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Dipartimento di Scienze della Terra, Universita` di Perugia, Piazza Universita` , I-06100 Perugia, Italy
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- PublicationRestrictedComposition and thermal structure of the lithosphere beneath the Ethiopian plateau: evidence from mantle xenoliths in basanites, Injibara, Lake Tana Province(2008)
; ; ; ; ; ; ; ; ;Ferrando, S.; Dipartimento di Scienze della Terra, Università di Siena ;Frezzotti, M. L.; Dipartimento di Scienze della Terra, Università di Siena ;Neumann, E. R.; Physics of Geological Processes, University of Oslo ;De Astis, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Peccerillo, A.; Dipartimento di Scienze della Terra, Università di Perugia ;Dereje, A.; Department of Geology and Geophysics, Faculty of Science, Addis Ababa University ;Gezahegn, Y.; Department of Geology and Geophysics, Faculty of Science, Addis Ababa University ;Teklewold, A.; ; ; ; ; ; ; Petrographic, minerochemical, and geothermobarometric data are reported for a suite of spinel-lherzolite pargasite xenoliths hosted in a Quaternary basanitic lava flow from the North-Western Ethiopian Plateau (Injibara, Lake Tana Province). Protogranular to porphyroclastic (deformed) rocks show evidence of a modal metasomatism, represented by a Cl-rich pargasitic amphibole, coupled with cryptic enrichment in Fe and Al. Equigranular rocks (granular) record a further cryptic metasomatism, represented by enrichment in Fe, Al, Na, and depletion in Ni, Cr and Cl. Some xenoliths (transitional) show intermediate textural and compositional characters, indicating that the granular samples represent an evolution of the deformed ones. All xenoliths give the same P–T equilibration conditions for Opx-Cpx pairs (947–1015 C and 1.3–2.0 GPa), but in granular samples, recrystallised olivine and spinel record T about 100 C higher. Two distinct metasomatic processes, probably connected with the emplacement of the Afar plume, are proposed. The first one is a pervasive modal metasomatism produced by water-rich fluids. The latter is a non-pervasive cryptic metasomatism, probably connected to migration of melts. The comparison the mantle beneath the Ethiopian Volcanic Plateau, the southern Main Ethiopian Rift and the central Main Ethiopian Rift suggests spatial heterogeneity of the mantle and variable mantle processes during asthenospheric upwelling.237 20 - PublicationRestrictedOrogens and slabs vs. their direction of subduction(1999)
; ; ; ; ; ; ;Doglioni, C.; Dipartimento di Scienze della Terra, Universita' La Sapienza ;Harabaglia, P.; Centro di Geodinamica, Universita' della Basilicata ;Merlini, S.; AGIP-ENI ;Mongelli, F.; Dipartimento di Geologia e Geofisica, Universita' di Bari ;Peccerillo, A.; Dipartimento di Scienze della Terra, Universita' di Perugia ;Piromallo, C.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; ; ; Subduction zones appear primarily controlled by the polarity of their direction, i.e., W-directed or E- to NNE-directed, probably due to the westward drift of the lithosphere relative to the asthenosphere. The decollement planes behave differently in the two end-members. In the W-directed subduction zone, the decollement of the plate to the east is warped and subducted, whereas in the E- to NNE-directed, it is ramping upward at the surface. There are W-directed subduction zones that work also in absence of active convergence like the Carpathians or the Apennines. W-directed subduction zones have shorter life 30–40 Ma.than E- or NE-directed subduction zones even longer than 100 Ma.. The different decollements in the two end-members of subduction should control different PTt paths and, therefore, generate variable metamorphic assemblages in the associated accretionary wedges and orogens. These asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast ‘eastward’ migrating structural wave along W-directed subduction zones, whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones. The magmatic pair calc-alkaline and alkaline–tholeiitic volcanic products of the island arc and the back-arc basin characterise the W-directed subduction zones. Magmatic rocks associated with E- or NE-directed subduction