A seismic source zone model for the seismic hazard assessment of the Italian territory   总被引：3，自引：0，他引：3  

Carlo Meletti  Fabrizio Galadini  Gianluca Valensise  Massimiliano Stucchi  Roberto Basili  Salvatore Barba  Gianfranco Vannucci  Enzo Boschi 《Tectonophysics》2008,450(1-4):85-108

We designed a new seismic source model for Italy to be used as an input for country-wide probabilistic seismic hazard assessment (PSHA) in the frame of the compilation of a new national reference map.

We started off by reviewing existing models available for Italy and for other European countries, then discussed the main open issues in the current practice of seismogenic zoning.

The new model, termed ZS9, is largely based on data collected in the past 10 years, including historical earthquakes and instrumental seismicity, active faults and their seismogenic potential, and seismotectonic evidence from recent earthquakes. This information allowed us to propose new interpretations for poorly understood areas where the new data are in conflict with assumptions made in designing the previous and widely used model ZS4.

ZS9 is made out of 36 zones where earthquakes with M_w > = 5 are expected. It also assumes that earthquakes with M_w up to 5 may occur anywhere outside the seismogenic zones, although the associated probability is rather low. Special care was taken to ensure that each zone sampled a large enough number of earthquakes so that we could compute reliable earthquake production rates.

Although it was drawn following criteria that are standard practice in PSHA, ZS9 is also innovative in that every zone is characterised also by its mean seismogenic depth (the depth of the crustal volume that will presumably release future earthquakes) and predominant focal mechanism (their most likely rupture mechanism). These properties were determined using instrumental data, and only in a limited number of cases we resorted to geologic constraints and expert judgment to cope with lack of data or conflicting indications. These attributes allow ZS9 to be used with more accurate regionalized depth-dependent attenuation relations, and are ultimately expected to increase significantly the reliability of seismic hazard estimates.  相似文献   

Trace element and isotope geochemistry of depleted peridotites from an N-MORB type ophiolite (Internal Liguride,N. Italy)   总被引：6，自引：0，他引：6  

E. Rampone  A. W. Hofmann  G. B. Piccardo  R. Vannucci  P. Bottazzi  L. Ottolini 《Contributions to Mineralogy and Petrology》1996,123(1):61-76

 Mantle peridotites of the Internal Liguride (IL) units (Northern Apennines) constitute a rare example of the depleted lithosphere of the Jurassic Ligurian Tethys. Detailed chemical (ICP-MS and SIMS techniques) and isotopic investigations on very fresh samples have been performed with the major aim to constrain the timing and mechanism of their evolution and to furnish new data for the geodynamic interpretation. The data are also useful to discuss some general geochemical aspects of oceanic-type mantle. The studied samples consist of clinopyroxene-poor spinel lherzolites, showing incipient re-equilibration in the plagioclase-facies stability field. The spinel-facies assemblage records high (asthenospheric) equilibration temperatures (1150–1250° C). Whole rocks, and constituent clinopyroxenes, show a decoupling between severe depletion in highly incompatible elements [light rare earth elements (LREE), Sr, Zr, Na, Ti] and less pronounced depletion in moderate incompatible elements (Ca, Al, Sc, V). Bulk rocks also display a relatively strong M(middle)REE/H(heavy)REE fractionation. These compositional features indicate low-degree (<10%) fractional melting, which presumably started in the garnet stability field, as the most suitable depletion mechanism. In this respect, the IL ultramafics show strong similarity to abyssal peridotites. The Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, indicate an extremely depleted signature (⁸⁷Sr/⁸⁶Sr=0.702203–0.702285; ¹⁴³Nd/¹⁴⁴Nd=0.513619–0.513775). The Sm/Nd model ages suggest that the IL peridotites melted most likely during Permian times. They could record, therefore, the early upwelling and melting of mid ocean ridge basalt (MORB) type asthenosphere, in response to the onset of extensional mechanisms which led to the opening of the Western Tethys. They subsequently cooled and experienced a composite subsolidus evolution testified by multiple episodes of gabbroic intrusions and HT-LP retrograde metamorphic re-equilibration, prior to their emplacement on the sea floor. The trace element chemistry of IL peridotites also provides useful information about the composition of oceanic-type mantle. The most important feature concerns the occurrence of Sr and Zr negative anomalies (relative to “adjacent” REE) in both clinopyroxenes and bulk rocks. We suggest that such anomalies reflect changes in the relative magnitude of Sr, Zr and REE partition coefficients, depending on the specific melting conditions. Received: 15 February 1995/Accepted: 4 August 1995  相似文献   

