Incompatible Element Patterns Research Articles

Spatial and temporal evolution of magmatic arcs: geochemical diversity and tectonic controls

We review the main petrologic and geochemical features of magmatic rocks from continental arcs compared to their intra-oceanic equivalents. Both types share a number of characteristics, including their typical HFSE-depleted incompatible element patterns and isotopic Sr, Nd, Pb and Hf features, intermediate between those of N-MORB and continental crust. They are thought to derive from the partial melting of the mantle wedge metasomatized by fluids emanating from the downgoing plate. They are dominantly hydrous fluids provided by the breakdown of amphibolites and antigorite serpentinites, but a significant role is attributed to felsic hydrous melts derived from oceanic metabasalts and associated sediments. The more enriched character of magmas from continental arcs with respect to those from intra-oceanic arcs is ascribed to their genesis from a less depleted mantle wedge in a milder thermal regime, to the enrichment of their source via subduction erosion of the continental margin, and finally to their contamination during ascent and storage within a thicker crust.

Tectonic dismemberment and plume-ridge interaction in the Sub-Antarctic South Atlantic Ocean Basin: Confirmation from in situ geochemical and Sr isotopic compositions of minerals

The Sub-Antarctic South Atlantic Ocean has been inferred to have undergone a complex tectonic history involving dismemberment and plume-ridge interaction. Here we report new in situ major (electron probe microanalysis, EPMA), trace element (laser ablation−inductively coupled plasma−mass spectrometry, LA-ICP-MS), and Sr isotopic (laser ablation−multicollector−inductively coupled plasma−mass spectrometry, LA-MC-ICP-MS) compositions for minerals (olivine, clinopyroxene, and plagioclase) from the Northeast Georgia Rise (NGR; Sites 698 and 699), Islas Orcadas Rise (IOR; Site 701), and Meteor Rise (MR; Site 703) volcanic samples in the Sub-Antarctic South Atlantic Ocean. Plagioclases and clinopyroxenes from Sites 698, 699, and 703 are characterized by similar incompatible element patterns and Sr isotopic characteristics, suggesting that they are likely to have originated from a cogenetic mantle source. The plagioclases and clinopyroxenes exhibit an oscillatory, reverse, and normal zoning texture, and display clear evidence of Sr isotopic disequilibrium, suggesting dynamic and open fractional-crystallization processes as well as extensive mixing of compositionally distinct magmas. Rims and groundmass of plagioclase exhibit much more highly radiogenic Sr isotopes than their cores, likely indicating the involvement of both continental lithospheric and recycled oceanic crust. The in situ geochemical and isotopic compositions of these minerals exhibit the features of both the oceanic-island basalt−type Tristan-Gough mantle plume track and the normal mid-oceanic-ridge basalt−type Mid-Atlantic spreading ridge (MAR) and Agulhas spreading ridge (AR) track. We speculate that the NGR, IOR, and MR were formed from the same mantle source with volcanic flow of the Tristan-Gough mantle plume. Subsequently, tectonic movement along the MAR and AR separated the originally combined MR-IOR-NGR, resulting in the incursion of depleted asthenospheric mantle and the contamination contributions of continental and recycled oceanic crust components.

Miocene olivine leucitites in southern Tibet: Implications for the recycling of carbonates into the mantle during continental subduction

We present a comprehensive geochronological, mineralogical, and geochemical study on previously unknown olivine leucitites occurring in the Maiga area of southern Tibet to examine the possibility of carbonate recycling during continental subduction. The Miocene (∼16 Ma) Maiga olivine leucitites are characterized by low SiO2 (44.6–46.3 wt%), high MgO (14.0–15.7 wt%), and ultra-calcic (CaO/Al2O3 = 1.1–1.2) compositions, along with arc-like incompatible element patterns (e.g., enrichment in large-ion lithophile and light rare earth elements, and depletion in high-field-strength elements). These rocks also have extremely radiogenic 87Sr/86Sr(i) (0.715647–0.715844), unradiogenic εNd(i) (−11.6 to −11.2) and εHf(i) (−10.5 to −10.0), and moderately radiogenic 206Pb/204Pb(i) (18.44–18.46) isotopic compositions. The studied samples are also characterized by slightly heavy O isotope ratios (clinopyroxene δ18O = +6.7‰ to +8.7‰) and relatively low water contents (0.75–1.75 wt%). The geochemical features of the Maiga olivine leucitites are compatible with an origin from a carbonated peridotite mantle source. Additionally, based on the regional tectonic evolution and mid-ocean-ridge-basalt-like Zn isotopic compositions (δ66Zn = +0.26‰ to +0.30‰), we hypothesize these magmas formed from interactions between ambient mantle peridotite and recycled Ca-rich carbonate sediments during Indian continental subduction. The transition from the neighboring Miocene lamproite-like rocks in southern Tibet to olivine leucititic magmas in Maiga is assumed to reflect the tapping of two different mantle sources. The first was modified by silicate-rich sediments, which generated lamproite-like rocks, whereas the Maiga mantle source was modified by carbonate-rich sediments. This study and previous research on other K2O-rich subduction-related magmas indicate that non-negligible amounts of carbonates can be recycled deep into the mantle during continental subduction, which may play an important role in global carbon recycling.

