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Abstract\nKEY POINTSNundorite is a unique and enigmatic rock type, known only from its type locality within the Koonenberry Belt of western NSW. The rock is highly anomalous in geochemical composition being peralkaline, with low silica and high alumina, but enriched in rare metals such as Zr, Nb and rare earth elements. This study presents a comprehensive mineralogical and geochemical study of nundorite to resolve its origins and its potential to inform on the geological processes that lead to critical metal concentration in the crust. Nundorite is found only at a single outcrop but shows a progression of alteration and deformation over ∼250 m from the northwest to southeast. The least deformed varieties consist of coarse aegirine grains within a matrix composed predominantly of natrolite and microcline with minor albite, nepheline and schizolite. With increasing deformation, the proportion of albite and aegirine decreases, the primary zeolite changes from natrolite to analcime, and the pyroxene and schizolite shift towards the aegirine and serandite (Mn-rich) endmember compositions, respectively. These transitions can be related to progressive hydration and recrystallisation of the rock in response to hydrothermal alteration associated with deformation. Nundorite also contains a diverse suite of accessory rare metal phases such as zircon, monazite, cerite-group minerals, fergusonite, fersmite, natroniobite, galgenbergite, epidote–allanite and a range of Zr alkali silicate minerals.Samarium–Nd isotopic compositions of nundorite (εNd ∼+5.5 at 585 Ma) are comparable with mafic volcanic rocks of the surrounding Mount Arrowsmith Volcanics and are consistent with derivation from a mantle source. These data, together with the uniform peralkaline composition, depletion in Ti and P, and enrichment in incompatible elements, are consistent with an origin as highly fractionated phonolite. Phonolites of similar composition are rare but have been reported from continental rift and intraplate alkaline provinces worldwide. Nundorite, therefore is not just a geological oddity, but also provides reaffirmation of the continental rift setting for the Mount Arrowsmith Volcanics and highlights the critical mineral resource potential of the region, as extensive crystal fractionation is a key mechanism to produce ore grade levels of rare metals in alkaline magmatic systems.Nundorite is a geological oddity found only in the Koonenberry Belt of western NSW.Nundorite has a high-alumina, peralkaline composition and is enriched in Zr, Nb and rare earth elements.Nundorite has a complex accessory mineral assemblage reflecting its unusual composition and variable alteration.The protolith of nundorite is phonolite formed after extensive crystal fractionation in a continental rift setting. [ABSTRACT FROM AUTHOR]

Alkaline–peralkaline igneous systems are promising sources of rare earth elements (REEs). Preservation bias has resulted in a gap in the geological record for alkaline–peralkaline magmatic systems, with the hypabyssal plumbing system linking magma chambers to extrusive volcanic rocks poorly represented. Large plutonic varieties of these systems are often proposed to have fed (now eroded) volcanoes, and current peralkaline volcanic systems obscure the plutonic system at depth. The alkaline to peralkaline Ampasibitika Intrusion in Madagascar is a rare example where the magmatic–volcanic interface between a deeper level magma reservoir and its genetically related caldera volcano is exposed. This c. 24 Ma sub-volcanic intrusive system comprises silica-undersaturated to silica-oversaturated units, of peralkaline to metaluminous and peraluminous characters, with varying styles of REE mineralisation, including supergene ion adsorption-style REE occurrences in the overlying weather profiles. There are two main intrusive suites: (1) the concentric Marginal Dyke Swarm (MDS) formed of quartz–microsyenite and peralkaline granite dykes (PGDs), and (2) the Ampasibitika Ring Dyke (ARD) comprising alkali feldspar syenites and subordinate nepheline syenites, trachytes and phonolites. We present new field observations and geochemical data to indicate that the MDS was emplaced as a series of low-viscosity, volatile-rich melt batches, which coalesced in the magma reservoir roof zone and intruded prior to caldera collapse, whereas the ARD was emplaced into the ring fault as a heterogeneous mix of variably evolved syenitic crystal mushes and phonolitic to trachytic-melt batches. As such, we suggest the MDS represents the residual melt fraction of the magma reservoir, whereas the ARD contains portions of the fractionating, silica-neutral to silica-undersaturated syenite, cumulate assemblage. In this revised framework, we assess the major and trace element geochemistry of amphibole- and clinopyroxene-group minerals to gain insight into the magmatic evolution of the Ampasibitika Intrusion and partitioning of REE between early cumulate and residual melt phases. Ultimately, the most REE-enriched units, the PGDs of the MDS, are identified as the product of the most volatile-rich, highly evolved melts from the roof zone of the magma reservoir. However, although REE enriched, the mineralogy does not always enable efficient release of REE for ion adsorption-style mineralisation; instead, lower REE-content protoliths with REE-host phases more amenable to decomposition release a greater proportion of REE. [ABSTRACT FROM AUTHOR]

The liquid line of descent from trachyte to pantellerite is controlled primarily by fractional crystallization of alkali feldspar, with whole-rock compositions following a fractionation path along the 'thermal valley' in the peralkaline haplogranite system Qz-Ab-Or-Ac-Ns and terminating at a minimum on the feldspar-quartz cotectic. Although whole-rock compositions for different pantelleritic suites follow nearly identical paths in a Qz-Ab-Or projection that terminate near the experimental minimum (Qz40.5Or34.5Ab25 at 100 MPa, projected from Ac-Ns), matrix glass from samples with near-minimum compositions record extreme differentiation and form a 'cotectic delta' beyond the terminus of the 'thermal valley'. Although each glass trend shows a continuing increase in Zr to >3000 μg/g, the most evolved compositions in each suite differ in peralkalinity (mol [Na + K] / Al) and in the proportions of FeOT, Qz, Ab, Or, and other components, which are related to subtle variations in the mafic phases controlled mainly by differences in oxygen fugacity (f O2) and pressure (P). To determine the controls over mafic mineral crystallization in pantelleritic magmas and the various paths these suites take beyond the apparent (whole-rock) minimum, amphibole-phyric suites from the ~159 ka Cala dell'Altura and Cala Gadir volcanic centres and the ~8–10 ka Cuddia Mida volcanic centre on Pantelleria have been analyzed and compared with each other and with the well-characterized and amphibole-free, compositionally zoned Green Tuff, the ~46 ka caldera-forming ignimbrite of the Cinque Denti caldera. Differences between the extended fractionation trends may be ultimately attributed to variations in oxygen fugacity, depth of emplacement, and water saturation. Shallower (lower pressure) magma reservoirs such as the one for the Green Tuff are water-saturated and undergo degassing, which leads to an increase in relative oxygen fugacity. Deeper (higher pressure) magma reservoirs remain water-undersaturated and retain water in the melt, which both maintains lower relative oxygen fugacities and enables the crystallization of amphibole. Amphibole formation appears to require melt water contents >4 wt%, low oxygen fugacity (<ΔNNO-1.5), and low temperatures (<700°C), although fluorine may stabilize it at higher temperatures in some rocks. [ABSTRACT FROM AUTHOR]

The numerous volcanic centres in the Main Ethiopian Rift (MER) present significant but poorly understood hazards to local populations. The MER is also an important site to gain insights into tectonic processes as it captures the transition from continental rifting (to the south) to incipient seafloor spreading (to the north). Peralkaline magmas account for around 90% of the volcanic products found in the MER. Determining the conditions under which these magmas evolve is critical to understanding rift-related volcanism and its associated hazards. Corbetti Caldera has an extensive record of large-scale, predominantly aphyric, peralkaline rhyolite eruptions. However, little is known about the mafic magmas from which these highly differentiated melts have evolved. Here we present data from the only basaltic deposit found within the caldera, coupled with whole rock, glass and mineral analysis of the peralkaline products, to investigate magma storage conditions at Corbetti. We demonstrate that magma mixing played a role in the evolution of the basaltic magmas and use RhyoliteMELTS modelling to show Corbetti's peralkaline magmas likely evolved at pressures between 100 and 250 MPa, from a magma with an initial water content of 0.5–1 wt%, at or below the QFM buffer. Mineral hygrometry on the sparse crystal populations corroborates the RhyoliteMELTS modelling, suggesting that the basaltic magma had 0.1–1.2 ± 0.32 wt% H 2 O, and the peralkaline magmas an average of ∼5.5 ± 1.25 wt% H 2 O. These results also match melt inclusion data for Corbetti and other peralkaline systems. We also provide new 40Ar/39Ar ages for two eruptions, a pre-caldera rhyolitic lava flow (206.7 ± 0.9 ka) and a post-caldera peralkaline ignimbrite (160 ± 0.8 ka). These results add to our understanding of the history of Corbetti and the storage conditions of peralkaline magmas within a continental rift setting and highlight the hydrous nature of Corbetti's magmas and the role that H 2 O plays during explosive eruptions. • We present new data from the only known basaltic units within Corbetti Caldera. • We determine shallow magma storage conditions between 100 and 250 MPa at temperatures ∼1100 °C for the basaltic magma and ∼ 793 °C for the peralkaline magma. • Corbetti's magmas are hydrous with ∼5.5 wt% H 2 O. • We date a pre-caldera lava flow to ∼207 ka and a post-caldera ignimbrite to 160 ka [ABSTRACT FROM AUTHOR]

