Magma Series Research Articles

Subsurface structure of the Tarfaya-Laayoune-Dakhla basin (West African Craton Margin, Morocco) revealed from the analysis of aeromagnetic data

The Tarfaya-Laayoune-Dakhla basin is the largest coastal basin in the Moroccan Atlantic margin. It is filled by a thick Meso-Cenozoic sedimentary cover that overlies unconformably the NW margin of the West African Craton and its Proterozoic to Paleozoic peripheral orogenic belt. In this study, we aim to decipher the structure of hidden part of these terranes and discuss the tectonic evolution of the basin. To achieve these objectives, aeromagnetic data were processed by the application of edge enhancement and depth estimation operators. The combination of their results with geological information and boreholes data allowed the construction of a detailed structural map of the subsurface of the study area. The resulted map delineates three distinct tectonic domains: the Northeastern Domain, the Western Domain and the Southern Domain. These domains exhibit geological evidences of four major tectonic periods: the Eburnean, Pan-African and Variscan orogenies, and the Triassic rifting. The Eburnean dextral transpression event formed a shear-zone with a large magmatic body in the Western Domain. This domain was not affected by the subsequent Pan-African orogeny and the following Ediacaran intracontinental rifting phase. Meanwhile, this event led to the formation of an N- to NNE-trending large half-graben structure in the Northeastern Domain, and an Ediacaran bimodal magmatic series in the Southern Domain. The Paleozoic Variscan orogeny has reactivated the N- to NNE-trending normal faults, allowing thrusting in these domains. Eventually, the Central Atlantic opening event reworked pre-existing reverse faults resulting increases in depth in the Northeastern and Western Domains, meanwhile, the extension was reduced in the Southern Domain.

Clinopyroxene Compositions of E-MORB-Type Gabbro from Bingdaban Ophiolites in Central Tianshan, NW China: Insights into the Evolution of the Magmatic System and Geodynamic Setting

Clinopyroxene, one of the primary rock-forming minerals in mafic rocks, is the major host of lithophile elements in the mantle lithosphere and plays a crucial role in understanding mantle evolution and rock petrogenesis. Taking the Bingdaban ophiolite as an example, this study employed electron probe microanalysis and in situ trace element analysis to investigate the geochemistry of clinopyroxene in gabbros to determine the magma series and evolution, constrain the physicochemical conditions of the magmatic processes and explore the petrogenesis and tectonic setting. Representative gabbro samples were subjected to zircon U–Pb isotopic analysis, yielding an age of 424.3 ± 5.9 Ma. Geochemical investigations revealed that the Bingdaban gabbros exhibit tholeiitic composition, suggesting a genesis associated with enriched mid-ocean ridge basalt (E-MORB). Mineralogical analyses indicated that the clinopyroxene in the gabbros was Mg-rich, Fe-poor, and alkali-poor, representing a subalkaline series. The compositional end members of clinopyroxene were calculated as Wo38.9–48.0En30.9–48.1Fs10.4–24.4, indicating a predominance of diopside with a minor amount of augite. Temperature–pressure conditions imply that these rocks formed in a high-temperature, low-pressure, and shallow-source environment. Compositional estimates of the melt in equilibrium with clinopyroxene are consistent with the overall characteristics of the host rock, reflecting an E-MORB setting. The Bingdaban gabbro likely originated from an initially depleted mantle source that later received an input of enriched mantle material, indicating formation in either an initial oceanic or immature back-arc basin tectonic setting.

Early zircon saturation in adakitic magmatic differentiation series and low Zr content of porphyry copper magmas

Porphyry copper ore-forming magmas worldwide are chemically distinguished from ordinary arc granitoids by lower Zr and by higher Sr/Zr ratios at equivalent SiO2 contents. Low ppm Zr in zircon-saturated melts and high whole-rock Sr/Zr in granitoid samples retaining igneous plagioclase are useful discriminants of Cu-fertile intrusive complexes. These and other chemical discriminants of porphyry-copper-forming melts of tonalite-granodiorite-adamellite composition cannot develop during crystallisation-differentiation at upper-crustal pressures. They indicate unusually high dissolved H2O (≥ 9 wt%) in residual melts at depths near the base of the crust. We compare Zr behaviour during mafic-to-felsic magmatic differentiation in orogenically deforming, copper-mineralised arc segments with behaviour during differentiation in non-orogenic, unmineralised arc segments laterally adjacent along the same subduction zones. In orogenically deforming segments, horizontal compressive stress provides resistance to opening of tensile dyke fractures and promotes entrapment of mantle-derived basaltic melts in magma chambers embedded in hot country rock at Moho-vicinity depths, where magmas cool slowly and residual melts tend to last long enough to experience intermittent chamber replenishment by basaltic melts from the deeper mantle. Over several cycles of replenishment and fractional crystallisation, residual melts acquire high concentrations of inherited chemical components that were largely excluded from cumulus minerals—H2O, Cl, CO2, SO3, etc. Accumulating H2O re-orders the high-pressure crystallisation sequence of igneous minerals in successive cycles and leads to early and prolific production of hornblende and early zircon saturation in mafic melts (< 60 wt% SiO2) of later cycles and leads to low Zr contents of zircon-saturated intermediate-composition residual melts.

