Nephelinite Research Articles

Micro-Raman spectroscopy and SIMS characterization of oxykinoshitalite in an olivine nephelinite from the Hyblean Plateau (Sicily, Italy)

A Ba-Ti rich oxymica occurs in an olivine nephelinite from S. Demetrio High in the northern margin of the Hyblean Plateau (Sicily, Italy). The rock sample exhibits a porphyritic texture formed by olivine, clinopyroxene, nepheline, titanian magnetite, apatite, and rare subhedral tabular mica crystals, which have a perfect cleavage on {001}, and a strong pleochroism with X pale brown and YZ brown. The studied oxykinoshitalite, characterized by micro-Raman spectroscopy, electron microprobe WDS (wavelength- dispersive system), and secondary-ion mass spectrometry (SIMS), was compared with the type material from the Fernando de Noronha island (Pernambuco, Brazil). Structural formula of the Hyblean oxykinoshitalite, calculated on the basis of 7 (Si, Al, Fe, Mg, Ti), is (Ba0.51K0.41Na0.04Ca0.01)0.97(Mg1.98Fe0.55Ti0.48)3.01(Si2.42Al1.56)3.98 O10(O1.17F0.62(OH)0.21)2.00. The lack of chemical zoning and the enrichment in Zr and Nb in the groundmass crystals of the Hyblean oxykinoshitalite suggest formation during the final crystallization stage of a basaltoid magma with ocean island basalt (OIB) affinity. Most likely, the magma originated by partial melting of metasomatized ultramafic rocks in the Hyblean crustal basement.

Reply to “Discussion of: Petrogenesis and Nd–Pb–Sr-isotope geochemistry of the olivine melilitites and olivine nephelinites (“ankaratrites”) in Madagascar”, by Robert D. Tucker and Bernard Moine

Reply to “Discussion of: Petrogenesis and Nd–Pb–Sr-isotope geochemistry of the olivine melilitites and olivine nephelinites (“ankaratrites”) in Madagascar”, by Robert D. Tucker and Bernard Moine

Nephelinite–melilitite–carbonatite relationships: Evidence from Pleistocene–recent volcanism in northern Tanzania

The younger (<1.2Ma) nephelinite–melilitite–carbonatite-volcanoes in the Gregory Rift Valley of northern Tanzania (termed the Younger Extrusives) comprise one of the most concentrated areas of carbonatite magmatism on Earth. The larger strato-volcanoes, characterised by strongly evolved magmatism, are surrounded by volumetrically much-smaller features (scoria and tuff-cones, explosion craters, lava flows) of carbonate-rich olivine-bearing nephelinites or melilitites that, in their relative primitiveness, are potential parents to the much-largerstrato-volcanoes. The bimodal size distribution is mirrored in bulk magma chemistry, and in their Sr, Nd and Pb isotopes, with the more primitive magmas lying close to a HIMU source; the isotope data for the larger strato-volcanoes cover a wider, linear array interpreted as due to variable degrees of interaction between HIMU magmas and enriched lithosphere, evidence for which is found in Tanzanian mantle xenoliths.An integrated model involves carbonated olivine–nephelinite magma generated in the asthenosphere by melting of carbonate-bearing peridotite. Small amounts of the melt migrate rapidly and are extruded as the olivine–nephelinites and –melilitites. Larger amounts of magma pond in the lithosphere with which they react and, during variable periods of stagnation, varying degrees of olivine–pyroxene–spinel fractionation take place before upward migration of now-differing magma batches. Variable residence time within, and reaction with, variably enriched lithosphere accounts for the array between the HIMU and enriched mantle end-members in the isotope mixing model.

Petrogenesis and Nd-, Pb-, Sr-isotope geochemistry of the Cenozoic olivine melilitites and olivine nephelinites (“ankaratrites”) in Madagascar

The Cenozoic ankaratrites of the Alaotra, Takarindoha–Vatomandry and Votovorona (NE Ankaratra) volcanic fields, Madagascar, range from olivine (±monticellite) melilitites, through olivine–melilite nephelinites to olivine (±leucite) nephelinites. The rocks show significant compositional ranges in their coexisting magmatic minerals (olivine-group minerals, melilite, clinopyroxene, nepheline, leucite, Ba-phlogopite, perovskite, ilmenite, spinels, apatite), and evidence of distinct parental magmas, often in different facies of the same vent. Primitive compositions (high Mg#, Cr and Ni concentrations) are found in each volcanic district, and a few lavas contain mantle xenoliths or xenocrysts. The rocks show enrichment in the most strongly incompatible elements (e.g., Ba and Nb up to 200 times primitive mantle, La/Ybn=24 to 40), with troughs at K and smooth, decreasing patterns towards the least incompatible elements in mantle-normalized diagrams. The Nd–Pb–Sr isotope geochemistry indicates a marked heterogeneity of the mantle sources of the various districts (e.g., 206Pb/204Pb=18.68–18.77, 87Sr/86Sr=0.704011–0.704207 for the Alaotra–Votovorona districts; 206Pb/204Pb=19.04–19.14, 87Sr/86Sr=0.703544–0.704017 for the Takarindoha–Vatomandry districts), with significant differences to other Cenozoic mafic volcanic rocks of northern Madagascar. The genesis of the Madagascan ankaratrites is related to rifting events which triggered low-degree partial melting of a garnet peridotite enriched in dolomite and incompatible-element-rich phases, in the lowermost lithosphere. Despite marked geochemical similarities, the source of the Madagascan melilitites bears no isotopic similarity to the HIMU-related sources of melilitites of eastern and southern Africa.

