Peralkaline Magmas Research Articles

Plume-related mantle source of super-large rare metal deposits from the Lovozero and Khibina massifs on the Kola Peninsula, Eastern part of Baltic Shield: Sr, Nd and Hf isotope systematics

The two world’s largest complexes of highly alkaline nepheline syenites and related rare metal loparite and eudialyte deposits, the Khibina and Lovozero massifs, occur in the central part of the Kola Peninsula. We measured for the first time in situ the trace element concentrations and the Sr, Nd and Hf isotope ratios by LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) in loparite, eudialyte an in some other pegmatitic minerals. The results are in aggreement with the whole rock Sr and Nd isotope which suggests the formation of these superlarge rare metal deposits in a magmatic closed system. The initial Hf, Sr, Nd isotope ratios are similar to the isotopic signatures of OIB indicating depleted mantle as a source. This leads to the suggestion that the origin of these gigantic alkaline intrusions is connected to a deep seated mantle source—possibly to a lower mantle plume. The required combination of a depleted mantle and high rare metal enrichment in the source can be explained by the input of incompatible elements by metasomatising melts/fluids into the zones of alkaline magma generation shortly before the partial melting event (to avoid ingrowth of radiogenic isotopes). The minerals belovite and pyrochlore from the pegmatites are abnormally high in 87Sr /86Sr ratios. This may be explained by closed system isotope evolution as a result of a significant increase in Rb/Sr during the evolution of the peralkaline magma.

The central Kenya peralkaline province: a unique assemblage of magmatic systems

The review focuses on the evolution of five contiguous peralkaline salic complexes in the south-central Kenya Rift Valley, stressing new developments of general significance to peralkaline magmatism. The complexes have evolved dominantly by combinations of fractional crystallization and magma mixing; volatile-melt interactions, remobilization of plutonic rocks and crystal mushes, and carbonate-silicate liquid immiscibility have been additional petrogenetic processes. Geochemical and experimental studies have shown that pantelleritic magmas can be generated by fractional crystallization of trachyte and high-silica rhyolite. Melts of comenditic composition were also formed by fractionation of trachyte but also locally by partial meltingof syenites. Studies of apparent partition coefficients have provided some of the first data on element distribution between phenocrysts and peralkaline silicic melts. Compositional zonation has been ubiquitous in the complexes, probably a result of the very low viscosity of the magmas.

Post-collisional tectonomagmatic evolution in the northern Arabian–Nubian Shield: time constraints from ion-probe U–Pb dating of zircon

Abstract: Ion-probe U–Pb dating of plutonic rocks from the northern Arabian–Nubian Shield in Sinai and southern Israel constrains the timing of late East African batholithic post-collisional calc-alkaline (CA2) magmatism and within-plate alkaline to peralkaline (AL) magmatism to c . 635–590 Ma and c . 608–580 Ma, respectively. The earliest dated CA2 rocks are slightly deformed to undeformed, indicating that penetrative deformation ceased by c . 630 Ma. Within the CA2 suite a change from mafic to felsic magmatism is manifested in most of the region, peaking in a voluminous pulse of granodiorite to granite intrusion at 610–600 Ma. The AL magmatism started contemporaneously with the peak in CA2 felsic activity at c . 608 Ma and lasted until 580 Ma. It includes mostly alkaline and peralkaline granites, probably representing variable degrees of differentiation of similar parental magmas. Thus CA2 and AL granites do not represent different tectonic settings, but coeval derivation from variable sources during crustal extension. The majority of rocks dated in this study show minor to non-existent zircon inheritance and thus indicate very minor interaction with previously formed felsic crust. The rare zircon xenocrysts span a typical East African age range (900–607 Ma) and confirm the absence of older crustal components in the juvenile Arabian–Nubian Shield. Supplementary material: Geochemical and geochronological methods, sample descriptions and data are available at http://www.geolsoc.org.uk/SUP18327 .

