Search

[1] A comprehensive geochemical study, including B, Pb, Sr, and Nd isotopes, has been carried out on El Salvador subduction-related lavas. The rocks have arc-type incompatible element distributions with high LILE/HFSE ratios, nearly constant 143Nd/144Nd (≈0.5130), and small differences in 207Pb/204Pb (15.53–15.57), whereas 87Sr/86Sr ranges from 0.7035 to 0.7039. Boron isotopic composition varies widely, between −2.7‰ and +6.3‰. The boron isotope signature points to involvement of fluid inputs from (1) a high-δ11B serpentinite fluid from serpentized mantle wedge dragged beneath the volcanic arc or from the subducting lithosphere and (2) a low-δ11B fluid from the progressive dehydration of subducted altered basaltic crust and/or sediments. The observed sample variability is explained with a model in which different proportions of serpentinite-derived (10–50%) and slab-derived fluids are added to an enriched-DMM source, triggering its partial melting. We suggest a model in which tectonic erosion, i.e., dragging down of slivers of serpentinized upper plate mantle, was responsible for the occurrence of serpentinite reservoir, 11B-enriched in the forearc by shallow fluids.

Evidence for the deep recycling of surficial materials through the Earth's mantle and their antiquity has long been sought to understand the role of subducting plates and plumes in mantle convection. Radiogenic isotope evidence for such recycling remains equivocal because the age and location of parent-daughter fractionation are not known. Conversely, while stable isotopes can provide irrefutable evidence for low-temperature fractionation, their range in most unaltered oceanic basalts is limited and the age of any variation is unconstrained. Here we show that delta(18)O ratios in basalts from the Azores are often lower than in pristine mantle. This, combined with increased Nb/B ratios and a large range in delta(11)B ratios, provides compelling evidence for the recycling of materials that had undergone fractionation near the Earth's surface. Moreover, delta(11)B is negatively correlated with (187)Os/(188)Os ratios, which extend to subchondritic values, constraining the age of the high Nb/B, (11)B-enriched endmember to be more than 2.5 billion years (Gyr) old. We infer this component to be melt- and fluid-depleted lithospheric mantle from a subducted oceanic plate, whereas other Azores basalts contain a contribution from approximately 3-Gyr-old melt-enriched basalt. We conclude that both components are most probably derived from an Archaean oceanic plate that was subducted, arguably into the deep mantle, where it was stored until thermal buoyancy caused it to rise beneath the Azores islands approximately 3 Gyr later.

During the Late Miocene to Pleistocene, Western Anatolia and the Aegean area were affected by scattered alkali basaltic activity that was temporally distinct from the older orogenic magmatism related to the subduction of Africa beneath the Anatolian and Aegean plates. On the basis of geochemical and isotopic data, two groups of alkali basalts have been distinguished. The fi rst group (Foca, Urla, Selendi, Samos, Chios, Patmos, and Psathoura) exhibits a wide variability in isotopic composition ( 87 Sr/ 86 Sr 0.7043–0.7079; 143 Nd/ 144 Nd 0.51278–0.51243) and trace-element distribution (Th/Ta 2.4–12.3; Ba/Nb 14–49) probably acquired from a subduction-related component. The second group (Kula, Biga, Kalogeri, and Thrace), on the other hand, retains typical intraplate features with no subduction-related imprinting ( 87 Sr/ 86 Sr 0.7031–0.7035; 143 Nd/ 144 Nd 0.5130–0.51275; Th/Ta 1.2–1.7; Ba/Nb 5–10). The fi rst group of basalts marks the transition between subduction-related and intraplate activity, characterized by the interaction of a mantle source with residual slab fl uids, whereas the second group is an expression of a mantle with any subduction signature. Within the second group, the geochemical and isotope variations highlight the involvement of both mid-ocean-ridge basalt (MORB)–like and ocean-island basalt (OIB)–like mantle domains. Overall, the intraplate character of this alkaline association indicates that the mantle wedge, previously metasomatized by slab-derived material, was replaced by the upwelling of subslab mantle. This process is considered to be the consequence of the extension of the hanging-wall Aegean-Anatolian lithosphere, coupled with the subducted African slab, which was stretched and torn. In this interpretation, the track of the alkali basalts would be a useful marker tool of ruptures in the slab.

