Nb and REE Distribution in the Monte Verde Carbonatite–Alkaline–Agpaitic Complex (Angola)
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Abstract:
The Angolan alkaline–carbonatite complex of Monte Verde has a semi-circular shape and is comprised of a central intrusion of foidolite rocks surrounded by concentrically arranged minor bodies of other alkaline rocks and carbonatite magmatic breccias. This rock association is hosted by fenitized Eburnean granites. Concentric swarms of alkaline dykes of late formation, mostly of nepheline trachyte composition, crosscut the previous units. Most high-field strength elements (HFSE) and rare earth elements (REE) are concentrated in pyrochlore crystals in the carbonatite and alkaline breccias. Magmatic fluornatropyrochlore is replaced and overgrown by five secondary generations of pyrochlore formed during subsolidus stages and have higher Th, REE, Si, U, Sr, Ba, Zr, and Ti contents. The second, third, and fourth pyrochlore generations are associated with late fluids also producing quartz and REE rich minerals; whereas fifth and sixth pyrochlore generations are linked to the fenitization process. On the other hand, minerals of the rinkite, rosenbuschite, wöhlerite, eudialyte groups, as well as loparite-(Ce), occur in accessory amounts in nepheline trachyte, recording low to moderate agpaicity. In addition, minor REE-bearing carbonates, silicates, and phosphates crystallize as late minor secondary minerals into carbonatite breccia and alkaline dykes. In conclusion, the scarcity of HFSE and REE minerals at the Monte Verde carbonatite-alkaline-agpaitic complex suggests low metallogenetic interest and economic potential for the outcrops analysed in this study. However, the potential for buried resources should not be neglected.Keywords:
Carbonatite
Nepheline
Nepheline syenite
Breccia
Trachyte
Cape verde
Peralkaline rock
Abstract The Late Cretaceous Itatiaia complex is made up of nepheline syenite grading to peralkaline varieties, quartz syenite and granite, emplaced in the metamorphic rocks of the Serra do Mar, SE Brazil. The nepheline syenites are characterized by assemblages with alkali feldspar, nepheline, Fe-Ti oxides, clinopyroxene, amphibole, apatite and titanite, while the peralkaline nepheline syenites have F-disilicates (rinkite, wöhlerite, hiortdahlite, låvenite), britholite and pyrophanite as the accessory phases. The silica-oversaturated rocks have alkali feldspar, plagioclase, quartz, amphibole, clinopyroxene and Fe-Ti oxides; the chevkinite-group minerals are the featured accessory phases and are found with allanite, fluorapatite, fluorite, zircon, thorite, yttrialite, zirconolite, pyrochlore and yttrocolumbite. The major- and trace-element composition of the Itatiaia rocks have variations linked to the amount of accessory phases, have smooth, enriched chondritenormalized rare-earth element ( REE ) distribution patterns in the least-evolved nepheline syenites and convex patterns in the most-evolved nepheline syenites. The REE distribution patterns of the quartz syenites and granites show a typical pattern caused by fractional crystallization of feldspar and amphibole, in an environment characterized by relatively high oxygen fugacity (>NiNiO buffer) and high concentrations of H 2 O and F, supporting the crystallization of hydrous phases, fluorite and F-disilicates. The removal of small amounts of titanite in the transition from the least-evolved to the most-evolved nepheline syenites stems from petrogenetic models involving REE , and is shown to be a common feature of the magmatic evolution of many other syenitic/ trachytic/ phonolitic complexes of the Serra do Mar and elsewhere.
Nepheline
Nepheline syenite
Peralkaline rock
Trachyte
Amphibole
Felsic
Alkali feldspar
Titanite
Fractional crystallization (geology)
Allanite
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Abstract This contribution explores the petrogenetic relationships between silicate and carbonatitic rocks in the Crevier Alkaline Intrusion (CAI, Québec, Canada). The CAI is located in the Proterozoic Grenville Province and is composed of a suite of undersaturated peralkaline rocks from ijolite to nepheline syenite and carbonatites. Petrogenetic relationships between different undersaturated alkaline igneous rocks, carbonate-bearing and carbonate-free nepheline syenite and carbonatites observed in the CAI suggest that (1) carbonate-bearing and carbonate-free silicate rocks are comagmatic with carbonatite, and that (2) both silicate and carbonatitic liquids are fractionated from an ijolitic parental magma that has undergone liquid immiscibility. One of the observed facies is characterized by spectacular ocelli of carbonate-bearing nepheline syenite in a matrix of carbonatite. The younger nepheline syenite facies can be divided into two groups based on the presence or absence of magmatic carbonates. Both groups are characterized by the presence of pyrochlore-group minerals that carry the Nb–Ta mineralization. We specifically use accessory minerals such as zircon, pyrochlore and apatite to constrain the temporal and physicochemical parameters of the immiscibility process. By coupling (1) mineral textures, (2) trace elements, (3) Ti-in-zircon thermometry, and (4) oxygen isotope compositions, we have traced the crystallization of zircon before, during and after the immiscibility process. The results allowed us to constrain the minimum temperature of this process at ∼815–865°C. In addition, two magmatic populations of pyrochlore are identified through their petrographic and geochemical characteristics within the younger nepheline syenite facies. Pyrochlore from the earlier ocelli facies of carbonate-bearing nepheline syenite follow a Nb–Ta differentiation trend, whereas pyrochlore from the younger carbonate-free nepheline syenite follow a more classical Nb–Ti trend. Following the complete immiscibility between the silicate and carbonatitic liquids, the fractionation between Nb and Ta stopped while a new generation of Nb-rich pyrochlore grew, displaying a more classical Nb–Ti fractionation trend and a higher Nb/Ta ratio in the nepheline syenite.
