Alkaline Magmatism Research Articles

The Cuyano proto-ocean between the Chilenia and Cuyania terranes: rifting and plume interaction during the Neoproterozoic – early Palaeozoic evolution of the SW Gondwana margin

AbstractThe Precordillera mafic–ultramafic belt (PMUB), located in central-western Argentina, comprises mafic and ultramafic bodies interlayered and/or in tectonic contact with marine siliciclastic units. Whole-rock, mineral geochemistry and Nd–Sr isotope analyses performed in magmatic rocks suggest a relatively different spatial and temporal evolution along the belt. The southern PMUB (south of 32° S) evolved as an intra-continental rifted margin with an enriched mid-ocean-ridge basalt (E-MORB) tholeiitic to alkaline magmatism, to a proto-ocean basin (the Cuyano proto-ocean) with tholeiitic normal-MORB geochemical signature. Based on neodymium model ages (TDM), the magmatic activity started during the late Neoproterozoic Era and continued into the early Palaeozoic Era. Instead, the northern PMUB (28–32° S) evolved as an intra-continental rifted margin with dominant tholeiitic E-MORB to continental flood basalt (CFB) magmatism during the early Palaeozoic Era. ϵNd values (+3.4 to +8.4), rare earth element trends and high-field-strength element systematics, together with an estimated potential mantle temperature of c. 50–100°C above ambient mantle, suggest the PMUB magmatism derived from an enriched mantle source related to the effect of a rising plume linked to the Iapetus Ocean opening. In particular, TDM estimations of 600–550 Ma agree with reported magmatism in central to southern Appalachians. The magmatism in the PMUB, and those registered in the Neoproterozoic Catoctin Formation and in the Southern Oklahoma Aulacogen in the conjugated Laurentian margin, seem to be contemporaneous, sharing a similar plume-enriched mantle source. In this context, the E-MORB signature identified along the PMUB can be described as a plume-distal ridge tectonic setting over an extended margin.

On the participation of natural salts in alkaline magmatism. Article 3. Genetic aspects of the model of salt-alkaline interactions

Research subject. An analysis of regional and global geological material characterizing the spatio-temporal relationships between alkaline magmatic and saline complexes allowed the author to propose and justify a new geological-genetic model of alkaline magmatism. This model considers saline complexes, located along the paths of the upward movement of deep magmas, as additional sources of alkaline and volatile components.Materials and methods. Three articles are devoted to the discussion and justification of this model. Two articles were devoted to geological aspects of the problem. The prerequisites and signs of the participation of ancient saline complexes in alkaline magmatism were characterized. It was shown that the presence of saline rocks in the deep zones of the earth's crust along the paths of the upward movement of deep magma flows is a geologically natural and common phenomenon. Natural alkaline-salt associations (spatio-temporal combinations of alkaline and salt objects) were indicated; their tectonic types were distinguished. A global overview of their different-age analogues (neo- and paleogeodynamic) was given.Results and discussion. The collected data made it possible to evaluate older (than magmas) salt-bearing complexes (deeply buried in the substrate) as a possible important and active participant in the ontogenesis of alkaline complexes, to give a positive assessment of the geological aspects of the “magma halocontamination” model and salt-magmatic interactions; to formulate the main geological-genetic provisions of this model.Conclusion. This article focuses on the discussion of the genetic aspects of the proposed model with an assessment of the probable role and significance of various halophilic components in the formation of alkaline magmas and their features. For this purpose, the similarity features in the spatial and quantitative distribution of halophilic and foydaphilic components in salt and alkaline rocks are considered; the probable role of various halophilic components in the formation of alkaline specialization of magmas, in the emergence of a rich set of unusual features of alkaline rocks (material, structural, morphological, etc.) is discussed. The probability of participation of the complex of paragenic (non-salt) members of the halophilic community (dolomites, anhydrites, black shales and associated ore components) in the interaction with hot magma is estimated. A comparative analysis of some basic provisions of the model under consideration with other geological-genetic models of alkaline petrogenesis is performed. The advantages of this model and its predictive capabilities are evaluated. A number of ideas have been proposed by the author for the first time, thus requiring further elucidation.

