Nephelinite Research Articles

Intervention of carbonate components in petrogenesis of the pyroxene nephelinites from the Jbel Saghro (Anti-Atlas, Morocco)

The Jbel Saghro alkaline complex was emplaced close to the eastern edge of the Moroccan Anti-Atlas. Within the northern part, two types of nephelinite were recognized [Ibhi and Nachit, 1999 and Ibhi, 2000]. The first type (olivine-rich nephelinite) constitutes the main volcanic mass south of the Bou Gafer granit (fig. 1), where the volcanism had been active at least during 2 Ma, between 9.6 and 7.5 ± 0.1 Ma [Berrahma et al., 1993]. The second group outcrops in the north (Foum El Kouss). It consists of pyroxene nephelinites which are younger (2.9 ± 0.1 Ma) [Berrahma et al., 1993], and bears carbonatitic xenoliths, melteigitic pyroxenites and metasomatised peridotite xenoliths. Geochemically, the pyroxene nephelinite is highly enriched in LILE compared with the first one. The mineralogical and geochemical characteristics may be explained by the incorporation of carbonatitic and melteigitic pyroxenite segregates of carbonatitic affinity.

The Pariquera Açu K-alkaline complex and southern Brazil lithospheric mantle source characteristics

Four of the most important K-alkaline magmatic complexes of southern Brazil are associated with two main NW-trending structures. The Lages occurrence is linked to the uplift of a large crustal block, the Lages Dome, tectonically controlled by old NW-trending fault zones. The Jacupiranga, Juquiá and Pariquera Açu (PGAM) occurrences are related to the Ponta Grossa Arch, a NW-trending uplift structure that produced tensional tectonic lineaments with the same direction and influenced the emplacement of both alkaline rocks and dyke swarms associated with Paraná flood basalt activity. The Pariquera Açu complex, petrographically and geochemically described here for the first time, comprises coarse- to fine-grained rocks representing two main suites with different degrees of alkalinity: theralite to essexite and syenogabbro to syenodiorite. In both series highly evolved products are lacking. The presence of carbonatites is inferred. In the Jacupiranga complex (ca. 130 Ma), clinopyroxenite (jacupirangite) cumulates with minor dunites are associated with ankaratrites, alkali gabbros and leucocratic rocks such as syenites, nepheline syenites and peralkaline phonolites. Many phases of both calcic and magnesian carbonatite intrusions, to which relevant fenitization processes of jacupirangite are related, characterize the district. The Juquiá complex (130–135 Ma) consists of prevalent olivine clinopyroxenite cumulates and nepheline syenites, the latter arranged in subanular concentric patterns together with subordinate ijolites–melteigites–urtites, essexites and syenodiorites. The central part of the intrusion is occupied by a small Mg-carbonatite body. The Lages complex (78–63 Ma) is made up of mafic–ultramafic rock-types (olivine melilitites, olivine nephelinites, basanites) and prevalent leucocratic bodies mainly consisting of peralkaline phonolites. Kimberlitic breccias and carbonatites are also present. Petrography, mineral and bulk-rock chemistry and mass balance calculations show that shallow-level fractional crystallization processes controlled the evolution from mafic to leucocratic rocks in all four complexes. Starting from a mantle source composition as suggested for the eastern Paraguay alkaline (ASU) magmatism, mass balance calculations indicate that primary magmas with high LILE contents and high La/Yb ratios are consistent with a garnet peridotite or phlogopite-bearing garnet peridotite solid residuum and less than 6% melting. The mantle sources are inferred to have been derived by different incompatible element enrichment probably associated with two late Proterozoic metasomatic processes: ASU (1.1 Ga) and Alto Paranaı́ba Igneous Province (APIP; 0.5 Ga). Mainly on the basis of geochemical evidence (e.g. Nb–Ta anomaly), mantle sources for PGAM and Lages Ol-melilitites and Ol-nephelinites seem to have been affected by an APIP-like event. On the other hand, an ASU-like metasomatic process is suggested for the Lages basanite mantle source, which may be indicative of vertical mantle heterogeneity. Geochemical data also point to a large-scale northward increase in concentration of incompatible elements (e.g. Lages Ol-melilitites and Ol-nephelinites vs APIP kamafugites). An attempt to correlate the chemical composition of silicate rocks and carbonate metasomatism shows that the mantle sources for PGAM and Lages occurrences plot on a mixing curve linking depleted peridotites and the average Ca-carbonatite. This fact supports the view that carbonatitic fluids/melts have been involved in the metasomatic processes responsible for variable incompatible element enrichment of the lithospheric mantle of SE Brazil.

