Alkali Basaltic Lavas Research Articles

Kinetic zinc isotope fractionation in olivine phenocrysts records magma evolution history of intra-plate basalts

In the past few years, zinc isotope systematics of basaltic magmas have been widely used as novel proxies for terrestrial mantle heterogeneity induced by recycled crustal materials and for planet formation and evolution. The influence of crystal-melt isotope disequilibrium during magma differentiation on the zinc isotopic composition of basaltic melts, nonetheless, has received little attention. In addition, no quantitative constraint has yet been given for the diffusion-driven Zn isotope fractionation between olivine crystals and melts. Here we present high-precision zinc isotope data (δ66ZnJMC-Lyon) for a series of olivine phenocrysts separated from intra-plate alkali basalts from the Jiaodong Peninsula in Eastern China, together with in-situ chemical analysis. Olivine phenocrysts have Zn isotopic compositions which are too light in comparison with the host basaltic melts (Δ66Znol-melt = −0.41‰ to −0.10‰; n = 16) to be explained by equilibrium isotope fractionation at magmatic temperatures. Instead, the decrease of δ66Zn values with decreasing Mg# (i.e., Mg/[Mg + Fe2+] × 100) and increasing Zn + Fe2+ contents in olivine phenocrysts suggests diffusion-driven kinetic fractionation during olivine crystallization. The data is well-fitted with a diffusion model in which Zn together with Fe2+ diffuse from surrounding melt into olivine crystals and approximately equal flux of Mg diffuses into melt due to the large chemical gradient, yielding a kinetic Zn isotope fractionation factor βZn of 0.07. By utilizing the fractional crystallization model under equilibrium or disequilibrium conditions, it is suggested that diffusion-induced isotope disequilibrium during olivine crystallization may drive Zn isotopic composition of the residual melt toward heavier values. When using zinc isotope systematics of any basaltic magma to probe the source heterogeneity, this impact must be taken into account if the Zn isotope disequilibrium widely exists in olivine phenocrysts. Based on the time-related quantitative diffusion model, our results show that the magma residence time for strongly alkali basaltic lavas (∼20 days) is 10–60 times shorter than that for weakly alkali basaltic lavas (6 months to 4 years). This supports the generation of weakly alkali basalts via interaction between silica-undersaturated melts and the surrounding lithospheric mantle, implying a probable common mechanism in intra-plate basalts that could be comparable globally. Thus, Zn isotope disequilibrium between olivine crystals and melts could provide valuable information on the evolution history of intra-plate basaltic magmas.

COMPOSITION, AGE AND GEODYNAMIC POSITION OF ALKALINE ROCKS IN THE BORGOY AND BOTSY MASSIFS (DZHIDA ALKALINE PROVINCE)

The Borgoy and Botsy massifs are a part of the Dzhida alkaline province of the Western Transbaikalia. It has been stated that the rocks of the Borgoy massif were formed during the period from 246 to 243 Ma, which coincides with the formation period of the Permian-Triassic alkaline magmatic rocks common in the Vitim province. The age obtained from the zircons in the Botsy massif (121±1.0 Ma) is typical of the final stage of the transformation of the rocks related to rifting and alkali basalt lava flow. The presence of negative Nb-Ta anomaly and a relative enrichment in Rb, Ba, Sr and U imply interaction between the material of the plume and the earlier accretionary complexes of the subduction zones.

Na/K Diversity of Primary Basaltic Magmas Induced by the Separation of Slab-derived Supercritical Liquid: Implications from Alkali Basaltic Lavas from Rishiri Volcano, Southern Kuril Arc

Abstract Primary basaltic magmas in subduction zones exhibit chemical diversity even in individual volcanoes. In this study, we aim to elucidate the dominant factors that determine the chemical composition, especially the Na/K, of primary basaltic magmas by conducting a comparative petrological study of the high-Na/K Horyosawa lava and low-Na/K Nozuka lava from Rishiri Volcano in the southern Kuril Arc. Both lava flows are basaltic and are suitable for estimating the magma generation conditions. The whole-rock SiO2 content of the high-Na/K Horyosawa lava, the main target of this study, ranges from 51·9 to 53·4 wt%. The petrological and geochemical features of this lava indicate that the effects of magma mixing and assimilation of crustal materials were limited, and the low-SiO2 magmas were essentially derived via olivine fractionation from the primary magma. The primary magma is estimated to have been generated by ~2·4 % partial melting of the source mantle with ~0·10 wt% water at ~1300 °C and ~2·4 GPa. By comparing the results of the high-Na/K Horyosawa lavas with those published for the low-Na/K basalts (Nozuka lava), we find that (1) the slab-derived fluids involved in their formation were supercritical liquids with similar compositions, including Na/K, when released from the subducting slab, and (2) the Horyosawa primary magma was generated at a shallower level in the mantle than the Nozuka primary magma. From these observations, it is inferred that the primary magma of the Horyosawa lava was formed by the influx melting of aqueous fluid separated from the supercritical liquid at depths shallower than the critical point, whereas the Nozuka lava was generated by supercritical liquid-fluxed melting at depths deeper than the critical point. These findings show that elemental partitioning during the separation of slab-derived supercritical liquid into aqueous fluid and hydrous melt can induce chemical diversity within the primary basaltic magmas of a single volcanic system.

