Alkaline Magmatism Research Articles

SOME REGULARITIES OF THE INTERRELATION OF THE GENESIS AND MINERALS DISTRIBUTION IN THE BOWELS

Some regularities of the interrelation of the genesis and minerals distribution in the bowels based on the analysis of information on the temporal and spatial distribution of minerals in geological complexes, primarily in Ukraine, were revealed. The distribution of minerals in magmatic complexes, pegmatites, hydrothermalites and metamorphites was studied. The relationship between tectonics and the distribution of minerals is noted. There is a clear direction of the geological development of the earth's crust: the pacification of tectonic processes — the growth of platforms — the differentiation of mineral matter. The number of formed mineral species increased rapidly from Archean to Phanerozoic complexes, from "basaltic" to "crustal" mineral formation, from ultrabasic rocks to acid ones. The Pre-Greenstone crust of Ukrainian Shield (USh) is predominantly represented by plagioclases and pyroxenes; with the development of granitoids, quartz and alkali feldspars joined them. From early to late stages of USh development, the number of species increased by an order of magnitude. Near-Azov megablock is in the first place. Maximum species formation is associated with alkaline magmatism and processes involving volatile components, in particular pegmatite formation. The number of minerals in pegmatites reaches hundreds of species. Mountain building led to the destruction of igneous rocks and the formation of new minerals. The appearance of free oxygen became a powerful factor in mineral formation. Superimposed processes with the supplying of deep fluids contributed to the transformation and redistribution of minerals and the formation of polygenic ores. The distribution of minerals makes it possible to detect typomorphic species for certain processes, which can be used to determine the criteria of mineralization, its scale, and the erosion section of ore bodies. The distribution of various mineral species, and the same species with identified macro- and microdefects, as a result of the conditions of mineral formation, is of practical importance.

Reappraisal of Duration and Eruptive Rates in Deccan Volcanic Province, India

Abstract Rapidly expanding geochronological, paleomagnetic and volcanological data of the Deccan Volcanic Province (DVP) has given new insights to the expansive knowledge on it that had been built up through the preceding decades. Precursory Late Cretaceous (80 – 68 Ma) magmatic activity across the Indian subcontinent preceded the main event of volcanism. The main Deccan event involved nearly continuous outpourings of ~1.5 million km3 of flood basalts in a span of less than 2 m.y. on either side of the Cretaceous-Paleocene Boundary (KPB), corresponding to 3 magnetic chrons. This was followed by basaltic and silicic flows and alkaline magmatism that continued up to ~61 Ma. Several earlier postulated long-distance correlations of lava sequences based on chemical parameters are rendered invalid by the precision geochronology, leading to doubts regarding earlier models of volumetric eruptive rates. We suggest that pending better flow-field maps and correlations across the province, stacking rates based on thickness between chronological sampling points, combined with volcanological parameters provide a more robust and reproducible estimates of the eruptive rates. The stacking rates in different parts of the province were largely in the magnitude range of 103 m/m.y. and are comparable to those observed in other continental flood basalt provinces.

Xenoliths Evidence of Alkaline Magmatic Infiltrations Beneath Lake Nyos (Cameroon Volcanic Line, West Africa)

Xenoliths enclosed in Lavas of the Nyos volcano (Cameroon Volcanic Line, continental sector) range from fertile lherzolites to harzburgites. One spinel-free wehrlite has been also sampled. The occurrence of phlogopites and pargasites in some harzburgites together with specific textural rock-type (lherzolites transitional porphyroclastic to equigranular), including major and trace element compositions both in peridotites bulk rocks and minerals point out interactions between the mantle and basaltic magmas responsible for the formation of wehrlites beneath the Nyos volcano. Hydrous minerals (phlogopites and pargasites) and metasomatic events are their main petrogeochemical signatures different from group 1 samples which are characterized by spoon-shaped REE patterns. Later on, hydrous phases, Ti-rich Cpx, CaO rich Ol, Ti, and V rich Ol wehrlite precipitated from melt enrichments due to the percolation of the mantle by basaltic magmas of alkaline affinity. The metasomatic liquid which percolates the Nyos mantle column was a dense alkaline silicate rich in volatile, displaying low HFSE abundances in the metasomatic hydrous melts compared to the LILE. It is suggested that Nyos mantle peridotites have experienced: 1) variable metasomatic events related to the percolating of the depleted mantle by a alkaline silicate liquid, 2) the spinel-free wehrlite is a group 2 sample corresponding to a cumulate of a similar melt, 3) amphibole may be a potassium-bearing mineral instead of or in addition to phlogopite at shallower levels of Nyos upper mantle and 4) transitional textural rock facies express also the fingerprint of rising mantle plume which were percolated by alkaline magma during their transit to the surface.