Sr-Nd-Pb-B isotope and incompatible trace element investigations were carried out on volcanics of the Phlegrean Volcanic District, which includes Campi Flegrei and the islands of Ischia and Procida (Campanian region, southern Italy). The results of these investigations allowed us to define several components in the genesis of magmas, including transitional mid-ocean-ridge basalt (MORB)???type asthenospheric mantle, aqueous fl uids, oceanic sediment melts derived from the subducted Ionian slab, and continental crust. A source contamination process that introduces variable proportions of slab-derived components into the mantle can explain the observed isotopic variations among the products of Ischia and Procida, and of Campi Flegrei older than 39 ka. Furthermore, comparison between Phlegrean Volcanic District and Aeolian arc volcanic rocks shows a different role of aqueous fluids and sediment melts from southeast to northwest. Energy-constrained assimilation and fractional crystallization modeling suggests that younger than 39 ka Campi Flegrei potassic trachybasalt magmas could have assimilated up to 2% of crustal melt, while evolving to trachyte during stagnation at ~8 km depth. Crustal assimilation and mixing/mingling processes among isotopically distinct magma batches at shallower depth account for geochemical and isotopic variations of younger than 39 ka Campi Flegrei volcanic rocks.

Different lithologies (impure marble, eclogite and granitic orthogneiss) sampled from a restricted area of the coesite-bearing Brossasco–Isasca Unit (Dora Maira Massif) have been investigated to examine the behaviour of 40Ar–39Ar and Rb–Sr systems in phengites developed under ultrahigh-pressure (UHP) metamorphism. Mineralogical and petrological data indicate that zoned phengites record distinct segments of the P–T path: prograde, peak to early retrograde in the marble, peak to early retrograde in the eclogite, and late retrograde in the orthogneiss. Besides major element zoning, ion microprobe analysis of phengite in the marble also reveals a pronounced zoning of trace elements (including Rb and Sr). 40Ar–39Ar apparent ages (~35-–62 Ma, marble; ~89–-170 Ma, eclogite; ~35-–52 Ma, orthogneiss), determined through Ar laserprobe data on phengites (step-heating and in situ techniques), show wide intra-sample and inter-sample variations closely linked to within-sample microchemical variations: apparent ages decrease with decreasing celadonite contents. These data confirm previous reports on excess Ar and, more significantly, highlight that phengite acted as a closed system in the different lithologies and that chemical exchange, not volume diffusion, was the main factor controlling the rate of Ar transport. Conversely, a Rb–Sr internal isochron from the same eclogite yields an age of ~36 Ma, overlapping with the time of the UHP metamorphic peak determined through U–Pb data and thereby corroborating the previous conclusion that UHP metamorphism and early retrogression occurred in close succession. Different phengite fractions of the marble yield calcite–phengite isochron ages of ~36 to ~60 Ma. Although this time interval matches Ar ages from the same sample, Rb–Sr data from phengite are not entirely consistent with the whole dataset. According to trace element variations in phengite, only Rb-–Sr data from two wet-ground phengite separates, yielding ages of ~36 and ~41 Ma, are internally consistent. The oldest age obtained from a millimetre-sized grain fraction enriched in prograde –peak phengites may represent a minimum age estimate for the prograde phengite relics. Results highlight the potential of the in situ 40Ar–-39Ar laser technique in resolving discrete P–T stages experienced by eclogite-facies rocks (provided that excess Ar is demonstrably a negligible factor), and confirm the potential of Rb–Sr internal mineral isochrons in providing precise crystallization ages for eclogite-facies mineral assemblages.