Secular evolution of the subcontinental lithospheric mantle beneath Indian cratons: Insights from geochemistry and geochronology of the Precambrian mafic dykes

The Indian Shield is comprised of the Dharwar, Bastar, Singhbhum, Bundelkhand, and Aravalli Cratons, intruded by distinct mafic dyke swarms of different generations (ca. 2.8–0.8 Ga). Most of these dykes are tholeiitic basalt to basaltic-andesite, including boninite. Some other subordinate dyke rocks are of komatiitic, picritic, and andesitic compositions. The vast areal extent of these dykes indicates that they are remnants of Precambrian Large Igneous Provinces (LIPs). The present study reviews the existing geochronological and geochemical data of various Precambrian mafic dyke swarms intruding in all Indian cratons, to track temporal changes in composition of the underlying subcontinental lithospheric mantle (SCLM). Most of these dykes have crust-like abundances of incompatible trace-element. Even the primitive dykes (Mg# = 82–64) exhibit crust-like incompatible element patterns. However, some also have depleted mantle-like abundances of incompatible elements. Most of the dykes also have high concentrations of compatible trace elements (e.g., Cr and Ni). The age-corrected radiogenic Nd isotope compositions (ɛ Nd(i) ) of these dykes vary between the upper continental crust and depleted mantle ɛ Nd growth arrays. The elemental composition and ɛ Nd(i) of these dykes suggest their derivation from a heterogeneous SCLM comprising enriched (metasomatized) and depleted mantle components. The negative as well as positive ɛ Nd(i) values exhibited by the ca. 2.8 Ga (oldest) dykes suggest the presence of enriched mantle materials distributed within a depleted-SCLM already by ca. 2.8 Ga. The enrichment must have occurred before ca. 2.8 Ga due to subduction-released fluids. Later, the SCLM beneath Indian cratons evolved with a composition consisting of an enriched/metasomatized mantle component and a depleted mantle component. • Geochemistry of 2.8–0.8 Ga mafic dyke swarms of Indian cratons have been reviewed. • They originated from heterogeneous SCLM containing enriched & depleted components. • Subduction-driven metasomatism of global SCLM occurred during the Archean.

The Quaternary monogenetic Bayuda Volcanic Field, Sudan – Insights into mantle and crustal processes during magma petrogenesis

The Holocene Bayuda Monogenetic Volcanic Field (BMVF) is located at the Great Bend of the river Nile in the north of Sudan. The analysed volcanic rocks are of basanitic to hawaiitic composition, characterising the BMVF as a sodic-alkaline volcanic field. Petrographic evidence documents olivine and clinopyroxene as the main fractionating phases as well as FC processes that were accompanied by assimilation of mantle and crustal rocks in addition to mingling and mixing processes. Utilising systematic differences in trace element variations, two primary magmas of Series A and B can be distinguished. Uniform normalised incompatible element patterns, e.g. LREE enrichment accompanied by HREE depletion, negative anomalies of K and Pb, and modelling based on partition coefficients of La, Dy and Yb in mantle residues, suggest partial melting of an amphibole-garnet lherzolite. The presence of amphibole provides a constraint, implying that the melt was derived from a metasomatised lithospheric mantle. The presence of the Pb-depletion (documented by Ce/Pb ratios of >30) suggests that all magmas are of HIMU-OIB type. This is supported by their uniform 87Sr/86Sr (0.703010–0.703347), 143Nd/144Nd (0.512869–0.512995), 206Pb/204Pb (19.787–19.895), 207Pb/204Pb (15.656–15.653) and 208Pb/204Pb (39.560–39.678) isotope characteristics. Additionally, we can describe the HIMU character of the NE African mantle with respect to 176Hf/177Hf (0.283006–0.283026). The mantle source of the BMVF magmas appears to have the composition of the common mantle “C” with an overprint by a HIMU component, resulting in higher radiogenic 206Pb/204Pb ratios above 19.5 (FOZO-HIMU-OIB type). We attribute this to the metasomatic character of the lithospheric mantle. The Hf/Nd isotope characteristics of the BMVF suggest that this metasomatism in the mantle source is most probably late Pan-African in age. When compared to circum-Mediterranean and regional volcanic fields, the high radiogenic Pb character of the BMVF indicates that their genesis cannot be related to magmatism associated with the activity of the Afar plume event and consequential magmatism along the Main Ethiopian Rift and the Red Sea Rift, as they are all of low radiogenic character.