Highlights: Middle Devonian zircon U-Pb crystallization age is found in the Tannuola terrane. The peralkaline A-type granites and coeval mafic rocks record the intraplate extensional setting. The granitic rocks have positive (+5.6 to +6.2) εNd(t) values and Ediacaran–Early Cambrian model ages. The geodynamic model is mantle plume activity beneath northern Central Asian Orogenic Belt areas during the Givetian period. The Tannuola terrane, located in the northern Central Asian Orogenic Belt, comprises magmatic rocks, attributed to island-arc and collisional settings during the Early Cambrian to the Late Ordovician. However, zircon U-Pb age, geochemical, and Sr-Nd isotopic constraints demonstrate that there was a short episode of peralkaline A-type granite magmatism in the northeast border area of the Tannuola terrane. The obtained zircon U-Pb age of 387.7 ± 3.3 Ma indicates emplacement of the peralkaline A-type granitic rocks in the Middle Devonian (Eifelian–Givetian period boundary). Petrologically, these rocks are mainly composed of riebeckite granites and aplites, which are approximately synchronous with augite-rich dolerites. The granitic rocks are ferroan and calc-alkalic to alkali-calcic in composition. They are characterized by a high content of SiO2, total alkali, Zr, and total REE. Significant depletion of Ba, Sr, P, Ti, and Eu indicates fractionation of plagioclase and/or K-feldspar. The values of εNd(t) in riebeckite granites range from +5.61 to +6.55, and the calculated two-stage model age ranges between 610 and 520 Ma. Coeval dolerites on the chondrite-normalized REE pattern, (Th/Yb)pm–(Nb/Yb)pm, and Th/Yb–Nb/Yb diagrams show compositional affinity between E-MORB and OIB. They are rich in incompatible elements with high HFSE/LREE ratios (Nb/La > 1), indicating that the primary magma originated from the lithospheric mantle metasomatized by asthenosphere-derived melt. Based on these geochemical characteristics, it can be reasonably inferred that the peralkaline A-type granitic rocks, and the coeval mafic rocks, are anorogenic and were emplaced in an extensional tectonic environment despite slightly higher Y/Nb values, which might be caused by a crustal contamination effect. The geochemistry of mafic rocks suggests that the magma originated from the enriched mantle sources. The results of a zircon-saturation thermometer show high initial magma temperatures between 923 and 1184 °C, with an average of 1030 °C, indicating this rock association might be related to a mantle plume beneath the northern Central Asian Orogenic Belt. [ABSTRACT FROM AUTHOR]

Alkaline-silicate complexes host some of the world's largest resources of rare-earth elements and high-field-strength elements (REE & HFSE) and represent the most fractionated magmatic systems on our planet. Geochemical evidence indicates that they are mantle melts, but while various studies highlight a role for lithospheric mantle, we do not know the precise origin of their contained REE and HFSE, and whether enrichment of the mantle source for these magmas can be attributed to specific geodynamic processes or events. We present new Nd-Hf isotope measurements (▪/▪ &▪/▪) made by LA-MC-ICP-MS, as well as a compilation of existing isotopic data for a suite of alkaline igneous rocks from the Gardar Province, a Mesoproterozoic continental rift in southern Greenland. Neodymium and hafnium isotopes are unaffected by crystal fractionation and can directly fingerprint the source of REE and HFSE. The dataset covers both phases of Gardar magmatism (1325–1261 and 1184–1140 Ma) and incorporates mafic dyke swarms and km-scale intrusive complexes, including Ilimmaasaq (Ilímaussaq) and Motzfeldt, which host some of the world's largest REE and HFSE deposits. The majority of Gardar complexes have a narrow range of positive median initial ε Nd (0 to +3.3) and ε Hf values (+0.2 to +6.0). Only two granite intrusions and the Eriksfjord basaltic lavas have crustally contaminated Nd-Hf isotope compositions, with the vast majority of Gardar igneous rocks preserving the isotope signature of their mantle source. Considering the diversity of rock types in the Gardar Province, initial ε Nd - ε Hf compositions are remarkably homogeneous, indicating a derivation of the Gardar's REE and HFSE from a laterally-extensive mantle melt source. Several Gardar systems have low initial ε Hf for a given ε Nd (▪ ε Hf to -9.7), a distinctive signature as few geological processes decouple the Nd and Hf isotope systems. The decoupled Nd-Hf isotope signatures are consistent with contributions from isotopically-matured phlogopite-bearing metasomatic veins (commonly known as PIC: phlogopite-ilmenite-clinopyroxene) in the lithospheric mantle. The metasomatising fluids that formed these source rocks were introduced via Palaeoproterozoic subduction, but the Gardar isotopic signatures indicate that REE and HFSE enrichment of these metasomes was not derived from subducted sediment; instead it is likely that metals were scavenged from the mantle wedge overlying the ancient subduction zone. The Gardar Nd-Hf isotope evolution trends overlap with a global compilation of kimberlites through time and allow us to tie the origin of the PIC metasomes to the regional geodynamic history of South Greenland. We identify PIC metasomes as a key metal source for the Gardar and by extension perhaps other REE-mineralised igneous provinces globally. • Nd Hf isotopes reported for alkaline rocks from REE rich Gardar continental rift. • Negative ▪ ε Hf discounts enrichment of mantle source by subducted sediment. • REE & HFSE derive from isotopically matured metasomatic veins in lithospheric mantle. • Origin of enriched source tied to regional geodynamic history. • Provides conceptual basis for distribution of REE HFSE provinces globally. [ABSTRACT FROM AUTHOR]

The recent (<190 ka) volcanic history of Pantelleria is characterized by the eruption of nine peralkaline ignimbrites, ranging in composition from comenditic trachyte to comendite to pantellerite. The ~46 ka Green Tuff (GT) was the last of these ignimbrites, which was followed by many effusive and explosive low-volume eruptions of pantellerite from vents within the caldera moat and along the caldera rim. Although recent studies have shed additional light on the age, petrochemistry, and volcanology of the older ignimbrites, there is very little knowledge of magmatism that occurred between these older ignimbrites, primarily due to the very scarce exposures. In this paper, we present new field descriptions and geochemical data for three local peralkaline centers never studied before, two pre-GT and one post-GT, which share a similar setting with respect to the caldera scarps but differ in terms of their age, composition, and eruptive style. These centers include: (i) the older (~125 ka) Giache center (comenditic trachyte), (ii) the ~67 ka Attalora center (comendite, pantellerite), and (iii) the younger (~14 ka) Patite center (pantellerite). [ABSTRACT FROM AUTHOR]

The Tannuola terrane, located in the northern Central Asian Orogenic Belt, comprises magmatic rocks, attributed to island-arc and collisional settings during the Early Cambrian to the Late Ordovician. However, zircon U-Pb age, geochemical, and Sr-Nd isotopic constraints demonstrate that there was a short episode of peralkaline A-type granite magmatism in the northeast border area of the Tannuola terrane. The obtained zircon U-Pb age of 387.7 ± 3.3 Ma indicates emplacement of the peralkaline A-type granitic rocks in the Middle Devonian (Eifelian–Givetian period boundary). Petrologically, these rocks are mainly composed of riebeckite granites and aplites, which are approximately synchronous with augite-rich dolerites. The granitic rocks are ferroan and calc-alkalic to alkali-calcic in composition. They are characterized by a high content of SiO2, total alkali, Zr, and total REE. Significant depletion of Ba, Sr, P, Ti, and Eu indicates fractionation of plagioclase and/or K-feldspar. The values of εNd(t) in riebeckite granites range from +5.61 to +6.55, and the calculated two-stage model age ranges between 610 and 520 Ma. Coeval dolerites on the chondrite-normalized REE pattern, (Th/Yb)pm–(Nb/Yb)pm, and Th/Yb–Nb/Yb diagrams show compositional affinity between E-MORB and OIB. They are rich in incompatible elements with high HFSE/LREE ratios (Nb/La > 1), indicating that the primary magma originated from the lithospheric mantle metasomatized by asthenosphere-derived melt. Based on these geochemical characteristics, it can be reasonably inferred that the peralkaline A-type granitic rocks, and the coeval mafic rocks, are anorogenic and were emplaced in an extensional tectonic environment despite slightly higher Y/Nb values, which might be caused by a crustal contamination effect. The geochemistry of mafic rocks suggests that the magma originated from the enriched mantle sources. The results of a zircon-saturation thermometer show high initial magma temperatures between 923 and 1184 °C, with an average of 1030 °C, indicating this rock association might be related to a mantle plume beneath the northern Central Asian Orogenic Belt.

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Abstract Understanding magma storage and differentiation in the East African Rift underpins our understanding of volcanism in continental rift settings. Here, we present the geochemistry of melt inclusions erupted in Main Ethiopian Rift transitional basalts, trachytes, and peralkaline rhyolites, produced by fractional crystallization. Basalts stored on‐ and off‐axis are saturated in an exsolved volatile phase at up to 18 km in the upper crust. Much of the CO2 outgassed from the magmas is likely lost through diffuse degassing. Observed CO2 fluxes require the intrusion of up to 0.14 km3 of basalt beneath the rift each year. On‐axis peralkaline rhyolites are stored shallowly, at ~4–8 km depth. In the Daly Gap, magmas saturate in sulfide and an exsolved volatile phase, which promotes magma rise to shallower levels in the crust. Here, magmas undergo further protracted fractional crystallization and degassing, leading to the formation of a substantial exsolved volatile phase, which may accumulate in a gas‐rich cap. The exsolved volatile phase is rich in sulfur and halogens: their projected loadings into the atmosphere during explosive peralkaline eruptions in the MER are predicted to be substantially higher than their metaluminous counterparts in other settings. The high fraction of exsolved volatiles in the stored magmas enhances their compressibility and must be considered when interpreting ground displacements thought to be caused by magma intrusion at depth; otherwise, intruding volumes will be underestimated. Pockets of exsolved volatiles may be present at the roof zones of magma reservoirs, which may be resolvable using geophysical techniques.

