Abstract. An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40 Ar-39 Ar (amphibole, micas) and U-Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos-Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos-Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been unroofed or exhumed.
International audienceIn their late stages of evolution, peraluminous granitic melts exsolve large amounts of fluidswhich can modify the chemical composition of granitic whole-rock samples. The niobium/tantalum (Nb/Ta) ratio is expected to decrease during the magmatic differentiation of graniticmelts, but the behavior of both elements at the magmatic-hydrothermal transition remainsunclear. Using a compilation of whole-rock geochemical data available in the literature, wedemonstrate that fractional crystallization alone is not sufficient to explain the distribution ofNb-Ta in most peraluminous granites. However, we notice that most of the granitic samplesdisplaying evidence of interactions with fluids have Nb/Ta < 5. We propose that the decreaseof the Nb/Ta ratio in evolved melts is the consequence of both fractional crystallization andsub-solidus hydrothermal alteration. We suggest that the Nb/Ta value of ~5 fingerprints themagmatic-hydrothermal transition in peraluminous granites. Furthermore, a Nb/Ta ratio of ~5appears to be a good marker to discriminate mineralized from barren peraluminous granites
The evidence for macroscopic life during the Palaeoproterozoic era (2.5-1.6 Gyr ago) is controversial. Except for the nearly 2-Gyr-old coil-shaped fossil Grypania spiralis, which may have been eukaryotic, evidence for morphological and taxonomic biodiversification of macroorganisms only occurs towards the beginning of the Mesoproterozoic era (1.6-1.0 Gyr). Here we report the discovery of centimetre-sized structures from the 2.1-Gyr-old black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, which we interpret as highly organized and spatially discrete populations of colonial organisms. The structures are up to 12 cm in size and have characteristic shapes, with a simple but distinct ground pattern of flexible sheets and, usually, a permeating radial fabric. Geochemical analyses suggest that the sediments were deposited under an oxygenated water column. Carbon and sulphur isotopic data indicate that the structures were distinct biogenic objects, fossilized by pyritization early in the formation of the rock. The growth patterns deduced from the fossil morphologies suggest that the organisms showed cell-to-cell signalling and coordinated responses, as is commonly associated with multicellular organization. The Gabon fossils, occurring after the 2.45-2.32-Gyr increase in atmospheric oxygen concentration, may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5 Gyr later, in the Cambrian explosion.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.