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The peralkaline Ilímaussaq Intrusion of the Gardar Province in South Greenland consists of several intrusive phases that have been related to different magmatic pulses and igneous cumulate processes. The lowermost exposed part of the intrusion is kakortokite, a eudialyte‐nepheline syenite, with distinctive sub‐horizontal layering which passes upwards into lujavrite, a fine‐grained melanocratic eudialyte‐nepheline syenite. Along the contact between kakortokite and lujavrite lies a transitional layered kakortokite, which has been defined previously based on textural and mineralogical criteria. However, published literature presents conflicting descriptions of this sequence in terms of thickness, layering, contacts, and mode of origin. Our own and previous field and petrographic investigations suggest that parts of the eudialyte‐rich zones in the transitional layered kakortokite comprise metasomatized and partially assimilated naujaite autoliths, a sodalite‐rich eudialyte nepheline syenite, detached probably from the roof of the intrusion. Here, we focus on the nature of the transitional layered kakortokite and conclude, in contrast to the conventional model, that it was formed by metasomatic transformation and assimilation of naujaite autoliths. We propose that the breakdown and replacement of sodalite locally increased chlorine and sodium concentrations, which may have catalyzed the precipitation of eudialyte. In detail, we suggest that the uppermost four eudialyte‐rich horizons were strongly influenced by metasomatic processes caused by mineral‐melt reaction, and were not simply the result of conventional igneous magmatic cumulate formation, as has been inferred for the underlying kakortokite layered units. Recognition of a significant metasomatic process has important bearing on the distribution of minerals with specific elements, like REE in eudialyte, both from a petrological and an economic perspective. [ABSTRACT FROM AUTHOR]

It is well known that oxidation of ferrous to ferric iron by water can generate molecular hydrogen (H 2), with the most widely recognized natural manifestation being serpentinization of olivine and pyroxene in ultramafic rocks. A less known yet extremely important source of natural H 2 are peralkaline igneous intrusions, where spectacular enrichments of H 2 are documented from fluid inclusions and as free gas migrating through fractured rocks and soils. Of these occurrences, the best studied are those at Strange Lake in Canada, Lovozero and Khibiny in Russia, and Ilímaussaq in Greenland. Based on petrographic observations and fluid inclusions analysis, it has been proposed that the hydrothermal alteration of arfvedsonite, an FeII-bearing amphibole, is the source of H 2 in this context, although it is yet to be unequivocally demonstrated. To investigate the generation of H 2 during alteration of peralkaline granites, we performed hydrothermal experiments on pure arfvedsonite grains and arfvedsonite-bearing granite (10 wt% arfvedsonite) from the Strange Lake pluton (Canada). These materials, in the presence of aqueous solutions, were sealed inside gold capsules or placed within titanium autoclaves, which allowed monitoring H 2 generation in function of temperature (280–400 °C), chlorinity (0 and 3 m NaCl), pH, and starting mineral assemblage. Blank experiments were conducted to quantify the background amounts of H 2 generated from Au/Ti oxidation, diffusing through the reaction cells, release from fluid inclusions or otherwise occluded in minerals. Solids were characterized by XRD, SEM, TEM on FIB foils, and STXM-XANES. Outcomes of this study demonstrate the production of H 2 in agpaitic peralkaline rocks by the hydrothermal alteration of arfvedsonite. The rate of H 2 production, normalized to the specific surface area of arfvedsonite, increases with temperature from 1100 to 2200 pmol cm−2 day−1 between 280 and 400 °C, respectively. Chlorinity tends to have a negative impact on the reaction rate, while circumneutral to alkaline conditions clearly promote H 2 generation. Altering whole granite samples instead of arfvedsonite grains only also enhances H 2 production rate. The presence of aluminum, released from microcline and albite dissolution, may increase both the solubility and the dissolution rate of arfvedsonite by promoting precipitation of phyllosilicates. At least two different types of phyllosilicates were observed, chlorite and smectite. Magnetite and secondary zircon were also identified at the surface of reacted arfvedsonite (Zr content = 1200 ppm). The H 2 production rates reported here at 280–400 °C are comparable, and even faster than those documented for serpentinization of olivine and harzburgite. A major feature of arfvedsonite alteration in peralkaline plutons is the formation of aegirine as a replacing mineral. However, aegirine was never observed in the reacted solids from our experiments, under the condition tested (up to 400 °C and 400 bar). This may be an effect of pressure, oxygen fugacity, or reaction progress, parameters that remain to be investigated to better constrain the reaction mechanism of arfvedsonite alteration. Agpaitic peralkaline igneous intrusions thus represent a fertile geological setting for deep microbial subsurface ecosystems, abiotic synthesis of organic molecules, and natural H 2 exploration for an alternative source of energy. [ABSTRACT FROM AUTHOR]

