Compositional evolution and cryptic variation in pyroxenes of the peralkaline Lovozero intrusion, Kola Peninsula, Russia
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Abstract The Lovozero alkaline massif is the largest of the world's layered peralkaline intrusions (∼650 km 2 ). We describe the evolution of clinopyroxene from the liquidus to the late residual stage throughout the whole vertical section (2.5 km thick) of the Lovozero Complex. Microprobe data (∼990 analyses) of the clinopyroxenes define a relatively continuous trend from diopside containing 15–20% hedenbergite and 10–12% aegirine components, to pure aegirine. The main substitutions during the evolution of the Lovozero pyroxenes are (Na,Fe 3+ ,Ti) for (Ca,Mg,Fe 2+ ). The composition of the pyroxene changes systematically upwards through the intrusion with an increase in Na, Fe 3+ and Ti and decrease in Ca and Mg. The compositional evolution of the Lovozero pyroxene reflects primary fractionation processes in the alkaline magma that differentiated in situ from the bottom to the top of the magma chamber as a result of magmatic convection, coupled with the sedimentation of minerals with different settling velocities. The temperature interval of pyroxene crystallization is very wide and probably extends from 970 to 450°C. The redox conditions of pyroxene crystallization in the Lovozero intrusion were relatively low, approximating the QFM buffer.Keywords:
Pyroxene
Peralkaline rock
Aegirine
Nepheline
Layered intrusion
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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The North Qoroq Centre is one of four major intrusive centres comprising the Igaliko Nepheline Syenite Coitplex. The centre is composed of a number of syenitic bodies all showing undersaturated character. Emplacement of the bodies was probably by ring fracture and block subsidence, combined with a degree of stoping. Petrographic, mineralogical and geochemical studies have demonstrated that the process of in situ fractionation accounts for the bulk of the variation in rock types seen in the centre. Clinopyroxene, olivine, Fe/Ti oxides and apatite were important early fractionating phases, followed by amphibole and biotite. The most important fractionating phase was, however, alkali feldspar. Its crystallization and separation resulted in peralkaline undersaturated syenites becoming more peralkaline and more undersaturated. Probe work on the majority of major mineral phases, present in the syenites, has enabled values to be placedon a number of important physical and chemical parameters. The temperature of the magma as it evolved and the values and effect of steadily varying silica activity, oxygen fugacity, water fugacity, and activity of sodium disilicate have all been considered. An alkali-rich aqueous phase probably co-existed with the more fractionated of the North Qoroq syenites. A reasonable idea of the nature and composition of this phase has been obtained and a number of features exhibited by the syenites attributed to its action. Influx of meteoric water at an early stage in the evolution of the magmas is suggested as an explanation for the common marginal pegmatites. This process could be instrumental in deciding whether magmas of trachytic composition proceed, with crystal fractionation, towards undersaturated or oversaturated residual compositions.
Peralkaline rock
Nepheline
Nepheline syenite
Alkali feldspar
Mineral redox buffer
Amphibole
Pyroxene
Trachyte
Fractional crystallization (geology)
Carbonatite
Pegmatite
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Clinopyroxenes and Coexisting Mafic Minerals from the Alkaline Ilimaussaq Intrusion, South Greenland
The Ilímaussaq intrusion consists of an augite syenite shell and a central cumulitic series of layered peralkaline (agpaitic) nepheline syenites with both roof and bottom cumulates. Microprobe analyses of the strongly zoned mafics show that the pyroxenes range from ferro-salite through hedenbergite to aegirine, the amphiboles from hastingsite through katophorite to arfvedsonite, and the olivines from Fo16.8 to FO0.2. Aenigmatite analyses are also presented. The crystal chemistry of the pyroxenes is discussed in detail. The behaviour of the various elements is discussed in relation to the conditions in the coexisting magma. The relation between oxygen fugacity and silica activity and the stability of fayalite and Na-poor pyroxenes in over-and undersaturated magmas is considered. The persistent stability of these minerals in the strongly undersaturated Ilimaussaq magma shows that the oxygen fugacities in Ilímaussaq were lower than in any other known alkaline suites. Based on the compositional development in the mafic minerals it is inferred that the intrusion formed from at least three separate pulses of successively more differentiated magma: (1) an augite syenite magma congealed inwards from the sides, (2) a peralkaline undersaturated magma gave rise to the roof cumulates which congealed successively downwards. The differentiating magma was water-undersaturated and underwent depletion in Mg and enrichment in Zr and Na. The contemporaneous bottom cumulates are hidden beneath the now visible bottom cumulates which possess a still more differentiated liquidus mineral assemblage and are ascribed to a third separate magma pulse.