Trace element partitioning between mica- and amphibole-bearing garnet lherzolite and hydrous basanitic melt: 2. Tasmanian Cainozoic basalts and the origins of intraplate basaltic magmas

Oligocene volcanics from Oatlands in Tasmania, Australia, include olivine tholeiites, alkali olivine basalts, nepheline basanites and olivine nephelinites. They have compositional characteristics that are typical of intraplate basalts worldwide. They are generally enriched in incom- patible elements relative to the primitive mantle and are strongly enriched in Nb, Ta and light rare earths, but not heavy rare earths. At the same time, they have Sr and Nd isotope compositions that are similar to those in some incompatible-element-depleted mid-ocean ridge basalts (E-type MORB). Experimentally obtained mineral/melt partition coefficients for an Oatlands basanite allow the relative concentrations of incompatible elements in the volcanics to be produced by small degrees of melting (B1%) of a source similar to the E-type MORB source of Workman and Hart (2005). However, the absolute concentrations that can be achieved in this way are much less than present in the most incompatible-element-enriched basanites and nephe- linites at Oatlands. This contradiction can be explained by open-system melting under the influence of a conductive geotherm. This would have involved upwardly migrating near-solidus melts from the asthenosphere cooling along a sub-adiabatic geotherm. Cooling of the melts would have caused them to re-crystallize and accumulate in the overlying mantle, thereby enriching both the new host rocks and any residual melts in incompatible elements. This would also have increased the buoyancy of the host rocks leading to upwelling and further (decompression) melting of incom- patible-element-enriched peridotite. We were able to use our partition coefficients to quantitatively model the develop- ment of incompatible-element enrichments in the Oatlands magmas by these processes. Our explanation is consistent with the characteristically scattered but widespread distri- butions and long time scales of intraplate volcanism in a broad variety of tectonic settings. This is because the con- ditions required to initiate volcanism (i.e. those of near-sol- idus melting of the asthenosphere) are relatively easy to produce and can therefore be caused by both near-surface tectonics and deeper mantle processes. Furthermore, the super-enrichments of incompatible elements in some intra- plate volcanics can be attributed to the influence of normal geothermal gradients on melting processes. Without the very strong fractionation imposed by this combination of factors, the Oatlands volcanics would more closely resemble mid- ocean-ridge basalts.

MINERAL COMPOSITIONS AND PETROGENETIC EVOLUTION OF THE ULTRAMAFIC-ALKALINE - CARBONATITIC COMPLEX OF SUNG VALLEY, NORTHEASTERN INDIA

The Sung Valley alkaline complex is a relatively small intrusion of Lower Cretaceous age emplaced slightly before or during the India–Antarctica break-up. It consists of ultramafic rocks (dunites, wehrlites, clinopyroxenites, uncompahgrites), mafic rocks (ijolites sensu lato ), felsic rocks (nepheline syenites) and carbonatites. The chemical composition of the mafic minerals indicates the expected enrichment in iron toward the felsic rocks. On the other hand, carbonatites feature very Mg-rich minerals, generally Cr-rich, indicating that their genesis is completely unrelated to that of mafic and felsic rocks (ijolites and nepheline syenites). The parageneses indicate that this complex was formed by batches of primitive magmas with a distinct magmatic affinity (olivine melilitites and olivine nephelinites, basanites, and possibly also carbonatites) which evolved independently, generating the observed spectrum of intrusive rocks. Clinopyroxenites have interstitial alkali feldspar and titanite, indicating that they formed from evolved feldspar-normative (phonotephritic, tephriphonolitic) magmas. The sequence perovskite–titanite and titanite–garnet noted in some ijolitic rocks indicates changes in the chemical composition of coexisting silicate melts and, most likely, an increasing f (O 2 ). The trace-element profiles of coexisting phases in interesting associations in a sample of ijolite were documented by means of LA–ICP–MS analyses.