Petrology and mineralogy of the martian olivine gabbroic shergottite Northwest Africa 13581: Insights into the enriched martian mantle source

Gabbroic and microgabbroic shergottites are intrusive igneous rocks on Mars and exhibit a wide variety of mineralogical and geochemical properties. However, their source reservoirs, magmatic processes and the link to other shergottite subtypes are not well constrained. Northwest Africa (NWA) 13581 is a newly found permafic olivine gabbro, representing a cumulate member in the shergottite group. This sample provides new critical constraints on the characteristics of mantle sources and magmatic evolution of shergottites. The texture and mineral chemistry of NWA 13581 indicate a polybaric crystallization condition, with an increase in oxygen fugacity of approximately 1 log unit after ascent and emplacement. Geochemical studies suggest that the sample is young (150 ± 25 Ma) and may share a similar enriched mantle reservoir (initial ε176Hfi = −18.4, initial ε143Ndi = −7.1) with shergottites like Zagami (basaltic shergottite), NWA 10169 (poikilitic shergottite) and Larkman Nunatak 06319 (olivine-phyric shergottite). In addition, the presence of potassium-rich melt inclusions enclosed in early-formed minerals of NWA 13581 implies a fertile source, probably originating from a metasomatized mantle. The occurrence of similar potassium-rich components in other enriched shergottites may suggest a common process in their mantle reservoir and during crystallization. Overall, NWA 13581 resembles poikilitic shergottites closer than typical gabbroic shergottites in several aspects such as poikilitic texture, mineral chemistry, magmatic evolutionary path, and emplacement conditions. A simplified model is proposed for the formation of NWA 13581 and poikilitic shergottites in the same magma series. Magma mixing and/or assimilation are not the major mechanism that account for their slightly varied mineral modal abundances and quantitative textural characteristics. For samples derived from similar mantle sources, the textural and mineralogical diversities of shergottites are largely related to crystallization at different crustal levels.

Tectonic Transition from Passive to Active Continental Margin of Nenjiang Ocean: Insight from the Middle Devonian-Early Carboniferous Granitic Rocks in Northern Great Xing’an Range, NE China

Northeast China occupies the majority of the eastern Central Asian Orogenic Belt, which mainly consists of continental blocks and accretionary terranes. The Devonian was a tectonic quiet period in the NE China region due to a lack of tectono-magmatism, but the tectonic background of this period has been unclear, especially for the Hegenshan-Heihe Suture between Xing’an and Songliao accretionary terranes, which represents the Paleozoic Nenjiang Ocean (a branch ocean of the eastern Paleo-Asian Ocean). Here we report granitic rocks from the Woluohe area, Northern Great Xing’an Range, NE China, to constrain the tectonic process of the transition from the Devonian quiet period to the Early Carboniferous active tectonic period. Three granitic rock samples produce zircon U-Pb ages of 389 Ma, 368 Ma, and 351 Ma, belonging to the Middle and Late Devonian and Early Carboniferous, respectively. They have high Si, Al, K, and Na contents, but with low Mg, Fe, and Ti contents, together with positive Hf isotopic features and low molar Al2O3/(MgO+FeOT) ratios, we suggest that they were derived from partial melting of lower crustal igneous rocks. Meanwhile, the narrow major element variation at odd with the fractionation process and their negative Nb and Ta anomalies imply the obvious contribution of crustal. Comprehensive tectonic setting analysis shows all samples are in calc-alkali magmatic series with rightward fractionated REE and trace element patterns that are enriched in LREE and LILE and depleted in HREE and HFS, indicating a subduction-related magmatic arc setting. Considering the regional tectonic setting and the small scale of the Devonian plutons, we suggest a limited subduction tectonic setting during the quiet period of the northern Great Xing’an Range, which might indicate the beginning of an initial northwestward subduction of the Nenjiang Oceanic lithosphere beneath the Xing’an Accretionary Terrane in the Middle Devonian, accelerated subduction in the Late Devonian, and bidirectional subduction in the Early Carboniferous.

Redox and mineral controls on Fe and Ti isotopic fractionations during calc-alkaline magmatic differentiation

Titanium and Fe isotopic compositions of lavas from a calc-alkaline differentiation suite and corresponding mineral separates from the Rindjani Volcano, Indonesia show that Fe and Ti isotopic fractionations between minerals and melts are lower than those recorded in other suites at all stages of differentiation. The limited isotopic fractionation for Ti is likely due to low-Ti magnetite and clinopyroxene being the dominant carriers of Ti in Rindjani lavas, as these minerals are thought to have limited equilibrium Ti isotopic fractionation relative to silicate magmas. Other magmatic differentiation suites controlled by removal of Ti-rich magnetite and characterized by a lesser role of clinopyroxene have larger Ti isotopic fractionations. This effect is an indirect consequence of the elevated Fe3+/Fe2+ ratio of calc-alkaline magmas such as Rindjani, which promotes Fe3+ incorporation into magnetite at the expense of Fe2+-Ti4+ pairs, such that increased oxygen fugacity will subdue Ti isotopic fractionation in global magmatic series. Similarly, we find negligible Fe isotopic fractionation in Rindjani bulk rocks and mineral separates, unlike previous studies. This is also likely due to the oxidized nature of the Rindjani differentiation suite, which leads to similar Fe3+/Fe2+ ratios in melt and minerals and decreases overall mineral-melt Fe fractionation factors. Paired Ti and Fe isotopic analyses may therefore represent a powerful tool to assess oxygen fugacity during differentiation, independent from Fe 3+ determinations of erupted samples.