Plio-Quaternary Volcanism in Northeastern Morocco: Petrography and Geochemistry of Outcrops with High Geothermal Potential

Cenozoic volcanic activity in Morocco started in the Miocene and proceeded, after a short break, during Pliocene and Quaternary. Calc-alkaline magmatic activity occurred first and was accompanied by the eruption of transitional lavas, followed by Plio-Quaternary alkali basaltic activity, which was emplaced within Neogene sedimentary basins of the Rif belt and its foreland. From calc-alkaline to alkaline magmatism, passing through a transitional term, volcanic activities in Morocco formed magnificent outcrops, very diversified in their affinities, their mineral compositions and their geochemistry. The petrographic study and geochemistry of Plio-quaternary volcanic rocks in the areas with high geothermal potential in northeast of Morocco show a dominance of Na-rich basaltic rocks (basanites, basalts, tephrites and basaltic trachy-andesites), followed by trachy-andesites. Most of the samples exhibit an enrichment of large-ion lithophile elements (LILEs) over high-field-strength elements (HFSEs). These results were used to test the global model of Circum-Mediterranean Anorogenic Cenozoic Igneous Province (CiMACI) and the regional model of [1].

Геология и геохимия щелочно-сиенитовых массивов Сыдо-Ербинской впадины (Минусинский прогиб)

This research work is devoted to the study of the chemical composition of the alkalinesyenite massifs of the Sydo-Erba depression based on the available geological and geochemical data. For about 100 years, the massifs have attracted the attention of various scientists not only from the practical point of view, as possible sources of aluminum raw materials, but also as objects for research, in particular, to determine the shape of the bodies of nepheline syenites occurring in them and models of the formation of alkaline magmas. The studied intrusions are composed mainly of syenites and alkaline syenites, as well as a few bodies of nepheline syenites. The paper will present and compare the geochemical data of alkaline nepheline syenites (foyaites) and nepheline syenites (tensbergites), which compose such massifs as Saibarsky, Burovsky, Vysokoy, Kosmatoy, in order to identify the influence of granite-diorite complexes on their composition. To achieve this goal, it is supposed to study the geological and petrographic features of the structure of the massifs, as well as to analyze the chemical composition of rocks of geochemical data presented in the form of contents of petrogenic and rare elements.

The origin of uranium deposits related to the Huangmeijian A-type granite from the Lu-Zong volcanic basin, South China: Constraints from zircon U-Pb geochronology and mineral chemistry

Although most of granite-related uranium deposits in South China are spatially and genetically associated with S-type granites, few are related to A-type granites. In this study, we present whole-rock major and trace elements, zircon U-Pb geochronology, and mineral chemistry data for the Huangmeijiang A-type granite in the Lu-Zong volcanic basin (South China) to investigate the origin of uranium deposits related to this pluton. The uranium ore bodies mainly occur within the Jurassic Luoling formation close to the Huangmeijian pluton. Whole-rock and biotite geochemistry indicates that the F-rich alkaline magmas with relatively high fO2 prompted the partitioning of U into the fluids released from the Huangmeijian pluton with an affinity of A-type granites, which would set the stage for the later concentration of the U into the Jurassic Luoling formation. Thorite, zircon, apatite, and bastnäsite are the dominant accessory minerals that host U in this pluton. Backscattered electron (BSE) images suggest that some zircon and thorite grains can be divided into the unaltered domains and altered domains. The textural features are interpreted as a later hydrothermal event superimposed on the magmatic zircon and thorite. In situ LA-ICP-MS U-Pb dating on the unaltered domains and altered domains of zircon grains yielded weighted mean ages of 127.8 ± 1.0 Ma and 65.7 ± 5.3 Ma, respectively. The former age represents the emplacement age of the Huangmeijian pluton, and the latter one overlaps the timing of a uranium mineralization event (∼66.6 Ma) in this area. The altered domains of thorite and high-(U + Th) zircon grains have lower U contents than those of the unaltered domains, indicating that uranium was mobilized from thorite and zircon during fluid infiltration, which might be the main source of uranium for the ∼ 66.6 Ma uranium mineralization. Our study suggests that the Huangmeijian pluton was the primary source of uranium for the formation of uranium deposits in this area.