In El Salvador, Central America, active deformation takes the form of a major dextral strike-slip fault system, the El Salvador Fault Zone, resulting from the oblique subduction of the Cocos Plate. The fault system is laterally discontinuous, being subdivided into different major en-echelon segments that partially overlap to form pull-apart structures. Volcanic activity is spatially confined to the fault segments and absent in the intervening pull-apart basins; no significant temporal gap exists in the erupted products, at least during the Plio-Quaternary. Detailed analyses within the geothermal fields of Berlín and Ahuachapán have revealed important volcano-structural and petrologic differences between the two areas. In the Berlín area active deformation is controlled by the regional transcurrent stress field, resulting in the development of systems of right-lateral E–W-trending strike-slip faults. Conversely, the structural setting of the Ahuachapán area is more complex, reflecting an interaction among different stress fields. Berlín products exhibit a marked geochemical and isotopic homogeneity indicating the presence of a single magmatic system. At Ahuachapán, on the other hand, the rocks display significant variations in both Sr isotopes and the LILE/HFSE ratios: this area is characterized by multiple volcanic centres, fed by different magma batches that reach the surface without reciprocal interactions in shallow reservoirs. Thus, the characteristics of the volcanic products at Berlín and Ahuachapán reflect their different tectonic settings, with important implications for geothermal investigations.

High-pressure (HP) metamorphic blocks enclosed in a mafic to ultramafic matrix from a melange on the island of Syros are rimmed by tourmaline-bearing reaction zones (blackwalls). The B isotopic composition of dravitic tourmaline within these blackwalls was investigated in situ by secondary ion mass spectrometry. Boron in these tourmalines is unusually heavy, with d 11 B values exceeding þ18‰ in all investigated samples and reaching an extreme value of þ28.4‰ in one sample. Blackwalls formed during exhumation of the HP melange at a depth of 20-25 km at temperatures of 400-430 � C, by influx of external hydrous fluids. The compositions of the fluids are estimated to be in the range of 100-300 mg/g B with d 11 B values of þ18 to þ28‰. The high d 11 B values cannot be explained by tourmaline formation from unmodified slab-derived fluids. However, such fluids could interact with the material in the exhumation channel on their way from the dehydrating slab to the site of tourmaline formation in the blackwalls. This could produce exceptionally high d 11 B values in the fluids, a case that is modelled in this study. The model demonstrates that subduction fluids may be effectively modified in both trace element and isotopic composition during their migration through the material over- lying the subducting slab. Blackwall tourmaline from Syros has a large grain size (several centimetres), high abundance, and an exceptionally high d 11 B value. The formation of tourmaline at the contact between mafic or felsic HP blocks and their ultramafic matrix involved fluids released during dehydration reactions in the subducting slab. It forms a heavy-boron reservoir in hybrid rocks overlying the subducting slab, and may, thus, have a significant impact on the geochemical cycle of B and its isotopes in subduction zones.

The Western Anatolia Miocene-to-Present Day magmatism evolved from calc-alkaline and shoshonitic rocks (21-16 Ma) to lamproites (16-14 Ma), and eventually into OIB-type magmas (2-0 Ma) represented by the Kula volcanics. In the calc-alkaline and shoshonitic association, Sr and Nd isotopic ratios and trace element variations suggest that the interaction with the crust was moderate, so that the geochemistry of these rocks is considered to reflect the heterogeneous chemical nature of their mantle source. The ultrapotassic and lamproitic rocks are characterised by a high Sr and low Nd isotopic composition and are strongly enriched in K and Rb with respect to Ba, indicating a phlogopite-bearing lithospheric source. Low Sr and high Nd isotopic compositions, together with low LILE/HFSE ratios, reveal the OIB-type nature of the Kula volcanics. Therefore, the products switch from supra-subduction orogenic suites to volcanics coming from sub-slab astenospheric mantle. The evolution is interpreted as being due to a 'horizontal' stretching of the slab (no slab pull break-off) generated by different velocities in the subduction hangingwall lithosphere. This triggered the extensional movement between Greece and Turkey and the stretching into two slabs of the NE-directed African subduction, due to the faster southwestward slab rollback of Africa underneath Greece relative to the slab segment below Cyprus and Anatolia. (c) 2005 Elsevier B.V. All rights reserved.