Understanding magma storage and differentiation in the East African Rift underpins our understanding of volcanism in continental rift settings. Here, we present the geochemistry of melt inclusions erupted in Main Ethiopian Rift transitional basalts, trachytes, and peralkaline rhyolites, produced by fractional crystallization. Basalts stored on‐ and off‐axis are saturated in an exsolved volatile phase at up to 18 km in the upper crust. Much of the CO2 outgassed from the magmas is likely lost through diffuse degassing. Observed CO2 fluxes require the intrusion of up to 0.14 km3 of basalt beneath the rift each year. On‐axis peralkaline rhyolites are stored shallowly, at ~4–8 km depth. In the Daly Gap, magmas saturate in sulfide and an exsolved volatile phase, which promotes magma rise to shallower levels in the crust. Here, magmas undergo further protracted fractional crystallization and degassing, leading to the formation of a substantial exsolved volatile phase, which may accumulate in a gas‐rich cap. The exsolved volatile phase is rich in sulfur and halogens: their projected loadings into the atmosphere during explosive peralkaline eruptions in the MER are predicted to be substantially higher than their metaluminous counterparts in other settings. The high fraction of exsolved volatiles in the stored magmas enhances their compressibility and must be considered when interpreting ground displacements thought to be caused by magma intrusion at depth; otherwise, intruding volumes will be underestimated. Pockets of exsolved volatiles may be present at the roof zones of magma reservoirs, which may be resolvable using geophysical techniques. Key Points: Alkali basalts are stored both on‐ and off‐axis at depths >15 km in the crust; on‐axis peralkaline rhyolites are stored at between ~4–8 km depthSaturation of mafic magmas in a sulfide and exsolved volatile phase in the Daly Gap promotes buoyancy and rise of magmas to shallower reservoirs, where protracted fractional crystallization and degassing occursCaldera‐forming, explosive eruptions in the MER may give rise to large emissions of SO2 and halogens, which may have significant environmental impacts [ABSTRACT FROM AUTHOR]

The impact of fluorine on phase relations and clinopyroxene–melt element partitioning has been explored experimentally to better understand the effect of this halogen on the residual enrichment of the REE and HFSE during crystallisation of alkaline and peralkaline magmas. Clinopyroxene was grown from three H 2 O -saturated synthetic glasses of tephriphonolite-to-phonolite composition in a rapid quench internally heated pressure vessel at 650–800 ∘ C and 200 MPa, with Δ log f O 2 fixed to ca. FMQ + 1. The fluorine content in the charges was varied and produced quenched melts from fluorine-free to 1.6 wt.% F. The experiments yield an assemblage of melt, fluid, sodic clinopyroxene, biotite, magnetite, ± K-feldspar, ± titanite, ± fluorite, and ± hiortdahlite (a Na-Ca-Ti-F sorosilicate). Addition of fluorine markedly increases the mode of biotite without significantly affecting the mode of clinopyroxene. Relative to fluorine-free compositions, experiments with 1.6 wt.% fluorine in the melt show a strong decrease in clinopyroxene–melt partition coefficients for the trivalent REE La–Dy and Y with a lesser decrease for the D HREE and D HFSE 4 + . The diminished uptake of these metals by clinopyroxene may reflect changes to their speciation in the melt phase, consistent with the formation of REE-F complexes and with modifications to the medium-range structural environment around HFSE 4 + ions in the melt. An increase in the fluorine content of the melt will thus make the REE and HFSE 4 + progressively less compatible and, therefore, available for residual enrichment. [ABSTRACT FROM AUTHOR]

Silicic volcanoes are increasingly understood to be underlain by crystal-rich and vertically extensive magma reservoirs within which disequilibrium is widespread. Observations from ignimbrite deposits demonstrate that silicic magma reservoirs are often compartmentalised and compositionally stratified. However, it is currently unclear whether the small (i.e., < 0.1 km3 dense rock equivalent) eruptions that dominate activity at many volcanoes, and peralkaline volcanoes in particular, are fed from similarly complex magma reservoirs as their larger counterparts. Here I report petrographic and geochemical observations from the products of a small peralkaline eruption on Pantelleria, Italy, with the aims of unravelling peralkaline magma assembly processes and evaluating the complexity of reservoirs feeding small silicic eruptions. Matrix glass and whole-rock compositions from the Khaggiar lava flow and Cuddia Randazzo pumice cone reveal that erupted magmas were assembled from at least three distinct magma types stored within a compartmentalised magma reservoir: trachytes, less-evolved pantellerites and evolved pantellerites. Chemical variability in the Khaggiar lava flow was created by at least three distinct processes: the accumulation of evolved macrocrysts into evolved pantellerites; the injection of trachytic magmas into less evolved pantellerites; and the accumulation of relatively primitive macrocrysts into trachytic magmas. Macrocryst textures indicate that both trachytic and pantelleritic domains of the magma reservoir experienced numerous recharge events prior to the one that ultimately triggered eruption. Overall, magmas forming the Khaggiar lava flow and Cuddia Randazzo pumice cone appear to have been assembled in analogous ways to those erupted in much larger events. My observations are in good agreement with those from other peralkaline volcanoes and confirm that magma mingling, crystal cannibalism and macrocryst entrainment are as ubiquitous in peralkaline systems as they are in their calc-alkaline counterparts. [ABSTRACT FROM AUTHOR]

Clinopyroxene is a key fractionating phase in alkaline magmatic systems, but its impact on metal enrichment processes, and the formation of REE + HFSE mineralisation in particular, is not well understood. To constrain the control of clinopyroxene on REE + HFSE behaviour in sodic (per)alkaline magmas, a series of internally heated pressure vessel experiments was performed to determine clinopyroxene–melt element partitioning systematics. Synthetic tephriphonolite to phonolite compositions were run H2O-saturated at 200 MPa, 650–825°C with oxygen fugacity buffered to log f O2 ≈ ΔFMQ + 1 or log f O2 ≈ ΔFMQ +5. Clinopyroxene–glass pairs from basanitic to phonolitic fall deposits from Tenerife, Canary Islands, were also measured to complement our experimentally-derived data set. The REE partition coefficients are 0·3–53, typically 2–6, with minima for high-aegirine clinopyroxene. Diopside-rich clinopyroxene (Aeg5–25) prefer the MREE and have high REE partition coefficients (D Eu up to 53, D Sm up to 47). As clinopyroxene becomes more Na- and less Ca-rich (Aeg25–50), REE incorporation becomes less favourable, and both the VIM1 and VIIIM2 sites expand (to 0·79 Å and 1·12 Å), increasing D LREE /D MREE. Above Aeg50 both M sites shrink slightly and HREE (VI r i ≤ 0·9 Å ≈ Y) partition strongly onto the VIM1 site, consistent with a reduced charge penalty for REE3+ ↔ Fe3+ substitution. Our data, complemented with an extensive literature database, constrain an empirical model that predicts trace element partition coefficients between clinopyroxene and silicate melt using only mineral major element compositions, temperature and pressure as input. The model is calibrated for use over a wide compositional range and can be used to interrogate clinopyroxene from a variety of natural systems to determine the trace element concentrations in their source melts, or to forward model the trace element evolution of tholeiitic mafic to evolved peralkaline magmatic systems. [ABSTRACT FROM AUTHOR]

We report a combined study of whole-rock major- and trace-element geochemistry, Sm Nd isotope composition, zircon U/Pb dating, and Lu Hf systematics of peralkaline and aluminous A-type granites from the Ririwai Ring Complex in north-central Nigeria. The Ririwai peralkaline and aluminous A-type granites are strongly ferroan, alkalic to alkali-calcic, and enriched in Hf, Zr, Ga, Rb, Y and REEs. They were emplaced between 176 ± 2.3 Ma and 169.6 ± 0.75 Ma. The peralkaline granites yield relatively higher ɛNd(t) (−2.3 to −1.2) and zircon ɛHf(t) values (−5.8 to −1.7) than the aluminous granites (ɛNd(t) = −3.6 to −3.3; zircon ɛHf(t) = −7.8 to −2.4). In addition, inherited zircons in the aluminous granites yield Pan-African (~590 Ma) ages and low ɛHf(t) values (−14.0). Taken together, these data suggest that the granites formed from extensive fractional crystallization of a transitional basaltic melt derived from an enriched OIB mantle source. The depletions of MgO, CaO, Ti 2 O, Sr and Ba in the granites indicate that Mg-rich olivine, calcic pyroxene, calcic amphibole, feldspars and Fe Ti oxides were the major fractionated phases during magma evolution. In addition, several types of evidence, e.g., moderately negative ɛNd(t) and ɛHf(t) values, and the presence of inherited zircons, imply that the parental melt was modified by assimilation of Pan-African upper crust into which the granites were emplaced. A transtensional regime generated prior to late Jurassic breakup of Gondwana, which led to reactivation of shear zones and opening of associated transcurrent faults, paved the way for emplacement of the A-type suite. The Sn–Nb–Zn mineralization of the Ririwai A-type suite was probably linked to complex magmatic evolutionary processes involving extensive fractional crystallization coupled with crustal assimilation and late-stage hydrothermal fluid activity. • The Ririwai granites show geochemical characteristics typical of A-type granite. • The A-type granite suites were emplaced between (176 ± 2.3 to 169.6 ± 0.75 Ma). • The granites formed from differentiation of magmas derived from enriched mantle. • Inherited zircon obtained from the granite suggests crustal contamination. • The Sn-Nb-Zn mineralization was linked to assimilation-fractional crystallization. [ABSTRACT FROM AUTHOR]