Experimentally Determined P-Wave Velocity Anisotropy for Rocks Related to the Western Bohemia Seismoactive Region

The elastic properties of granites from Western Bohemia which we measured and published earlier have been supplemented with measurements of olivine nephelinite from the same region and with data on lherzolitic xenolith from the vicinity of Železný Brod. The set of velocities measured under laboratory conditions has been compared with depth profiles suggested for the purpose of locating seismic swarm events which occur in this region. P-wave velocities were measured under pressures of up to 400 MPa. This pressure corresponds to depths of about 15 km for this region. The data were extrapolated for larger depths. Comparing laboratory measurements and seismic profiles, we can conclude that the rocks under study may be constituents of crust structures.

Silvialite, a new sulfate-dominant member of the scapolite group with an Al-Si composition near the 14/m–P42/nphase transition

AbstractSilvialite, ideally Ca4Al6Si6O24SO4, is tetragonal,I4/m, Z = 2, witha= 12.160(3),c= 7.560(1) Å,V= 1117.9(8) Å3,c:a= 0.6217:1, ω = 1.583, ε = 1.558 (uniaxial negative), Dm= 2.75 g/cm3, Dcalc= 2.769 g/cm3and H (Mohs) = 5.5. It is transparent and slightly yellow, has a good {100} cleavage, chonchoidal fracture, white streak and a vitreous lustre. It occurs in upper-mantle garnet-granulite xenoliths hosted by olivine nephelinite, from McBride Province, North Queensland, Australia. The empirical formula, derived from electron-microprobe analysis, is (Na1.06Ca2.86)(Al4.87Si7.13)O24[(SO4)0.57(CO3)0.41]. Crystal-structure refinement shows disordered carbonate and sulfate groups along the fourfold axis. Silvialite is a primary cumulate phase precipitated from alkali basalt at 900–1000°C and 8–12 kbar under highfSO2andfO2. The name silvialite, currently used in literature to describe the sulfate analogue of meionite, was suggested by Brauns (1914).

The Petrology of a Melilite-Olivine Nephelinite from Hamada, SW Japan

A melilite–olivine nephelinite lava flow occurring in Hamada, SW Japan, is the most silica-undersaturated lava known to have erupted in Japan (SiO2 content ∼37 wt %), and may be a least fractionated melt from mantle depths. Heterogeneity of elements such as Si, Al, Ca, and alkaline elements is one of the characteristic features of the Hamada nephelinites and is attributed to hydrothermal alteration, because these elements’ concentrations correlate with H2O contents. Melting phase relations at high pressures on the least altered nephelinite sample in the presence both of H2O and CO2 demonstrated that the nephelinite melt is multiply saturated with olivine and clinopyroxene but not with orthopyroxene, suggesting that the nephelinite magma was not derived from a lherzolite source, but possibly from a wehrlite source. Such an unusual mantle material may have been formed by carbonate metasomatism of a lherzolitic upper mantle, which is a process consistent with the geochemical characteristics of the Hamada nephelinite lava.

The Petrology of a Melilite-Olivine Nephelinite from Hamada, SW Japan

A melilite–olivine nephelinite lava flow occurring in Hamada, SW Japan, is the most silica-undersaturated lava known to have erupted in Japan (SiO2 content ∼37 wt %), and may be a least fractionated melt from mantle depths. Heterogeneity of elements such as Si, Al, Ca, and alkaline elements is one of the characteristic features of the Hamada nephelinites and is attributed to hydrothermal alteration, because these elements’ concentrations correlate with H2O contents. Melting phase relations at high pressures on the least altered nephelinite sample in the presence both of H2O and CO2 demonstrated that the nephelinite melt is multiply saturated with olivine and clinopyroxene but not with orthopyroxene, suggesting that the nephelinite magma was not derived from a lherzolite source, but possibly from a wehrlite source. Such an unusual mantle material may have been formed by carbonate metasomatism of a lherzolitic upper mantle, which is a process consistent with the geochemical characteristics of the Hamada nephelinite lava.

