Single Parental Magma Research Articles

Magmatic-hydrothermal evolution and rare metal enrichment of the Huoshibulake B-rich rare metal granite in the Southern Tianshan: Insights from texture, geochemistry, and Hf[sbnd]O isotopes of zircon

Deciphering magmatic-hydrothermal evolution and rare metal mineralization mechanisms in granitic systems is always challenging, particularly when elucidating the intricate details of magmatic-hydrothermal transition, the critical stage when the system changes from melt-driven to fluid-driven with associated metal extraction. Here we present a case study of the Huoshibulake Nb-REE-mineralized alkali pluton in the north margin of the Tarim Carton, China. The pluton exhibits abundant tourmaline-quartz orbicules and tourmaline-quartz veins, which are the products of immiscible B-rich melt exsolved during the magmatic-hydrothermal transition. Three types of zircon crystals with different morphology have been recognized in the pluton: antecrystic, autocrystic and hydrothermal zircons, the latter two representing the melt and fluid parts of the system. LA-ICP-MS UPb dating of autocrystic zircon from granite and orbicule samples (206Pb/238U ages of 274.8 ± 1.5 and 273.6 ± 2.0 Ma, respectively) and cassiterite from the vein sample (lower 206Pb/238U intercept age of 271.4 ± 4.1 Ma) overlap within error. The similar trace-element patterns and near-identical O-isotopic compositions between hydrothermal and autocrystic zircons indicate that the hydrothermal zircons were crystallized from magmatic fluids. Zircon HfO isotopic systematics suggests that the coeval A1-type granites in the Halajun area evolved from a single parental magma chamber which originated from ∼30% of the upper crustal material mixed with ∼70% mantle-derived mafic magma. Whole-rock compositional trends reveal that fractional crystallization of felsic minerals contributed to the rare metal enrichment. The rare metal mineralization in the veins and the close association of rare metal mineralization with fluorite indicate the critical role of F-rich hydrosilicate liquids in rare metal mineralization, which is also recorded by the significant increase of rare metal concentrations from the autocrystic to the hydrothermal zircon grains.

Petrogenesis and REE mineralogical characteristics of Shitouping granites in southern Jiangxi Province: Implication for HREE mineralization in South China

The enrichment of heavy rare earth elements (HREE) in the granitic host-rocks is crucial for generating ion-adsorption HREE deposits in South China. The Shitouping pluton, located in the southern part of Jiangxi Province, South China, develops large-scale HREE deposits in its weathering profile identified by recent geological surveys. These granites can be divided into medium to coarse-grained biotite monzogranite (MGBG), medium to coarse-grained syenogranite (MGSG) and fine-grained alkali feldspar granite (FGAG) based on field observations and mineral assemblages. New geochronologic results show that this composite pluton intruded during the Early Cretaceous (ca. 140 Ma). Three granitic units displaying comparable whole-rock Nd [εNd(t) = −4.5 to −3.4] and zircon Hf compositions [εHf(t) = −4.4 to +1.2], combined with mineral association, hydrothermally altered zircon and whole-rock geochemistry, indicate they were derived from partial melting of hybridized crustal source, followed by intense fractional crystallization and fluid metasomatism in the highly evolved granitic system. The MGSG and FGAG have higher Rb, Nb, HREE contents and Rb/Sr ratios, as well as lower Nb/Ta, K/Rb ratios in comparison with the MGBG, implying they are more evolved units. The overlapping magmatic zircon trace element compositions between MGBG and MGSG, coupled with the subvertical contact boundary between the FGAG and the MGBG, suggest that the generation of Shitouping pluton should be attributed to the periodic magma replenishment, remobilization, and differentiation process in the long-lived magmatic system rather than simple fractional crystallization from a single parental magma. Hydrothermally altered zircon with elevated concentration of HREE and the paragenetic relationship between LREE-phosphate and HREE minerals in the MGSG suggest that the fluid metasomatism promotes the migration and accumulation of HREE. This is consistent with the high HREE concentration recorded in the MGSG, which contains abundant HREE minerals associated with fluorites. Thus, this study highlights that magma differentiation and fluid metasomatism during the late magmatic stage could achieve the relatively high HREE contents in granites to favor the formation of ion-adsorption HREE deposits.