For many magmatic systems, crystal compositions preserve a complex and protracted history, which may be largely decoupled from their carrier melts. The crystal cargo may hold clues to the physical distribution of melt and crystals in a magma reservoir and how magmas are assembled prior to eruptions. Here we present a geochemical study of a suite of samples from three peralkaline volcanoes in the Main Ethiopian Rift. While whole‐rock data show strong fractional crystallization signatures, the trace element systematics of feldspars, and their relationship to their host glasses, reveals complexity. Alkali feldspars, particularly those erupted during caldera‐forming episodes, have variable Ba concentrations, extending to high values that are not in equilibrium with the carrier liquids. Some of the feldspars are antecrysts, which we suggest are scavenged from a crystal‐rich mush. The antecrysts crystallized from a Ba‐enriched (more primitive) melt, before later entrainment into a Ba‐depleted residual liquid. Crystal‐melt segregation can occur on fast timescales in these magma reservoirs, owing to the low‐viscosity nature of peralkaline liquids. The separation of enough residual melt to feed a crystal‐poor postcaldera rhyolitic eruption may take as little as months to tens of years (much shorter than typical repose periods of 300–400 years). Our observations are consistent with these magmatic systems spending significant portions of their life cycle dominated by crystalline mushes containing ephemeral, small (< 1 km3) segregations of melt. This interpretation helps to reconcile observations of high crustal electrical resistivity beneath Aluto, despite seismicity and ground deformation consistent with a magma body. Key Points: Alkali feldspars have variable trace element (Ba) concentrations, some far from equilibrium with their carrier liquids (melts)Some feldspars are antecrysts (in equilibrium with a liquid on the line of descent, but not the host melt), picked up from a crystal mushIgnimbrites erupted during caldera‐forming events are dominated by antecrysts, perhaps scavenged from deep parts of the magma reservoir [ABSTRACT FROM AUTHOR]

The oceanic-island magmatic systems of the Atlantic Ocean exhibit significant diversity in their respective sizes, ages, and the compositional ranges of their eruptive products. Nevertheless, almost all of the Atlantic islands and island groups have produced peralkaline felsic magmas, implying that similar petrogenetic regimes may be operating throughout the Atlantic Ocean, and arguably elsewhere. The origins of peralkaline magmas are frequently linked to low-degree partial melting of enriched mantle, followed by protracted differentiation in the shallow crust. However, additional petrogenetic processes such as magma mixing, crustal melting, and contamination have been identified at numerous peralkaline centers. The onset of peralkalinity leads to magma viscosities lower than those typical for metaluminous felsic magmas, which has profound implications for processes such as crystal settling. This study represents a compilation of published and original data which demonstrates trends that suggest that the peralkaline magmas of the Atlantic Ocean islands are generated primarily via extended (up to ∼ 95%), open system fractional crystallization of mantle-derived mafic magmas. Crustal assimilation is likely to become more significant as the system matures and fusible material accumulates in the crust. Magma mixing may occur between various compositional end-members and may be recognized via hybridized intermediate magmas. The peralkaline magmas are hydrous, and frequently zoned in composition, temperature, and/or water content. They are typically stored in shallow crustal magma reservoirs (∼ 2–5 km), maintained by mafic replenishment. Low melt viscosities (1 × 101.77 to 1 × 104.77 Pa s) facilitate two-phase flow, promoting the formation of alkali-feldspar crystal mush. This mush may then contribute melt to an overlying melt lens via filter pressing or partial melting. We utilize a three-stage model to account for the establishment, development, and termination of peralkaline magmatism in the ocean island magmatic systems of the Atlantic. We suggest that the overall control on peralkaline magmatism in the Atlantic is magma flux rate, which controls the stability of upper crustal magma reservoirs. The abundance of peralkaline magmas in the Atlantic suggests that their development must be a common, but not inevitable, stage in the evolution of ocean islands.

Graphical abstract Highlights • Guinean-Nubian lineament. • High concentrations of REE. • High concentrations of Th and U. • Bastnäsite-(Ce), parisite-(Ce), monazite–(Ce), apatite with LREE&Na. • Xenotime, pyrochlore, columbite-(Fe), Nb-rutile and thorite. Abstract The El Garra El Hamra complex is a small high-level plug (with associated volcanics) that was emplaced along an ENE-WSW trending dextral strike-slip fault corresponding to the Guinean-Nubian lineament. Owing to continued movement along this fault, the intrusion split into two parts that were displaced horizontally by approximately 580 m. The intrusion consists mainly of syenites (fresh and altered) and granites (altered) that are locally overlain by alkaline trachyte and rhyolite and are hosted by Nubian sandstones. The fresh syenites are composed of perthitic K-feldspars, albite, alkali pyroxenes (aegirine-augite and aegirine) and secondary alkali amphiboles (arfvedsonite and riebeckite). Accessory minerals include opaques, apatite, zircon, xenotime-(Y) and bastnäsite-(Ce). Calcite and hematite are alteration products. Geochemically, the syenites are peralkaline; their CIPW norms classify them as ranging between syenite and quartz syenite in compositions. They were emplaced in a within-plate tectonic setting along the Guinean-Nubian lineament (Kalabsha), close to the extension of the Chephren-Kom Ombo trend (a branch of the Trans-African Shear Zone). They were evolved from a magma compositionally similar to Ocean island basalt (OIB). Numerous dextral and sinistral strike-slip faults cut the intrusion and may have facilitated the circulation of fluids that altered them hydrothermally. These faults developed during or shortly after emplacement of the intrusion or perhaps during subsequent reactivation of the Guinean-Nubian and Trans-African shear zones and were accompanied by dykes that range in compositions from intermediate to acidic composition; most of which strike ENE-WSW. Hydrothermal alteration of the syenites locally led to extreme enrichment in rare metals (e.g. LREE, HREE, Zr, Hf, Th, U, Nb and Be) as the minerals zircon, gittinsite, bastnäsite-(Ce), parisite-(Ce), monazite–(Ce), REE-rich fluorapatite, chevkinite-(Ce), xenotime-(Y), pyrochlore, columbite-(Fe), Nb-rutile and thorite. Fluorite and barite are important gangue minerals. [ABSTRACT FROM AUTHOR]

Magma plays a vital role in the break-up of continental lithosphere. However, significant uncertainty remains about how magma-crust interactions and melt evolution vary during the development of a rift system. Ethiopia captures the transition from continental rifting to incipient sea-floor spreading and has witnessed the eruption of large volumes of silicic volcanic rocks across the region over ∼45 Ma. The petrogenesis of these silicic rocks sheds light on the role of magmatism in rift development, by providing information on crustal interactions, melt fluxes and magmatic differentiation. We report new trace element and Sr–Nd–O isotopic data for volcanic rocks, glasses and minerals along and across active segments of the Main Ethiopian (MER) and Afar Rifts. Most δ 18 O data for mineral and glass separates from these active rift zones fall within the bounds of modelled fractional crystallization trajectories from basaltic parent magmas (i.e., 5.5–6.5‰) with scant evidence for assimilation of Pan-African Precambrian crustal material ( δ 18 O of 7–18‰). Radiogenic isotopes ( ε Nd = 0.92 – 6.52 ; 87 Sr/ 86 Sr = 0.7037–0.7072) and incompatible trace element ratios (Rb/Nb <1.5) are consistent with δ 18 O data and emphasize limited interaction with Pan-African crust. However, there are important regional variations in melt evolution revealed by incompatible elements (e.g., Th and Zr) and peralkalinity (molar Na 2 O + K 2 O / Al 2 O 3 ). The most chemically-evolved peralkaline compositions are associated with the MER volcanoes (Aluto, Gedemsa and Kone) and an off-axis volcano of the Afar Rift (Badi). On-axis silicic volcanoes of the Afar Rift (e.g., Dabbahu) generate less-evolved melts. While at Erta Ale, the most mature rift setting, peralkaline magmas are rare. We find that melt evolution is enhanced in less mature continental rifts (where parental magmas are of transitional rather than tholeiitic composition) and regions of low magma flux (due to reduced mantle melt productivity or where crustal structure inhibits magma ascent). This has important implications for understanding the geotectonic settings that promote extreme melt evolution and, potentially, genesis of economically-valuable mineral deposits in ancient rift-settings. The limited isotopic evidence for assimilation of Pan-African crustal material in Ethiopia suggests that the pre-rift crust beneath the magmatic segments has been substantially modified by rift-related magmatism over the past ∼45 Ma; consistent with geophysical observations. We argue that considerable volumes of crystal cumulate are stored beneath silicic volcanic systems (>100 km 3 ), and estimate that crystal cumulates fill at least 16–30% of the volume generated by crustal extension under the axial volcanoes of the MER and Manda Hararo Rift Segment (MHRS) of Afar. At Erta Ale only ∼1% of the volume generated due to rift extension is filled by cumulates, supporting previous seismic evidence for a greater role of plate stretching in mature rifts at the onset of sea-floor spreading. We infer that ∼45 Ma of magmatism has left little fusible Pan-African material to be assimilated beneath the magmatic segments and the active segments are predominantly composed of magmatic cumulates with δ 18 O indistinguishable from mantle-derived melts. We predict that the δ 18 O of silicic magmas should converge to mantle values as the rift continues to evolve. Although current data are limited, a comparison with ∼30 Ma ignimbrites (with δ 18 O up to 8.9‰) supports this inference, evidencing greater crustal assimilation during initial stages of rifting and at times of heightened magmatic flux. [ABSTRACT FROM AUTHOR]

