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1.
The Rhodiani ophiolites are represented by two tectonically superimposed ophiolitic units: the “lower” Ultramafic unit and the “upper” Volcanic unit, both bearing calcareous sedimentary covers. The Ultramafic unit consists of mantle harzburgites with dunite pods and chromitite ores, and represents the typical mantle section of supra-subduction zone (SSZ) settings. The Volcanic unit is represented by a sheeted dyke complex overlain by a pillow and massive lava sequence, both including basalts, basaltic andesites, andesites, and dacites. Chemically, the Volcanic unit displays low-Ti affinity typical of island arc tholeiite (IAT) ophiolitic series from SSZ settings, having, as most distinctive chemical features, low Ti/V ratios (< 20) and depletion in high field strength elements and light rare earth elements.The rare earth element and incompatible element composition of the more primitive basaltic andesites from the Rhodiani ophiolites can be successfully reproduced with about 15% non-modal fractional melting of depleted lherzolites, which are very common in the Hellenide ophiolites. The calculated residua correspond to the depleted harzburgites found in the Rhodiani and Othrys ophiolites. Both field and chemical evidence suggest that the whole sequence of the Rhodiani Volcanic unit (from basalt to dacite) originated by low-pressure fractional crystallization under partially open-system conditions. The modelling of mantle source, melt generation, and mantle residua carried out in this paper provides new constraints for the tectono-magmatic evolution of the Mirdita–Pindos oceanic basin. 相似文献
2.
Abstract Ophiolites of the Mirdita–Subpelagonian zone form a nearly continuous belt in the Albanide–Hellenide orogen, including mid‐ocean ridge basalt (MORB) associations in the western Mirdita sector and supra‐subduction zone (SSZ) complexes, with prevalent island arc tholeiitic (IAT) and minor boninitic affinities in the eastern part of the belt (i.e. eastern Mirdita, Pindos, Vourinos). In addition, basalts with geochemical features intermediate between MORB and IAT (MORB/IAT) are found in the central Mirdita and in the Aspropotamos sequence (Pindos). These basalts alternate with pure MORB and are cut by boninitic dykes. The distinctive compositional characteristics of the mafic magmas parental to the different ophiolitic suites can be accounted for by partial melting of mantle sources progressively depleted by melt extractions. Partial melting processes (10–20%) of lherzolitic sources generated pure MORB, leaving clinopyroxene‐poor lherzolite as a residuum. Approximately 10% water‐assisted partial melting of this latter source, in an SSZ setting, may in turn generate basalts with MORB/IAT intermediate characteristics, whereas IAT basalts and boninites may have been derived from 10–20% and 30% partial melting, respectively, of the same source variably enriched by subduction‐derived fluids. In addition, boninites may also have been derived by comparatively lower degrees of hydrated partial melting of more refractory harzburgitic sources. A generalized petrologic model based on mass balance calculations between bulk rock and mineral compositions, indicate that most of the intrusives (from ultramafic cumulates to gabbronorites and plagiogranites), as well as sheeted dykes and volcanics (from basalts to rhyodacites) forming the bulk crustal section of the SSZ ophiolites, may be accounted for by shallow fractional crystallization from low‐Ti picritic parental magmas very similar in composition to IAT picrites from Pacific intraoceanic arcs. The most appropriate tectono‐magmatic model for the generation of the SSZ Tethyan ophiolites implies low velocity plate‐convergence of the intraoceanic subduction and generation of a nascent arc with IAT affinity and progressive slab roll‐back, mantle diapirism and extension from the arc axis to the forearc region, with generation of MORB/IAT intermediate basalts and boninitic magmas. 相似文献
3.