U-Pb ion microprobe dating and Sr and Nd isotope geology of the Galiñeiro Igneous Complex: A model for the peraluminous/peralkaline duality of the Cambro-Ordovician magmatism of Iberia

The pre-Variscan basement of Iberia contains two contrasting associations of silicic igneous rocks, one composed of intermediate to felsic peraluminous rocks with ages clustering around 486 Ma, and the other dominated by felsic peralkaline rocks considered to be of similar age. The most voluminous manifestation of peralkaline magmatism is the Galiñeiro Igneous Complex, mainly consisting of A1-type granitoids locally extremely enriched in HFSE and REE, which underwent medium-grade metamorphism during the Variscan. Previous attempts at dating these rocks using zircon concentrates have not succeeded. Using zircon U-Pb ion microprobe we found these granitoids have a crystallization age of 482 ± 2 Ma, and they are therefore coeval with the spatially related peraluminous magmas. Although the Rb-Sr isotope system composition was totally reset at 325 Ma, coincidental with the peak of the Variscan metamorphism, the Nd isotope composition has not been perturbed, and yielded εNd 482 Ma ≈ 1.6 and T DM ≈ 0.9–1.25 Ga, thus indicating mantle-derived magmas with a perceptible crustal component. The chemical features of these rocks: A1-type chemistry, elevated fluorine, exceptionally high HFSE and REE with a large Eu negative anomaly, constant Nb/Ta ≈ 9.4, and (Th/Ta) PM increasing from ca. 0.1 to > 3 as Th and Ta increase, cannot be explained by magmatic differentiation of mantle magmas plus bulk-assimilation of continental crust. They, however, can be reasonably explained as the result of the percolation through lower crustal materials of highly mobile F-HFSE-REE-rich alkaline hydrous fluids released from a mafic alkaline magma in an extensional environment. Since previous work based on zircon inheritance and dissolution kinetics has revealed that the magmas of the coeval peraluminous association were formed by fast melting caused by underplating on mafic magmas in an extensional environment, we conclude that the duality between peralkaline and peraluminous Cambro-Ordovician magmas of Iberia does not reflect different geodynamic settings, as previously believed, but is the result of the same phenomenon of rifting and crustal melting caused by the arrival of small batches of hot mantle magmas at the base of the crust. If these magmas were tholeiitic or moderately alkaline, they rapidly induced crustal melts that simply inherited the geochemical signature of the protolith. If, on the other hand, the underplating magmas were more alkaline, they released hydrothermal alkaline fluids that percolated through the lower crust to form such compositionally anomalous rocks as the peralkaline Galiñeiro gneisses.

Heterogeneous initial Sr isotope compositions of highly evolved volcanic rocks from the Main Ethiopian Rift, Ethiopia

Many individual mineral Sr isotope studies have revealed the complex evolution of highly evolved rocks in upper crustal magmatic systems, casting doubts on the meaning of whole-rock Sr isotopes in such samples. In this paper, whole-rock Sr isotope measurements were replicated (three to 13 times) on six highly evolved peralkaline rhyolites from the Main Ethiopian Rift (MER) to appraise their internal heterogeneity and, thus, the significance of such data. These rocks were all fresh samples of pumice, obsidian and lava. Their maximum Sr contents and ages were 15 μg/g and 1.7 Ma, respectively. Significant small-scale heterogeneities of both Sr isotopes and Rb and Sr contents were observed in most samples, although not necessarily associated with petrological characteristics suggesting possible inheritance processes. Only two, almost crystal-free, obsidian give fairly homogeneous Sr isotope ratios. These results outline the ambiguity of a single whole-rock Sr isotope determination on highly evolved peralkaline rocks, especially when no simultaneous accurate determination of the Rb/Sr is performed. They also suggest that the limitations of Sr whole-rock analyses are not restricted to phenocryst-rich samples as phenocryst-poor obsidian and almost aphyric pumices and lava are also concerned. These data further underscore that unreplicated whole-rock Sr isotope measurements should always be used with great caution in the petrogenetic modeling of highly evolved rocks. However, multiple determinations of whole rock 87Sr/86Sr in the same samples, combined with other geochemical and isotopic data, may provide constraints on the shallow level evolution of these magmas. It is suggested that selective upper crustal contamination and/or interactions with halogen-bearing hydrous fluids, typical of evolved peralkaline magmas, were probably involved in the late magmatic evolution of these MER rhyolites. The more pervasive character of fluid interaction processes would probably better account for the small-scale association of uncontaminated and contaminated signatures in a single sample. Thus, even fresh samples may have their Rb–Sr isotopic system significantly modified by fluid interactions, not as a secondary process but at the late magmatic stage.