Magnetic properties of high-Ti basaltic rocks from the Krušné hory/Erzgebirge MTS. (Bohemia/Saxony), and their relation to mineral chemistry

This study provides new thermomagnetic and petrographic data on specific basaltic rock association from the broader vicinity of the Loucna-Oberwiesenthal volcanic centre, western Bohemia/Saxony. Two types of volcanic rocks were recognized there: (i) high-Ti types (3.5–5.2 wt% TiO2) represented by (mela)nephelinite s.s., and sporadically present (ii) medium-Ti types (2.5–3.5 wt% TiO2) of olivine nephelinite, nepheline basanite and phonotephrite compositions. In order to examine the rock-magnetic behaviour, they were studied for their variations in the Curie temperature (TC) and field-dependent susceptibility, spinel group minerals, chemistry and petrology.

Evolution of a small intraplate basaltic lava field: Jerrabattgulla Creek, upper Shoalhaven River catchment, southeast New South Wales

The Jerrabattgulla Creek basalts are in the upper catchment of the Shoalhaven River of southeastern New South Wales. The basalts erupted into a narrow, north-draining valley and modified the local drainage system, re-routing the paleo-Jerrabattgulla Creek, preserving a series of sub-basaltic quartzose gravels with silcretes in the paleovalley. The paleovalley indicates that a north-flowing drainage existed in this place in the Miocene. The high-relief, narrow valley has preserved a volcanic stratigraphy allowing the magmatic evolution of this small lava field to be determined. The lavas have a large compositional range from olivine nephelinite through to quartz tholeiite, which is unusual in such a small lava field. They represent three distinct magma batches, most likely from an amphibole–apatite metasomatised sub-continental lithospheric mantle, and underwent fractional crystallisation of olivine, clinopyroxene and plagioclase and assimilated upper crust. The lava field underwent temporal change from dominantly alkaline, to mixed alkaline and subalkaline, to dominantly alkaline magmatism over the course of its evolution.

Unusual pegmatoid crystallisations in a nephelinite plug, near Round Lagoon, eastern Central Plateau, Tasmania

Pegmatoids in a Late Oligocene olivine nephelinite plug near Round Lagoon form a complex low-pressure fractionation suite. The host nephelinite contains meta-peridotite and meta-wehrlite mantle xenoliths and its composition (Mg# 0.63) may reflect both mantle and then limited fractionation. The pegmatoids range from ultramafic through mafic to feldspathic assemblages in a progressive, but discontinuous, fractionation sequence ~ olivine ~ sodalite ijolite, nepheline syenite ~ alkali syenite). Within this sequence, olivine and clinopyroxene compositions decrease in content, while clinopyroxene becomes increasingly Na- and Fe-rich to produce late stage aegirine-augite and aegirine. Nepheline is prominent in the sequence and crystallised over a wide temperature range from 10000 to <500°C. The presence of sodalite suggests volatile Cl-rich fluxing. Mg-rich spinel crystallised in assemblages, distinct from Fe- and Ti-rich oxides of the magnetite-ulvospinel series in later assemblages. The Round Lagoon low-pressure pegmatoids developed by fractionation in a narrow, vertical feeder rather than in broad lava ponds such as those noted in nephelinite flows at Inverell, New South Wales, and at La Madera, Argentina.

Late Cretaceous to Paleocene melilitic rocks of the Ohře/Eger Rift in northern Bohemia, Czech Republic: Insights into the initial stages of continental rifting

The volcanic rocks of the Ohře/Eger Rift (northern Bohemia, Czech Republic), the easternmost part of the Cenozoic Volcanic Province of western and central Europe, include rare occurrences of Late Cretaceous to Paleocene (68 to 59 Ma) ultramafic melilitic rocks in the Osečná Complex and associated Devil's Dyke swarm. These melilitic suites, related to the initial stage of rifting, occur in the outer parts of the rift zone. Magmatism during the main stage of the rifting is represented by a voluminous Eocene to Miocene (40 to 18 Ma) bimodal suite of basanites and phonolites which is present in the inner part of the Ohře Rift zone. The Osečná Complex is a lopolith-like subvolcanic intrusion, composed mainly of olivine melilitolite with rare pegmatoids, ijolites and glimmerites, accompanied by numerous cone-sheets and dykes of olivine micro-melilitolite and melilitic lamprophyre. The NNE–SSW trending Devil's Dyke swarm consists predominantly of melilite-bearing olivine nephelinite. The primitive melilitic rocks have a primary olivine + melilite + spinel ± clinopyroxene association and are characterized by low contents of SiO 2, Al 2O 3 and total alkalis but high CaO, MgO, Cr, Ni, CO 2 and strongly incompatible trace elements including light REE. High initial ɛ Nd values of + 3.2 to + 5.2 accompanied by variable 87Sr/ 86Sr ratios of ∼ 0.7033 to ∼ 0.7049 are interpreted as evidence for melting of a heterogeneous veined mantle. A portion of a depleted mantle source was overprinted by carbonate-rich fluids with enriched Sr isotopic composition. Mantle metasomatism was probably related to carbonatitic magmatism associated with incipient rifting of the Bohemian Massif lithosphere.