Major- and trace-element, Sr-Nd isotopes, and mineral chemistry data were obtained for a collection of volcanic rock samples erupted by the Cay and Maca Quaternary volcanoes, Patagonian Andes (45°S, Chile). Cay and Maca are two large, adjacent stratovolcanoes that rise from the Chiloe block at the southern end of the southern volcanic zone (SVZ) of the Andes. Samples from the two volcanoes are typical medium-K, calc-alkaline rocks that form two roughly continuous, largely overlapping series from subalkaline basalt to dacite. The overall geochemistry of the samples studied is very similar to that observed for most volcanoes from the southern SVZ. The narrow range of Sr-Nd isotope compositions (87Sr/86Sr=0.70389-0.70431 and 143Nd/144Nd=0.51277-0.51284) and the major- and trace-element distributions indicate that the Cay and Maca magmas differentiated by crystal fractionation without significant contribution by crustal contamination. This is in accordance with the thin (

In 1999 the Istituto di Geoscienze e Georisorse (IGG), with the support of the International Atomic Energy Agency (IAEA), undertook the collection, preparation and distribution of eight geological materials intended for a blind interlaboratory comparison of measurements of boron isotopic composition and concentration. The materials came from Italian sources and consist of three natural waters (Mediterranean seawater and two groundwaters) and five rocks and minerals (tourmaline, basalt, obsidian, limestone and clay). The solid materials were crushed, milled and mixed, in preparation for distribution. Extensive assays performed at the IGG on these materials demonstrated that their boron isotopic and chemical compositions are homogeneous. Additional homogeneity tests were carried out on solid material fragments at the GeoForschungsZentrum Potsdam, with the specific objective of investigating the suitability of some of them for the calibration in situ of micro-analytical techniques. Two materials, B4 (tourmaline) and B6 (obsidian), proved to be isotopically homogeneous and may become excellent references for in situ microanalyses of boron isotopes. The materials described here were used as the basis of a major laboratory intercomparison study and are now available for further distribution from either the IAEA (solid materials) or the IGG (waters). L'Istituto di Geoscienze e Georisorse (IGG), avec le soutien de l'Agence Internationale de l'Energie Atomique (AIEA), a entrepris en 1999 la recolte, la preparation et la distribution de huit materiaux geologiques dans le but de conduire une comparaison en aveugle des mesures de composition isotopique et de concentration du bore entre differents laboratoires. Les materiaux, qui derivent tous de sites italiens, comprennent trois eaux naturelles (une eau de la Mer Mediterranee et deux eaux souterraines) et cinq roches et mineraux (tourmaline, basalte, obsidienne, calcaire et argile). Pour la distribution, les materiaux solides ont ete reduits en fragments, broyes et melanges et leur homogeneite de composition isotopique et chimique du bore a ete verifiee par des essais exhaustifs. D'autres essais d'homogeneite ont ete effectues au GeoForschungsZentrum Potsdam sur des fragments des materiaux solides avec le but particulier d'etudier la possibilite d'utilisation de ces materiaux pour la calibration in situ des techniques microanalytiques. Deux materiaux, le B4 (tourmaline) et le B6 (obsidienne), se sont reveles isotopiquement homogenes et donc peuvent devenir d'excellents materiaux de reference pour les microanalyses isotopiques in situ du bore. Les materiaux, qui sont decrits dans cette note, ont ete utilises pour une etude d'intercomparaison des mesures isotopiques et chimiques du bore entre differents laboratoire. Ils sont maintenant a disposition a l'AIEA (materiaux solides) et a l'IGG (eaux) pour leur distribution ulterieure.