Chemical and physical evolution of the ‘lower layered sequence’ from the nepheline syenitic Ilímaussaq intrusion, South Greenland: Implications for the origin of magmatic layering in peralkaline felsic liquids

The Mid-Proterozoic composite Ilímaussaq complex, South Greenland, is a classic locality to study magmatic layering in evolved peralkaline magmas. Most of the rock units show magmatic layering to differing extents, but ‘kakortokites’ – generally medium-grained agpaitic nepheline syenites – show a spectacular recurrence of black, red and white layers, which is due to regular changes in the modal contents of arfvedsonitic amphibole, eudialyte (sensu lato), alkali feldspar and nepheline, respectively. These three-layer units are found in the lower part of the intrusion and recur 29 times before grading into the overlying lujavrites (melanocratic agpaitic nepheline syenites), which are generally fine-grained and fissile with less-developed layering. The compositional trends observed in amphibole and eudialyte throughout the stratigraphic sequence reflect various processes including the chemical evolution of the melt by crystal fractionation, changes in the crystallising mineral assemblage and sub-solidus alteration. Eudialyte is the first mineral to crystallise in the investigated sequence and is therefore appropriate for recording evolution trends within the melt. Amphibole, on the other hand, always crystallises later and is therefore affected by other crystallising minerals. A detailed microprobe study of both minerals through the whole kakortokite stratigraphy displays surprisingly little change in mineral compositions within the kakortokites, but strong fractionation trends in the overlying lujavrites. Although various models have been proposed to explain the recurrence of the 29 rhythmic units, the origin of this prominent magmatic layering in the kakortokites and the lack of mineralogical and strong mineral chemical changes has not been quantitatively explained. We propose, that in the kakortokites, minerals were probably separated from each other as a result of their different densities. The interior cooled, resulting in crystallization but only a very small proportion of crystals (0.1–0.3% for each of the four minerals) could remain suspended in the melt before gravity forced them to settle down in a stagnant layer of reduced convection. A combination of volatile pressure variations caused by eruptive activity and repeated replenishment can explain the oscillating liquidus temperature, the small changes in mineral compositions and such a process would produce enough crystals to form the 29 layers.

Age and evolution of late Mesozoic metamorphic core complexes in southern Siberia and northern Mongolia

Numerous Cretaceous metamorphic core complexes (MCCs) extend from Transbaikalia in Russia to northern Mongolia within the Central Asian Orogenic Belt. We investigated the Buteel and Zagan MCCs in detail. Shear sense indicators in mylonitized rocks show footwall-to-the-NW tectonic transport. Single zircon dating of footwall rocks in the Buteel MCC establishes the emplacement of granitoid orthogneiss precursors at 240–211 Ma, a felsic metavolcanic rock at 265.0 ± 1.2 Ma, a syenite at 265.5 ± 1.2 Ma and a metarhyolite of the pre-granitoid basement at 553.6 ± 2.9 Ma. A peralkaline granite intruding orthogneisses of the Zagan MCC has a new U–Pb zircon age of 151.6 ± 0.7 Ma. 40 Ar/ 39 Ar ages of 133.5 ± 1.8 Ma of hornblende from amphibolite and 122.6 ± 1.8 Ma of biotite from mylonitized gabbro–dolerite of the Buteel MCC are interpreted as cooling ages representing the time of deformation in the footwall. Geological data suggest that the MCCs in Transbaikalia and northern Mongolia formed as a result of extension in a crust that had previously been thickened by abundant calc-alkaline magmatism in an Andean-type setting on the border of the closing Mongol–Okhotsk ocean, by widespread collisional to post-collisional thrusting, and by extensive alkaline–peralkaline magmatism.

Rb-Cs-rich rasvumite and sector-zoned "loparite-(Ce)" from Mont Saint-Hilaire (Quebec, Canada) and their petrologic significance

Rasvumite and “loparite-(Ce)” from the Mont Saint-Hilaire alkaline complex in Quebec were re-examined using a variety of analytical techniques. Rasvumite crystals from a marble xenolith and tawite (“sodalite xenolith”) entrained in nepheline syenite contain significant amounts of Rb and Cs (up to 7.2 and 2.6 wt.%, respectively). Our data indicate that these elements are more compatible with respect to rasvumite than sodalite, tainiolite, or perovskite-type phases. Cubo-octahedral crystals and penetration twins of “loparite-(Ce)” from the tawite comprise {100} growth sectors composed of loparite-(Ce) and {111} sectors composed of lueshite; the proportion of Na 0.5 Ce 0.5 TiO 3 and NaNbO 3 components varies by ≥15 mol.% between the sectors. In addition to the light rare-earth elements and Ti, the {100} sectors are enriched in K, Sr, Ba, Y, Th, U, Fe, Si and Zr with respect to the {111} sectors, which show higher levels of Na, Ca, Nb and Ta. Some elements (Ba, Th and U) exhibit a two-fold or greater difference in D between the sectors. Crystal-chemical analysis of the sector zoning indicates that higher-charged cations partition into surface protosites with fewer bonds satisfied (in agreement with Dowty’s model). Among isovalent A -site cations, the larger partition into the {100} sectors. This observation is at variance with Dowty’s predictions, but can be readily explained in terms of the relative differences in bond strength between large and small cations (estimated from their bond-valence parameters). The distribution of B -site cations is highly charge-dependent (but size-independent) and constrained mostly by heterovalent substitutions in the A site within a given sector. Comparison with the published data shows that the inter-sectorial distribution of cations in the perovskite structure is controlled not only by their charge, radius and involvement in coupled substitutions, but also by the chemistry of crystallization environment ( e.g. , availability of Nb). The implications of these data for the study of element partitioning in perovskites are discussed. The loparite-lueshite intergrowths and Rb-Cs-rich rasvumite in the tawite are interpreted to have crystallized in equilibrium with sodalite, aegirine and tainiolite from halogen-rich peralkaline magma. The tawite and its host nepheline syenite may have formed from cognate immiscible magmas, as proposed earlier by Piilonen, McDonald and Lalonde.