New trace, major and rare earth element data for the Early Pleistocene alkali olivine basalts and olivine nephelinites from Kuantan, Pahang: Plume-related rift volcanics or wrench-related crustal extension?

The Kuantan Basalts are one of the very few bodies of basic extrusives in Peninsular Malaysia. It was erupted too late to have been caused by the Albian rifting that formed the sedimentary basins off the Peninsula's East Coast, the mid-Oligocene extension caused by the collision of the Indian subcontinent and Asia, or by the compression beginning in the mid-Miocene that followed it. This paper presents new trace element data for alkali olivine basalts and olivine nephelinites belonging to the Kuantan Basalt. Both are enriched in incompatible and light rare earth elements, with signatures comparable to Oceanic Island Basalts and East African Rift basaltoids. They plot in the Intraplate Basalt field on a Zr-Ti-Y discrimination plot. The geochemical evidence, as well as the timing, points to a mantle plume-related genesis, rather than one related to wrench tectonics-induced extension.

Entrained Macrocryst Minerals as a Key to the Source Region of Olivine Nephelinites: Humberg, Kaiserstuhl, Germany

Olivine nephelinites commonly contain macrocrysts of olivine and clinopyroxene. Some of these macrocrysts might represent fragments of the source region of the host magma transported to the Earth’s surface. If this hypothesis is correct these fragments can be used to characterize the composition of the source region and to put constraints on the magma generation process. In this study, we investigate the origin of macrocrysts and mineral aggregates from an olivine nephelinite from the Kaiserstuhl, Germany. We focus on clinopyroxenes (Cpx), which can be divided into three groups. Cpx I is relict Cpx from aggregates with deformed olivine that is depleted in Ca and characterized by strong light rare earth element (LREE) fractionation, low Ti/Eu and negative high field strength element (HFSE) anomalies. Its geochemical signature is consistent with formation by carbonatite metasomatism and with equilibration in the presence of orthopyroxene. Cpx II is Ca-rich Cpx, forming both aggregates with deformed olivine and individual macrocrysts. The LREE, as for Cpx I, are strongly fractionated. Convex REE patterns may be present.The depletion in HFSE is less pronounced. Cpx III is oscillatory zoned Cpx phenocrysts showing enrichment in Ca, convex REE patterns and no HFSE anomalies.The transition in the trace element abundances between the Cpx of the three groups is gradual. However, Cpx I and II did not crystallize from the host magma, as demonstrated by the presence of kink-bands and undulose extinction in the associated olivine and by the composition of alkali aluminosilicate glass inclusions in Cpx II. Based on the Cpx relationships, we interpret the studied suite of macrocrysts and mineral aggregates as a mixture of disintegrated fragments of the source region of the host olivine nephelinite.The process of melt generation was multi-stage. A primary carbonatite melt ascending from deeper levels in the mantle, probably from the dolomite^garnet peridotite stability field, reacted with mantle peridotite along the solidus ledge in the system lherzolite^CO2 (520^22 kbar) and started to crystallize carbonate minerals. Because of its low solidus temperature, the resulting carbonate-wehrlite assemblage melted incongruently with the formation of additional clinopyroxene. The carbonatite melt evolved during crystallization of carbonate minerals and concomitant incongruent melting of the carbonate-wehrlite, accompanied by the segregation of incipient alkali aluminosilicate melts. As a consequence of fast reaction rates in the presence of a carbonatite melt, this process probably took place under disequilibrium conditions. Further melting of the assemblage wehrliteþ alkali aluminosilicate melt led to the generation of the olivine nephelinite magma. It entrained fragments of the wehrlite and brought them to the surface.

The petrogenesis of basic and ultrabasic alkaline rocks of western Makhtesh Ramon, Israel: geochemistry, melt and fluid inclusion study