Pre-eruption magmatic processes and magma plumbing system at Hachijo-Nishiyama volcano, Izu–Bonin arc, Japan

Nishiyama volcano on Hachijojima Island is an active basaltic volcano located in the Izu–Bonin arc. In this study, petrological and geochemical analyses were conducted on mafic lavas and pyroclastics to understand the magma plumbing system and pre-eruption magmatic processes. Whole-rock major element compositions show significant variations (49.4–54.9 wt.% SiO2), and the samples contain variable amounts of plagioclase phenocrysts (1–40 vol.%). The whole-rock Sr, Nd, and Pb isotopic compositions of samples from the youngest volcanic stage (< 0.7 ka) are homogeneous, whereas some samples from the older stage (3–1 ka) have relatively low Pb isotopic ratios. This observation suggests that the younger magmas were derived from a single parental magma, but another parental magma with distinct geochemical features was involved in the magmatic system before 1 ka. The temporal variation in the FeO*/MgO ratios of the volcanic products is complex and is considered to reflect the intermittent injection of primitive magmas into the main magma chamber in which fractional crystallization occurred. Two-pyroxene geobarometry suggests that the main magma chamber was located at a depth of 9–12 km. The core region of some plagioclase phenocrysts consists of a glass inclusion-free inner core and an inclusion-rich outer mantle, suggesting that some plagioclase crystallized in the main magma chamber, which was followed by overgrowth during magma ascent because of increasing liquidus temperatures due to decompression-induced water exsolution from the melt. The whole-rock compositions of some eruption units with different Al2O3/MgO ratios exhibit distinct plagioclase-controlled trends, which negates the possibility that plagioclase accumulation occurred in a stable magma chamber. In addition, the density of plagioclase was higher than that of the melt during the magma ascent to the surface. From these observations, it is suggested that the accumulation of plagioclase phenocrysts occurred in ascending magmas as the plagioclase settled relative to the surrounding melt. The estimated depth of 9–12 km for the main magma chamber coincides with the depth range over which earthquake swarms occurred in 2002, suggesting that the magma chamber is still active, and that the earthquake swarms may reflect the injection of primitive magma into the magma chamber.Graphical

Petrological and geochemical characteristics of the mafic–ultramafic Americano do Brasil Complex, central Brazil, and the implications for its genesis

The Americano do Brasil Complex occurs in the Neoproterozoic Goias Magmatic Arc, central Brazil. It is composed of two mafic–ultramafic cumulate sequences, intruded into granodioritic gneisses. Although deformed and partially recrystallized by a regional metamorphic overprint, the complex still preserves relict igneous features, such as adcumulate to heteradcumulate textures. The Northern sequence is mostly composed of olivine and olivine-clinopyroxene cumulates, whereas the Southern consists mainly of two-pyroxene cumulate rocks, with plagioclase and olivine cumulates occurring in lesser amounts. The complex has three main orebodies, with textures that range from disseminated to massive sulfide breccias with durchbewegung texture. Thermodynamic modeling using a single picrite parental magma composition can predict cumulate rock compositions and mineral modes similar to all of the observed cumulate rock compositions of the Americano do Brasil Complex. Equilibrium crystallization of the liquid and assimilation-batch-crystallization involving up to 45 % of the host gneisses in the upper crust produces solids similar to the cumulates described in the Northern and Southern sequences, respectively. Modeled pressure–temperature emplacement conditions of the magma were c.a. 2.5 kbar and 1310 °C. Both sequences have similar incompatible trace element patterns which, together with the results of the modeling, imply a broadly comagmatic origin.

Petrology of the late Pan-African Guider syenite pluton (Cameroon): Implications for magma genesis and emplacement

The Guider pluton, located in the eastern border of the North-West Cameroon domain close to the Mayo-Kebbi domain, is one of the rare high potassic magmatic bodies in the northern portion of Central African Orogenic Belt. Previously studied for its structural and geochronological aspects, its petrogenesis remained uncertain. Petrography, mineralogy, and whole rock geochemistry data are presented. The pluton is made of pink and grey quartz syenites and contains numerous enclaves of quartz diorites. These enclaves are highly concentrated at the center of pluton, previously identify as the pluton's feeder zone. The transition between syenitic types is gradational while the contact between grey quartz syenite and quartz diorite is sharp. Geochemically, they display a high-K signature. Our results including field observations, petrography, mineral chemistry and geochemistry suggest that these rocks derive from a single parental magma differentiated by fractional crystallization and sedimentation in a magmatic chamber. These results also suggest that the parental magma was generated by partial melting of metasomatized lithospheric mantle. Combining previous results with former structural data, we envisage that the Guider pluton derive from two successive steps: (i) a static step of fractional crystallization and sedimentation in a deep magmatic chamber; and (ii) a dynamic step during which the stratified magma ascent toward the upper crust, the evolved syenitic magma being tapped first and the diorite later. The magmatic structures are consistent with this magma emplacement scenario.

