Bernard Laumonier

On the non-existence of a Cadomian basement in southern France (Pyrenees, Montagne Noire): implications for the significance of the pre-Variscan (pre-Upper Ordovician) series

The deepest Hercynian metamorphic terrains in the Pyrenees and in the nearby Montagne Noire are made up of medium-grade orthogneisses and micaschists, and of high-grade, often granulitic, paragneisses. The existence of a granitic-metamorphic Cadomian basement and of its sedimentary Lower Paleozoic cover was advocated from the following main arguments: (i) a supposed unconformity of the Lower Cambrian Canaveilles Group (the lower part of the Paleozoic series) upon both granitic and metamorphic complexes; (ii) a ca . 580 Ma U-Pb age for the metagranitic Canigou gneisses. A SE to NW transgression of the Cambrian cover and huge Variscan recumbent (“penninic”) folds completed this classical model. However, recent U-Pb dating provided a ca . 474 Ma, early Ordovician (Arenigian) age for the me-tagranites, whereas the Vendian age (581 ± 10 Ma) of the base of the Canaveilles Group was confirmed [Cocherie et al. , 2005]. In fact, these granites are laccoliths intruded at different levels of the Vendian-Lower Cambrian series. So the Cadomian granitic basement model must be discarded. In a new model, developed in the Pyrenees and which applies to the Montagne Noire where the orthogneisses appear to be Lower Ordovician intrusives too, there are neither transgression of the Paleozoic nor very large Hercynian recumbent folds. The pre-Variscan (pre-Upper Ordovician) series must be divided in two groups: (i) at the top, the Jujols Group, mainly early to late Cambrian, that belongs to a Cambrian-Ordovician sedimentary and magmatic cycle ; the early Ordovician granites pertain to this cycle; (ii) at the base, the Canaveilles Group of the Pyrenees and the la Salvetat-St-Pons series of the Montagne Noire, Vendian (to earliest Cambrian?), are similar to the Upper Alcudian series of Central Iberia. The Canaveilles Group is a shale-greywacke series with rhyodacitic volcanics, thick carbonates, black shales, etc. The newly defined olistostromic and carbonated, up to 150 m thick Tregura Formation forms the base of the Jujols Group, which rests more or less conformably on the Canaveilles Group. The high-grade paragneisses which in some massifs underlie the Canaveilles and Jujols low- to medium grade metasediments are now considered to be an equivalent of the Canaveilles Group with a higher Variscan metamorphic grade; they are not derived from metamorphic Precambrian rocks. So, there is no visible Cadomian metamorphic (or even sedimentary) basement in the Pyrenees. However, because of its age, the Canaveilles Group belongs to the end of the Cadomian cycle and was deposited in a subsident basin, probably a back-arc basin which developed in the Cadomian, active-transform N-Gondwanian margin of this time. The presence of Cadomian-Panafrican ( ca. 600 Ma) zircon cores in early Ordovician granites and Vendian volcanics implies the anatexis of a thick (> 15 km?) syn-Cadomian series, to be compared to the very thick Lower Alcudian series of Central Iberia, which underlies the Upper Alcudian series. Nd isotopic compositions of Neoproterozoic and Cambrian-Ordovician sediments and magmatites, as elsewhere in Europe, yield Paleoproterozoic ( ca. 2 Ga) model-ages. From the very rare occurrences of rocks of this age in W-Europe, it can be envisionned that the thick Pyrenean Cadomian series lies on a Paleoproterozoic metamorphic basement. But, if such a basement does exist, it must be “hidden”, as well as the lower part of the Neoproterozoic series, in the Variscan restitic granulites of the present (Variscan) lower crust. So a large part of the pre-Variscan crust was made of volcano-sedimentary Cadomian series, explaining the “fertile” characteristics of this crust which has been able to produce the voluminous Lower Ordovician and, later, Upper Carboniferous granitoids.

Emeralds in the Eastern Cordillera of Colombia: Two tectonic settings for one mineralization

Colombian emeralds are formed through a hydrothermal-sedimentary process. On the western side of the Eastern Cordillera, the deposits are linked by tear faults and associated thrusts developed during a compressive tectonic phase that occurred at the time of the Eocene-Oligocene boundary, prior to the major uplift of the Cordillera during the Andean phase (middle Miocene). On the eastern side of the Cordillera, emerald mineralization occurred earlier, at the time of the Cretaceous-Tertiary boundary, during a thin-skinned extensional tectonic event linked to evaporite dissolution. This event predates the Andean phase, during which this part of the chain was folded and thrust over the Llanos foreland.