Neogene Metasomatism in the Subcontinental Lithosphere beneath SE Asia—Evidence from Modal and Cryptic Phosphorus Enrichment in Peridotites and Pyroxenites from Southern Laos

AbstractMetasomatism is the prime process to create compositional heterogeneity of the upper mantle. Mineralogical and mineral chemical changes of the mantle triggered by metasomatism can be used to deduce the nature of the metasomatic agent(s) and to constrain the timing of metasomatism. This information is vital for an understanding of the secular evolution of a given mantle segment and the magmatic processes occurring therein. For this study spinel-lherzolites and -websterites were collected from ∼16 Myr old alkali-basaltic lava flows that were extruded on the Bolaven Plateau in south–central Laos. These xenoliths are fragments of the shallow continental lithosphere of the SE Asian peninsula and originate from a mantle segment that acted as source for Cenozoic basaltic volcanism in the wake of the India–Asia collision. In both rock types modal metasomatism formed apatite ± whitlockite ± phlogopite ± calcic amphibole ± calcite ± orthopyroxene. The principal metasomatic phase is apatite, which appears in three varieties. Type-I apatite is ±inclusion-free and associated with phlogopite, calcic amphibole, calcite and lamellar orthopyroxene. It is high in Na and low in P and shows low analytical totals indicating a type-B carbonate–apatite component. Type-I apatite presumably precipitated from a P-alkali-rich mixed H2O–CO2 fluid with low large ion lithophile element (LILE)–light rare earth element (LREE) contents. Type-II apatite shows a spongy texture and has lower Na and higher P contents with higher analytical totals. Crosscutting discontinuous zones of type-II characteristics within type-I apatites indicate type-II formation through an exchange Na+ + CO32– = PO43– + Ca2+ by a later fluid with lower aCO2. REE-rich type-III apatite is the youngest type and formed by infiltration of basaltic melts as part of spongy rims around clinopyroxene. One lherzolite contains whitlockite in addition to apatite. Whitlockite formation is ascribed to a short-lived metasomatic event involving a fluid with extremely low aH2O. Disequilibrium between whitlockite and the bulk assemblage is indicated by hydrous silicates in the immediate vicinity of whitlockite and by substantial H2O contents of 250–370 µg g–1 in clinopyroxenes and 170–190 µg g–1 in orthopyroxenes. High-density (1·15–≥1·17 g m–3) CO2–fluid inclusions in the whitlockite-bearing sample provide evidence for the presence of low-aH2O fluids at mantle depths. The spinel-herzolites may also show cryptic metasomatism evidenced by P zoning in olivine, which is characterized by P-poor (<20–130 µg g–1) cores and P-rich (170–507 µg g–1) rims, the latter in part with oscillatory zoning on a µm scale. Element correlations indicate [4]Si4+ + [6](Mg, Fe)2+ = [4]P5+ + [6]Li+, 2 [4]Si4+ + 4 [6](Mg, Fe)2+ = 2 [4]P5+ + 3 [6](Mg, Fe)2+ + [6]vac and/or 5 [4]Si4+ = 4 [4]P5+ + [4]vac as major P incorporation mechanisms. High P–T experiments conducted at 2 GPa and 950–1050 °C yield apatite-saturated P contents of olivine in the range ∼360–470 µg g–1. Most P concentrations in olivines from the xenoliths including those in the P-rich rims, however, are significantly lower than the apatite-saturated values, which indicates disequilibrium uptake of P during growth of the P-rich rims by dissolution–reprecipitation. Diffusion modeling indicates that the P zoning must have formed within decades prior to the eruption of the host basalts. This is consistent with the preservation of Li disequilibrium partitioning between olivine and pyroxenes in some of the xenoliths. All metasomatic phenomena were assigned to two metasomatic events, both of which were in close temporal relation with the eruption of the xenolith host basalts: an older event-1 formed type-I apatite, hydrous silicates, calcite and orthopyroxene and caused the modification of type-I apatite composition towards that of type-II. It is also likely to be responsible for whitlockite formation and P zoning in olivine. A younger event-2 comprises all paragenetic, textural and compositional modifications of the xenolith assemblages associated with the infiltration of basaltic melts.

Micro-CT investigation of garnet-spinel clusters in mantle peridotite xenoliths

We have investigated the origin of garnet in garnet-spinel peridotite mantle xenoliths from two well-known localities, Pali-Aike (Patagonia) and Vitim (Siberia). We used micro-CT scanning to examine the textures of garnet-spinel clusters and the relationship between garnet and spinel. CT scanning yields different mineral modes in terms of spinel and garnet than observed in thin-section, because distribution of these phases is inhomogeneous, and a larger volume is visible in a CT scan than in a thin-section. We conclude that garnet formed after spinel in both localities, but the xenoliths record a complex tectonic history, in which the original garnets in garnet peridotites were transformed to spinel-pyroxene clusters in spinel peridotites, via lithospheric thinning. This stage was followed by deformation of the spinel peridotites, producing a lineation. A final stage of isobaric cooling caused garnet to re-grow around the spinel clusters. Aspect ratios of garnets differ from those of garnet-spinel clusters; the latter are more similar to those of spinel pyroxene clusters. Kelyphite is a secondary feature resulting from heating and rapid decompression during entrainment in the host alkali basaltic lavas and has no relationship with the formation of the garnet-spinel clusters.