Petrogenesis of Ta-Nb mineralization related Early Cretaceous Lingshan granite complex, Jiangxi Province, southeast China: Constraints from geochronology, whole-rock and in-situ mineral geochemistry, and Nd-Hf isotopic compositions

The Lingshan granite complex, located in the northeast part of the Qin-Hang Belt, Jiangxi Province, southeast China, has produced the largest Ta-Nb deposits in China, i.e., the Huangshan Nb-Ta and Songshugang Ta-Nb-(W-Sn) deposits. This complex is spatially a ring-like multiphase batholith, and comprises a series of granitic rocks from the central to marginal units. Mafic microgranular enclaves (MMEs) occur only in the central unit. Geochronologic results show that the magmatic activities at Lingshan district took place at Early Cretaceous, with similar emplacement ages (133–134 Ma) between the central and marginal units. The Lingshan granites are metaluminous to weakly peraluminous (most A/CNK < 1.1), and display typical A-type affinity. However, the whole-rock REE contents and Zr/Hf and (La/Yb)N ratios exhibit an abnormal increase from the central to marginal units. The Li, Rb and Cs elements of biotite as well as the Zr/Hf ratios of zircon do not exhibit a continuous enrichment trend from the central towards marginal granites. All these features suggest that the generation of Lingshan granites is more complex than the fractional crystallization from a single parental magma. Alternatively, a crystal mush model could be applied to better interpret the geochemical and isotopic contrasts between different granite phases. The marginal granites were likely formed by later crystallization events after the rejuvenation and extraction from the crystal mush reservoirs, while the central granites were formed from the residual crystal-rich mush with a relatively low terminal porosity. Moreover, the Nb and Ta contents of biotite exhibit a gradual enrichment with increasing fluorine contents in the marginal granites. The enrichment of lithium and fluorine in the granitic melt can significantly promote the proportion of non-bridging oxygens (NBOs) for coordinating Nb and Ta in the melt, further enhancing the solubility of columbite and tantalite, and eventually causing the Nb and Ta elements either incorporate into the mineral lattice of micas in the early marginal granites or form columbite-group minerals in the pegmatite/altered granite. In summary, we propose that the Ta-Nb enrichment event in the Lingshan district is spatially and genetically related to the extensive fractionation evolution from central to marginal units.

Geochemical Characterization of Intraplate Magmatism from Quaternary Alkaline Volcanic Rocks on Jeju Island, South Korea

The major and trace elements of Quaternary alkaline volcanic rocks on Jeju Island were analyzed to determine their origin and formation mechanism. The samples included tephrite, trachybasalts, basaltic trachyandesites, tephriphonolites, trachytes, and mantle xenoliths in the host basalt. Although the samples exhibited diversity in SiO2 contents, the relations of Zr vs. Nb and La vs. Nb indicated that the rocks were formed from the fractional crystallization of a single parent magma with slight continental crustal contamination (r: 0–0.3 by AFC modeling), rather than by the mixing of different magma sources. The volcanic rocks had an enriched-mantle-2-like ocean island basalt signature and the basalt was formed by partial melting of the upper mantle, represented by the xenolith samples of our study. The upper mantle of Jeju was affected by arc magmatism, associated with the subduction of the Pacific Plate beneath the Eurasian Plate. Therefore, we inferred that two separate magmatic events occurred on Jeju Island: one associated with the subduction of the Pacific Plate beneath the Eurasian Plate (represented by xenoliths), and another associated with a divergent setting when intraplate magmatism occurred (represented by the host rocks). With AFC modeling, it can be proposed that the Jeju volcanic rocks were formed by the fractional crystallization of the upper mantle combined with assimilation of the continental crust. The xenoliths in this study had different geochemical patterns from previously reported xenoliths, warranting further investigations.

Four Pan-African plutonic sets of the Colomines gold district (East-Cameroon): Petrogenesis, K-Ar dating and geodynamic significance