EVIDENCE FOR THE PRESENCE OF A CARBONATED MANTLE AND CARBONATITE COMPLEX BENEATH SAGHRO (ANTI-ATLAS, MOROCCO)

The Plio-quaternary Foum El Kouss volcano (Anti-Atlas, Morocco) is composed by pyroclastic rocks and two volcanic phases of Pleistocene to Recent age: massive lava flows of olivine nephelinites first, and pyroxene-nephelinites towards the top. Carbonatite inclusions have been found in the pyroxene nephelinites. Major mineral phases are SrO-rich (6 wt%) calcite, SrO-rich (1.8 wt%) fluorapatite, ZrO-rich (24.1 wt%) pyrochlore (19.4 wt% Nb2O5), barian titanian biotite (21 wt% BaO and 14 wt% TiO2), magnetite (with ilmenite exsolution) and salite. Reaction rims around these enclaves, made of fluorine rich glass, confirm their xenolithic origin. Spinel lherzolitic xenoliths are also found in these nephelinites and display mineralogical evidence of interaction between the lithospheric mantle and the carbonated fluids. The destabilization of these spinel peridotites can be summarized by the reaction: orthopyroxene1 + spinel1 + diopside1 + olivine1 + “carbonated” fluid Þ diopside2 + olivine2 + chromite + apatite + K- feldspar. Such an association of carbonatite inclusion and spinel lherzolite with Foum El Kouss nephelinites suggests the presence of an underlying carbonatite complex and carbonated mantle in the Anti-Atlas region.

NA-ALKALI SILICATE METASOMATISM IN MANTLE XENOLITHS FROM SOUTHERN ALPS (VENETO VOLCANIC PROVINCE)

