Separated and leached minerals from >40 samples from Eastern Pyrenean ultramafic massifs have been analysed for rare earth elements (REEs), and Sr and Nd isotope ratios. The sub-continental lithospheric mantle in this region has interacted with several different episodes of niagmatic activity since at least the Palaeozoic. The range of isotopic composition in the massifs is extreme for mantle material (87Sr/86Sr =0·7020−0·7050; 143Nd/144Nd = 0·5139−0·5121), although the dominant lithologies, spinel lherzolite and spinel websterite, are isotopically much more homogeneous. Light rare earth element (LREE)-depleted patterns occur in clinopyroxenes from protogranular and porphyroclastic spinel lherzolites. These rocks have high ɛNd and lowɛSr values (mid-ocean ridge basalt—MORB-type) and represent lithospheric mantle unaffected or only slightly affected by subsequent magmatic events. With increasing refractory nature of the peridotites, heavy rare earth element (HREE) content and ɛNd of clinopyroxene decrease, and ɛSr and (Ce/Yb)N increase. Harzburgite bands may be formed by the passage of tholeitic melts unrelated to the host peridotites. Layered pyroxenites originated as crystal segregates from tholeiitic magmas. Clinopyroxenes from garnet clinopyroxenites have bell-shaped REE patterns, but websterites have reacted with host peridotites, acquiring similar REE patterns. Layered pyroxenites generally have higher ɛSr values than spinel peridotites, and show a wide range of ɛNd, resulting in part from time-integrated Sm/Nd fractionation. Some layered pyroxenites have extremely low ɛNdvalues (−5·6 to −10·2) resembling enriched mantle 1 (EMI); others have high ɛNd values (+ 25 to +28), although many have ɛNd values similar to those of their host peridotites. Cross-cutting amphibole pyroxenite and hornblendite veins were formed by 100-Ma-old alkali basaltic magmas which have reacted with pre-existing peridotite. The metasomatism of the Caussou massif is also related to this magmatism. REE patterns in clinopyroxene and amphibole from hydrous veins and metasomatized peridotites resemble those of megacrysts from alkali basalts. Magmatic minerals in hydrous veins have homogeneous #x025B;Nd values (+ 5·7 to +7·2), whereas metasomatic minerals have slightly higher values (+ 6·5 to +9·4). The highɛSr(+ 6·4 to +7·8) of some amphiboles from Caussou may be related to crustal fluids.
La distribution des elements en trace, et des terres rares est etudiee dans les lherzolites des massifs ultrabasiques d'origine mantellique de lanzo (alpes piemontaises, italie) et de l'ariege (pyrenees, france), ainsi que dans les differents facies petrographiques associes. Elle permet de connaitre les diapirs asthenospheriques, les modalites de la fusion partielle, les mecanismes de la circulation magmatique et l'importance des interactions peridotite-magma lors de cette circulation et les mecanismes du metasomatisme mantellique alcalin dans la mesure ou les roches affectees presentent une large gamme de concentrations en terres rares, qui reproduit le domaine de variation des xenolites mantelliques remontees par le volcanisme alcalin
Research Article| August 01, 2007 Origin of the island arc Moho transition zone via melt-rock reaction and its implications for intracrustal differentiation of island arcs: Evidence from the Jijal complex (Kohistan complex, northern Pakistan) Carlos J. Garrido; Carlos J. Garrido 1Departamento de Mineralogía y Petrología & Instituto Andaluz de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Granada & CSIC, 18002 Granada, Spain Search for other works by this author on: GSW Google Scholar Jean-Louis Bodinier; Jean-Louis Bodinier 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Search for other works by this author on: GSW Google Scholar Bruno Dhuime; Bruno Dhuime 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Search for other works by this author on: GSW Google Scholar Delphine Bosch; Delphine Bosch 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Search for other works by this author on: GSW Google Scholar Ingrid Chanefo; Ingrid Chanefo 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Search for other works by this author on: GSW Google Scholar Olivier Bruguier; Olivier Bruguier 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Search for other works by this author on: GSW Google Scholar Shahid S. Hussain; Shahid S. Hussain 3Pakistan Museum of Natural History, Garden Avenue, Shakarparian, 44000 Islamabad, Pakistan Search for other works by this author on: GSW Google Scholar Hamid Dawood; Hamid Dawood 3Pakistan Museum of Natural History, Garden Avenue, Shakarparian, 44000 Islamabad, Pakistan Search for other works by this author on: GSW Google Scholar Jean-Pierre Burg Jean-Pierre Burg 4Structural Geology and Tectonics, ETH Zürich & Universität Zürich, Geologisches Institut, Leonhardstrasse, 19/LEB, CH-8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Author and Article Information Carlos J. Garrido 1Departamento de Mineralogía y Petrología & Instituto Andaluz de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Granada & CSIC, 18002 Granada, Spain