Melt Percolation, Concentration and Dyking in the Hawaiian Mantle Plume and Overriding Lithosphere: Links to the Evolution of Lava Composition along the Volcanic Chain

Abstract Oceanic island basalts and related magmatic rocks from Hawaii are derived from a compositionally heterogeneous mantle plume. Here we describe how this heterogeneity results from the transport of filaments of a specific composition in the plume, representing a relatively small volume of rocks (~15 %) interbedded inside a dry peridotite mantle. Four types of filaments are considered: sub-primitive mantle, ultralow-velocity zone, fertilized-harzburgite and eclogite type filaments. We present a model that describes the flow within a plume and the stress field in the overriding viscoelastic lithosphere and that can determine, from depth to the surface, the melting rate, composition and trajectory of melts produced within each type of filament. Our model shows that (1) the filaments melt at a depth corresponding to >5 GPa, where the temperature gap between the solidus and liquidus is narrow (~40–80 °C), and (2) the volume of filaments is small relative to the total volume of mantle, which therefore allows the latent heat required for the partial melting to be provided via conduction inside the hot plume. The primitive melts produced inside the filaments, occasionally mixed with the melt derived from an eclogite filament, represent a volume comparable with that expected in a plume composed only of dry peridotite that partially melts to a degree of ~10 % at the interface between the spinel and garnet fields (60–70 km depth). In particular, in the centre of the plume, sub-primitive mantle filaments produce up to 30 % tholeiite–picrite melts, whereas in fertilized-harzburgite filaments, the mantle melts completely to produce a melt having a meimechite-like composition. A key finding is that the fractional crystallization of these melts probably forms the so-called ‘primary mantle-derived alkaline magmas’ along with dunites and olivine-rich cumulates. Our plume model shows that the mantle flow divides into two parts. The first corresponds to hot flowlines that originate at a depth of ~200 km and at a distance of less than 25 km from the plume axis. Along these flowlines, when the mantle reaches a pressure of 5 GPa, the partially molten horizon in filaments is sufficiently thick for the interstitial melt to be squeezed out via dykes. This melt eventually ponds as sills in a subrectangular zone that is located inside the overlying lithosphere, between 70 and 50 km depth and centred over a distance of less than 40 km on either side of the axis. This zone is designated as the shield magmatic reservoir. The volatile-rich melt inside the sills infiltrates the surrounding mantle lithosphere and partially melts it. After ~0·1 Myr, the melt resumes its vertical ascent via dykes and eventually ponds and differentiates within subcrustal magma chambers located below active shield volcanoes. This sequence of processes matches the expected volume, petrology and geochemistry recorded for shield volcanoes. The second part of the melt flow does not pond within the shield magmatic reservoir. Rather, the mantle cold flowlines, originating at ~200 km depth and at 25–35 km from the plume axis, discharge their interstitial melt through dykes that were initially generated deeper, at ~5 GPa. The melt reaches the Moho at 100–150 km from the plume axis, where it forms magmatic bodies within which the melt differentiates. This melt probably represents that observed in pre- and postshield volcanoes. Finally, at ~70 km from the plume axis and at a depth greater than 200 km, the flowlines are subvertical. They then deflect at ~180 km depth and rotate toward the horizontal and eventually transit at 10–20° to the horizontal across an ~200 km distance from the axis and reach ~140 km depth. The fertilized-harzburgite and sub-primitive mantle/ultralow-velocity zone filaments that flow along these elbows partially melt by a few to several per cent. The resulting interstitial melt has a kimberlite-like composition. Thereafter, the excess pressure at the top of the filament at ~200 km from the axis overcomes the threshold for dyking and thus allows the escape of the interstitial melt via dykes ponding in subcrustal magma chambers or emerging directly at the surface. These melts have a composition similar to that associated with rejuvenated volcanism. We use the nature and the composition of whole erupted magmas and the seismic structure along the Hawaiian chain to validate this model.