A survey of accessory mineral assemblages in peralkaline and more aluminous A-type granites of the southeast coastal area of China

AbstractAn extensive belt of A-type granite exists along the southeast coast of China. The granites are divided into peralkaline and more aluminous subgroups which differ in mineral assemblages, mineral compositions and textures. In the peralkaline subgroup, primary magmatic Th-rich zircon is typically overgrown by Th-poor zircon containing thorite micro-inclusions. REE minerals in this subgroup are dominated by allanite-(Ce), chevkinite-(Ce), titanite and pyrochlore. Fe-Ti oxides are titanian magnetite and Mn-rich ilmenite. In contrast, in the more aluminous subgroup rocks, zircon is weakly zoned and exhibits very low Th but relatively high U contents. The REE minerals are dominated by Th-rich monazite-(Ce). Titanium-poor magnetite, pyrophanite and rutile are the major Fe-Ti oxides. These occurrences indicate that peralkaline magmas favour the formation of REE silicates, whereas magmas with higher alumina saturation stabilize REE phosphates. Peralkaline granites crystallized at temperatures 50–100°C greater than the more aluminous granites, but under lower oxidation conditions. These differences in formation conditions of the two A-type granite subgroups, deduced by accessory mineralcharacteristics, are inferred to be related to magma derivation at different crustal levels, with peralkaline magma deriving from a deeper crustal level with more mantle input.

A high-pressure experimental study on the evolution of the silicic magmatism of the Main Ethiopian Rift

In this contribution we report the results of an experimental study that investigated equilibrium and fractional crystallization of hydrous, transitional alkaline basalt at low oxygen fugacity, under lower to middle crustal conditions to constrain the generation of subaluminous and peralkaline differentiation products that typically occur in rift systems. The experiments reveal that liquids produced by equilibrium crystallization in the range 0.7–1 GPa cannot cross the subaluminous/peralkaline compositional divide. In contrast, fractional crystallization experiments under isobaric and polybaric conditions approach closer the naturally observed trend from subaluminous to evolved peralkaline products suggesting that polybaric differentiation starting at elevated pressures can indeed lead to the transition from subaluminous to peralkaline derivative liquids. The presence of water in the parental magmas of silicic derivative products is of prime importance for the fractionation equilibria as well as for the mobility of such magmas toward shallow crustal levels. We suggest that peralkaline magmas in rift environments are indicative for differentiation under relatively low oxygen fugacity conditions in an extensional environment characterized by a high degree of crustal fracturing that allows rapid upward migration of mafic parental magmas and formation of shallow magma reservoirs. Crystallization–differentiation of parental, hydrous transitional alkaline basalt in such reservoirs is controlled by low pressure phase equilibria that typically evolve through early saturation of anorthite-rich plagioclase and suppressed amphibole crystallization resulting in ‘low-alumina’, peralkaline derivative liquids.

Mantle Xenoliths and Their Host Magmas in the Eastern Alkaline Province, Northeast Mexico