PETROGRAPHY AND GEOCHEMISTRY OF SOME PALEOPROTEROZOIC GRANITOIDS AT THE NORTH-EASTERN MARGIN OF THE KUMASI BASIN IN GHANA

This study investigates basin-type granitoid samples from the north-eastern margin of the Kumasi Basin in Ghana to establish their source and geodynamic setting. Petrographic analysis, TAS and A/NK-A/CNK plots classify the granitoids as metaluminous quartz diorite, metaluminous granodiorite, and peraluminous monzogranite; and exhibiting I-type signatures. These rocks are formed by magma differentiation and/or partial melting at various stages. Distribution patterns of incompatible elements and the positive Eu/Eu* anomalies of 1.15 and 1.47 exhibited by quartz diorite and granodiorite, respectively, the values suggest the rocks crystallized from melts formed in a water-saturated environment. The negative Eu/Eu* anomaly exhibited by monzogranite indicate fractionation of plagioclase in the final stages of the magma evolution. The water-rich environment is probably due to dewatering of the basin’s foreland volcaniclastic sediments during regional subsidence, burial and metamorphism. K2O enrichments and wide variations suggest that the granodiorite and monzogranite are formed from fractional crystallization and/or crustal assimilation of the continental crust by under-plating dioritic magma. The higher Al2O3/TiO2 enrichment and the shift from metaluminous to peraluminous in the monzogranite suggest a longer residence time within the continental crust, during which fractional crystallization and the assimilation of pre-existing crustal components into the dioritic magma that resulted in the formation of the monzogranite. The study requires replication at other areas within the basin to generate enough data to enhance metallogenic studies in the terrain.

Geochemistry of volcanic rocks from Oldoinyo Lengai, Tanzania: Implications for mantle source lithology

Oldoinyo Lengai is the only active carbonatite volcano within the East African Rift Valley in northern Tanzania. The volcano is dominated by peralkaline silicate rocks with natrocarbonatites. This study presents new mineralogical and geochemical data, including Sr–Nd–Pb–Hf–Mg isotopic compositions, for volcanic rocks at Oldoinyo Lengai and lavas from the nearby Gregory Rift Valley. The samples analyzed in this study include olivine melilitite, melanephelinite, wollastonite nephelinite, and phonolite. The olivine melilitites and melanephelinites have highly fractionated rare-earth-element (REE) patterns with (La/Yb)N values of 26.4–64.9, suggesting that they formed from magmas generated by low-degree (up to ∼7%) of partial melting within the garnet stability field. The wollastonite nephelinites have much higher (La/Sm)N values but lower (Sm/Yb)N values relative to typical oceanic island basalts (OIB), with flat heavy REE patterns [(La/Yb)N = ∼22]. The phonolites have elevated REE abundances but with patterns intermediate between the other two sample groups [(La/Yb)N = ∼41]. All samples have primitive-mantle-normalized incompatible element patterns that are characterized by negative K and Rb anomalies but no significant Eu anomalies. They also have elevated Yb contents relative to the compositions of modeled garnet peridotite-derived melts, suggesting that they were derived from a sublithospheric source containing enriched HIMU-like recycled oceanic crustal material. However, the wollastonite nephelinites have significantly positive Ba, U, Sr, and Pb anomalies similar to those found within the Oldoinyo Lengai natrocarbonatites. The wollastonite nephelinites might have been sourced from a region of sub-continental lithospheric mantle (SCLM) that was previously metasomatized by interaction with carbonatite melts. The phonolites in the study area have also weakly positive Pb and Sr anomalies indicative of some interaction with the SCLM. All samples have δ26Mg values (−0.39‰ ± 0.07‰) lighter than the composition of normal mantle material (−0.25‰ ± 0.04‰). In addition, a negative correlation between δ26Mg values and MgO concentrations suggests derivation from a source region containing recycled carbonate. The samples from the study area define a mixing array between HIMU- and EM1-type OIB in Sr–Nd and Pb–Pb isotopic correlation diagrams, and have pronounced Nd–Hf isotopic decoupling, plotting below the mantle regression line in Nd–Hf isotopic space. The negative deviation from the Nd–Hf isotopic mantle array and the presence of an EM1-type mantle component in the Sr–Nd isotopic compositions of the Oldoinyo Lengai volcanic rocks can be generated by recycling of E-MORB-type oceanic crustal material with an age of 1.5–1.0 Ga.

