Chemical and Textural Peculiarities of Zircon from Peralkaline Granites and Quartz-Bearing Syenites
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Zircon from four plutons of peralkaline granites and quartz-bearing syenites, differing in geotectonic positions, petrological and mineralogical compositions, and contents of volatile and trace elements, was studied using SEM, CL, and EPMA with the intention to define typical textural and chemical features of zircon from peralkaline rocks. In strongly peralkaline Na-pyroxene-bearing rocks represented by the Khan Bogd and Khalzan Buregte plutons (Mongolia), the primary zircon is scarce or missing. Most zircon grains are secondary, originating in hydrothermal stage from primary Zr silicates. They often form globular or radial aggregates. Chemical compositions of zircon in these rocks typically show high contents of Y, moderate contents of REE (thus high Y/Yb values) together with low contents of U and Th and low analytical totals. In mildly peralkaline mica-bearing rocks represented by Ivigtut stock (Groenland) and Madeira pluton (Brazil), the exclusive primary Zr mineral is zircon, mostly of orthomagmatic origin. Its analytical totals approach 100 wt%, enrichment in HREE, resulting in low Y/Yb values, is typical. Zircon populations from two types of peralkaline granitoids can be distinguished from each other and from zircon from S-type granites based on combination of the Zr/Hf, Y/Yb, and U/Th values, or on the Y-Hf-P ternary diagram.Keywords:
Peralkaline rock
Pyroxene
Aegirine
Geochronology
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Exploitable or potentially exploitable deposits of critical metals, such as rare-earth (REE) and high-field-strength elements (HFSE), are commonly associated with alkaline or peralkaline igneous rocks. However, the origin, transport and concentration of these metals in peralkaline systems remains poorly understood. This study presents the results of a mineralogical and geochemical investigation of the Na-metasomatism of alkali amphiboles and clinopyroxenes from a barren peralkaline granite pluton in NE China, to assess the remobilization and redistribution of REE and HFSE during magmatic-hydrothermal evolution. Alkali amphiboles and aegirine-augites from the peralkaline granites show evolutionary trends from sodic-calcic to sodic compositions, with increasing REE and HFSE concentrations as a function of increasing Na-index [Na#, defined as molar Na/(Na+Ca) ratios]. The Na-amphiboles (i.e., arfvedsonite) and aegirine-augites can be subsequently altered, or breakdown, to form hydrothermal aegirine during late- or post-magmatic alteration. Representative compositions analyzed by in-situ LA-ICPMS show that the primary aegirine-augites have high and variable REE (2194–3627 ppm) and HFSE (4194–16,862 ppm) contents, suggesting that these critical metals can be scavenged by alkali amphiboles and aegirine-augites. Compared to the primary aegirine-augites, the presentative early replacement aegirine (Aeg-I, Na# = 0.91–0.94) has notably lower REE (1484–1972) and HFSE (4351–5621) contents. In contrast, the late hydrothermal aegirine (Aeg-II, Na# = 0.92–0.96) has significantly lower REE (317–456 ppm) and HFSE (6.44–72.2 ppm) contents. Given that the increasing Na# from aegirine-augites to hydrothermal aegirines likely resulted from Na-metasomatism, a scavenging-release model can explain the remobilization of REE and HFSE in peralkaline granitic systems. The scavenging and release of REE and HFSE by Na-metasomatism provides key insights into the genesis of globally significant REE and HFSE deposits. The high Na-index of the hydrothermal aegirine might be useful as a geochemical indicator in the exploration for these critical-metals.
Aegirine
Peralkaline rock
Amphibole
Metasomatism
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Aegirine
Peralkaline rock
Amphibole
Pyroxene
Carbonatite
Nepheline
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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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Zargat Na' 是戒指建筑群外面在埃及的东南的沙漠收割 Shalatin 城市的 90 km NW。戒指建筑群形成在包围 mafic-ul-tramafic 山上面高站的一条突出的山脉。它被主要打击 NNW-SSE 和 E-W 的关节和差错的二个集合切，并且被堤，斑状的碱的正长岩，和霏细岩斑岩注射。它由碱正长岩，碱石英正长岩，和花岗石的忍受 peralkalinearfvedsonite 和 pegmatitic 堤和基石组成。建筑群被极端丰富在 REE，黑钨矿和稀罕的、高域力量金属(HFSM ) 局部地描绘，例如 Zrand Nb。最高的集中(1.5 wt% Zr， 0.25 wt% Nb， 0.6 wt%Σ R EE ) 发生在钠角闪石伟晶岩附近形成了的 inaegirine 钠长石细晶岩。招待石英在钠角闪石融化包括花岗石和伟晶岩提供不含糊的证据碱的作文和稀罕金属丰富是的 per 主要 magmatic 特征。在石英晶体的玻璃包括也包括 Nb 有不兼容的痕量元素的高集中(750 x 10 ~(-6)), Zr (2500 x10~(-6)) 和 REE (1450 x 10 ~(-6)) 。REE， Nb 和 aegirine 钠长石细晶岩的 Zr 作文主要作为 melt 包括，和 Y/Ho 比率阴谋一样的线性丰富趋势 displayunfractionated， near-chondritic 价值。化学药品并且组织上 aegirine-albiteaplites 的特征从在从 residualperalkaline 的不稳定的损失以后的快速的结晶化是显然结果的花岗石融化在到 melt 包括的作文类似。
Aegirine
Peralkaline rock
Pegmatite
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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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A group of slightly peraluminous Variscan plutons in Northern Portugal were selected from the study of zircon composition. The selected plutons are: the Vila Pouca de Aguiar and the Lavadores-Madalena plutons with I-type affinities and the Vieira do Minho pluton, an l-S transitional type. Zircon occurs as euhedral to subhedral crystals and exhibit finely concentric oscillatory magmatic zoning mainly related to variations of Hf, Y, U and Th concentrations. Most zircon crystals show the dominant “xenotime” substitution. The zircon crystals have Zr/Hf ratio in the range of 21 to 52, with no significant differences between the different granites. These values are in the same range of other peraluminous granites and are in accordance with a crustal signature of zircon. Moreover, the range of Zr/Hf valu es in zircon crystals overlaps with that of crustal sources and consequently to the potential protoliths proposed in the genesis of the Vieira do Minho and the Vila Pouca de Aguiar plutons, namely meta-igneous crustal sources at different levels. Although zircon from the Lavadores-Madalena pluton has a compositional range similar to the other plutons, an origin by hibridisation has been proposed. However, similar zircon chemistry between this pluton and Vila Pouca de Aguiar and Vieira do Minho plutons could also suggest a similar crustal source.
