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Origin of the gabbroic sequences from the Ligurian Jurassic ophiolites: implications for lower crust generation at slow spreading settings
No full textOlivine-rich troctolites from Ligurian ophiolites (Italy): evidence for impregnation of replacive mantle conduits by MORB-type melts
No full textThe gabbroic plutons of the Internal Ligurian ophiolites (Northern Apennines, Italy) include olivine-rich troctolite bodies up to 80 m thick. The rounded to embayed morphology of olivine (Fo = 87-88 mol %) and spinel from the olivine-rich troctolites, together with the high Mg-number (88-90) and Cr contents (2400-3000 ppm) of associated poikilitic clinopyroxene, indicates their formation through reaction between an olivine-spinel matrix and an infiltrating melt. This interpretation is corroborated by the occurrence of plagioclase-rich veins that exhibit diffuse margins against the host olivine-rich troctolites and centimetre-scale spinel-bearing dunitic clots, which are interpreted as the crystallization products of the infiltrating melts and relics of pre-existing olivine-rich material, respectively. The incompatible element signature of clinopyroxene from the olivine-rich troctolites shows that the infiltrating melts were chemically similar to mid-ocean ridge basalt (MORB). Spinel from the olivine-rich troctolites contains inclusions of Ti-pargasite/kaersutite, phlogopite/aspidolite and orthopyroxene; inclusions of ilmenite and loveringite are also locally present. These spinels most probably formed from hybrid melts derived from interaction between melts rich in incompatible elements and new injections of primitive melt. We propose that this interaction occurred in the mantle, in melt conduits of replacive origin near the base of the conductive thermal boundary layer, and led to formation of a matrix made up of olivine and minor inclusion-bearing spinel. This matrix was subsequently impregnated by MORB-type melts saturated in plagioclase + clinopyroxene to produce the olivine-rich troctolites. The amount of melt added to the olivine-spinel matrix to produce the olivine-rich troctolites is estimated to be ~25 wt%
The magmatic-hydrothermal transition in the lower oceanic crust: clues from the Ligurian ophiolites, Italy
No full textThe gabbroic bodies from the Jurassic Ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the Mid Atlantic Ridge. Initial cooling of the Ligurian gabbros is associated with local development of hornblende-bearing felsic dykes and hornblende vein networks. The hornblende veining is correlated with the widespread development of hornblende as coronas/pseudomorphs after the clinopyroxene in the host gabbros. In addition, the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins. The hornblendes from the felsic dykes and the hornblende-rich rocks within the mantle sliver show a similar geochemical signature, characterized by low Mg#, CaO and Al2O3, negligible Cl, and high TiO2, K2O, REE, Y, Zr and Nb concentrations. The whole-rock Sm-Nd isotopic compositions of the felsic dykes and the hornblende-rich rocks define a Sm-Nd isochron corresponding to an age of 154 ± 20 Ma and an initial εNd of 9.2 ± 0.5. The δ18O of the hornblendes and coexisting zircons from these rocks (about +4.5‰ and +5.8‰, respectively) do not indicate the presence of a seawater component in these melts. The formation of the felsic dykes and of the hornblende-rich rocks within the mantle sliver involved SiO2-rich silicate melts with negligible seawater component, which presumably were derived from high degree fractional crystallization of MOR-type basalts. The vein and the coronitic/pseudomorphic hornblendes show high Mg# and CaO, significant Cl (0.02-0.17 wt%) and low TiO2 and K2O concentrations. The coronitic/pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and δ18O values (+1.0 to 0.7 ‰) close to seawater, suggesting an origin by reaction between migrating seawater-derived fluids and the host gabbros. The vein hornblendes commonly show slight LREE enrichment, relatively high concentrations of Nb (up to 2.5 ppm) and δ18O ranging from +3.7 to +0.8‰. The crystallization of these hornblendes most likely involved both seawater and magmatic components
Magmatic-hydrothermal interactions at a fossil slow-spreading centre (Internal Ligurian ophiolites, Italy)
No full textHigh temperature alteration of the gabbroic oceanic crust (Ligurian ophiolites, Italy): evidence for hydrothermal-magmatic interactions
No full textRole of melting process and melt–rock reaction in the formation of Jurassic MORB-type basalts (Alpine ophiolites)