The study of Colomines area (eastern Cameroon) is designed to furnish petrological, mineralogical, geochemical and K-Ar data on granitoids in order to discuss the thermodynamic conditions of crystallization of different magmas, the magma series and differentiation processes, the characteristics of magmas’ sources and the geodynamic implications. The Colomines area is made up of four intrusive sets (heterogeneous granodiorites and their mafic enclaves, mafic plutonic suite (MPS), porphyritic biotite granites, Ms-Bt granites). Mineralogical assemblage is constant in MPS and mafic enclaves (Cpx+Hbl+Bt+Pl+Qtz+Op+Sph+Zrn+Ap), and granodiorite and granites sets (Hbl+Bt+Ms+Pl+Kfs+Op+Sph+Zrn+Ap), except the leucocratic Ms-Bt granites which are amphibole-free. Amphibole in diorite (MPS) and heterogeneous granodiorite has a composition of edenite and ferroedenite, respectively, whereas biotite composition varies with an increase of Fe content from basic to acidic rocks. Concomitantly, plagioclase show an increase of albite component. Crystallization of magmas started at great depths >22 km but the coexistence of heterogeneous granodiorites, MPS plutonic suite rocks, porphyritic biotite-granites and Ms-Bt granites at the same crustal level indicates that magmas migrated upwards and emplaced at a depth <4.5 km, in the upper crust. Chemical analyses show that all rocks are magnesian except ferroan Ms-Bt granites and a few samples of microgranites and porphyritic granites. Granodiorites, few monzonites of MPS and all granites are peraluminous whereas mafic enclaves in heterogeneous granodiorites are metaluminous. Heterogeneous granodiorites, gabbros and diorites of the MPS are calcic and calc-alkalic while other rock types are alkali-calcic and alkalic. Porphyritic granites and Ms-Bt granites trend from calc-alkaline to shoshonitic series. It is suggested that a fractional crystallization process may have contributed to the transition from blind primary magmas to these evolved granitoids. However, each group of rocks does not correspond to a magmatic sequence resulting from a single parent magma by fractional crystallization. They are rather indicative of the heterogeneity of magmatic sources and complex melting processes. The granitoids and associated rocks of Colomines correspond to complex mixtures in varying proportions of magmas resulting from the melting of the subducted crust and associated sediments, the mantle wedge and the lower crust. Biotite and rock compositions typical of volcanic-arc and syn-collision-related granites, and the compositions of Ms-Bt granites typical of post-collisional granites point to an active continental margin setting. The Colomines area has recorded most of the main magmatic events of the Pan-African orogeny from ante-collisional D1 (e.g. > 640 Ma heterogeneous granodiorites of Kaleka hill), syn-collisional D2 (620-600 Ma migmatisation and anatexis: leucosomes of Kaleka hill), late collisional D3 (590-580 Ma MPS) to Post-collisional D4 (580-560 Ma porphyritic biotite granites and Mu-Bt granites) intrusions.

Petrology and geochemistry of three Early Holocene eruptions from Makushin Volcano, Alaska

Makushin stratovolcano, Alaska, produced three, highly explosive, andesitic eruptions between ~ 9292 and 6215 yBP. Those eruptions are informally named the CFE (“crater-forming eruption”), Nateekin, and Driftwood Pumice, and they deposited significant tephra fallout in the present-day port of Dutch Harbor and City of Unalaska area. The focus of this study is to examine the geochemistry and petrology of those eruptions to better understand Makushin volcano hazards, andesite petrogenesis and eruption triggering by mafic recharge processes. The CFE, Nateekin, and Driftwood Pumice samples range from basaltic andesite to dacite but are predominantly andesitic (SiO2 = 55.6 to 63.5 wt%). The CFE deposits are slightly compositionally stratified, with the top CFE samples slightly more mafic (55 to 60 wt% SiO2) than the basal deposits (58 to 60 wt% SiO2). Disequilibrium mineral compositions and textures in the CFE, Nateekin, and Driftwood Pumice samples, combined with two pyroxene thermometry and An-rich plagioclase microlites (An80) found only in the top of the CFE deposits, provide evidence for repetitive mafic recharge triggering those eruptions, consistent with prior studies. We compare the Makushin geochemical data with data from select satellite vents and cones in the Makushin Volcanic Field (MVF) from prior studies, to examine possible genetic relationships. The geochemical data and Rhyolite-MELTS models run at crustal storage conditions (2 kbar, fO2 = Ni-NiO, and 1.5 and 3.5 wt% H2O) indicate that no single parental magma supplies the MVF satellite cones and Makushin volcano. Instead, two component mixing models better fit the MVF geochemical array. Our Makushin results compare well with models of predominantly andesitic volcanoes that require mafic recharge to mobilize the andesites and trigger eruptions.

