Eric Gloaguen

U-Pb LA-ICP-MS dating of apatite in mafic rocks: Evidence for a major magmatic event at the Devonian-Carboniferous boundary in the Armorican Massif (France)

Abstract Apatite is a ubiquitous accessory mineral found in most magmatic rocks and is often the only U-bearing mineral available to date mafic rocks because primary zircon and/or baddeleyite are not present. In this paper, U-Pb LA-ICP-MS dating of apatite was applied to seven different dike and sill samples of dolerite from the Variscan belt of Brittany (Armorican Massif, western France). These dolerites, which are characterized by a within-plate tholeiite geochemical signature, are organized in several dense swarms across the belt. Their geochemical compositions are homogeneous although they intrude a large geographical area subdivided into several domains each characterized by different tectonic-metamorphic settings. Their emplacement ages were so far poorly constrained due to the difficulty to date these mafic rocks using either the 40Ar/39Ar or the U-Pb methods on classical minerals like mica, plagioclase, or zircon. Although the closure temperature of apatite is lower than the emplacement temperature of the magma, physical models show that the time needed to solidify and cool these mafic dikes and sills below the apatite closure temperature is basically of the order of 100 years or less. Consequently, the U-Pb dates obtained on apatite can be interpreted as the emplacement ages for these mafic intrusions. Our results demonstrate that, in all cases, the apatite grains do carry enough radiogenic Pb to be dated by in situ U-Pb analyses and yield a 207Pb-corrected mean age of 363.4 ± 5.8 Ma. These results reveal the existence of a major and short-lived magmatic event in the Variscan belt of Brittany during the Devonian-Carboniferous transition, a feature further highlighted by field evidence. Beyond the geological implications of these results, U-Pb LA-ICP-MS dating of apatite appears to represent an ideal tool to date small size mafic intrusions.

Syn-thinning pluton emplacement during Mesozoic extension in eastern Mongolia

[1]xa0This paper documents relationships between deformation and magmatic activity that occurred in the central part of eastern Mongolia during late Mesozoic continental-scale NW-SE extension. Two coarse-grained, biotite-bearing, syn-tectonic intrusions are described. The Nartyn granite that extends over an area greater than 30 by 10 km was emplaced within low-grade metasediments and shows a weak pervasive, magmatic fabric reworked by solid-state deformation along its margins. The northwestern roof of the granite is marked by a normal shear zone, the Choyr Shear Zone, characterized by top-to-the-NW motions. The shear zone is overlain by the Choyr Basin, which is filled with unmetamorphosed continental sedimentary rocks of early Cretaceous ages. From structural and geochronological data, we propose that the Nartyn massif was emplaced as a flat laccolith-type intrusion at ca. 136–130 Ma during crustal thinning. The Altanshiree granite, located ∼140 km east of the Nartyn granite, is a syn-kinematic pluton of similar age (134–128 Ma), also emplaced during crustal thinning. In the Nartyn and Altanshiree areas, extension implies pervasive crustal thinning, combined with limited exhumation. These areas are different from classical metamorphic core complexes, where strong strain localization along detachments induces exhumation of hot middle to lower crust. Results also suggest that early Cretaceous syn-extensional intrusions are an important feature of the tectonic history of eastern Mongolia.

Global Lithium Resources and Sustainability Issues

Abstract This chapter provides data and information on the factors underpinning the current lithium demand/supply balance and its long-term evolution from a sustainable development perspective. Many issues outlined in this chapter are also applicable to other rare metals of strategic importance to advanced economies, including for the transition toward a “green,” low-carbon economy. After a warning about the issues related to data availability and reliability, it starts with an analysis of the current lithium demand through a presentation of the main lithium uses and an estimation of the future demand that could result from these uses. It then presents the supply side, by providing information on lithium minerals and carriers (brines), lithium deposit types, and their respective importance, with an estimate of the currently known resources, reserves, and production processes. Indications of capital expenditure and of operating costs related to the main production processes are given. Further on, issues related to Li recycling are discussed. In the concluding part, several factors that will determine the long-term demand/supply balance (2025–2030 and beyond) are discussed, highlighting the need for a mix of policy decisions, governance, and scientific innovations if the objective of a long-term balance is to be met.