Determination of Lithium, Beryllium and Boron at Trace Levels by Laser Ablation-Inductively Coupled Plasma-Sector Field Mass Spectrometry   总被引：1，自引：2，他引：1  

Massimo Tiepolo  Alberto Zanetti  Riccardo Vannucci 《Geostandards and Geoanalytical Research》2005,29(2):211-224

The analytical capabilities of laser ablation (LA)-ICP-MS in determining Li, Be and B at trace levels in geological samples have been tested on a series of glass reference materials and natural samples. The LA-ICP-MS instrument used consisted of a sector-field ICP-MS coupled with a laser ablation microprobe operating at either 266 or 213 nm wavelength. Reference glasses from NIST (SRM 612, 614 and 616) and MPI-DING (KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, T1-G and ATHO-G) were selected to develop the analytical method and to assess the best instrumental configuration. A series of calcic amphiboles with different Li, Be and B concentrations were also analysed using both LA-ICP-MS and SIMS to test the applicability of the method to natural minerals. Results indicated that with a spot size of 40 μm the agreement between measured and reference values of Li, Be and B is generally better than 10% for NIST SRM 612 and 20% for NIST SRM 614. Average reproducibility at the 2s level was 10% for Li, 20% for Be and 15% for B. Limits of detection were approximately 100 ng g^-1 for Be and B and 200 ng g^-1 for Li. These results were confirmed by analyses carried out on natural amphiboles and compared well in terms of precision and accuracy with those commonly achieved by SIMS.  相似文献   

Spinel-peridotite xenoliths from Kapfenstein (Graz Basin,Eastern Austria): A geochemical and petrological study     

O. Vasellil  H. Downes  M. F. Thirlwall  R. Vannucci  N. Coradossi 《Mineralogy and Petrology》1996,57(1-2):23-50

Summary Ultramafic xenoliths from the basaltic tuff of Kapfenstein (Graz Basin) are mainly protogranular and protogranular- to- porphyroclastic spinel lherzolites. In addition to these common rocks, a coarse-grained spinel harzburgite and a heterogeneous spinel peridotite consisting of a clinopyroxene-poor harzburgite and a clinopyroxene-rich lherzolite have been found. Interstitial amphiboles (pargasite and kaersutite) are present in most xenoliths. The magmatic activity which brought these xenoliths to the surface is part of the Plio-Pleistocene extension-related alkali basalt volcanism which took place in the Carpatho-Pannonian Region, post-dating the subduction-related calc-alkaline volcanism which formed the Inner Carpathian arc.Mineralogical and bulk geochemical data for Kapfenstein xenoliths are similar to those for other sub-continental shallow mantle xenolith suites. Correlation diagrams for major and trace elements indicate that their chemical variations are related to different degrees of partial melting due to the extraction of mafic melts. Rare earth element (REE) patterns of separated clinopyroxenes from the xenoliths are generally light rare earth element (LREE)-depleted, although LREE-enriched and more complex patterns have also been found.⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/.¹⁴⁴Nd isotopic ratios of separated clinopyroxenes vary between 0.70193 and 0.70330 (^Sr, = – 36.4 to –17.0) and 0.512824 and 0.513589 (_NdNd = 4.2 to 18.l), respectively. The highest⁸⁷Sr/⁸⁶Sr and lowest¹⁴⁴³Nd/¹⁴⁴Nd.Nd ratios occur in those samples which have the most refractory character and also have the most unusual REE patterns. REE determined by ion microprobe on separated interstitial amphibole grains indicate equilibrium with the host peridotite in most of the xenoliths but disequilibirum conditions have been observed, including some extreme LREE enrichments.Striking textural and some geochemical and isotope geochemical similarities have been recognised between Kapfenstein xenoliths and those from the Eastern Transylvanian Basin, both of which are situated on the edges of the diapiric upwelling located in the central part of the Pannonian Basin. The deformation, depletion and enrichment processes which occurred in these two areas are thought to be similar. The LREE-enriched interstitial amphiboles and the presence of cross-cutting amphibole veins suggest that the shallow depleted lithospheric mantle beneath the Graz Basin has been affected by metasomatising events related to the mafic alkaline host magmas rather than by subduction-related fluids or melts.