Petrology of Nickeliferous Mafic–Ultramafic Bodies of the Southeastern Aldan-Stanovoy Shield

Consideration is given to the nature of mafic–ultramafic bodies in the southeastern part of the Aldan-Stanovoi Shield using the Kun-Manie ore field as an example. These bodies played an important role in the formation of the Kun-Manie sulfide copper–nickel deposit as conduits of sulfide-bearing magmas. A detailed mineralogical and geochemical study with interval sampling was carried out on core samples from the drilling of two intrusive bodies, Treugolnik and Iken, using drill holes 21 and 25. The drill–core data present evidence for their origin from a single parent magma. The Treugolnik and Iken minor intrusions were emplaced in magmatic chambers at different depths during the cotectic crystallization of dark minerals from the primary picrite (Treugolnik body) and intermediate picrobasalt (Iken body) magmas. A comparative study of petrogeochemical and mineralogical data suggests that the Treugolnik intrusion formed as a closed system with magma emplaced in a single, relatively rapid injection and crystallized without losses; the Iken sill formed as an open system as a result of magma emplacement in several pulses with time intervals between injections. These magma bodies, together with the host metamorphic rocks, were involved in tectonic deformations after they crystallized.

Stable isotope composition of minerals in the Belaya Zima plutonic complex, Russia: Implications for the sources of the parental magma and metasomatizing fluids

The Belaya Zima alkaline–carbonatite complex in the East Sayan Mountains, Russia, is a multiphase concentric intrusion, which comprises diverse alkaline silicate rocks and carbonatites. Melteigites are the earliest products of crystallization and they are followed by ijolites–urtites, nepheline syenites, and finally, calcite, calcite–dolomite and ankerite carbonatites. In this paper, we present new geochronological data (Ar–Ar method), chemical and stable isotope (H, C and O) compositions of the main rock-forming minerals in carbonatites and the silicate rocks of the complex. The Ar–Ar dating of phlogopite from ankerite carbonatites is consistent with the age of syenite emplacement and implies that the silicate rocks and carbonatites are likely to be comagmatic. The evolutionary trends of pyroxene, fluorapatite, amphibole and phlogopite are consistent with fractional crystallization from a single parental magma. Primary Nb mineralization is represented by pyrochlore, whereas columbite-(Fe) is a replacement product of pyrochlore at the post magmatic, hydrothermal stage. At that stage, interaction of calcite carbonatites with hydrothermal fluids resulted in the formation of hydrothermal paragenesis comprising dolomite–ankerite, monazite-(Ce), ancylite-(Ce), minerals of burbankite group and Ca–REE fluorcarbonates. The hydrothermal processes were also responsible for cation leaching from pyrochlore, the replacements of phlogopite by tetraferriphlogopite and early Ca-rich pyroxene by aegirine. The stable isotope data suggest that the rocks crystallized from a 18O-depleted magma. Shifts toward higher δ18O values relative to the primary compositions are commonly observed in minerals from carbonatites, which is attributed to hydrothermal processes.

Mineralogical Study of Gabbro-Anorthosite from Dumka, Chhotanagpur Gneissic Complex, Eastern Indian Shield

Abstract The Chhotanagpur Gneissic Complex (CGC), bearing imprints of widespread high grade metamorphic and magmatic history since Palaeoproterozoic, represents an integral crustal segment of Eastern Indian Shield. The gabbro-anorthosite intrusives constitute a part of mafic-ultramafic magmatism in the CGC. The study area around Dumka (24°16' to 24°20'N: 87°13' to 87°22'E) predominantly comprises of granite gneiss and charnockitic country rocks within which gabbro-anorthosite intrusions occur as lenses. Field relations and structural studies reveal that the country rocks of Dumka have suffered three phases of deformation represented by F1, F2 and F3 folds. The gabbro-anorthosite intrusives maintain a sharp contact with the host rocks, deformed and metamorphosed. Relict igneous layering or primary igneous foliation (Sig) is recorded where metamorphic overprint is minimal. Mineral phases of gabbro-anorthosite rocks suggest that clinopyroxene compositions from gabrro correspond to diopside and clinoferrosilite, while those from anorthosite are clinoferrosilite. Amphiboles from the gabbro-anorthosite rocks are calcic, and range from ferroan pargasite in gabbro to ferroan pargasitic hornblende in anorthosite. Plagioclase from gabbro and anorthosite belong to bytownite and andesine respectively. Chemical composition of garnet in gabbro is almandine. Thermobarometric estimates for Dumka gabbro-anorthosites correspond to 511°C to 915°C and 5.0-7.5 kb pressure, comparable to that estimated for Bengal Anorthosite (593-795°C, 4.1-7.3 kb). Fractionation trend of plagioclase substantiates a single parental magma in the evolution of Dumka gabbro-anorthosite intrusives.