Carbonatite
Nepheline syenite
Nepheline
Peralkaline rock
Petrogenesis
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The Crevier alkaline intrusion (CAI, QC) is located in the Proterozoic Grenville Province and is composed of a suite of undersaturated alkaline rocks from ijolite to nepheline syenite and carbonatites. We present (i) field relationships; (ii) comagmatic carbonatite, carbonate-free nepheline syenite and carbonate-rich syenite; (iii) textures of interstitial carbonates in nepheline syenite, silicate rims around carbonates in carbonatite, primary carbonates at the edges of nepheline and feldspar or as inclusion in albite in the carbonate-rich syenite, an orbicular facies with spheres of carbonate-bearing nepheline syenite in a carbonatitic matrix; (iv) coupled to LA-ICP-MS U-Pb geochronology on zircon from nepheline syenite and apatite from carbonatite, that argue in favor of a coeval emplacement of the silicate rocks and the carbonatites, and of their parental linkage from liquid immiscibility. The (i) petrography, (ii) textures and (iii) trace elements contents of pyrochlore provide evidence for the coexistence of these two magmatic differentiation trends. In the early carbonate-rich syenite, pyrochlore defines a Nb-Ta differentiation trend, whereas they follow a more classical Nb-Ti trend in the younger nepheline syenite. The later crystallization of Nb-rich pyrochlore with more constant Nb/Ta ratio and a correlative classic Nb-Ti trend in the nepheline syenite suggests that the Nb and Ta fractionation has stopped in response to the end of the immiscibility between the two liquids. In addition, the (i) textures, (ii) trace elements, (iii) Ti-in-zircon thermometry, and (iv) oxygen isotope compositions of zircon grains from the CAI, allows to constrain the temperature range of ca. 1000-815°C at which the immiscibility process occurred. Accordingly, the deposit to microscopic multiscale evidence provided by petrographic textures, whole rock, accessory mineral geochemical signatures and isotopic compositions of the various lithologies demonstrate the petrogenesis of the CAI through a silicate-carbonate liquid immiscibility from a parental silicate liquid of ijolite composition.
Carbonatite
Nepheline syenite
Nepheline
Petrogenesis
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Perovskite-group minerals from the Lovozero peralkaline complex, on the Kola Peninsula, Russia, are primarily loparite-rich members of the perovskite-lueshite-loparite-(Ce) solid-solution series. From early-forming poikilitic nepheline syenite to late eudialyte lujavrite, loparite compositions evolve by enrichment in Na, Sr, and Nb, and depletion in Ca, Ti and light rare-earth elements. The evolutionary trend is from calcian niobian loparite-(Ce) in the poikilitic nepheline syenite and rocks of the differentiated complex through niobian calcian loparite-(Ce) in the differentiated complex and eudialyte lujavrite to cerian lueshite in eudialyte lujavrite. This trend coincides with the proposed order of crystallization of the major intrusive series of the massif. Intra- and intergrain compositional variation and diverse patterns of core-to-rim zonation exhibited by loparite grains from the same sample are characteristic of most parageneses and may result from a combination of re-equilibration phenomena and late-stage metasomatic processes.