The peridotitic xenoliths hosted by the Paleogenic lavas of the Veneto Volcanic Province (Southern Alps, Italy) are mainly constituted by spinel-bearing lherzolites, with subordinate harzburgites. All the xenoliths show protogranular textures represented by well-equilibrated assemblage of olivine, orthopyroxene, clinopyroxene and spinel. Superimposed pyrometamorphic textures are widespread, with the development of secondary phases in reaction relationships with primary paragenesis: Type A texture is characterised by reaction areas around orthopyroxene (in contact with spinel), generating a secondary assemblage made up of small crystals of olivine, clinopyroxene, feldspar, spinel and glass in decreasing order of abundance. Type B1 and B2 textures are similar to the previous one, but comparatively greater glass abundance within the secondary phases is observed. In B1 glass is usually black or dark brown in colour with skeletal feldspar often associated with it. In B2 glass is pale brown, with subeuhedral crystals of olivine, spinel, feldspar and clinopyroxene as reaction products. Type C texture is constituted by spongy clinopyroxene, sometimes replaced by secondary assemblages of clinopyroxenes and glass. In-situ analysis (EMP and SIMS) of the phases in the different textural domains allowed to constrain the metasomatic processes evidenced by the pyrometamorphic textures. Secondary olivines show a marked enrichment in FeO and CaO with respect to primary olivines. Primary clinopyroxenes are compositionally homogeneous from core to rim and show restricted ranges of Al2O3 (6.17-8.09 wt% in lherzolites; 4.00-6.56 wt% in harzburgites) Na2O (1.16-1.86 wt% in lherzolites; 0.67-1.37 wt% in harzburgites) and TiO2 (0.36-0.58 wt% in lherzolites; 0.05-0.62 wt% in harzburgites) contents. The primary clinopyroxenes in lherzolites are unzoned with LREE depleted patterns ((La/Yb)N= 0.014-0.025), whereas they are slightly zoned and enriched in LREE ((La/Yb)N=0.64-3.99) in harzburgites. Secondary clinopyroxenes show a restricted range of variation in major elements compositions with lower amounts of Al2O3 and Na2O with respect to the primary clinopyroxenes. However, the clinopyroxenes recrystallized from pre-existent clinopyroxenes (type C texture) show more marked enrichment in FeO and TiO2 with respect to the clinopyroxenes grown at the expense of orthopyroxenes (type A and B textures). Both clinopyroxenes show typical LREE-enriched patterns ((La/Yb)N=3.36-5.38). Spongy clinopyroxenes (type C texture) are characterised by a wide range of compositional variation and always appear markedly zoned from core to rim. Their major element compositions encompass the entire compositional range of primary and secondary clinopyroxenes. The inner portions of the crystals do not appear to be involved in any reaction and have LREE depleted patterns ((La/Yb)N=0.01-0.08) similar to those of primary crystals, whereas the marginal and more “spongy” portions show generally LREE enriched patterns ((La/Yb)N=0.27-1.97). Spinels show a typical inverse correlation between Cr# (Cr/(Cr+Al)*100) and Mg# (Mg/(Mg+Fe)*100) values, the secondary spinels having the higher Cr# values and lower Mg# values. Feldspars range from andesine to K-oligoclase, anorthoclase and Na-sanidine, very similar to those reported in the xenoliths of Lake Bajkal (Ionov et al., 1995) and the Ytong region (Xu et al., 1996). Glasses are tipically silica- (55.9-61.52 wt%), alkali- (8.38-16 wt%) and Al2O3-rich (20.49-23.65 wt%) with low MgO (0.17-1.36 wt%), CaO (0.37-2.06 wt%), FeO (0.77- 2.24 wt%) and TiO2 (0.89-1.91 wt%) contents, and relatively constant Na2O/K2O ratios (between 1 and 2). They are moderately enriched in LREE ((La/Yb)N =24-40) and do not show any Zr or Ti negative anomalies. A multistage evolution model can be invoked to explain the complex textural and chemical features shown by these xenoliths. The LREE depleted patterns in primary clinopyroxenes of lherzolites suggest that the initial process was a partial melting that can be modelled using the equation of non-modal fractional melting proposed by Johnson et al. (1990). The calculation results give a degree of melting variable from 3% to 5% for lherzolites and around 26% for harzburgites. Successively the depleted mantle was enriched by interaction with a metasomatic agent, responsible for the development of the pyrometamorphic textures, whose nature was defined using major and trace elements mass-balance calculation model. The best fit was obtained using a nephelinite erupted in the Veneto Volcanic Province or a lamprophyre of Calceranica (Southern Alps; Galassi et al. 1994) as hypothetical metasomatic agents. In order to tentatively constrain the timing of metasomatic event, the inward diffusion model of Griffin et al. (1989), using the linear equation of Crank (1975) and assuming a solid-solid diffusion coefficient (Ds) of 10-15 for the REE (Sneeringer et al. 1984), was applied. Taking into account size and enrichment profile of spongy and secondary clinopyroxenes, it may be put forward that the most recent metasomatic processes occurred shortly (3,000-79,000 y) before the xenoliths were entrapped by the host magmas.

