1,721,143 research outputs found
Upper mantle evolution during continental rifting and ocean formation: evidences from peridotite bodies of the Western Alpine -Northern Apennine system.
No full textLigurian peridotites and ophiolites: from rift to ocean formation in the Jurassic Ligure-Piemontese basin
No full textEvolution of the lithosphere beneath eastern Paraguay: geochemical evidence from mantle xenoliths in the Asuncion-Nemby nephelinites
No full textPetrological constraints on the evolution of mantle peridotites during passive lithospheric rifting: examples from ancient and modern oceanic basin.
No full textMafic and ultramafic pods with eclogitic relics from the Proterozoic Nagssugtoqidian mobile belt of East Greenland
No full textUpper mantle evolution during continental rifting and ocean formation: evidences from peridotite bodies of the Western Alpine - Northern Apennine system
No full textEvolution of mantle peridotities during passive continental rif¬ting: examples from ancient and present situations
No full textSAPPHIRINE-BEARING AMPHIBOLE GABBRO FROM THE MANTLE SEQUENCE OF FINERO (SOUTHERN ALPS): PETROGRAPHY, GEOCHEMISTRY AND GEODYNAMIC CONTEXT
No full textA late swarm of sapphirine-bearing amphibole gabbroic veins discordantly crosscut the main layering of the Finero phlogopite-peridotite massif, Western Italian Alps. Sapphirine locally occurs in a melanocratic zone placed between a leucocratic gabbroic band, forming the central part of the intrusion, and the host peridotite. The melanocratic zones are observed on both sides of the leucocratic gabbroic vein and consist of (i) an outer orthopyroxene-rich zone along the host peridotite and (ii) an inner amphibole-rich zone placed along the leucocratic gabbroic band side. Sapphirine either overgrows spinel or occurs as isolated inclusion within large amphiboles in the amphibole-rich melanocratic zone. Spinels without sapphirine envelopes also microtexturally co-exist with independent sapphirine grains. EMPA and LA-ICP-MS analyses of minerals from the melanocratic and leucocratic bands evidence significant differences in terms of both major and trace elements in the composition of the parent melts. In particular, the amphiboles in the melanocratic zones show higher TiO2, Na2O and K2O, M-HREE and HFSE than those in the leucocratic ones. The Al2O3 content of amphibole and the Fo in olivine of the host peridotite are significantly higher and lower, respectively, than those in other Finero peridotites far from the amphibole gabbroic veins. Moreover, amphiboles from the host peridotite are characterised by LREE-enriched convex-upward patterns significantly different with respect to those documented in literature for the Finero phlogopite-peridotites [1]. Mineral assemblages and mineral chemistry in both the melanocratic zone and the host peridotites are interpreted as the result of different stages of melt migration associated with melt-rock interaction. In particular, the major and trace element compositions of the amphiboles from the wall peridotite suggest that during an early stage, possibly before the opening of the conduits, the peridotite suffered porous flow migration of a melt more enriched in REE with respect to those forming the gabbroic bands. The crystallisation of the melanocratic bands is related to a second stage, in which the precipitation of large amphiboles was accompanied by a strong reaction between host peridotite and melt flowing into the conduit that determined the complete substitution of peridotite olivine with orthopyroxene at the peridotite-vein contact. A third stage was characterised by with the precipitation of the leucocratic band, associated to a further enlargement of the vein. Petrographic survey highlights that parent melt of leucocratic zone reacted with the minerals of the melanocratic one, inducing sapphirine growth around spinels. The genetic relationships occurring between the parent melts of the melanocratic and leucocratic zones must be yet established. Working hypotheses consider the parent melt of the leucocratic zone either related to a late injection or a residual differentiate after precipitation of melanocratic band in the frame of flow differentiation process. Quantitative considerations