Nepheline syenite
Peralkaline rock
Nepheline
Kola peninsula
Massif
Pegmatite
Metasomatism
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The 1341 Ma old Spitskop Complex in South Africa is one of a series of intrusions of alkaline affinity, which were emplaced into the central Kaapvaal Craton over the time period 1.4–1.2 Ga. Spitskop contains calcite and dolomite carbonatite closely associated with pyroxenite, ijolite and nepheline syenite, and provides an ideal opportunity to study the petrogenetic relationships between alkaline silicate and carbonatite magmatism. The pyroxenites are not alkalic and are preserved as xenoliths within a plug-like intrusion of ijolite. Nepheline syenites are highly peralkaline, though not agpaiitic, and intrude the ijolites as a series of sheets. These units are cut by a plug of carbonatite composed of an incomplete marginal zone of calcite and dolomite–calcite carbonatite, and a larger central zone of ferroan dolomite carbonatite. Clinopyroxene compositions change systematically from diopside-rich compositions in the pyroxenites to aegirine-augite–hedenbergite in the ijolites to acmite-dominated compositions in the nepheline syenites. Whole-rock chemical data indicate, however, that the nepheline syenites and ijolites are unlikely to be related through fractional crystallization of any reasonable combination of their component minerals (clinopyroxene, nepheline, perthite) from a common parental magma. Low total rare earth element (REE) concentrations and flat to convex-up normalized patterns in the syenites contrast strongly with the steep, light REE (LREE)-enriched patterns in the ijolites. The silicate and carbonatite components differ markedly in their εSr–εNd compositions, the carbonatites having more depleted values (εSr –10 to +10; εNd –1 to –8) than the silicates (εSr 0 to +33; εNd –8 to –13). In addition, the calcite-rich carbonatites have more negative εNd (–6 to –8) than the dolomite carbonatites (–1 to –4). Contrasting isotopic compositions along with the geochemical variations within and between the silicates and carbonatites argue against them being derived from conjugate immiscible liquids. Instead, it is proposed that the carbonatites evolved from primitive carbonate liquids produced directly by low-degree melting of carbonated mantle peridotite. A preliminary model is presented to explain how mantle carbonatite melts can ascend through the mantle and into the crust. It is proposed that the silicate magmatic rocks associated with the carbonatite are produced by melting of enriched mantle lithosphere induced by the influx of deeper-sourced carbonatite melts.
Carbonatite
Nepheline
Nepheline syenite
Xenolith
Baddeleyite
Fractional crystallization (geology)
Peralkaline rock
Aegirine
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Nepheline
Nepheline syenite
Carbonatite
Peralkaline rock
Pyroxene
Alkali feldspar
Aegirine
Trachyte
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Carbonatite
Nepheline syenite
Nepheline
Massif
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Sr, Nd, C and O isotopic compositions were determined for carbonatite and associated peralkaline silicate rocks from the Zhidoy complex in the East Sayan province, Russia. The complex consists predominantly of alkali pyroxenite with a small amount of ijolite, nepheline syenite and carbonatite. Rb-Sr isotopic data for two carbonatite samples, ijolite, and nepheline syenite give a whole rock isochron age of 569 Ma with initial εSr and εNd values of about −6.1 and 1.1, respectively. This indicates that the rocks were derived from a single parental magma by magmatic differentiation. Judging from the mode of occurrence, it is presumed that the carbonatites were produced by carbonate-silicate liquid immiscibility. Another two carbonatite samples have εSr and εNd values of −9.3 and −11.8, 2.3 and 3.4, respectively. They show a wide range of variation in initial εSr and εNd values. The concentration of Sr in the rock varies from 4660 to 13600 ppm. The chemical and isotopic variations in the carbonatite are not due to crustal contamination but are the result of the mixing of two different carbonatitic components. It is concluded that alkaline magmatism occurred at least twice in the Zhidoy area: (1) the first magmatism produced the alkali pyroxenite cumulate and the carbonatite with low initial 87Sr/86Sr ratio; (2) the second one produced the ijolite, nepheline syenite and high-εSr carbonatite. The generation of magmas with different 87Sr/86Sr and 143Nd/144Nd ratios may reflect an isotopic heterogeneity of the mantle source region.
Carbonatite
Peralkaline rock
Nepheline syenite
Nepheline
Radiogenic nuclide
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The 360-370-Ma-old Lovozero peralkaline massif (NW Russia) is a layered nepheline syenitic–foidolitic pluton. In the rocks of the massif, late-stage (auto)metasomatic alterations of rock-forming minerals are quite intense. We studied the products of the alteration of nepheline and sodalite via microtextural, microprobe, and spectroscopic methods. We found that these minerals are extensively replaced by the association between natrolite + nordstrandite ± böhmite ± paranatrolite in accordance with the following reactions: 3Nph + 4H2O → Ntr + Nsd + NaOH; 6Nph + 9H2O → Ntr + Pntr + 2Nsd + 2NaOH; Sdl + 4H2O → Ntr + Nsd + NaOH + NaCl, where Nph is nepheline, Ntr is natrolite, Nsd is nordstrandite, Pntr is paranatrolite, and Sdl is sodalite. As a result, about one-third of the sodium from nepheline (and sodalite) is set free and passes into the fluid. This leads to an increase in the Na/Cl ratio and, hence, the pH of the fluid. An increase in pH stabilizes hyperagpaitic minerals (e.g., ussingite, villiaumite, thermonatrite, and trona), which can crystallize in close proximity to pseudomorphized nepheline and sodalite. Thus, the alteration of feldspathoids increases the pH of late-magmatic fluids, which in turn can lead to the crystallization of hyperagpaitic minerals.
Nepheline
Peralkaline rock
Sodalite
Massif
Nepheline syenite
Aegirine
Metasomatism
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Nepheline
Nepheline syenite
Aegirine
Peralkaline rock
Ilmenite
Carbonatite
Alkali feldspar
Amphibole
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