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Spinel-peridotit Xenolithe von Kapfenstein (Grazer Becken, Ostösterreich): Eine geochemische und petrologische Studie

Zusammenfassung Die ultramafischen Xenolithe aus dem Basaltuff von Kapfenstein (Grazer Becken) bestehen großteils aus protogranularem und porphyroklastischem Spinell-Lherzolith. Abgesehen von diesen weitverbreiteten Gesteinen wurden auch ein grobköringer Spinell-Harzburgit und ein heterogener Spinell-Peridotit, der aus einem Klinopyroxen-armen Harzburgit und einem Klinopyroxen-reichen Lherzolith besteht, gefunden. Interstitielle Amphibole (Pargasit und Kaersutit) sind in den meisten Xenolithen vorhanden. Die magmatischen Prozesse, die diese Xenolithe an die Erdoberfläche befördeten, stehen mit dem mit Extension assoziierten Alkali-Basalt-Vulkanismus, der während des Pliozän/ Pleistozäns dem durch Subduktion ausgelösten Kalk-alkalischen Vulkanismus des inneren Karpathenbogens folgte, in Zusammenhang.Die mineralogischen und geochemischen Vollanalysen der Kapfensteiner Xenolithe sind mit den Daten von anderen Gruppen von subkontinentalen Xenolithen des oberen Mantels vergleichbar. Korrelationsdiagramme für Haupt- und Spurenelemente zeigen, daß die chemische Variation auf verschiedene Grade von Teilschmelzbildung zurück-zuführen ist. Die Verteilungsmuster der Seltenen Erden in Klinopyroxenen der Xenolithe sind generall LREE-verarmt, aber LREE angereichte und kompliziertere Verteilungsmuster kommen vor. Die⁸⁷Sr/⁸⁶Sr und¹⁴³Nd/1⁴⁴Nd Isotopenverhältnisse der Klinopyroxene schwanken zwischen 0.70193 und 0.70330 (_Sr = von – 36.4 bis –17.0), beziechungsweise zwischen 0.512854 und 0.513589 (_Nd=von –4.2 bis 18.l). Die höchsten⁸⁷Sr/⁸⁶Sr und niedrigsten¹⁴³Nd/¹⁴⁴Nd Isotopenverhältnisse treten in den Proben mit dem stärksten refraktären Charakter und den ungewöhnlichsten REE-Verteilungsmustern auf. Die mittels Zonensonde ermittelten Gehalte an REE der interstitiellen Amphibole deuten ein Gleichgewicht mit dem Wirt-Peridotit an, obwohl auch Anzeichen von gestörtem Gleichgewicht, unter anderem extreme Anreicherungen an LREE, beobachtet wurden.Ein Vergleich der Kapfenstein-Xenolithe mit denen aus dem östlichen Transsylvanischen Becken zeigt bemerkenswerte strukturelle und einige geochemische und isotopengeochemische Ähnlichkeiten. Beide Gegenden befinden sich am Rande des Erdmantel diapirs, im Zentrum des Pannonischen Beckens. Die Deformationen und Verarmungsund Anreicherungsprozesse, die in beiden Gebieten stattfanden, sind vergleichbar. Die an LREE angereicherten interstitiellen Amphibole und das Auftreten von diskordanten Amphibolgängen legen nahe, daß der obere verarmte lithosphärische Mantel unter dem Grazer Becken von, mit den mafischen alkalischen Grund-Magmen verwandten, metasomatischen Prozessen erfaßt worden ist, und nicht von mit Subduktion assoziierten Flüssigkeiten oder Schmelzen.