Precise Pb-Pb baddeleyite geochronology, geochemistry, and Sr-Nd isotopic constraints on the 2.36 & 1.88 Ga mafic dykes from the Bastar craton, India: Implications for their petrogenesis in conjunction with the Dharwar mafic dykes

This research focuses on the geochronology and geochemistry of WNW-NW-NNW trending Paleoproterozoic mafic dykes from the Bastar craton in India. The weighted mean 207Pb/206Pb ages of baddeleyite grains obtained from the Tuskal, Pakhanjore, and Gariaband regions were 2365.25 ± 0.54 Ma, 2365.70 ± 0.48 Ma, and 2363.85 ± 0.79 Ma, respectively. Baddeleyite grains from a mafic dyke near Narayanpur-Kondagaon yielded a weighted mean 207Pb/206Pb age of 1886.5 ± 1.8 Ma. In the TAS (Total Alkali Silica) diagram, the 2.36 Ga and 1.88 Ga mafic dykes have basalt to basaltic andesite composition. Dykes from both age groups are represented by the tholeiitic magma series in the AFM diagram. In the spidergrams, the dykes show LREE and LILE enrichment, with the 2.36 Ga dykes showing flat HREE and the 1.88 Ga dykes showing slightly inclined HREE patterns. The REE non-modal batch melting model indicates derivation of the 2.36 Ga mafic dykes from a shallower spinel lherzolitic source, whereas the 1.88 Ga mafic dykes were derived from a deeper spinel-garnet lherzolitic mantle source. The 2.36 Ga dykes have near chondritic to slight super chondritic initial Nd isotopic values (εNd(2.36 Ga) = −0.95 to 2.36), whereas the 1.88 Ga dykes have sub chondritic initial Nd isotopic values (εNd(1.88 Ga) = −2.37 to −9.67). Present-day orientations and similar precise ages of the mafic dykes reported from the Bastar and Dharwar cratons indicate the counter-clockwise rotation of the Bastar craton during the Paleoproterozoic.

Magnesium isotope fractionation in differentiation of mafic-alkaline-carbonatitic magma and Fe-P-REE-rich melt at Bayan Obo, China

The Bayan Obo area in the North China Craton (NCC) contains the largest rare earth element (REE) resources in the world. The key to this gigantic and efficient REE accumulation has been the focal point of century-long research efforts. Intense differentiation of mantle-derived alkaline magmas is envisaged as a highly viable process for REE enrichment based on the first discovery of mafic-alkaline-carbonatitic suite in the Bayan Obo region. Geochronological and Sr-Nd-Hf-O isotopic analyses revealed that these rocks derived from a common mantle source and evolved with magmatic differentiation at 1.31 Ga to 1.32 Ga. Elemental geochemistry and Mg isotopes in this magma series (alkaline diabase sill, syenite dyke and carbonatite) unambiguously support the differentiation model. The earliest ferrocarbonatite, with nearly undifferentiated REE concentrations, has mantle-like δ26Mg (−0.27 ± 0.03‰), and therefore can be considered as the original product of the mantle derived alkaline magma at Bayan Obo. The subsequent magnesiocarbonatites have a LREE enrichment feature, which is consistent with the alkaline diabase sills. The magnesiocarbonatites possess light Mg isotopic composition (from − 0.42‰ to − 0.50‰), whereas the alkaline diabase sills have heavy Mg isotopic composition (from − 0.03‰ to − 0.05‰). This rock assemblage should present two end members, which differentiated likely by immiscibility in the chamber from a common alkaline magma. The subsequent syenite dyke has no obvious Mg isotopic variation (from − 0.44‰ to − 0.39‰) during differentiation from the above alkaline magma. Notably, as the most important REE hosts, fine-grained ferrocarbonatites show large Mg isotopic variations (from − 0.85‰ to + 0.13‰) and present positive correlations with total MgO and CaO contents, but negative with total Fe2O3, P2O5 and REE contents. In fine-grained ferrocarbonatite, closely associated apatite, monazite, hematite and magnetite minerals occur as a special intergrowth texture. This unique paragenetic assemblage is likely the mineralization record of a Fe-P-REE-rich melt, which shows particular affinity of hydrothermal fluid and light Mg isotope in carbonatitic magma. By contrast, magnesian siderite phenocrysts as the early cumulates in carbonatitic magma are enriched in heavy Mg isotope (from − 0.07‰ to + 0.21‰). Immiscibility of Fe-P-REE-rich melt and fractional crystallization of carbonate minerals are responsible for large Mg isotope fractionation and giant REE enrichment in the Bayan Obo deposit.