The Case for Primary, Mantle-derived Carbonatite Magma

There is much debate about whether carbonatite magmas are derived in ‘secondary’ fashion through the advent of liquid immiscibility operating in the crust on evolved nephelinitic magma, or whether they are derived in the mantle by direct partial melting of a carbonated peridotite. This paper briefly summarizes the εSr–εNd data fo carbonatites in general and evaluates the isotopic relationships between carbonatites and alkaline silicate rocks in several well-studied complexes from Africa. Available data for carbonatites younger than 200 Ma have a range in εSr–εNd that is less than that found in oceanic basalts despite the fact that carbonatites traverse lithospheres that ar much more complex than those in the oceans. By contrast, for the Napak, Kerimasi, Shombole, Dorowa, Shawa and Spitskop complexes the alkaline silicate rocks show greater variability and have more enriched εSr–εNd (higher εSr, lower εNd) values than their associated carbonatites. In general, the carbonatites hav isotopic compositions that are closer to the more primitive silicate rocks, such as melilitites a olivine nephelinites, than to more evolved nephelinites and phonolites. In the case of the Napak Complex the enriched component was introduced from the lower crust whereas for the Dorowa and Shawa complexes of SE Zimbabwe, the component was derived from the sub-continental lithospheric mantle. These relationships indicate that the carbonatites must have existed as discrete magmas i themantle and argue against a derivation by liquid immiscibility in the crust. Although a contrast i isotopic composition does not rule out an immiscibility relationship at mantle depths and early in the evolutionary history of a melilititic or nephelinitic magma, there is little experimental support for it. Existing experimental data indicate that immiscibility between carbonate an silicate liquids is favoured at low, crustal, pressures but that immiscibility is unlikely to occur in realistic mantle melts or their derivatives at mantle pressures. Many experimental data exist to show that magnesian carbonatite liquids form as the near-solidus melts of carbonated mantl peridotite at depths in excess of 75 km. We conclude that the calcitic and dolomitic carbonatite magma discussed in this paper are best considered as being derived from primary carbonatite magmas generated in the mantle by partial melting of carbonated peridotite.

Geology and geochemistry of Gannet (Karewa) Island, Tasman Sea: A rift‐related nephelinitic tuff ring

Gannet (Karewa) Island is a small (0.06 km2) island situated in the Tasman Sea, northwest of Kawhia Harbour, western North Island, New Zealand. It consists of well indurated, palagonitic tuff and lapilli tuff with subordinate scoriaceous basalt bombs and blocks (Karewa Volcanic Formation) which are considered to represent the eroded remnants of a tuff ring. Evidence for this includes such features as cross‐bedded coarse ash units, lensoidal beds, U‐channels, and impact sag structures. Juvenile bombs and blocks consist of fine‐grained olivine nephelinite, comprising microphenocrysts of olivine and acicular clinopyroxene, set in a devitrified glassy groundmass. Accidental lithic fragments are abundant and include ultramafic xenoliths (up to 5 cm), sedimentary rock fragments (15 cm), and hornblende andesite crystal tuff (20 cm). The island has previously been mapped as part of the Alexandra Volcanics, but the major and trace element abundances and isotopic composition indicate that it has lower SiO2 (40–42%) and higher TiO2 (3.2–3.5%), MgO (14–16%),Nb(110–125 ppm), Zr(307–343 ppm)andLREE abundances, and lower 87Sr/86Sr (0.7030) and relatively higher SNd (+6.8) compared with the alkaline intraplate basaltic fields of Okete, Ngatutura, or South Auckland, or the convergent margin basalts of the Alexandra Volcanics and Egmont. Juvenile olivine nephelinite bombs have been dated by K‐Ar determination at 0.51 ± 0.05 Ma, much younger in age than the nearby Alexandra Volcanics (2.74–1.60 Ma) or offshore middle‐late Miocene volcanics. Gannet Island is located approximately above the Taranaki Fault, which is shown on offshore seismic profiles as extending northwards, parallel to the coast off western North Island. Movement on the Taranaki Fault, which forms the eastern boundary of the North Taranaki Graben, ceased in the late Miocene. Since the Pliocene, this region of western North Island has been in extension, and hence Gannet Island is interpreted on geochemical constraints and tectonic position to be an isolated rift‐related volcano. This supports the interpretation that the North Taranaki Graben formed as a back‐arc rift rather than an actively spreading back‐arc basin. The nephelinites of Gannet Island, and also those in the South Auckland volcanic field, which have similar ages, lie just to the northwest of the limit of seismic activity on the subducting Pacific plate, and are probably close to the most distal expression of the contemporary plate boundary.