The Boset-Bericha Volcanic Complex (BBVC) is one of the largest stratovolcanoes of the northern Main Ethiopian Rift (MER). However, very little is known about its eruptive history, despite the fact that approximately 4 million people live within 100 km of the complex. Here, we combine field observations, morphometric analysis using high-resolution LiDAR data, geochemistry and 40 Ar/ 39 Ar geochronology to report the first detailed account of the geological evolution of the BBVC, with a focus on extensive young lava flows covering the two edifices, Gudda and Bericha. These lavas exhibit a bimodal composition ranging dominantly from basaltic rift floor lavas and scoria cones, to pantelleritic trachytes and rhyolite flows at Gudda, and comenditic rhyolites at Bericha. Further, several intermediate compositions are associated with fissure vents along the Boset-Kone segment that also appear to link the silicic centres. We divide the BBVC broadly into four main eruptive stages, comprising: (1) early rift floor emplacement, (2) formation of Gudda Volcano within two main cycles, separated by caldera formation, (3) formation of the Bericha Volcano, and (4) sporadic fissure eruptions. Our new 40 Ar/ 39 Ar geochronology, targeting a representative array of these flows, provides evidence for episodic activity at the BBVC from ~ 120 ka to the present-day. We find that low-volume mafic episodes are more frequent (~ 10 ka cyclicity) than felsic episodes (~ 100 ka cyclicity), but the latter are more voluminous. Over the last ~ 30 ka, mafic to intermediate fissure activity might have reinvigorated felsic activity (over the last ~ 16 ka), manifested as peralkaline lava flows and pyroclastic deposits at Gudda and Bericha. Felsic episodes have on average a higher eruption rate (2–5/1000 years) and productivity at Gudda compared to Bericha (1–2/1000 years). The young age of lavas and current fumarolic activity along the fault system, suggest that the BBVC is still potentially active. Coincident episodic activity within the BBVC and at several rift segments in the MER is observed, and facilitates continental rifting. [ABSTRACT FROM AUTHOR]

The recent (<100ka) volcanic stratigraphy of Terceira, Azores, includes at least seven peralkaline trachytic ignimbrite formations, attesting to a history of explosive eruptions. In this study, the petrogenesis and pre-eruptive storage conditions of the ignimbrite-forming magmas are investigated via whole-rock major and trace element geochemistry, melt inclusion and groundmass glass major element and volatile compositions, mineral chemistry, thermobarometric models, and petrogenetic modelling. Our primary aims are to develop a model for the magmatic plumbing system from which the ignimbrite-forming trachytes of Terceira were produced by evaluating various petrogenetic processes and constraining pre-eruptive magma storage conditions. We also place the ignimbrite-forming magmas into the context of the Terceira suite and discuss the potential implications of pre-eruptive magma conditions for eruptive behaviour. Results indicate that ignimbrite-forming, comenditic trachytes are generated predominantly by extended fractional crystallization of basaltic parental magmas at redox conditions around 1 log unit below the fayalite-magnetite-quartz buffer. This is achieved via a polybaric fractionation pathway, in which mantle-derived basalts stall and fractionate to hawaiitic compositions at lower crustal depths (~15km), before ascending to a shallow crustal magma storage zone (~2-4km) and fractionating towards comenditic trachytic compositions. The most evolved pantelleritic magmas of Terceira (not represented by the ignimbrites) are plausibly generated by continued fractionation from the comenditic trachytes. Syenite autoliths represent portions of peralkaline trachytic melt that crystallized in situ at the margins of a silicic reservoir. Trachytic enclaves hosted within syenitic autoliths provide direct evidence for a two-stage mingling process, in which ascending hawaiites are mixed with trachytic magmas in the shallow crustal magma storage zone. The resulting hybridized trachytes then ascend further and mix with the more evolved peralkaline trachytes in the uppermost eruptible cap of the system, passing first through a syenitic crystal mush. The reduced viscosities of the peralkaline silicic magmas of this study in comparison with their metaluminous counterparts facilitate rapid crystal-melt segregation via crystal settling, generating compositionally zoned magma bodies and, in some instances, relatively crystal-poor erupted magmas. Reduced viscosity may also inhibit highly explosive activity (e.g. formation of a sustained eruption column), and limit the majority of explosive eruptions to low pyroclastic fountaining or 'boil-over' eruption styles. The formation of intermediate composition magmas within the system is considered to be limited to episodic mixing between mafic and silicic magmas. [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 silicic peralkaline volcanoes of the East African Rift are some of the least studied volcanoes on Earth. Here we bring together new constraints from fieldwork, remote sensing, geochronology and geochemistry to present the first detailed account of the eruptive history of Aluto, a restless silicic volcano located in a densely populated section of the Main Ethiopian Rift. Prior to the growth of the Aluto volcanic complex (before 500 ka) the region was characterized by a significant period of fault development and mafic fissure eruptions. The earliest volcanism at Aluto built up a trachytic complex over 8 km in diameter. Aluto then underwent large-volume ignimbrite eruptions at 316 ± 19 ka and 306 ± 12 ka developing a ~ 42 km 2 collapse structure. After a hiatus of ~ 250 ka, a phase of post-caldera volcanism initiated at 55 ± 19 ka and the most recent eruption of Aluto has a radiocarbon age of 0.40 ± 0.05 cal. ka BP. During this post-caldera phase highly-evolved peralkaline rhyolite lavas, ignimbrites and pumice fall deposits have erupted from vents across the complex. Geochemical modelling is consistent with rhyolite genesis from protracted fractionation (> 80%) of basalt that is compositionally similar to rift-related basalts found east of the complex. Based on the style and volume of recent eruptions we suggest that silicic eruptions occur at an average rate of 1 per 1000 years, and that future eruptions of Aluto will involve explosive emplacement of localised pumice cones and effusive obsidian coulees of volumes in the range 1–100 × 10 6 m 3 . [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]

The Triassic Saima alkaline complex on the Liaodong Peninsula, northeastern China, consists mainly of potassic phonolite, nepheline syenite, and sodic lujavrite. The lujavrite shows significant Zr-REE mineralization, which is present in the form of early magmatic, Zr-REE-enriched clinopyroxene (30%–40%), titanite (5%), and loparite-(Ce), and late magmatic to hydrothermal wadeite, widespread eudialyte group minerals (5%–10%), and catapleiite. Ultimately, the fractionation of the alkaline magma leads to the crystallization of mosandrite and hezuolinite. Textural relations and compositional variation among the characteristic Zr-REE-bearing minerals record that both Zr and REEs were strongly incompatible in the sodic melt, but that Zr mineralization preceded REE mineralization. The main Zr-REE mineralization in the Saima lujavrite resulted from the high peralkalinity, Na/K ratio and HFSE content, low oxygen fugacity, and the intensive activity of water and volatiles of its evolving magma. The discontinuous and abrupt changes in melt composition and mineral assemblage from the potassic nepheline syenite of the complex to the sodic lujavrite suggest that their magma was derived from different episodes of magmatic activity with different physico-chemical characteristics, rather than from the continuous evolution of a single magmatic event. [ABSTRACT FROM AUTHOR]

West of the southern, Archean, part of the Reguibat Rise of the West African Craton the Oulad Dlim Massif consists of metamorphic nappes stacked during the Mauritanides (Variscan) orogeny. In the Derraman region, about 12 km west of the nappes, we have found strongly deformed hypersolvus aegirine-riebeckite A1-type granites with SHRIMP zircon U–Pb ages of ca. 525 ± 3 Ma, ε(Nd) 525Ma (− 5.2 to − 6.8.) and Nd model ages T CR ≈ 1.85 Ga. These granites define two km-sized bodies and a few smaller satellites. One body is emplaced within a 3.12 Ga leucocratic gneiss. The other body and its satellites are emplaced within an Archean low-grade metasedimentary sequence with detrital zircons that have ages that peak at 2.84 Ga, 2.91 Ga, and 3.15 Ga. These Archean gneisses and metapelite rocks define a tectonic unit, hereafter called the Derraman-Bulautad-Leglat (DBL) unit, which was formed from the Reguibat basement at the very margin of the WAC. The ~ 525 Ma Derraman granites are the oldest post-Archean rocks in this unit and were generated in an intraplate rifting environment from melting of crustal fenites during the ubiquitous Cambrian rifting event that affected this part of northern Gondwana. At the present level of knowledge, however, we cannot decide whether the “old” Nd isotope signature of Derraman granites resulted from melting of an old (Paleoproterozoic) fenite source or reflects the signature of the mantle-derived metasomatising fluids. The just-discovered Derraman granites are strikingly similar to other rift-related Cambrian–Ordovician hypersolvus aegirine–riebeckite granites widespread in North Gondwana. Understanding the potential connections between them would help to understand the Cambrian–Ordovician breakdown of northern Gondwana. [ABSTRACT FROM AUTHOR]