Reconstruction of primary alkaline magma composition from mineral archives: Decipher mantle metasomatism by carbonated sediment

Alkaline magmas play a key in understanding crust-mantle interaction and deep carbon recycling. Here, petrography and in situ mineral analyses of evolved alkaline rocks from the North China Craton are reported to decipher mantle metasomatism by recycled carbonated sediments via reconstruction of primary magma composition. The alkaline rocks are nepheline syenites for which new, consistent zircon and titanite UPb ages of ~235 Ma are presented. The rocks are highly evolved (SiO2 = 58.1–60.0 wt%) with negative anomalies for Sr, Ba, Ti, P and Eu, and relative depletion of MREE, indicating fractional crystallization of feldspar, apatite, titanite and amphibole. Core-rim variation of 87Sr/86Sr (0.7052–0.7092) of late-crystallized low Mg# clinopyroxene and feldspar are attributed to crustal assimilation.The origin of primary alkaline magma is recorded in early high Mg# (70–84) clinopyroxenes with weak Eu anomalies (Eu/Eu* = 0.94–1.29), suggesting that they crystallized from a little-differentiated mantle-derived magma (Mg# > 60). Their variable 87Sr/86Sr ratios (0.7031–0.7060) correlate positively with Sr/Y and (La/Yb)N and negatively with Ti/Eu ratios, but are not correlated with Mg# or Eu/Eu*. Moreover, the REE patterns of melt equilibrated with high 87Sr/86Sr clinopyroxene is similar to coeval alkaline rocks from adjacent areas. These features indicate that the primary alkaline magma was derived from the mixing of depleted mantle and enriched mantle metasomatized by carbonatite melts from subducted crustal materials. Low zircon εHf(t) values (−1.4 to −2.8) imply that the subducted crustal material was carbonated sediments. Calcite and CO2 inclusions occur in early-crystallized olivine, clinopyroxene and zircon, and amphibole and fluorapatite both reveal a volatile-rich (H2O, F and CO2) alkaline magma derived from mantle enriched by carbonate metasomatism. The mineral archives of alkaline rocks not only record mantle metasomatism by recycled carbonated sediments, but also fractional crystallization and assimilation during magma evolution.

B–Sr–Nd–Pb isotopic constraints on the origin of the Maoniuping alkaline syenite–carbonatite complex, SW China

The Maoniuping giant rare earth element deposit is genetically related to a Cenozoic alkaline syenite–carbonatite complex. However, the origin of the alkaline complex and the genesis of the Maoniuping deposit are still unclear. In this study, the origin of the Maoniuping complex was constrained through B–Sr–Nd–Pb isotopic data for fresh/unaltered alkaline syenites. Our results indicate that the Maoniuping alkaline syenite has arc-like trace element patterns with elevated initial 87Sr/86Sr (0.706040–0.706541), 206Pb/204Pb (18.00–18.32), 207Pb/204Pb (15.53–15.60), and 208Pb/204Pb (38.25–38.78) ratios, and relatively low εNd(t) values (–4.92 to –2.66), suggesting derivation by partial melting of fertile mantle source. It also has higher B contents (1.06–7.65 ppm) and heavier δ11B values (−5.38‰ to − 2.46‰) than the upper mantle, indicating modification of the mantle source by interaction with carbonatitic melts/fluids derived from marine sediments and altered oceanic crust within the subducting Indian slab. Our results, together with recently acquired seismic tomographic data, indicate that hybridisation in the mantle source was likely induced by subduction input of marine sediments to the mantle transition zone. Boron isotopes provide a novel indicator of the deep recycling of subducted crustal materials in the sources of alkaline magmas.

Combined In Situ Chemical and Sr Isotopic Compositions and U–Pb Ages of the Mushgai Khudag Alkaline Complex: Implications of Immiscibility, Fractionation, and Alteration

The Mushgai Khudag complex consists of numerous silicate volcanic-plutonic rocks including melanephelinites, theralites, trachytes, shonkinites, and syenites and also hosts numerous dykes and stocks of magnetite-apatite-enriched rocks and carbonatites. It hosts the second largest REE–Fe–P–F–Sr–Ba deposit in Mongolia, with REE mineralization associated with magnetite-apatite-enriched rocks and carbonatites. The bulk rock REE content of these two rock types varies from 21,929 to 70,852 ppm, which is much higher than that of syenites (716 ± 241 ppm). Among these, the altered magnetite-apatite-enriched rocks are characterized by the greatest level of REE enrichment (58,036 ± 13,313 ppm). Magmatic apatite from magnetite-apatite-enriched rocks is commonly euhedral with purple luminescence, and altered apatite displays variable purple to blue luminescence and shows fissures and hollows with deposition of fine-grained monazite aggregates. Most magmatic apatite within syenite is prismatic and displays oscillatory zoning with variable purple to yellow luminescence. Both magmatic and altered apatite from magnetite-apatite-enriched rocks were dated using in situ U–Pb dating and found to have ages of 139.7 ± 2.6 and 138.0 ± 1.3 Ma, respectively, which supports the presence of late Mesozoic alkaline magmatism. In situ 87Sr/86Sr ratios obtained for all types of apatite and calcite within carbonatite show limited variation (0.70572–0.70648), which indicates derivation from a common mantle source. All apatite displays steeply fractionated chondrite-normalized REE trends with significant LREE enrichment (46,066 ± 71,391 ppm) and high (La/Yb)N ratios ranging from 72.7 to 256. REE contents and (La/Yb)N values are highly variable among different apatite groups, even within the same apatite grains. The variable REE contents and patterns recorded by magmatic apatite from the core to the rim can be explained by the occurrence of melt differentiation and accompanying fractional crystallization. The Y/Ho ratios of altered apatite deviate from the chondritic values, which reflects alteration by hydrothermal fluids. Altered apatite contains a high level of REE (63,912 ± 31,785 ppm), which are coupled with increased sulfur and/or silica contents, suggesting that sulfate contributes to the mobility and incorporation of REEs into apatite during alteration. Moreover, altered apatite is characterized by higher Zr/Hf, Nb/Ta, and (La/Yb)N ratios (179 ± 48, 19.4 ± 10.3, 241 ± 40, respectively) and a lack of negative Eu anomalies compared with magmatic apatite. The distinct chemical features combined with consistent Sr isotopes and ages for magmatic and altered apatite suggest that pervasive hydrothermal alterations at Mushgai Khudag are most probably being induced by carbonatite-evolved fluids almost simultaneously after the alkaline magmatism.