Ancient-depleted and enriched mantle lithosphere domains in northern Madagascar: geochemical and isotopic evidence from spinel-to-plagioclase-bearing ultramafic xenoliths

Mantle xenoliths hosted in Cenozoic alkaline rocks of northern Madagascar (Massif d'Ambre and Bobaomby volcanic fields) are spinel lherzolites, harzburgites and rare websterites. Petrography, electron microprobe, LA-ICP-MS and thermal ionization mass spectrometry techniques allowed to recognize domains characterized by variable degree of partial melting and extent of re-enrichment processes: 1) refractory spinel-to-spinel+plagioclase-lherzolites, with clinopyroxenes having marked LREE (Light Rare Earth Elements) depletion ((La/Yb)N~0.2) and very high 143Nd/144Nd (0.513594), which represent a limited and shallow portion of old mantle that suffered low degree partial melting (2–3%) and was later accreted to the lithosphere. These lherzolites acted as a low-porosity region, being, in places, percolated by small volumes of melts shortly before eruption; 2) lherzolites and harzburgites that suffered variable degrees of partial melt extraction (up to 15%), assisted and/or followed by pervasive, porous flow infiltration of alkaline melts in a relatively large porosity region, leading to the creation of a wide area rich in secondary mineral phases (i.e. olivine, clinopyroxene and pargasitic amphibole), enriched in incompatible elements (e.g., LaN/YbN in clinopyroxene up to 15) and having radiogenic Sr and unradiogenic Nd; 3) websterites and wehrlite-bearing samples that record differentiation processes of alkaline melts highly enriched in Th, U and LREE, not yet documented in the erupted volcanics of northern Madagascar. The mantle xenoliths of northern Madagascar show a regional decrease of the equilibration temperature from to SW (up to 1180°C, Nosy Be Archipelago) to the NE (up to 900°C, Bobaomby district). A significant lithologic and geochemical variation of the shallow lithospheric mantle beneath northern Madagascar is noted, in contrast with the relatively uniform geochemical and isotopic composition of the host alkali basalt and basanite lavas.

Marco geológico y tectónico de la Isla del Coco y la región marítima circunvecina, Costa Rica

La Isla del Coco es la única isla oceánica y el único afloramiento subaéreo de la Cordillera Volcánica del Coco, el rasgo geográfico y geológico más extenso en aguas territoriales costarricenses. Desde el punto de vista geológico, está conformada por rocas volcánicas, predominantemente coladas de lavas basálticas y traquíticas en menor cantidad, con rocas piroclásticas y epiclásticas subordinadas. Posee suelos, coluvios y depósitos de playas (arenas y cantos) superficiales; su topografía es muy variable pero predomina el relieve quebrado y rugoso. La Isla del Coco es la parte emergida de un volcán submarino de evolución compleja, desarrollado a partir y durante el Pleistoceno Inferior (entre 2.2 y 1.5 millones de años, Ma), producto de una anomalía térmica en el manto a través de varias fisuras, que originaron varios alineamientos de volcanes submarinos. La Isla del Coco se encuentra en un estadio de erosión activo, y sus arcos y plataformas sumergidas (90-110m y 183m) son probablemente el producto de una erosión subaérea durante las dos últimas máximas glaciaciones, combinados quizás con la subsidencia debida al enfriamiento del escudo volcánico y de la corteza oceánica. Al ser la isla geológicamente joven, posee importantes implicaciones para la comprensión de la evolución y el endemismo de su biodiversidad. Las amenazas en la geodinámica principales identificadas son los deslizamientos, los tsunamis y, en menor grado, la sismicidad, poco frecuente y con magnitudes moderadas (≤ 5.8 Mw hasta la fecha) dentro de un radio de alrededor de 300km, predominantemente asociada con fallas dextrales de rumbo N-S. Los pocos datos obtenidos hasta el momento indican que hay un cierto grado de actividad sísmica en los alrededores de la Isla del Coco, relacionados con las fallas locales. Algunos pequeños tsunamis históricos y prehistóricos han afectado a la Isla del Coco y los sismos así como la precipitación pluvial elevada han generado deslizamientos. La licuefacción está restringida a las dos playas arenosas (bahías Chatham y Wafer).