suggest that the selective addition of Al-rich phases, like amphiboles and micas, to a basalt can determine the large Al/Si ratios required for sapphirine precipitation. Modelling results indicate that the eutectic T of Finero phlogopite-peridotite is <1000°C and that the first partial melts are saturated in corundum. Thus, it is proposed that injection of basaltic melts triggered the partial melting of limited volumes of phlogopite peridotite producing Al-rich melts: the mixing of such Al-rich components with the migrating basalt is believed to have played a fundamental role in favouring the precipitation of sapphirine.References. [1] Zanetti, A., Mazzucchelli, M., Rivalenti, G., Vannucci, R. (1999): Contrib. Mineral. Petrol., 134, 107-122
INSIGHTS INTO THE TRIASSIC GEODYNAMIC EVOLUTION OF THE ADRIA PLATE: THE STUDY CASE OF THE MAFIC-ULTRAMAFIC SEQUENCE OF FINERO
No full textA strategic lithologic sequence for the study of the Middle-Upper Trias magmatic events in the South-Alpine domain (Adria plate) is outcropping in the Finero area, located in the north-easternmost sector of the Ivrea-Verbano Zone (Western Alps). Such a sequence is located along the Insubric line and is formed by a strongly-metasomatised mantle body, surrounded by a mafic-ultramafic intrusive sequence [1,2,3], which documents the alternation of mantle and crustal rocks placed at the bottom of the Adria plate before the opening of Ligurian-Piedmontese branch of the Jurassic Tethys.Unlike the intrusive sequences of the central and southern sectors of the Ivrea-Verbano Zone, characterised by Permo-Carboniferous emplacement ages, the Finero massif shows abundant radiometric evidence of intrusion of basic melts at the bottom of the continental crust during Trias, which formed the cumulitic sequences of the so-called Basic Complex of Finero. Besides, in Triassic times, the mantle sequence of Finero suffered a virtually complete metasomatic recrystallisation triggered by several episodes of pervasive to channelled porous flow migration of (mostly hydrous) melts. Later on, but yet in Triassic time, the mantle sequence experienced the intrusion of basic veins-dykes (locally characterised by the presence of sapphirine), which discordantly cut the mantle foliation. Thus, the mafic–ultramafic Finero sequence represents a unique opportunity to characterise the composition of Triassic melts migrating through the Adria realm escaping significant interaction with the continental crust. Notwithstanding that several papers have been devoted to the petrologic investigation of the mafic-ultramafic Finero sequence since the beginning of the seventies, its petrochemical and geodynamic evolution is presently very poorly constrained. Crucial issues still debated are: 1) the sources of the liquids that percolated the mantle sequence, the timing and geodynamic setting of the mantle metasomatism; 2) the age of accretion of the mantle sequence to the bottom of the continental crust; 3) the geochemical composition of the parent melts of the Basic Complex, their differentiation processes, the timing of the different melt injections and their potential relationships with the melt-related events recorded by the associated mantle sequence. In the frame of this contribution, new data about the major and trace mineral chemistry of the three main units of the Basic Complex (i.e Internal Gabbro, Amphibole Peridotite, External Gabbro) and of the various peridotitic (e.g. phlogopite harzburgites, dolomite-apatite-bearing wehrlites, dunites with chromitites bands and/or pyroxenite-hornblendite veins), pyroxenitic (e.g. phlogopite-bearing websterite, orthopyroxenites, clinopyroxenites) and femic (e.g. sapphirine-bearing amphibole gabbros) lithologies of the mantle sequence will be provided, in order to constrain the geodynamic setting of the melt-related processes.References. [1] Siena, F., Coltorti, M. (1989): Jb. Miner. Mh., 6, 255-274; [2] Zanetti, A., Mazzucchelli, M., Rivalenti, G., Vannucci, R. (1999): Contrib. Mineral. Petrol., 134, 107-122.; [3] Morishita, T., Hattori, K.H., Terada, K., Matsumoto, T., Yamamoto, K., Takebe, M., Ishida, Y., Tamura, A., Arai, S. (2008): Chem. Geol., 251, 99-111
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