Academic literature on the topic 'Alkali-rich felsic magmas'

The Itapuranga alkali granite and Uruana quartz syenite are large K-rich EW-elongated intrusions, in the central part of the Neoproterozoic Brasília Belt, central Brazil. They are associated with Pireneus lineaments, which cut the regional NNW-SSE structures of the southern part of the belt. SHRIMP and conventional U-Pb data for the Itapuranga and Uruana intrusions indicate crystallization ages of 624 ± 10 Ma and 618 ± 4 Ma, respectively. Three zircon cores from the Itapuranga granite yielded U-Pb ages between 1.79 and 1.49 Ga. Sm-Nd T DM ages for both intrusions are 1.44 Ga and epsilonNd(T) values are -5.1 and -5.7, suggesting the input of material derived from older (Paleo- to Mesoproterozoic) sialic crust in the origin of the parental magmas. Magma mixing structures indicate co-existence of mafic and felsic end-members. The felsic end-member of the intrusions is dominantly represented by crust-derived melts, formed in response to the invasion of Paleo/Mesoproterozoic sialic crust by alkali-rich mafic magmas at ca. 620 Ma. These intrusions are roughly contemporaneous with, or perhaps slightly younger than, the peak of regional metamorphism in the southern Brasília Belt. Their emplacement along the Pireneus lineament suggest a syn-tectonic origin for them, most probably in transtensional settings along these faults.

AbstractXenoliths in an olivine nephelinite from the McBride Province, North Queensland, include Cr-diopside lherzolites, spinel and garnet websterites, felsic, 2-pyroxene and garnet granulites, and hornblendites. The spinel and garnet websterites are interpreted as crystal segregations from olivine basalt or alkali olivine basalt magma at ∼ 12 kbar followed by isobaric cooling (to approximately 900–1000°C) and subsolidus reequilibration. Garnet and 2-pyroxene granulites are mineralogically and texturally distinct and are considered to represent relatively large degrees of crystallization of basaltic magmas at comparable or slightly lower pressures (8–12 kbar). Mafic and ultramafic xenoliths have been modified to varying degrees following the relatively recent influx of a H2O- and CO2-bearing fluid. Variable amounts of amphibole and mica developed in response to the introduced fluid and it is argued that some hornblendites are the end-products of this process acting on spinel websterites. Felsic and 2-pyroxene granulite xenoliths display only minor evidence of increased PH2O. Mineralogical and textural evidence indicates high-sulphur Ca-rich scapolite in several garnet granulites did not form in response to the increased fluid activities. It is proposed the scapolite was a primary cumulate phase precipitated from alkali basaltic magma under elevated fo2 and fso2 conditions.

The Mesozoic Monteregian alkaline province of southern Quebec includes mafic alnöite, monchiquite, basanite, camptonite, and alkali basalt dykes. Most carry phenocrysts of clinopyroxene that generally zone towards Ti–AlIV–Fe–Mn-rich and Mg–AlVI–Cr-poor rims. The zoning can best be explained through polybaric crystallization and differentiation during ascent from the upper mantle. In intermediate and leucocratic dykes, clinopyroxene AlIV–Ti contents decrease with the Mg/(Mg + ΣFe) ratio, probably reflecting concurrent fractionation of a Ti-rich phase. Pyroxene phenocrysts in Monteregian mafic dykes commonly have green clinopyroxene cores that are richer in Na and Fe and poorer in Mg and Cr than the enclosing titansalite phenocrysts. Some cores are euhedral and sector zoned, implying crystallization from a melt more evolved than their present hosts. The high AlVI contents of these cores imply high pressures of crystallization. The abundance of crustal xenoliths and evolved pyroxene cores indicates that the host magmas hybridized with felsic melts, cumulates, or metasomatites within the crust or an anomalously Fe–Na-rich upper mantle. This implies that the host dykes are not primary magmas but hybrids. Consequently, dyke chemistry cannot simply be inverted to determine the composition and mineralogy of the mantle source.