L. Beccaluva D. Ohnenstetter M. Ohnenstetter A. Paupy 《Contributions to Mineralogy and Petrology》1984,85(3):253-271
Within the Vourinos ophiolite evidence of two magmatic series has been preserved in cognate cumulates and in effusive and hypabyssal rocks, which constitute the earlier Krapa sequence and the younger Asprokambo sequence. The Asprokambo dyke basic magmas which are poor in incompatible elements and relatively Ni and Cr rich, bear some resemblance to very low Ti basalts (transitional to boninites) found in subduction related arcs or interarc basins. Krapa series magmas from sills, massive and pillow lavas are best equated with low-K tholeiites of island arc suites. Compositions of Al- and Ti- poor Cpx in lavas from both series are comparable to those in island arc basalts, the Asprokambo Cpx being richer in Ca and Cr than those from Krapa.The large volume of cumulates from the Krapa sequence displays the following crystallization order: Ol±Sp, Cpx, Pl±Opx, Mt. Periodic influx of fresh magma batches into the magma chamber occurred mainly during the formation of the lower cumulates (wehrlite, Ol-clinopyroxenite and melagabbro). The upper cumulates, gabbronorite and leucogabbronorite with minor Mt-bearing gabbronorite, crystallized in the upper levels of a magma chamber which became progressively smaller with time. In the Asprokambo sequence, Ol+Sp, Opx, Cpx, PI and Amph are the successively crystallizing phases. The ortho to heteradcumulates consist of websterite, Pl-websterite, gabbronorite, amphibole bearing leuconorite, diorite and granophyre. In cumulates, especially in the lower Krapa sequence, significant subsolidus reaction was probably induced by the persistence of high geothermal gradients linked to continuous magmatism. Petrological features indicate that the evolution of the Krapa series is more compatible with an intermediate fractional/equilibrium crystallization history in an initially open system, whereas nearly perfect fractional crystallization in closed system may have occurred in the small Asprokambo magma chambers. Chemical variations in the lavas of both series can be explained in terms of crystallization of the observed cumulates. Significantly, the Asprokambo intrusives have igneous Mg-hornblende and vanadium bearing, chromian, aluminous titaniferous magnetite, crystallization of which is responsible for the calcalkaline evolutionary trend of these rocks. Major and trace element modelling necessitates a two stage model for the petrogenesis of the Vourinos parental melts, involving high-degree remelting of previously depleted mantle sources favoured by the influx of subduction derived hydrous fluids. The primary magmas parental to the Krapa and Asprokambo series could have been derived respectively by 20 and 30% equilibrium partial fusion of variably depleted lherzolitic sources, leaving residua having a harzburgitic to dunitic composition. 相似文献
4.
Luigi Beccaluva Gianluca Bianchini Massimo Coltorti William Perkins Franca Siena Carmela Vaccaro Marjorie Wilson 《Contributions to Mineralogy and Petrology》2001,142(3):284-297
Peridotite xenoliths entrained in Plio-Pleistocene alkali basalts from Sardinia represent fragments of the uppermost lithospheric mantle, and are characterised by an anhydrous four-phase mineral assemblage. They range in bulk rock composition from fertile spinel-lherzolites to residual spinel-harzburgites. The Sr-Nd isotope and trace element composition of clinopyroxene mineral separates varies between LREE-depleted samples with 87Sr/86Sr as low as 0.70262 and 143Nd/144Nd up to 0.51323 and LREE-enriched samples with 87Sr/86Sr up to 0.70461 and 143Nd/144Nd down to 0.51252. The available data suggest that all the studied peridotite samples suffered variable degrees of partial melting during Pre-Mesozoic times (based on Nd model ages relative to CHUR and DMM). The overprinted enrichment is related to a subsequent metasomatism, induced by fluids rising through the lithosphere that preferentially percolated the originally most depleted domains. Despite the occurrence of orogenic volcanism in the area, preferential enrichment in elements typically associated with slab derived fluids/melts (K, Rb, Sr, Th) relative to LREE has not been detected, and metasomatism seems to be more likely related to the infiltration of highly alkaline basic melts characterised by an EM-like Sr-Nd isotopic composition. Similar 87Sr/86Sr-143Nd/144Nd compositions, characterised by an EM signature, are observed in anorogenic mafic lavas and peridotite xenoliths from widespread localities within the "European" plate, whereas they have not previously been recorded in peridotite xenoliths and associated alkaline mafic lavas from the stable "African" lithospheric domain. 相似文献
5.
Two basaltic fragments dredged from a volcanic seamount in the Sicilian continental slope of the Strait of Sicily show petrochemical features typical of within plate alkalibasaltic lavas and K/Ar age of about 10 m.y. The data represent the first volcanic evidence of Upper Miocene tensional tectonics in this sector of the African plate. 相似文献
6.
