Petrogenesis of Mafic Garnet Granulite in the Lower Crust of the Kohistan Paleo-arc Complex (Northern Pakistan): Implications for Intra-crustal Differentiation of Island Arcs and Generation of Continental Crust

We report the results of a geochemical study of the Jijal and Sarangar complexes, which constitute the lower crust of the Mesozoic Kohistan paleo-island arc (Northern Pakistan). The Jijal complex is composed of basal peridotites topped by a gabbroic section made up of mafic garnet granulite with minor lenses of garnet hornblendite and granite, grading up-section to hornblende gabbronorite. The Sarangar complex is composed of metagabbro. The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like, light rare earth element (LREE)-enriched REE patterns similar to those of island arc basalts. Together with the Jijal garnet granulite, they define negative covariations of LaN, YbN and (La/Sm)N with Eu* [Eu* = 2 × EuN/(SmN + GdN), where N indicates chondrite normalized], and positive covariations of (Yb/Gd)N with Eu*. REE modeling indicates that these covariations cannot be accounted for by high-pressure crystal fractionation of hydrous primitive or derivative andesites. They are consistent with formation of the garnet granulites as plagioclase-garnet assemblages with variable trapped melt fractions via either high-pressure crystallization of primitive island arc basalts or dehydration-melting of hornblende gabbronorite, provided that the amount of segregated or restitic garnet was low (30 km (equivalent to c. 1·0 GPa), together with the hot geotherms now postulated for lower island arc crust, should cause dehydration-melting of amphibole-bearing plutonic rocks generating dense garnet granulitic roots in island arcs. Dehydration-melting of hornblende-bearing plutonic rocks may, hence, be a common intracrustal chemical and physical differentiation process in island arcs and a natural consequence of their maturation, leading to the addition of granitic partial melts to the middle-upper arc crust and formation of dense, unstable garnet granulite roots in the lower arc crust. Addition of LREE-enriched granitic melts produced by this process to the middle-upper island arc crust may drive its basaltic composition toward that of andesite, affording a plausible solution to the ‘arc paradox' of formation of andesitic continental-like crust in island arc setting

Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle

Subduction-transform edge propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, northwest Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with lithology. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790–1165 ºC) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second-phase particles. In the coarse-grained peridotites, well-developed olivine crystal-preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010] fiber and the [010] and [001] axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths.This research has been supported by the Agencia Estatal de Investigación (grant nos. CGL2016-75224-R, CGL2016-81085-R and CGL2015-67130-C2-1-R), the Junta de Andalucía research groups RNM-131 and RNM-148, and the International Lithosphere Program (grant no. CC4-MEDYNA)

Geological mapping of carbonatites and related ores from the Oulad Dlim massif (Dakhla Province, Morocco) using remote sensing, portable X-ray fluorescence, and mineralogical data

Abstract and Poste

Les massifs basiques et ultrabasiques de Najac (Aveyron) et de Chamrousse ( Alpes) - Etude géologique et géochimique

Ce travail est composé de trois chapitres, qui correspondent respectivement à trois publications : -le premier présente le cadre géologique et structural du massif de Najac (Bodinier et Burg,1981) et discute ses relations avec l 'ensemble des séries cristallophyliennes du Rouergue occidental; -le deuxième concerne l'étude géochimique de ce massif, menée afin de définir son affinité géochimique et d'élucider les relations pétrogénétiques entre les différents termes qui le constituent (Bodinier,1981); -enfin,le troisième chapitre concerne l'étude strictement géochimique du massif de Chamrousse , avec des objectifs identiques à ceux envisagés pour le massif de Najac .Pour une meilleure compréhension du problè, les données géologiques mises aimablement à notre disposition par F. Carme y sont présentées succintement (Bodinier et al.,1981) . Le but de cette étude, les résultats acquis et les implications majeures sont résumés dans les premières pages.pas de résum

Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites, French Pyrenees

International audienceIn the Pyrenees, the lherzolites nowhere occur as continuous units. Rather, they always outcrop as restricted bodies, never more than 3 km wide, scattered across Mesozoic sedimentary units along the North Pyrenean Fault. We report the results of a detailed analysis of the geological setting of the Lherz massif (central Pyrenees), the type-locality of lherzolites and one of the most studied occurrences of mantle rocks worldwide. The Lherz body is only 1.5 km long and belongs to a series of ultramafic bodies of restricted size (a few metres to some hundreds of metres), occurring within sedimentary formations composed mostly of carbonate breccias originating from the reworking of Mesozoic platform limestones and dolomites. The clastic formations also include numerous layers of polymictic breccias reworking lherzolitic clasts. These layers are found far from any lherzolitic body, implying that lherzolitic clasts cannot derive from the in situ fragmentation of an ultramafic body alone, but might also have been transported far away from their sources by sedimentary processes. A detailed analysis of the contacts between the Lherz ultramafic body and the surrounding limestones confirms that there is no fault contact and that sediments composed of ultramafic material have been emplaced into fissures within the brecciated carapace of the peridotites. These observations bear important constraints for the mode of emplacement of the lherzolite bodies. We infer that mantle exhumation may have occurred during Albian strike-slip deformation linked to the rotation of Iberia along the proto-North Pyrenean Fault

Comportement des radiochronomètres Rb/Sr, Ar/Ar et Sm/Nd au cours du métamorphisme (Cas des éclogites de l'arc de Bergen (Norvège))

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Caractérisation géochimique de l'arc du Kohistan (Nord Pakistan) (implication pour l'initiation et l'évolution de subduction océanique)

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Géochimie du niobium et du tantale (distribution et fractionnement de ces deux éléments dans les différents réservoirs terrestres)

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Sr-Nd-Pb isotopic compositions of peridotite xenoliths from Spitsbergen: Numerical modelling indicates Sr-Nd decoupling in the mantle by melt percolation metasomatism

Several spinel peridotite xenoliths from Spitsbergen have Sr-Nd isotopic compositions that plot to the right of the 'mantle array' defined by oceanic basalts and the DM end-member (depleted mantle, with low 87 Sr/86 Sr and high 143Nd/144 Nd). These xenoliths also show strong fractionation of elements with similar compatibility (e.g. high La/Ce), which cannot be produced by simple mixing of light rare earth element-depleted peridotites with ocean island basalt-type or other enriched mantle melts. Numerical simulations of porous melt flow in spinel peridotites applied to Sr-Nd isotope compositions indicate that these features of the Spitsbergen peridotites can be explained by chemical fractionation during metasomatism in the mantle. 'Chromatographic' effects of melt percolation create a transient zone where the host depleted peridotites have experienced enrichment in Sr (with a radiogenic isotope composition) but not in Nd, thus producing Sr-Nd decoupling mainly controlled by partition coefficients and abundances of Sr and Nd in the melt and the peridotite. Therefore, Sr-Nd isotope decoupling, earlier reported for some other mantle peridotites worldwide, may be a signature of metasomatic processes rather than a source-related characteristic, contrary to models that invoke mixing with hypothetical Sr-rich fluids derived from subducted oceanic lithosphere. Pb isotope compositions of the Spitsbergen xenoliths do not appear to be consistently affected by the metasomatism.SCOPUS: ar.jinfo:eu-repo/semantics/publishe