ABSTRACT:Recent water-undersaturated phase equilibrium data on the subsystems of the granite-H2O system have provided important new constraints on the topology of the cotectic surfaces and hence on the compositional evolution of felsic magmas. The effect of water on phase relations can be deduced from a comparison of anhydrous and H2O-saturated data or from data obtained in the presence of a CO2-bearing fluid. However, although new experimental evidence indicates that the silica enrichment of evolving H2O-undersaturated, H2O-unbuffered melts during the co-precipitation of quartz and feldspar is as previously thought for orthoclase-rich compositions, it suggests that such a trend is considerably less for Ab-rich compositions. For water-poor trachytic melts, the newly recognised strong destabilisation of the sanidine melt component relative to the anorthite melt component with increasing water content indicates that the co-precipitation of two feldspars will result in saturation of the melt with ternary alkali feldspar at an earlier stage (i.e. higher melt anorthite content) than previously thought. This, in turn, implies that the melt differentiation path will have a greater component of anorthite depletion during the equilibrium co-precipitation of ternary feldspars and that the melt will remain in the peritectic region of the two feldspar plus liquid surface over a greater interval of crystallisation, thereby enhancing the possibility that the resoption of plagioclase during the early stages of equilibrium with alkali feldspar may go to completion. Comparison of CO2-free and CO2-bearing haplogranitic phase equilibrium data suggests that CO2 may be playing an independent part in the modification of phase equilibria and may induce a significant destabilisation of the orthoclase melt component.

The Gabro de Cerro Frontino was emplaced in the Cañasgordas Block, located in the Northern Segment of the Colombian Western Cordillera. It corresponds to a pluton composed of at least three magmatic pulses, emplaced during a short period of time. Gabbros and diorites are more common in the unit than clinopiroxenites, monzodiorites and monzonites. These rocks are composed of calcic to intermediate plagioclase, augite-egerine type clinopyroxene and biotite; olivine and flogopite may be present in some mafic rocks and alkali feldspar and quartz may be present in some felsic rocks. Sphene, magnetite and apatite are common accessory minerals. The silica content in the rocks varies between 37.08% and 54.4%, with constant values of MnO (0.1% 0.4%), impoverishment of Fe2O3, MgO, CaO, TiO2 and P2O5 as SiO2 increases, and enrichment of K2O, Na2O and Al2O3 as SiO2 increases. The basic and ultrabasic rocks fall in the sub-alkaline series, the rest of the samples fall in the medium to K-rich calc-alkaline series and in the shoshonitic series. The Gabro de Cerro Frontinocorresponds to magmas impoverished on heavy rare earth elements with respect to light rare earth elements, which suggests the contribution of a subduction component in the magma genesis. The LILE (Sr, K, Rb, Pb, Ba) are enriched with respect to the HFSE values that are relatively flat and impoverished; the unit also exhibits a negative anomaly of Nb with respect to Th and Ce, being a magmatic arc the environment of generation. The ages obtained in biotite using the Ar-Ar method fall between 9.87±0.18 Ma and 11.44±0.36 Ma, Middle to upper Miocene (Tortonian-Serravallian), similar to age of other plutons that are part of the Botón Arc.