Peralkaline syenite and granite dykes cut the Straumsvola nepheline syenite pluton in Western Dronning Maud Land, Antarctica. The average peralkalinity index (PI = molecular Al/[Na + K]) of the dykes is 1.20 ( n = 29) and manifests itself in the presence of the Zr silicates eudialyte, dalyite and vlasovite, and the Na-Ti silicate, narsarsukite. The dykes appear to have intruded during slow cooling of the nepheline syenite pluton, and the petrogenetic relationship of the dykes and the pluton cannot be related to closed-system processes at low pressure, given the thermal divide that exists between silica-undersaturated and oversaturated magmas. Major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition, and internal mineral segregation prior to final solidification. The distribution of accessory minerals is consistent with late-stage crystallization of isolated melt pockets. The dykes give an Rb-Sr isochron age of 171 ± 4.4 Ma, with variable initial Sr/Sr ratio (0.7075 ± 0.0032), and have an average ε of − 12.0. Quartz phenocrysts have δO values of 8.4-9.2‰, which are generally in O-isotope equilibrium with bulk rock. Differences in the δO values of quartz and aegirine (average Δ = 3.5‰) suggest aegirine formation temperatures around 500 °C, lower than expected for a felsic magma, but consistent with poikilitic aegirine that indicates subsolidus growth. The negative ε (< − 10) and magma δO values averaging 8.6‰ (assuming Δ = 0.6‰) are inconsistent with a magma produced by closed-system fractional crystallization of a mantle-derived magma. By contrast, the nepheline syenite magma had mantle-like δO values and much less negative ε (average − 3.1, n = 3). The country rock has similar δO values to the granite dykes (average 8.0‰, n = 108); this means that models for the petrogenesis of the granites by assimilation are unfeasible, unless an unexposed high-δO contaminant is invoked. Instead, it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali-metasomatised gneiss that surrounds the nepheline syenite, followed by fractional crystallization. [ABSTRACT FROM AUTHOR]