The rare potassium zirconium silicate dalyite has been identified for the first time on Terceira, Azores, within syenitic ejecta of the Caldeira-Castelinho Ignimbrite Formation. New quantitative analyses of this dalyite were combined with the small number of published analyses from various locations worldwide to evaluate the mineral's compositional variability. Additionally, solid-state modelling has been applied to assess the site allocations of substituting elements. The new analyses yield the average formula (K1.84Na0.15)Σ=1.99(Zr0.94Ti0.012Hf0.011Fe0.004)Σ=0.967Si6.03O15. Model results predict the placement of substituting Hf and Ti in the octahedral site, and highlight the overall complexity in the incorporation of Fe, Mg and Ba. The combined dataset reveals that dalyite found within peralkaline granites and syenites is generally defined by higher Na↔K substitution and lower Ti↔Zr substitution relative to dalyite from highly potassic rocks. The Terceira dalyite exhibits a bimodal variation in the degree of Na↔K substitutionwhich is attributed to a K-enrichment trend induced by late-stage pore wall crystallization and albitization, coupled with the control of pore size upon the degree of supersaturation required to initiate nucleation of dalyite in pores of varying size. [ABSTRACT FROM AUTHOR]

Peraluminous (P.I. < 1) and peralkaline (P.I. > 1) rhyolites from SE Lake Chad consisted of quartz, alkali feldspar and oxides–hydroxides phenocrysts, display negative Eu anomaly. F-arfvedsonite, augite–hedenbergite, aegirine and aegirine-augite are present in peralkaline rhyolites. The peralkaline and peraluminous rhyolitic magmas of the SE Lake Chad derive likely from the same source, according to their coexistence on the same volcano (Hadjer Bigli); their similar Zr/Nb, Zr/Hf ratios and their linear trends established in Zr vs Nb, Zr vs Ta and Zr vs Y diagrams, which linked to removal of successive batches of magma from a mantle source or to different degrees of partial melting. These magmas generated from the partial melting model of the underplated mantle, which would have induced the intrusion of crustal materials, triggering the hydrothermal reactions. Although, the high Zr amounts are not related to bulk assimilation of partially melted crustal materials. [ABSTRACT FROM AUTHOR]

Bulk-rock and mineralogical characterization of massive samples (lava flows, lava domes, scoria, spatter), chosen to be representative of different activity periods and of the whole known compositional range of Ischia island (Roman Magmatic Province, Campanian district) is reported in this paper. The rocks vary in composition from shoshonites to peralkaline trachyphonolites. Crystallization started with Al-rich chromite inclusions and their host magnesian olivine in shoshonites and latites, and ended with låvenite, rinkite, kochite, hiortdahlite, hainite, Mn-aenigmatite, britholite and Ti–Zr-rich aegirine in the groundmass of the peralkaline trachyphonolites. Removal of feldspar-bearing assemblages (with calcic-to-sodic plagioclase in shoshonites and latites, sodic sanidine and anorthoclase in trachytes and trachyphonolites) is established throughout the compositional range, and is accompanied by interaction between variably evolved magmas (with their differing phenocryst assemblage), mostly in mafic and intermediate compositions. This led to enrichment in Mn, alkalis, Zr, Nb, REE, Rb, Th, U, Cl and F, and depletion in Mg, Fe, V, Ca, Ba, Sr and Eu in the most evolved magmas. The Ischian rocks have a tendency to sodic affinity and by peculiar mineral compositions and compositional trends, which do not indicate extremely oxidizing conditions. The Ischian rocks thus differ from the Phlegrean Fields analogues and the ultrapotassic, more silica undersaturated rocks of Somma–Vesuvius. Overall, variations in the chemical compositions of the rocks and their trends imply unrelated feeding systems and thus are inconsistent with the hypothesis of a common magma reservoir beneath the main volcanic areas of the Campanian Plain. [ABSTRACT FROM AUTHOR]

The St. Lawrence Granite (SLG), located in the Avalon Zone of the Appalachian Orogeny, is a peralkaline to metaluminous granite, best known for its association with extensive, and economically significant, fluorite deposits. Genetic models for the fluorite mineralization have been proposed, but major gaps and misconceptions still remain, such as the lack of fluorite in the country rocks, the source of Ca to form fluorite and the locally abundant calcite in the fluorite veins and the granite. Petrography, geochemistry and electron microprobe analysis revealed new details on the late-magmatic processes in the granite that played a significant role in fluorite mineralization. The SLG intruded at shallow levels and comprises several phases, each having different textures, mineralogy, and geochemistry. Most of the fluorite deposits are associated with the East lobe (E lobe), which is the only peralkaline and one of the most intensely altered phases of the granite. Alteration and fluorite mineralization are the results of devolatilization of the granite. The volatiles included F, Cl, CO 2 and water. Early exsolution of a F-rich vapour, followed by a F-rich acidic fluid led to the fluorite mineralization. In the later stages, a F-poor and CO 2 -rich fluid was left and was unable to leave the granite, migrated along the upper contact of the granite and accumulated in the cupolas resulting in late carbonatization and extensive autometasomatism. Exsolution of a F-rich vapour, coupled with the shallow depth of intrusion of the SLG, allowed the F to leave the granite leading to fluorite mineralization in the country rocks, as demonstrated by the AGS fluorite deposit, which is partially hosted in sedimentary rocks overlying the granite; this encourages further exploration in the surrounding rocks. The source of the Ca is from the alteration of primary Ca-bearing feldspar and amphiboles in the granite, implying the lack of spatial correlation between fluorite mineralization and Ca-containing country rocks. This is significant for pursuing further exploration for fluorite in the St. Lawrence area, and elsewhere. • Peralkaline phase of St. Lawrence Granite associated with most of the fluorite • Volatile exsolution resulted in alteration and fluorite mineralization. • F-rich vapour, followed by F-rich fluid responsible for fluorite mineralization • Source of Ca is from alteration of primary Ca-bearing minerals in granite. [ABSTRACT FROM AUTHOR]

The East African Rift (EAR) hosts the highest density of peralkaline volcanoes of any region globally, making it an ideal location to study the subaerial and magmatic processes of peralkaline volcanism. Corbetti Caldera is one such peralkaline centre found within the southern part of the Main Ethiopian Rift (MER), a segment of the EAR. Corbetti is actively deforming and has previously undergone large-scale Plinian eruptions. However, our broader understanding of Corbetti's evolution is limited. Here we present a detailed study of the pre-, syn-and post-caldera eruptive deposits and compile them into a composite stratigraphy of the volcanic sequence. We find evidence for multiple previously undocumented large-scale eruptions, including the deposition of two pre-caldera lithic lag breccias, and we identify three additional post-caldera obsidian lavas. We constrain the age of a young Tuff Cone, Biftu, which sits outside the caldera walls, to <7375 ± 54 cal BP through 14C dating of shells within an associated PDC deposit. The Wendo Koshe Cone, the most recent cone within the complex, is interpreted as the remains of two cones that opened in close proximity to each other and formed the vents for two pyroclastic eruptions: Bedded Pumice and Wendo Koshe Younger Pumice (WKYP). There is abundant evidence for the occurrence of pyroclastic density currents (PDC) within the caldera, usually associated with pumice cone-forming eruptions. These flow deposits rarely extend beyond the caldera walls. From our compiled stratigraphy, we estimate a recurrence rate of one eruption per 300–400 years over the last 2.3 ky. These findings are especially significant as Corbetti is earmarked for potential geothermal exploration, and an understanding of eruptive frequency and style provides vital context to assess the potential hazards associated with this future infrastructure. • We present a detailed composite stratigraphy of eruptive deposits from Corbetti Caldera, Ethiopia • Previously undocumented deposits are identified, indicating Corbetti commonly generates large-scale, dilute PDCs • The Wendo Koshe Cone comprises of two overlapping vents from which the Bedded Pumice and WKYP eruptions were sourced • The age of the Biftu Tuff Cone has been constrained to <7375 ± 54 cal BP • We estimate Corbetti has a recurrence rate of one eruption per 300–400 years over the last 2.3 ka [ABSTRACT FROM AUTHOR]

The Katherina ring complex (KRC) in the central part of south Sinai, Egypt, is a typical ring complex of late Neoproterozoic age (605–580 Ma). It was developed during the final tectono-magmatic stage of the north Arabian–Nubian Shield (ANS) during evolution of the Pan-African crust. The KRC includes Katherina volcanics, subvolcanic bodies, ring dykes and Katherina granitic pluton. The Katherina volcanics represent the earliest stage of the KRC, which was subsequently followed by emplacement of the subvolcanic bodies and ring dykes. The Katherina granitic pluton depicts as the latest evolution stage of the KRC that intruded all the early formed rock units in the concerned area. The Katherina volcanics are essentially composed of rhyolites, ignimbrite, volcanic breccia and tuffs. Mineralogically, the peralkaline rhyolites contain sodic amphiboles and aegirine. The rhyolite whole rock chemistry has acmite-normative character. The subvolcanic bodies of the KRC are represented by peralkaline microgranite and porphyritic quartz syenite. The ring dykes are semicircular in shape and consist mainly of quartz syenite, quartz trachyte and trachybasalt rock types. The Katherina subvolcanic rocks, volcanic rocks as well as the ring dykes are alkaline or/and peralkaline in nature. The alkaline granitic pluton forms the inner core of the KRC, including the high mountainous areas of G. Abbas Pasha, G. Bab, G. Katherina and G. Musa. These mountains are made up of alkaline syenogranite and alkali feldspar granite. The mantle signature recorded in the KRC indicates a juvenile ANS crust partial melting process for the generation of this system. The evolution of the KRC rocks is mainly dominated by crystal fractionation and crustal contamination. Mineral geothermometry points to the high temperature character of the KRC, up to 700–1100 °C. [ABSTRACT FROM AUTHOR]