A comparison between the sub-continental lithospheric mantle of Libya, Morocco and Cameroon: Evidences from structural data and trace element of mantle xenolith Cr-diopsides

We analysed the crystallography and the chemistry (major and trace elements) of 16 Cr-diopsides belonging to mantle xenoliths from northern (Waw-En-Namus, Libya; Bou-Ibalrhatene and Tafraoute, Morocco) and central Africa (Nyos and Barombi maars, Cameroon). These mantle xenoliths were extracted from mantle domains underlying a metacraton (Libya), a pericraton (Morocco), and a Pan-African mobile belt (Cameroon). The clinopyroxenes from pericratonic and mobile belt occurrences show similar geochemical features. They usually have high R3+ content and low Ca. When normalized to chondrite abundances, their incompatible trace elements show a wide spectrum of patterns, from slightly depleted to slightly enriched and U-shaped patterns. Such features may indicate interaction with carbonatitic and silicic melts. Their Vcell suggest different equilibrature pressures that are compatible with an ascending diapir and/or entrapment by magmas at different depths within the lithospheric mantle. It is important to stress that variable cell and tetrahedron volumes suggest that Libyan clinopyroxenes formed at variable depths. Those arising from shallower depth are less depleted and may display enriched and U-shaped patterns, while clinopyroxenes deriving from deeper zones display depleted and flat incompatible element patterns, possibly suggesting a melt extraction preceding the delamination of the SCLM.

Geology and petrology of the potassic and ultrapotassic rocks from the northern part of Senirkent (Isparta-SW Turkey): evidence of magma–carbonate wall-rock interactions

Potassic, ultrapotassic lava domes, flows, and volcano-sedimentary deposits including leucite-bearing and leucite-free grains crop out to the north of Senirkent (Isparta), at the apex of the Isparta Angle (southwestern Turkey). Ultrapotassic rocks are nepheline-foidolite, italite, leucitite, melilite leucitite, melilite-gehlenite leucitite, diopside leucitite, and leucitophyre with lowest SiO2 content (38.71–44.72 wt%, but the leucitophyre has 51.4 wt%), higher K2O (4.36–10.19 wt%), MgO (3.25–6.98 wt%), and K2O/Na2O ratio (2.65–32.8). Potassic rocks are phonolitic leucitites, sanidine-phyric phonolite, and trachytes with SiO2 (49.35–64.46 wt%), K2O (8.28–11.38 wt%), MgO (0.17–1.39 wt%), and K2O/Na2O ratio (2.25–11.8). Some leucitites, diopside leucitites, and phonolitic leucitites enclose partly carbonatite occurences or associated with carbonate-rich part including carbonatite globules, phlogopite, clinopyroxene, and/or leucite, melanite crystals embedded in glassy volcanic matrix and/or carbonate matrix. Primitive mantle-normalized incompatible element patterns display strong enrichments in LILEs (Cs, Rb, Ba, U, Th, Sr) relative to HFSEs (Nb, Ti) with distinct negative Nb–Ti anomalies, suggesting subduction-related lavas. Chondrite-normalized REE patterns are characterized by light REE enrichment (251–493 times chondrite for La) and flat heavy REE patterns (10–27 times chondrite for Yb). All the leucite-bearing and leucite-free rocks display similar, subparallel chondrite-normalized and primitive mantle-normalized trace element patterns, suggesting cogenetic origin. Mineralogical, petrological, and geochemical data suggest magma–carbonate wall-rock interaction and tapping of different parts of the melts within the magma chamber along the fault lines formed during neotectonic period and extension-related block faulting.

Petrogenesis of a composite dike system in the Wadi Rimthi area, South Sinai, Egypt

Composite dike systems are a common feature in the basement complex of the Egyptian Eastern Desert and Sinai. These dike systems cut several lithological units, but mostly concentrated in post-orogenic calc-alkaline and alkaline granitoid rocks. New field observations, petrographic studies, and whole rock geochemical data are used to reveal the genesis and evolution of these dikes as well as to examine the genetic relationship between the parent mafic and felsic melts. The investigated composite dike system is characterized by a felsic core and mafic margins. The mafic margins are basalt to andesite in composition, whereas the felsic core is mainly rhyolitic in composition. The mafic dike samples are typically enriched in rare earth elements (REEs) (up to 305 ppm) with fractionated patterns ((La/Lu)N = 7.81–9.25) and mildly fractionated heavy rare earth elements (HREEs) ((Gd/Lu)N = 2.53–2.69). The felsic dike samples have low REE contents (up to 234 ppm) with less fractionated patterns ((La/Lu)N = 4.05–5.59) and weak to unfractionated HREE patterns ((Gd/Lu)N = 1.35–1.53). Major and trace element data of the examined mafic and felsic dikes do not show a clear mutual fractionation history. The marked variations in the K/Rb, Rb/Zr, and La/Sm ratios suggest that mafic and felsic rocks were formed from two individual melts and partly contaminated with crustal materials. According to the incompatible trace element patterns, mafic magma is assumed to be generating from the melting of a garnet-bearing peridotite, modified by the fractionation of amphibole, clinopyroxene, olivine, and zircon. The felsic magma, on the other hand, might have been derived from the melting of garnet-free crustal material dominated by feldspar, apatite, and Fe-Ti oxides, followed by pyroxene and amphibole fractionation crystallization.