Migration of Late Miocene to Quaternary alkaline magmatism at the Alpine-Pannonian transition area: Significance for coupling of Adria plate motion with the Alpine-Carpathian front

Within-plate migration of alkaline basaltic centers is generally related to translation of the lithosphere with a hot spot above a largely stationary mantle plume. Here, we report, for the first time, a hitherto unrecognized migration of small-sized Late Miocene to Early Quaternary alkali-basaltic volcanic centers at the transition from Eastern Alps to Pannonian Basin. The volcanic centers migrated along the South Burgenland High in a regular sequence over a 95 km distance from NNE to SSW between 11 Ma and 1.7 Ma. The basaltic magmatism was also associated with thinning of the crust and lithosphere and an increase of the thermal gradient. In detail, three stages of migration are recognized as follows: (i) Stage 1 with a 55 km SSW-towards shift of volcanism between 11 and 5 Ma; (ii) Stage 2 with a ~ 35 km-WSW shift and enlargement of the distribution of volcanic centers between ca. 5 and 3.5 Ma; and (iii) Stage 3 with a S-directed shift of ca. 25 km between 3.5 and 1.7 Ma. The Stage 1 shift of alkali-basaltic volcanism can be also observed in the Little Hungarian Plain and South Slovakia–Nógrád volcanic fields, and the Stage 2 in the Balaton-Bakony of the Pannonian Basin (north of Lake Balaton). We propose that this pattern and mechanism of migration of the volcanic centers along the South Burgenland High resulted from thermally induced progressive thinning of the lithosphere over a mantle plume underneath the ALCAPA block, which was moving from SSW to the NNE between 11 and 1.7 Ma. The migration was interrupted by a marked eastward shift between 5 and 3.5 Ma (Stage 2). The migration of volcanic centers correlates with orientation and timing of regional shortening phases and inversion of the Alpine-Carpathian-Pannonian system although the overall amount of shortening remains uncertain. The deformation events allow to correlate Adria motion with deformation along the frontal parts of Eastern Alps, and Western and Eastern Carpathians across the western Pannonian Basin.

Compositional Evolution of the Variscan Intra-Orogenic Extensional Magmatism in the Valencia del Ventoso Plutonic Complex, Ossa-Morena Zone (SW Iberia): A View from Amphibole Compositional Relationships

The Ossa-Morena Zone (OMZ), SW Iberia, has numerous Lower Carboniferous compositionally zoned plutons that formed in a Variscan intra-orogenic extensional setting. This magmatism shows a wide compositional variation comprising alkaline, transitional, and calc-alkaline suites. The calc-alkaline suite was produced by hybridization of alkaline magmas with felsic melts generated by crustal anatexis related to the intrusion of mafic magmas in the middle crust. In this work, we present a textural and mineralogical study of the Variscan Valencia del Ventoso main pluton from the OMZ to track the compositional evolution of magmas during hybridization using constraints from amphibole compositions and to determine the P-T conditions of emplacement using amphibole-based thermobarometry. This pluton exhibits reverse zoning with an inner facies containing alkaline dolerites, gabbros, and quartz diorites, an intermediate facies with transitional diorites, and an outer facies with calc-alkaline quartz diorites to monzogranites. Magmas from the intermediate and border facies crystallized under oxidizing conditions at relatively low temperatures (range: 640–760 °C) and ca. 280–300 MPa, implying near H2O-saturated conditions. These rock facies show mineralogical evidence of hybridization between alkaline to mildly alkalic and calc-alkaline magmas. The former is inferred from the occurrence of antecrysts of labradorite-andesine, high-Ti pargasite-hastingsite, and biotite with deficiency in tetrahedral-site occupancy, a distinctive feature of biotite from the inner facies alkaline dolerites. This contrasts with later crystallization from the calc-alkaline magma of andesine-oligoclase, low-Ti magnesiohornblende-edenite, and biotite with full tetrahedral-site occupancy. Constraints from amphibole-melt compositional relationships in antecrystic high-Ti amphibole suggest that the alkaline magmatic component could have a high- to ultra-K affinity.