zones have higher abundances of incompatible elements, and mainly consist of calc-alkaline– shoshonitic suites, with large volumes of batholithic intrusions and porphyry copper ore deposits. The subduction zones surrounding the Adriatic plate in the central Mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab. The western margin of the Adriatic plate contemporaneously overridden and underthrust Europe toward the ‘west’ to generate, respectively, the Alps and the Apennines, while the eastern margin subducted under the Dinarides–Hellenides. These belts confirm the characters of the end-members of subduction zones as a function of their geographic polarity similarly to the Pacific subduction zones.293 57 - PublicationOpen AccessEvolution of the volcanic plumbing systemof Alicudi (Aeolian Islands - Italy): evidence from fluid and melt inclusionsin quartz xenoliths(2004)
; ; ; ; ;Bonelli, R.; Dipartimento di Scienze della Terra, Università degli Studi di Siena, Siena, Italy ;Frezzotti, M. L.; Istituto di Geologia Ambientale e Geoingegneria (IGAG) - CNR, Roma, Italy ;Zanon, V.; Dipartimento di Scienze della Terra, Università degli Studi di Perugia, Perugia, Italy ;Peccerillo, A.; Dipartimento di Scienze della Terra, Università degli Studi di Perugia, Perugia, Italy; ; ; Quartz-rich xenoliths in lavas (basalts to andesites; 90-30 ka) from Alicudi contain abundant melt and fluid inclusions. Two generations of CO2-rich fluid inclusions are present in quartz-rich xenolith grains: early (Type I) inclusions related to partial melting of the host xenoliths, and late Type II inclusions related to the fluid trapping during xenolith ascent. Homogenisation temperatures of fluid inclusions correspond to two density intervals: 0.93-0.68 g/cm3 (Type I) and 0.47-0.26 g/cm3 (Type II). Early Type I fluid inclusions indicate trapping pressures around 6 kbar, which are representative for the levels of partial melting of crustal rocks and xenolith formation. Late Type II fluid inclusions show lower trapping pressures, between 1.7 kbar and 0.2 kbar, indicative for shallow magma rest and accumulation during ascent to the surface. Data suggest the presence of two magma reservoirs: the first is located at lower crustal depths (about 24 km), site of fractional crystallization, mixing with source derived magma, and various degrees of crustal assimilation. The second magma reservoir is located at shallow crustal depths (about 6 km), the site where magma rested for a short time before erupting.154 333 - PublicationRestrictedModeling the magma plumbing system of Vulcano (Aeolian Isalnds, Italy) by integrated fluid inclusion geo-barometry, petrology and geophysics.(2006-01)
; ; ; ; ;Peccerillo, A.; Dipartimento di Scienze della Terra, Universita` di Perugia, Piazza Universita` , I-06100 Perugia, Italy ;Frezzotti, M. L.; Dipartimento di Scienze della Terra, Universita` di Siena, Via Laterina 8, I-53100 Siena, Italy ;De Astis, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Ventura, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; An integrated petrological, geophysical, and fluid-inclusion model is presented for the internal structure of the active Vulcano Island, Southern Tyrrhenian Sea. The present structure of the magma storage system in the crust consists of two major deep accumulation zones located at 17–21 km and 8–13 km depth, plus a minor one at 1–5 km depth, beneath Fossa Cone. The deepest magma accumulation zone contains mafic melts and is located at the transition between the upper mantle and a granulitic lower crust. This reservoir has been active since the onset of the exposed volcanism, and has undergone continuous fractional crystallization, crustal assimilation, and mixing with primary melts from the mantle. Slightly differentiated magmas from the deep reservoir feed a shallower accumulation zone in the middle and upper crust, or erupt directly to the surface through lateral vents. Deep melts probably enter the shallowest reservoir shortly before magma outbreak at the surface, and may represent the trigger of eruptions. According to our model, magmatic eruptions at Vulcano are related to deep magma dynamics, whereas most of the changes in the geochemical and geophysical parameters observed at the active cone in the last century could be due to shallow depth modification of rock permeability, possibly as a result of cone gravitative instability and/or tectonic events. Implications for strategies of volcano monitoring and for forecasting eruptions are briefly discussed.1716 30 - PublicationRestrictedKinematics of slab tear faults during subduction segmentation and implications for Italian magmatism(2008)