Liquid immiscibility between nephelinite and carbonatite from 1.0 to 2.5 GPa compared with mantle melt compositions

In order to define the conditions for the formation of immiscible carbonatite magmas in the lithosphere and in the crust, we have conducted phase equilibrium experiments to determine the effect of pressure and temperature on the silicate-carbonate liquid miscibility gap in bulk compositions appropriate for magmas in the upper mantle. A primitive (magnesian) nephelinite (NEPH) was used as a starting material, mixed with carbonates. Experiments were made with mixtures in the joins NEPH-dolomite-Na2CO3 (NEPH-Dol-NC) at 1.0 to 2.5 GPa, and NEPH-calcite (NEPH-CC) at 1.0 GPa. The miscibility gap was intersected by the join NEPH-Dol-NC (liquids with olivine), but not by NEPH-CC. Together with previous results for the Mg-free system (Na2O-CaO-Al2O3-SiO2-CO2), it was established that the size of the miscibility gap for magnesian compositions increases with decreasing pressures from depths of ˜100 km to ˜ 35 km; it increases further as compositions are changed by decreasing Mg/Ca. The maximum CaCO3 in liquids associated with the miscibility gap is 50 wt % for Mg-bearing liquids, and 80 wt % for Mg-free liquids. There is no experimental evidence for nearly pure-CaCO3 immiscible liquids, but abundant evidence for the precipitation of rounded calcite crystals from carbonate-rich liquids. The join NEPH-CC locates a piercing point on the liquidus field boundary for coprecipitation of olivine and calcite at NEPH50CC50 (wt %), part of the silicate-carbonate liquidus field boundary which defines the locus of liquids formed from carbonate-peridotites. The miscibility gap results are compared with magmas formed during partial fusion of CO2-bearing mantle peridotites, and during fractional crystallization of mantle-derived magmas. None of the probable magma paths in mantle processes intersects the miscibility gap. CO2-bearing mantle-derived alkalic magmas such as nephelinites and melilitites may fractionate during uprise through the mantle and crystallization within the crust. The compositions of these evolved nephelinites and phonolites approach the silicate side of the miscibility gap, confirming the probable generation of immiscible, alkalic carbonate-rich liquids at crustal pressures.

Richterite-bearing peridotites and MARID-type inclusions in lavas from North Eastern Morocco: mineralogy and D/H isotopic studies

K-richterite/phlogopite-bearing peridotite xenoliths and MARID inclusions have been found in Late Cretaceous (67±0.2 million years) monchiquites and an olivine nephelinite from North Eastern Morocco. It is the first evidence of MARID rocks and K-richterite/phlogopite-bearing peridotites outside the kimberlitic context. In the hydrous xenoliths, textural features suggest that K-richterite, phlogopite and Al-poor diopside are replacement minerals. K-richterites contain 2–5 wt% FeO, 0.1–1.5 wt% TiO2 and <0.5 wt% Cr2O3. Micas contain 5.4–7.4 wt% FeO and 0.3–2.2 wt% TiO2, with Cr and Ni contents <0.2 wt%. Diopsides are Al-poor (<0.2 wt% Al2O3) and contain 0.1–0.2 wt% TiO2, 0.9–1.1 wt% Na2O and 1.3–1.7 wt% Cr2O3. Compared to known K-richterites and micas from metasomatised peridotite nodules (PKP types), the Moroccan minerals are more Fe rich, K-richterites have higher Ti and micas less Cr and Ni. They are thus closer to MARID than to PKP minerals. K-richterites and mica from the MARID inclusions show typical characteristics, e.g. high FeO (4.3–4.7 wt% in richterite and 7.2 wt% in mica), low NiO and/or Cr2O3 and the incomplete filling of the tetrahedral site by Si+Al. Ion probe D/H analyses of amphiboles and micas from both xenolith types give high δD values ranging from –8 to –73, with large variations within single grains (up to 50‰). Both the D-enrichment and the δD variations are inherited from the mantle. The similar chemical composition and δD values of K-richterite/phlogopite from the hydrous peridotites and MARID minerals suggest a genetic link between the two types of xenoliths. The conditions required for producing MARIDs and K-richterite/phlogopite-bearing peridotites may thus exist in contexts other than stable cratonic settings. MARID rocks and the associated metasomites may result from a hydrous fluid interaction with a peridotite, the metasomatic agent being characterised by a high K and low Al signature and a high δD value. A D-rich source is involved in the metasomatic event producing the hydrous minerals, and the scatter observed in the δD values suggests a mixing between this source and another one with typical upper mantle D/H composition. As indicated by the low δD (–74) values of micas from the host lava, metasomatism predated and is unrelated to the alkaline volcanism.