Peralkaline igneous rocks are defined by a molar (Na + K)/Al ratio > 1 and are subdivided into miaskitic and agpaitic varieties depending on their mineralogy. In the more common miaskitic types, rare earth elements (REEs) and high field strength elements (HFSEs) are largely stored in zircon and titanite, while agpaitic varieties contain a wealth of mostly halogen-bearing Na-Ca-HFSE minerals instead. Among those, minerals of the eudialyte, rinkite, and wöhlerite groups are the most common ones. The present review on the geological and mineralogical information available on agpaitic rocks provides a summary of the fluid inclusion record of miaskitic and agpaitic rocks as fluids play a key role in the evolution of peralkaline rocks. Magmas that crystallize peralkaline rocks are generally believed to originate from low-degree partial melting of geochemically enriched mantle lithologies, combined with prolonged differentiation processes at shallow crustal levels. Agpaitic and hyperagpaitic rocks (the latter containing appreciable amounts of water-soluble minerals) represent the most evolved stages of peralkaline systems. They form either parts of plutonic to subvolcanic composite magmatic complexes, which consist of several agpaitic and/or miaskitic intrusive units, or they occur as sills, laccoliths, domes, dykes, or even as lavas. However, as agpaitic rocks are notably rare compared to miaskitic rocks (about 100 vs. several thousand occurrences worldwide), their formation requires special conditions that are not generally met during the evolution of peralkaline rocks. Despite their rarity, agpaitic and hyperagpaitic rocks form important deposits of critical metals such as REE, Zr, Nb, and U and are interesting targets for otherwise rare elements such as F, Be, Sn, Zn, and Ga. The relative timing when magmas reach their agpaitic stage is highly variable. Magmatic–agpaitic assemblages can form only if early-magmatic crystallization conditions were reduced enough (low f O 2 ) to enable subsequent Fe enrichment, an increase in peralkalinity, retention of halogens, and extreme enrichment of HFSEs in the evolving magmas, as only these contribute to the direct crystallization of agpaitic minerals. Late-magmatic interstitial agpaitic assemblages indicate that the required enrichment levels of the above-mentioned constituents were reached only during the final differentiation stages of magmas. Hydrothermal agpaitic assemblages precipitate from highly saline brines released from peralkaline magmas and are capable of transporting HFSEs. All three varieties of agpaitic assemblages occur in plutonic, subvolcanic, and volcanic rocks. Although many modern and detailed studies have dealt with plutonic–subvolcanic agpaitic rocks, most volcanic occurrences are insufficiently studied, mainly because of difficult outcrop or logistic situations. However, especially the volcanic examples raise questions on the details of why and how degassing of halogens at such shallow emplacement levels is sufficiently prevented to precipitate halogen-bearing agpaitic assemblages. Thus, a thorough geochemical and petrological investigation of such localities is desired. The presently valid definitions of agpaitic and miaskitic rocks are not appropriate, and therefore, alternative definitions are suggested. Similarly, the frequently used classification scheme for nepheline syenites must be abandoned as it is inconsistent and excludes very similar mineral assemblages observed in other partly even quartz-bearing rock types. Variable processes may produce sequences of mineral assemblages that belong to different groups of this classification. Therefore, we suggest that rather than dividing agpaitic rocks into specific subgroups, careful textural studies that distinguish early-magmatic, late-magmatic, and hydrothermal phase assemblages are warranted. This is the only way to understand the effect of various physicochemical parameters (such as P , T , f O 2 , a SiO 2 , a H 2 O , peralkalinity, and the activity of other compounds including halogens) during different evolutionary stages of these mineralogically and texturally diverse rocks. [ABSTRACT FROM AUTHOR]

The peralkaline to agpaitic Ilímaussaq Complex, S. Greenland, displays spectacular macrorhythmic (> 5 m) layering via the kakortokite (agpaitic nepheline syenite), which outcrops as the lowest exposed rocks in the complex. This study applies crystal size distribution (CSD) analyses and eudialyte-group mineral chemical compositions to study the marker horizon, Unit 0, and the contact to the underlying Unit − 1. Unit 0 is the best-developed unit in the kakortokites and as such is ideal for gaining insight into processes of crystal formation and growth within the layered kakortokite. The findings are consistent with a model whereby the bulk of the black and red layers developed through in situ crystallisation at the crystal mush–magma interface, whereas the white layer developed through a range of processes operating throughout the magma chamber, including density segregation (gravitational settling and flotation). Primary textures were modified through late-stage textural coarsening via grain overgrowth. An open-system model is proposed, where varying concentrations of halogens, in combination with undercooling, controlled crystal nucleation and growth to form Unit 0. Our observations suggest that the model is applicable more widely to the layering throughout the kakortokite series and potentially other layered peralkaline/agpaitic rocks around the world. [ABSTRACT FROM AUTHOR]

This work describes rare accessory minerals in volcanic and subvolcanic silica-undersaturated peralkaline and agpaitic rocks from the Permo-Triassic Cerro Boggiani complex (Eastern Paraguay) in the Alto Paraguay Alkaline Province. These accessory phases consist of various minerals including Th-U oxides/silicates, Nb-oxide, REE-Sr-Ba bearing carbonates-fluorcarbonates-phosphates-silicates and Zr-Na rich silicates. They form a late-stage magmatic to deuteric/metasomatic assemblage in agpaitic nepheline syenites and phonolite dykes/lava flows made of sodalite, analcime, albite, fluorite, calcite, ilmenite-pyrophanite, titanite and zircon. It is inferred that carbonatitic fluids rich in F, Na and REE percolated into the subvolcanic system and metasomatically interacted with the Cerro Boggiani peralkaline and agpaitic silicate melts at the thermal boundary layers of the magma chamber, during and shortly after their late-stage magmatic crystallization and hydrothermal deuteric alteration. [ABSTRACT FROM AUTHOR]