; ; ; ; ; ;Rosenbaum, G.; School of Physical Sciences, University of Queensland, Brisbane, Queensland, Australia. ;Gasparon, M.; School of Physical Sciences, University of Queensland, Brisbane, Queensland, Australia. ;Lucente, F. P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia ;Peccerillo, A.; Dipartimento di Scienze della Terra, Universita` degli Studi di Perugia, Perugia, Italy. ;Miller, M. S.; Department of Earth Science, Rice University, Houston, Texas, USA.; ; ; ; Tectonic activity in convergent plate boundaries commonly involves backward migration (rollback) of narrow subducting slabs and segmentation of subduction zones through slab tearing. Here we investigate this process in the Italian region by integrating seismic tomography data with spatiotemporal analysis of magmatic rocks and kinematic reconstructions. Seismic tomography results show gaps within the subducting lithosphere,which are interpreted as deep (100–500 km) subvertical tear faults. The development of such tear faults is consistent with proposed kinematic reconstructions, inwhich different rates of subduction rollback affected different parts of the subduction zone. We further suggest a possible link between the development of tear faults and the occurrence of regional magmatic activity with transitional geochemical signatures between arc type and OIB type, associated with slab tearing and slab breakoff.We conclude that lithospheric-scale tear faults play a fundamental role in the destruction of subduction zones. As such, they should be incorporated into reconstructions of ancient convergent margins, where tear faults are possibly represented by continental lineaments linked with magmatism and mineralization.237 31 - PublicationRestrictedGeology, volcanic history and petrology of Vulcano (central Aeolian archipelago)(2013)
; ; ; ; ; ; ; ;De Astis, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Lucchi, F.; Università di Bologna ;Dellino, P.; Università di Bari ;La Volpe, L.; Università di Bari ;Tranne, C.; Università di Bologna ;Frezzotti, M. L.; Università di Milano - Bicocca ;Peccerillo, A.; Università di Perugia; ; ; ; ; ; Vulcano is an active NW–SE-elongated composite volcano located in the central Aeolian archipelago. Based on available radiometric ages and tephrochronology, the exposed volcanism started at c. 127 ka and spread through eight Eruptive Epochs separated by volcano-tectonic events and major quiescent stages. Various eruptive centres and two intersecting multi-stage calderas resulted from such evolution. Vulcano geological history displays several changes of eruption magnitude, eruption styles and composition of magmas through time. Vulcano rocks range from basalt to rhyolite and show variable alkali contents, roughly increasing during time. Magmas with low to intermediate SiO2 contents and high-K to shoshonite affinity prevail in the early Epochs 1–5 (c. 127–28 ka), whereas intermediate to high-SiO2 shoshonite and potassic alkaline products dominate the last three Epochs (<30 ka). This sharp increase in silicic products is related to the shallowing of the plumbing system and resulting major role of the differentiation processes in shallow level reservoirs. Radiogenic isotope compositions are variable (87Sr/86Sr = 0.70424–0.70587, 143Nd/144Nd = 0.51254–0.51276, 206Pb/204Pb = 19.305–19.759, 207Pb/204Pb = 15.659–15.752, 208Pb/204Pb = 39.208–39.559) as a result of both source heterogeneities and shallow-level interaction of magmas with continental crust. The compositional variations of mafic magmatism with time suggest that the source zone changed from a metasomatized, fertile, ocean island basalt- (OIB-) like mantle to a metasomatized depleted lithospheric mantle.1269 45 - PublicationRestricted(Garnet)-spinel peridotite xenoliths