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With 7 Figures  相似文献   

The relationship between websterite and peridotite in the Balmuccia peridotite massif (NW Italy) as revealed by trace element variations in clinopyroxene   总被引：7，自引：0，他引：7  

Rivalenti  Giorgio  Mazzucchelli  Maurizio  Vannucci  Riccardo  Hofmann  Albrecht W.  Ottolini  Luisa  Bottazzi  Piero  Obermiller  Walter 《Contributions to Mineralogy and Petrology》1995,122(3):275-288

 Field, mineralogical and petrological data are presented on a newly found carbonatite occurrence associated with “kamafugite” lava at Cupaello, central Italy. This carbonatite occurrence is part of the Late Pleistocene Umbria-Latium ultra-alkaline district (ULUD) which extends southwards within the Apennines to Mount Vulture, delineating an important magmatic province along the most peripheral belt of the Tyrrhenian extensional tectonic system. This province is distinct, but probably related genetically with the more abundant and common leucite-bearing assemblages of the Roman Comagmatic Region and represents the first reported occurrence of carbonatite assemblages in the Mediterranean Basin. The Cupaello suite indicates that primary or near-primary mantle silicate melts of “kamafugitic” composition are transitional with Ca-carbonatite liquid and provides direct evidence of immiscibility of carbonatite from “kamafugite” magma. It is inferred that a primary mantle origin of Ca-carbonatites is conditional upon a potential silicate magma that may be coupled with the carbonatite, but may not have reached the surface. The data indicate a strong genetic link between ULUD Ca-carbonatites and some African analogues, supporting the view that their genesis depends on similar source and associated tectonic conditions. Received: 17 January 1995 / Accepted: 14 June 1995  相似文献   

Subsolidus reactions monitored by trace element partitioning: the spinel- to plagioclase-facies transition in mantle peridotites     

E. Rampone  G. B. Piccardo  R. Vannucci  P. Bottazzi  L. Ottolini 《Contributions to Mineralogy and Petrology》1993,115(1):1-17

Mantle peridotites of the External Liguride (EL) Units (Northern Apennines) mainly consist of fertile spinel-lherzolites partially recrystallized to plagioclase-facies assemblages, and are consequently appropriate to investigate the interphase element partitioning related to the transition from spinel- to plagioclase-facies stability field. Evidence for the development of the plagioclase-facies assemblage is mainly given by: (1) large exsolution lamellae of orthopyroxene and plagioclase within spinel-facies clinopyroxene; (2) plagioclase rims around spinel; (3) granoblastic domains made up of olivine+plagioclase±clino-and orthopyroxene. In situ major and trace [REE (rare-earth elements), Ti, Sc, V, Cr, Sr, Y, Zr and Ba] element mineral analyses have been performed, by electron and ion probe, on selected samples which show the progressive development of the plagioclase-bearing assemblage. The main compositional variations observed during the change from spinel- to plagioclase-facies minerals are as follows: (1) clinopyroxenes decrease in Al, Na, Sr, Eu/Eu^* and increase in Y, V, Sc, Cr, Zr and Ti; (2) amphiboles decrease in Eu/Eu^*, Sr, Ba and increase in Zr and V; (3) spinels decrease in Al and increase in Cr and Ti. The most striking feature is the decoupling in the behaviour of similarly incompatible elements (D about 0.1) in clinopyroxene, e.g. Sr decrease is mirrored by Zr increase. Massbalance calculations indicate that the trace element interphase redistribution documented in the EL peridotites occurred in a closed system and in response to the metamorphic reaction governing the transition from the spinel- to the plagioclase-facies stability field. The observed element partitioning reveals, moreover, that subsolidus re-equilibration processes in the upper mantle produce HFSE (high-field-strength element)/REE fractionation in minerals, which must be evaluated for a reliable determination of mineral-melt distribution coefficients. The results of this study furnish evidence for subsolidus metamorphic evolution during decompression, without concomitant partial melting processes. This is consistent with the interpretation that the EL peridotites represent subcontinental lithospheric mantle emplaced at the surface in response to lithospheric thinning and tectonic denudation mechanisms related to the Triassic-Jurassic rifting of the Ligure-Piedmontese basin.  相似文献   

N-MORB crust beneath Fuerteventura in the easternmost part of the Canary Islands: evidence from gabbroic xenoliths     

Else-Ragnhild?NeumannEmail author  Riccardo?Vannucci  Massimo?Tiepolo 《Contributions to Mineralogy and Petrology》2005,150(2):156-173