Beryl mineralization in the Nugrus-Sikait domain in the South Eastern Desert (SED) of Egypt occurs as disseminated crystals in granitic pegmatite and quartz, as well as pegmatite veins crosscutting mélange schist and ophiolitic rocks. When granitic pegmatite comes into contact with the ophiolitic rocks, phlogopite and amphibole schists are formed due to K metasomatism. The ophiolitic mélange is intruded by leucogranite and related pegmatite along the NNW to NW Nugrus shear zone. Beryl samples have been collected from Um Sleimat, Madinat Nugrus, Wadi Abu Rusheid, and Wadi Sikait. Major oxides and in situ trace and rare earth elements (REEs) of beryl and associated minerals were analyzed through EPMA and LA-ICP-MS, respectively. The investigated beryl, based on its color and chemical compositions, can be classified into the two following types: pegmatitic beryl (type I) and schist-related beryl (type II). The former is colorless to pale green, and is mainly restricted in pegmatite veins; it is poor in Cr2O3 (up to 0.03 wt%) and MgO (Nil). The latter, deep green in color, is rich in Cr2O3 (up to 0.27 wt%) and MgO (up to 2.71 wt%), and occurs within quartz veins, phlogopite schists, and tremolite schists. The abundant beryl mineralization in phlogopite schists and their related quartz veins suggests that granite and associated pegmatite are the source rocks for the Be-bearing fluids that migrate along the NW-SE trending deep-seated tectonic zone, such as the Nugrus shear zone. Therefore, the formation of beryl in schists is attributed to the interaction of granitic/pegmatitic-derived Be-bearing fluids with serpentinite and gabbro interlayered with mélange schists. Variations in the trace and REE contents of both beryl types (I and II) indicate their two-stage formation from different compositions of Be-rich fluids, where light REEs, Zr, Nb, Ba, and Th decrease from type I beryl to type II. These two phases of beryl could be attributed to the magmatic/hydrothermal fluids associated with the pegmatite emplacement. The early phase of the late-stage magmatic-derived fluids was closely related to magma evolution and pegmatite formation, forming euhedral type I beryl. The late phase of pegmatite-derived fluids was mixed with serpentinite/schist-derived fluids that cause high V and Cr content in type II beryl. The composition of parent magmas of felsic rocks, the high degree of magma fractionation or the late stage melts, fluid compositions (rich in Be, Li, Cs, Rb, K), and alkali metasomatism, as well as the linear NW-SE trending deep-seated shear zone, are all factors possibly influencing beryl mineralization in the SED of Egypt.

The West Barneys River Plutonic Suite consists of gabbro, syenite-monzonite, alkali-feldspar syenite to quartz alkali-feldspar syenite, and alkali-feldspar granite outcropping in an area of ∼100 km2 in the southern Antigonish Highlands. Magma mixing and mingling textures indicate a comagmatic relationship between some of the mafic and intermediate–felsic lithologies. However, nine U–Pb (zircon) ages, three by thermal ionization mass spectrometry (TIMS) and six by laser-ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS), from the West Barneys River suite and the lithologically similar Cape Porcupine Complex located 60 km to the east range from ca. 495 to 460 Ma, indicating that emplacement occurred over a significant span of time. Intermediate to felsic rocks consist mainly of perthitic K-feldspar and variable amounts of quartz; interstitial granophyre is present in some samples, consistent with shallow emplacement. Mafic phases are Fe-rich amphibole and clinopyroxene, and in some units, fayalite. Intermediate and felsic samples have chemical characteristics of within-plate ferroan A-type granitoid rocks. Gabbroic rocks consist of plagioclase (oligoclase–labradorite) and augite/diopside with less abundant orthopyroxene, olivine, biotite, and ilmenite/magnetite. Their chemical compositions are transitional from tholeiitic to alkalic and characteristic of continental within-plate mafic rocks. The εNd values are similar in gabbroic, syenitic, and granitic samples, ranging between 0.9 and 4.9, consistent with a co-genetic origin for the mafic and intermediate/felsic components of the suite, and derivation from Avalonian subcontinental lithospheric mantle in an extensional environment.

Gabal El-Ineigi fluorite-bearing rare-metal granite with A-type affinity, located in the Central Eastern Desert of Egypt, is distinguished by its abundance of large fluorite-quartz veins and mafic enclaves. Plagioclase (labradorite to oligoclase), Mg-rich biotite, and Mg-rich hornblende are the main components of mafic enclaves, with significant amounts of fluorite as essential phases, and titanite and Fe-Ti oxides (Nb-free rutile and ilmenite-rutile solid solution) as the main accessories. These enclaves are monzodioritic in composition, Si-poor, and highly enriched in Ca, Fe, Mg, and F compared to the host alkali feldspar F-poor Si-rich granites. Given the conflicting evidence for a restitic, xenolithic, magma mixing/mingling, cumulate, or bimodal origin for these enclaves, we propose that the mafic enclaves and felsic host granites are two conjugate liquids, with contrasting compositions, of a single parental melt. This is inferred by the normalized REE patterns that are similar. As a result, liquid immiscibility is proposed as a probable explanation for this mafic–felsic rock association. These enclaves can be interpreted as transient melt phases between pure silicate and calcium-fluoride melts that are preserved from the early stages of separation before evolving into a pure fluoride (Ca-F) melt during magma evolution. Due to element partitioning related to melt unmixing, the enclaves are preferentially enriched in Ca, F, Li, Y, and REE and depleted in HFSE (such as Zr, U, Th, Ta, Nb, Hf, and Ga) in comparison to the host granites. Furthermore, mafic enclaves exhibit W-type tetrad effects, while host granites exhibit M-type tetrad effects, implying that the REE partitioning, caused by liquid immiscibility, is complementary.

