Multi-stage metamorphism recorded in crustal xenoliths from Permian dykes of the region of Mrirt (Moroccan Central Massif)

The Permian magmatic rocks from Morocco contain crustal xenoliths that sample the Variscan crust in a context of widespread magmatism. A series of such xenoliths was collected in Permian dykes of the Central Massif, in the region of Mrirt. The metapelitic xenoliths are silica poor to intermediate (44 < SiO2 <57 wt%) and alumina-rich (17 < Al2O3 < 34 wt%) and are notably enriched in some HFSE (Nb, Ta, Ti) and some transition elements (Cr, V, W, Ni). Their petrographic evolution depicts a multi-stage evolution from an early, subsolidus, metamorphic history related to regional metamorphism, of which biotite, garnet and sillimanite are the witnesses toward a late thermal evolution coeval with the entrapment in the magma, marked by pervasive partial melting and development of peritectic spinel and cordierite together with K-feldspar and ilmenite. The overall presence of corundum, which relates to the high Al2O3 content, accounts for an initial stage of partial melting and magma escape, prior to the entrapment in the magma. Textural and chemical observation suggests further xenolith digestion and melt flux from the xenoliths toward the magma during ascent. Thermodynamic modelling allowing the determination of the pressure-temperature history of each xenolith shows that the initial pressure varies from ca. 1.5 to ca. 6 kbar, which, considering lithostatic pressure, corresponds to sampling depths of ca. 5–25 km. The temperatures recorded by the parageneses coeval with partial melting in the presence of spinel are in the range 800–900 °C whatever the pressure, which accounts for rapid heating due to thermal equilibration with the magma. U–Th/Pb dating of monazite by EMPA gives a poorly resolved Permian age of 293 ± 25 Ma for metamorphism in the xenoliths, in accordance with the stratigraphic age of the host rock. The pressure conditions are similar to those recorded for regional metamorphism in the other Variscan outcrops in Morocco, namely the Jebilet, the Rehamna and the Aouli-Mibladen granitic complex, while the maximal temperatures are much higher. The xenoliths thus appear as typical of the Late Variscan geological evolution of the deep crust in the Moroccan Mesetas, where abundant magmatism was responsible for local crustal heating of a crust previously affected by regional metamorphism

Multi-stage metamorphism recorded in crustal xenoliths from Permian dykes of the region of Mrirt (Moroccan Central Massif)

The Permian magmatic rocks from Morocco contain crustal xenoliths that sample the Variscan crust in a context of widespread magmatism. A series of such xenoliths was collected in Permian dykes of the Central Massif, in the region of Mrirt. The metapelitic xenoliths are silica poor to intermediate (44 < SiO2 <57 wt%) and alumina-rich (17 < Al2O3 < 34 wt%) and are notably enriched in some HFSE (Nb, Ta, Ti) and some transition elements (Cr, V, W, Ni). Their petrographic evolution depicts a multi-stage evolution from an early, subsolidus, metamorphic history related to regional metamorphism, of which biotite, garnet and sillimanite are the witnesses toward a late thermal evolution coeval with the entrapment in the magma, marked by pervasive partial melting and development of peritectic spinel and cordierite together with K-feldspar and ilmenite. The overall presence of corundum, which relates to the high Al2O3 content, accounts for an initial stage of partial melting and magma escape, prior to the entrapment in the magma. Textural and chemical observation suggests further xenolith digestion and melt flux from the xenoliths toward the magma during ascent. Thermodynamic modelling allowing the determination of the pressure-temperature history of each xenolith shows that the initial pressure varies from ca. 1.5 to ca. 6 kbar, which, considering lithostatic pressure, corresponds to sampling depths of ca. 5–25 km. The temperatures recorded by the parageneses coeval with partial melting in the presence of spinel are in the range 800–900 °C whatever the pressure, which accounts for rapid heating due to thermal equilibration with the magma. U–Th/Pb dating of monazite by EMPA gives a poorly resolved Permian age of 293 ± 25 Ma for metamorphism in the xenoliths, in accordance with the stratigraphic age of the host rock. The pressure conditions are similar to those recorded for regional metamorphism in the other Variscan outcrops in Morocco, namely the Jebilet, the Rehamna and the Aouli-Mibladen granitic complex, while the maximal temperatures are much higher. The xenoliths thus appear as typical of the Late Variscan geological evolution of the deep crust in the Moroccan Mesetas, where abundant magmatism was responsible for local crustal heating of a crust previously affected by regional metamorphism