Highlights: [•] Rare metals are hosted in granitic dykes emplaced in the rims of the complex. [•] Granites are ferroan and have a peralkaline affinity. [•] Mineralization is agpaitic and miaskitic magmatic, and hydrothermal. [•] The hydrothermal mineralization consists of pseudomorphs after primary minerals. [•] These granites represent one of very few known occurrence of eudialyte in granite. [ABSTRACT FROM AUTHOR]

Fluorine and chlorine affect phase stabilities in magmatic rocks. We present phase equilibrium experiments investigating a peralkaline and iron-rich phonolitic composition with variable F and Cl contents. The starting composition represents a dyke rock, which is a possible parental melt to the peralkaline Ilímaussaq plutonic complex (South Greenland). Experiments were performed at 100 MPa, 1,000-650 °C and low oxygen fugacity adjusted with graphite-lined gold capsules in an internally heated argon pressure vessel and rapid quench cold seal pressure vessels. To cover this large T interval, we applied a two-step fractional crystallization strategy where glasses representing residual melt compositions at 800 °C were synthesized as starting material for consecutive experiments at lower T. In these experiments, oxidized starting glasses allocate oxygen by reduction of ferric iron and up to 1.2 wt% dissolved OH form through reaction with hydrogen provided by the pressure medium (HO) in initially dry experiments. For OH determination, hydrated super-liquidus experiments in Au capsules were performed to calibrate the extinction coefficient for the fundamental OH stretching vibration using infrared spectroscopy ( ε = 48 ± 3 L mol cm). Observed mineral phases in our experiments are titanomagnetite, fayalitic olivine, clinopyroxene, aenigmatite (NaFeTiSiO), alkali feldspar and nepheline (±native iron) coexisting with residual melt. Above 1.5 wt% F concentrations, fluorite (CaF) and hiortdahlite (CaZrSiOF) are stable in favor of Ca-rich clinopyroxene. Sodalite (NaAlSiOCl) and eudialyte (NaCaFeZrSiO(OH)Cl) form at Cl concentrations of 0.2-0.5 wt% (depending on T) and ZrO concentrations >0.7 wt% are additionally needed to stabilize eudialyte and hiortdahlite. Therefore, F and Cl may become compatible in such systems and have the potential to influence F/Cl melt ratios in evolving magmas. [ABSTRACT FROM AUTHOR]

Abstract: The north-western sector of the Gharyan volcanic field (northern Libya) consists of trachytic-phonolitic domes emplaced between ~41 and 38Ma, and small-volume mafic alkaline volcanic centres (basanites, tephrites, alkali basalts, hawaiites and rare benmoreites) of Middle Miocene-Pliocene age (~12–2Ma). Two types of trachytes and phonolites have been recognized on the basis of petrography, mineralogy and geochemistry. Type-1 trachytes and phonolites display a smooth spoon-shaped REE pattern without negative Europium anomalies. Type-2 trachytes and phonolites show a remarkable Eu negative anomaly, higher concentration in HFSE (Nbph name="sbnd" />Zrand Ti than Type-1 rocks. The origin of Type-1 trachytes and phonolites is compatible with removal of clinopyroxene, plagioclase, alkali feldspar, amphibole, magnetite and titanite starting from benmoreitic magmas, found in the same outcrops. Type-2 trachytes and phonolites could be the result of extensive fractional crystallization starting from mafic alkaline magma, without removal of titanite. In primitive mantle-normalized diagrams, the mafic rocks (Mg#=62–68, Cr up to 514ppm, Ni up to 425ppm) show peaks at Nb and Ta and troughs at K. These characteristics, coupled with low 87Sr/86Sr(i) (0.7033-0.7038) and positive εNd (from +4.2 to +5.3) features typical of the mafic anorogenic magmas of the northern African plate and of HIMU-OIB-like magma in general. The origin of the mafic rocks is compatible from a derivation from low degree partial melting (3–9%) shallow mantle sources in the spinel/garnet facies, placed just below the rigid plate in the uppermost low-velocity zone. The origin of the igneous activity is considered linked to passive lithospheric thinning related to the development of continental rifts like those of Sicily Channel (e.g., Pantelleria and Linosa) and Sardinia (e.g., Campidano Graben) in the Central-Western Mediterranean Sea. [Copyright &y& Elsevier]