The basic and ultrabasic alkaline rocks of western Makhtesh Ramon, Israel crop out in numerous lava flows and subvolcanic bodies. The rock suite is composed of tephrite, basanite, basanitic nephelinite, analcimite, olivine nephelinite, and melilite-olivine nephelinite and in many outcrops is represented by glass-bearing varieties. Melt and fluid inclusions have been studied in olivine, clinopyroxene, and plagioclase phenocrysts. The EP, SIMS and microthermometry methods were used for inclusion study. The geochemical data obtained on glasses of melt inclusions (major, REE, trace elements, volatiles) are compared with the data on whole-rock and groundmass glass compositions. The compositions of melt inclusions reflect the different stages of rock crystallization: the initial products of crystallization are similar to whole-rock compositions whereas final portions of melts are usually enriched in SiO2, Al2O3, and alkalis, and depleted in mafic components. The data on contemporaneous melt and CO2 inclusions were used for the evaluation of the P–T conditions of rock generation. The following parameters were obtained: tephrite: P = 6.3–7.7 kbar and T = 1,150–1,250°C; basanite: P = 6.6–9.2 kbar and T = 1,150–1,250°C; olivine and analcime-olivine nephelinite: P = 5.6–8.2 kbar and T = 1,150–1,250°C; melilite-olivine nephelinite: 4.0–5.4 kbar and T mainly between 1,150 and 1,200°C. Magma genesis was restricted to P–T conditions of spinel- and plagioclase-lherzolite fields. These data suggest the shallowest depth of magma genesis occurred in Makhtesh Ramon compared to other occurrences of Early Cretaceous magmatism at the Middle East. Differences in the degree of batch partial melting of the same source rocks best explain the diversity of the igneous suite in western Makhtesh Ramon.

The Tamar Trough revisited: correlations berween sedimentary beds, basalts, their ages and valley evolution, North Tasmania

The Tamar Trough, an Early Palaeogene fault structure, contains sedimentaty beds and interleaved basaltic flows that infill the structure along its 70 km length. These infills represent a complex interplay between sedimentation, channel erosion, eruptive dislocations, and even 'out of trough' diyersions of the ancestral Tamar drainage. Several areas of resistant basalt flows remain in the south, upper, middle and lower Tamar reaches. Although some palynological control was known, radiometric dating of previously untested basalts now allows close integration and age-pegging for observed palynological biozones. The K-Ar and Ar-Ar ages of the basalt bodies indicate eruptive events at 47, 33-37 and 25 Ma, correlating with Proteacidites asperopolus-Malvacipollis diversus, Nothofagites asperus and Proteacidites tuberculatus biozone age sedimentary beds respectively. Basanite, alkali basalt and hawaiite flows dominate basalt lithology with lesser olivine nephelinite, transitional olivine basalt, olivine tholeiite and quartz tholeiite. Basalt geochemistry suggests derivation from different degrees of partial mande melting (from 7 to 35%), with alkaline and tholeiitic basalts being derived from separate source regions. Most alkaline basalts have high-jl (HIMU) related trace element signatures, which are absent in the tholeiitic rocks. A basalt plug on the trough margin at Loira gave a Jurassic age and has Jurassic dolerite-like geochemistry. The Tamar sequence suggests that the initial fluvio-Iacustrine and later channel-fill sedimentation from 65(?) to 24(?) Ma was then punctuated in places by periods of alkaline volcanism between 47 to 33(?) Ma, and alkaline and tholeiitic volcanism between 33 to 24(?) Ma. No Neogene fossils are known, so this later period was probably one of net erosion. These contrasting quiet sedimentary and more volcanic intervals are related here to a tectonic model that involves northerly drift of Victorian and Tasmanian lithosphere over several former Tasman metasomatised mantle plume sources.

OXYKINOSHITALITE, A NEW SPECIES OF MICA FROM FERNANDO DE NORONHA ISLAND, PERNAMBUCO, BRAZIL: OCCURRENCE AND CRYSTAL STRUCTURE