New Age and Geochemical Data from the Southern Colville and Kermadec Ridges, SW Pacific: Insights into the recent geological history and petrogenesis of the Proto-Kermadec (Vitiaz) Arc

The intra-oceanic Kermadec arc system extends ~1300 km between New Zealand and Fiji and comprises at least 30 arc front volcanoes, the Havre Trough back-arc and the remnant Colville and Kermadec Ridges. To date, most research has focussed on the Kermadec arc front volcanoes leaving the Colville and Kermadec Ridges virtually unexplored. Here, we present seven 40Ar/39Ar ages together with a comprehensive major and trace element and Sr-, Nd-, and Pb-isotope dataset from the Colville and Kermadec Ridges to better understand the evolution, petrogenesis and splitting of the former proto-Kermadec (Vitiaz) Arc to form these two remnant arc ridges. Our 40Ar/39Ar ages range from ~7.5–2.6 Ma, which suggests that arc volcanism at the Colville Ridge occurred continuously and longer than previously thought. Recovered Colville and Kermadec Ridge lavas range from mafic picro-basalts (MgO = ~8 wt%) to dacites. The lavas have arc-type normalised incompatible element patterns and Sr and Pb isotopic compositions intermediate between Pacific MORB and subducted lithosphere (including sediments, altered oceanic crust and serpentinised uppermost mantle). Geochemically diverse lavas, including ocean island basalt-like and potassic lavas with high Ce/Yb, Th/Zr, intermediate 206Pb/204Pb and low 143Nd/144Nd ratios were recovered from the Oligocene South Fiji Basin (and Eocene Three Kings Ridge) located west of the Colville Ridge. If largely trench-perpendicular mantle flow was operating during the Miocene, this geochemical heterogeneity was likely preserved in the Colville and Kermadec sub arc mantle. Between 4.41 ± 0.35 and 3.40 ± 0.24 Ma some Kermadec Ridge lavas record a shift from Colville Ridge- to Kermadec arc front-like, suggesting the proto-Kermadec (Vitiaz-) arc split post 4.41 ± 0.35 Ma. The Colville and Kermadec Ridge data therefore place new constraints on the regional tectonic evolution and highlight the complex interplay between pre-existing mantle heterogeneities and material fluxes from the subducting Pacific Plate. The new data allow us to present a holistic (yet simplified) picture of the tectonic evolution of the late Vitiaz Arc and northern Zealandia since the Miocene and how this tectonism influences volcanic activity along the Kermadec arc at the present.

The Oortsog Peridotite–Troctolite–Gabbro Intrusion, Western Mongolia:New Petrological and Geochronological Constraints

Abstract —New petrological, isotope-geochronological, and geophysical data reveal two phases of magmatism in the Oortsog mafic–ultramafic intrusion in Western Mongolia. The intrusion consists of (i) rhythmically layered peridotite gabbro (278.7 ± 2.5 Ma) and (ii) poorly differentiated biotite-containing amphibole–olivine gabbro and gabbronorite (272 ± 2 Ma). The inverse upward succession from leucocratic to melanocratic lithologies within each rhythm indicates that the intrusion was tectonically overturned. The earlier rocks (phase 1) have lower contents of major oxides (Na2O + K2O, TiO2, and P2O5) than the later ones (phase 2) and show different patterns of incompatible elements. The intrusive rocks of phases 1 and 2 were derived from depleted (positive εNd) and enriched (negative εNd) mantle sources, respectively. The calculated parental melts of both phases belong to picritic (Mg-rich) basaltic magma. More insight into the intrusion structure was gained by mapping magnetic anomalies.

Leucitites within and around the Mediterranean area

Leucite-bearing volcanic rocks are commonly found within and around the Mediterranean area. A specific type of this rock group are leucitites. They are found both in a hinterland position of active and fossil subduction systems as well as in foreland tectonic settings, but none have been found in the Maghreb (N Africa) and Mashreq (Middle East) areas. Here a review of the main leucitite occurrences in the circum-Mediterranean area is presented, with new whole-rock, mineral chemical and Sr-Nd-Pb isotopic ratios on key districts, with the aim of clarifying the classification and genesis of this rock type.Many of the rocks classified in literature as leucitites do not conform to the IUGS definition of leucitite (i.e., rocks with >10 vol% modal leucite and with foids/(foids + feldspars) ratio > 0.9, with leucite being the most abundant foid). Among circum-Mediterranean rocks classified as leucitites in the literature, we distinguish two types: clinopyroxene-olivine-phyric (COP) and leucite- phyric (LP) types. Only the second group can be truly classified as leucitite, being characterized by the absence or the very rare presence of feldspars, as well as by ultrapotassic composition.The COP group can be distinguished from the LP group on the basis of lower SiO2, Na2O + K2O, K2O/Na2O, Al2O3, Rb and Ba, and higher MgO, TiO2, Nb, Cr and Ni. The LP group shows multi-elemental patterns resembling magmas emplaced in subduction-related settings, while COP rocks are much more variable, showing HIMU-OIB-like to subduction-related-like incompatible element patterns. COP rocks are also characterized generally by more homogeneous isotopic compositions clustering towards low Sr and high Nd isotopic ratios, while LP leucitites plot all in the enriched Sr-Nd isotopic quadrant. LP rocks usually have lower 206Pb/204Pb and higher 207Pb/204Pb.This study shows that the geochemical signal of mantle melts does not always reflect the tectonic setting of magma emplacement, suggesting paying extreme attention in proposing geodynamic reconstructions on the basis of chemical data only.