The Cenozoic volcanism in the Kivu rift: Assessment of the tectonic setting, geochemistry, and geochronology of the volcanic activity in the South-Kivu and Virunga regions

The Kivu rift is part of the western branch of the East African Rift system. From Lake Tanganyika to Lake Albert, the Kivu rift is set in a succession of Precambrian zones of weakness trending NW-SE, NNE-SSW and NE-SW. At the NW to NNE turn of the rift direction in the Lake Kivu area, the inherited faults are crosscut by newly born N-S fractures which developed during the late Cenozoic rifting and controlled the volcanic activity. From Lake Kivu to Lake Edward, the N-S faults show a right-lateral en echelon pattern. Development of tension gashes in the Virunga area indicates a clockwise rotation of the constraint linked to dextral oblique motion of crustal blocks. The extensional direction was W-E in the Mio-Pliocene and ENE-WSW in the Pleistocene to present time.The volcanic rocks are assigned to three groups: (1) tholeiites and sodic alkali basalts in the South-Kivu, (2) sodic basalts and nephelinites in the northern Lake Kivu and western Virunga, and (3) potassic basanites and potassic nephelinites in the Virunga area. South-Kivu magmas were generated by melting of spinel + garnet lherzolite from two sources: an enriched lithospheric source and a less enriched mixed lithospheric and asthenospheric source. The latter source was implied in the genesis of the tholeiitic lavas at the beginning of the South-Kivu tectono-volcanic activity, in relationships with asthenosphere upwelling. The ensuing outpouring of alkaline basaltic lavas from the lithospheric source attests for the abortion of the asthenospheric contribution and a change of the rifting process. The sodic nephelinites of the northern Lake Kivu originated from low partial melting of garnet peridotite of the sub-continental mantle due to pressure release during swell initiation. The Virunga potassic magmas resulted from the melting of garnet peridotite with an increasing degree of melting from nephelinite to basanite. They originated from a lithospheric source enriched in both K and Rb, suggesting the presence of phlogopite and the local existence of a metasomatized mantle. A carbonatite contribution is evidenced in the Nyiragongo lavas.New K-Ar ages date around 21 Ma the earliest volcanic activity made of nephelinites. A sodic alkaline volcanism took place between 13 and 9 Ma at the western side of the Virunga during the doming stage of the rift and before the formation of the rift valley. In the South-Kivu area, the first lavas were tholeiitic and dated at 11 Ma. The rift valley subsidence began around 8–7 Ma. The tholeiitic lavas were progressively replaced by alkali basaltic lavas until to 2.6 Ma. Renewal of the basaltic volcanism happened at ca. 1.7 Ma on a western step of the rift. In the Virunga area, the potassic volcanism appeared ca. 2.6 Ma along a NE-SW fault zone and then migrated both to the east and west, in jumping to oblique tension gashes.The uncommon magmatic evolution and the high diversity of volcanic rocks of the Kivu rift are explained by varying transtensional constraints during the rift history.

Mantle xenolith-xenocryst-bearing monogenetic alkali basaltic lava field from Kutch Basin, Gujarat, Western India: Estimation of magma ascent rate

Kutch rift basin of northwestern India is characterized by a topography that is controlled by a number of fault controlled uplifted blocks. Kutch Mainland Uplift, the largest uplifted block in the central part of the basin, contains alkali basalt plugs and tholeiitic basalt flows of the Deccan age. Alkali plugs often contain small, discoidal mantle xenoliths of spinel lherzolite and spinel wehrlite composition. Olivine occurs as xenocrysts (coarse, fractured, broken olivine grains with embayed margin; Fo>90), phenocrysts (euhedral, smaller, and less forsteritic ~Fo80), and as groundmass grains (small, anhedral, Fo75) in these alkali basalts. In a few cases, the alkali plugs are connected with feeder dykes. Based on the width of feeder dykes, on the sizes of the xenocrysts and xenoliths, thickness of alteration rim around olivine xenocryst, we estimate that the alkali magmas erupted at a minimum speed of 0.37km per hour. The speed was likely greater because of the fact that the xenoliths broke up into smaller fragments as their host magma ascended through the lithosphere.

Origin and evolution of Neogene alkali-basaltic magmas in the southwestern flank of the Baikal rift system (Heven lava plateau, northern Mongolia)

The Heven lava plateau in the Hövsgöl field of the South-Baikal igneous province formed in the Early–Middle Miocene between 20 and 15.5 Ma. It consists of Early Miocene hawaiites and trachybasalts and Middle Miocene basanites erupted, correspondingly, during two major events in its history. The Heven alkali-basaltic lavas are compositionally similar to their counterparts from other volcanic fields in the southern flank of the Baikal rift system and are richer in Ba, K, Pb, and Sr than oceanic island basalts (OIB). The basanitic, hawaiitic, and trachybasaltic magmas were generated at pressures from 25 to 15 kbar and at temperatures in the range from 1434 to 1358 ºC. The magma sources occurred at 74 to 41 km in asthenospheric and lithospheric mantle and were ~200 ºC hotter than the ambient lithospheric mantle in the surrounding areas and the continental geotherm. The crystallization history of dark-colored began with liquidus highly magnesian olivine and Cr-spinel, and then several other parageneses formed successively as pressures and temperatures decreased: Ol + Cpx and Ol + Cpx + TiMgt ± Pl phenocrysts and subphenocrysts, Cpx + TiMgt + Ilm + Pl microphenocrysts, and finally interstitial Ne + Kfs alkali aluminosilicates. There were two crystallization stages with different mineral chemistry trends. The chemistry of minerals changed as the rising magmas first reached the crust–mantle region and then moved to shallow depths, erupted, and solidified. The generation of the Heven hawaiite–trachybasalt and basanite magmas was controlled by the depth of the reservoirs and the melt fraction in garnet-bearing asthenospheric and lithospheric mantle associated with progressive and regressive dynamics of the lower heterogeneous mantle plume consisting of PREMA and EMI components.