Onset of the anorogenic alkaline magmatism in the Nigerian Younger Granite province: Constraints from the Daura and Dutse complexes

The Younger Granite province of Niger and Nigeria exemplifies a classical anorogenic alkaline ring granite belt with southward age migration from ca. 470 Ma to 140 Ma. However, the precise age transition from the older Nigerien Younger Granite (NrYG) to the younger Nigerian Younger Granite (NaYG) is poorly constrained. The Daura and Dutse complexes, located at the northernmost tip of the NaYG province, represent a temporal and spatial link between the NrYG and NaYG provinces. The Daura complex is composed of amphibole syenite and biotite granite, while the Dutse complex consists of fayalite syenite and arfvedsonite aegirine granite. Similar to other alkaline rocks of the NrYG and NaYG provinces, the Daura and Dutse samples all chemically show typical A1-type granite features, such as high alkalis (K2O + N2O = 8.33–11.27 wt%.), Ga/Al ratios, Zr + Nb + Ce + Y contents (527–2345 ppm) and FeOt/(FeOt+MgO) ratios (0.03–0.70) as well as elevated HFSEs. The NdHf and Pb isotopic systematics show that all the syenite-granites yield εNd (t) = −2.9 to +1.5, ɛHf (t) = −3.8 to +16.7, and 206Pb/204Pb = 17.818 to 19.555, suggestive of magmas derived from a slightly depleted mantle source with OIB imprints. A reduced to slightly oxidized redox state is invoked for the parental melt based on estimations from mineral chemistry. The Daura amphibole syenite is composed of >60% alkali feldspar, show high Eu/Eu* (>1), Ba/Zr (>3.5) values and cumulate textures, resembling an alkali feldspar cumulate rock. The other alkaline granitic rocks were likely generated from variable fractional crystallization of the residual melt with distinct crustal contaminations. Zircon UPb dating reveal that the Daura amphibole syenite and biotite granite were emplaced at 324 ± 1 Ma and 321 ± 3 Ma, respectively, whereas the Dutse granitoids showed episodic intrusive periods of ca. 264 Ma and 232 Ma. All these results predate other anorogenic alkaline complexes of the NaYG and update the oldest age (ca. 214 Ma in previous references) recorded in the alkaline granite province. To the north, it connects well with ages from the Zinder complex (~330 Ma) in the southernmost NrYG province, reflecting a significant link between the NrYG and NaYG provinces. From a broad view, a linear age migration from north to south in the Niger-Nigeria Younger Granite province remains well preserved, although temporal and spatial intervals exist in local region. This emplacement pattern can be traced to the periodic and repeated reactivation of lithospheric shear zones and transcurrent faults.

Quantifying metasomatic high-field-strength and rare-earth element transport from alkaline magmas

Abstract Alkaline igneous rocks host many global high-field-strength element (HFSE) and rare-earth element (REE) deposits. While HFSEs are commonly assumed to be immobile in hydrothermal systems, transport by late-stage hydrothermal fluids associated with alkaline magmas is reported. However, the magnitude of the flux and the conditions are poorly constrained and yet essential to understanding the formation of REE-HFSE ores. We examined the alteration of country rocks (“fenitization”) accompanying the emplacement of a syenite magma at Illerfissalik in Greenland, through analysis of changes in rock chemistry, mineralogy, and texture. Our novel geochemical maps show a 400-m-wide intrusion aureole, within which we observed typically tenfold increases in the concentrations of many elements, including HFSEs. Textures suggest both pervasive and structurally hosted fluid flow, with initial reaction occurring with the protolith's quartz cement, leading to increased permeability and enhancing chemical interaction with a mixed Ca-K-Na fenitizing fluid. We estimated the HFSE masses transferred from the syenite to the fenite by this fluid and found ~43 Mt of REEs were mobilized (~12% of the syenite-fenite system total rare-earth-oxide [TREO] budget), a mass comparable to the tonnages of some of the world's largest HFSE resources. We argue that fenite can yield crucial information about the tipping points in magma evolution because retention and/or loss of volatile-bonded alkali and HFSEs are key factors in the development of magmatic zirconosilicate-hosted HFSE ores (e.g., Kringlerne, at Ilímaussaq), or the formation of the syenite-hosted Nb-Ta-REE (Motzfeldt-type) roof-zone deposits.