AbstractThe change in chemical composition trend of magmatic nepheline through magma evolution has been characterized from the alkaline series of the Messum complex in which nepheline occurs in a succession of different mineral parageneses from mafic-rich (theralites) to strongly evolved felsic-rich rock types (nepheline syenites). The nepheline compositions are dependent on those of coexisting feldspar(s). They record an evolution parallel to that of the melt schematized according to experimental phase diagrams, from initially Ca-rich compositions in equilibrium with calcic plagioclase towards increasingly Ca-poor, Na-rich and Si-rich compositions. The K contents show a maximum that corresponds to the appearance of alkali feldspar in the parageneses. This evolution is qualitatively preserved in spite of the low-T Na/K re-equilibration typical of plutonic nephelines. Although a slight increase in the silica content of nepheline is consistent with the experimentally defined magmatic trend, several high-silica nephelines from the Messum rocks as well as from other reported occurrences, cannot be reconciled with the experimental data. The nepheline solid-solution model available suggests that such ‘abnormal’ compositions might be related to different crystallization mechanisms between natural nephelines and some synthetic analogues.

Le comportement des halogènes (F, Cl, Br et I) dans les systèmes magmatiques est loin d'être clairement compris. La communauté scientifique n'a qu'une connaissance fragmentaire des processus qui influencent le comportement de ces éléments pendant le stockage et l'ascension des magmas. Les études récentes du comportement des halogènes lourds comme le brome et l'iode se sont principalement concentrées sur les magmas liés à la subduction, dans le contexte du cycle géochimique des halogènes depuis la subduction jusqu'à l'atmosphère. Les études antérieures sur la solubilité des halogènes (à saturation en saumure) ont montré que les liquides calco-alcalins felsiques, fortement polymérisés, présentent une solubilité des halogènes (à l'exception du F) plus faible que les liquides felsiques riches en alcalins. En outre, les magmas riches en alcalins peuvent produire des éruptions de grand volume (par exemple, le système du rift est-africain), ce qui peut conduire à une émission massive d'halogènes dans l'atmosphère. Les émissions d'halogènes dans l'atmosphère sont donc potentiellement sous-estimées dans ces contextes, en raison d'un manque de compréhension détaillée du comportement des halogènes (et notamment du brome et de l'iode) pendant le dégazage magmatique. Dans ce contexte, l'objectif de cette thèse est de contraindre expérimentalement le partage des halogènes entre la phase fluide magmatique et le liquide silicaté dans les systèmes alcalins felsiques et de conduire une étude préliminaire de l'abondance des halogènes (notamment du brome et de l'iode) dans les verres alcalins naturels mafiques à felsiques provenant de différents contextes géodynamiques. Nous avons réalisé des expériences HP-HT (800 -1100 °C ; 10-200 MPa ; NNO-0.6 - NNO+3.4) en utilisant quatre compositions de liquide silicaté de teneur en SiO2 et de rapport molaire [(Na₂O+K₂O)/Al₂O₃] variables (phonolite, comendite, pantellerite et un analogue synthétique de la composition phonolitique). Nos résultats montrent que la composition du liquide a un impact important sur le partage des halogènes entre le fluide et le liquide silicaté. Le Dhalogens (avec Dhalogens = concentration de l'halogène dans la phase fluide/concentration de l'halogène dans le liquide silicaté) augmente avec la teneur en SiO₂ et diminue avec l'alcalinité des liquides, en accord avec les données de solubilité. Nous avons effectué une étude systématique de l'influence de la température et de la pression sur le partage des halogènes entre le fluide