No full textThis study reports a geochemical investigation of two thick basalt sequences, exposed in the Bracco–Levanto ophiolite (northern Apennine, Italy) and in the Balagne ophiolite (central-northern Corsica, France). These ophiolites are considered to represent an oceanward and a continent-near paleogeographic domain of the Jurassic Liguria–Piedmont basin. Trace elements and Nd isotopic compositions were examined to obtain information about: (1) mantle source and melting process and (2) melt–rock reactions during basalt ascent. Whole-rock analyses revealed that the Balagne basalts are slightly enriched in LREE, Nb, and Ta with respect to the Bracco–Levanto counterparts. These variations are paralleled by clinopyroxene chemistry. In particular, clinopyroxene from the Balagne basalts has higher CeN/SmN (0.4–0.3 vs. 0.2) and ZrN/YN (0.9–0.6 vs. 0.4–0.3) than that from the Bracco–Levanto basalts. The basalts from the two ophiolites have homogeneous initial Nd isotopic compositions (initial εNd from + 8.8 to + 8.6), within typical depleted mantle values, thereby excluding an origin from a lithospheric mantle source. These data also reject the involvement of contaminant crustal material, as associated continent-derived clastic sediments and radiolarian cherts have a highly radiogenic Nd isotopic fingerprint (εNd at the time of basalt formation = − 5.5 and − 5.2, respectively). We propose that the Bracco–Levanto and the Balagne basalts formed by partial melts of a depleted mantle source, most likely containing a garnet-bearing enriched component. The decoupling between incompatible elements and Nd isotopic signature can be explained either by different degrees of partial melting of a similar asthenospheric source or by reaction of the ascending melts with a lower crustal crystal mush. Both hypotheses are reconcilable with the formation of these two basalt sequences in different domains of a nascent oceanic basin
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
New insights into the evolution of the Finero Mafic Complex, north-eastern Ivrea-Verbano Zone
No full textThe Finero Mafic Complex outcrops in the northern sector of the Ivrea-Verbano Zone (IVZ, Southern Alps). It occurs at the flank of an antiform which core is constituted by mantle peridotites, and it consists of mafic/ultramafic rocks subdivided in three units: a) the Layered Internal Zone (LIZ), in tectonic contact with the mantle unit; b) the Amphibole Peridotite (Amph-Pd); c) the External Gabbro (EG), which is in tectonic contact with the Variscan crystalline basement (Rivalenti et al., 1984; Siena & Coltorti, 1989). Recent studies point to a Middle Triassic emplacement age for the EG unit (Zanetti et al., 2013), suggesting that it is unrelated to the Permian Mafic Complex of the central IVZ. Owing to the lack of a detailed petrochemical characterisation of the FMC, we performed new major (EMP) and trace element (LA-ICPMS) analyses on representative samples from the LIZ and Amph-Pd. The LIZ mainly consists of hornblende-gabbros; anorthosites and garnet hornblendites with minor pyroxenites. The Amph-Pd is mostly made up of Amph-bearing harzburgites and dunites with minor piroxenites. Locally, Amph-rich veins with a variable thickness from a few cm to about 1 m crosscut the magmatic layering. Olivine (Fo87-82) only occurs in the perfrom Amph-Pd, whereas amphibole and clinopyroxene are common throughout the entire sequence. The Mg# of Cpx and Amph tends to increase from the LIZ towards the upper part of the Amph-Pd whereas the Al2O3 content in Cpx and Amph is up to 11 and 18 wt.%, respectively and show an opposite trend. In garnet-free pyroxenites and hornblendites from LIZ, Amph and Cpx have slightly LREE-depleted patterns with flat HREE (at 2xCI in Cpx) and marked positive Eu, Sr, Pb and U anomalies. Similar features are shown by the Cpx and Amph from the associated gabbros, but they are strongly depleted in HREE indicating chemical equilibration with garnet. Cpx and Amph from the Amph-bearing peridotites (Amph-Pd) have instead LREE-enriched spoon-shape patterns with HREE contents comparable with those of the LIZ lithologies, being also characterised by Eu, Sr and U enrichments. The LILE enrichments and fractionation can be reconciled by an interaction dominated by ion exchange chromatographic-type process with strongly LILEenriched melts: the composition of the latter is recorded by the amphibole-dominated lithologies. The new data suggest that the LIZ with a clear crustal signature. Instead, the trace element variations in the Amph-Pd cannot be explained via a closed-system evolution, pointing to the presence of significant changes in the composition of the uprising mantle melts
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