Alkaline magmas along the periphery of Sierra de San Carlos-Cruillas and Sierra de Tamaulipas (Eastern Alkaline Province, northeast Mexico) contain ultramafic xenoliths. Most of the host rocks are mafic (e.g., basalts, basanites, trachybasalts, phonotephrites), and have geochemical characteristics of nearly primary magmas generated in subcontinental zones (SiO2 = 42.6-48.4%, MgO = 6.3-11.2%, Mg# = 59.2-69.7). MORB-normalized patterns are similar to those displayed by extensionrelated mafic magmas. This hypothesis is supported by an enrichment in light REE ([La/Yb]N = 10.8-27.1; chondrite-normalized ratios) and the behavior of relatively immobile trace elements. Using a partial melting model for REE, the source for the mafic magmas apparently was in the garnet P-T stability field. On the other hand, peralkaline phonolites and tephriphonolites (SiO2 = 52.9-54.1%, MgO = 1.5-1.8%, Mg# = 33.1-39.8) also contain mantle xenoliths. These evolved magmas show MORB-normalized multi-element diagrams characterized by enrichment in highly incompatible elements (e.g., Rb, Sr, Ba) and positive anomalies for HFSE. Ultramafic xenoliths are predominantly protogranular spinel-lherzolites as well as harzburgites and rare dunites, websterites, and wherlites. A few samples display transitional and porphyroclastic textures, indicating that the xenoliths were carried to the surface from stable mantle zones with little or no deformation. The mineralogy (olivine + orthopyroxene + clinopyroxene ± spinel) of xenoliths contained in mafic magmas is typical of unaltered mantle nodules. However, small garnet crystals exhibiting disequilibrium textures occur in one sample. The core and rim compositions in olivine range from Fo90 to Fo94, whereas orthopyroxenes are characterized by En88-93 and clinopyroxenes by En44-51Fs3-10Wo43-50. Chromiferous spinels have Mg/(Mg + Fe+2) = 0.76-0.83 and Cr/(Cr + Al) = 0.10-0.25. In comparison to the nodules contained in mafic magmas, mantle xenoliths included in phonolitic liquids show some differences: (1) smaller size (diameter < 1 cm); (2) clinopyroxene is less common (< 5% volume) and spinel is absent; and (3) partial alteration to micaceous minerals. Equilibrium temperatures for mantle xenoliths contained in mafic magmas range from 850 to 1170°C, as calculated applying different geothermometers, whereas mantle nodules sampled by peralkaline liquids show only lower equilibrium temperatures (<900°C). A first approximation to equilibrium pressure, based on mineralogical constrains, indicates values from 10 to 25 kbar. Summing up, we consider that the mafic magmas were generated within the spinel-garnet domain of the lithospheric mantle, having little or no interaction with their wall rocks. The ultramafic xenoliths were probably sampled during magma ascent above its source. In contrast, the geochemistry of phonolitic rocks and their altered mantle xenoliths reveal that such magmas cannot be explained by direct mantle melting. The petrogenesis of these peralkaline magmas can be described by a two-step model: (1) partial melting of metasomatized mantle which produced an alkaline magma enriched in LILE and HFSE; and (2) subsequent fractional crystallization of this magma at upper mantle pressures, producing phonolites. While ascending, the peralkaline magmas sampled altered nodules in a shallow level of the subcontinental mantle. The rise and eruption of the mantle-bearing mafic and evolved magmas were facilitated by the post-Laramide extensional regime established during Tertiary time in northeast Mexico.

Zircon age and occurrence of the Adaatsag ophiolite and Muron shear zone, central Mongolia: constraints on the evolution of the Mongol–Okhotsk ocean, suture and orogen

The Adaatsag ophiolite in eastern Mongolia is situated in the Mongol–Okhotsk suture zone, which extends from central Mongolia through Transbaikalia to the Sea of Okhotsk and separates the Siberian and Amurian (Mongolian) plates. The ophiolite sequence passes upwards from serpentinite mélange and serpentinized dunite and harzburgite, through layered gabbro (with leucogabbro pegmatite dykes), wehrlite and clinopyroxenite, to isotropic gabbro and leucogabbro, sheeted mafic dykes, and olivine-rich basaltic lavas, overlain by red chert and meta-clastic sediments. A single-zircon mean 207 Pb/ 206 Pb evaporation age of 325.4 ± 1.1 Ma for a leucogabbro pegmatite dyke records the time of igneous crystallization of the plutonic suite, and thus the time of formation of oceanic crust in the Mongol–Okhotsk ocean that gave rise to the ophiolite. A U–Pb secondary ionization mass spectrometry (SIMS) and evaporation zircon age of 172 Ma for a mylonitized granite provides a maximum age for the left-lateral Muron shear zone, which occurs close to the left-lateral Mongol–Okhotsk suture, and indicates that the suture in eastern Mongolia formed at least by the mid-Jurassic. We review the evidence and models for subduction tectonics that gave rise to major calc-alkaline batholiths along active continental margins of the bordering plates and to extensive, post-collisional, alkaline to peralkaline magmatism. Whereas the magmatic history of the orogen is better known, the age of the ocean, and the time of formation and deformation of the suture zone are not. Our new data on the Adaatsag ophiolite and Muron shear zone provide key constraints on the early and late stages of development of the Mongol–Okhotsk ocean and orogen.