The almost unknown Ambohimirahavavy ring complex in the Cenozoic alkaline province of northwestern Madagascar has recently attracted considerable interest because of the discovery of important rare-metal mineralization. The complex consists of arc-shaped bodies made up of silica-under- and -oversaturated syenites and extremely evolved peralkaline granitic dykes, as well as several mafic to felsic volcanic units, including basalt, phonolite and trachyte, all of which have an alkaline affinity. Uranium–lead zircon ages of 24.2 ± 0.6 Ma and 23.5 ± 6.8 Ma have been obtained for nepheline syenites and peralkaline granitic dykes, respectively, which, together with field data and ages of neighboring complexes, support emplacement controlled by regional lithospheric structures, rather than an evolving hot spot. Whole-rock major and trace-element and Sr–Nd isotopic data for the mafic suite suggest that the parental melt of this complex was generated by low degrees of melting of a metasomatized mantle source with residual amphibole. Fractional crystallization of this alkali basaltic melt likely produced the silica-undersaturated suite. We propose that the silica-oversaturated suite evolved from the undersaturated melt after contamination of the latter by crustal material. Further evolution to peralkaline compositions in both suites is attributed mainly to plagioclase and alkali feldspar segregation. Nepheline and feldspar compositions, as well as considerations of mineral equilibria among mafic silicates and Fe–Ti oxide minerals indicate crystallization temperatures of 1000 to 700 °C and an oxygen fugacity of 0.4 to 0.8 log units below the fayalite–magnetite–quartz (FMQ) buffer at 1 kbar for the silica-undersaturated melt, and temperatures of 860 to 570 °C and an oxygen fugacity of 1.5 to 3.8 log units below FMQ for the oversaturated syenitic melt. The undersaturated melt evolved towards a more peralkaline composition. Crystallization of arfvedsonite plus aegirine further reduced the melt the evolution of which ended with fluid exsolution. At late stages of crystallization, the oversaturated melt departed from the reducing trend of the undersaturated melt, evolving towards high oxygen fugacity. Very late exsolution of the fluid permitted concentration of the HFSE in the last stages of magmatic evolution with local production of low-temperature pegmatitic phases extremely enriched in these elements. [ABSTRACT FROM AUTHOR]

Abstract: We use thallium (Tl) concentrations, K/Rb, K/Tl and Rb/Tl ratios and Tl isotopes in minerals from the alkaline to peralkaline Ilimaussaq complex (South Greenland) to trace magmatic differentiation, crustal assimilation, magmatic degassing, ore precipitation and hydrothermal metasomatism. Closed-system magmatic differentiation is marked by a coherent decrease of K/Tl- and K/Rb-ratios, whereas crustal assimilation results in a strong Tl-enrichment, causing low K/Tl-ratios compared to K/Rb-ratios. Thallium isotopes show only slight changes during orthomagmatic differentiation and the assimilation of crustal material cannot be traced, since the isotopic composition of the average crust is within the range of the mantle and mantle-derived rocks. Magmatic degassing, however, increases Rb/Tl-ratios and changes the isotopic composition of Tl. The released fluids are enriched in Tl, characterized by high 205Tl/203Tl ratios and can precipitate Tl-rich sulfide and silicate minerals as indicated by some late-magmatic hydrothermal veins, which contain a conspicuous assemblage of Tl–Fe–Cu-sulfides (thalcusite, djerfisherite, chalcothallite). The oxidative alteration of these assemblages at high pH results in small-scale redistribution of Tl. Thallium released by this process is entrained into late-stage Tl-enriched astrophyllite. [Copyright &y& Elsevier]

Abstract: The Late Cretaceous bimodal Yunshan (Yongtai) volcanics in Fujian province contain peralkaline rhyolites, the only presence of such rhyolites in southeastern China. Whole-rock and mineral chemical compositions are analyzed for the coexisting aluminous (metaluminous to weakly peraluminous)-peralkaline high-silica rhyolites from the Yunshan volcanics. They are sparsely porphyritic, and contain K-feldspar, ferromagnesian minerals, quartz, magnetite, and titanomagnetite phenocrysts, as well as accessory minerals such as fayalite, chevkinite, apatite and zircon. The mineral assemblage indicates an oxidizing pre-eruption condition. These rhyolites exhibit diagnostic geochemical features of A-type granites, such as elevated 104 *Ga/Al (mostly greater than 2.6) and FeOT/(FeOT +MgO) ratios, enrichment in high field strength elements (HFSE) such as Zr (>400ppm) and Nb, and strong depletion in Al2O3 (<13wt%), CaO, Ba and Sr. On the basis of their petrographic and geochemical characteristics, it is suggested that the rhyolite magmas are derived from partial melting of H2O-poor (meta) granitic igneous rocks in the deep crust, and cannot be fractionated from the coeval Yunshan mafic magmas. Geochemical variations of major and trace elements indicate the possible fractionation of K-feldspar, calcium-rich pyroxene, Fe–Ti oxides and minor chevkinite during the magma evolution. In peralkaline rhyolites, we found that the pre-existing Fe–Ti oxide and hedenbergite phenocrysts had been transformed into aegirine+oxide and aegirine+oxide+fluorite assemblages, respectively. These mineral assemblages are the products of the subsolidus reaction of pre-existing phenocrysts and extraneous Na–F-rich fluids. Such Na–F-rich fluids may be derived from the degassing of the subvolcanic rocks. The reactions indicate that the Yunshan peralkaline rhyolites could be generated through the reaction of highly fractionated aluminous silica magmas and Na–F-rich fluids. [Copyright &y& Elsevier]

We experimentally investigated the phase relations of a peralkaline phonolitic dyke rock associated with the Ilímaussaq plutonic complex (South Greenland). The extremely evolved and iron-rich composition (magnesium number = 2, alkalinity index = 1.44, FeO* = 12 wt%) may represent the parental magma of the Ilímaussaq complex. This dyke rock is therefore perfectly suited for performing phase-equilibrium experiments, since in contrast to the plutonic rocks of the complex, no major cumulate formation processes complicate defining a reasonable starting composition. Experiments were carried out in hydrothermal rapid-quench cold-seal pressure vessels at P = 100 MPa and T = 950-750 °C. HO contents ranging from anhydrous to HO saturated (~5 wt% HO) and varying fO (~ΔlogFMQ −3 to +1; where FMQ represents the fayalite-magnetite-quartz oxygen buffer) were applied. Reduced and dry conditions lead to substantial crystallization of alkali feldspar, nepheline, hedenbergite-rich clinopyroxene, fayalite-rich olivine and minor amounts of ulvøspinel-rich magnetite, which represent the phenocryst assemblage of the natural dyke rock. Oxidized and HO-rich conditions, however, suppress the crystallization of olivine in favor of magnetite and clinopyroxene with less or no alkali feldspar and nepheline formation. Accordingly, combined low fO and aHO force the evolution of the residual melt toward decreasing SiO, increasing FeO* and alkalinity index (up to 3.55). On the contrary, high fO and aHO produce residual melts with relatively low FeO*, high SiO and a relatively constant alkalinity index. We show that variations of aHO and fO lead to contrasting trends regarding the liquid lines of descent of iron-rich silica-undersaturated peralkaline compositions. Moreover, the increase in FeO* and alkalinity index (reduced and dry conditions) in the residual melt is an important prerequisite to stabilize late-magmatic minerals of the dyke rock, for example, aenigmatite (NaFeTiSiO), coexisting with the most evolved melts at 750 °C. Contrary to what might be expected, experiments with high aHO and interlinked high fO exhibit higher liquidus T's compared with experiments performed at low aHO and fO for experiments where magnetite is liquidus phase. This is because ulvøspinel-poor magnetite crystallizes at higher fO and has a higher melting point than ulvøspinel-rich magnetite, which is favored at lower fO. [ABSTRACT FROM AUTHOR]

Abstract: Phenocrysts of garnet, pyroxene and nepheline in peralkaline nephelinite from the Nasira parasitic cones at Oldoinyo Lengai contain quenched immiscible silicate (peralkalinity=2–13) and Na–Ca-carbonate melts. Their bulk compositions further define the limits of liquid immiscibility for peralkaline carbonated nephelinite magmas and confirm this process was operative at Oldoinyo Lengai during older stages of activity. Groundmass glasses in Nasira nephelinites are peralkaline (peralkalinity=5.5–9.5) but less evolved than melt inclusion glasses (peralkalinity=8–13) in nepheline phenocrysts, implying that these magmas are hybrids formed by magma mixing. Groundmass glass in diverse peralkaline combeite nephelinite ash clasts with and without melilite and/or wollastonite formed in the January–June 2008 eruptions of Oldoinyo Lengai are also exceptionally peralkaline. Two trends in their compositions are evident: (1) increasing peralkalinity from 6 to 10 with SiO2 decreasing from 42 to 33wt.%; (2) increasing peralkalinity from 6 to 16 with SiO2 decreasing from 45 to 40wt.%. All recent glasses are considered to be more evolved than groundmass glass in Nasira combeite nephelinite. These data indicate that several varieties of nephelinite exist at Oldoinyo Lengai. Their parental magmas are considered to have been initially enriched in alkalis during partial melting of their metasomatized asthenospheric sources and further by subsequent assimilation, or re-solution, of previously exsolved natrocarbonatite melt in the magma chamber(s) underlying Oldoinyo Lengai. On this basis, none of the bulk compositions of peralkaline stage II lavas at Oldoinyo Lengai, including Nasira, are considered to represent those of liquids as their compositions are determined by rheological factors (phenocryst accumulation; cumulate disruption) and assimilation processes. The formation of combeite is considered to be a consequence of natrocarbonatite melt assimilation. [Copyright &y& Elsevier]

