.

The genus Parastriatopora SOKOLOV, 1949 (Tabulata) in the Lower Devonian of Argentina : palaeobiogeographic implications

Abstract Some specimens of tabulate corals from Lower Devonian of the Precordillera Central in San Juan province (Argentina) are described. These branches are assigned to the genus Parastriatopora Sokolov , 1949 and they are found close to Parastriatopora gigantea Knod , 1908 . Other material, probably conspecific with this, has been reported in the bibliography such as Favosites argentina Thomas , 1905 . The age of this finding is discussed and some palaeobiogeographic considerations on their ocurrence in South America are proposed.

Conodont colour alteration indices (CAI) of Upper Ordovician limestones from the Iberian Peninsula

Conodont colour alteration index (CAI) data in Upper Ordovician rocks from several areas of the Variscan domain in the Iberian Peninsula indicate conditions ranging from diagenesis to low-grade metamorphism. In most of the areas, where studies using other indicators, such as illite crystallinity (IC) or where vitrinite reflectance are lacking, the CAI method has permitted a preliminary estimation of the metamorphic grade. In the Almadén syncline (Central-Iberian Zone), where IC studies are available, the thermal conditions inferred from CAI data agree with those obtained by the IC method. In the Puertollano–Almuradiel syncline, the thermal interval obtained primarily from fluid inclusions (270–370°C) overlaps considerably with that obtained from CAI data (180–340°C). In general, cleavage in rocks is present in anchizonal or epizonal conditions, whereas in diagenetic conditions with CAI ≥ 2.5, cleavage is scarce. The conodont texture changes with increasing metamorphism, and apatite recrystallisation appears in general with CAI ≥ 5. Variation of CAI values within a single sample and/or within short stratigraphic distances observed at several localities is due to hydrothermal activity.

Gondolellidae (Conodonta), del Carbonífero Superior de la Zona Cantábrica (N. de España). Sistemática e implicaciones paleotérmicas

Resumen Los hallazgos de gondolelidos en el Carbonifero Superior son escasos. Este trabajo se ocupa del estudiode tres especies de los generos Gondolella y Neogondolella encontrados en tres secciones de la Zona Cantabrica (N. de Espana). La fauna consiste en elementos pectiniformes de Gondolella laevis Kossenko & Kozitskaya , Gondolella nov. sp. 1 y Neogondolella clarki ( Koike ). Tanto G. Laevis como N. clarki se conocen en Norteamerica y la antigua URSS. La segunda de estas especies, ha sido descrita originalmente del Japon. Los valores de CAI de estos elementos y de otros procedentes de las mismas unidades estructurales permiten discutir las condiciones termicas a las que han estado sometidas las rocas de donde proceden.

Tectonothermal analysis of a major unit of the Cantabrian Zone: the Ponga unit (Variscan belt, NW Spain)

The paleothermometric evolution of the Ponga unit at the core of the Cantabrian Zone, the arcuate foreland fold and thrust belt of the Variscan orogen in NW Spain, is analyzed using the conodont Color Alteration Index and the Kübler Index of illite. The results indicate that the distribution of the thermal maturity patterns is independent of the stratigraphy and the structure of the unit. These include the occurrence of dominant diagenetic values and very low-to-low-grade metamorphism in the southern area. The main thermal event is interpreted to be related to hydrothermal fluids ascending through late-Variscan extensional faults. The anomalous high heat flow in the southern sector of the Ponga unit is part of a previously identified large band in the Cantabrian Zone, where the common occurrence of low-grade metamorphic rocks, hydrothermal dolostones, and ore mineralizations is related to an extensional orogenic episode that started near the Carboniferous–Permian boundary, soon after the closure of the Asturian arc.