Gabbro xenoliths reported in this paper were collected in northern Fuerteventura, the Canary Island located closest to the coast of Africa. The xenoliths are very fresh and consist of Ti–Al-poor clinopyroxene + plagioclase (An_87–67) + olivine (Fo_72–86) ± orthopyroxene. Clinopyroxene and orthopyroxene are constantly and markedly depleted in light rare earth elements (LREE) relative to heavy REE (HREE), as expected for cumulus minerals formed from highly refractory N-MORB-type melts. In contrast, whole-rock Primordial Mantle-normalized trace element patterns range from mildly S-shaped (mildly depleted in Pr–Sm relative to both the strongly incompatible elements Rb–La and the HREE) to enriched. Estimates show that the trace element compositions of the rocks and their minerals are compatible with formation as N-MORB gabbro cumulates, which have been infiltrated at various extents (≤1% to >5%) by enriched alkali basaltic melts. The enriched material is mainly concentrated along grain boundaries and cracks through mineral grains, suggesting that the infiltration is relatively recent, and is thus associated with the Canary Islands magmatism. Our data contradict the hypothesis that a mantle plume was present in this area during the opening of the Atlantic Ocean. No evidence of continental material that might reflect attenuated continental crust in the area has been found. Gabbro xenoliths with REE and trace element compositions similar to those exhibited by the Fuerteventura gabbros are also found among gabbro xenoliths from the islands of La Palma (western Canary Islands) and Lanzarote. The compositions of the most depleted samples from these islands are closely similar, implying that there was no significant change in chemistry during the early stages of formation of the Atlantic oceanic crust in this area. Strongly depleted gabbros similar to those collected in Fuerteventura have also been retrieved in the MARK area along the central Mid-Atlantic Ridge. The presence of N-MORB oceanic crust beneath Fuerteventura implies that the continent–ocean transition in the Canary Islands area must be relatively sharp, in contrast to the situation both further north along the coast of Morocco, and along the Iberian peninsula.  相似文献   

Crystal recycling in the steady-state system of the active Stromboli volcano: a 2.5-ka story inferred from in situ Sr-isotope and trace element data     

Lorella Francalanci  Riccardo Avanzinelli  Isabella Nardini  Massimo Tiepolo  Jon P. Davidson  Riccardo Vannucci 《Contributions to Mineralogy and Petrology》2012,163(1):109-131

In situ Sr-isotope data by microdrilling, coupled with major and trace element analyses, have been performed on plagioclase and clinopyroxene from seven samples collected during the 2002–2003 eruptive crisis at Stromboli volcano (Aeolian Islands, Italy). On 28 December 2002, the persistent moderate explosive activity was broken by an effusive event lasting about 7 months. A more violent explosion (paroxysm) occurred on 5 April 2003. Two magma types were erupted, namely a volatile-poor and highly porphyritic magma (HP-magma) poured out as scoria or lava and a volatile-rich, phenocryst-poor magma (LP-magma) found as pumice. LP-magma differs from the HP-magma also for its slightly less-evolved chemistry, the groundmass composition and the lower Sr-isotope ratios. Micro-Sr-isotope data show the presence of zoned minerals in strong isotope disequilibrium, as previously found in products erupted in 1984, 1985 and 1996 AD, with ⁸⁷Sr/⁸⁶Sr values generally decreasing from cores to rims of minerals. Only some outer rims testify for equilibrium with the host groundmass. The internal mineral zones with high Sr-isotope ratios (0.70665–0.70618) are interpreted as ‘antecrysts’, crystallised during the previous activity and recycled in the present-day system since the opening shoshonitic activity of the Recent Period, which occurred at about 2.5 ka ago. This result has implications for the dynamics of the present-day plumbing system of Stromboli at intermediate pressure (about 2–3 km depth) and allows us to propose a model whereby an HP-magma reservoir is directly interconnected at the bottom with a cumulate crystal much reservoir. Efficient mixing between residing HP- and input LP-magmas can occur in this reservoir, due to more similar rheological characteristics of the two magmas than in the conduit, where crystallisation is enhanced by degassing. Antecrysts (and possibly melts) re-enter in the HP-magma reservoir both from the bottom, recycled by ascending LP-magmas crossing the crystal mush, and from the top, recycled by descending degassed and dense HP-magma, residual of the periodic Strombolian explosions at the surface. The isotope variation measured in the groundmasses allows calculating the proportion of the LP-magma entering the shallow HP-magma reservoir at ~20%. From this proportion, we estimate that the total volume of LP-magma input during 2002–2003 closely matches the magma volume erupted in the effusive event, suggesting a steady-state system at broadly constant volume. The comparison with estimates of the LP-magma volume ejected by the paroxysm indicates that the LP-magma amount directly reaching the surface during the 5 April paroxysm is minimal with respect to that entering the system.  相似文献   