Oxykinoshitalite, ideally Ba (Mg 2 Ti 4+ ) (Si 2 Al 2 ) O 10 O 2 , is a new species of mica from Fernando de Noronha Island, Pernambuco, Brazil; it is found in an olivine nephelinite with olivine, clinopyroxene, Fe–Ti oxide, nepheline, calcite, apatite and K-rich feldspar. Oxykinoshitalite forms corroded irregularly shaped grains 0.03 to 0.05 mm across, and its color varies from bright orange to brown. It is brittle, H = 2½, D obs = 3.3(1), D calc = 3.45 g/cm 3 , has a brown streak, vitreous luster and does not fluoresce in ultraviolet light; it has a perfect cleavage on {001} and a splintery fracture. In transmitted plane-polarized light, oxykinoshitalite is strongly pleochroic, X pale brown, Y ≈ Z deep red-brown; X ∧ c = 2° (in β obtuse), Y = b , Z ∧ a = 8° (in β obtuse), with absorption X Y ≈ Z. It is biaxial positive, α 1.708 ± 0.001, β 1.710 ± 0.001, γ 1.719 ± 0.001, 2 V obs = 56 ± 2°, 2 V calc = 51°. Oxykinoshitalite is monoclinic, space group C 2/ m , a 5.3516(7), b 9.2817(11), c 10.0475(13) A, β100.337(3)°, V 490.98(18) A 3 , Z = 2. The strongest ten X-ray-diffraction lines in the powder pattern [ d in A( I )( hkl )] are: 2.637(10)(131), 2.172(9)(133), 3.646(7)(112), 3.130(7)(112), 3.383(6)(002), 2.902(5)(113), 2.435(5)(201), 1.988(5)(133), 1.661(5)(135), 1.547(5)(312), and 1.526(5)(206). Analysis by a combination of electron microprobe, SIMS and crystal-structure refinement gives SiO 2 28.92, Al 2 O 3 15.26, TiO 2 11.94, Cr 2 O 3 0.03, Fe 2 O 3 9.07, FeO 3.84, MnO 0.14, MgO 10.74, CaO 0.03, BaO 15.49, Na 2 O 0.39, K 2 O 4.05, F 0.91, H 2 O 0.55, sum 100.97 wt.%; V, Ni, Zn, Sr, Cl were not detected. The formula unit, calculated on the basis of 12(O,OH,F) is (Ba 0.50 K 0.43 Na 0.07 ) ∑ 1.00 (Mg 1.33 Fe 2+ 0.27 Mn 2+ 0.01 Fe 3+ 0.57 Ti 4+ 0.74 ) ∑2.92 (Si 2.40 Al 1.49 ) ∑3.89 O 10 [O 1.46 (OH) 0.30 F 0.24 ] ∑2.00 . Refinement of the crystal structure shows it to be the 1 M polytype. The refinement converged to R 1 = 4.3% for 585 unique ( F o > 4σ F o ) reflections, collected on a Bruker single-crystal P 4 diffractometer with a CCD detector and Mo K α X-radiation. Electron-microprobe and SIMS analysis of the crystal used to collect the data on X-ray intensities gave the empirical formula (Ba 0.47 K 0.42 Na 0.06 Ca 0.01 ) ∑0.96 (Mg 1.35 Fe 2+ 0.28 Fe 3+ 0.59 Ti 4+ 0.75 Mn 0.01 ) ∑2.98 [Si 2.31 Al 1.56 ] ∑3.97 O 10 (O 1.44 OH 0.31 F 0.25 ) ∑2.00 . The oxygenian Ti-dominant analogue of kinoshitalite occurs in olivine nephelinite and is associated with olivine, clinopyroxene, Fe–Ti oxide, nepheline, calcite, apatite and K-rich feldspar. There is one tetrahedrally coordinated T site, T –O> = 1.671 A, occupied by (Si 2.31 Al 1.56 ). There are two octahedrally coordinated sites, M (1) occupied by (Mg 0.52 Fe 2+ 0.13 Fe 3+ 0.27 Ti 4+ 0.08 ) with M (1)–O> = 2.110 A, and M (2) occupied by (Mg 0.84 Fe 2+ 0.15 Fe 3+ 0.33 Mn 0.01 Ti 4+ 0.67 ) with M (2)–O> = 2.065 A. The interstitial [12]-coordinated I site is occupied by (Ba 0.47 K 0.42 Na 0.06 Ca 0.01 ), with I –O> = 3.107 A. There are two main coupled substitutions in this structure: (1) Ti 4+ for Mg, primarily at the M (2) site, and O 2− for (OH) − and F − at the O(4) site, and (2) K for Ba at the I site, and Si for Al at the T site.

Origin of Recent alkaline lavas by lithospheric thinning beneath the northern Canadian Cordillera

Recent alkaline lavas that have erupted across the disparate terranes of the northern Canadian Cordillera provide natural probes with which to interrogate the underlying lithosphere. The lavas range between two compositional end members, olivine nephelinite (NEPH) and hypersthene-normative olivine (Hy-NORM) basalt. The chemical signature of amphibole in the incompatible element enriched NEPH end member indicates that it is derived in the lithospheric mantle. The Hy-NORM end member is characterized by lower incompatible trace element contents but is still relatively enriched relative to primitive mantle. Although the Hy-NORM end member is always more radiogenic in Pb and Sr isotopes and less radiogenic in Nd isotopes than the NEPH end member, its isotopic signature varies with tectonic belt. In particular, Hy-NORM basalts in the Omineca Belt are strikingly more radiogenic in Sr and Pb isotopes and less radiogenic in Nd isotopes than otherwise equivalent Hy-NORM basalts in the adjacent Intermontane Belt, indicating the existence of a major lithospheric boundary between the two belts. Cordilleran and other continental Hy-NORM basalts have distinctly low Ca and high Na contents compared with their equivalents in oceanic hot spots or at mid-ocean ridges. A comparison with experimental melts of mantle peridotite indicates that these characteristics reflect smaller degrees of partial melting (&lt;10%) in the stability field of garnet in the lower lithospheric mantle beneath the northern Cordillera. Contrary to the conclusion commonly drawn from experimental results, the Cordilleran NEPH lavas may be derived from similar or shallower depths than coeval Hy-NORM basalts.

