The Upper Pennsylvanian is characterised by significant climatic fluctuations and marks the expansion of complex dryland ecosystems. However, these ecosystems are much less well-documented than the coal-forming environments of the Carboniferous, since plant fossils preserve poorly in dryland environments. One of the markers of Upper Pennsylvanian mesophytic to xerophytic environments is Lesleya Lesquereux. Here we report a new assemblage of Lesleya from the Westphalian D of the La Mure Basin (Isère, France). Nearly 68 specimens were recovered, belonging to two taxa, L. cf. grandis and L. belledonnensis sp. nov. In addition, the informal taxon Lesleya Ře 3, described previously from the Westphalian B–D of the Bohemian Massif (Czech Republic), is reassigned to L. belledonnensis sp. nov. Lesleya belledonnensis sp. nov. occurs in plant fossil assemblages dominated by giant leaves of Cordaites and rare taxa exhibiting xerophilous characters both in France and Czech Republic. Moreover, the extensive sampling at Vaulnaveys-le-Bas allows us to identify successive stages of taphonomic degradation of Lesleya leaves and provided insights into the taxonomic and paleoecological significance of margin-related leaf characters of the genus Lesleya.
This study presents new U–Pb–Hf isotopic data and zircon ages from the Ediacaran to Ordovician Ötztal Complex of the Eastern Alps in Austria to provide new constraints on the evolution of the northern Gondwana active margin in the “proto-Alpine” realm. The multistage tectonic evolution of the complex started with siliciclastic deposition presumably in an accretionary wedge that may have lasted from ca. 600 Ma to ca. 517 Ma. The age spectra are dominated by Neoproterozoic zircon grains indicating that the complex was most likely sourced from the Arabian–Nubian shield, with a contribution of older Proterozoic and Archean grains from the more westerly Saharan metacraton. The deposition was partly overlapping in time with Cambrian to Early Ordovician mafic magmatism that formed either as mafic underplate below the accretionary wedge or outboard, being later accreted as part of the lower plate. The wedge was then intruded by compositionally diverse granitoids from ca. 500 Ma until ca. 440 Ma. By comparing the Ediacaran and Early Paleozoic evolution of the Ötztal Complex with originally more westerly Cadomian-basement terranes (e.g., those now found in the Bohemian Massif), we concluded that the Cenerian orogeny was generally younger in the proto-Alps than elsewhere in the former Cadomian belt. This was possibly due to a significantly curved geometry of the northern Gondwana margin and/or due to an eastward ridge–trench–transform triple point migration. Arrival of a warmer part of the oceanic plate then may have caused mantle melting, mafic underplating, and voluminous granitic plutonism in the forearc, perhaps finally terminated by ridge–trench interaction and slab break-off.
Serpentinization significantly alters orogenic peridotites, but the processes controlling the magnetic fabric development remain incompletely understood, particularly the roles of mineralogy, fluid availability, and deformation. This study addresses this gap by combining petrography, rock magnetic measurements, and numerical modeling on samples representing moderate to complete serpentinization. Magnetic properties indicate a mix of paramagnetic serpentine and ferrimagnetic magnetite, with anisotropy of magnetic susceptibility fabrics showing spatial patterns linked to serpentinization degree and structural setting. Microstructural observations reveal that serpentine and magnetite grow parallel or at characteristic angles to primary fabrics, while deviations reflect post-serpentinization deformation. Numerical modeling confirms that magnetic fabric orientation depends on the interaction of primary mineral crystallographic preferred orientation and topotactic serpentinite growth. These results demonstrate the complex interplay of mineralogy, fluid-rock interaction, and deformation in controlling magnetic fabric evolution during serpentinization in orogenic peridotites.
Chromium (Cr) and its isotopic composition (δ53Cr) are emerging as valuable proxies in both environmental and paleoenvironmental studies, yet their redox behaviour is complicated by the presence of organic ligands. This study performed laboratory-controlled batch dissolution and flow-through column experiments on soils/sediments to examine the effects of low-molecular-weight organic acids on Cr mobility and isotopic fractionation. Our results reveal that citric and oxalic acids can enhance the dissolution of Cr(III) under strongly reducing conditions where relatively high levels of ferrous iron are present, highlighting an overlooked pathway of Cr mobilisation. The Cr isotope variability (δ53Cr = −1.39‰ to +0.11‰) in batch leachates and column effluents is not directly driven by redox changes; the very light δ53Cr signatures likely reflect effects of incongruent dissolution and kinetic isotope fractionation. Ligand-bound Cr(III) can potentially be transported across environmental interfaces with distinct isotopic signature, complementing conventional views of Cr and Cr isotope biogeochemical cycling. Citric and oxalic acids can enhance the dissolution of chromium(III) under strongly reducing conditions where relatively high levels of ferrous iron are present, according to laboratory-controlled batch dissolution and flow-through column experiments on soils and sediments.
In zircon, transgressive textures cutting oscillatory zoning and associated chemical modifications are usually attributed to fluid-mediated coupled dissolution-precipitation (CDP). Here, we show an example of melt-mediated CDP in zircon from metagranite, with consequences on calculated ages and chemical composition. Zircon oscillatory zoning is commonly blurred and truncated by embayments and channels of replaced zircon with irregular and sharp boundaries. These are interpreted as replacement fronts and may be spatially associated with micro-porosity. Some superimposed replacement fronts indicate repeated CDP and porosity healing. Micro-porosity and inclusions are associated with replaced domains and thus are epigenetic. We highlight the necessity of combined high-resolution back-scattered electron imaging, otherwise the CDP textures may be overlooked in cathodoluminescence images. Inclusions of Ph–Grt–Ttn–Ap–Qz–Bt are compatible with the matrix assemblage Grt−Ph−Bt−Ttn−Kfs−Pl−Qz ± Rt ± Ilm, which equilibrated at c. 15−17 kbar, 690–740 °C with partial re-equilibration to c. 12 kbar, 680 °C. Since the conditions were above the wet solidus, the zircon changes happened through melt-mediated CDP. The replaced domains show an overall decrease of REE, Y, Th and Th/U, P, a large variation in Yb/Gd, shallower Eu anomaly, and increased U, and Hf. The chemical changes indicate a tendency of zircon purging trace elements during reequilibration with migrating melt. Spots with high LREE are probably due to the effect of micro-inclusions. Based on textures, inherited zircons are c. 540–600 Ma, protolith granite is c. 507–496 Ma, and CDP replacement occurred down to c. 335 Ma. But, the melt-mediated CDP resulted in a smear of mostly concordant, but spurious spot dates between inherited, protolith and modified domains, spanning > 200 Myr.