Geochronology, geochemistry and Sr-Nd isotope compositions of the Niubiziliang mafic-ultramafic complex in the South Altyn orogen, NW China: Implications for Ni-Cu mineralization and tectonic setting

The Niubiziliang mafic-ultramafic complex located in the South Altyn orogen consists of lherzolite, plagioclase-bearing lherzolite, olivine websterite, and gabbro. The intrusion contains a newly discovered zone of magmatic Ni-Cu sulfide mineralization, which is categorized into three units (Nos. 1, 2, and 3) based on discernible morphological features, with drilling investigations revealing sulfide mineralization in complexes Nos. 2 and 3 in both mafic and ultramafic rocks. The rocks of the Nuibiziliang complex have olivines with Fo 76.9–82.4 and Ni contents between 79 and 1957 ppm. The clinopyroxenes have variable Al2O3 contents between 0.52 and 5.05 wt%, TiO2 contents between 0.02 and 1.30 wt%, and NaO contents from 0.08 to 1.27 wt%. The geochemical variations are consistent with the intrusion belonging to a subalkaline magma series and the major element variations are consistent with igneous differentiation from a single magma type. The rocks display enrichment in light rare-earth elements and large-ion lithophile elements (K, Ba, and Sr) and depletion in high-field-strength elements (Ti, Zr, Hf, Ta, and Nb) and heavy rare-earth elements, which are typical features of enriched mid-ocean ridge basalt (E-MORB). These rocks exhibit enriched Sr–Nd isotope compositions with low (87Sr/86Sr)i ratios and mostly low positive εNd(t) values, indicating that the Niubiziliang complex originated from the partial melting of the asthenospheric mantle with up to 10% contamination by continental crust. Fractional crystallization enriched sulfur in the magma during the initial stage of magmatic evolution, but crustal contamination was the main factor causing sulfide saturation in the magma. Laser ablation inductively coupled plasma-mass spectrometry U-Pb dating of baddeleyites in the ultramafic rocks of the Niubiziliang complex yielded an age of 452±12 Ma for the crystallization of the magma. This age is similar to the age of eclogites formed during exhumation for the South Altyn orogen and indicates that the mafic-ultramafic magmatism formed in a rift setting by a process of break-up of an oceanic crustal slab at depth in the mantle.

Petrogenetic Study on Ultramafic Rocks from Waturapa and Surrounding Areas, South Konawe Regency, Southeast Sulawesi Province

The petrogenesis study of ultramafic igneous rocks in the South Konawe Region has been carried out by several previous researchers, however, petrogenesis of ultramafic igneous rocks in the Waturapa Region has never been carried out in detail. This study aims to determine the characteristics and petrogenesis of ultramafic igneous rocks in the Waturapa area using petrographic and geochemical analysis using the XRF method. Petrographic analysis was carried out to determine the relative abundance percentage of primary minerals in the form of olivine, clinopyroxene, orthopyroxene, and opaque minerals as well as secondary serpentine minerals which were formed later. Meanwhile, XRF geochemical analysis is used to determine the major and minor oxide content in rocks. This geochemical data is used to determine ultramafic rock types, and magma series and to interpret the tectonic setting of the research location. The results showed that the ultramafic rocks in the study area consisted of olivine websterite and lherzolite, both of which have been serpentinized which is characterized by the presence of serpentine minerals such as lizardite and chrysotile. These serpentine minerals are present as replacement minerals and fracture-filling minerals. The geochemical characteristics of the analyzed rocks showed a SiO2 content of less than 45%, high MgO content, and low K2O, TiO2, Na2O3, and P2O5 compounds. The igneous rocks in the study area are classified as ultrabasic or ultramafic rocks (peridot gabbro). Ultramafic rocks in the study area belong to the tholeiitic magma series that formed in oceanic islands or oceanic intraplate margins.

The Coorg Block, southern India: Insights from felsic and mafic magmatic suites on Mesoarchean plate tectonics and correlation with supercontinent Ur

The Coorg Block in southern India is one of the oldest crustal fragments on the globe and is composed of Mesoarchean magmatic suites, dominantly charnockites, gabbros and their variants that were metamorphosed under granulite facies conditions. Here we investigate a suite of charnockites, metagabbros and amphibolite from this block by undertaking detailed field studies, petrological and geochemical analysis, and zircon and monazite isotope geochronology. Phase equilibria modeling using pseudosection computations shows ultrahigh-temperature metamorphic conditions yielding a P-T range of 10–11 kbar at 1000 °C for the charnockite assemblage and 8–9 kbar at 950 °C for the metagabbro. Geochemistry indicates that the felsic and mafic rocks are sub-alkaline and represent a tholeiitic magma series. The rocks were derived possibly through fractionation of a reduced primary magma in a Mesoarchean subduction-related setting. Zircon grains from the charnockites and gabbros show magmatic textures, with LREE depletion and HREE enrichment, negative La, Pr, Nd, Eu and positive Ce, Sm, Gd anomalies, and Th/U ratios consistent with magmatic crystallization. We present zircon and monazite U–Pb data from a comprehensive suite of samples that indicate timing of peak of magmatic emplacement for both the charnockite and gabbroic suites occurred at ∼ 3.15 Ga, closely followed by metamorphism at ∼ 3.0 Ga. The presence of magmatic xenocrysts of up to ∼ 3.5 Ga in some of these rocks suggest older crustal components in the basement. Zircon Lu–Hf analyses suggest that the amphibolite and gabbro were mostly derived from depleted mantle sources whereas the charnockite suite involved both juvenile and reworked Paleoarchean components. The tectonic setting of these felsic and mafic magmatic rock suites correlates with a Mesoarchean subduction system, complementing recent models on the emergence of modern-style plate tectonics in the early Earth. We correlate the Mesoarchean arc magmatic suites in the Coorg Block to the building blocks of the core of the Earth’s oldest supercontinent Ur, with the surrounding Neoarchean granulite blocks and their equivalents in other cratonic fragments to the ‘expanded Ur’.

