Sector zoned aegirine from the Il�maussaq alkaline intrusion, South Greenland
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Aegirine
Peralkaline rock
Nepheline
Abstract At least five successive intrusions of nepheline-bearing magmas fractionated in situ and developed repeatedly overlapping, yet distinct chemical trends in the mafic silicates. Representative electron probe analyses are given for olivine, clinopyroxene, amphibole, biotite, and aenigmatite and their variations discussed. Increasing Na with fractionation produced aegirine-rich clinopyroxene and caused Na/K in amphibole to increase from 2.4 to 7.0. Si z /Al z increased in both amphibole and in the less abundant mica. Mn-enrichment is common to all of the mafic silicates, including the olivines and it is suggested that the parent magmas were initially Mn-rich. Mineralogically the Motzfeldt centre is evolved and similar to the other centres in the Igaliko complex. However, the Motzfeldt centre contains the most fractionated rocks (lujavrites) and is in this respect similar to the peralkaline Ilimaussaq intrusion. The compositional ranges and trends of the mafic silicates in Motzfeldt thus provide links between the uncommon peralkaline plutons and the more abundant and typical nepheline syenites.
Nepheline
Peralkaline rock
Amphibole
Aegirine
Nepheline syenite
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Abstract A peralkaline nephelinite lava ([Na+K]/Al 2.15) from the active carbonatite volcano Oldoinyo Lengai, contains combeite, Ba lamprophyllite, a phase with affinities to delhayelite, CeSrNb perovskite, a CaNa phosphate high in Sr, Ba and K, and peralkaline glass; in addition to Fe-rich nepheline, aegirine-rich clinopyroxene and FeK-rich sodalite. The high alkali concentrations relative to alumina in the bulk rock could not have been achieved by fractionational crystallisation of the known Al-rich phenocryst phases (nepheline and sodalite) and some other process must be invoked.
Peralkaline rock
Aegirine
Nepheline
Carbonatite
Sodalite
Nepheline syenite
Phenocryst
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Peralkaline rock
Aegirine
Nepheline
Nepheline syenite
Carbonatite
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Peralkaline rock
Nepheline syenite
Nepheline
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Hyperagpaitic rocks are highly peralkaline nepheline syenites in which minerals such as ussingite [Na2AlSi2O8(OH)] and naujakasite (Na6FeAl4Si8O26) crystallize instead of, or in addition to, feldspars and feldspathoids; eudialyte is succeeded by steenstrupine-(Ce) and members of the lovozerite and lomonosovite groups; highly water-soluble minerals such as villiaumite (NaF) and natrosilite (Na2Si2O5) form part of magmatic mineral assemblages. Hyperagpaitic magmatic rocks in the Ilímaussaq alkaline complex in South Greenland include intrusive bodies of villiaumite- and naujakasite-bearing lujavrite (i.e. melanocratic, silica-undersaturated syenite), and late veins and pegmatites. The transition from agpaitic to hyperagpaitic magmatic conditions is controlled by increasing peralkalinity (or aNa2Si2O5) and changes in activities of water, halogens and phosphorus. The relative importance of these parameters can be evaluated by chemographic analysis based on observed mineral assemblages and mineral compositions. Highly peralkaline melts can crystallize agpaitic albite + microcline + nepheline + arfvedsonite + eudialyte ± sodalite magmatic assemblages over a wide range of alkali, water and halogen activities. In most cases, a net increase in HF activity is needed to induce villiaumite crystallization, suggesting hyperagpaitic conditions, but because of the orientation of the villiaumite-saturation surface in log aNa2Si2O5–log aH2O–log aHF space, loss of an aqueous fluid may stabilize villiaumite without increasing peralkalinity. Naujakasite indicates elevated aNa2Si2O5, ussingite elevated aH2O, and steenstrupine-(Ce) and vuonnemite high aNa2Si2O5 and aP2O5. Closed-system fractionation of agpaitic magma may lead to hyperagpaitic residual liquids if aegirine, eudialyte or sodalite do not crystallize early, which is not applicable to the main line of magma evolution in Ilímaussaq, with abundantly eudialyte-bearing kakortokite and sodalite-bearing naujaite cumulates. Hyperagpaitic residual liquids can have developed if fractionation of these minerals were suppressed, or by open-system processes such as intraplutonic assimilation of sodalite-bearing cumulates in lujavritic magma at low aHCl.
Peralkaline rock
Nepheline
Aegirine
Sodalite
Nepheline syenite
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Pegmatite
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Peralkaline rock
Nepheline
Aegirine
Nepheline syenite
Analcime
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Aegirine
Peralkaline rock
Nepheline
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Aegirine
Peralkaline rock
Nepheline
Nepheline syenite
Amphibole
Titanite
Alkali feldspar
Ilmenite
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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.
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Peralkaline rock
Sodalite
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Nepheline syenite
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Nepheline
Nepheline syenite
Aegirine
Peralkaline rock
Ilmenite
Carbonatite
Alkali feldspar
Amphibole
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