Mantle source heterogeneity in subduction zones: constraints from elemental and isotope (Sr, Nd, and Pb) data on Vulcano Island, Aeolian Archipelago, Italy

Vulcano is part of the Aeolian volcanic arc in the southern Tyrrhenian Sea. Its products were emplaced through multiple episodes of edifice building and collapse since about 120 ka B.P. to present. A major discontinuity in the activity occurred after about 28 ka, while the focus of volcanism moved from SE to NW. The older products are basalts to shoshonites, and have lower K2O than the younger ones, shoshonites to rhyolites. Between these two groups, Lentia latites-rhyolites, Spiaggia Lunga basalts, and Quadrara shoshonite-trachytes, erupted along the western side of Vulcano Island. The Spiaggia Lunga basalts (i) are the most primitive magmas erupted at Vulcano after 28 ka (ii) mark the change between older and younger phases, and (iii) overlap geochemically with a monzogabbroic intrusion of similar age. This work focuses on the Spiaggia Lunga products, discussed within a large dataset of geochemical and radiogenic isotope analyses on the entire Vulcano sequence. Older products have more primitive geochemical and isotope characteristics, and lower incompatible element contents, than younger ones. The Spiaggia Lunga basalts exhibit intermediate geochemical characteristics between the older and the younger groups, and can likely be regarded as a third magmatic phase, which represents a distinct mantle reservoir active during the magmatic history of Vulcano. Significant variations of Sr, Nd, and Pb isotope ratios, and isotopic disequilibrium between phenocrysts and groundmass, are present among the Vulcano products. This variability suggests crustal assimilation in shallow-level magma chambers, which also accounts for the formation of evolved products by combined assimilation and fractional crystallization, particularly in the younger series. Considering only the mafic products, incompatible element patterns with high LILE/HFSE and enriched signatures of Sr, Nd, and Pb isotope ratios, indicate enriched mantle sources. Besides, chemical and isotope variability among older, younger, and Spiaggia Lunga mafic rocks, suggests an origin from geochemically diverse primary melts, derived from distinct mantle reservoirs. Their parent magmas, based on geochemical and isotope patterns, were from both MORB- and OIB-type mantle sources, subject to variable degrees of metasomatism by subducted sediments.

Petrology and geochemistry of sheeted dykes and pillow lavas from the Sabzevar Ophiolitic Mélange (Northeast Iran): new Constraints for the Late Cretaceous evolution of the Neo-Tethys Oceanic Basin between the Central Iranian Microcontinent and Eurasia

TheSabzevarophiolite-bearingmelangemainlyincludestectonic slicesofophiolitesregardedas remnants of the Late Cretaceous oceanic basin located between the northern margin of the Central Iran Microcontinent and the southern margin of the Turan (Eurasia) plate, as well as remnants of its northern continental margin. We present here new mineral chemistry and whole rock chemical data on the sheeted dyke complex and pillow lavas series from this ophiolite. The sheeted dykes consist of basalts and basaltic andesites showing island arc tholeiitic (IAT) affinity (Group 1). They display low TiO 2 (0.37-0.81 wt%) contents and N-MORB normalized incompatible element patterns featuring Th enrichment and Ta and Nb depletion. The pillow lavas show different chemical features compared to the sheeted dykes and can be subdivided in two distinct geochemical groups. The first Group of pillow lavas (Group 2) mainly consists of andesites showing a clear calc-alkaline (CA) affinity with low TiO 2 (0.71-0.81 wt%) contents. Their N-MORB normalized incompatible element patterns display marked enrichments in Th, U, La, and Ce, and depletion in Ta, Nb, and Ti. The second Group of pillow lavas (Group 3) is mainly represented by alkaline basalts with high TiO 2 (1.54-3.45 wt%) contents. N-MORB normalized incompatible element patterns show significant Th, Ta, and Nb enrichments. Trace element modeling shows that CA primary melts were generated by 20% partial melting of a depleted lherzolite mantle source that had been enriched by continental crust chemical components. IAT basalts were generated from various degrees of partial melting of either depleted lherzolitic or harzburgitic mantle sources bearing no or negligible influence from continental crust components. Alkaline basalts were generated by polybaric partial melting of an enriched within-plate oceanic mantle source. A new tectono-magmatic model that can explain the different rock association cropping out in the Sabzevar ophiolite is proposed. This model implies the existence, on the southern margin of the Turan plate during the Late Cretaceous, of a continental arc-forearc system where CA and IAT rocks were erupted, respectively. Alkaline basalts were erupted in a subduction-unrelated oceanic basin and then tectonically accreted to the forearc as a consequence of the collision between the seamount and the arc-forearc system.