Age and petrogenesis of Ni-Cu-(PGE) sulfide-bearing gabbroic intrusions in the Lausitz Block, northern Bohemian Massif (Germany/Czech Republic)

The Lausitz Block, located in the northernmost part of the Bohemian Massif, hosts a large number of dike- to stock-shaped gabbroic intrusions that mainly comprise brown hornblende-poor (Group I; i.e. olivine gabbronorite, olivine gabbro, gabbro and diorite) and subordinately brown hornblende-rich lithologies (Group II; i.e. olivine-hornblende gabbro and hornblende gabbro). Several of these intrusions host small-scaled magmatic Ni-Cu-(PGE) sulfide accumulations. The intrusions are part of interconnected mafic–(ultramafic) plumbing systems that intruded Cadomian granodiorites of Lausitz Block in the Middle to Late Devonian during the early stages of the Variscan Orogeny. The previously inferred Devonian age of the intrusions is refined by biotite Ar-Ar dating that yield ages between 389.1 ± 3.9 Ma and 372.2 ± 3.7 Ma (2σ). Group I and Group II lithologies differ in their mineralogical and geochemical composition. Compared to the Group I lithologies those of Group II are characterized by higher modal contents of primary brown hornblende, Fe-Ti oxides and apatite, by Ti- and Al-enriched clinopyroxene and by lower contents of SiO2 and increased contents of TiO2, P2O5, LILE, HFSE and LREE. The differences suggest at least two different magmatic series where Group I rocks are linked to tholeiitic basaltic magmas with low to moderate Ti and volatile contents, whereas Group II rocks are derived from Ti- and volatile-enriched alkaline basaltic magmas. The magmas experienced clinopyroxene fractionation during their crustal ascent and storage, but were only minor affected by crustal contamination (< 5%) according to Sr-Nd-Pb isotope systematics. The tholeiitic and alkaline basaltic magmatism could be explained by a tectonic environment between a subduction and intraplate setting. Group II rocks have an intraplate chemical signature and their parental magmas are clearly originated from a deeper-seated mantle source with melting of garnet peridotite, likely associated with an asthenospheric upwelling. By contrast, precursor magmas of Group I rocks are originated from partial melting of a less-enriched source, presumably spinel peridotite, from a subduction-related environment. Trace elements systematics suggest that both groups at least partly resulted from an interaction (mixing) of the alkaline and tholeiitic magmas. Partial melting estimates for both groups vary between 5 and 20%. The proposed tectonic setting could have been related to a subduction slab retreat within the framework of the Variscan orogeny, which led to an upwelling of the asthenospheric mantle. Cu/Zr ratios <1 of gabbroic rocks from several intrusions suggest a previous segregation of magmatic sulfides in other sections of the magmatic plumbing system and give rise for a vertical and lateral Ni-Cu exploration potential.

Extreme Differentiation along Multiple Liquid Lines of Descent in Strongly Peralkaline Magma Series at Pantelleria (Italy)

AbstractThe 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 &amp;gt;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 (fO2) 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 &amp;gt;4 wt%, low oxygen fugacity (&amp;lt;ΔNNO-1.5), and low temperatures (&amp;lt;700°C), although fluorine may stabilize it at higher temperatures in some rocks.

Zircon U-Pb dating and petrogenesis of the São José do Campestre Granite Complex, NE Brazil: an example of neoarchean mantle-derived post-collisional magmatism

This article discusses the geochemical and petrological evolution of the São José do Campestre granite complex (SJCgr), the last Neoarchean plutonic event so far described in the São José do Campestre massif in NE Brazil. We report field, petrographic, zircon U-Pb dating, and whole rock and mineral chemistry for representative SJCgr samples. Laser ablation zircon U-Pb data indicate that the granite emplacement took place at 2664 ± 13 Ma. The rocks comprising the SJCgr have relatively well-preserved primary textures and fabrics and compositions varying from gabbro to syenogranite. Major and trace element contents reveal a metaluminous, calc-alkaline through transitional to alkaline signature, and LILE- and LREE-enriched series analogous to late Archean sanukitoid and modern arc granitoid. The evolution of the SJCgr is envisaged as follows: (1st) partial melting of a metasomatized mantle (2.5–3.0 GPa, ~85–102 km, 1,000–1,200°C), generating a basaltic to basaltic andesitic magma; (2nd) fractional crystallization (FC) of olivine at mantle or lower crustal depth, leading to the parental magmas of the magmatic series; and (3rd) 40–15% FC of olivine gabbro-norite and olivine monzonite cumulates (400–600 MPa, 15–23 km). The SJCgr shares similarities with post-collisional granitoids and, thus, would represent the last Neoarchean episode of mantle-derived magma in Northeastern Brazil.