Jean-Louis Bodinier 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Bruno Dhuime 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Delphine Bosch 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Ingrid Chanefo 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Olivier Bruguier 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Shahid S. Hussain 3Pakistan Museum of Natural History, Garden Avenue, Shakarparian, 44000 Islamabad, Pakistan Hamid Dawood 3Pakistan Museum of Natural History, Garden Avenue, Shakarparian, 44000 Islamabad, Pakistan Jean-Pierre Burg 4Structural Geology and Tectonics, ETH Zürich & Universität Zürich, Geologisches Institut, Leonhardstrasse, 19/LEB, CH-8092 Zürich, Switzerland Publisher: Geological Society of America Received: 19 Jan 2007 Revision Received: 12 Mar 2007 Accepted: 18 Mar 2007 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2007) 35 (8): 683–686. https://doi.org/10.1130/G23675A.1 Article history Received: 19 Jan 2007 Revision Received: 12 Mar 2007 Accepted: 18 Mar 2007 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Carlos J. Garrido, Jean-Louis Bodinier, Bruno Dhuime, Delphine Bosch, Ingrid Chanefo, Olivier Bruguier, Shahid S. Hussain, Hamid Dawood, Jean-Pierre Burg; Origin of the island arc Moho transition zone via melt-rock reaction and its implications for intracrustal differentiation of island arcs: Evidence from the Jijal complex (Kohistan complex, northern Pakistan). Geology 2007;; 35 (8): 683–686. doi: https://doi.org/10.1130/G23675A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract If the net flux to the island arc crust is primitive arc basalt, the evolved composition of most arc magmas entails the formation of complementary thick ultramafic keels at the root of the island arc crust. Dunite, wehrlite, and Cr-rich pyroxenite from the Jijal complex, constituting the Moho transition zone of the Kohistan paleo–island arc (northern Pakistan), are often mentioned as an example of high-pressure cumulates formed by intracrustal fractionation of mantle-derived melts, which were later extracted to form the overlying mafic crust. Here we show that calculated liquids for Jijal pyroxenites-wehrlites are strongly rare earth element (REE) depleted and display flat or convex-upward REE patterns. These patterns are typical of boninites and are therefore unlike those of the overlying mafic crust that have higher REE concentrations and are derived from light rare earth element (LREE)–enriched melts similar to island arc basalt. This observation, along with the lower 208Pb/204Pb and 206Pb/204Pb ratios of Jijal pyroxenites-wehrlites relative to gabbros, rejects the hypothesis that gabbros and ultramafic rocks derive from a common melt via crystal fractionation. In the 208Pb/204Pb versus 206Pb/204Pb diagram, ultramafic rocks and gabbros lie on the same positive correlation, suggesting that their sources share a common enriched mantle 2 (EM2) signature but with a major depleted component contribution for the ultramafic rocks. These data are consistent with a scenario whereby the Jijal ultramafic section represents a Moho transition zone formed via melt-rock reaction between subarc mantle and incoming melt isotopically akin to Jijal gabbroic rocks. The lack in the Kohistan arc of cogenetic ultramafic cumulates complementary to the evolved mafic plutonic rocks implies either (1) that a substantial volume of such ultramafic cumulates was delaminated or torn out by subcrustal mantle flow from the base of the arc crust in extraordinarily short time scales (0.10–0.35 cm/yr), or (2) that the net flux to the Kohistan arc crust was more evolved than primitive arc basalt. 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Fluorapatite (FAp), which occurs in various geological settings, contains rare earth elements (REE) for which unveiling the crystal chemistry is a key geochemical issue, especially for unravelling the conditions of fractionation and crystallisation.However, no consensus has been reached regarding their binding modes in FAp, with studies suggesting REE 3þ -Ca 2þ substitution at the Ca(1) site, or at the Ca(2) site, or involving adsorption mechanisms.Our contribution here is based on yttrium K-edge X-ray absorption spectroscopy (XAS), performed on two genetically contrasting minerals: a hydrothermal FAp (from Durango, Mexico), and a sedimentary phosphorite (from Morocco).The results clearly show that Y substitutes for the Ca(2) site in both FAp.However, the spectral differences observed for the sedimentary FAp (B-type carbonated-FAp) suggest a sorption model that is either i) a mixture of Y-Ca( 2) substitution and Y-adsorbed as an inner shell complex, or ii) Y-Ca(2) substitution along with carbonate groups replacing phosphate groups in the surrounding atomic shell.These models of yttrium sorption in FAp shed new light on the understanding of rare earth partitioning and enrichment processes, with major geochemical implications such as i) the identification of crystallising fluids and temperature in magmatic-hydrothermal settings, and ii) preservation of past seawater-porewater conditions in sedimentary settings.