Petrology of Mt Etinde Nephelinite Series

Petrology of Mt Etinde Nephelinite Series

High- 3 He Plume Origin and Temporal-Spatial Evolution of the Siberian Flood Basalts

An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a (40)Ar/(39)Ar plateau age of 253.3 +/- 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed (3)He/(4)He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-(3)He SFBP plume with a suboceanic-type upper mantle beneath Siberia.

Isotopic characteristics of mantle sources for Quaternary continental alkaline magmas in the northern Canadian Cordillera

Three mantle compositions are identified as potential source end members for Quaternary to recent alkaline volcanic rocks from Fort Selkirk, Llangorse-Hirschfeld, Alligator Lake and Mt. Edziza in the northern Canadian Cordillera. These are: (1) an amphibole-rich source, characterised by unradiogenic Sr, Nd and Pb, from which the olivine nephelinite lavas formed, (2) the continental lithospheric mantle which is characterised by high 207 Pb 204 Pb and appears to be involved in the formation of the alkali olivine basalts of Fort Selkirk, and (3) a mantle with radiogenic Pb and unradiogenic Sr (HIMU-type) represented by lavas from Mt. Edziza. The Mt. Edziza volcano is the largest of the volcanic centres in the region, and is considered to reflect melting of sublithospheric mantle of HIMU composition below central British Columbia. Incipient melting of amphibole-veined subcontinental mantle lithosphere resulted from plume upwelling and/or transtensional pressure release and produced the small nephelinite to olivine basalt centres of the northern Cordilleran Province. The source of the nephelinite magmas is slightly more radiogenic than present-day Pacific MORB, and is best represented by the most depleted component of the Aleutian magmas. This suggests enrichment of the subcontinental lithosphere in the northern Cordillera by melts of this isotopic composition during Cretaceous subduction. The Alligator Lake complex is anomalous and characterised by the most radiogenic lavas. Despite the presence of crustal xenoliths there is no clear geochemical signature for crustal contamination and, in contrast to the other volcanic centres which were erupted through the Intermontain Belt, the lavas of this centre may have been derived from a highly radiogenic lithospheric mantle beneath the Coast Plutonic complex.

Cenozoic alkali basaltic magmas of western Germany and their products of differentiation

Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhon, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr−Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of δD with 87Sr/86Sr in amphibole and phlogopite and a slightly larger δ18O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.

Premières données géochronologiques sur le volcanisme alcalin du Jbel Saghro, Anti-Atlas, Maroc

The Jbel Saghro is one of the two late Cenozoic, large volume strato-volcano of the so-called Anti-Atlas. Saghro (east) and Siroua (west) lie within a zone of local extension but in a regional context of compression, related to a major E-W fault system that defines the southern rim of the Haut-Atlas. The tectonic regime is trans-tensional. Lavas composition range from olivine nephelinites to phonolithes, but major and trace element criteria permit distinctions between one group which includes only phonolites versus a separate more diverse group which includes nephelinites and phonolites. K-Ar dating of 22 whole rock samples demonstrate a temporal-spatial progression from early volcanism in the centre of the area (10-6.4 Ma) to activity focused in the northern part of the complex (5.5-2.8 Ma; includes the youngest eruption). The sequence is successively more evolved from nephelinite through tephrite to phonolite, but the younger, northern sequence is confined to olivine-nephelinites.

Petrography of alkaline volcanic-subvolcanic rocks from the Brazilian Fernando de Noronha Archipelago, Southern Atlantic Ocean