GEOLOGIA E GEOQUÍMICA DO STOCK SERRA DO CAPUXU: MAGMATISMO CÁLCIO-ALCALINO DE ALTO-K NO DOMÍNIO RIO PIRANHAS-SERIDÓ, PROVÍNCIA BORBOREMA

O Stock Serra do Capuxu (SSC) está situado na porção setentrional da Província Borborema, na porção sudoeste do Domínio Rio Piranhas-Seridó. O stock intrude rochas gnáissicas do Complexo Caicó, dos quais guarda xenólitos, e trunca feições dúcteis prévias. Ocorrem enclaves tipo MME, localmente com feições de coexistência de magmas (mixing e mingling); e schlierens múltiplos e complexos. O SSC caracteriza-se por rochas leucocráticas (ΣMáficos=7,5-13%) de composição monzogranítica, textura equigranular fina/média, ocorrendo localmente fenocristais de K-feldspato (microclina). K-feldspato+plagioclásio+quartzo constituem mais de 85% modal; biotita é o principal máfico (6-10%); titanita, opacos, allanita e zircão são acessórios; e clorita, mica branca e carbonato ocorrem como produtos de transformações tardi-magmáticas. São rochas bastante evoluídas (SiO2=71,80-73,14), com teores de álcalis elevados (Na2O+K2O=8,25-8,77) e baixos teores de cálcio (CaO=1,23-1,69). Os espectros de elementos terras raras (ETR’s) mostram enriquecimento dos ETR’s leves, em relação aos pesados, com pronunciada anomalia negativa de Európio (Eu/Eu*=0,34-0,44). O índice de saturação em alumina do SSC revela um caráter transicional metaluminoso a peraluminoso (A/CNK=0,98-1,03). Diagramas discriminantes de séries magmáticas revelam, na sua maioria, afinidade química com rochas da série subalcalina (cálcio-alcalina de alto-K) e diagramas classificativos de ambiente tectônico associam o SSC a ambiente dominantemente pós-colisional. Esses dados são corroborados pela idade U-Pb em zircão de 579 Ma encontrada para o stock, sugerindo um alojamento tardi-tectônico para o SSC, consonante com os aspectos de campo do pluton. Postula-se uma fonte litosférica, mas diferente dos dioritos associados, para o SSC, possivelmente fusão de gnaisses do embasamento (baixo grau de fusão, face seus elevados e pouco variados teores de SiO2) gerando um magma já de natureza cálcio-alcalina de alto-K. Rochas com assinaturas similares têm sido denominadas de “Granitos Tipo-I Fracionados”

The petrogenesis of chemically zoned, phonolitic, Plinian and sub-Plinian eruptions of Somma-Vesuvius, Italy: Role of accessory phase removal, independently filled magma reservoirs with time, and transition from slightly to highly silica undersaturated magmatic series in an ultrapotassic stratovolcano

A mineralogical, major, LA-ICP-MS trace element mineral chemistry and bulk-rock geochemical study of juvenile samples of the Mercato, Avellino, Pompeii and Pollena eruptions, collected in stratigraphically and volcanologically well-characterized sections of the Somma-Vesuvius stratovolcano (Roman Magmatic Province), along with reference data on the 1944 CE and the Pomici di Base eruptions, highlights the compositional variability of bulk-rock and glass from leucite phonotephrite to garnet-bearing phonolite . The latter products have extreme fractionation of trace elements (e.g., La/Yb n = 126, Zr/Y = 89, Zr/Hf =78, Nb/Ta = 40; Th/U = 2.3), very low Sc, V, Y, HREE and very high As, Tl, Cs, Pb, Th and U. The Pomici di Base products, older than the eruptions described above, range from leucite-bearing shoshonites to trachytes , are devoid of garnet and belong to an independent liquid-line-of-descent, having also different fractionation between trace elements (e.g., La/Yb n = 15; Zr/Y = 12.4, Zr/Hf = 50, Nb/Ta = 15.6; Th/U = 3 in the Pomici di Base trachytes). A marked chemical variability of the observed phases is found. The geochemistry of garnet, amphibole , clinopyroxene and other phases shows wide variations of concentrations and elemental ratios (e.g., La/Yb n up to 520 in the sadanagaite coexisting with garnet). The magmatic evolution is dominated by low-pressure, oxidized, nearly closed-system fractional crystallization of clinopyroxene, plagioclase , leucite, ±magnetite, ±biotite, ±olivine and apatite in the transition tephrite-phonotephrite, and of potassic sanidine (±hyalophane), Fe-clinopyroxene, melanite garnet±Fe-amphibole in the transition tephriphonolite-phonolite. Mineralogic and geochemical evidence and modelling points out the existence of independent, zoned magma batches throughout the activity of the stratovolcano, which possibly started to crystallize at similar depths. The Somma-Vesuvius magmas thus evolved in shallow independent reservoirs with respect to those of the neighbouring volcanic complexes (Campi Flegrei, Ischia) as shown, for instance, by the contrasting compositional trends of clinopyroxene and amphibole, and have very limited evidence of crustal contamination (and/or carbonate assimilation). • Pollena, Pompeii, Avellino and Mercato are zoned eruptions with phonolites • The magmas which formed the various eruptions evolved independently • Different garnets dominate HREE, Y and HFSE; amphibole is strongly HREE-poor • The phonolites and their mineral compositions have no tendency to become peralkaline • F, Cl and H 2 O dominate the volatile budget