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The El Picacho Tertiary intrusive complex is located in the west-central part of the Sierra de Tamaulipas, in the alkaline province of northeast Mexico. The complex cuts Cretaceous limestone, and it consists of gabbro, kaersutite diorite, syenite, nepheline-rich foidolites (ijolite, urtite, juvite), and trachytic and phonolitic dikes. The nephelinerich foidolites are affected by alkali metasomatism and contain apatite-rich veins enriched in the rare-earth elements. The nepheline-rich foidolites consist of nepheline (55-91 9o), pyroxene (4-35V0) (Di36Hd3eAe25 to Dil8Hd37Ae45) and orthoclase (0-22V0), They are silica-undersaturated rocks (44-50 wt.olo SiOt with a strongly alkaline character (11-15 wt.9o Na2O + K2O) and low contents of Mg (l-4 w.q0 MgO), corresponding to fractionated products of a silica-undersaturated parental magma. The alkaline metasomatism is represented by a slightly peralkaline aegirine fenite, which consists of albite, orthoclase, cancrinite pseudomorphs after nepheline, and Na-pyroxene (DilaHd27Ae5e to Die1Hd27Ae72), Mafic-mineral-free fenite also is found in the complex and consists of orthoclase and albite, with minor amounts of interstitial quartz. The apatite-rich veins are composed of radial aggregates of apatite, spherulitic chalcedony, quartz, siderite, calcite, barite, and britholite, which is the main R.EE-mineral in the veins. The total REE content in the veins varies from 14600 to 296@ ppm, with strong enrichment in ZREE (La,/Lu ratios range from 308 to2258). A silicate-carbonate liquid immiscibility model is postulated and discussed in order to explain coherently the petrogenetic relationship of the nephelinerich foidolites with fenite metasomatism and apatite-rich veins enriched in LREE.
Nepheline
Orthoclase
Metasomatism
Nepheline syenite
Aegirine
Trachyte
Pyroxene
Peralkaline rock
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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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The Aillik Bay alkaline intrusive suite comprises dykes of alkaline lamprophyre (sannaite and olivine sannaite), kimberlite and carbonatite. Structural and mineralogical criteria indicate that the dykes are related to an intrusive centre of nephelinite-carbonatite type situated beneath the Labrador Sea to the northeast of the study area. -- Dykes were emplaced in three structural episodes; two concentric sets are separated in time by a dominant radial set. Sannaites make up the first set and the bulk of the second whereas kimberlites and carbonatites exclusively occupy the third dyke set. Formation of segmented dykes is attributed to flow instabilities enhanced by a volatile-rich fluid moving ahead of the magma. This fluid was presumably exsolved from the magma as a result of pressure reduction during emplacement, and also assisted in the formation of parallel fracture zones adjacent to kimberlites and carbonatites. -- Sannaites are characterised by leucocratic ocelli which are frequently zoned: a central zone dominated by carbonate and analcite gives way to an outer zone of Fe-mica, pyroxene, nepheline, K-feldspar and analcite. The outer zones were formed by segregation of late-stage melt. One sample bears globules which are clearly the result of liquid immiscibility. Immiscibilty and segregation are accompanied by concentration of incompatible elements. Groundmass mineralogy shows chemical evolution similar to nepheline syenites. -- Minerals in kimberlite delineate a more complex history, beginning at depth in a low oxygen fugacity environment. Kimberlites lack high pressure equilibrated diamond 'marker' minerals, and thus diamond potential is low. Carbonatites typically exhibit relict kimberlitic textures. Sannaite and olivine sannaite were derived by flow differentiation from a parental magma, the composition of which is defined. All rock types were derived by partial melting of an incompatible element enriched mantle source. Structural inheritance permitted successive emplacement of rocks representing progressively smaller and deeper derived melt fractions via the intrusive centre.