Petrogenesis of Grove Mountains 020090: An enriched “lherzolitic” shergottite

Abstract–Grove Mountains (GRV) 020090 is a “lherzolitic” shergottite found in the Grove Mountains, Antarctica. It exhibits two distinct textures: poikilitic and nonpoikilitic. In poikilitic areas, large pyroxene oikocrysts enclose subhedral olivine and chromite chadacrysts. Pyroxene oikocrysts are zoned from pigeonite cores to augite rims. In nonpoikilitic areas, olivine, pyroxene, and interstitial maskelynite occur as major phases, and minor phases include chromite and merrillite. Compared with typical “lherzolitic” shergottites, GRV 020090 contains a distinctly higher abundance of maskelynite (19 vol%). Olivine and pyroxene are more ferroan (Fa28–40, En57–72Fs24–31Wo4–14 and En46–53Fs17–21Wo26–35), and maskelynite is more alkali‐rich (Ab43–65Or2–7). The major phases, whole‐rock (estimated) and fusion crust of GRV 020090, are relatively enriched in light rare earth elements (LREE), similar to those of the geochemically enriched basaltic shergottites, but distinct from those of LREE‐depleted “lherzolitic” shergottites. Combined with a high oxygen fugacity of log fO2 = QFM − 1.41 ± 0.04 (relative to the quartz‐fayalite‐magnetite buffer), it is clear that GRV 020090 sampled from an oxidized and enriched mantle reservoir similar to those of other enriched shergottites. The calculated REE abundances and patterns of the melts in equilibrium with the cores of major phases are parallel to but higher than that of the whole rock, suggesting that GRV 020090 originated from a single parent magma and experienced progressive fractional crystallization in a closed system. The crystallization age recorded by baddeleyite is 192 ± 10 (2σ) Ma, consistent with the young internal isochron ages of enriched shergottites. Baddeleyite dating results further demonstrated that the young ages, rather than ancient ages (&gt;4 Ga), appear to represent the crystallization of Martian surface lava flow. GRV 020090 shares many similarities with Roberts Massif (RBT) 04261/2, the first enriched “lherzolitic” shergottite. Detailed comparisons suggest that these two rocks are petrologically and geochemically closely related, and probably launch paired.

Geochemical Constraints on the Petrogenesis and Tectonic Environment of Gabbroic Intrusives in the Siang Window of Eastern Himalaya, Northeast India

Abstract Carbonate and calcareous-quartzite of Miri-Buxa Group in the Siang Window of Eastern Himalaya intruded by mafic rocks of gabbroic affinity. These intrusive rocks are low-Ti tholeiites (Ti/Y = 379-478; Nb/La = 0.99-1.88) and characterized by enriched LILE-LREE, depleted in HFSE with minor REE fractionation [(La/Yb)N = 2.72-3.35)]. Geochemical behaviour of the incompatible trace elements with the rare earth elements abundances indicates their cogenetic nature and their emplacement in a continental rift tectonic environment. The liquidus olivine temperature of these mafic rocks ranges from 1262°C to 1380°C showing a gentle decrease of [Mg] with a steep increase of [Fe]. These charters thus imply that the rocks are either related to the extent of common source or fractionational crystallization of plagioclase and clinopyroxene from a single batch parental magma. Petrogenetic modeling of [Mg]-[Fe] and REE indicates that these mafic intrusives probably derived from a mantle source similar to komatiitic composition at moderate to high degree (8%-20%) of partial melting.

Differentiation and Source Processes at Mt Pelée and the Quill; Active Volcanoes in the Lesser Antilles Arc

Volcanic rocks erupted at Mt Pelee (Martinique; central Lesser Antilles) and the Quill volcano (Statia; northern Lesser Antilles) define distinct differentiation trends, each of which can be accounted for largely by fractional crystallization of plagioclase, amphibole and Fe–Ti oxides. This assemblage is seen commonly in associated cumulate nodules, although the petrography of the lavas is pyroxene + plagioclase + Fe–Ti oxides. Thus differentiation is controlled, in part, by cryptic amphibole fractionation. At a given degree of differentiation incompatible trace element abundances tend to be higher at Mt Pelee, and REE patterns are more fractionated than is the case at the Quill. Isotopic ratios of Sr and Pb correlate with indices of differentiation (e.g. wt % SiO2) at both volcanoes (more convincingly at Mt Pelee), indicating that differentiation is also an open-system process. When the differentiation trends for the two volcanoes are compared they do not converge towards a single parental magma composition, suggesting that the primary magmas for the Quill and Mt Pelee are different. This difference is most probably due to mantle source variations. The source of the Mt Pelee magmas appears to be more enriched in incompatible elements, consistent with a greater proportion of admixed subducted sediment. This observation is in agreement with previous studies that have documented an increasing sediment contribution southwards along the arc. However, comparison with available data for other volcanoes along the arc does not reveal a consistent along-arc trend, suggesting that a model of sediment–source mixing is oversimplified and that additional factors such as variable fluid contributions from the subducted slab may be important.

