Description: The Cenozoic igneous activity of Sardinia is essentially concentrated in the 38-0.1 Myr time range. On the basis of volcanological, petrographic, mineralogical, geochemical and isotopic considerations, two main rock types can be defined. The first group, here defined SR (Subduction-Related) comprises Late Eocene-Middle Miocene (~ 38-15 Ma) igneous rocks, essentially developed along the Sardinian Trough, a N-S oriented graben developed during the Late Oligocene-Middle Miocene. The climax of magmatism is recorded during the Early Miocene (~ 23-18 Ma) with minor activity before and after this time range. Major and trace element indicators, as well as Sr-Nd-Pb-Hf-Os-O isotope systematic indicate complex petrogenetic processes including subduction-related metasomatism, variable degrees of crustal contamination at shallow depths, fractional crystallization and basic rock partial melting. Hybridization processes between mantle and crustal melts and between pure mantle and crustally contaminated mantle melts increased the isotopic and elemental variability of the composition of the evolved (intermediate to acid) melts. The earliest igneous activity, pre-dating the Early Miocene magmatic climax, is related to the pushing effects exerted by the Alpine Tethys over the Hercynian or older lower crust, rather than to dehydration processes of the oceanic plate itself. The second group comprises volcanic rocks emplaced from ~ 12 to ~ 0.1 Ma. The major and, partially, trace element content of these rocks roughly resemble magmas emplaced in within-plate tectonic settings. From a Sr-Nd-Pb-Hf-Os isotopic point of view, it is possible to subdivide these rocks in two subgroups. The first, defined RPV (Radiogenic Pb Volcanic) group comprises the oldest and very rare products (~ 12-4.4 Ma) occurring only in the southern sectors of Sardinia. The second group, defined UPV (Unradiogenic Pb Volcanic), comprises rocks emplaced in the remaining central and northern sectors during the ~ 4.8-0.1 Ma time range. The origin of the RPV rocks remains quite enigmatic, since they formed just a few Myr after the end of a subduction-related igneous activity but do not show any evidence of slab-derived metasomatic effects. In contrast, the complex origin of the mafic UPV rocks, characterized by low 206Pb/204Pb (17.4-18.1), low 143Nd/144Nd (0.51232-0.51264), low 176Hf/177Hf (0.28258-0.28280), mildly radiogenic 87Sr/86Sr (~ 0.7044) and radiogenic 187Os/188Os ratios (0.125-0.160) can be explained with a mantle source modified after interaction with ancient delaminated lower crustal lithologies. The strong isotopic difference between the RPV and UPV magmas and the absence of lower crustal-related features in the SR and RPV remain aspects to be solved.

Description: A diffuse and voluminous (〉1400 km3) Miocene-Quaternary volcanic activity developed around the Karlıova Triple Junction in East Anatolia as a consequence of collisional tectonics among Anatolia, Arabia and Eurasia continental plates. The volcanic rocks of this region are grouped into three phases of activity: 1) Early Phase (Solhan volcanism; ~7.3–4.4 Ma), with emplacement of alkali basalt to trachyte lava flows and pyroclastic successions; 2) Middle Phase (Turnadağ and Varto volcanism; ~3.6–2.6 Ma), mostly with products with the same compositional range plus minor dacites and rhyolites, and 3) Late Phase (Özenç volcanism; ~2.6–0.5 Ma), with emplacement of alkali basaltic, hawaiitic and mugearitic lavas and dykes. Primitive Mantle-normalized patterns of the three rock groups share an enriched LILE and depleted HFSE contents, with overall positive spikes of Pb and mildly fractionated LREE/HREE trends showing more similar affinity to global subducting sediments rather than to magmas emplaced in mid-plate settings (i.e., OIB). Initial Sr isotopic ratios of the least evolved compositions range from values lower than BSE (87Sr/86Sri = 0.7041) to radiogenic compositions (87Sr/86Sri = 0.7050). They reflect either FC-like processes, with 87Sr/86Sri up to 0.7064, or closed system fractional crystallization, with 87Sr/86Sri = 0.7046–0.7049. Initial Nd are higher than ChUR estimate for the most and the least evolved compositions (143Nd/144Ndi = 0.51267–0.51280), indicating provenance from isotopically depleted sources. Lead isotopic ratios are characterized by a remarkable homogeneous 206Pb/204Pb (18.95–19.04), with 207Pb/204Pb (15.65–15.72) and 208Pb/204Pb (38.87–39.21) slightly above the Northern Hemisphere Reference Line, pointing towards the EMII end-member. Geochemical modelling for the least evolved volcanic units indicate the likely generation from an amphibole-bearing spinel-lherzolitic source. P-T calculations for partial melting calculations gave lithospheric pressures for initial magma generation (0.8–1.3 GPa). Possible cause of melting might be related to passive upwelling of asthenosphere as a response to the local extension linked to the development of North Anatolian and East Anatolian Fault Zones. Anyhow, volcanic units from the KTJ display only limited geochemical signatures of garnet-bearing sources, or any HIMU-OIB like characteristics, as instead observed in the other portions of the Eastern Anatolia. The long-lasting complex tectonic evolution of the Eastern Anatolia is responsible for the large geochemical variability of the magmatic products. However, the general characteristics of KTJ volcanic rocks are mainly dominated by subduction-related signatures, with most of the primary magma characteristics having been heavily masked by fractionation and crustal assimilation processes.