Constraints on the Origin and Evolution of Magmas in the Payún Matrú Volcanic Field, Quaternary Andean Back-arc of Western Argentina

The Payún Matrú Volcanic Field (Pleistocene–Holocene) is located in the Andean back-arc of the Southern Volcanic Zone, western Argentina, and is contemporaneous with the Andean volcanic arc at the same latitude. It includes two polygenetic, mostly trachytic volcanoes: Payún Matrú (with a summit caldera 8 km wide) and Payún Liso (a smaller stratovolcano). The volcanic field includes about 200 scoria cones and alkali basaltic and trachybasaltic lava flows, forming two basaltic fields around Payún Matrú. New 40Ar–39Ar ages extend the activity of Payún Matrú up to 700 ka. The major and trace element and Sr–Nd isotopic compositions of the basaltic lavas and Payún Matrú rocks indicate that the trachytes of Payún Matrú are the result of fractional crystallization of basaltic parent magmas without significant upper crustal contamination, and that the basalts have a geochemical similarity to ocean island basalt (La/Nb = 0·8–1·5, La/Ba = 0·05–0·08). The Sr–Nd isotopic compositions of the basaltic to trachytic rocks range between 0·703813 and 0·703841 (87Sr/86Sr) and 0·512743 and 0·512834 (143Nd/144Nd). Mass-balance and Rayleigh fractionation models support the proposed origin of the trachytes, and an assimilation–fractional crystallization model indicates a low degree of upper crustal contamination in the youngest trachytes. Magnesium numbers (45–55) and contents of Ni (<20–90 ppm) and Cr (30–180 ppm) in the lavas in the basaltic fields indicate that these are not primary magmas. The data also suggest that the basaltic lavas originated in the asthenospheric mantle, probably within the spinel stability field and beneath an attenuated continental lithosphere in the back-arc area. The lack of a slab-fluid signature in the Payún Matrú Volcanic Field rocks, along with unpublished and published geophysical results (mantle tomography and electrical conductivity anomalies) suggest that magmas were generated by decompression-induced melting of upwelling mantle.

The Pliocene–Quaternary Buffalo Valley volcanic field, Nevada: Post-extension, intraplate magmatism in the north-central Great Basin, USA

The Buffalo Valley volcanic field consists of Pliocene through Quaternary lava flows and spatter cones located south of Battle Mountain and adjacent to the Fish Creek Mountains, north-central Nevada. The volcanic rocks are split into two groups by age and geochemistry. The Pliocene sequence (4.02 to 2.75Ma) consists of olivine- and plagioclase-bearing alkali basaltic lava flows with minor pyroclastic deposits, found primarily along the south flank of Battle Mountain and also at the north end of the Fish Creek Mountains and within the Fish Creek Mountains caldera. The Quaternary series (1.99 to 1.14Ma) includes nearly a dozen trachybasaltic spatter cones with short lava flows erupted along the northwest flank of the Fish Creek Mountains. Normalized rare earth element and incompatible element plots for both groups are light rare earth and Nb–Ta enriched, resembling alkali basalts from ocean islands, but the Quaternary lavas are more light rare earth element-enriched and cross the Pliocene basalt patterns at Eu. Radiogenic and stable isotope ratios are consistent with an asthenospheric mantle source, and the rare earth element patterns indicate a shift from melting in the spinel to garnet peridotite field with time. Basaltic rocks from other intraplate fields in the Great Basin, including the Lunar Crater and Cima fields, only include lavas that originated at depth in the garnet peridotite field. Buffalo Valley is located at the margin of a proposed lithospheric drip (delamination) and within a zone of lithospheric thinning that extends across northern Nevada, both of which may control where melting in the asthenosphere may occur. The proximity to the edge of Precambrian–Phanerozoic lithosphere boundary may also be a factor in melt generation.

