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 wasc. 35 °C km−1and the temperature reached by the older Cambrian rocks wasc. 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.

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 Kübler 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 <2, indicating diagenetic conditions. In spite of the low CAI values, textures show great variety and were mainly originated by diagenetic processes of apatite dissolution and precipitation. The conodonts underwent a long period of heating (probably from the Pennsylvanian to the Cenozoic) to low temperatures (<100°C) to reach the low CAI values measured. Assuming a normal geothermal gradient, these temperatures required an overburden <3 km that in part was due to burial and in part to tectonic superimposition. Minor local anomalies in the CAI values and some textural alterations, related to dissolution and precipitation of authigenic minerals, could be due to epithermal activity that gave rise to various ore deposits in the studied area mainly during Permian times.

Conodont palaeothermometry of contact metamorphism in Middle Ordovician rocks from the Precordillera of western Argentina

The Yerba Loca Formation (Middle–Upper Ordovician), exposed in the Western Precordillera, San Juan, Argentina, is made up of clastic–carbonate turbidites, and basic–ultrabasic rocks. It is affected by regional Siluro-Devonian very low-grade metamorphism that locally reaches greenschist facies. At Ancaucha creek, 45 conodont samples were taken from two sections that include 30 to 50 m thick sills. In order to analyse the thermal alteration patterns produced by these intrusive bodies, conodont Colour Alteration Index (CAI) is contrasted with optical petrography and X-ray diffraction analyses of clay minerals. The intrusions are dated as post-Darriwilian (Da2), as determined by conodont biostratigraphy of the host rock, which indicates theParoistodus horridusSubzone of theLenodus variabilisZone. The distribution of CAI values defines a thermal aureole of about 2.5 times intrusion thickness that prevailed over the later very low-grade metamorphism. Metasomatism at Ancaucha creek is recorded by CAI values of 4 to 7, particularly restricted to a few layers close to the intrusions, as indicated by conodont textures and rock fabric. One-dimensional thermal computer simulation conforms to empirical data indicating temperatures greater than 600 °C for the contact zone, although it points out slightly narrower thermal aureoles. The clay mineral assemblage of most of the analysed samples (chlorite, illite, smectite and I/S mixed-layers) is complex and probably derives from several superimposed processes, thus representing non-equilibrium assemblages. In turn, KI values (0.27–0.32) indicate anchizone metamorphism, in agreement with regional CAI values of 4; consequently, the occurrence of smectite and I/S probably resulted from retrograde diagenesis processes.

Superposition of tectono-thermal episodes in the southern Cantabrian Zone (foreland thrust and fold belt of the Iberian Variscides, NW Spain)

The combined use of the illite “crystallinity” Kübler index (KI) and the conodont colour alteration index (CAI) has revealed the existence of three thermal episodes in an area affected by thin-skin tectonics, close to the internal zones of the Variscan orogen in NW Spain. In the southernmost part of the study area, the first episode gave rise to a regional syntectonic Variscan metamorphism. The associated deformation involves the development of a slaty cleavage, which is mainly recognized in Precambrian rocks. Towards the foreland, the syntectonic metamorphism disappears and only an incipient burial metamorphism, giving rise to anchizonal conditions in the basal part of the thrust units, is observed. Another metamorphic episode occurred close to the Carboniferous-Permian boundary in an extensional tectonic regime. This metamorphism is restricted to the northern part of the study area, where it reached anchizonal or epizonal conditions. It is associated with a subhorizontal or moderately north-dipping cleavage and can be considered as a late-Variscan episode. The last thermal episode occurred during the Permian. It was produced by heat flow due to hydrothermal fluids, whose migration was favoured by faults. The effects of this episode are irregularly distributed, and they are apparent in the unconformable Stephanian rocks in which anchizonal or epizonal conditions were reached. It is interpreted as a post-Variscan episode.

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 Kübler 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–Carrión 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.

Conodonts and corals in kimberlite xenoliths confirm a Devonian seaway in central Ontario and Quebec

Eighteen samples containing sedimentary rock xenoliths were obtained from cores drilled into eight Mesozoic kimberlite pipes in the Kirkland Lake area, Ontario, and in Ontario and Quebec near Lake Timiskaming. Nine samples from five pipes contained fossils that were used for age determinations. These fossils are Middle or Late Ordovician graptolites, inarticulate brachiopods, and conodonts; Silurian and (or) Devonian conodonts; Early Devonian colonial corals; a Devonian stromatoporoid; and Early to Middle Devonian conodonts. Regionally, conodonts are unaltered (conodont colour alteration index, CAI 1). Conodont CAI values from the xenoliths are elevated (CAI 2), and a few conodonts have surface colour changes, suggesting hydrothermal alteration. Age determinations allow stratigraphic correlation between xenoliths and Paleozoic outcrops. For the Ordovician and Silurian samples, correlations are made to exposures in the nearby Lake Timiskaming outlier. For the Devonian samples, the closest possible correlative outcrops are about 300 km away. These fossils provide the first physical evidence of a connection between a Lake Timiskaming "basin" and other Ontario basins during at least part of the Devonian. These strata persisted at least until the Mesozoic before they were removed by erosion.

Simultaneous use of thermal indicators to analyse the transition from diagenesis to metamorphism: an example from the Variscan Belt of northwest Spain

The transition from diagenesis to very low-grade metamorphism has been characterized by conodont colour alteration index (CAI), illite crystallinity (IC), clay-mineral assemblages and rock fabric in a transect (Cape Peñas–Cape Torres) located in the Cantabrian Zone (northwest Spain). IC values show a northwest to southeast change from epizonal to diagenetic values. CAI values show a general congruence for low- and high-grade areas. This correlation indicates that CAI values for the epizone are >5.5, the anchizone 5.5≥CAI>4, and the diagenetic zone ≤4. Cleavage develops in incompetent rocks with CAI values greater than 3, mainly by pressure solution. Temperature increases towards the inner parts of the orogen (towards the northwest) and is considered to be related to the tectonic superposition caused by thrusting and folding. A gradient of 35°C/km is inferred for the transect. In the southeastern sector of the section there are several thermal anomalies that are attributed to the presence of epithermal fluids.