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Nepheline
Pyroxene
Peralkaline rock
Petrogenesis
Baddeleyite
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Nepheline
Nepheline syenite
Carbonatite
Peralkaline rock
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Alkali feldspar
Aegirine
Trachyte
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Abstract The Lovozero alkaline massif is the largest of the world's layered peralkaline intrusions (∼650 km 2 ). We describe the evolution of clinopyroxene from the liquidus to the late residual stage throughout the whole vertical section (2.5 km thick) of the Lovozero Complex. Microprobe data (∼990 analyses) of the clinopyroxenes define a relatively continuous trend from diopside containing 15–20% hedenbergite and 10–12% aegirine components, to pure aegirine. The main substitutions during the evolution of the Lovozero pyroxenes are (Na,Fe 3+ ,Ti) for (Ca,Mg,Fe 2+ ). The composition of the pyroxene changes systematically upwards through the intrusion with an increase in Na, Fe 3+ and Ti and decrease in Ca and Mg. The compositional evolution of the Lovozero pyroxene reflects primary fractionation processes in the alkaline magma that differentiated in situ from the bottom to the top of the magma chamber as a result of magmatic convection, coupled with the sedimentation of minerals with different settling velocities. The temperature interval of pyroxene crystallization is very wide and probably extends from 970 to 450°C. The redox conditions of pyroxene crystallization in the Lovozero intrusion were relatively low, approximating the QFM buffer.
Pyroxene
Peralkaline rock
Aegirine
Nepheline
Layered intrusion
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Abstract The Pandé massif is a small (4.9×63.4 km) subvolcanic complex of the Cameroon Line striking W – E and intrudes a Panafrican granite basement. It comprises a syenite-granite suite, where coarse- to finegrained syenites are predominant and the granites are the product of residual melt after syenite crystallization, and two volcanic (trachyte-rhyolite and trachyte) sequences. Amphibole and pyroxene are the dominant mafic silicates, the first occurring mainly in rhyolites and coarse- to medium-grained syenites, and the second, principally in all syenites, trachytes and granites. Rare biotite flakes are encountered in the coarse-grained or alkaline syenites and fayalite rimmed with oxides occurs in trachyte from the first volcanic sequence (T1). Apatite and zircon are common accessories, whereas some titanite occurs in the medium-grained syenites. The plutonic rocks are drusy, intrude the first volcanic sequence but pre-date the second (T2). All the mafic minerals are Fe-rich. Detailed studies of amphibole and pyroxene show that their compositions define relatively limited trends, amphibole varying from ferro-richterite to arfvedsonite and pyroxenes along the acmite-hedenbergite join of the Ac-Hd-Di diagram, in both the intrusive suite and volcanic rocks. Where the two minerals coexist, pyroxene crystallized subsequent to amphibole, a situation generally found in late-stage or subsolidus aegirines. The overlap in plutonic and volcanic pyroxene trends suggests their crystallization from magmas of the same composition. However, the presence of quartz and fayalite in T1 and of pure aegirine in T2 and the occurrence of Zr-bearing aegirine (NaZr 0.5 Fe 0.5 2+ Si 2 O 6 ) in the early crystallizing alkaline syenites evolving towards pure aegirine from medium- to fine-grained quartz syenites and granites, are consistent with changes in oxygen fugacities during magmatic differentiation. Two stages are distinguished: f O 2 increasingly decreased from T1 to alkaline syenite emplacement (from 10 −16 to 10 −24 bracketed by WM and QFM buffers) where a disequilibrium, probably caused by water dissociation with volatile loss (H 2 ) during magma degassing, favoured crystallization of Zr-bearing aegirine; a decrease in amphibole proportions towards medium-grained quartz syenites and an increase in f O 2 from the medium-grained quartz syenites to granites and T2 sequence. The Mg-poor nature of all the mafic silicates, subsolidus origin of amphiboles, crystallization of pyroxene subsequent to amphibole and subsolidus trends defined by pyroxenes are compatible with the parental magma having itself been a late-stage derivative magma, e.g. the last product of an alkaline melt from which the voluminous Mayo Darlé granite bodies crystallized.
Trachyte
Peralkaline rock
Amphibole
Pyroxene
Aegirine
Massif
Nepheline
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