Cretaceous lower crust of the continental margins of the northern pacific: Petrological and geochronological data on lower to middle crustal xenoliths

Despite the exposures of Precambrian and Paleozoic rocks and the accretionary tectonic history of the northern Pacific (northeastern Asia, Alaska, and Kamchatka), it is likely that a considerable portion of the lower crust of the continental margins is much younger and was generated by Cretaceous postaccretion magmatic events. Data on xenoliths suggest that Late Cretaceous and Paleocene mafic intrusions and cumulates of calc-alkaline magmas may become more important with increasing depth. This conclusion is based on the petrological and geochronological investigation of lower-middle crustal xenoliths borne by mantlederived alkali basalt lavas and U-Pb dating of zircon cores from the igneous rocks of the region. We studied deep mafic xenoliths of granulites and gabbroids (accounting for <2% of the general xenolith population) from the Late Neogene alkali basalt lavas of the Enmelen and Viliga volcanic fields (Russia) and the Imuruk volcanic field in the Seward Peninsula, St. Lawrence Island, and Nunivak Island (Alaska). Depleted MORB-like varieties and relatively enriched in radiogenic isotopes and LREE rocks were distinguished among plagioclase-bearing xenoliths. The most representative collection of Enmelen xenoliths was subdivided into three groups: LREE enriched charnockitoids and mafic melts, pyroxene-plagioclase cumulates with a positive Eu anomaly, and LREE depleted garnet gabbroids. Mineral thermobarometry and calculated seismic velocities (P = 5–12 kbar, T = 740–1100°C, and V p = 7.1 ± 0.3 km/s) suggest that the xenoliths were transported from the lower and middle crust, and the rocks show evidence for their formation through the magmatic fractionation of calc-alkaline magmas and subsequent granulite-facies metamorphism. The U-Pb age of zircon from the xenoliths ranges from the Cretaceous to Paleocene, clustering mainly within 107–56 Ma (147 crystals from 17 samples were dated). The zircon dates were interpreted as reflecting the magmatic and metamorphic stages of the growth and modification of the regional crust. The distribution of the obtained age estimates corresponds to the main magmatic pulses in two largest magmatic belts of the region, Okhotsk-Chukchi and Anadyr-Bristol. The absence of older inherited domains in zircons from both the xenoliths and igneous rocks of the regions is a strong argument in favor of the idea on the injection of juvenile material and underplating of calc-alkaline magmas in the lower crust during that time interval. This conclusion is supported by isotope geochemical data: the Sr, Nd, and Pb isotope ratios of the rocks and xenolith minerals show mantle signatures (87Sr/86Sr = 0.7040–0.70463, 143Nd/144Nd = 0.51252–0.51289, 206Pb/204Pb = 18.32–18.69) corresponding to an OIB source and are in general similar to those of the Cretaceous calc-alkaline basalts and andesites from continental-margin suprasubduction volcanoplutonic belts. Xenoliths from Nunivak Island and Cape Navarin show more depleted (MORB-like) geochemical and isotopic characteristics, which indicates variations in the composition of the lower crust near the southern boundary of the Bering Sea shelf.

Stratigraphy, geochronology and evolution of the Mt. Melbourne volcanic field (North Victoria Land, Antarctica)

Mt. Melbourne (2,732 m a.s.l.) is a large quiescent stratovolcano located in Northern Victoria Land (Antarctica) and is one of a handful of volcanoes on the Antarctic plate with the potential for large-scale explosive eruptions. During the XVIII Italian Expedition in 2002–2003, the Mt. Melbourne volcanic succession was studied in terms of stratigraphy and sampled for 40Ar/39Ar age determinations and geochemistry. The early, Lower Pleistocene, volcanism was largely alkali basaltic to hawaiitic in composition and monogenetic in style, producing tens of small scoria cones and lava flows scattered over a wide area across the Transantarctic Mountains (Random Hills Period). During the Middle Pleistocene, volcanic activity focused to the area of the Mt. Melbourne stratovolcano, where several monogenetic centres show the transition from early sub-glacial/subaqueous conditions to emergent subaerial conditions (Shield Nunatak Period). The oldest exposed deposit associated with the early activity of the Mt. Melbourne stratovolcano (Mt. Melbourne Period) is a trachytic subaerial ignimbrite dated at 123.6 ± 6.0 ka, which reflects the establishment of a crustal magma chamber. Above the ignimbrite a succession of alkali basaltic, hawaiitic, and subordinate benmoreitic lavas and scoria cones is exposed, dated at 90.7 ± 19.0 ka. The Holocene deposits are exposed at the top of Mt. Melbourne, where the crater rim is composed of trachytic to rhyolitic pumice fall deposits, which are also extensively dispersed around the volcano, likely originated from Plinian-scale eruptions. The most recent explosive deposit proved difficult to date accurately because very low quantities of radiogenic 40Ar were released, resulting in imprecise plateau ages of 50 ± 70 and 35 ± 22 ka.

Cenozoic Volcanism on the Hangai Dome, Central Mongolia: Geochemical Evidence for Changing Melt Sources and Implications for Mechanisms of Melting