Permian-Triassic magmatic evolution of granitoids from the southeastern Central Asian Orogenic Belt: Implications for accretion leading to collision

Many orogenic belts in the world exhibit accretionary and collisional orogenic phases to varying extents. How accretion evolves into collision of the Central Asian Orogenic Belt (CAOB), the largest Phanerozoic accretionary orogenic belt, is an intriguing question. In this paper, we present new U-Pb age, geochemical and isotopic data for Permian-Triassic granitoids from middle Inner Mongolia, Northern China in the southeastern CAOB, and delineate the magmatic transition from subduction to (soft) collision. The magmatic record of soft collision is identified and characterized by thickened lower crust-derived high Sr/Y granitoids with a sub-linear distribution along the Solonker suture zone. Granitoids from Early Permian to Late Permian became more enriched in whole-rock Nd and zircon Hf isotopic compositions (eNd(t) values from 2.4 to −19.5, eHf(t) values from 11.6 to −33.7), indicating increasing incorporation of old crust. The change in peak timing of magmatism from west (ca. 264 Ma) to east (ca. 251 Ma) along the Solonker suture zone implies “scissor-like” closure of the Paleo-Asian Ocean. Integrated with previous studies, a three-stage tectonic model from the Permian to Triassic by accretion leading to collision on the south-eastern margin of CAOB is proposed. (1) Early Permian (> ca. 285 Ma): Juvenile magmatism on an active continental margin with double-sided subduction of the Paleo-Asian Ocean; (2) Middle Permian to Middle Triassic (ca. 285–235 Ma): Magma source transition from juvenile to old crust induced by a tectonic switch from arc to “scissor-like” closure and subsequent intra-continental orogenic contraction; (3) Late Triassic (< ca. 235 Ma): A-type and alkaline magmatism in response to post-collisional extension.

Origin of Antecrysts in Igneous Rocks from the Salavat Range (NW Iran): an Explanation for the Geochemical Signature of Potassic Alkaline Rocks

AbstractAbundant silica-undersaturated potassic lavas are found in the centre of the Turkish–Iranian plateau (NW Iran) as flows, pillows and dykes. They display abundant zoned clinopyroxene macrocrysts and xenoliths of igneous cumulates. We determined four types of zoned crystals (Type-I, -II, -III and -IV) on the basis of their composition and zoning patterns. Use of in situ compositional data, together with whole-rock major and trace elements and the isotopic signatures of the host lavas provided evidence for the derivation of the different types of zoned clinopyroxenes from at least two contrasting parental melts. Our findings are consistent with an origin of the ultrapotassic and sodic alkaline melts from the deep-seated compaction pockets inferred from our previous studies of the alkaline magmatism throughout the Turkish–Iranian plateau. The ultrapotassic melt, which accumulated at the top of the compaction pockets, eventually ponded close to the spinel–garnet mantle transition and generated colourless antecrysts (Type-I and Type-II) and clinopyroxenite cumulates. When the compaction pocket impinged on the continental lithosphere, interstitial melts segregated and flowed inside dykes where grass green antecrysts (Type-III) and zoned phenocrysts (Type-IVa) crystallized from a melt having a geochemical signature of sodic alkaline melt. Later, at the crustal level, melt crystallization processes produced Type-IVb zoned phenocrysts. Our results are at odds with the paradigm of potassic magmas in NW Iran being derived strictly from a single mantle source.

Volcaniclastic deposit (tonstein) in the Balcomie Beds, Lower Carboniferous, East Fife

A volcanogenic clay bed (tonstein) has been identified in the Balcomie Beds of the Inverclyde Group near Crail, East Fife. Its chemical composition suggests an undersaturated alkaline magma source. This horizon may be contemporaneous with the early Carboniferous Garleton Hills trachytic lavas of East Lothian (346 Ma). This would make it the earliest expression of Carboniferous volcanism preserved in Fife, and also the earliest occurrence of a tonstein in Fife.