Transition between Variscan and Alpine cycles in the Pyrenean-Cantabrian Mountains (N Spain): Geodynamic evolution of near-equator European Permian basins

In the northern Iberian Peninsula, the Pyrenean-Cantabrian orogenic belt extends E-W for ca. 1000 km between the Atlantic Ocean and Mediterranean Sea. This orogen developed from the collision between Iberia and Eurasia, mainly in Cenozoic times. Lower-middle Permian sediments crop out in small, elongated basins traditionally considered independent from each other due to misinterpretations on incomplete lithostratigraphic data and scarce radiometric ages. Here, we integrate detailed stratigraphic, sedimentary, tectonic, paleosol and magmatic data from well-dated lithostratigraphic units. Our data reveal a similar geodynamic evolution across the Pyrenean-Cantabrian Ranges at the end of the Variscan cycle. Lower-middle Permian basins started their development under an extensional regime related to the end of the Variscan Belt collapse, which stars in late Carboniferous times in the Variscan hinterland. This orogenic collapse transitioned to Pangea breakup at the middle Permian times in the study region. Sedimentation occurred as three main tectono-sedimentary extensional phases. A first phase (Asselian-Sakmarian), which may have even started at the end of the Carboniferous (Gzhelian) in some sections, is mainly represented by alluvial sedimentation associated with calc-alkaline magmatism. A second stage (late Artinskian–early Kungurian), represented by alluvial, lacustrine and palustrine sediments with intercalations of calc-alkaline volcanic beds, shows a clear upward aridification trend probably related to the late Paleozoic icehouse-greenhouse transition. The third and final stage (Wordian-Capitanian) comprised of alluvial deposits with intercalations of alkaline and mafic beds, rarely deposited in the Cantabrian Mountains, and underwent significant pre- and Early Mesozoic erosion in some segments of the Pyrenees. This third stage can be related to a transition towards the Pangea Supercontinent breakup, not generalized until the Early/Middle Triassic at this latitude because the extensional process stopped about 10 Myr (Pyrenees) to 30 Myr (Cantabrian Mountains). When compared to other well-dated basins near the paleoequator, the tectono-sedimentary and climate evolution of lower-middle Permian basins in Western and Central Europe shows common features. Specifically, we identify coeval periods with magmatic activity, extensional tectonics, high subsidence rates and thick sedimentary record, as well as prolonged periods without sedimentation. This comparison also identifies some evolutionary differences between Permian basins that could be related to distinct locations in the hinterland or foreland of the Variscan orogen. Our data provide a better understanding of the major crustal re-equilibration and reorganization that took place near the equator in Western-Central Europe during the post-Variscan period. • The beginning of the Iberian Variscan orogenic collapse is late Carboniferous in the hinterland and early Permian towards the foreland. • Variscan orogenic collapse is associated with calc-alkaline magmatism that shifts to alkaline magmatism in middle Permian times. • The beginning of the Alpine cycle of sedimentation in the whole Pyrenean ranges occurred as three main Permian tectono-sedimentary extensional phases. • The Permian tectono-sedimentary evolution of the Pyrenean ranges shows common features to the ones of Central and W Europe basins.

REE mineralization related to carbonatites and alkaline magmatism in the northern Tarim basin, NW China: implications for a possible Permian large igneous province

REE mineralization related to carbonatites and alkaline magmatism in the northern Tarim basin, NW China: implications for a possible Permian large igneous province

Trachyte-phonolite transition at Dunedin Volcano: Fingerprints of magma plumbing system maturity and mush evolution