Description: The Pleistocene (~460–265 ka) San Venanzo volcanic complex belongs to the IAP (Intra-Apennine Province) in central Italy, which comprises at least four small Pleistocene monogenetic volcanoes plus several unrooted pyroclastic deposits with peculiar mineralogical and whole-rock chemical compositions. San Venanzo products are strongly SiO2-undersaturated, CaO- and MgO-rich and show ultrapotassic serial character. The relatively common occurrence of calcite in the pyroclastic rocks and the overall high CaO content are interpreted in literature as primary mineral. The main rock facies at San Venanzo are calcite-rich scoria and lapilli tuffs, with minor massive lava flows, and a rare pegmatoid variant (melilitolitic pockets). All the San Venanzo rocks are feldspar-free, with a typical paragenesis of forsteritic olivine, non-stoichiometric Ca-rich diopside, melilite, leucite, kalsilite, opaque minerals, nepheline, phlogopite, calcite, apatite, cuspidine, wollastonite, kirschsteinite-monticellite s.s. ± glass and other minor and very rare minerals typical of agpaitic melts. Based on petrographic analyses, the studied rocks can be classified as olivine melilitites, olivine leucite melilitites, venanzites (a local variant of kamafugites), calcite leucite melilitolites and Ca-rich olivine leucite melilitite tuffs. Mass balance calculations indicate a direct genetic link between the lava bodies and the pegmatoid melilitolitic pocket through a fractional crystallization process characterized by the removal of ~74% of a melilite-bearing uganditic cumulate made up of melilite, leucite, olivine, kalsilite and chromite. Primitive mantle-normalized patterns of the lavas and tuffs are rather spiked and share negative anomalies for Ba, Nb, Ta, P and Ti resembling typical magmas generated by supra-subduction mantle wedge. These compositions are very different from the only two other kamafugite localities outside Italy (Toro Ankole and Virunga in the East Africa Rift and Alto Paranaiba Igneous Province in SE Brazil). The melilitolite sample is more incompatible element-enriched than the other San Venanzo volcanic rocks, coherently with its evolved liquid composition proposed here. Major and trace element contents indicate a general depletion proportional to the amount of CaO content. The negative trends in Harker-type diagrams with CaO as abscissa are compatible with a process of variable interaction between a silicate magma with sedimentary marly carbonates/limestones. The presence of Mg-rich (Fo97–92) and rim-ward CaO-enriched (up to 1.72 wt%) euhedral olivine, as well as the presence of thin kirschsteinite rim around olivine crystals agree with a process of crustal carbonate assimilation by an originally strongly SiO2-undersaturated silicate magma. On the other hand, the lack of feldspars even in the rocks with the highest SiO2, the high CaO content, and the extreme SiO2-undersaturation of San Venanzo rocks exclude their derivation from a simple peridotitic source. In order to generate these peculiar compositions, the presence of a SiO2-K2O-CaO-rich H2O-bearing component, identified in a carbonated phlogopite peridotite is required. The results of different isotopic systematics (Sr-Nd-Pb-He-Ne-Ar) presented here are compatible with a process of crustal contamination both at mantle source levels (to explain the general N-S isotopic trends recorded in Quaternary volcanic rocks of Italian peninsula and Sicily) and with interaction of ultrabasic melts with limestones at shallow crustal depths.

Description: Published

Description: 103256

Description: 3V. Proprietà chimico-fisiche dei magmi e dei prodotti vulcanici

Description: JCR Journal

Description: The small Plio-Quaternary volcanic centre of Cabezo Negro de Tallante in SE Spain includes a thick deposit of polymictic pyroclastic tuff-breccia, whose fragments are agglutinated by a carbonate-rich component. This feature is also observed in other monogenetic volcanic centres cropping out in the Tallante-Cartagena volcanic district. The carbonate fraction has been recently interpreted in literature as representing a mantle component, therefore pointing to the existence of a diffuse carbonatitic activity in the area. Based on detailed sedimentological (presence of pisoids and root remnants), petrographic (presence of plagioclase and absence of euhedral silicate minerals in the calcite plagues), mineral chemistry (Ba-Sr-poor calcite composition), whole-rock chemistry (overall low incompatible element content in the pure carbonate fraction and a monotonous trace element negative correlation with CaO) as well as isotopic constraints (perfect correlations between Sr-Nd-Pb isotopic ratios with CaO in the basaltic and carbonate fraction, as well as heavy δ18O and light δ13C isotopic composition of the carbonate fraction), we propose a secondary origin for the carbonate component, excluding any contribution of mantle carbonatite melts. The presence of carbonates infiltrating the abundant mantle and crustal xenolith fragments found in the pyroclastic breccia is not related to the presence of carbonatitic melts at mantle to lower crustal depths, but to in-situ fragmentation of the Strombolian tuffbreccia deposit, followed by secondary carbonate infiltration. The pyroclastic breccia was indeed affected by an alternation of carbonate precipitation and dissolution in a vadose zone, where the activity of bacteria, fungi, roots and meteoric water led to the formation of a calcrete (caliche)-type deposits. Basaltic rocks (hawaiites and basanites) occur in the area as scoria and lava fragments in the pyroclastic breccia as well as small lava flows. They have been modelled with a low-degree partial melting of an amphibole-bearing peridotitic mantle close to the lithosphere-asthenosphere boundary. The origin of the mildly alkaline sodic basaltic activity in SE Spain post-dates the abundant and long-lasting subduction-related volcanic phase in the Betic Chain. Its origin is explained without requiring the presence of any thermal anomaly, but simply as consequence of the difference of lithospheric depths and edge-driven-type small-scale convection

Description: Published

Description: 106140

Description: 5V. Processi eruttivi e post-eruttivi

Description: JCR Journal