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Chlorite/smectite - alkali feldspar metasomatic xenoliths from hyblean miocenic diatremes (Sicily, Italy): evidence for early interaction between hydrothermal brines and ultramafic/mafic rocks at crustal levels.
No full textUpper-Miocene tuff-breccia pipes from the Hyblean area (south-eastern Sicily) bear a number of ultramafic and mafic xenoliths (spinel-peridotites, pyroxenites, gabbroic rocks) whose study has led to conclude that the Meso-Caenozoic carbonatic and volcanic succession stands upon a fossil oceanic core-complex, probably connected with the adjacent Ionian crust.
Evidence for hydrothermal metasomatism is common in the Hyblean ultramafic and mafic xenoliths. In some cases, extreme metasomatic transformation and shearing have deleted original mineralogy and textures yielding unusual rocks, hereafter called “metasomatites”. These rocks generally exhibit cataclastic texture, their hydrothermal mineral assemblage consisting of Na-rich alkali feldspar, chlorite/smectite (C/S) and/or smectite/illite (S/I) mixed layers, Fe-Ti oxide/ hydroxide ± sieve-textured aegirine-augite ± titanite ± zircon. In addition, relics of Fo91 olivine, probably related to peridotite protoliths, rarely occur in these rocks. Comparing metasomatite whole chemistry to typical Hyblean oxide-gabbro, overall gain in H2O, alkalis, HFSE, U, Th, HREE and significant loss in Ca and Mg are remarkable. The studied xenoliths probably represent fragments of deep-seated, mafic/ultramafic fault-breccia, recording the effects of longlasting hypersaline hydrous fluids circulation in the Hyblean crustal basement
1988. Ultramafic nodules from Mount Etna (Sicily): Fluid inclusion and petrological studies.
No full textProceeding
1988. Ultramafic nodules from Mount Etna (Sicily): Fluid inclusion and petrological studies.
No full textProceeding
Geochemical features of eastern Sicily lithosphere as probed by Hyblean xenoliths and lavas
No full textUpper mantle and lower crust xenoliths of Quaternary basanitoid lavas and Miocene tuff-breccias of the Hyblean Volcanic Plateau (HVP), southeastern Sicily, have been studied for their REE contents and Sr-Nd isotope composition. Mantle rocks mainly consist of spinel-harzburgites, spinel-websterites and monomineralic assemblages of Al-augite, Plagioclase-bearing xenoliths are two-pyroxene mafic granulites and subordinate anorthosites. Pressure estimates point to a depth ranging from the lower crust down to the spinel-peridotite field (at least 45 km); temperature varies between 800 and 1050 degrees C. Mantle xenoliths have LREE(n) > HREE(n), high Nd (0.51289-0.51299) and low Sr (0.70298-0.70334) isotopic ratios, suggesting the occurrence of LREE enrichment in relatively recent times. The possibility that contamination is induced in the xenoliths by host lavas is discussed against the hypothesis of a Permo-Triassic age for the metasomatic event. Latter model is preferred due to the occurence of igneous activity in SE Sicily since Trias, to whole-rock T-DM(Nd) (200-270 Ma) and to the linear correlation of mantle samples in the Sm/Nd vs. Nd-143/Nd-144 diagram pointing to an ''age'' of 170 +/- 110 Ma. In this case, initial epsilon(Nd) = 6.8 +/- 0.9 denotes an asthenospheric origin for the metasomatic agent. Plagioclase-bearing xenoliths have Nd and Sr isotope ratios ranging between 0.51297-0.51288 and 0.70278-0.70292, respectively. In these lower crust mafic granulites, Sm/Nd whole-rock and mineral pairs yield ''ages'' ranging between 370 +/- 110 and 190 +/- 40 Ma. This suggests that, since Hercynian time, products of subalkaline and alkaline affinity have been adding to the crust and merges the evolution of the lower crust of SE Sicily to the cycle of igneous activity linked to the extensional tectonics affecting the northern margin of the African Plate. Sr-Nd isotopic and trace element data on Plio-Quaternary primitive lavas from the HVP provide a further sampling of the upper mantle of southeastern Sicily. Nd-Sr isotopic data of tholeiitic (Nd-143/Nd-144 = 0.51315-0.51306; Sr-87/Sr-86 = 0.70273-0.70285) and alkaline rocks (Nd-143/Nd-144 = 0.51299-0.51293; Sr-87/Sr-86 = 0.70302-0.70307) suggest that the genesis of HVP lavas is controlled by both a component akin to the source of MORB and a relatively enriched source similar to mantle xenoliths. A model is proposed in which both sources are present in the Hyblean lithosphere due to veining of the Thermal Boundary Layer
New petrological data for the Mt Iblei volcanic rocks, southeast Sicily: genetic and geotectonic implications)
No full textDuring the Cretaceous, Miocene and Plio-Quaternary time the SE Sicily was affected by widespread volcanism. Cretaceous volcanic rocks outcrop in the southern part of Sicily. They consist of basaltic flows, sometimes subaqueous, with a dominant alkaline character. Upper Miocene volcanism is characterized by numerous subaqueous eruptive centres, comprising volcaniclastic deposits interbedded with carbonate sediments. Alkaline and subalkaline magmas were contemporaneously erupted. In the Middle Upper Pliocene sequence alkaline and tholeiitic basalts are interbedded in a thick sequence of subaereal lava flows. Geochemical data suggest that the alkaline rocks could be derived by 4% and 10% of partial melting, whereas higher degrees of partial melting (35%) are necessary for the tholeiitic basalts. The association between alkaline and tholeiitic basalts can be related to the tectonics of eastern Sicily. -English summar
Gabbroic xenoliths in tuff-breccia pipes from the Hyblean Plateau: insights into the nature and composition of the lower crust underneath South-eastern Sicily, Italy
No full textCrust-derived xenoliths hosted by Miocene basaltic diatremes in the Hyblean Plateau (south-eastern Sicily, Italy) provide new information regarding the nature of a portion of the central Mediterranean lower crust. These xenoliths can be divided in three groups: gabbros (plagioclaseclinopyroxeneFe–Ti oxides +/- apatite +/- amphibole +/- Fe-rich green spinel), diorites (An-poor plagioclase, clinopyroxene +/- Fe–Ti oxides +/- orthopyroxene) and mafic granulites (plagioclaseclinopyroxenegreen spinel +/- orthopyroxene +/- Fe–Ti oxides). Gabbros form the main subject of this paper. They represent cumulates whose igneous texture has been locally obliterated by metamorphic recrystallization and shearing. They were permeated by Fe–Ti-rich melts related to tholeiitic-type fractional crystallisation process. Incompatible element ratios (Zr=Nb1⁄4 23 5–26; Y=Nb1⁄41.4–11) indicate that these cumulate gabbros derived from MORB liquids. Late-stage and hydrothermal fluids caused diverse, sometimes important, metasomatic trasformations. Petrographic and geochemical comparison with gabbroids from well-known geodynamic settings show that the Hyblean lower crustal xenoliths were probably formed in an oceanic or oceanic-continent transition environment
Geobarometria delle inclusioni fluide e indagini petrologiche in noduli e xenocristalli di lave alcaline etnee.
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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
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