Recent studies of mantle xenoliths and orogenic peridotite bodies have emphasized the consistent agreement between Re-Os melt-depletion (TRo) model ages for the mantle and ages of overlying and/or surrounding crust. Moreover, Re-Os isotopic investigations of sub-continental lithospheric mantle underlying non-cratonic areas suggest that these mantle segments have remained stable for long periods of time (>1 Ga). However, several important questions remain to be answered. Does the lithospheric mantle remain coupled to the overlying continental crust through major and multiple orogenic events? Is there any age stratigraphy preserved within the continental mantle root? We have carried out petrographic, trace element and Re-Os isotopic studies of six shallow mantle peridotite xenoliths sampled by alkali basalts from localities scattered throughout the French Massif Central (FMC). The FMC is an appropriate lithospheric segment for assessing the behaviour of Re-Os isotopic systematics during major orogenic and magmatic events. The non-cratonic crust of the FMC has undergone at least two main orogenies (Cadomian and Variscan) during the past 700 Ma. More recently, the FMC lithosphere has been affected by a mantle plume which induced extensive Quaternary alkali volcanism. The six samples were subdivided into two groups. The first group consists of olivine-rich harzburgites displaying a poikiloblastic microtexture. They have high equilibrium temperatures, ranging from 1150~ to 1250~ and very refractory geochemical characteristics with low A1203 and Lu contents (0.1-1.8 wt.% and 9 -20 ppb, respectively). The geochemistry of constituent silicate minerals (e.g. olivine with low mg#) as well as whole rock trace element patterns suggest that these poikiloblastic olivine-rich peridotites have experienced ' lbasaltic' melt infiltration at high melt/rock ratio. Their high equilibration temperatures may be consistent with a deep lithospheric mantle source. The second group of samples does not display a poikiloblastic microtexture, but have a porphyroclastic to protogranular microtexture. They are clinopyroxene-poor lherzolites ( 4 6 % cpx) or lherzolites (11% cpx). Equilibrium temperatures within this group range from 950-1100~ A1203 contents vary from 1.3-3.5 wt.%, and Lu contents range from 10-57 ppb. Trace element diagrams show that the cpx-poor lherzolites are L R E E enriched, whereas the lherzolites display 'depleted MORB mantle' trace element patterns. However, samples from both groups display large positive U/ Th anomalies in both whole rocks and separated clinopyroxenes. These xenoliths display a large range in Os concentrations (1.74-3.60 ppb) that is inversely correlated with indices of mantle fertility, and exhibit a large range in Os isotopic compositions (lS7Os/lS8Os = 0.1122--0.1349; 7os = --13 to +4) that is inversely correlated with Re/Os (Fig. 1). Refractory harzburgite xenoliths have the lowest Os i s o t o p i c c o m p o s i t i o n s (187Os/188Os =
Resume Les peridotites protogranulaires remontees par les basanites miocenes du Forez representent un manteau lithospherique initialement equilibre dans le facies a grenat, a une profondeur de 50 a 60 km. Elles ont subi une reequilibration dans le facies a spinelle, temoignant d'un rechauffement isobare jusque vers 1 100–1 200 °C. A ce processus est associe un enrichissement en elements a grand rayon ionique, resultant de la percolation de faibles fractions de liquides riches en volatils, voire carbonates. Ces deux processus sont attribues aux effets thermiques et chimiques de l'erosion lithospherique en bordure du panache mantellique du Massif central.
Complex multi-stage models involving silicate, hydrous and carbonate melts of distinct provenance have been invoked to explain the metasomatism observed in mantle rocks. In contrast, relatively simple models requiring polybaric crystallization of alkaline silicate melts have been proposed to explain the occurrence of veined mantle rocks. To address the spatial and temporal relationships between veins and wall-rocks, a sequence of drill cores was obtained from Lherz, France. In outcrop the vein (amphibole–garnet pyroxenite dyke) is spatially associated with hornblendite veinlets (lherzite), and proximal amphibole-bearing and distal apatite-bearing wall-rock peridotite. Considerable elemental and isotopic heterogeneity exists in these wall-rock peridotites, in many instances equivalent to, or greater than, that observed in mantle xenoliths from worldwide localities. A single stage of reactive porous flow best explains the elemental and isotopic heterogeneity in the wall-rock. In essence it is proposed that emplacement of the silicate melt (dyke) was inextricably linked to chromatographic fractionation/reaction of derivatives which led to the coexistence, in space and time, of silicate, hydrous and carbonate melts. This model elegantly and simply describes the formation of complex metasomatic aureoles around mantle veins and negates the need, in the case of basalt-hosted (and kimberlite-hosted) xenoliths, for complex multi-stage models involving several episodes of melt influx with each melt being of different provenance.