Trace element and Pb–B–Li isotope systematics of olivine-hosted melt inclusions: insights into source metasomatism beneath Stromboli (southern Italy)     

Federica?SchiaviEmail author  Katsura?Kobayashi  Eizo?Nakamura  Massimo?Tiepolo  Riccardo?Vannucci 《Contributions to Mineralogy and Petrology》2012,163(6):1011-1031

We studied the elemental and isotopic (Pb, B and Li isotopes) composition of melt inclusions hosted in highly forsteritic (Fo_83–91) olivines that were collected from San Bartolo lava and pumice (ST79p, ST82p and ST531p) samples erupted by Stromboli in historical times. The studied melt inclusions have primitive calcalkaline to shoshonitic basaltic compositions. They cover a compositional range far wider than that exhibited by the whole-rocks and differ in key trace element ratios. San Bartolo melt inclusions are characterized by lower incompatible trace element abundances, higher ratios between fluid-mobile (B, Pb, U and LILE) and less fluid-mobile (REE, Th, HFSE) elements and lower La/Yb ratios relative to the pumice-hosted melt inclusions and pumiceous melts erupted during paroxysmal events. Trace elements, along with different Pb, B and Li isotopic signatures, attest to source heterogeneity on the small scale and provide new insights into subducted components beneath Stromboli. Results of a mixing model suggest that metasomatism of the mantle source of pumice-hosted melt inclusions was driven by solute-rich high-pressure fluids (<20%) expelled from the deep portion of the slab. Heterogeneous Pb isotopic composition together with light δ¹¹B (−8.6 to −13.7‰) and δ⁷Li (+2.3 to −1.7‰) indicates that high-pressure liquids were released in variable proportions from highly dehydrated metabasalts and metasediments. On the other hand, the elemental and isotopic (δ¹¹B ~ −1.9 to −5.9‰) composition of San Bartolo melt inclusions is better explained by the addition of a prevalent aqueous component (~2 to 4%) escaped at shallower depths from sediments and altered basaltic crust in almost equivalent proportions, with a smaller contribution by high-pressure fluids. Owing to the high-angle dip of the subducted cold Ionian slab, aqueous fluids and high-pressure fluids would rise through the mantle wedge and locally superimpose on each other, thus giving origin to variously metasomatized mantle domains.  相似文献   

The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism   总被引：13，自引：0，他引：13  

Alberto Zanetti  Maurizio Mazzucchelli  Giorgio Rivalenti  Riccardo Vannucci 《Contributions to Mineralogy and Petrology》1999,134(2-3):107-122

The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite (websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (La_N/Yb_N = 8–17) and depleted in Nb. It has high radiogenic Sr (⁸⁷Sr/⁸⁶Sr₍₂₇₀₎ = 0.7055–0.7093), low radiogenic Nd (ɛNd₍₂₇₀₎= −1 to −3) and EMII-like Pb isotopes. Two pyroxenite – peridotite sections examined in detail show the virtual absence of major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes exhibit LREE-enriched patterns (La_N/Yb_N ∼16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative Sr anomaly, but incompatible element concentration ∼1.9 (Sr) to ∼7.9 (Ti) times higher than that of coexisting clinopyroxene. Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg## and higher in Na₂O, and spinels and phlogopite are richer in Cr₂O₃. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical signatures of “carbonatite” metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and Sr concentrations than amphibole (^amph/cpxD_REE,Sr = 0.7–0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling, the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics. In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process, which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence of some CO₂ in the slab-derived melt. The CO₂/H₂O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and with the occurrence of modal metasomatism before peridotite incorporation in the crust. Received: 20 July 1998 / Accepted: 28 October 1998  相似文献