A classification scheme for the volcanic-subvolcanic rocks of the Fernando de Noronha Archipelago is presented, following the IUGS recommendations, based on the petrographic study of a large number of samples. In Fernando de Noronha two main volcanic events were defined by Almeida (1955): an older Remédios Formation (12 to 8 Ma old) composed of domes, plugs and dikes of tephritic-basanitic to trachytic and phonolitic compositions, with essexite porphyries, limburgites and alkaline lamprophyres intruding basal pyroclastic rocks, and a younger Quixaba Formation (3 to 2 Ma), made up mostly by flows of nephelinitic composition (ankaratrites). The lamprophyres were separated in two groups: tephritic or melanocratic lamprophyres and typical lamprophyres, the first ones belonging to a suite characterized by continuous increase in amphibole content. Most of the rocks of the Remédios Formation appear to belong to two distinct petrographic series, one represented by an undersaturated sodic basanite-tephrite (essexite)-phonolite trend and the other by a potassic alkali basalt-trachyandesite-trachyte series, while the limburgites and lamprophyres are of uncertain ancestry. Some ankaratrite and olivine nephelinite dikes that only cut the Remédios rocks are also attributed to the Quixaba Formation. The basanitic flow with mantle xenoliths of the São José Island, considered by Almeida (1955) to form the youngest São José Formation, is here tentatively interpreted as representing the waning stages of the Quixaba volcanism.

An Ultramafic Inclusion from the Weeks Island Salt Dome, South Louisiana

Mines in Gulf Coast salt domes commonly encounter entrained clastic sedimentary material. The clastic sediments include red, brown, green and yellow sands, silts and clays in brecciated seams which may represent shear zones within or between salt stocks. In January of 1992, miners encountered a hard, dark-gray block of distinctive lithology within a clastic inclusion zone at the 1,000 foot level of the Morton Salt Company's mine at Weeks Island, Iberia Parish, Louisiana. The square block measured approximately 18 inches on a side and had a depth into the freshly exposed salt face of a few inches. Thin-section petrography established that the dark gray is an ultramafic porphyry. Phenocrysts of serpentinitized olivine, pyroxene and amphibole occur in a groundmass dominated by serpentine and opaque materials. This new occurrence is significant because it extends the known range of mafic and ultramafic igneous activity within the Gulf area. Olivine nephelinites and their differentiation products are widely distributed in the Austin-San Antonio-Brackettville areas of central Texas, both as intrusions and extrusions and are probably related to the Balcones fault zone. Potassium-Argon dates for these rocks published by Adams and Baldwin (1971) range between 85 and 63 million years. Other igneous p lugs are known or inferred from subsurface data. In 1976, Braunstein and McMichael briefly described an intrusive body of altered porphyritic basic rock from offshore St. Bernard Parish, Louisiana (Chandeleur Sound), at a depth of 7,200 feet. The was dated at 82 M.Y., ± 8 m.y., although the published cross section implies intrusion through (or possibly onlap of) early Tertiary Midway strata. There is no published record of foraminifera or other paleontological materials to enable the sediments to be dated, although late Triassic or possibly Jurassic sporomorphs from the salt have been reported by Jux (1961). It is normally concluded that the entrained clastic sediments are younger than the salt and were picked up during the salt's upward diapirism, although salt domes in the Arabian Gulf contain blocks of material that are clearly from the older underlying basement rocks, apparently sucked from below the rising salt mass. The age of the new block of porphyritic peridotite is problematical. If a real consanguinity exists between the Weeks Island sample and these other examples, a late Cretaceous age similar to the ages discussed above may be realistic; the sample itself is not a promising subject for dating. Another possible interpretation exists. The serpentinitized peridotite (and the associated red elastics) may have been derived from beneath the salt, in a manner comparable with the Arabian Gulf examples. The red sands may therefore correlate with the Triassic Eagle Mills Formation of North Louisiana and the peridotite may thus represent a sample derived ultimately from the upper mantle beneath the Gulf, brought near to the late Triassic surface by tectonics or volcanism early in the Mesozoic opening phase. This scenario is highly speculative but raises some interesting possibilities for future consideration.

Sanidine-olivine-diopside association in a metasomatized garnet lherzolite xenolith from southeastern China

A garnet lherzolite xenolith hosted in olivine nephelinite from Xilong, Zhejiang Province, southeastern China, shows clear evidence of modal metasomatism involving a special sanidine-olivine-diopside (SOD) association which was produced by replacement of primary orthopyroxene. The fluid responsible for the metasomatism was a silica-undersaturated vapour, rich in K, Ba, Sr and Ti. It is suggested that the SOD assemblage is the product of metasomatism of a depleted lherzolite precursor, and formed in the upper mantle prior to entrainment and eruption.