Radiogenic Isotopic Ratio Variations in Carbonatites and Associated Alkaline Silicate Rocks: Role of Crustal Assimilation

Carbonatites commonly occur together with alkaline silicate rocks. It is frequently argued in the literature that the varied Sr/Sr, Nd/Nd and Pb isotopic ratios in the alkaline silicate rocks reflect source heterogeneity, and that in many cases bear no genetic significance to the coexistence of silicates and carbonatites. Such a hypothesis may not be universal as it fails to explain the observations that in numerous carbonatite^alkaline complexes both rock types are contemporaneous, they exhibit complementary trace element patterns, and most importantly their initial radiogenic isotopic ratios overlap, all of which when considered together suggest a common parentage. In addition, the alkaline silicate rocks have more variable and higher Sr/Sr, and lower Nd/Nd and Pb isotopic ratios compared with those of the carbonatites and show hyperbolic trends in isotopic ratio vs concentration plots, hinting at a possible contamination of their parental magma by crustal or lithospheric material. Using a mathematical model that quantifies the isotopic ratios and concentrations of an element during concurrent assimilation and fractional crystallization of silicate rocks combined with immiscible separation of carbonate melt (AFCLI), I propose that the isotopic ratio variations in most carbonatite^alkaline silicate complexes can be explained by assimilation of crustal material by parental carbonated silicate parental magmas. A highly plausible scenario that emerges from this exercise is that not only are the carbonate and associated silicate magmas derived from a single parental magma, but also that the lower crust plays an important role in their diversification.

Pyroxenites of the Krestovskaya alkaline-ultramafic intrusion: Composition of parental magmas and their sources

The main rock-forming minerals of pyroxenites in the Krestovskaya intrusion in the Maimecha-Kotui alkaline-ultramafic province are Al- and Ti-fassaite and low-Al high-Mg diopside. Both clinopyroxene varieties bear primary inclusions of alkaline-ultramafic melts enriched in incompatible elements, F (up to 0.3–0.4 wt %), and probably also CO2. The homogenization temperatures of the inclusions are approximately equal and lie within the range of 1200–1300° C. However, the melts preserved in the diopside are undersaturated in Si and Al and richer in Fe, Ba, Sr, Na, and incompatible elements than melt inclusions in the fassaite; they are free in H2O (no more than 0.003 wt %); and are close in composition to katungite-mafurite. Melt inclusions in the fassaite are richer in Si, Mg, and Al; contain up to 0.435 wt % H2O; and compositionally approach alkaline picritoids. Melts of such composition cannot be produced by the differentiation of a single parental magma and were most probably derived from different mantle sources. Judging from the high concentrations of incompatible elements and their distribution in the melt inclusions, these sources were localized in the undepleted mantle at various depths (the picritoid melts were derived from a deeper source) and underwent different degrees of partial melting, with garnet and plagioclase remaining in the residue. The coexistence of diopside and fassaite in a single rock can be explained by the concurrent development of magmatic chambers at different depths during rifting, when this process was repeatedly reactivated and it facilitated the arrival of primitive melts derived from different mantle sources into the same magmatic chambers, in which these melts mixed and evolved. These processes probably predetermined the origin of the alkaline-ultramafic carbonatite intrusions and perhaps also the potassic series in the East African Rift.

Three differentiation stages of a single magma at Piton de la Fournaise volcano (Reunion hot spot)

We present major element and volatile concentration analyses in melt and gas inclusions from two recent picrite eruptions (February 2005 and December 2005) at Piton de la Fournaise volcano (La Reunion Island, Indian Ocean). Combined with literature data, our new data show that the large variability of major element compositions in Piton de la Fournaise lavas may be explained by three depth‐dependent differentiation stages of a single transitional parental magma (9–11 wt% MgO; 0.5–0.8 wt% K2O, and 10–12 wt% CaO). The deepest differentiation (&gt;7.5 km) is controlled by the fractional crystallization of clinopyroxene + plagioclase and yields gabbros and basalts enriched in K and depleted in Ca relative to the parental magma. In a shallower storage zone (&lt;2.5 km), differentiation of the transitional parental magma is dominated by the fractionation/accumulation of Fo83–85 olivine phenocrysts, yielding Mg‐poor basalts at the top of the reservoir and picrites at the base. In cooling dykes and subsurface pockets (&lt;0.6 km), Mg‐poor basalts may themselves evolve into more differentiated melts by clinopyroxene + plagioclase fractionation. The incorporation of dunitic xenocrysts or other xenoliths is not necessary to explain the major element chemical diversity of the volcano. The same superposition of differentiation mechanisms may occur at similar depths in other shield volcanoes.