Multistage gold mineralization in the Hadamengou gold deposit in the northern margin of the North China Craton: Insights from in-situ trace element contents and sulfur isotope analyses of pyrite

Abstract The Hadamengou gold deposit is one of the largest gold deposits in the Wulashan-Daqingshan gold mineralization cluster, in the northern margin of the North China Craton. The gold deposit, which is hosted by Archean gneiss, comprises potassic alteration and abundant quartz-pyrite veins, with five types of hydrothermal pyrite associated with gold, tellurides, and sulfides. The earliest stage of pyrite (Py1) occurs as random disseminations within the limited volume of modified quartz-pyrite veins and was formed during or prior to the last episode of metamorphism. Py2, from the potassic and chlorite-sericite alteration zones, is related to the activity of potassic fluids, and contains inclusions of native gold, galena, melonite, electrum, and petzite. Py3, without inclusions, occurs as randomly- disseminated grains within the limited volume of grey quartz-pyrite veins. A few native gold grains are found along fractures of quartz associated with Py3. Py4 occurs as veinlets or aggregates in the milky quartz-pyrite veins and contains the highest Au concentrations. Abundant native gold, calaverite, and electrum occur along the grain boundaries and fractures, or as inclusions within Py4 and quartz. Py5 occurs as rounded and cataclastic shapes within the limited volume of quartz-carbonate veins and is depleted in Au. LA-ICP-MS trace element analysis of the pyrite shows that Py1 has a low content of gold (median of 0.07 ppm) and other trace elements (Ag, Te, Pb, Cu, Bi, and As). Potassic alteration, grey quartz-pyrite veins, and milky quartz-pyrite veins make a very large contribution to the gold endowment, with a relatively high gold content in pyrite, including Py2 (median of 0.8 ppm), Py3 (4.3 ppm), and Py4 (2.5 ppm). Other trace elements (e.g., Ag, Te, Pb, Mo, and Bi) are also significantly enriched within Py2 to Py4. Little or no gold was detected in Py5. Thus the potassic alteration and milky quartz-pyrite veins were major contributors to gold mineralization, based on the volume and Au content of pyrite within the Hadamengou deposit. In-situ LA-MC-ICP-MS sulfur isotope analyses of pyrite show negative values and a narrow range of variation of δ34S (averages of −11.4‰, −11.8‰, −10.5‰, and −9.4‰ for Py2-Py5, respectively), indicating an oxidized magmatic sulfur source for the pyrites in Hadamengou. Additionally, the occurrence of tellurium minerals associated with various pyrites, and the high Te concentration of various pyrites, also suggest that ore-forming fluids were derived from an oxidized alkaline magma. These results, combined with published geochronological data, suggest that during two episodes, Au-rich potassic and SiO2-rich fluids separated from the deep-lying Devonian alkaline magma during ascent along the EW-trending faults, and reacted together with the wall rocks to form potassic and chlorite-sericite alteration zones, resulting in gold precipitation.

Subduction: The recycling engine room for global metallogeny

The formation of ore deposits, within mineral systems that represent highly anomalous concentrations of economic elements in localized parts of the Earth’s crust, first requires an enriched and available source of those elements. In early Earth history, initial enrichment in the upper mantle was arguably through mantle plumes that even recycled metals from the core-mantle boundary. From the Mesoarchean onwards, the development of plate tectonics allowed subduction processes to be increasingly important in both recycling of elements from the upper mantle into the lower crust and from there into the upper crust where mineral deposits could form and be exploited. The most obvious expression of this recycling process is the direct association of porphyry-high-sulfidation epithermal-skarn Cu-Au-Ag +/- Mo systems and inboard fractionated granite-associated Sn-W systems with magmatic-hydrothermal fluids derived from subduction-related magmas in convergent margins, where epithermal Au-Ag systems also developed due to high thermal gradients. Syn-subduction VMS Cu-Zn-Pb(Au) systems were accreted at convergent margins. Most orogenic gold systems relate to fluids released through devolatilization of down-going subduction slabs and overlying metal-rich sediment wedges. More indirect associations relate to the ability of subduction zones to cause structural damage and metasomatize and fertilize sub-continental mantle lithosphere along the margins of bordering craton margins. Orogen collapse in the final stages of the subduction-related orogenic event led to asthenosphere upwelling and formation of proximal intrusion-related and more distal Carlin-type gold systems in continental basins on the craton margins. During subsequent orogenic events, previously metasomatized and fertilized mantle lithosphere was reactivated to release hydrothermal fluids to form many of the orogenic gold systems in eastern China. Reactivation of the metasomatized lithosphere also resulted in development of post-subduction hybrid or alkaline magmas that generated giant magmatic Kiruna-type Fe-P, carbonatite-related REE-Nb or Cu-P, and lamproite-related diamond systems, together with magmatic-hydrothermal IOCG mineral systems near craton margins. Subduction-damaged craton margins and their rift-related marginal basins became the sites of formation of Zambian Copper Belt-type deposits and the craton margins were reactivated during subsequent rifting allowing intrusion of giant Ni-Cu mafic–ultramafic bodies along lithosphere-tapping fault zones at those margins. Thus, subduction acts as the recycling engine room or factory responsible for many of the global giant mineral systems that are currently exploited.

