Mn and Ca enriched olivines from nepheline syenites of the South Qôroq Centre, south Greenland
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Keywords:
Fayalite
Mineral redox buffer
Nepheline
Nepheline syenite
Pigeonite
Peralkaline rock
Fractional crystallization (geology)
Abstract The North Qôroq centre comprises a series of concentric nepheline syenite intrusions and forms part of the Igaliko Nepheline Syenite Complex, in the rift-related Gardar Province of South Greenland. The North Qôroq syenites range from mildly undersaturated augite syenite to strongly peralkaline agpaitic nepheline syenite. Extensive in situ fractional crystallization has been postulated for the chemical variation both within units and throughout the centre. Many of the rocks have been affected by metasomatic fluids associated with the emplacement of younger syenite units, and this complicates their interpretation. In this study, the trends and compositions exhibited by pyroxene and amphibole from North Qôroq are examined and related to either primary crystallization or metasomatic activity (e.g. controls of f O2 , peralkalinity). Implications thus drawn are used to interpret the chemical processes inherent in the chemical and fluid evolution of alkaline magmas, and, in particular, the transition from miaskitic to agpaitic magmatism. In general, the major phases of the North Qôroq syenites records the increasing evolution of the units by crystal fractionation, towards peralkaline compositions. The composition of olivine, in the least evolved syenites, also points to a relatively high state of fractionation of the parent magma, whilst pyroxene and amphibole record an overall decrease in Mg/Mg+Fe), and a general increase in Fe 3+ and alkali content, with increased fractionation. The increasing peralkalinity of the magma also governs the evolution of pyroxene and, to a lesser degree, amphibole towards higher Zr and Ti contents in the more Na-rich compositions. The trends for pyroxene from metasomatized syenite show similar patterns, but lower Fe 2+ enrichment, suggesting the source of the metasomatic fluids is similar to the evolving syenites. The presence of amphiboles in metasomatic rocks, and high F contents attest to the F-rich nature of the metasomatic fluids, which is in agreement with results previously reported for metasomatic fluorapatite.
Peralkaline rock
Nepheline
Nepheline syenite
Amphibole
Pyroxene
Metasomatism
Alkali feldspar
Fractional crystallization (geology)
Carbonatite
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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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Abstract It is now generally agreed that the term ‘agpaitic’ should be restricted to peralkaline nepheline syenites (and phonolites) containing minerals such as eudialyte and rinkite, that is complex silicates of Zr, Ti, the rare earth elements ( REE ), and F and other volatiles. There are, however, cases of transition into more common types of nepheline syenites containing zircon, titanite, ilmenite, etc. The agpaitic rocks are characterized by extremely high contents of rare elements such as Li, Be, Nb, Ta, REE , Zr, Th, etc. and of volatiles, first of all F and Cl. This gives rise to a wealth of mineral species, more than 500 in the Lovozero and Khibina complexes of the Kola peninsula, about 250 in Mont Saint-Hilaire, Quebec, Canada, and about 200 in the type locality, the Ilímaussaq complex, South Greenland. These rocks have very long melting intervals and solidus temperatures as low as 500 to 400°C. They are accompanied by a gas phase rich in methane and other hydrocarbons and most probably also by sodium-rich fluids as indicated by the presence of minerals such as ussingite (NaAlSi 3 O 8 ·NaOH) and villiaumite (NaF) and of pegmatites and hydrothermal veins rich in sodium and rare and volatile elements. Agpaitic nepheline syenites are considered to have been formed by consolidation of melts oversaturated in alkalis, especially sodium, under conditions preventing the volatiles from escaping. These melts have been derived by extreme fractionation processes in alkali basaltic or nephelinitic magmas. The main stage of crystallization of the melts is characterized by minerals such as nepheline (sometimes also sodalite), alkali feldspars, arfvedsonite, aegirine and eudialyte, but the most highly developed, hyperagpaitic lujavrites of the Ilímaussaq complex have been formed from melts with extreme concentrations of sodium and volatiles resulting in the formation of naujakasite instead of nepheline, ussingite instead of sodalite and alkali feldspars, and steenstrupine instead of eudialyte. During the late stages of crystallization, sodium-rich fluids are the cause of late- and postmagmatic alteration and of the formation of hydrothermal mineralizations. The late stages are characterized by water-soluble sodium-rich minerals of which more than 80 have been found in the Khibina and Lovozero complexes.
Nepheline
Nepheline syenite
Peralkaline rock
Titanite
Pyroxene
Pegmatite
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Peralkaline rock
Aegirine
Nepheline
Nepheline syenite
Carbonatite
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Eudialyte-group minerals are important accessory minerals of peralkaline rocks of nepheline-syenite massifs and alkaline–ultramafic complexes. Here, we report the complex study of a eudialyte-group mineral (EGM) from peralkaline pegmatite of the alkaline-ultrabasic Odikhincha massif (Polar Siberia). The chemical composition of the studied EGM is intermediate between those of taseqite and eudialyte, with small admixtures of other members of the eudialyte group. The crystals of EGMs were formed during the postmagmatic stage in the temperature range of 300–350 °C and partly replaced by late eudialite along cracks during the zeolite stage (~230 °C). The chemical compositions, structural features and mineral association of the studied EGM are similar to those of Sr-Nb-dominant EGM found in other nepheline-syenite massifs, such as Khibiny, Lovozero and Pilansberg. The EGM studied in this work is a Cl-deficient taseqite variety (“monochlore taseqite”), which differs from “dichlorotaseqite” (found only in the Ilimaussaq massif) by a lower amount of chlorine.
Peralkaline rock
Massif
Nepheline
Nepheline syenite
Pegmatite
Ultramafic rock
Aegirine
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Nepheline
Nepheline syenite
Carbonatite
Peralkaline rock
Pyroxene
Alkali feldspar
Aegirine
Trachyte
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Peralkaline rock
Nepheline syenite
Nepheline
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Nepheline syenite
Peralkaline rock
Nepheline
Aegirine
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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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