Éditeur(s) : HAL CCSD Elsevier Elsevier Masson
Résumé : Thalattosuchia was a diverse clade of marine crocodylomorphs known from the Early Jurassic to the Early Cretaceous. Recent studies have hypothesized that their extinction was two-phased: (1) habitat loss near/at the Jurassic-Cretaceous boundary heavily reduced their morphofunctional diversity, particularly in Europe, while (2) climate change and a shift in marine fauna during the Early Cretaceous (either at the Valanginian-Hauterivian boundary or during the early Hauterivian) finished off the already stressed clade. Unfortunately, the Cretaceous fossil record of thalattosuchians is poor, with only one putative “teleosaurid” specimen and approximately ten metriorhynchid specimens. Here we re-describe the youngest known teleosaurid from the Cretaceous (Valanginian of south-eastern France). Originally considered to be a teleosaurid (possibly Steneosaurus), we demonstrate that it belongs to Metriorhynchidae, and a newly discovered subclade, Plesiosuchina. It differs from Plesiosuchus in the pattern of tooth enamel ornamentation and the variation in dentary alveoli size. Referring this specimen to Metriorhynchidae means there are no definitive Cretaceous teleosaurid specimens. Furthermore, it suggests that both durophagous and piscivorous teleosaurids became extinct at the end of the Jurassic. Interestingly, this is the fourth metriorhynchid lineage known to cross the Jurassic-Cretaceous boundary. As such, it would appear that the two thalattosuchian families responded very differently to the lowering sea levels at the end of the Jurassic: teleosaurids possibly became extinct, while metriorhynchids were seemingly unaffected.
ISSN: 0753-3969

hal-01122803
https://hal.archives-ouvertes.fr/hal-01122803
DOI : 10.1016/j.annpal.2014.01.002

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Madeira Sn-Nb-Ta deposit corresponds to the albite-enriched facies of Madeira granite (Madeira Suite) that is part of a NE-SW alignment of three granitic bodies. Structural analyses suggest that the albite-enriched granite emplacement and development of related structures occurred in a different tectonic setting than earlier facies of the granitic bodies in the NE-SW alignment and that the structure was dominated by a N-NE-trending shortening related to a left-lateral motion along a NE-SW corridor within a Riedel shear. Three types of structures (flat-lying, near vertical fractures, and geodes/miaroles) concentrated the last magmatic-related fluid in the albite-enriched granite. U-Pb in situ zircon dating of the albite-enriched granite yielded an age of 1,822 ± 22 Ma. The other granitic plutons of the Madeira Suite, Água Boa granite (topaz-granite facies), and Europa per-alkaline granite formed at 1,825 ± 14 Ma, 1,831± 11 Ma, and 1,839 ± 10 Ma, respectively. The granite bodies of the Madeira Suite occur in an area occupied by a cauldron complex composed of volcanic sequences (Iricoumé Group) and associated A-type granites (Mapuera Suite), and their formation was controlled by the reactivation of the cauldron structures and the pre-existing structures. The gap between the cauldron complex and the Madeira Suite is no greater than 35 Ma. The geological evolution was reanalyzed from the perspective of an entire extensional environment where three successive episodes of A-type magmatism and basin formation (Urupi Formation) occurred. The data and interpretations of this work imply crucial constraints on the petrologic models proposed for the albite-enriched granite and, consequently, on the genesis of the Madeira deposit
ISSN: 0301-9268

insu-00939888
https://hal-insu.archives-ouvertes.fr/insu-00939888
https://hal-insu.archives-ouvertes.fr/insu-00939888/document
https://hal-insu.archives-ouvertes.fr/insu-00939888/file/BastosNeto-PrecambrianResearch-2014.pdf
DOI : 10.1016/j.precamres.2013.12.021

Éditeur(s) : HAL CCSD
Résumé : International audience
The Madeira deposit (Sn, Nb, Ta) at Pitinga, Amazonia, in Brazil, is associated with the albite-enriched facies of the A-type Madeira granite (~1,820 Ma). Fluorine (cryolite), Y, REE, Li, Zr, and Th are potential by-products of the disseminated ore. We studied the xenotime from the core albite-enriched (CAG), transitional albite-enriched granite (TAG) and pegmatitic albite-enriched granite (PAG). In all these rocks, xenotime is magmatic, has high HREE contents (0.37 to 0.54 atoms per formula unit; LREE are not detected), and low U, Th and Ca content. The CAG xenotime is the richest in REE, followed by that in the PAG and TAG units. Fluorine was detected in all xenotime crystals from CAG (up to 5.10%) and from PAG (up to 1.40%) and in most crystals from TAG (up to 0.68%). Where the xenotime is richer in F, the Na and Si increase, the P, Ca, and the effectiveness of a thorite-type substitution decrease. Fluorine controls the ratio REE/Y; the richer the xenotime in F, the richer it is in REE, in particular Er and Yb. The cell parameters a and c are significantly shortened in F-rich xenotime. There is no evidence of OH in the structure. We contend that fluorine substitutes for O to form PO3F tetrahedra, as in bobdownsite. The formation of the PO3F tetrahedra in the xenotime accounts well for the compatibility between the crystallographic characteristics and the chemical composition.
The Canadian Mineralogist

insu-00805907
https://hal-insu.archives-ouvertes.fr/insu-00805907
DOI : 10.3749/canmin.50.6.1453