1,721,038 research outputs found
N-MORB crust beneath Fuerteventura in the easternmost part of the Canary Islands: evidence from gabbroic xenoliths
No full textOrigin of titanian pargasite in gabbroic rocks from the Northern Apennine ophiolites (Italy): insights into the late-magmatic evolution of a MOR-type intrusive sequence
No full textInteraction between basic and acid magmas during the latest stages of the post-collisional Variscan evolution: Clues from the gabbro-granite association of Ota (Corsica-Sardinia batholith)
No full textThe intrusive association of Ota (post-Variscan Corsica-Sardinia batholith) consists of mafic rocks and granites displaying mingling relations. The mafic rocks are olivine gabbros to quartz-diorites, and the granites are subsolvus and characterised by biotite as the only mafic phase. Minor amounts of amphibole-bearing granodiorites to granites develop along the contacts between mafic rocks and granites. The rather high initial εNd (+4.5) of the most primitive mafic sample indicates that parental basic melts had an extensive contribution from a depleted mantle source. In the mafic rocks, the decrease of initial εNd (to +2.1) is associated with an increase of Th and U concentrations in clinopyroxene, thus indicating that the Ota basic melts underwent a process of crustal contamination. Associated biotite granites have low initial εNd (+0.4) and high Th and U concentrations. The chemical evolution of Ota basic melts most likely occurred through fractional crystallisation and concomitant contamination with the acid magma that gave rise to associated biotite granites. The incompatible element signature of late-stage anhedral amphibole from variably evolved mafic rocks is similar, thus suggesting that the whole Ota mafic sequence recorded the contamination process, possibly by percolation within the gabbroic crystal mush of a melt contaminated by the acid magma. Hybridisation between basic and acid magmas is also documented by contact amphibole-bearing granitoids. In particular, major and trace element zoning of amphibole from granodiorites reveals that such a hybridisation was associated with reactions between crystals from the basic magma (amphibole and clinopyroxene) and the acid melt. The occurrence of chemically homogeneous amphiboles in associated granites suggests that “contact” hybridisation comprised also a process of gradient-induced chemical diffusion
U-Pb chronological constraints for the late- and post-orogenic episodes in the Ligurian Variscides
No full textTrace-element partitioning in olivine: modelling of a complete data set from a synthetic hydrous basanite melt
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LAM U-Pb zircon Early Jurassic exhumation age of the Finero Phlogopite Peridotite (Ivrea-Verbano Zone, Western Alps) and its geodynamic consequences
No full textA new LA-ICP-HRMS investigation of transparent zircons, unzoned and smoky at cathodoluminescence (CL), separated from chromitite layers segregated in mantle dunite bodies belonging to the Phlogopite Peridotite unit (hereafter PP) of the Finero Complex (Ivrea –Verbano Zone, Southern Alps) provides single-spot 206Pb/238U Lower Jurassic ages between 200 to 180 Ma, with a pronounced peak at around 190 Ma. Relevant exception is represented by two pinky zircons showing relics of zoning at CL, with darker cores that give Triassic ages from 240 to 230 Ma. The presence of continental crust component(s) evidenced by the negative EpsilonHf of the zircons, the strict similarity of the trace element contents shown by clinopyroxenes and amphiboles from chromitites and the phlogopite harzburgites and pyroxenites hosting the dunite bodies, as well as the complete to partial disappearance of olivine replaced by orthopyroxene, indicate that the parent melts of the chromitites had a cognate origin with the hydrous LILE-enriched silica-saturated melts responsible of the pervasive metasomatism recorded by the Finero mantle sequence. The combination of our data with those reported in literature for the PP chromitite zircons determines a large age interval ranging from 290 to 180 Ma. However, zircon populations with different U-Pb ages show EpsilonHf very similar to that found in this study. The latter evidence, together with the rejuvenation of the ages with the disappearing of the internal structures suggest that the large age variability is the result of a prolonged residence at mantle/lower crustal depths of the PP, characterised by progressive reequilibration stages of the U-Pb zircon system. Thus, it is here proposed that the segregation of the zircon-bearing chromitite layers was related to the pervasive metasomatic event, which occurred at ~290 Ma or before. Successively, the U-Pb zircon system remained virtually unperturbed until Middle Triassic,when the area was affected by at least two main magmatic cycles with tholeiitic to Na-alkaline geochemical affinity associated to tectonic instability. The consequent thermal perturbations induced re-equilibration stages of the chromitite zircons, which ended with the Early Jurassic exhumation documented by the U-Pb ages of chromitite zircons of this study. Our data are in agreement with the interpretation that the Early Jurassic extensional tectonics was characterised by an important reheating event at 190 Ma, possibly due to lithospheric hyperextension. Such a scenario considers that the PP unit resided at mantle depths during Early Permian, being possibly emplaced at crustal levels only thanks to trans-lithospheric faults during the Early Jurassic. This evolution is completely different with respect to the present day interpretation of the geodynamic history of the mantle bodies in the Val Sesia area, which are believed to have been emplaced within the continental crust, as part of accretionary prisms, since the end of the Variscan orogeny or before. This evidence confirms that the northernmost part of the Ivrea-Verbano Zone underwent peculiar Paleozoic to Mesozoic geodynamic processes, thus unravelling important additional complexities to the interpretation of the geodynamic evolution of the area now related to the Southern Alps
U-PB CHRONOLOGICAL CONSTRAINTS FOR THE LATE TOPOST-VARISCAN MAGMATISM AND METAMORPHISM IN THELIGURIAN ALPS (ITALY)
No full textSlab-melting during Alpine orogeny: evidence from mafic cumulates of the Adamello batholith (Central Alps, Italy)
No full textThe Adamello batholith is the major Tertiary calc-alkaline intrusion of the Italian Alps. It consists primarily of tonalites to granodiorites and locally contains bodies of amphibole-rich mafic to ultramafic cumulates (hornblendites to amphibole gabbros). In these cumulates, two different parageneses of igneous origin can be distinguished. The first consists of euhedral brown amphibole with olivine, spinel and clinopyroxene inclusions. The second paragenesis consists of poikilitic plagioclase containing fine-grained euhedral clinopyroxene, minor green amphibole and accessory titanite, calcite, quartz, zircon, apatite and Fe-Ti-oxide phases. Minerals of the second paragenesis have been analys ed for trace elements by laser ablation ICP-MS and ion microprobe (SIMS). Trace element compositions of liquids in equilibrium with clinopyroxene and green amphibole have been calculated by applying suitable sets of solid/ liquid partition coefficients. Computed liquids show a marked LREE enrichment over HREE, abrupt enrichment in U and Th (up to 300 times N-MORB) and negative Nb-Ta anomalies. These chemical features are consistent with titanite, plagioclase and calcite trace element compositions. Literature data show that the liquid in equilibrium with the first paragenesis was less LREE enriched and had lower concentrations of Be, B, Th and U. The contrasting trace element signature between the two parageneses cannot be related to fractional crystallisation. Oxygen isotope compositions of mineral separates have been determined by laser fluorination. δ18O values of brown and green amphiboles (+5.4 to +5.7‰) and of titanite (+4.0‰) indicate that crustal contamination played a negligible role in the origin of parental melts of both parageneses. The peculiar trace element signature of the liquids in equilibrium with the second paragenesis (e.g., extremely high Th/Nb and La/Yb values) has been attributed to slab-derived melts with a high sediment component. Equilibration of the ascending slab melts with the peridotitic mantle wedge would explain the observed low δO18 of amphibole and their relatively high Mg contents
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