Geochemically, the large family of alkaline plutonic rocks (both Qtz-undersaturated and -oversaturated compositions) can be subdivided into metaluminous [(Na2O + K2O) < Al2O3] and peralkaline [(Na2O + K2O) > Al2O3] types. In this paper, we discuss two important aspects of the mineralogical evolution of such rocks. With respect to their Fe–Mg phases, a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite (± ilmenite). The relative stability of these phases is controlled by oxygen fugacity and Na activity in the crystallizing melts. If Na activity in the melt is high enough, arfvedsonite + aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions. Major mineralogical differences within this rock group exist with respect to their high field strength element (HFSE)-rich minerals: most syenitic rocks, known as miaskites, contain zircon, titanite or ilmenite as HFSE-rich minerals, whereas in agpaites complex Na–K–Ca–(Ti, Zr) silicates incorporate the HFSE. Similarly, only a small group of peralkaline granites are found to lack zircon, titanite or ilmenite but instead contain Na–K–Ca–(Ti, Zr) silicates. Here, we present a detailed phase petrological analysis of the chemical parameters (µNa2O, µCaO, µK2O) that influence the transition from miaskitic to agpaitic rocks. Based on the occurrence of Ti and Zr minerals, several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished: a high-Ca trend, which is exemplified by the alkaline rocks of the Kola Province, Russia, and a Ca-depletion trend, which is displayed by the alkaline rocks of the Gardar Province, South Greenland. Both trends show significant Na-enrichment during magmatic evolution. High-Ca agpaites evolve from nephelinitic parental melts that did not crystallize large amounts of plagioclase. In contrast, agpaites showing Ca-depletion originate by extensive fractionation of plagioclase from basaltic parental melts. In some peralkaline granites evolutionary trends are observed that culminate in agpaite-like HFSE-mineral associations in the most evolved rocks. [ABSTRACT FROM AUTHOR]

Abstract: Apatite from the augite syenite and the roof sequence of layered nepheline syenites (pulaskite, foyaite, sodalite foyaite and naujaite) of the Ilímaussaq alkaline complex has been analysed by electron microprobe for Ca, P, Na, Si, selected rare earth elements (REE), F and Cl. In the augite syenite, chilled facies and middle zone, euhedral apatite crystals are relatively abundant as inclusions in other liquidus phases. The REE content is ca. 1 wt.% REE2O3. The same level is found in interstitial apatite in the coarse-grained innermost facies. REE-enriched intercumulus apatite overgrowths are common and can have up to ca. 18 wt.% REE2O3. Apatite is a common phase in the pulaskite and foyaite, whereas it is rare in the sodalite foyaite and naujaite. Based on compositional variation and zoning, it is possible to distinguish four generations of apatite in the layered roof sequence: 1) primary apatite belonging to the high-temperature mineral assemblage. This apatite occurs in the pulaskite, foyaite and sodalite foyaite as oscillatory-zoned crystals, whereas only remnants are found in the naujaite. The REE enrichment is dominated by the Ca2+ +P5+ =REE3+ +Si4+ substitution and maximum REE2O3 exceeds 20 wt.%. 2) irregularly zoned apatite occurs as rim zones on the primary apatite in the foyaite and as the dominant type of apatite in the sodalite foyaite and naujaite. Most irregularly zoned grains are altered primary apatite. During the dissolution and recrystallisation process the composition gradually changed towards a lower REE content. 3) overgrowths of homogeneous apatite on the irregularly zoned apatite in the naujaite. 4) Na- and REE-rich apatites occur in the foyaite, sodalite foyaite and naujaite either as overgrowths on earlier generations of apatite or as individual crystals. Calculated chondrite normalised patterns show that the later generations of apatite are relatively enriched in LREE and depleted in HREE. The most pronounced evolution is found in Na- and REE-rich apatite. Apatite/melt partition coefficients for La, Ce, Nd, Sm, Dy, Y and Yb are calculated for the augite syenite and nepheline syenites of the roof sequence. The coefficients show the characteristic convex shape with a maximum around Nd and Sm. Using the calculated partition coefficients and the chemical composition of the Na- and REE-rich apatite, the REE content of a late aqpaitic melt in the roof sequence is estimated. The shape of the chondrite normalised pattern is similar to that of a late arfvedsonite lujavrite dyke and steeper than the patterns of the primary magma. [Copyright &y& Elsevier]