Regional Variations in the Mineralogy of Metasomatic Assemblages in Mantle Xenoliths from the West Eifel Volcanic Field, Germany

Xenoliths record two distinct events in the mantle below the Quarternary West Eifel Volcanic Field, Germany. The first, during the Hercynian Orogeny, led to widespread formation of secondary, Ti-poor amphibole, clinopyroxene and phlogopite. The signature of the second event, related to Quaternary volcanism, varies across the field. At Dreiser Weiher and Meerfelder Maar, this event is characterized by amphibole–phlogopite–clinopyroxene veins, hosted in lherzolite and harzburgite xenoliths brought to the surface by sodic olivine nephelinite–basanite suite lavas. These veins formed from crystallization of sodic magma that flowed along fractures in the mantle. At Rockeskyller Kopf, Gees and Baarley, the Quaternary event is characterized by wehrlite xenoliths, many of which have phlogopite–clinopyroxene veins, that were transported by potassic foid suite lavas. Wehrlite formed by reaction of lherzolite– harzburgite, with a large volume of potassic magma that flowed along grain boundaries rather than in fractures. During reaction, orthopyroxene was consumed and secondary clinopyroxene, olivine and phlogopite precipitated. Veins formed in wehrlites only during periodic over-pressure events. The composition of the magmas parental to the veins is similar to the lavas that carried the xenoliths to surface, indicating that the source of foid and olivine nephelinite–basanite suite magma is domainal, as was the flow regime and magma flux.

LITHOSPHERIC MANTLE EVOLUTION IN THE WESTERN-CENTRAL MEDITERRANEAN AREA: EVIDENCE FROM WITHIN-PLATE CENOZOIC VOLCANISM AND ASSOCIATED MANTLE XENOLITHS

Extension-related within-plate Cenozoic volcanism in the central-western Mediterranean mostly occurs in three provinces: Veneto, Adria plate, i.e. African northern promontory; Iblei (Sicily), African lithosphere; and Sardinia, a drifted fragment of the European lithosphere. In Veneto (Paleogene) and Iblei (Neogene-Quaternary), transtensional rift volcanism developed as foreland reaction to collisional processes along the Alpine and Maghrebian chains respectively, generating prevailing basic magmas - from tholeiites to Na-alkali basalts and nephelinites. In Sardinia, the Pliocene-Quaternary volcanism - related to general tensional tectonics of the central Mediterranean – produced comparatively more potassic magmas from subalkaline basalts, alkali basalts/trachybasalts to basanites, locally associated with rhyolitic and phonolitic differentiates. Integrated petrogenetic study, based on incompatible element and Sr-Nd-Pb isotope systematics for the three volcanic provinces, leads to the following constraints: 1) the primary magmas, from tholeiites, alkali basalts to basanites and nephelinites, were generated by decreasing melting degrees of progressively deeper lithospheric mantle sources (ca. 30 to 100 km depth); 2) extensive and multiple enrichment processes by alkali-silicate metasomatizing components widely affected all mantle sources, which invariably require 5 – 10% metasomatic volatile-bearing phases, mainly amphibole for Veneto and Iblei, and phlogopite + amphibole for Sardinia; 3) a previously depleted lithospheric mantle (DM) is enriched by prevailing HIMU (with subordinate EMII) geochemical components in both Veneto and Iblean Provinces, whereas in Sardinia distinctly more potassic EMI signature predominates, in addition to HIMU; 4) in all the investigated provinces, the more alkaline deeper basic magmas (compared to sub-alkaline basalts) show relatively more marked HIMU signatures, also with involvement of carbonatitic components in the lowest lithospheric sources where nephelinites were generated. Mantle xenoliths associated to alkaline lavas of the three volcanic provinces, generally represent shallow portions of the lithospheric mantle column (< 40-50 km depth, i.e. in the Mechanical Boundary Layer, MBL) according to thermobarometric estimates and rheologic characteristics (e.g. grain size). Petrological data invariably indicate that these mantle xenoliths underwent a complex compositional evolution, characterized by depletions due to extraction of basic magmas (mostly occurring in the pre-Palaeozoic), and multiple metasomatic enrichments reflected in compositional variations of the constituent mineral phases, particularly pyroxenes, and the variable occurrence of pyrometamorphic textures and new phases, including glass + feldspar + amphibole + phlogopite. The resulting isotopic signatures conform well to those of their host magmas, being dependent on the variable contributions of the DM, HIMU, and EM components: a prevailing HIMU imprint is recorded for both Veneto and Iblean mantle materials, whereas a predominant EMI component, in addition to HIMU, is observed for the Sardinian mantle. A comparison at a circum-Mediterranean scale reveals that the geochemical features of Sardinia are also observed in the Cenozoic anorogenic magmas and associated mantle xenoliths located within the Variscan basement of centralwestern Europe. Peridotite massifs, tectonically emplaced during the Alpine orogeny, are similarly characterized (in addition to HIMU) by a prevalent EMI signature, which could thus have been active at least since the pre-Mid Mesozoic. By contrast, EMI signature is lacking in Veneto and Iblei, as well as in the mantle xenoliths and host alkaline lavas from other occurrences of the North African domain, where the HIMU component predominates. The widespread presence of HIMU in the European/North African domains may be related to the existence of a Cenozoic plume region extending from the eastern Atlantic to Central Europe and Central-Western Mediterranean, also suggested by the seismic tomographic data. Accordingly, the more recent HIMU metasomatizing agents rising from the convecting mantle appear to have been effectively accumulated in the lower lithospheric portions (i.e. Thermal Boundary Layer, TBL), whereas older metasomatic components (e.g. EMI for Europe) may have been better preserved in the upper, more rigid lithospheric mantle (i.e. MBL).