from Mega (Ethiopia): Evidence for rejuvenation and dynamic thinning of the lithosphere beneath the southern Main Ethiopian RiftPetrological and geothermobarometric data from (garnet)-spinel peridotite and pyroxenite xenoliths in Quaternary alkali-basalts at Mega (Sidamo region, Ethiopia) provide evidence for refertilization and thinning of the lower subcontinental lithospheric mantle (SCLM) during Tertiary rifting events in the southern Main Ethiopian Rift (MER). Samples of lherzolites, harzburgites, and olivine websterites contain spinel-pyroxene symplectites thatwere formed by garnet breakdown reactions. P-T equilibration conditions, in part calculated using the reconstructed garnet composition (pyrope), indicate the evolution of peridotites from the P-T conditions fromthe garnet stability field (2.9–2.2 GPa; 945–1025 °C), through garnet-spinel transition (b 2.1–1.7 GPa at 906–1017 °C), to the final equilibration in the spinel stability field (1.3 GPa at 862–1056 °C), corresponding to a nearly adiabatic decompression path of about 50 km. Peridotite chemical compositions plot in an off-craton trend, with variable fertility degrees, only in part due to partial melting processes. Rocks showvariable CaO contents (0.96–4.12 wt%), CaO/Al2O3 ratios (1.42–4.46), and clinopyroxene contents (6–31 vol.%), reaching compositions more enriched than pyrolite. Clinopyroxene shows evident LREE, U, Th, and Sr enrichments, particularly in harzburgites. Geochemical data suggest heterogeneous refertilization of a refractory lower SCLM, induced by infiltration and migration through porous flow of silicate-carbonate melts. Metasomatic processes drove melt compositions towards low-viscosity highly-mobile C-O-H-richmelts, inducing cryptic and hydrousmodalmetasomatismin refractory peridotites. Heterogeneous refertilization of the SCLM beneath the southern MER seems to have been induced by upwelling of asthenospheric melts and/or by reactivation of ancient lithospheric metasomes. Rejuvenation processes could have rheologically weakened the base of SCLM through gravitational instabilities, and increased the extensional stresses, causing a destabilizing effect at the onset of the lithospheric thinning and Tertiary rifting.
89 4 - PublicationRestrictedTrace element and isotopic variations from Mt. Vulture to Campanian volcanoes: constraints for slab detachment and mantle inflow beneath southern Italy(2006)
; ; ; ; ;De Astis, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Kempton, P. D.; Natural Environment Research Council, Polaris House, North Star Avenue, Swindon, SN2 1EU, UK ;Peccerillo, A.; Dipartimento di Scienze della Terra, University of Perugia, Piazza Universita` , 06100 Perugia, Italy ;Wu, T. W.; Department of Geology, University of Western Ontario, London, ON, Canada N6A 5B7; ; ; New Sr–Nd–Pb isotopic ratios and trace element data for volcanic mafic rocks outcropping along a E–W transect in southern Italy, from Mt. Vulture to Neapolitan volcanoes, are reported. The variation of LILE/HFSE, HFSE/HFSE and radiogenic isotopes along this transect indicates that all of these volcanoes contain both intra-plate and subduction-related signatures, with the former decreasing from Mt. Vulture to Campanian volcanoes. New data are also reported for the Paleocene alkaline rocks from Pietre Nere (Apulia foreland), which show isotopic ratios mostly overlapping the values for Mediterranean intra-plate volcanoes as well as the Eocene–Oligocene alkaline mafic lavas from the northern Adria plate. Pietre Nere provides evidence for an OIB mantle composition of FOZO-type, free of subduction influences, that is present beneath the Adria plate (Africa) before its collision with Europe. After this collision, and formation of the southern Apennines, westward inflow of mantle from the Adria plate to the Campanian area occurred, as a consequence of slab break off. Interaction of subduction components with inflowing Adria mantle generated hybrid sources beneath the Vulture–Campania area, which can explain the compositional features of both Mt. Vulture and the Campanian mafic rocks. Therefore, mafic magmas from these volcanoes represent variable degrees of mixing between different mantle components.307 27