Geological, petrological and geochemical framework of Miravalles-Guayabo caldera and related lavas, NW Costa Rica

The Miravalles-Guayabo caldera is located in the Guanacaste Cordillera and its genesis and evolution is related with the geodynamic setting of northwestern Costa Rica. The development of a N-S graben, and related NE-trending fracture zones, was essentially coeval with caldera forming eruptions and the multiple collapses which dismembered earlier volcanoes.The lavas of the Miravalles-Guayabo Complex (MGC) are two-pyroxene calc-alkaline andesites and basaltic andesites with subordinate high alumina basalts and rare dacites. Interstitial matrix glasses span from andesite (in basalts) to dacite (in andesite s.l.) and rhyolite (in dacite) that overlap the bulk chemical compositions of the lavas. REE patterns and spider diagrams show typical features of a subduction-related volcanism at convergent plate boundaries with spiked incompatible-element patterns, negative anomalies for Nb and Ti in spider diagrams, enrichment in LILE (with the exclusion of Sr) and LREE, and relative depletion in HFSE. The geochemical signatures of the MGC volcanic rocks are essentially consistent with recent petrogenetic models.Thermobarometric estimates suggest that basaltic and magmas equilibrated with their phenocrystic phases at 1080–1120 °C, and pressures ranges of 380–430 MPa, whereas andesitic melts are confined at 200–230 MPa and temperatures of 1050–1070 °C. In turn, amphibole-biotite dacites equilibrated at 240–290 MPa and temperatures of 920–950 °C approaching water saturated conditions (with H2O contents up to 5.7–7.2 wt%). Thermobarometry indicate that contiguous andesitic-dacitic magma lenses are stored within the subvolcanic region at depths below 10 km, whereas basaltic magmas are stored and degas at slightly higher depths (up to ~15 km). Single eruptions associated with the emplacement of lava flows seem to be related to the tapping of portions of single magma lenses during minor unrest episodes. Conversely, caldera forming eruptions involved the ejection of differentiated rhyolitic pumices (of La Ese member) and likely occurred during recharge and pressurization of this heterogeneous reservoir under critical stress conditions.

Tectonic control on the genesis of magmas in the New Hebrides arc (Vanuatu)

We present here new bathymetric, petrological and geochemical whole rock, glass and mineral data from the submarine Epi volcano in the New Hebrides (Vanuatu) island arc. The structure has previously been interpreted to be part of a larger caldera structure but new bathymetric data reveal that the volcanic cones are aligned along shear zones controlled by the local tectonic stress field parallel to the recent direction of subduction. We aim to test if there is an interaction between local tectonics and magmatism and to what extent the compositions of island arc volcanoes may be influenced by their tectonic setting. Primitive submarine Epi lavas and those from the neighbouring Lopevi and Ambrym islands originate from a depleted mantle wedge modified by addition of subduction zone components. Incompatible element ratios sensitive to fluid input (e.g., Th/Nb, Ce/Yb) in the lavas are positively correlated with those more sensitive to mantle wedge depletion (e.g., Nb/Yb, Zr/Nb) amongst the arc volcanoes suggesting that fluids or melts from the subducting sediments have a stronger impact on the more depleted compositions of the mantle wedge. The whole rock, glass and mineral major and trace element compositions and the occurrence of exclusively normally zoned clinopyroxene and plagioclase crystals combined with the absence of inversely zoned crystals and water-bearing phases in both mafic and evolved lavas suggest that the erupted melt was relatively dry compared to other subduction zone melts and has experienced little disequilibrium modification by melt mixing or assimilation. Our data also imply that differentiation of amphibole is not required to explain the incompatible element patterns but may rather result from extensive clinopyroxene fractionation in agreement with petrographic observations. Thermobarometric calculations indicate that the melts fractionated continuously during ascent, contrasting with fractionation during stagnation in an established crustal magma reservoir. We interpret the occurrence of this fractional crystallisation end-member in a relatively thick island arc crust (~30 km thickness) to result from isolated and relatively rapid ascent of melts, most likely through a complex system of dykes and sills that developed due to the tectonic positioning of Epi in a complex tectonic zone between a compressional environment in the north and an extensional setting in the south. We can show that the alignment of the cones largely depends on the local tectonic stress field at Epi that is especially influenced by a large dextral strike-slip zone, indicating that structural features have a significant impact on the location and composition of volcanic edifices.