Abstract: Composition and zoning of amphibole in agpaitic pegmatites of the 1.16 Ga Ilímaussaq complex, South Greenland record the chemical evolution of the final stages of an already extremely fractionated melt. Our results show that the general differentiation trends found in the earlier rocks of the complex are continued in the pegmatites, albeit with some significant modifications: the dominating exchange mechanism of Na+Si ⇔ Ca+Al in the amphiboles of the magmatic stage changes to K+Si ⇔ Ca+Al and K ⇔ Na in some pegmatitic samples. Na/K ratios in amphiboles, which generally increase in the course of the Ilímaussaq fractionation, partly display a reversal during the crystallization of the most differentiated amphiboles. The alkali trends are probably related to the buffering of Na+and K+activity by the co-crystallization of albite and microcline. This buffering favors Na+in cooling fluids. This mechanism is lost when analcime replaces feldspar as a stable phase in the late stages of crystallization, e.g. due to locally elevated H2O activity. Analcime does not incorporate significant amounts of K and accordingly, amphibole incorporates more K in analcime-bearing assemblages. The Na–K variation in amphiboles in the Ilímaussaq pegmatites allow a detailed view into the late-stage evolutionary trends of a textbook agpaitic complex. The transition from silicate melt to aqueous fluid is recorded by the change of the dominant alkali ion in the pegmatitic amphiboles from Na to K. Only in the very latest stage, virtually K-free mineral assemblages in analcime–aegirine veins support the existence of a Na-dominated aqueous fluid. [Copyright &y& Elsevier]

Abstract: Microrhythmic layering is locally developed in agpaitic arfvedsonite lujavrite from the Ilímaussaq alkaline complex, South Greenland. Three–15-cm-thick laminated dark layers alternate with 1–10-cm-thick, light-coloured granular urtitic layers. Dark layers are uniform (isomodal) but the urtitic layers are enriched in early nepheline and eudialyte in their lower parts and in late analcime and REE phosphate minerals in the upper parts. The layers are separated by sharp contacts; they are draped around rafts from the overlying roof zone and lack structures indicative of current processes or post-cumulus deformation. Compared with the background arfvedsonite lujavrite of the complex, the dark layers are richer in sodalite, microcline and arfvedsonite and poorer in analcime and eudialyte. They have higher K2O, Cl, FeO⁎ and S but lower Na2O, H2O+, Zr and P contents, the opposite of the light-coloured layers. The complementary chemistry of the two types of layers oscillates about the composition of the background arfvedsonite lujavrite. Layers probably formed in a stagnant bottom layer of the lujavrite magma chamber. Each layer started as a liquid layer which exchanged components with the underlying crystallization front. On cooling, it crystallized primocrysts and exchanged components with the overlying magma which became a new, complementary liquid layer and, during further cooling and burial within the sequence of layers, it underwent largely closed-system interstitial crystallization. Exhaustion of Cl and a sharp decrease in a NaCl relative to a H2O terminated the crystallization of a sodalite-rich dark layer and initiated abundant crystallization of nepheline in the overlying liquid layer (urtitic layer). The layered sequence represents a local K2O-, Cl-rich but Na2O-, H2O-poor facies of arfvedsonite lujavrite and may have formed by exchanging components with sodalite-bearing rafts from the roof zone. [Copyright &y& Elsevier]