The Canigou orthogneisses (Eastern Pyrenees, France, Spain): an Early Ordovician rapakivi granite laccolith and its contact aureole

From new field and single zircon geochronological data, we reinterpret the structure of the Canigou orthogneisses (Eastern Pyrenees) as an Early Ordovician rapakivi granite laccolith. This laccolith is surrounded by a contact metamorphic aureole predating the Hercynian tectono-metamorphic events. This aureole consists of migmatites at the contact with unmelted orthogneisses, of skarns in the carbonate beds and of retrogressed Al-rich porphyroblasts over some hundred metres beyond the contact. The metallogenic activity (Fe, F) observed around the massif might be related to the laccolith emplacement.

Comment to “Latest-Cretaceous/Paleocene karsts with marine infillings from Languedoc (South of France); paleogeographic, hydrogeologic and geodynamic implications by P. J. Combes et al.”

1 Laboratoire des Mécanismes et Transferts en Géologie, 39 allées Jules-Guesde, 31062 Toulouse cedex 4, France. 2 INRAP, ZA les Champs Pinsons, 13, rue du Négoce, 31650 Saint-Orens-de-Gameville, France. 3 37, avenue de Cousse, 31750 Escalquens, France. 4 École des Mines de Nancy, Département des Sciences de la Terre, LAEGO-Mines, 54042 Nancy, France. 5 École Nationale Supérieure des Mines de Paris, Géosciences, 35, rue Saint-Honoré, 77305 Fontainebleau, France.

Comment on Duplex at the lateral tip of a thrust fault: the "La Cagalière" example (NE Pyrenees, France) by C. Souque et al.

The Corbieres are the weakly deformed foreland of the Eastern Corbieres Thrust Sheet, NE of the Pyrenees. Alpine deformation is somewhat intense in the Lagrasse area, where a thrusted fold, the Lagrasse Fold, is known for long [1, 2]. Though outcropping conditions are good, some disagreement persists about the exact geometry of the structure (number of thrusts, place of the small but complex Cagaliere Duplex) and the timing of thrusting relative to the overturned N60-70 directed Lagrasse Fold [1, 3, 4 hereafter LAU95, 5, 6, 7 hereafter SOU03]. (i) Ellenberger et al . [1] proposed a one-thrust model, with a front and a rear thrusts being parts of a single thrust (the Lagrasse Thrust); the place of the Cagaliere Duplex is unclear (not the same on the map and on the cross-section). (ii) LAU95, followed by Frizon de Lamotte et al . [5], advocated a three-thrusts model, the front thrust (la Bade Thrust) being distinct from, and later than the rear thrust (Col Rouge Thrust); the Cagaliere Duplex is linked to the la Bade Thrust (Figs. 5 and 6). (iii) SOU03 developped a different, intermediate two-thusts model, with an early la Bade Thrust disappearing to the SW in the Cagaliere Duplex and a late, out-of-sequence Col Rouge Thrust extending to part of the front thrust and hiding the la Bade Thrust (Figs. 3 and 4A). However, SOU03, in their paper focusing on the Cagaliere duplex development, do not explain why the LAU95’model is wrong and must be given up. Consequently, we intend to comment briefly on the SOU03’model which, according to us, is weakened by several inconsistencies and errors, and hence cannot be a valid substitute for our (LAU95’) model. 1. SOU03 use an inaccurate stratigraphic section (Fig. 2 left, to be compared with the true section to the right): (i) Rognacian and Thanetian limestones thickness is overestimated, hence the thickness of the mainly marly Lagrasse Formation is underestimated; (ii) Rognacian is supposed to rest on the Paleozoic basement, but actually it overlies a thick (tens and possibly hundreds meters), mainly marly Begudian; Begudian rests directly on the Paleozoic basement, as more to the North, or possibly on Upper Cretaceous, as more to the East [2]. In both cases, in the Lagrasse area, the basal floor of the thrust system is not situated in the basement (SOU03) but in its sedimentary cover (LAU95), and there is no evidence of thick-skinned tectonics in the Lagrasse area. 2. Concerning the trusts geometry . A key point is the local but prominent, almost vertical Cagaliere Fault (Fig. 1), which separates the overturned Thanetian Limestone of the la Bade crest and the one of the la Cote crest (all these structures are located in the overturned, short limb of the Lagrasse Fold). If this fault was a late, high angle fault, it should continue to the SW and the NE, which is not. For this reason, all authors have tried to interpret this fault as a portion of a thrust, but, in this case and at this very place, this thrust must be near vertical. LAU95’solution was to introduce a thrust (Cagaliere Thrust) earlier than the Lagrasse Fold and folded by it (Fig. 5 and 6); in this model, the Cagaliere Fault is a ramp duplicating the Thanetian Limestone. For Ellenberger et al . [2] and SOU03, the Cagaliere Fault belongs to their Col Rouge Thrust; however, no vertical fault appears on their cross-sections (Fig. 4A, redrawn from SOU03’Fig. 3B): the Cagaliere Fault is drawn near horizontal, which does not conform to the field observations. Fig. 4B (redrawn from the Fig. 4A but with the true stratigraphic section and the true dips) is a more realistic cross-section consistent with the SOU03’model: it can be seen that the alleged Col Rouge