et le liquide silicaté et les résultats montrent que la température a un effet plus prononcé sur le partage que la pression. L'influence des conditions redox a également été étudiée et les résultats montrent que le D_I diminue avec la diminution de la fO2, tandis que DBr et le DCl montrent un effet inverse. Nous présentons la première détermination des abondances en halogènes lourds (Br et I) dans des verres felsiques riches en alcalins, avec des concentrations de l'ordre de ~10 ppm de Br et jusqu'à ~1 ppm de I dans les rhyolites riches en alcalins. Les concentrations en iode de ces verres sont au moins un ordre de grandeur plus élevées que les concentrations déterminées par l'analyse roche totale des produits volcaniques calco-alcalins publiées par ailleurs, mettant en exergue la nécessité de quantifier davantage les halogènes lourds dans les magmas afin de mieux évaluer les émissions atmosphériques d'halogènes et leur impact sur l'environnement
The behavior of halogens (F, Cl, Br and I) in magmatic systems is far from being clearly understood. The scientific community has only a fragmentary understanding of the processes that influence the behaviour of these elements during magma storage and ascent to the surface. Recent studies of heavy halogens (Br and I) behaviour have focused mainly on subduction-related magmas, in the context of the geochemical cycle of the halogens from subduction to the atmosphere. Previous studies of halogens solubility (at brine saturation) have shown that felsic, highly-polymerized calc-alkaline melts have lower halogens (except F) solubility than felsic alkali-rich melts. In addition, alkali-rich melts can produce large volume eruptions (e.g., East Africa Rift System), leading to potential massive release of halogens into the atmosphere. Emissions of halogens to the atmosphere are therefore likely to be underestimated, due to the lack of detailed understanding of the behavior of halogens (and in particular bromine and iodine) during magmatic degassing. In this work, we have addressed this gap by experimentally constraining fluid/melt halogen partitioning in felsic alkali-rich systems, with a focus on Br and I, and by a preliminary study of halogens abundances in natural mafic to felsic alkali-rich glasses from different geodynamic contexts. We performed HP-HT experiments (800°-1100 °C; 10 -200 MPa; NNO-0.6 -NNO+3.4) using four melt compositions, with variable SiO2 contents and [(Na₂O+K₂O)/Al₂O₃] molar ratios (natural phonolite, comendite and pantellerite and a synthetic analogue of phonolitic composition). Our results show that melt composition has a strong effect on the partitioning of halogens between fluid and melt. Dhalogens (with Dhalogens = halogen concentration in the fluid phase / halogen concentration in the silicate melt) increases with SiO₂ content and decreases with melt alkalinity, in agreement with the solubility data. We have carried out a systematic investigation of the influence of temperature and pressure on the fluid-melt partitioning of halogens and the results show that temperature has a more pronounced effect on partitioning than pressure. The effect of the redox conditions on halogens fluid/melt partitioning was also explored and the results indicate that DI decreases with decreasing fO₂, whereas DBr and DCl show the opposite effect. We present the first determination of heavy halogens (Br and I) abundances in felsic alkali-rich glasses, with concentrations in the order of ~10 ppm of Br and up to ~1 ppm of I in alkali-rich rhyolites. Iodine concentrations for these melts are at least an order of magnitude higher than concentrations determined by bulk rock analysis of calc-alkaline volcanic rocks in previous studies, highlighting the need for further quantification of heavy halogens in magmas to better assess their atmospheric emission and impact