Paleoproterozoic metaluminous syenites synchronous with the c. 2.21 Ga mafic dyke swarms from the Eastern Dharwar Craton, India: implications for alkaline magmatism associated with the breakup of supercraton Superia

Abstract The Eastern Dharwar Craton (EDC) hosts numerous Late Neoarchean to Paleoproterozoic syenites whose genesis remains unclear. Here we present a petrological and geochemical study on the syenites from Peddavaduguru, Danduvaripalle and Vannedoddi, towards the western margin of the Paleo-Mesoproterozoic Cuddapah Basin in the EDC. These syenite bodies constitute a part of the Paleoproterozoic (2210 ± 110 Ma) Dancherla alkaline complex and are contemporaneous with a mafic dyke swarm emplacement in the EDC. The Danduvaripalle and Vannedoddi syenites display geochemical affinities to adakites. The Peddavaduguru syenite shows a negative Eu anomaly and relatively gentle chondrite-normalized rare earth element (REE) pattern, suggesting an origin by a different process. The ɛ Nd values range between 0.5 and 0.8 for the Peddavaduguru syenite, − 5.2 and − 4.2 for the Danduvaripalle syenite, and − 6.0 and − 1.0 for the Vannedoddi syenite. In tectonic discrimination diagrams, the Peddavaduguru syenite shows affinities to within-plate granitoids similar to syenites from the Deccan large igneous province whereas the others show geochemical similarities to arc-related alkaline rocks and volcanic arc granitoids. The adakitic syenites show deficiency of MgO, Ni, Cr and Sc, highly fractionated REE patterns and negative Nb–Ta–Ti anomalies along with low Nb/U and high Th/U ratios. These geochemical traits are compatible with their origin by the partial melting of a mafic crustal source that subsequently underwent fractionation of amphibole and garnet responsible to impart an adakitic character. We suggest that the crustal source of these syenites underwent partial melting by heat from the rising plume during the Paleoproterozoic extension of the Superia supercraton. The non-adakitic Peddavaduguru syenite, on the other hand, is suggested to have originated from direct fractionation of mafic magma.

Transformation from oxidized to reduced alkaline magmas in the northern North China Craton

The oxygen fugacity of a magma controlling element distribution and mobility is influenced by multiple factors including source composition and magmatic processes (e.g., assimilation and degassing). Here, whole-rock and in situ mineral analyses of the alkaline complex from the northern North China Craton are reported to investigate the influence of source composition and magmatic processes on the evolution of the melt oxidation state. The alkaline complex comprises orthoclase pyroxenites, pyroxene syenites, and syenites with similar mineral assemblages and trace element patterns. Consistent zircon and titanite U-Pb age of ~234 Ma is presented for the alkaline rocks. Their high large ion lithophile elements content, negative high field strength element anomalies, and high 87Sr/86Sr ratio suggest a metasomatized mantle origin by recycled crustal material. Magnetite and hematite inclusions were found in the early-crystallized minerals including clinopyroxene and apatite. Fluorapatites are characterized by high S (SO3 up to 0.64 wt%) and low Mn contents, with estimated ΔFMQ higher than +3. These features together indicate a primary oxidized magma inherited from the metasomatized mantle.However, inclusion-rich fluorapatites with decreasing S and increasing Mn content from the core to the rim were found in some pyroxene syenites. Moreover, the S content of inclusion-rich fluorapatite from these pyroxene syenites is lower than that of other rock types in the complex, which may correspond to a decrease in oxygen fugacity of ~2.5 log units. These features suggest that the oxidized magma was later transformed into reduced magma. Oriented Cu-rich sulfide in inclusion-rich fluorapatite formed during the reducing event. δ65Cu of the pyroxene syenite with inclusion-rich fluorapatite (−0.06 to −0.24‰) and sulfides in fluorapatite (−0.26‰) are lighter than those of other rock types (+0.30 to +0.43‰), suggesting that the sulfides were derived from extraneous materials via magma assimilation. Variable and high 87Sr/86Sr ratios (0.7071–0.7085) in titanites and high-207Pb/206Pb rims of zoned K-feldspar in the pyroxene syenite imply that the reduced extraneous materials were derived from ancient crust. Overall, in situ mineral analyses and Cu isotopes indicate a direct link between the transformation of alkaline magma from an oxidized to a reduced state with source composition and magmatic processes, and improve our understanding of element mobility and distribution.