Silica-poor, mafic alkaline lavas from ocean islands and continents: Petrogenetic constraints from major elements

Strongly silica-poor (ne-normative), mafic alkaline lavas generally represented by olivine nephelinites, nephelinites, melilitites, and olivine melilitites have erupted at various locations during Earth's history. On the basis of bulk-rock Mg#, high concentrations of Na2O, TiO2, and K2O, and trace element geochemistry, it has been suggested that these lavas represent low-degree melts that have undergone little crystal fractionationen route to the surface. Many of these lavas also carry high-pressure mantle material in the form of harzburgite, spinel lherzolite, and variants of websterite xenoliths, and rare garnet-bearing xenoliths. However, phenocryst phases instead indicate that these magmas cooled to variable extents during their passage. We note subtle, yet important, differences in terms of CaO, Al2O3, CaO/AlP2O3, and CaO/MgO. High-pressure experimental melting studies in CMAS-CO2 (3-8 GPa) and natural lherzolitic systems (3GPa) demonstrate that at an isobar increasing F leads to a moderate decrease in CaO + MgO, whereas CaO/MgO and CaO/Al2O3 sharply decrease. Relatively high CaO/Al2O3 indicates melting in the presence of garnet (>- 85 km). Studies also demonstrate that CO2-bearing lherzolitic systems, when compared with anhydrous ones, also have higher CaO content in the coexisting melt at a given P and T. Comparison of the bulk-rock major-element chemistry of silica-poor, mafic alkaline lavas with experimentally determined high-pressure melts indicates that melting of anhydrous mantle lherzolite or garnet pyroxenite is not able to explain many of the major element systematics of the lavas. However, high-pressure partial melts of carbonated lherzolite have the right major element trends. Among ocean islands, lavas from Samoa and Hawaii are perhaps the products of very low degree of partial melting. Lavas from Gran Canaria and Polynesia represent products of more advanced partial melting. On continents, lavas from South Africa and certain localities in Germany are the products of a very low degree of partial melting, and those from Texas and certain other localities in Germany are products of a slightly more advanced degree of partial melting of a carbonated lherzolite. Lavas from Deccan, Czech Republic, and Freemans Cove are the products of even more advanced degree of partial melting. The mere presence of mantle xenoliths in some of these lavas does not necessarily mean that the erupted lavas represent direct mantle melts.

Derivation of potassic (shoshonitic) magmas by decompression melting of phlogopite+pargasite lherzolite

A model metasomatized lherzolite composition contains phlogopite and pargasite, together with olivine, orthopyroxene, clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at ≥1.5 GPa, phlogopite is stable above the dehydration solidus, determined by the melting behaviour of pargasite and coexisting phases. At 2.8 GPa, melts with residual phlogopite+garnet lherzolite mineralogy at 1195 °C and with garnet lherzolite mineralogy at 1250 °C are both olivine nephelinite with K/Na (atomic)=0.51 and K/Na=0.65, respectively. Recent work shows that melting along the dehydration (fluid-absent) solidus of the phlogopite+pargasite lherzolite at pressures <1.5 GPa is very different with the presence of phlogopite, decreasing the solidus below that of pargasite lherzolite. At 1.0 GPa, both phlogopite and pargasite disappear at temperatures at or slightly above the solidus. The compositions of two melts at 1.0 GPa, 1075 °C (with different water contents), in equilibrium with residual spinel lherzolite mineralogy are silica-saturated trachyandesite (∼5% melt fraction, ∼3% H 2O) to silica-oversaturated basaltic andesite (∼8% melt fraction, 4.5% H 2O). Both compositions may be classified as ‘shoshonites’ on the basis of normative compositions, silica-saturation, and K/Na ratio. Decompression melting of metasomatized lithospheric lherzolite with minor phlogopite and pargasite may produce primary ‘shoshonitic’ magmas by dehydration melting at ∼1 GPa, 1050–1150 °C. Such magmas may be parental to Proterozoic batholithic syenites occurring in Brazil.