Composition and conditions of crystallization of zircon from the rare-metal ores of the Gremyakha–Vyrmes Massif, Kola Peninsula

The first data on the composition and inner structure of zircon, one of the main ore minerals of the rare-metal metasomatites of the Gremyakha–Vyrmes alkaline-ultramafic massif, are reported. Early zircon generations are enriched in Y and REE and contain numerous inclusions of rock-forming and accessory minerals of metasomatites, as well as syngenetic fluid inclusions of calcite, thorite and thorianite. Late generations differ in the elevated Hf content and contain no inclusions. The elevated concentrations of Ca and Th in the central zones of crystals are related to the presence of numerous micron-sized inclusions of calcite and thorium phases. All zircon varieties have extremely low U and Pb contents. Concentrations and distribution patterns of incompatible and rare-earth elements in zircon from the metasomatites of the Gremyakha–Vyrmes Massif are similar to those of syenite pegmatites and magmatic carbonatites around the world. Mineral from these associations shows a positive Ce anomaly and elevated HREE contents. According to the compositions of zircon and thorite inclusion in it and experimental data on the simultaneous synthesis of these minerals, the crystallization temperature of zircon was 700–750°С. Using Ti-in-zircon temperature dependence, late zurcon was formed at temperature of 700–750°С. The rare-metal metasomatites are formed at the final stages of the massif formation, presumably after foidolites. Carbonatites could initiate metasomatic reworking of foidolites and accumulation of trace metals in them. The evolution of the primary alkaline–ultramafic melt toward the enrichment in trace elements was mainly controlled by crystallization differentiation.

New age and geochemical constraints on the origin of Quaternary adakite-like lavas in the Arabia–Eurasia collision zone

Adakite-like lavas from the Iranian segment of the Arabia–Eurasia collision zone have been considered as partial melts derived from a subducted Neotethyan slab, a hypothesis that cannot be further evaluated due to the lack of systematic age and radiogenic isotopic data. Here, we revisit the Anar region, Iran where such lavas were reported by providing these data to examine their probable origin as slab melts. New SIMS zircon U–Pb ages attest to eruption at ca. 1 to 2Ma, postdating the Arabia–Eurasia collision. The lavas are characterized by dacitic to rhyolitic, metaluminous to slightly peraluminous and trondhjemitic compositions, and incompatible element patterns similar to continental crustal rocks. Extreme depletion in Nb, Ta, Y and the heavy REE relative to the light and middle REEs reflects control by garnet and rutile. Except one sample presumably affected by secondary gain of radiogenic Sr, the depleted Sr–Nd–Hf isotopic compositions relative to the Bulk Silicate Earth [(87Sr/86Sr)i=0.7040 to 0.7043; εNd(t)=+3.2 to +5.3; εHf(t)=+10.5 to +11.3] indicate magma genesis from juvenile source rocks. A slab melt origin for the Anar lavas is very unlikely and the reason is twofold. First, the Y and HREE of the source rocks estimated by inversed batch melting models assuming garnet pyroxenite residue are far too low compared with MORB-like lithologies and subducted sediments. Second, the low MgO, Ni and Cr of the lavas point to limited interaction with peridotites in a mantle wedge. Fractionation-related models are also unlikely due to the apparent lack of broadly coeval, mafic precursor magmas without adakitic signature. The low-Y and low-HREE source rocks revealed by the melting models, together with the low MgO, Ni and Cr of the Anar lavas, can be explained by lower crustal melting under eclogite-facies conditions. These source rocks were most likely high-pressure cumulates in the root of the Urumieh–Dokhtar magmatic arc, a Neotethyan arc that was active in the Paleogene. Lower crustal melting might be related to localized destruction of the continental mantle, giving rise to the temporally and spatially associated ultrapotassic magmas. The hypothesis of an ‘adakitic province’ near Anar where slab detachment and melting occurred does not hold in light of available data and observations.

New perspectives on the origin of olivine-rich troctolites and associated harrisites from the Ligurian ophiolites (Italy)

The mid-ocean ridge (MOR)-type gabbroic sequences from the Internal Ligurian ophiolites include olivine-rich troctolite lenses up to tens of metres thick. For one of these lenses, a portion of metre-scale thickness characterized by skeletal to dendritic olivines (harrisites) was observed near the contact with host gabbros. Spinels from the olivine-rich troctolites locally include Ti-pargasite to kaersutite frequently associated with phlogopite to Na-phlogopite. Spinel-hosted amphibole displays rare earth element (REE) patterns characterized by a negative Eu anomaly, thereby recording a magmatic process associated with plagioclase segregation. The amphiboles show variable depletion of light REE (LREE) relative to middle REE (MREE), and heavy REE (HREE) that are weakly enriched to depleted with respect to MREE. Crystallization of the inclusion-bearing spinels is attributed to cooling of hybrid melts that originated by interaction between primitive melts and gabbro-related melts relatively rich in SiO 2 and incompatible elements. Clinopyroxene and amphibole from the harrisites show extensive variability for trace element compositions, albeit characterized by subparallel incompatible element patterns. This chemical variability was most probably acquired in response to rapid crystal growth related to undercooling of the harrisite parental melt. We propose that the melt undercooling was related to interaction of a primitive melt batch with a gabbroic crystal mush. Supplementary material : Details of the analytical technique are available at http://doi.org/10.6084/m9.figshare.c.3284312