No abstract

As operational concept, geodiversity implies a measurement and its application narrowed to a given spatial area, allowing the identification of clusters for prioritization and planning purposes. This study proposes a first geodiversity index assessment for French Guiana, a French Oversea territory located within the Guiana Shield (South America). Almost entirely covered by the Amazon rainforest associated with an exceptional biodiversity, French Guiana appears as an international conservation and land-planning challenge facing multiple issues (e.g. urban, agricultural and industrial growth, forest management, mining planning).Geodiversity assessment is performed through a grid-based approach. A geodiversity index is calculated as the sum of four sub-indices (lithodiversity and unlithified diversity, mineral diversity, hydrodiversity, geomorphodiversity). The index calculation is based on the number of entities within each grid-cell. Spatial correlation is used to identify geodiversity clusters and finally the index is aggregated at different spatial units relevant for land-planning (e.g. municipalities, hydrographic sectors, areas of ecological interest, the Guiana Amazonian Park and the Departmental Mining Plan).The results show that the geodiversity index is mainly controlled by lithodiversity and that high geodiversity clusters are located along the gold-bearing greenstone belts crossing the territory. However, spatialized data concerning geodiversity are still inadequate or unavailable to perform proper operational assessments in French Guiana. Furthermore, based on qualitative examples, we discuss that the assessment of diversity alone might not be always enough for geoconservation nor land-planning perspectives. It is pivotal to account for the geonfunctionalityi.e. the contribution of geodiversity to the functioning of socio-ecological systemsand its interaction with anthropic activities.

Summary Due to a too sparse permanent seismic coverage during the last decades, the crustal structure of western France and the surrounding regions is poorly known. In this study, we present a 3-D seismic tomographic model of this area obtained from the analysis of 2-year continuous data recorded from 55 broad-band seismometers. An unconventional approach is used to convert Rayleigh wave dispersion diagrams obtained from ambient noise cross-correlations into posterior distributions of 1-D VS models integrated along each station pair. It allows to avoid the group velocity map construction step (which means dispersion curve extraction) while providing meaningful VS posterior uncertainties. VS models are described by a self-adapting and parsimonious parameterization using cubic Bézier splines. 1268 separately inverted 1-D VS profiles are combined together using a regionalization scheme, to build the 3-D VS model with a lateral resolution of 75 km over western France. The shallower part of the model (horizontal cross-section at 4 km depth) correlates well with the known main geological features. The crystalline Variscan basement is clearly associated with positive VS perturbations while negative heterogeneities match the Mesocenozoic sedimentary basins. At greater depths, the Bay of Biscay exhibits positive VS perturbations,which eastern and southern boundaries can be interpreted as the ocean-continent transition. The overall crustal structure below the Armorican Massif appears to be heterogenous at the subregional scale, and tends to support that both the South-Armorican Shear Zone and the Paris Basin Magnetic Anomaly are major crustal discontinuities that separate distinct domains.

Twenty three serpentinite samples collected from five outcrops in south Brittany (Audierne near Quimper, Champtoceaux near Nantes), western France, were studied using optical microscopy, electron microprobe (EMP), inductively-coupled optical emission spectroscopy (ICP-OES), inductively-coupled plasma mass spectrometry (ICP-MS), and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). Their bulk-rock major and trace element contents recomputed on an anhydrous basis are broadly characteristic of mantle-derived peridotites, as are the covariation trends of inert elements despite evidence of serpentinization-related remobilization of some fluid-mobile elements (FME; e.g., Ca, La, Ce, Sr, U). One outcrop near Champtoceaux shows fertile lherzolite compositions and chondritenormalized rare earth element patterns consistent with low (5-7%) degree of mantle partial melting. The other four occurrences are harzburgites displaying higher partial melting degrees (15-25%). Regardless of their degree of fertility, our South Armorican peridotites bear evidence of high-temperature melt/fluid -rock metasomatic interaction yielding to an overall enrichment in highly incompatible elements (HIE; Cs, Rb, Ba, Th, U, Pb, La). Hydrous modal metasomatism has been identified in both lherzolites and harzburgites. The lherzolites reacted with HIE-enriched small-volume fluids at P = 1.5-2 Gpa for T >900°C that produced a Ti-poor pargasite. The Audierne harzburgites were pervasively refertilized by alkali-rich hydrous melts that precipitated K-and Cr-rich pargasite. Taken as a whole, South Armorican peridotites record a great diversity of protoliths, from supra-subduction zone ophiolites (Audierne) to arcfore arc provenance. 20 25 30 35 40 45 50 55 60 trace elements by inductively-coupled plasma mass spectrometry (ICP-MS) and in-situ analyses of preserved minerals by laser ablation-ICPMS (LA-ICPMS). It is well known that geochemical signatures recorded in serpentinites are a complex combination of protolith partial melting, melt-rock interaction history, and serpentinization conditions (Deschamps et al., 2013; Cooperdock et al., 2018 and references therein). Some major elements (CaO, SiO 2 , MgO), and light REE (LREE) can be mobilized during serpentinization and should be interpreted with caution ( Paulick et al., 2006; Snow and Dick, 1995) while strong enrichment in fluid-mobile elements (FME; e.g., B, Li, Cl, As, Sb, Pb, U, Cs, Sr, Ba) may occur. All this secondary remobilization may alter the petrogenetic features of pre-serpentinization processes.Our first aim was therefore to unravel any potential alteration of protolith mineral assemblages by serpentinizing fluids. Our second goal was to provide tighter constraints on putative protolith compositions by identifying petrogenetic processes that typically occur in the upper mantle, such as partial melting events and fluid/melt-rock interactions (metasomatism).

Knowledge of the Guiana Shield evolution during the Gondwana break-up is key to a better understanding of craton dynamics and margin response to transtensional opening. To improve this knowledge, we investigated the dynamics and thermal evolution of French Guiana, using several low-temperature thermochronology methods applied to basement rocks, including apatite and zircon (U-Th)/He and apatite fission tracks. Inverse modelling of results allows us to reconstruct the Phanerozoic thermal history of French Guiana margin and to give a preview of the Guiana Shield evolution. Three main events are inferred: firstly, a long-term period of relative stability since ~1.2 Ga, with no strong evidence for any erosional or burial event (>5-7 km); secondly, a heating phase between ~210 and ~140 Ma consistent with the Central Atlantic Magmatic Province-related event. Finally, an exhumation phase between ~140 and ~90 Ma, triggered by the Equatorial Atlantic opening, brought samples close to the surface (<40°C).