Two subgroups of A-type granites in the coastal area of Zhejiang and Fujian Provinces, SE China: age and geochemical constraints on their petrogenesis

ABSTRACTLate Cretaceous (90–100 Ma) A-type granites are widespread in the coastal area of the Zhejiang and Fujian Provinces, SE China. According to mineralogical and geochemical characteristics, the A-type granites in this belt can be further divided into aluminous and peralkaline subgroups. The aluminous subgroup often contains aluminous-rich minerals (e.g. spessartine and Mn-rich muscovite), while the peralkaline subgroup usually contains riebeckite, arfvedsonite and aegirine. Geochemically, the aluminous A-type granites show lower Nb, Zr, Ga, Y and REE abundances, and lower FeO*/MgO and Ga/Al than the peralkaline subgroup. When they occur in the same area, the two subgroups of A-type granites display quite similar initial Nd isotopic compositions, which are indicative of mixing of ancient basement crustal rocks with variable amounts of mantle materials. Integrated geological and geochemical investigations indicate that both the aluminous and the peralkaline magmas are highly evolved and reflect the residual liquids left after high degrees of fractional crystallisation in a deep magma chamber. The present authors suggest that the mineralogical and geochemical differences between the aluminous and peralkaline subgroups are likely to have been generated via different differentiation paths controlled by varying fluorine contents of the parent magmas.

Crystallisation and genesis of peralkaline magmas from Pantelleria Volcano, Italy: an integrated petrological and crystal-chemical study

Pantelleria, the type locality of pantellerite, is a Late-Pleistocene magmatic system with a bimodal association of alkali basalts and peralkaline rocks showing a large compositional gap at SiO 2=50–67 wt.%. Major and trace element data show a strong change in all element patterns with two distinct liquid lines of descent for the mafic and the felsic end-members for many elements (Al 2O 3, FeO tot, Ba, and Cr). The mineral chemistry data also show compositional gaps for many mineral phases, similar to those observed in the whole-rock data. Trace element models show that a trachytic melt can be generated from a basaltic parental magma either by a two step fractional crystallisation process, or by 30% partial melting of gabbroic cumulate. Pantelleritic magmas are derived by fractional crystallisation of mainly K-feldspar from a trachytic end-member. The crystal-chemical parameters of the clinopyroxene crystals show two clearly distinct patterns for the mafic and felsic rocks, which are unlikely to be generated by continuous crystal fractionation process from basalt to pantellerite. Clinopyroxene crystals in the mafic rocks show trends comparable with patterns observed for some Italian potassic suites (e.g., Sabatini volcano), with an increase of Al in the T site and Ca in M2 with differentiation. On the other hand, crystals in the peralkaline felsic rocks follow the patterns of the African Na-rich products (e.g., Nyambeni, Boseti) with Si increasing in T and Na in M2. Chemical parameters of the host rocks plotted vs. structural data of the clinopyroxene crystals divide mafic and felsic rocks into two well distinguished groups of undersaturated and oversaturated magmas, but do not explain the transition between the two groups. The data suggest that simple fractional crystallisation by itself cannot explain the generation of peralkaline magmas in Pantelleria starting from parental basalts. Other processes such as partial melting of gabbroic cumulates are more viable and have to be considered in the genesis of peralkaline magmas.

Relationships between Mafic and Peralkaline Silicic Magmatism in Continental Rift Settings: a Petrological, Geochemical and Isotopic Study of the Gedemsa Volcano, Central Ethiopian Rift

Petrological and geochemical data are reported for basalts and silicic peralkaline rocks from the Quaternary Gedemsa volcano, northern Ethiopian rift, with the aim of discussing the petrogenesis of peralkaline magmas and the significance of the Daly Gap occurring at local and regional scales. Incompatible element vs incompatible element diagrams display smooth positive trends; the isotope ratios of the silicic rocks (Sr/Sr ˆ 0 70406---0 70719; Nd/Nd ˆ 0 51274---0 51279) encompass those of the mafic rocks. These data suggest a genetic link between rhyolites and basalts, but are not definitive in establishing whether silicic rocks are related to basalts through fractional crystallization or partial melting. Geochemical modelling of incompatible vs compatible elements excludes the possibility that peralkaline rhyolites are generated by melting of basaltic rocks, and indicates a derivation by fractional crystallization plus moderate assimilation of wall rocks (AFC) starting from trachytes; the latter have exceedingly low contents of compatible elements, which precludes a derivation by basalt melting. Continuous AFC from basalt to rhyolite, with small rates of crustal assimilation, best explains the geochemical data. This process generated a zoned magma chamber whose silicic upper part acted as a density filter for mafic magmas and was preferentially tapped; mafic magmas, ponding at the bottom, were erupted only during post-caldera stages, intensively mingled with silicic melts. The large number of caldera depressions found in the northern Ethiopian rift and their coincidence with zones of positive gravity anomalies suggest the occurrence of numerous magma chambers where evolutionary processes generated silicic peralkaline melts starting from mafic parental magmas. This suggests that the petrological and volcanological model proposed for Gedemsa may have regional significance, thus furnishing an explanation for the large-volume peralkaline ignimbrites in the Ethiopian rift.