C. Natali L. Beccaluva G. Bianchini F. Siena 《Contributions to Mineralogy and Petrology》2013,166(2):351-370
The Axum–Adwa igneous complex consists of a basalt–trachyte (syenite) suite emplaced at the northern periphery of the Ethiopian plateau, after the paroxysmal eruption of the Oligocene (ca 30 Ma) continental flood basalts (CFB), which is related to the Afar plume activity. 40Ar/39Ar and K–Ar ages, carried out for the first time on felsic and basaltic rocks, constrain the magmatic age of the greater part of the complex around Axum to 19–15 Ma, whereas trachytic lavas from volcanic centres NE of Adwa are dated ca 27 Ma. The felsic compositions straddle the critical SiO2-saturation boundary, ranging from normative quartz trachyte lavas east of Adwa to normative (and modal) nepheline syenite subvolcanic domes (the obelisks stones of ancient axumites) around Axum. Petrogenetic modelling based on rock chemical data and phase equilibria calculations by PELE (Boudreau 1999) shows that low-pressure fractional crystallization processes, starting from mildly alkaline- and alkaline basalts comparable to those present in the complex, could generate SiO2-saturated trachytes and SiO2-undersaturated syenites, respectively, which correspond to residual liquid fractions of 17 and 10 %. The observed differentiation processes are consistent with the development of rifting events and formation of shallow magma chambers plausibly located between displaced (tilted) crustal blocks that favoured trapping of basaltic parental magmas and their fractionation to felsic differentiates. In syenitic domes, late- to post-magmatic processes are sometimes evidenced by secondary mineral associations (e.g. Bete Giorgis dome) which overprint the magmatic parageneses, and mainly induce additional nepheline and sodic pyroxene neo-crystallization. These metasomatic reactions were promoted by the circulation of Na–Cl-rich deuteric fluids (600–400 °C), as indicated by mineral and bulk rock chemical budgets as well as by δ18O analyses on mineral separates. The occurrence of this magmatism post-dating the CFB event, characterized by comparatively lower volume of more alkaline products, conforms to the progressive vanishing of the Afar plume thermal effects and the parallel decrease of the partial melting degrees of the related mantle sources. This evolution is also concomitant with the variation of the tectono-magmatic regime from regional lithospheric extension (CFB eruption) to localized rifting processes that favoured magmatic differentiation. 相似文献
7.
Geochronology and magmatic character of the pliocene-pleistocene volcanism in Sardinia (Italy) 总被引：2，自引：0，他引：2
L. Beccaluva M. Deriu G. Macciotta C. Savelli G. Venturelli 《Bulletin of Volcanology》1977,40(3):153-168
The Pliocene-Pleistocene subaerial volcanic activity of the island of Sardinia developed from about five million years ago to the Pleistocene. Volcanism was mainly fissural, related to rifting of the Sardinian crustal block and connected to intraplate tensional tectonics involving at the same time the area of the Tyrrhenian Sea. Areally the most abundant rocks are basic, ranging in serial character from alkaline to subalkaline types. In some areas intermediate and salic lavas also occur; trachytic and phonolitic rocks are mainly associated with basalts of alkaline affinity, whereas rhyolites and dacites are mainly related to subalkalic basalts. K/Ar data show that lavas of different serial character (from alkalic to subalkalic) are produced on the island within the same time range, from about four to two million years; it is to be noted, however, that the early products (about 5 million years) are mildly alkalic in character whereas most of the youngest products (0.6–0.14 million years) are strongly alkalic. 相似文献
8.
9.
L. Beccaluva M. Coltorti P. Di Girolamo L. Melluso L. Milani V. Morra F. Siena 《Mineralogy and Petrology》2002,74(2-4):277-297
Summary The Late Pleistocene Mt. Vulture strato-volcano developed at the intersection of NE-SW and NW-SE lithospheric fault systems,
on the easternmost border of the Apennine compressional front overthrust onto the Apulian foreland. The initial phase of the
volcanic activity is represented by pyroclastic deposits, including lava blocks, and subordinate eccentric domes, mostly phonolitic
in composition. The later stages of activity formed the bulk of the strato-volcano (pyroclastic products and subordinate lavas),
mostly tephritic in composition, with minor intercalations of basanite, mela-foidite and melilitite lavas and dikes.