Cenozoic volcanism within Mongolia forms part of a large central Asian province of intra-plate magmatism. Numerous small-volume volcanic cones and alkali basalt lava flows have been formed since c. 30 Ma; from c. 12 Ma activity has been focused on the uplifted Hangai dome. A mechanism for melting beneath the dome has, however, thus far remained enigmatic. Some of the oldest basalts on the Hangai dome erupted at its centre at ∼6 Ma and their geochemistry suggests a garnet lherzolite source region at 90–100 km depth. These lavas have Pb isotope compositions similar to those of depleted Indian mid-ocean ridge basalts (MORB) ( 206 Pb/ 204 Pb = 17·822, 207 Pb/ 204 Pb = 15·482, 208 Pb/ 204 Pb = 37·767), which may be indicative of the involvement of ambient asthenospheric mantle in their petrogenesis. Younger basalts exhibit a gradual shift in isotopic composition towards a source that has less radiogenic Pb and more radiogenic Sr, evidenced by the eruption of lavas with 206 Pb/ 204 Pb = 16·991 and 87 Sr/ 86 Sr = 0·704704. The youngest lavas, dated as younger than ∼8 ka, have the highest K 2 O contents (up to 5·2 wt %) and are characterized by the most enriched trace-element signatures; they are interpreted to represent melting of a metasomatically altered sub-continental lithospheric mantle containing phlogopite. Concurrent with progressive melting of the lithosphere, melting appears to propagate outwards from the centre of the dome to its margins; by 0·7 Ma the marginal magmatism is interpreted to result from melting of a depleted MORB-source mantle component with a smaller contribution from the lithospheric mantle. The spatial and temporal variations in melting beneath the Hangai dome may be explained by either lithospheric delamination or the presence of a small-scale thermal anomaly in the upper mantle. Although it is not possible to distinguish between these models on the basis of geochemistry alone, the lack of a viable mechanism to generate small-scale upwelling lends support to a model involving delamination of the lithospheric mantle beneath the Hangai dome.

The evolution of the Arabian lower crust and lithospheric mantle — Geochemical constraints from southern Syrian mafic and ultramafic xenoliths

Geochemical compositions of lower crustal and lithospheric mantle xenoliths found in alkali basaltic lavas from the Harrat Ash Shamah volcanic field in southern Syria place constraints on the formation of the Arabian–Nubian Shield in northern Arabia. Compositions of lower crustal granulites are compatible with a cumulate formation from mafic melts and indicate that they are not genetically related to their host rocks. Instead, their depletion in Nb relative to other incompatible elements points to an origin in a Neoproterozoic subduction zone as recorded by an average depleted mantle Sm–Nd model age of 630 Ma. Lithospheric spinel peridotites typically represent relatively low degree (< 10%) partial melting residues of spinel lherzolite with primitive mantle compositions as indicated by major and trace element modelling of clinopyroxene and spinel. The primary compositions of the xenoliths were subsequently altered by metasomatic reactions with low degree silicate melts and possibly carbonatites. Because host lavas lack these signatures any recent reaction of the lherzolites with their host magma can be ruled out. Sm–Nd data of clinopyroxene from Arabian lithospheric mantle lherzolites yield an average age of 640 Ma suggesting that the lithosphere was not replaced since its formation and supporting a common origin of the Arabian lower crustal and lithospheric mantle sections. The new data along with published Arabian mantle xenolith compositions are consistent with a model in which the lithospheric precursor was depleted oceanic lithosphere that was overprinted by metasomatic processes related to subduction and arc accretion during the generation of the Arabian–Nubian Shield. The less refractory nature of the northern Arabian lithosphere as indicated by higher Al, Na and lower Si and Mg contents of clinopyroxenes compared to the more depleted nature of the south Arabian lithospheric mantle, and the comparable low extent of melt extraction suggest that the northern Arabian lithosphere formed in a continental arc system, whereas the lithosphere in the southern part of Arabia appears to be of oceanic arc origin.

Caldera-forming eruptions of the Quaternary Kone Volcanic Complex, Ethiopia

Quaternary volcanism in the northern Main Ethiopian Rift is characterized by rift-axis eruptive centres with complicated collapse structures. Despite their association with major explosive eruptions, few modern studies have been carried out on them. We present the results of fieldwork and laboratory analyses of one of the largest of these centres, the Kone Volcanic Complex (KVC). It consists of nested calderas, trachyte lava flows, trachyte and alkali rhyolite pyroclastic deposits, alkali rhyolite lava domes and numerous small transitional to alkali basalt lava vents and flows. The largest subsidence structure, Birenti Caldera, has a diameter of ∼11 km and an area of ∼95 km 2. Post-collapse volcanism in the interior of Birenti Caldera produced a predominantly trachytic cone that was partially destroyed by a further eruption, forming the well-preserved but irregularly-shaped Kone Caldera (diameter of ∼8 km and an area of ∼24 km 2). Rhyolite lava dome eruptions and tectonically-controlled basalt lava effusion represent the most recent volcanism in the KVC. We report here the first detailed geologic map, along with petrologic and stratigraphic analyses, in order to reconstruct the eruptive history of this large but previously little-known volcanic complex. The findings have implications for understanding the relationships between rifting, tectonics and volcanism in the northern Main Ethiopian Rift, and for future efforts to trace widespread tephra horizons for regional stratigraphic correlation.