The REE-Ti mineral chevkinite in comenditic magmas from Gran Canaria, Spain: a SYXRF-probe study

The REE-Ti silicate chevkinite has been recognised previously in Miocene ignimbrites from Gran Canaria, and in correlative offshore syn-ignimbrite turbidites. We have estimated the partition coefficients of REE, Y, Zr and Nb for chevkinite and co-existing peralkaline rhyolitic (comendite) glass using synchrotron-XRF-probe analyses (SYXRF) in order to evaluate the role of this mineral in the REE budget of felsic peralkaline magmas. The Zr/Nb ratio of the chevkinite is 1.55–1.7, strongly contrasting with Zr/Nb of 6.5 in the associated glass. Zr shows a three-fold enrichment in chevkinite relative to the residual melt, whereas Nb is enriched by a factor >10. The enrichment of Ce and La in chevkinite is even more significant, namely 19 wt(%) Ce and 12 wt(%) La, compared to 236 ppm Ce and 119 ppm La in the glass. Chevkinite/glass ratios are 988±30 for La, 806±30 for Ce, 626±30 for Pr, 615±40 for Nd, 392±50 for Sm, 225±30 for Eu, 142±25 for Gd, 72±20 for Dy. For trace elements, we derived KdTE of 74±25 for Y, >8 for Hf, >50 for Th, 15±5 for Nb and 3.55±0.4 for Zr. Mineral/glass ratios for co-existing titanite are 28±10 for La, 86±20 for Ce, 98±30 for Pr, 134±35 for Nd, 240±50 for Sm, 50±20 for Eu, 96±25 for Gd, 82±25 for Dy, 99±30 for Y, 45±10 for Nb and 3±0.5 for Zr. Based on these data, the removal of only 0.05 wt% of chevkinite from a magma with initially 300 ppm Ce would deplete the melt by 93 ppm to yield 207 ppm Ce in the residual liquid. Chevkinite thus appears, when present, to be the controlling mineral within the LREE budget of evolved peralkaline magmas.

Chemical evolution and petrogenetic implications of loparite in the layered, agpaitic Lovozero complex, Kola Peninsula, Russia

Lovozero, the largest of the world’s layered peralkaline intrusions, includes gigantic deposits of Nb + REE-loparite ore. Loparite, (Na,Ce,Ca)2(Ti,Nb)2O6, became a cumulus phase after crystallisation of about 35% of the ‘Differentiated Complex’, and its compositional evolution has been investigated through a 2.35 km section of the intrusion. The composition of the cumulus loparite changes systematically upwards through the intrusion with an increase in Na, Sr, Nb and Th and decrease in REE and Ti. This main trend of loparite evolution records differentiation of the peralkaline magma through crystallisation of 1600 m of the intrusion. The formation of the loparite ores was the result of several factors including the chemical evolution of the highly alkaline magma and mechanical accumulation of loparite at the base of a convecting unit. At later stages of evolution, when concentrations of alkalis and volatiles reached very high levels, loparite reacted with the residual melt to form a variety of minerals including barytolamprophyllite, lomonosovite, steenstrupine-(Ce), vuonnemite, nordite, nenadkevichite, REE, Sr-rich apatite, vitusite-(Ce), mosandrite, monazite-(Ce), cerite and Ba, Si-rich belovite. The absence of loparite ore in the “Eudialyte complex” is likely to be a result of the wide crystallisation field of lamprophyllite, which here became a cumulus phase.

Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilimaussaq Complex, South Greenland

The 1·13 Ga Ilímaussaq intrusive complex, South Greenland, is composed of various types of alkali granite and silica-undersaturated alkaline to agpaitic nepheline syenites related to three subsequently intruded magma batches. Mineral chemistry indicates continuous fractionation trends within each rock type, but with distinct differences among them. The last, peralkaline magma batch is the most fractionated in terms of XFemafic mineral, feldspar composition and mineral assemblage. This indicates that an evolving magma chamber at depth discontinuously released more highly fractionated alkaline melts. Fluid inclusions in some sodalites record a pressure drop from 3·5 to 1 kbar indicating that crystallization started during magma ascent and continued in the high-level magma chamber. On the basis of phase equilibria and preliminary fluid inclusion data, crystallization temperature drops from >1000°C (augite syenite liquidus) to <500°C (lujavrite solidus) and silica activity decreases from ∼0·8 to <0·3. An almost pure methane fluid phase at high temperatures and an almost pure aqueous fluid phase in the last crystallization stages of the agpaitic rocks indicate a strong increase in water activity. NaCl activity drops from 0·4 during magmatic sodalite crystallization to <0·01 (3 wt % NaCl equiv) in the late magmatic aqueous fluids. Relative oxygen fugacity [ΔFMQ, where FMQ is fayalite–magnetite–quartz)] depends on silica and water activity via two solid–solid buffer reactions. It decreases during fractionation in the augite syenite from about FMQ – 1 to below FMQ – 4, but increases in the peralkaline stage. The extreme peralkaline fractionation trend appears to be governed by low water activity and low SiO2 activity in the parental melt. Only then is methane a stable fluid phase during most of the crystallization history, which prevents early unmixing of an aqueous NaCl-bearing fluid phase.

In situ trace-element analysis of individual silicate melt inclusions by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS)

This paper reports the successful application of laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) to the in situ analysis of a diverse suite of twenty trace elements including Zr, Hf, Nb, Ta, Y, and REEs, in individual silicate melt inclusions in phenocrysts from Fantale volcano, Ethiopia. The UV laser, a frequency quadrupled Nd: YAG operating at 266 nm, significantly improves the ablation characteristics of minerals that do not absorb strongly at near-IR wavelengths (e.g., quartz and feldspar). Furthermore, it allows for a significant reduction in ablation pit size to ca. 10 μm, thereby permitting numerous applications that require high-resolution sampling. Multiple ablations in individual melt inclusions in the size range 10–50 μm demonstrate both the effectiveness of the technique and the generally homogeneous character of the inclusions. Comparison of the LAM-ICP-MS data for international reference material RGM-1 (a rhyolite), with recommended values, indicates an analytical precision of <10% for most of the trace elements determined in this study. The trace element abundances of the Fantale melt inclusions, determined by LAM-ICP-MS, are typical of those of pantellerites (i.e., peralkaline rhyolites), and are consistent with their origin as tiny volumes of melt trapped in quartz and alkali-feldspar phenocrysts during the final stage of fractional crystallization of the host peralkaline magma.

UPb zircon age of the Strange Lake peralkaline complex: implications for Mesoproterozoic peralkaline magmatism in north-central Labrador

A fine-grained, exotic-poor granite of the Strange Lake peralkaline complex contains zircons that are dated at 1240 ± 2 Ma. This granite is one of the oldest intrusions of this ring complex and one of only a few that contain primary zircon; others contain abundant alkali zircono-silicates. Mesoproterozoic anorogenic rocks in north-central Labrador form three lithotectonic associations: Association I, anorthosite suite; II, mafic magmatism; and III, peralkaline felsic magmatism. A compilation of precise age data for a large number of these anorogenic rocks indicates that there are two main temporal-spatial episodes of anorogenic magmatism comprising Association I and II intrusions: an interior trend (A) from 1.46 to 1.42 Ga and a coastal trend (B) from 1.34 to 1.27 Ga. Temporal-spatial relationships between Association III and Association I-II intrusions suggest a genetic relationship between the Flowers River peralkaline granite (Association III, 1289 Ma) and the spatially associated Char Lake quartz monzonite (Association I, 1291 Ma). No anorthositic or mafic rocks of comparable age occur near the Strange Lake peralkaline complex (Association III) and the Red Wine Intrusive Suite (Association III). However, the close temporal relationship (1224–1250 Ma vs. 1240 Ma) between the spatially widely separated Seal Lake gabbros (Association II) and the Strange Lake peralkaline granite (∼200 km) may indicate a cotectonic relationship between these intrusions. The three episodes of anorogenic magmatic activity that occurred in north-central Labrador correlate with similar Mesoproterozoic magmatism in the Grenville Province. The Strange Lake peralkaline complex, in particular, was emplaced nearly contemporaneously with several other anorogenic felsic rocks throughout the Grenville Province, suggesting the occurrence of a large-scale event at this time.