Alkali pyroxenes and amphiboles: a window on rare earth elements and other high field strength elements behavior through the magmatic-hydrothermal transition of peralkaline granitic systems

The Early Cretaceous (~132 Ma) Anitapolis stock is made up of cumulate pyroxenites, minor nepheline syenites and ijolites and scarce dykes of nephelinitic composition. Carbonatites form a small body, but are also widespread as dykes and veins. The complex is entirely surrounded by Late Proterozoic granitic-gneissic rocks (570–630 Ma). The Late Cretaceous (70–77 Ma) Lages complex constitutes a proeminent dome structure underlined by a concentric arrangement of Permian to Triassic sediments. It consists mainly of peralkaline phonolites and nepheline syenites with subordinate ultramafic rocks (melilitite, olivine nephelinite, basanite and tephrite). Other important rock-types are kimberlitic breccias and carbonatites as a minor intrusion. Chemical data suggest that the Anitapolis carbonatites are essentially Ca-carbonatites and the Lages ones Fecarbonatites. Normalized IE and REE diagrams for both carbonatites display different patterns. d 18 O and d 13 C isotopes for the Anitapolis carbonatites plot near the primary carbonatite box, whereas the Lages rocks point to a large spreading of d 18 O values. Radiogenic isotope data indicate that the carbonatites and the associated silicate rocks present similar Sr i and Nd i values, spanning from time integrated depleted to enriched types. Nd-model ages for the Early (Anitapolis) and Late Cretaceous (Lages) carbonatites are 1.3 ± 0.1 Ga and 1.2 ± 0.2 Ga, respectively, but the Lages rocks seem to have been affected by two different Proterozoic mantle metasomatic events. Considerations on the geodynamic implications of the alkaline and alkaline-carbonatitic magmatism are made on the basis of models other than mantle plume.

Carbonatite

Nepheline

Peralkaline rock

Metasomatism

Ultramafic rock

10.5327/rbg.v32i1.959

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Citations (22)

Mineral chemistry of a peralkaline combeite-lamprophyllite nephelinite from Oldoinyo Lengai, Tanzania

Mineralogical Magazine (1998)

J. B. Dawson Peter Hill

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

10.1180/002646198547567

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Citations (36)

Paragenesis and decomposition assemblage of a Mn-rich eudialyte from the Sushina peralkaline nepheline syenite gneiss, Paschim Banga, India

Lithos (2012)

Roger H. Mitchell Aniket Chakrabarty

Peralkaline rock

Aegirine

Nepheline

Nepheline syenite

Carbonatite

10.1016/j.lithos.2012.02.003

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Citations (15)

The miaskitic-to-agpaitic transition in peralkaline nepheline syenite (white foyaite) from the Pilanesberg Complex, South Africa

Chemical Geology (2016)

Tom Andersen Marlina Elburg Muriel Erambert

Peralkaline rock

Nepheline syenite

Nepheline

10.1016/j.chemgeo.2016.08.020

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Citations (32)

The Transition from Agpaitic to Hyperagpaitic Magmatic Crystallization in the Ilímaussaq Alkaline Complex, South Greenland

Journal of Petrology (2015)

Tom Andersen Henrik Friis

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

Microcline

Pegmatite

Magmatic water

10.1093/petrology/egv039

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Citations (17)

pH changes in peralkaline late-magmatic fluids