Protolith
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Pyroxene
Peralkaline rock
Aegirine
Kola peninsula
Massif
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A suite of samples with eudialyte and eudialyte decomposition minerals from the kakortokite and associated pegmatites of the Ilímaussaq complex in South Greenland has been investigated by electron microprobe analysis. Extensive decomposition of eudialyte has resulted in the formation of catapleiite as host for a number of rare and hitherto unknown REE minerals besides known minerals such as monazite and kainosite. Mineral A1 is present in very small amounts in nearly all eudialyte decomposition aggregates and comprises two varieties: Ca-rich A1 with composition HCa3REE6(SiO4)6(F◊) and presumed apatite structure, and Ca-poor A1 with composition (Fe,Mn,Ca)1.5REE6Si6FO22 and unknown structure. Mineral A2 with composition (Ca,Fe)1.2REE4Si6O19−y(OH)2y · nH2O is indistinguishable from A1 in EMP-backscattered light and has only been found at a limited number of localities. Mineral A2 also occurs as a primary mineral at one locality. Additional rare and new REE-minerals are mineral A3 with composition Na0.2Ca0.6Fe0.2Mn0.5 Al0.5REE2.8Si6F0.5O)18-y(OH)2y · nH2O; mineral Uk2 with composition REE2.00F1.50O2.25-y(OH)2y · nH2O; mineral Uk3 with composition CaREE4O7-yOH)2y · nH2O; and mineral Y1 with composition Na2Ca4Y2.7REE1.3F18 (OH)4. The Ce:(Y+La+Pr+Nd+Sm+Gd) molar ratio for A1, A2, A3, Uk2, Uk3 and monazite is close to 1:1. Characteristic for A1, A2 and monazite are substantial solid solutions between La and (Pr+Nd+Sm+Gd) with slowly increasing content of Ce as the content of La increases. A similar pattern does not exist for the REE in fresh eudialyte. Kainosite, identified in one decomposition aggregate, has not previously been found in the Ilímaussaq complex.
Peralkaline rock
Aegirine
Nepheline syenite
Nepheline
Pegmatite
Carbonatite
Ilmenite
Sodalite
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The formation of peralkaline melts is generally recognized to be related to enrichment of the mantle in sodium or potassium during a metasomatic event that predated magma generation. The wide range in mineral assemblages, mineral compositions, and fluid compositions in peralkaline melts are, to a large extent, a function of the sodium/potassium content of these mantle-metasomatizing fluids, which also governs redox conditions. We propose that sodic fluids will reduce the mantle assemblage by depleting ferric iron from garnet to form the aegirine component in pyroxene. In contrast, potassic fluids would oxidize the mantle assemblage by extracting Al2O3 from the garnet to make phlogopite. We suggest that during differentiation of peralkaline melts, once Fe–Ti oxides have been depleted from the assemblage, simple crystallization reactions of common solid phases such as aegirine or arfvedsonite control the oxygen fugacity by equilibria such as Hence, the more persodic a melt is, the more aegirine will be crystallized and the more reduced this melt will be. This is because without a mineral donor for ferric iron, the crystal chemical forcing of the coupled crystallization of Na with Fe3+ drives oxidation of the dominant Fe2+ in the melt and thereby reduction of the C–H–S–O-bearing fluid and/or melt phase. This is in convincing agreement with observations from a variety of silica-undersaturated peralkaline complexes such as Ilimaussaq in Greenland, Mt. St. Hilaire in Canada, Lovozero and Khibina in Russia, and Tamazeght in Morocco, and explains the occurrence of magmatic methane in the most sodic of these complexes. In silica-oversaturated peralkaline melts such as comendites and pantellerites, however, this equilibrium appears to be less influential and these melts are accordingly less reduced. In perpotassic melts, in contrast, no K–Fe3+ pyroxene is stable and K can only be incorporated into dominantly Fe2+-bearing phases such as biotite or K-amphibole, if no K-feldspar is stable. Here, redox conditions in the coexisting fluid and/or melt are either unaffected or even driven to more oxidized values by the crystallization reactions, which is in accordance with observations on the ultrapotassic rocks of the Roman Province (Italy) or the kalsilite-bearing Murun complex in Siberia. We conclude that peralkalinity in magmatic rocks is tightly connected to oxygen fugacity, that both the generation and the evolution and crystallization of peralkaline melts are governed by redox equilibria, and that these depend strongly on the Na/K ratio as well as on the (Na + K)/Al ratio of the melt. The specific features of peralkaline rocks, such as the difference between agpaitic and miaskitic rocks, the unusual enrichment of high field strength elements and the occurrence of magmatic hydrocarbons in some of them, can all be explained by the oxygen fugacity-controlling model reactions discussed in this study.
Peralkaline rock
Aegirine
Mineral redox buffer
Pyroxene
Phlogopite
Metasomatism
Fractional crystallization (geology)
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