Age, geochemistry and mantle source of the Alto Diamantino basalts: Insights on NW Paraná Magmatic Province

The Paraná-Etendeka is one of the largest and most thoroughly investigated large igneous provinces in the world formed in a very narrow time-range (i.e., 1.6–3.0 Ma) between 135 and 132 Ma. The South American portion of this province, i.e. the Paraná Magmatic Province, is mainly characterized by a mafic magmatism of coeval low- and high-Ti magmatic terms. This bimodal activity has been explained by different degrees of melting of a homogenous mantle source or by plume activity and metasomatism of the sub-continental mantle. Despite the large body of literature, the true origin of the mantle source and processes leading to this bimodal magmatism are still debated. In this work, we present the first geochronological, geochemical and isotopic data of basalts from Alto Diamantino (southern Mato Grosso, Brazil). These basalts have tholeiitic affinity and can also be classified in two different suites based on TiO 2 content (L-Ti < 3 wt%; H-Ti > 3 wt%) and Ti/Y (450 ppm), similarly to other volcanics from the Paraná Magmatic Province. Other geochemical parameters as Zr/Y and Nb/Yb ratios range from 3.78 to 8.60 and from 3.82 to 8.31, respectively. The L-Ti and H-Ti series have significant geochemical similarities, including 87 Sr/ 86 Sr ratios, with the Ribeira and Pitanga basalts of the Paraná Magmatic Province, respectively. The 40 Ar/ 39 Ar ages of the two series are statistically equivalent ranging between 134.4 ± 1.6 and 132.9 ± 2.0 Ma. Our data suggest that the Alto Diamantino basalts constitute the northwestern most expression of the Paraná Magmatic Province. Sr and Nd isotope ratios suggest that the original source of the Alto Diamantino basalts was a EM-I metasomatized mantle characterized by small amounts of different components (DMM and EM-II). Positive Ba and lack of Pb anomalies, together with K/Rb ratio, indicate the presence of amphibole in the mantle source of the Alto Diamantino basalts. Th/Nb and TiO 2 /Yb ratios suggests that the compositional difference between the L-Ti and H-Ti basalts could be explained by different degree of melting at different depths of a heterogeneously metasomatized mantle source. The H-Ti group of basalts is compatible with low degrees of partial melting at depth while the L-Ti suite records larger degrees of melting. The metasomatic event occurred after the Mesoproterozoic, possibly during the Neoproterozoic formation of the Gondwana supercontinent. • Alto Diamantino basalts are northernmost part of Paraná Magmatic Province. • Low- and High-Ti basalts have similar geochemical affinity but different evolution. • Magmatic series are coeval between 134.4 ± 1.6 and 132.9 ± 2.0 Ma. • Mantle source was refertilized after Mesoproterozoic subduction by hydrous melts. • L- and H-Ti basalts derived from different melting degree of a heterogeneous source.

Mafic dikes of the Mariinsky Taiga Alkaline Province, Kuznetsk Alatau terrane, southwestern Siberia: Intraplate alkaline magmatism in the Central Asian Orogenic Belt