The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75-76 wt% SiO) in comparison with the pumice (64-75 wt% SiO), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50-60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and PO highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50-60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas-intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal-proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit. [ABSTRACT FROM AUTHOR]

A variety of methods exist to constrain sub-volcanic storage conditions of magmas. Petrological, seismological and satellite geodetic methods are integrated to determine storage conditions of peralkaline magmas beneath Dabbahu Volcano, Afar, Ethiopia. Secondary ion mass spectrometry (SIMS) analysis of volatile contents in melt inclusions trapped within phenocrysts of alkali feldspar, clinopyroxene and olivine from pantellerite obsidians representing the youngest eruptive phase (<8 ka) show HO contents ≤5.8 wt.% and CO contents generally below 500 ppm, although rarely as high as 1,500 ppm. Volatile saturation pressures (at 679-835°C) are in the range 43-207 MPa, consistent with published experimental data for similar pantellerites, which show that the phenocryst assemblage of alkali feldspar + cpx + aenigmatite ± ilmenite is stable at 100 to 150 MPa. Inferred magma storage depths for these historic eruptions are ~1-5 km below sea-level, consistent with the depths of earthquakes, associated with magma chamber deflation following a dyke intrusion in the period Oct 2005-Apr 2006. Interferometric synthetic aperture radar (InSAR) data for the same period reveal a broad ~20 km diameter area of uplift. Modelling of different geometries reveals that a series of stacked sills over a 1-5 km depth range best matches the InSAR data. The consistency of depth estimates based on petrological study of ancient eruptions and the seismicity, inflation and deflation of Dabbahu observed in relation to the dyking event of 2005, suggest a small but vertically extensive and potentially long-lived magma storage region. [ABSTRACT FROM AUTHOR]

Abstract: The Late Cretaceous–Palaeocene (71–61Ma) Kap Washington Group (KWG) volcanic sequence is exposed at the north coast of Greenland. The sequence is bimodal and was erupted in a continental rift setting during the opening of the Arctic Ocean. The succession exposed on the Kap Washington peninsula, which forms the bulk of the KWG sequence (>5km thick), has been sampled along four traverses with a combined stratigraphic thickness of ca. 1500m. The sampled sequence is dominated by silicic ignimbrites (69–79wt.% SiO2) showing geochemical features typical of ferroan, A-type granitoids. The ignimbrites range from sparsely phyric, mildly peraluminous compositions [ASI=Al2O3/(CaO+Na2O+K2O)=1.05–1.20] to feldspar+quartz±sodic amphibole±Fe–Ti oxide phyric peralkaline compositions [PI=(Na2O+K2O)/Al2O3 =1.00–1.40]. The peraluminous ignimbrites appear to overlie the peralkaline ignimbrites, although stratigraphy is complicated by faulting. Fiamme imbrication indicates that both types were erupted from a vent area located north of the Kap Washington peninsula. The peralkaline ignimbrites have Sr–Nd–Pb isotopic compositions which overlap with the compositions of KWG basalts, indicating a dominantly basaltic source. The more peralkaline compositions were generated by up to ca. 50% fractional crystallisation of alkali feldspar–quartz-dominated assemblages from mildly peralkaline parental magmas, themselves probably derived by fractionation of trachytic magmas. The peraluminous ignimbrites have slightly negative εNd(i) and more radiogenic 207Pb/204Pbi and 208Pb/204Pbi. Modelling indicates that they are not cogenetic with the peralkaline ignimbrites and they are inferred to have originated by partial melting of hybridised mafic crust. Petrographic evidence suggests that magma mixing was an important process and variations in Nd–Pb isotopes and trace element ratios indicate mixing between peralkaline and peraluminous magma batches. [Copyright &y& Elsevier]

Abstract: Microgranular enclaves are common within intermediate to felsic granitic rocks that have I- and S-type affinity however they are rare within alkaline anorogenic granitoids of A-type affinity. The Permian (~260Ma) Emeishan large igneous province (ELIP) of southwest China contains two peralkaline silica saturated A-type granitic plutons that host microgranular enclaves. The enclaves from the Baima pluton are intermediate in composition and have lower SiO2 and higher TiO2, CaO and Mg# (SiO2 =57.2 to 63.0wt.%; TiO2 =0.8 to 1.8wt.%; CaO=1.7 to 3.3wt.%; Mg#=28 to 44) than their host (SiO2 =62.6 to 67.8wt.%; TiO2 =0.5 to 1.4wt.%; CaO=0.4 to 1.8wt.%; Mg#=15 to 31). The enclaves from the Taihe pluton are more felsic (SiO2 =63.8 to 71.3wt.%; TiO2 =0.3 to 0.6wt.%; CaO=0.6 to 2.3wt.%; Mg#=8 to 22) but are still less evolved than their host (SiO2 =69.8 to 75.1wt.%; TiO2 =0.2 to 0.6wt.%; CaO=0.4 to 0.8wt.%; Mg#=3 to 12). In both cases, the enclaves have very similar εNd(T) values (Baima εNd(T) =+2.8 to +3.2; Taihe εNd(T) =+1.0 to +2.0) as their hosts (Baima εNd(T) =+3.0 to +3.2; Taihe εNd(T) =+1.5 to +1.9). The major and trace element trends of the enclave-host pairs suggest that fractional crystallization occurred and that element diffusion was likely minimal. The enclaves are interpreted as entrained accumulations of early formed crystals of a silicic magma which was originally produced by fractional crystallization of a mafic magma. [Copyright &y& Elsevier]

Abstract: The Mongolian–Transbaikalian Belt (MTB) is the largest and most representative component of the Central-Asian network of Late Paleozoic and Mesozoic granites and syenites and associated volcanic rocks. The MTB incorporates over 350 individual plutons and numerous volcanic fields, predominantly of alkaline and peralkaline composition. We present new U–Pb zircon and titanite geochronological data from five plutonic complexes located in the central part of the MTB. Four of them (Bryansky, Khorinsk, Kharitonovo and Malo-Kunalei) are peralkaline and alkali-feldspar granitoids, and one plutonic complex (Ust-Khilok) represents a shoshonitic series (K-rich gabbro, monzonite and syenite). The new data confirm the presence of three distinct stages of highly alkaline magmatic activity within the MTB: Early Permian (280–275Ma), Late Triassic (230–219Ma) and Late Jurassic (∼152Ma). The dated shoshonitic plutonic complex formed between 280 and 278Ma, overlapping with the formation of the Early Permian alkali-feldspar–peralkaline granitoid rocks. The ages of the alkali-feldspar and peralkaline magmatic suites are indistinguishable within a single magmatic complex. This suggests that several episodes of syenite and granite magma emplacement occurred within <2myr intervals. The close association in space and time of geochemically distinct igneous rocks points to co-existence of discrete magma chambers comprising magma from different sources. Early Permian magmatic activity in the MTB completed a continuous post-collisional cycle of magmatism that started in the Carboniferous (at ∼330Ma) and lasted for about 50myr. The Late Triassic stage marks the beginning of anorogenic tectonic setting in the MTB evolution. Late Jurassic peralkaline magmatism in the territory of the MTB is related to the formation of Mesozoic metamorphic core complexes, which are abundant in northern Mongolia and Transbaikalia and formed in an anorogenic within-plate environment. [Copyright &y& Elsevier]

Abstract: Pantelleria Island, located in the Sicily Channel Rift Zone (Italy), is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50ka, after the large Green Tuff caldera-forming eruption, volcanic activity at Pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits. Basaltic cinder cones and lava flows are only present outside the caldera in the NW sector of the island. The most recent basaltic (Cuddie Rosse, ∼20ka) and pantelleritic (Cuddia Randazzo and Cuddia del Gallo, ∼6ka) pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions. The melt inclusions in pyroxene and olivine phenocrysts of Cuddie Rosse scoriae have an alkali basalt composition. The dissolved volatiles comprise 0.9–1.6wt.% H2O, several hundred ppm of CO2, 1600–2000ppm of sulphur and 500–900ppm of chlorine. The water–carbon dioxide couple gives a confining pressure ∼2kbar prior to the eruption. This result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption. The melt inclusions in feldspar, fayalite and aenigmatite phenocrysts of Cuddia del Gallo and Cuddia Randazzo pumice have a pantelleritic composition (Agpaitic Indices 1.3–2.1), up to 4.4wt.% H2O, 8700ppm Cl, 6000ppm F, and CO2 below the detection limit. Sulphur averaging 420ppm has been measured in Cuddia Randazzo melt inclusions. These data indicate relatively high volatile contents for these low-energy Strombolian-type eruptions. Melt inclusions in Cuddia del Gallo pumice show the most evolved composition (Agpaitic Indices 2–2.1) and the highest volatile content, in agreement with fluid saturation conditions in the magma chamber prior to the eruption. This implies a confining pressure of ∼1kbar for the top of the pantelleritic reservoir. The composition of melt inclusions and mineralogical assemblage of Cuddia Randazzo pumice indicate that it has a lower evolutionary degree (Agpaitic Indices 1.3–1.8) and lower pre-eruptive Cl and H2O contents than Cuddia del Gallo pumice. An increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the Cuddia del Gallo explosive eruption. Evidence of widespread pre-eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the Cuddia Randazzo event. [Copyright &y& Elsevier]