Abstract: The distribution of boron has been studied in rocks and minerals of the Ilímaussaq complex, South Greenland, using optical emission spectrometry. In the silica-undersaturated rocks of intrusive phases 1 and 3, average B contents increased from 5.6 ppm in augite syenite to a maximum of 8.9 ppm in sodalite-rich agpaitic nepheline syenite (naujaite roof cumulate) and then decreased to 5.4 ppm in the final lujavrites. Boron only behaved as an incompatible element during certain stages of the fractionation history. Starting at the naujaite stage, sodalite crystals (60–45 ppm B) were fractionated by flotation and were also trapped among the heavy cumulus phases of the bottom cumulates. This prevented the significant build-up of B in late derivatives as seen in other nepheline syenites. Nevertheless, in late pegmatites and veins associated with the agpaitic rocks, B was locally concentrated in certain Be minerals and metamict/reworked minerals. In the silica-oversaturated rocks of intrusive phase 2, average B contents increased from 8.6 ppm in quartz syenite to 13 ppm in alkali granite. [Copyright &y& Elsevier]

Abstract: Lujavrites are rare meso- to melanocratic agpaitic nepheline syenites that are characterized by elevated contents of elements such as Li, Be, Zr, REE, Nb, Th and U. They are the most evolved members of the three large composite agpaitic complexes – Lovozero, Kola Peninsula, Russia; Pilansberg, South Africa; and Ilímaussaq, South Greenland – and are inferred to stem from the same deep fractionating magma sources that fed the earlier members of the complexes. The composition of the melts that evolved into lujavrites is, however, not well known. The agpaitic part of the Ilímaussaq complex is divided into a roof series, a floor series of cumulates and an intermediate series of lujavrites sandwiched between the two. In the traditional view, the lujavrites formed from residual melts left between the downward crystallizing roof series and the floor cumulates. New field observations and geochemical data suggest that the floor cumulates and the main mass of lujavrites constituted a separate intrusive phase which was emplaced into the already consolidated roof series rocks largely by piecemeal stoping. Studies of the contact facies of the floor cumulates indicate that the initial magma of the floor cumulate–lujavrite sequence was peralkaline nepheline syenitic with enhanced contents of Zr, Hf, HREE, Y, Nb, Ta, F, Ba and Sr. Subsequent crystallization in a closed system resulted in the formation of the floor cumulates and lujavrites. Chemical analyses of dykes within and outside the complex represent stages in the magmatic evolution of the agpaitic rocks. [Copyright &y& Elsevier]

Lujavrites are meso- to melanocratic agpaitic nepheline syenites and are the most highly evolved rocks of the Ilı´maussaq complex, South Greenland. Spheroidal bodies measuring up to 20 cm in diameter occur locally in arfvedsonite lujavrite. They consist of a core rich in analcime and arfvedsonite and a rim rich in analcime, aegirine and a low temperature potassium feldspar. The host lujavrite is dominated by albite and arfvedsonite. The igneous lamination of the host continues undisturbed through rim and core of spheroids and the arfvedsonite crystals in host, rim and core have identical chemical compositions. There are sharp contacts between host and rim and rim and core. Host, rim and core contain the same accessory minerals and have almost identical chemical compositions, the main differences are high H2O and Na2O but low SiO2 in the core and high Fe2O3 in the rim. The spheroids are proposed to have been formed by separation of immiscible interstitial H2O-rich globules of magma from the host lujavrite magma at a late stage in the crystallization of the lujavritic melt. [Copyright &y& Elsevier]