The Mariinsky Taiga Alkaline Province (MTAP) is located in the Kuznetsk Alatau terrane of southwestern Siberia, which is part of Altai-Sayan orogenic system, of the Central Asian Orogenic Belt. The MTAP consists of a wide distribution of Paleozoic alkaline-basite magmatism (volcanic, subvolcanic and intrusive), which has been recently characterized with the recognition of at least two main timings, ca. 500 and ca. 400 Ma (herein labeled MTAP-1 and MTAP-2, respectively). We newly characterize the petrology, geochemistry, and age constraints of NNW and NNE trending alkaline dikes which cross-cut the alkaline intrusions and host volcanogenic and carbonate units and which are inferred to belong to MTAP-2. The dikes consist of ultrabasic foidolites (urtites, urtite-porphyry, ijolite-porphyry, microijolites), basic foidolites (plagioclase ijolites), alkaline basites (gabbro-diabase, labradorite porphyrites, camptonites and tamaraites), and nepheline, analcime and alkaline syenites. Their chemical composition is characterized by a wide range in silica (SiO 2 = 38.7–60.7 wt%), and alkalinity (Na 2 O + K 2 O = 4–18.7 wt%; Na 2 O/K 2 O = 0.5–8.3 wt%), low titanium (TiO 2 = 0.07–2.15 wt%) and high alumina content (Al 2 O 3 = 11.1–28.5 wt%), which corresponds to the K-Na differentiates of the basic alkaline magmatic series. The sum of rare-earth elements (ΣREE) increases from alkaline basite (ΣREE = 103.58–218.94 ppm), to the main foidolites (ΣREE = 137.74–217.61 ppm), and then to the ultrabasic foidolites (ΣREE = 55.56–268.88 ppm). Maximum REE content occurs in nepheline syenites and microsyenites (ΣREE = 73.24–1566.96 ppm) and they are the final product of the differentiation of foidite and basite melts. All dike varieties are characterized by low concentrations of the most highly charged elements (Th, Nb, Ta, Hf, and Y), which, according to their concentrations, plot between oceanic island basalt (OIB) and island arc basalt (IAB). The presence of a negative Nb-Ta anomaly and the relative enrichment of Rb, Ba, Sr and U indicate probable interaction of a mantle plume melts with lithospheric mantle metasomatized in an earlier subduction event. The primary isotope ratio 87 Sr/ 86 Sr(t) = 0.705388–0.706025 and the range of ɛNd(t) from +3.5 to +6.53 in the rocks indicate interaction of mantle derived melts with crustal components. Sm-Nd and U-Pb dating of the alkaline-basite dikes matches the boundary of the Early-Middle Devonian (406–389 Ma) and indicates membership in MTAP-2, which can be linked to the regional ca. 400 Ma Altai-Sayan Large Igneous Province/Rift System. Both the NNW and NNE trending dikes are associated with, and probably fed from, nearby alkaline intrusions such as i.e. Kiya-Shaltyr pluton which are also ca. 400 Ma (part of MTAP-2), but are independent of older ca. 500 Ma intrusions (which belong to MTAP-1) such as the University pluton and which are suggested to belong to a older plume-generated regional event. • Characterization of NNW and NNE trending dike belts in the MTAP, western Siberia. • They formed in the Middle Paleozoic (406–389 Ma) in a plume-rifting event. • Isotopic parameters ɛSr(t) indicate a strong contamination by crustal components. • The dike represent intraplate magmas emplaced in a shear setting.

A reconnaissance study of potentially important scandium deposits associated with carbonatite and alkaline igneous complexes of the Permian Emeishan Large Igneous Province, SW China

• The ELIP in SW China has a component of carbonatite-alkaline complexes that were not previously well known. • Pyroxenites of these complexes are composed of clinopyroxene with high Sc contents and can form low grade but high tonnage of Sc ores. • Exploration Targets for Sc in pyroxenites in three representative complexes have ∼12,000 to ∼36,000 tonnes of Sc metal at grades of ∼30 to ∼70 ppm. • Carbonatite-alkaline complexes of the ELIP may become a potentially important Sc metallogenic province and may be targets for exploration of REEs . Economically viable Sc deposits in the world are scarce and new and alternative resources are now urgently needed to ensure future sustainable supplies. We report here an essentially unknown, yet potentially important, type of Sc deposit that is associated with carbonatite and alkaline igneous complexes of the Permian Emeishan Large Igneous Province (ELIP), SW China. The ELIP is well known for its variably mineralized intrusions and flood basalts. Associated alkaline igneous complexes are minor in volume but important as possible sources of Sc and other REEs. Several such complexes occur in the N-S-trending Kangdian belt, where they are typically composed of pyroxenite, gabbro, and nepheline syenite. Some complexes also contain jacupirangite, melteigite, ijolite, and urtite. We show that some of these complexes contain carbonatites. The alkaline mafic-ultramafic rocks typically belong to either sodic or potassic magmatic series with high alkali compositions (up to 15.4 wt% Na 2 O + K 2 O). Pyroxenites in these complexes are composed of clinopyroxene with variable amounts of apatite and magnetite and have 39 to 71 ppm Sc. The Sc concentrations of these rocks may be high enough to justify mining under the current prices of Sc, which is nearly 5 times that of gold. Our preliminary study indicates a minimum of ∼12,000 to ∼36,000 tonnes of Sc metal in three representative complexes. This type of mineralization may ultimately contribute to a reliable and sustainable Sc supply chain in the future. In addition to Sc in the pyroxenites, carbonatites in these complexes will also be potentially important targets for REE exploration.

Onset of long-lived silicic and alkaline magmatism in eastern North America preceded Central Atlantic Magmatic Province emplacement

Abstract The White Mountain magma series is the largest Mesozoic felsic igneous province on the eastern North American margin. Previous geochronology suggests that magmatism occurred over 50 m.y., with ages for the oldest units apparently coeval with the ca. 201 Ma Central Atlantic Magmatic Province, the flood basalt province associated with the end-Triassic mass extinction and the opening of the Atlantic Ocean. We use zircon U-Pb geochronology to show that emplacement of White Mountain magma series plutons was already underway at 207.5 Ma. The largest volcanic-plutonic complex, the White Mountain batholith, was emplaced episodically from ca. 198.5 Ma to ca. 180 Ma and is ~25 m.y. older than published ages suggest, and all samples we dated from the Moat Volcanics are between ca. 185 Ma and 180 Ma. The Moat Volcanics and the White Mountain batholith are broadly comagmatic, which constrains the age of a key Jurassic paleomagnetic pole. Our data indicate that a regional mantle thermal anomaly in eastern North America developed at least 5 m.y. prior to the main stage of Central Atlantic Magmatic Province flood basalt volcanism and suggest a geodynamic link between the White Mountain magma series and the Central Atlantic Magmatic Province.