How do social status and tree architecture influence radial growth, wood density and drought response in spontaneously established oak forests?

AbstractKey messageDuring the past decades, a multitude of oak stands have spontaneously established across the pine-dominated landscapes of the French Landes de Gascogne. Yet their future performance under modern climate change is unknown. We show that coppiced, dominant trees are most prepared to cope with drought episodes, displaying higher basal area increment and lower sensitivity to extreme events.ContextForest stands dominated by pedunculate oak (Quercus robur L.) have spontaneously established across the pine-dominated landscapes of the French Landes de Gascogne. These oak stands are typically unmanaged and unsystematically coppiced, resulting in mixtures of single- and multi-stemmed (coppiced) trees.AimsTo determine the ability of spontaneous oak forest stands to face climate change–related hazards, by analysing differences in growth (tree-ring width and basal area increment—BAI), wood density and climate sensitivity depending on their tree architecture (single- vs multi-stemmed trees) and their social status in the forest.MethodsWe exhaustively cored 15 oak stands (n = 657 trees). We compared stand characteristics and climate sensitivity between tree architectures considering two sampling designs, either all sampled trees (the exhaustive sampling) or those with a dominant status (dominant sampling). At the tree level, we used linear mixed effects models to compare wood density and growth between tree architectures and the trees’ social status within the canopy layer (dominant- vs non-dominant trees).ResultsMulti-stemmed trees exhibited higher wood density than single-stemmed trees for diameters > 30 cm. Dominant multi-stemmed trees showed lower sensitivity to extreme events (pointer years), higher BAI but lower annual growth rates than dominant single-stemmed trees.ConclusionDominant multi-stemmed trees are potentially the most prepared ones to cope with increasing soil water deficit following drought episodes, at least during the first 60 years of the life of the tree. The vulnerability to face harsher climate conditions for Q. robur stands can be misled when using a dominant sampling design

Insect – Tree Interactions in Thaumetopoea pityocampa

The pine processionary moth is, by far, the most important insect defoliator of pine forests in Southern Europe and North Africa, both in terms of its temporal occurrence, geographic range and socioeconomic impact. Monitoring and pest management actions are therefore required on a regular basis, to ensure the detection, evaluation and mitigation of potential risks to forest and public health. However, we still lack some of the basic knowledge required for relevant analyses of the risk posed by the pine processionary moth. Pest risk is defined as a combination of three components: (1) pest occurrence, which depends on the spatiotemporal dynamics of populations; (2) plant vulnerability to the pest, resulting in a certain amount of damage; and (3) the socioeconomic impact of damage, depending on the potential value of the plants damaged (Jactel et al. 2012). The population dynamics of the processionary moth has been extensively studied, in particular within the context of climate change (see Battisti et al. 2014, Chap. 2, this volume). Several studies have recently addressed the question of tree and forest vulnerability to pine processionary attacks but a comprehensive review of evidence was missing. This is the first objective of this chapter. In particular we were interested in a better understanding of the ecological mechanisms responsible for the host tree selection, at both the species and individual tree levels. In a second part we show that pine susceptibility to the pine processionary moth could be reduced by improving forest diversity at different spatial scales. In the last part of this chapter we provide quantitative estimate of the growth losses caused by defoliations of the pine processionary moth. Altogether this information paves the way for quantitative risk analyses on pine processionary moth infestations based on forest growth models