The contemporaneous emission of low-K and high-K trachybasalts and the role of the NE Rift during the 2002 eruptive event, Mt. Etna, Italy

Mount Etna volcano erupted almost simultaneously on its northeastern and southern flanks between October 27 and November 3, 2002. The eruption on the northeastern flank lasted for 8 days, while on the southern flank it continued for 3 months. The northeastern flank eruption was characterized by the opening of a long eruptive fracture system between 2,900 and 1,900 m.a.s.l. A detailed survey indicates that the fractures’ direction shifted during the opening from N10W (at the NE Crater, 2,900 m) to N45E (at its lowest portion, 1,900 m) and that distinct magma groups were erupted at distinct fracture segments. Based on their petrological features, three distinct groups of rocks have been identified. The first group, high-potassium porphyritic (HKP), is made up of porphyritic lavas with a Porphyritic Index (P.I.) of 20–32 and K2O content higher than 2 wt%. The second group is represented by lavas and tephra with low modal phenocryst abundance (P.I. < 20) named here oligo-phyric (low-phyric), and K2O content higher than 2 wt% (HKO, high-potassium oligophyric). The third group, low-potassium oligophyric (LKO), consists of tephra with oligophyric texture (P.I. < 20) but K2O content < 2 wt%. K-rich magmas (HKP and HKO) are similar to the magma erupted on the southern flank, and geochemical variations within these groups can be accounted for by a variable degree of fractionation from a single parent magma. The K-poor magma (LKO), erupted only in the upper segment of the fracture, cannot be placed on the same liquid line of descent of the HK groups, and it is similar to the magmas that fed the activity of Etna volcano prior to the eruption of 1971. This is the first time since then that a magma of this composition has been documented at Mt. Etna, thus providing a strong indication for the existence of distinct batches of magma whose rise and differentiation are independent from the main conduit system. The evolution of this eruption provides evidence that the NE Rift plays a very active role in the activity of Mt. Etna volcano, and that its extensional tectonics allows the intrusion and residence of magma bodies at various depths, which can therefore differentiate independently from the main open conduit system.

Multistage evolution of the Jijal ultramafic–mafic complex (Kohistan, N Pakistan): Implications for building the roots of island arcs

The ultramafic–mafic complexes located at the base of obducted paleo-island arcs are commonly interpreted as evidence for intra-crustal fractionation of primitive (high-Mg#) mantle melts. The present study, realized on the Jijal mafic–ultramafic basal section of the Cretaceous Kohistan oceanic arc (N Pakistan), discards a crystal fractionation model from a single parental magma to take into account the geochemical and isotope variations observed between the plutonic crust and the underlying ultramafic section. The basal ultramafic rocks, i.e. dunites, wehrlites and clinopyroxenites, show high Mg#, extremely depleted REE patterns, no marked HFSE anomalies and high LILE enrichment. Cpx and leached whole rocks from this unit yield restricted ε Nd values (+ 5.5 to + 6.8), a large ε Sr range (− 5.9 to + 8.8) and mainly radiogenic 207Pb/ 204Pb ratios. The overlying mafic section, i.e. gabbroic rocks, displays low Mg# (46 < Mg# < 54), enriched REE patterns and marked HFSE anomalies. These plutonic rocks have also very homogeneous Sr and Pb isotopic compositions defining a restricted domain, which does not overlap that of the ultramafic samples. Moreover, Cpx from the basal ultramafics yield an Sm–Nd isochron at 117 ± 7 Ma interpreted as dating crystallization of the ultramafic section. Together, these results indicate that the mafic rocks on one hand and the ultramafic rocks on the other hand, originate from separate sources with specific emplacement underneath the subduction zone. A lherzolite lens, collected from the top of the mafic section, plots on the early Cretaceous Sm–Nd isochron and therefore yields the same 143Nd/ 144Nd initial ratio. This suggests that the ultramafic part of the lower Kohistan arc complex sampled the initiation stages of the subduction with production of magmas with boninite-like features. At this stage, the mantle wedge has been already modified by percolation of the first slab-derived fluids. On such a scenario the lherzolite lens would correspond to the lithospheric mantle underneath the arc before its initiation. A multi-stage history is proposed for the evolution for the Kohistan arc through ∼ 30 Ma. Stage #1 reflects the spontaneous initiation of subduction associated with extensive boninitic affinity magmatism, stages #2 and #3 are related to arc building by tholeiitic melts and, a fourth step (stage #4) corresponds to the intra-crustal differentiation.