Variations in rock and mineral composition suggest that the volumetrically predominant basanite-tephrite (foidite)-phonotephrite-phonolite
series can be accounted for by fractional crystallization processes starting from basanitic parental magmas, in agreement
with the remarkably constant 87Sr/86Sr isotopes (0.70586–0.70581). Mass-balance calculations indicate that the variably differentiated magmas may have been produced
by removal of wehrlite, clinopyroxenite and syenite cumulates, some of which are occasionally found as cognate xenoliths in
the volcanics. Fractionation processes probably developed in multiple-zoned magma chambers, at depths of 3–5 km, corresponding
to the tectonic discontinuity between the allochthonous Apennine formations and the underlying Apulian platform. Highly differentiated
phonolitic magmas capping the magma chambers and their conduits thus appear to have fed the initial volcanic activity, whereas
dominantly tephritic products were erupted in later stages.
The least evolved mafic magmas, namely basanites, mela-foidites and melilitites, are characterized by diverse Na/K ratios
and critical SiO2-undersaturation, which indicate their derivation as independent melts generated from distinct, heterogeneously enriched mantle
sources and by variable partial melting degrees.
Primitive mantle-normalized incompatible element patterns of Vulture mafic lavas invariably share analogies with both orogenic
subduction-related magmas (high Low Field Strength Elements/High Field Strength Elements ratios, K, Rb and Th contents and
marked Ti and Nb negative anomalies) and alkaline lavas from within-plate and rift settings (high Light Rare Earth Elements,
P, Zr, Nb and Na). These geochemical features may be accounted for by magma generation from deep lithospheric mantle sources,
enriched in Na-alkali silicate/carbonatite anorogenic components, subsequently affected by orogenic subduction-related K-metasomatism,
analogous to that which modified magma sources of the Roman Magmatic Province along the internal Apennine Chain.
Received April 12, 2000; revised version accepted June 7, 2001 相似文献
10.
Hydrogeochemical study in the Main Ethiopian Rift: new insights to the source and enrichment mechanism of fluoride 总被引：2，自引：0，他引：2
Tewodros Rango Gianluca Bianchini Luigi Beccaluva Tenalem Ayenew Nicolò Colombani 《Environmental Geology》2009,58(1):109-118
The central Main Ethiopian Rift suffers a severe water quality problem, characterized by an anomalously high fluoride (F)
content that causes an endemic fluorosis disease. The current study, conducted in the Ziway–Shala lakes basin, indicates that
the F content exceeds the permissible limit for drinking prescribed by the World Health Organization (WHO; 1.5 mg/l) in many
important wells (up to 20 mg/l), with even more extreme F concentration in hot springs and alkaline lakes (up to 97 and 384 mg/l
respectively). The groundwater and surface water from the highlands, typically characterized by low total dissolved solids
(TDS) and Ca (Mg)–HCO3 hydrochemical facies, do not show high F content. The subsequent interaction of these waters with the various rocks of the
rift valley induces a general increase of the TDS, and a variation of the chemical signature towards Na–HCO3 compositions, with a parallel enrichment of F. The interacting matrixes are mainly rhyolites consisting of volcanic glass
and only rare F-bearing accessory minerals (such as alkali amphibole). Comparing the abundance and the composition of the
glassy groundmass with other mineral phases, it appears that the former stores most of the total F budget. This glassy material
is extremely reactive, and its weathering products (i.e. fluvio/volcano-lacustrine sediments) further concentrate the fluoride.
The interaction of these “weathered/reworked” volcanic products with water and carbon dioxide at high pH causes the release
of fluoride into the interacting water. This mainly occurs by a process of base-exchange softening with the neo-formed clay
minerals (i.e. Ca–Mg uptake by the aquifer matrix, with release of Na into the groundwater). This is plausibly the main enrichment
mechanism that explains the high F content of the local groundwater, as evidenced by positive correlation between F, pH, and
Na, and inverse correlation between F and Ca (Mg). Saturation indices (SI) have been calculated (using PHREEQC-2) for the
different water groups, highlighting that the studied waters are undersaturated in fluorite. In these conditions, fluoride
cannot precipitate as CaF2, and so mobilizes freely without forming other complexes. These results have important implications for the development of
new exploitation strategies and accurate planning of new drilling sites.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献