Intrusion-Associated Gold Systems and Multistage Metallogenic Processes in the Neoarchean Abitibi Greenstone Belt

In gold-endowed greenstone belts, ore bodies generally correspond to orogenic gold systems (OGS) formed during the main deformation stage that led to craton stabilization (syntectonic period). Most OGS deposits postdate and locally overprint magmatic-hydrothermal systems, such as Au-Cu porphyry that mostly formed during the main magmatic stage (synvolcanic period) and polymetallic intrusion-related gold systems (IRGS) of the syntectonic period. Porphyries are associated with tonalite-dominated and sanukitoid plutons, whereas most IRGS are related to alkaline magmatism. As reviewed here, most intrusion-associated mineralization in the Abitibi greenstone belt is the result of complex and local multistage metallogenic processes. A new classification is proposed that includes (1) OGS and OGS-like deposits dominated by metamorphic and magmatic fluids, respectively; (2) porphyry and IRGS that may contain gold remobilized during subsequent deformation episodes; (3) porphyry and IRGS that are overprinted by OGS. Both OGS and OGS-like deposits are associated with crustal-scale faults and display similar gold-deposition mechanisms. The main difference is that magmatic fluid input may increase the oxidation state and CO2 content of the mineralizing fluid for OGS-like deposits, while OGS are characterized by the circulation of reduced metamorphic fluids. For porphyry and IRGS, mineralizing fluids and metals have a magmatic origin. Porphyries are defined as base metal and gold-bearing deposits associated with large-volume intrusions, while IRGS are gold deposits that may display a polymetallic signature and that can be associated with small-volume syntectonic intrusions. Some porphyry, such as the Côté Gold deposit, demonstrate that magmatic systems can generate economically significant gold mineralization. In addition, many deposits display evidence of multistage processes and correspond to gold-bearing or gold-barren magmatic-hydrothermal systems overprinted by OGS or by gold-barren metamorphic fluids. In most cases, the source of gold remains debated. Whether magmatic activity was essential or marginal for fertilizing the upper crust during the Neoarchean remains a major topic for future research, and petrogenetic investigations may be paramount for distinguishing gold-endowed from barren greenstone belts.

Microscale textural heterogeneity and tip-streaming instability in alkaline magmas: Evidence in tube pumices from Campi Flegrei, Italy

We present a textural investigation of compositionally homogeneous alkaline tube pumices from the plinian fallouts of the Agnano Monte Spina eruption (ca. 4.1 ka), Campi Flegrei, Italy. The pumice clasts present strong textural complexity from hand specimen to microscopic scale of inspection. The clasts are characterized by large vesicle tubes that extend up to several millimetres along the clast main axis. They also exhibit small and elongated vesicles arranged in stretched bands around textural domains of larger, sub-spherical vesicles associated with pyroxene, feldspar, and biotite phenocrysts. Clasts, vesicle tubes, vesicle deformation, major axes of pyroxene phenocrysts and crystals with size ≤100 μm align in the same direction. In addition, we observe chains of gradually smaller stretched vesicles likely generated from break-up of a larger parent vesicle (i.e., tip-streaming phenomenon). We use image analysis to quantify the heterogeneity of vesicle sizes and shapes. Then, we deduce the Capillary number (Ca) of the each single vesicle, which represents the ratio between the deforming viscous stress on a bubble and the restoring stress supplied by surface tension, and obtain the spatial distribution of Ca values within pumice clasts. As Ca depends on strain-rate and viscosity, the spatial distribution of Ca values provides us with a snapshot of the sub-millimetre scale variation in magma strain-rate and rheology before fragmentation. Ca in highly stretched vesicles is close to values (0.45 to 0.6) required for bubble-breakup induced by tip-streaming. Instead, low Ca values (0.3 to 0.45) are calculated for large sub-spherical vesicles mostly distributed around phenocrysts. We suggest that sub-millimetre heterogeneities in vesicle sizes and shapes recorded in tube pumices are caused by a combination of tip-streaming, heterogeneous bubble nucleation and second-boiling processes prior to fragmentation. The heterogeneous spatial distribution of Ca values may be the result of local micrometric shear localisation and consequent deformation upon break-up experienced by magma while ascending in the conduit. This deformation suggests that a variation of magma strain-rate and viscosity occurred before fragmentation at the clast microscale and this is likely to affect magma dynamics and fragmentation. Our results claim for future experimental and numerical modelling studies that investigate and consider the effect of such micrometric heterogeneities on magma rheology and flow condition variations inside volcanic conduits.

Petrogenesis of Proterozoic alkaline ultramafic rocks in the Yilgarn Craton, Western Australia