ABSTRACT Ferroan granite is a characteristic rock type of the Laurentian margin. It is commonly associated with anorthosite and related rocks. Ferroan granites are strongly enriched in iron, are alkalic to alkali-calcic, and are generally metaluminous. These geochemical characteristics reflect their tholeiitic parental magma source and relatively reducing and anhydrous conditions of crystallization. Their compositions distinguish them from arc magmas, which are magnesian and calcic to calc-alkalic. Ferroan granite magmas are hot, which promotes partial melting of their crustal wall rocks. Assimilation of these silica-rich and peraluminous melts drives the resulting magmas to higher silica and aluminum saturation values. Where Proterozoic ferroan granites intrude Archean crust, their mantle component is readily identified isotopically, but this is more difficult where they intrude relatively juvenile crust. Ferroan granite forms in tectonic environments that allow partial melts of tholeiitic mantle to pond and differentiate at or near the base of the crust. Phanerozoic examples occur in plume settings, such as the Snake River Plain and Yellowstone, or under certain conditions involving slab rollback, such as those that formed the Cenozoic topaz rhyolites of the western United States or ferroan rhyolites of the Sierra Madre Occidental. It is possible that the long-lived supercontinent Nuna-Rodinia, of which Laurentia was a part, formed an insulating lid that raised underlying mantle temperatures and created a unique environment that enabled emplacement of large volumes of mafic melt at the base of the crust. Ascent of felsic differentiates accompanied by variable crustal assimilation may have created large volumes of Proterozoic ferroan granite and related rocks.

ABSTRACT The Providencia island group comprises an extinct Miocene stratovolcano located on a shallow submarine bank astride the Lower Nicaraguan Rise in the western Caribbean. We report here on the geology, geochemistry, petrology, and isotopic ages of the rocks within the Providencia island group, using newly collected as well as previously published results to unravel the complex history of Providencia. The volcano is made up of eight stratigraphic units, including three major units: (1) the Mafic unit, (2) the Breccia unit, (3) the Felsic unit, and five minor units: (4) the Trachyandesite unit, (5) the Conglomerate unit, (6) the Pumice unit, (7) the Intrusive unit, and (8) the Limestone unit. The Mafic unit is the oldest and forms the foundation of the island, consisting of both subaerial and subaqueous lava flows and pyroclastic deposits of alkali basalt and trachybasalt. Overlying the Mafic unit, there is a thin, minor unit of trachyandesite lava flows (Trachyandesite unit). The Breccia unit unconformably overlies the older rocks and consists of crudely stratified breccias (block flows/block-and-ash flows) of vitrophyric dacite, which represent subaerial near-vent facies formed by gravitational and/or explosive dome collapse. The breccias commonly contain clasts of alkali basalt, indicating the nature of the underlying substrate. The Felsic unit comprises the central part of the island, composed of rhyolite lava flows and domes, separated from the rocks of the Breccia unit by a flat-lying unconformity. Following a quiescent period, limited felsic pyroclastic activity produced minor valley-fill ignimbrites (Pumice unit). The rocks of Providencia can be geochemically and stratigraphically subdivided into an older alkaline suite of alkali basalts, trachybasalts, and trachyandesites, and a younger subalkaline suite composed dominantly of dacites and rhyolites. Isotopically, the alkali basalts together with the proposed tholeiitic parent magmas for the dacites and rhyolites indicate an origin by varying degrees of partial melting of a metasomatized ocean-island basalt–type mantle that had been modified by interaction with the Galapagos plume. The dacites are the only phenocryst-rich rocks on the island and have a very small compositional range. We infer that they formed by the mixing of basalt and rhyolite magmas in a lower oceanic crustal “hot zone.” The rhyolites of the Felsic unit, as well as the rhyolitic magmas contributing to dacite formation, are interpreted as being the products of partial melting of the thickened lower oceanic crust beneath Providencia. U-Pb dating of zircons in the Providencia volcanic rocks has yielded Oligocene and Miocene ages, corresponding to the ages of the volcanism. In addition, some zircon crystals in the same rocks have yielded both Proterozoic and Paleozoic ages ranging between 1661 and 454 Ma. The lack of any evidence of continental crust beneath Providencia suggests that these old zircons are xenocrysts from the upper mantle beneath the Lower Nicaraguan Rise. A comparison of the volcanic rocks from Providencia with similar rocks that comprise the Western Caribbean alkaline province indicates that while the Providencia alkaline suite is similar to other alkaline suites previously defined within this province, the Providencia subalkaline suite is unique, having no equivalent rocks within the Western Caribbean alkaline province.