The nature of the subcontinental lithospheric mantle beneath the Arabian Shield: Mantle xenoliths from southern Syria

Rare earth element and Sr- and Nd-isotopic data are presented for clinopyroxenes and amphiboles from mantle harzburgite and lherzolite xenoliths from the Harrat As Shaam volcanic field in southern Syria. These rocks have sampled the subcontinental lithospheric mantle (SCLM) from beneath the Arabian Shield. Initial ɛ-Nd values for the mantle xenoliths from the Arabian SCLM plot within the field of crustal rocks from the region supporting the view that the felsic rocks of the Arabian Shield and the SCLM were derived from a common mantle source between about 750 and 950Ma. Nd-isotope errorchrons for clinopyroxenes and for clinopyroxene–amphibole pairs indicate an event in the SCLM at around 750Ma. This is identified as a melt infiltration event which resulted in the enrichment in light REE of the harzburgitic and lherzolitic clinopyroxenes and the lherzolitic amphiboles and coincides with the time of crust formation in the Arabian Shield (620–870Ma). The present-day Nd–Sr isotopic composition of amphiboles in the lherzolite xenoliths overlaps the composition of one of the proposed source regions for alkali basalt lavas in this region. Hence it is proposed that the mantle equivalent in composition to the amphibole–lherzolites melted in the shallow SCLM, during Cenozoic lithospheric stretching. The Harrat As Shaam xenoliths provide a picture of SCLM refertilisation during the formation of juvenile crust.

A geochemical approach to Neogene–Quaternary volcanic activity of western Anatolia: An example of episodic bimodal volcanism within the Selendi Basin, Turkey

New radiometric age and geochemical data are presented from the late Cenozoic volcanic activity in the Selendi Basin of western Anatolia. Stratigraphical and age data show that the volcanism occurred in four distinct stages from the early Miocene to Quaternary. The first early Miocene stage includes two volcanic units with contrasting compositions: the Eğreltidağ volcanic unit and the Kuzayır lamproite. These volcanic units interfinger with the sedimentary rocks of the early Miocene Hacıbekir Group. The Eğreltidağ volcanic unit consists of high-K calc-alkaline dacitic and rhyolitic lavas and associated pyroclastic rocks, whilst the Kuzayır lamproite is composed of alkaline, ultrapotassic mafic lavas, classified as lamproites. The second volcanic stage during the middle Miocene is also represented by two contrasting volcanic units: the Yağcıdağ volcanic unit and the Orhanlar basalt. These interdigitate with middle Miocene lacustrine sedimentary rocks of the İnay Group. The Yağcıdağ volcanic unit consists of high-K to shoshonitic and calc-alkaline andesitic lavas and pyroclastic rocks, whilst the Orhanlar basalt is composed of syn-sedimentary mafic lavas of shoshonitic composition. The geochemical features of the early to middle Miocene volcanic rocks indicate a lithospheric mantle source that was most probably amphibole-bearing (not phlogopite) and was metasomatized by fluid-dominated processes during subduction events. The late Miocene volcanic activity in the region consists solely of the alkali basaltic lavas of the Kabaklar basalt. Geochemically these lavas suggest that the effects of subduction zone processes were begun to wane after the late Miocene. Finally, during the Plio–Quaternary, sodic alkaline basaltic volcanic activity occurred in the region, represented by the Kula volcanics. Geochemical characteristics of the Kula volcanics indicate an asthenosphere-dominated mantle source. Melting models for the Miocene lavas show that the mantle source was depleted with time, whilst for the Plio–Quaternary, there was a transition from destructive-continental margin magmatism to intraplate volcanic activity.

Neogene volcanism and extension in Western Anatolian-Aegean area: A new geodynamic model

The widespread Western Anatolian-Aegean Neogene volcanism presents a complex geochemical evolution reflecting the uncommon space-time variability of the geodynamic setting of the region. In the Western Anatolian and Central Aegean, a widespread supra-subduction magmatism, with calc-alkaline to shoshonitic affinity, took place from Early to Middle Miocene; this phase of activity ends with spots of ultra-K lavas and dykes. From Late Miocene onwards scattered alkali basaltic lavas with intraplate affinity were emitted, while calc-alkaline activity occurred in the South Aegean arc. Since Late Oligocene-Early Miocene, the region was, and still is, affected by extensional tectonics generally ascribed to a backarc rift. However the Aegean region should rather be considered as an unconventional backarc since its characteristics rather differ from ‘typical’ backarcs. In fact, in spite of a long lasting(>40Ma) active NE-directed subduction of Africa, the backarc area still maintains a relatively thick continental crust (>20–25 km). Moreover, the upper Eurasian plate is overriding the lower Africa plate with separate segments, with Greece moving faster, and Turkey moving slower. The differential velocity between Greece and Turkey determines extension in the upper plate, unrelated to the loss of subducted retreating lithosphere, which is the usual setting for the origin of ‘classic’ backarc settings. The geodynamic framework is supported by the geochemical and isotopic features of the supra-subduction magmas revealing the occurrence of a trapped, drying slab, with progressive decreasing of Fluid Mobile Elements/Fluid Immobile Elements ratios, δ11B and δ7Li, coupled with scarce variations of Sr and Nd isotopes. Moreover, the differential motion between the Greek and Anatolian micro-plates creates tear zones with the formation of slab ruptures or vertical slab windows. The occurrence of such windows is, in fact, outlined by the presence of alkali basalts with intraplate affinity sourced in the sub-slab mantle.