This paper is focused on the role of boron coordination in determining the 11B/10B isotopic fractionation between melt/glass and biotite at magmatic temperatures. For this purpose, three evolved volcanic rocks from Roccastrada, Mt. Amiata, and Mt. Cimini belonging to the Neogene-Quaternary magmatism of central Italy were studied. In these samples, the measured boron biotite-glass partition coefficient ranges between 0.004 and 0.011, indicating that boron behaves as an incompatible element during biotite crystallization. The 11B magic-angle spinning nuclear magnetic resonance (NMR) spectra reveal the presence of trigonal BO3/2 units, tetrahedral BO4/2− sites, and three-coordinated BO2/2O− species containing one nonbridging oxygen. The relative contributions of these different boron sites were estimated by spectral deconvolution, and it was observed that the fraction of trigonally coordinated boron decreases with increasing K2O concentration in the glass. The 11B/10B isotopic fractionation between biotite and melt/glass was observed to be large even at magmatic temperatures and was found to be 1.0066 (Roccastrada sample), 1.00535 (Mt. Amiata sample), and 1.00279 (Mt. Cimini sample). Fractionation is mostly related to the relative amount of trigonal and tetrahedral boron sites in the glass network rather than to other processes, including the speciation of hydrous species in the glass structure. The measured α values are significantly higher than the calculated ones obtained using the reduced partition function ratios (RPFRs) for B(OH)3 and B(OH)4− as reported by Kakihana et al. (1977) and the abundance of trigonal and tetrahedral boron obtained by 11B NMR spectra. Furthermore, a nonlinear relationship is observed between the percentage of BO4 in the glass structure and the measured 1000lnα, suggesting that the approximation of monomeric B(OH)3 and B(OH)4− species contributions through ideal mixing in calculating the RPFRs in polyanions (Oi et al., 1989) probably does not apply to silicate glasses. The large B isotopic fractionation measured between glass and biotite and its dependence on the boron coordination in the glass are a limitation to the use of δ11B in the mineral to characterize magmas. Nonetheless, the high incompatible behavior of boron in the most common magmatic minerals rules out that fractional crystallization significantly modified the B isotopic composition of the melt.

At Elba Island (Italy), nine shallow-levei late Miocene granite porphyry layers connected by feeder dikes built up three nested Christmas-tree laccoliths. The intrusive units have been defined and correlated between exposures in western and centrai Elba on the basis of field,petrographic, and geochemical data, along with intrusive relationships. Detailed mapping and reconstuction of tectonic history led to restoration of the originai 5-km-thick sequence and determination of the dimensional parameters of each intrusive layer. Thè laccolith layers were emplaced at depths between 1.9 and 3.7 km, exploiting physical discontinuities that served as crustal magma traps inside a stack of nappes. The intrusive layers are 50-700 m thick, with diameters between 1.6 and 10 km. Length to thickness relationships for individuai laccolith layers show a power-law correlation that does not fìt the known dimensional distribution for laccoliths, but instead fit a line with a slope typical of the theoretical vertical-inflation stage of laccolith development This is interpreted as the first reported natural example of the occurrence of a vertical-inflation stage during laccolith growth. The dimensional data for Elba intrusive layers also suggest that laccoliths and plutons commonly form by amalgamation of smaller sheet-like bodies, while multilayer laccoliths form when coalescence faiis, possibly owing to the large availability of crustal magma traps.