The Yilgarn Craton and its northern margin contain a variety of petrogenetically poorly defined small-volume alkaline ultramafic rocks of Proterozoic age. This study documents the petrography, mineral and bulk-rock geochemistry and Nd-Hf-Sr-Pb isotope compositions of a selected suite of these rocks. They comprise ~2.03–2.06 Ga ultramafic lamprophyres (UML) and carbonatites from the Eastern Goldfields Superterrane (EGS), ~0.86 Ga UML from Norseman, and orangeites from the Earaheedy Basin, including samples from Jewill (~1.3 Ga), Bulljah (~1.4 Ga) and Nabberu (~1.8–1.9 Ga). The Proterozoic UML and carbonatites from the EGS and Norseman display very consistent chondritic to superchondritic Nd-Hf isotope compositions and trace-element ratios similar to modern OIBs, which are indicative of a common mantle source across this wide alkaline province. These Nd-Hf isotope compositions overlap with the evolution trends of global kimberlites through time, thus suggesting that this mantle source could be deep and ancient as that proposed for kimberlites. Conversely, the orangeites located in the Earaheedy Basin along the northern margin of the Yilgarn Craton display trace element signatures similar to subduction-related calc-alkaline magmas. Taken together with their highly enriched Sr-Nd-Hf isotope compositions, these characteristics indicate an ancient lithospheric mantle source, which was probably metasomatised by subduction-related fluids. As the ages of the Bulljah and Jewill orangeites overlap with the breakup of the Columbia supercontinent, it is proposed that orangeite magmatism was triggered by changes in plate stress conditions associated with this event. This study provides a comprehensive picture of the genesis of Proterozoic alkaline magmatism in the Yilgarn Craton, highlighting the complex tectono-magmatic evolution of this lithospheric block after its assembly in the Archean.

Nepheline syenite and related rocks at Meruda Takkar hill, northern Kachchh: Neoproterozoic Malani basement or Mesozoic alkaline magmatism?

An extremely small but significant exposure of possibly Precambrian or Pre-Deccan Mesozoic plutonic rocks spanning 200 × 90 m in size with 15 m elevation, remotely located amidst vast salt flats of the Great Rann of Kachchh received attention when it was first reported by Biswas in 1968; however, systematic petrological and petrographical studies remained undone. Therefore, in the present study, we addressed complete geological accounts to establish its possible origin. Our detailed geological study indicates that the complex is made up of nepheline and aegirine bearing alkali feldspar syenite with numerous fine-to-medium grained felsic dykes. It lays two viewpoints: (1) it has affinity with Trans Aravalli Belt anorogenic felsic magmatism, i.e., Precambrian and (2) it is genetically related to the pre-Deccan Mesozoic plutonic intrusives like Nir Wandh in Pachham Island and Mundwara and Sarnu–Dandali complexes in Rajasthan. The nearest felsic magmatic suite is an exposure of gray and pink granites of Nagar Parkar of Neoproterozoic age. Presence of undersaturated minerals and xenoliths of diorites within syenite parent body may suggest partial melting of lower crust probably during Rodinia supercontinent rifting. However, presence of alkali-rich minerals, alkali feldspars, kaersutite, aegirine, apatite and rutile needles and the complex being part of the Kachchh rift basin suggests its genetic resemblance with Pre-Deccan Mesozoic plutonic events. The radiometric age will confirm the age of this plutonic massif. Based on the field geological and petrographical studies of Meruda Takkar syenitic complex, two viewpoints have been raised: Radiometric age determination and geochemical analysis of the Meruda syenite are needed to establish its age and origin.

Features of Eudialyte Ore Formation in High-Alkaline Magmas of the Lovozero Deposit (Kola Peninsula)

Detailed studies have shown that a change in the eudialyte occurrence forms (and the moment of its crystallization) is a new geochemical criterion for rare metal ore content in alkaline magmas (eudialyte ores). A new principle of the presence of ores in alkaline magmas has been formulated: a prerequisite for the formation of an ore deposit is early saturation of alkaline magmas with an ore mineral. If the ore component concentration is significantly lower than the cotectic (saturation) one, then melt saturation and crystallization of an ore mineral will take place at later stages of rock formation in a small volume of the interstitial melt, when the phenomena of convective–gravity differentiation and segregation of mineral phases in the form of ore deposits are hampered. This leads to dispersion of the ore components in the form of xenomorphic grains of accessory minerals. Rocks of the differentiated complex (lower zone of the Lovozero deposit) and rocks of the Khibiny massif contain xenomorphic eudialyte and are not promising for eudialyte ores. Eudialyte deposits are associated with the upper zone of the Lovozero intrusion where euhedral early eudialyte occurs. The initial magma is saturated with eudialyte after crystallization of about 85% of the intrusion. The proposed criterion is applicable to the largest alkaline massifs in the world. The Ilimaussaq massif (Greenland), the rocks of which contain early crystallized euhedral eudialyte, hosts a superlarge eudialyte ore deposit. Unlike the Khibiny massif and the Pilanesberg alkaline complex, the rocks of which contain late xenomorphic eudialyte, this massif has no deposits of this type.

Paleoproterozoic metaluminous syenites synchronous with the c. 2.21 Ga mafic dyke swarms from the Eastern Dharwar Craton, India: implications for alkaline magmatism associated with the breakup of supercraton Superia

Paleoproterozoic metaluminous syenites synchronous with the c. 2.21 Ga mafic dyke swarms from the Eastern Dharwar Craton, India: implications for alkaline magmatism associated with the breakup of supercraton Superia