The Late Cretaceous (~78 Ma) Poços de Caldas massif is the largest alkaline complex in Brazil and the second in the world by extension (>800 km2). It is considered the westernmost outcrop of the Cabo Frio magmatic lineament, in the northern sector of Serra do Mar potassic-ultrapotassic igneous province, central-eastern Brazil. The outcropping rocks are peralkaline phonolites (~80%) and nepheline syenites (~15%) with rarer (<5%) basic-ultrabasic rocks (leucite basanites, basanites, basalts and lamprophyres). The phonolites have different levels of volatile (F and Cl) and other trace elements, which tend to decrease with increasing evolution, due to removal of F-, Mn-, HFSE- and REE-rich accessory phases accompanying cotectic alkali feldspar and nepheline. The geochemical variability of titanite, eudialyte, F-disilicates, aenigmatite, lamprophyllite, clinopyroxene, amphibole and other phases indicate the effects of removal of accessory phases and the presence of independent liquid lines of descent in the various intrusive pulses. The initial Sr−Nd isotopic ratios of the basic-ultrabasic lavas [87Sr/86Sr i = 0.70440–0.70498; ε Ndi = −3.7 to −1.2] are within the range of the other Late Cretaceous-Paleogene rocks of the northern sector of Serra do Mar, indicating a highly metasomatized K-rich lithospheric source. Nepheline syenites and phonolites have Sr Nd isotopes largely overlapping with that of the basic-ultrabasic lavas (87Sr/86Sr i = 0.70503–0.70540 and ε Ndi = −3.9 to −2.5 for nepheline syenites; 87Sr/86Sr i = 0.70511–0.70527 and ε Ndi = −3.4 to −3.2 for phonolites). Prolonged fractional crystallization processes dominated by clinopyroxene removal from melts with compositions resembling those of the basic-ultrabasic compositions can produce residual liquids qualitatively comparable with those represented by the phonolites and nepheline syenites. • Different pulses of magmas generated nepheline syenites and phonolites. • The rare basic rocks may represent the parental liquid of the evolved compositions. • Titanite, eudialyte, and F-disilicates dominate the REE concentration budget. • Different REE patterns suggest the different stages of crystallization. [ABSTRACT FROM AUTHOR]

The Oligocene Peak Range Volcanics (PRV) of central Queensland have intruded through, and erupted onto, Permian sedimentary rocks of the Bowen Basin above what is likely to be a lithospheric-scale suture zone. The southern PRV consists of domes and flows of augite (bearing) trachyte, hornblende trachyte, hornblende rhyolite, arfvedsonite rhyolite and aegirine rhyolite. The aegirine rhyolite bodies represent the most evolved rock types, with peralkaline index (molar Na + K/A) values of >1.3 and extreme enrichment in trace elements, including the rare metals Zr, Hf, Nb, Ta, and REE. Clarrys Dome, one of three aegirine rhyolite domes, features a late stage magmatic agpaitic assemblage of dalyite (K 2 ZrSi 6 O 15), a eudialyte-like mineral and aegirine, which is variably replaced and overprinted by secondary REE carbonates and Zr silicates. Major and trace element geochemistry are used to show that the suite of PRV rock types represent a fractionation sequence from the least evolved augite trachytes to the most evolved aegirine rhyolites. This rock type range can be modelled by extended alkali-feldspar-dominated fractional crystallisation of an alkali basalt parental melt at a shallow crustal levels. Initial εNd values across the range of rock types are between +3 to +4, which is consistent with a common mantle source. Thus, the extreme trace element enrichment and high peralkalinity of the aegirine rhyolites is the product of the extended fractionation of a melt formed by low-degree partial melting of a mildly depleted mantle source. The secondary ore mineral assemblage and REE redistribution observed in the core of Clarrys Dome is interpreted to be due to a combination of hydrothermal alteration by fluids derived from subjacent devolatilising magma bodies, and late-stage weathering. The extreme enrichment of rare metals in the aegirine rhyolite bodies in the southern PRV support their potential to represent low grade, but large tonnage (100 s of Mt) resources of rare metals. Clear evidence of hydrothermal alteration across the region also raised the potential that higher grade rare metal mineralisation produced by hydrothermal activity may exist in the region. • The southern Peak Range Volcanics are related to each other via fractional crystallisation. • The peralkaline rhyolites display a late stage magmatic agapitic assemblage of dalyite and a eudialyte-like mineral with aegirine • The data presented indicates the most evolved bodies may be low grade, bulk tonnage rare metal resources [ABSTRACT FROM AUTHOR]