New whole-rock B, Sr, Nd isotope ratios and 87Sr/86Sr on clinopyroxenes have been collected to study the enrichment of fluid mobile elements (FMEs) observed in Mt. Etna volcanics. Etna volcano, one of the most active in the world, is located in an extremely complex tectonic context at the boundary between colliding African and European plates. The analytical work focuses on current (1974–1998) and historic (1851–1971) eruptive activity, including some key prehistoric lavas, in order to interpret the secular shift of its geochemical signature to more alkaline compositions. Boron is used as a tool to unravel the role of fluids in the genesis of magmas, revealing far-reaching consequences, beyond the case study of Mt. Etna. Small variations are observed in δ11B (−3.5 to −8.0‰), 87Sr/86Sr (0.70323–0.70370), and 143Nd/144Nd (0.51293–0.51287). Moreover, temporal evolution to higher δ11B and 87Sr/86Sr, and to lower 143Nd/144Nd, is observed in the current activity, defining a regular trend. Sr isotopic equilibrium between whole-rock and clinopyroxene pairs indicates the successive introduction of three distinct magma types into the Etna plumbing system over time; these are characterized by differing degrees of FME enrichment. In addition, certain lavas exhibit evidence for country rock assimilation, magma–fluid interaction, or magma mixing in the shallow feeding system; at times these processes apparently lowered magmatic δ11B and/or induced Sr isotopic disequilibrium between whole rock and clinopyroxene. The regular increase of δ11B values is correlated with Nb/FME and 87Sr/86Sr ratios; these correlations are consistent with simple mixing between the mantle source and aqueous fluids derived from nearby Ionian slab. The best fit of Mt. Etna data is obtained using an enriched-MORB mantle source and a fluid phase with δ11B of about −2‰ and 87Sr/86Sr of 0.708. We argue that the slab window generated by differential roll-back of subducting Ionian lithosphere (with respect to Sicily) allows the upwelling of asthenosphere from below the subduction system and provides a suitable path of rise for subduction-related fluids. The increasing geochemical signature of fluid mobile elements enrichment to Mt. Etna lavas is considered a consequence of the progressive opening of slab window through time.

The regional distribution of geochemical and isotopic compositions of granitoid rocks from a Gondwana continental margin is studied to highlight its structure and geodynamic evolution. The intrusive rocks emplaced during the early Palaeozoic Ross Orogeny in northern Victoria Land (Antarctica) constitute a high-K calc-alkaline association. The geographic patterns of isotope and geochemical data on granitoid rocks allow the distinction of two portions of the continental margin, separated by a sharp discontinuity. The portion towards the palaeo-Pacific Ocean (Oceanward Side) displays strongly regular inland increase of Sr- and decrease of Nd-isotope ratios, coupled with analogous variations in major and trace elements; on this basis we infer a NW-SE-trending margin affected by SW-directed subduction. The portion towards the East Antarctic Craton (Continentward Side) shows a similar regular variation only for Nd isotope compositions, consistent with a hypothesis of a N-S-striking margin with west-ward subduction. In the Oceanward Side, isotope and trace-element characteristics suggest that the granites were generated by extensive interaction of mantle-derived magmas with high-level crustal melts. The origin of Continentward Side intrusives is compatible with a process of interaction between mantle-derived melts and a mafic granulite lower crust. The granitoids of the two crustal sectors share the same Proterozoic Sm-Nd model ages, suggesting that they both belong to the same crustal province. We interpret this arrangement of crustal segments as due to the shift and rotation of a forearc sliver of the Gondwana margin. This movement was likely enhanced by oblique subduction under an irregular margin weakened by the presence of a magmatic arc.

Two flank eruptions took place at Mt. Etna during 1989–1993. The former (September–October 1989) produced lavas among the most primitive of this century while the latter (December 1991–March 1993) was unusually long lasting and the largest of the last three centuries. Alkali basalts of 1989 displayed initial enrichments of Rb and K accompanied by an higher value of radiogenic-Sr and isotopic disequilibrium between host-rock and clinopyroxene (0.70364 and 0.70356, respectively). Subsequently, within one week, K, Rb and 87Sr86Sr ratio decreased to constant values (87Sr86Sr = 0.70355). Nd isotopic ratio did not show significant variations. These features can be explained by a selective contamination process involving essentially Rb, K and radiogenic-Sr. The bulk from the 1991–1993 eruption hawaiites, were erupted between January and May 1992. In this period, lavas (here called JML) were characterised by uniform geochemical and isotopic compositions and Sr-isotopic equilibrium between whole rock and pyroxene (0.70355). In other periods (December 1991 and June 1992) we observed lower and variable Sr isotopic compositions coupled with disequilibria between lavas and their clinopyroxenes (0.70351 in the w.r. and 0.70345 in the cpx of the sample of 15 December 1991). Starting from June 1992, the products seem to reveal fractionation effects as suggested by the continuous rise in concentrations of incompatible elements. When the observed variability cannot be ascribed to fractionation effects, we are not able to discriminate mixing from selective contamination. However, lavas erupted during December 1991 evolved towards the isotope composition of JML that was never exceeded, moreover the regular behaviour of Rb, K and radiogenic-Sr of 1989 lavas was not observed during the 1991–1993 eruption. Thus, we prefer to interpret these variations as mainly due to mixing processes between an end-member of JML composition and another one having the same, or lower, isotopic composition of the clinopyroxene found in the sample of December 1991. Several petrogenetic processes are contemporaneously active at Mt. Etna; they encompass magma mixing, selective contamination and fractional crystallization. Their relative importance varies according to the original volume of each magma batch and to the status of the shallow feeding system. The complexity of these processes, acting in the short time span of a few months, claims caution about the geochemical representativity of a single lava sample for each eruption.

Geological magazine, 127 (2), ISSN:0016-7568, ISSN:1469-5081

[1] Geochemical and chronological data for Cenozoic plutons and dikes from northern Victoria Land (Antarctica), were used to propose a tectonic-magmatic model for this portion of the West Antarctic Rift System (WARS). The seven major plutons are compositionally bimodal, with gabbroic and syenitic portions. Among the 180 studied dikes, most are 1 m thick and have alkali basalt-basanite-tephrite compositions, along with minor intermediate rocks. Trachytic-rhyolitic dikes (up to 50 m thick) are by far less common. The 40Ar-39Ar data for dikes indicate middle Eocene to early Oligocene ages, the oldest found to date for igneous activity throughout the WARS. The geochronological-structural framework provides evidence for coeval emplacement of dikes on two main NW-SE and N-S striking trends, whereas plutonic-subvolcanic activity occurred in adjacent crustal sectors at different times. Mafic dikes display major and trace element distribution typical of basalts from oceanic islands, such as high ratios of Nb and Ta to large ion lithophile and heavy rare earth elements, coupled with prominent negative K and Pb anomalies in the primitive mantle-normalized multielement diagrams. Initial isotopic compositions are within the ranges of 0.70299–0.70372 for 87Sr/86Sr and 4.2–6.3 for eNd. These features are shared by younger Neogene lavas. Geochemical modeling for both mafic dikes and lavas indicates an enriched magma source characterized by a residual potassic hydrous phase, thus pointing out a mantle source nearly uniform throughout 50 m.y. Comparable geochemical signature are reported for the magma source of the other Cenozoic volcanic provinces of the WARS and the whole Antarctic Plate. The new data for the early igneous rocks of the rift put in evidence a chronologic-structural link between magmatic evolution, regional tectonics and plate dynamics which suggests a model for WARS magmatism that is alternative to current plume hypotheses. We propose that magma genesis and emplacement are related to reactivation of preexisting translithospheric faults, which promoted local decompression melting of an enriched mantle that was previously veined during the decompression episode associated with the amagmatic late Cretaceous extensional rift phase.

At Monte Arci alkaline (hawaiites to trachytes), subalkaline with a marked calc-alkaline character (basalts to dacites) and rhyolitic lavas were erupted almost simultaneously in Late Pliocene time. Major- and trace-element chemistry, microprobe mineralogy and isotopic data suggest a partial melting origin for both rhyolites and subalkaline rocks. Different sources are however inferred for two rock series: homogeneous, calc-alkaline in nature for subalkaline rocks; unhomogeneous, richer in 87 Sr, for rhyolitic ones. Crystal fractionation differentiation from subcrustal alkali-basalts should have been the main process in the genesis of alkaline rocks. Large-scale contaminations with rhyolitic and/or alkaline rocks are evident in many of these lavas. Such a complicated magmatic association characterizes an area where volcanism related to post-collisional tensional movements in Pliocene time superimposes to Middle Miocene calc-alkaline basic volcanism related to previous subduction processes. The Pliocene volcanic history of Monte Arci emphasizes the influence of the paleogeodynamic environment on the nature of magmas erupted in post-continental collision areas, that are frequently difficult to arrange in the usual schemas connecting magma composition with tectonic setting.