Phonolite-trachyte associations are a common feature of alkaline volcanoes in intraplate settings, and their coexistence challenges closed-system magmatic differentiation scenarios. Here we have investigated the mineralogical and petrochemical features of dikes, lavas, pyroclastic deposits, and comagmatic crystal-rich enclaves outcropping at Dunedin Volcano (Otago region, southern New Zealand). These alkaline magmatic products show both highly and mildly alkaline affinities, trending towards phonolitic and trachytic end-members, respectively. Intermediate rocks are phonotephrites + tephriphonolites (highly alkaline series) and mugearites + benmoreites (mildly alkaline series) with a phenocryst assemblage of clinopyroxene + plagioclase ± amphibole formed at low to mid-crustal levels (i.e., ~29–16 km). Phonolites are porphyritic rocks characterized by alkali feldspar ± amphibole ± clinopyroxene. Their whole-rock compositions are highly enriched in incompatible elements, with variable Ba + Sr contents. A weak negative to slightly positive Eu anomaly is also associated with 87Sr/86Sr ratios of 0.7028–0.7031, which are comparable to those of parental magmas. Geochemical models indicate that phonolites originate as interstitial melts that are generated via abundant alkali feldspar crystallization from a shallow crystalline mush (i.e., ~14–5 km). Strong melt differentiation and extraction is testified by crystal-rich enclaves, as remnants of the mush region. On the other hand, trachytes are phenocryst-poor products strongly depleted in Ba + Sr and with a marked negative Eu anomaly. Trachytes are characterized by 87Sr/86Sr ratios of 0.7040–0.7060, which are different from intermediate rocks and phonolites, and trend towards crustal isotopic compositions. Integrated mass balance, trace element, and energy-constrained modeling confirm that trachytes originate from mildly alkaline magmas interacting with the country rock during feldspar fractionation. We interpret the transition from trachyte to phonolite formation and eruption resulting from the maturation of the plumbing system through accumulation, cooling, and degassing of both highly and mildly alkaline magmas.

Coeval calc-alkaline and alkaline Cadomian magmatism in the Bafq, central Iran: Insights into their petrogenesis

The Chah-Gaz and Mishdovan areas in the Bafq magmatic complex, central Iran, contain thick series of terrigenous sediments (the Rizu-Dezu complex), and arc-related calc-alkaline and alkaline igneous rocks. Geochemical analyses of igneous rocks from both areas indicate two distinct rock clans: (1) high-K, calc-alkaline-shoshonitic rocks with strong depletions in Nb, P, and Ti, and (2) an alkaline quartz gabbro-diorite, with trace element patterns resembling oceanic island basalts (OIB). New geochronological data reveal that magmatic rocks of both clans crystallized almost simultaneously, with zircon UPb ages of 534 Ma and 537 Ma, respectively. The whole-rock NdSr isotopic data ((87Sr/86Sr)(I) = 0.7052 to 0.7064 and ɛNd(t) = +1.3 to +2.7) of alkaline quartz gabbro-diorite indicate an enriched OIB-like mantle source, while the high-K, calc-alkaline-shoshonitic rocks have εNd(t) = −5.5 to −7.6, clearly reflecting significant contributions from pre-existing Proterozoic basement. Apatite in both the Chah-Gaz and Mishdovan magmatic rocks is of magmatic origin, with light rare earth element (LREE) enrichment patterns. The low Sr/Y and Eu/Eu⁎ values in apatite demonstrate the non-adakitic character of the investigated rocks, while the moderately negative Eu anomaly and inverse correlation between δCe and δEu in the analyzed apatites may reflect reduced parental magmas. The geochemical and isotopic results presented here indicate that slab rollback and opening of an extensional basin could have initiated concurrent Cadomian arc-related calc-alkaline and rift-associated alkaline magmatism in the Chah-Gaz and Mishdovan areas.

Mineralogical and Geochemical Features of Native Gold From Placers of the Evota Gold-Bearing Region (Russia, Aldan Shield)

Evota gold-bearing region is located in south of Republic Sakha (Yakutia) within the Nimnyr terrane of Aldan shield. A large numbers of gold-bearing placers are known on studied territory, but the primary sources for them have not been established. In this work, based on the study of the mineralogical and geochemical features of gold from alluvial deposits of the Evota gold-bearing region, possible genetic types of primary sources are considered. Obtained data showed that native gold in the studied objects has a very high, high and medium fineness. The roundness of gold is different. Almost ore crystals with sharp edges and well-rounded individuals with polished faces were found. The fineness of the studied gold grains was determined by microprobe analyzer Cameca Camebax-micro and varies in the range from 812 to 1000 %⁰. A thin (up to 20 μm) high-grade rim was found in two grains (cr. Zolotoy). The central part of one of them has a fineness of 865 %o, and in the edge part it reaches 1000 %⁰, which indicates that this gold was in the hypergenesis zone. Admixture elements determined as traces and are presented Cu, Pd, Fe, Ni. Minerals-microinclusions - quartz, potassium feldspar, pyroxene, staurolite, maldonite, bismuthite revealed in gold grains. In some watercourses, for example, in the creek Sukhoi, only very high fineness gold (993-1000 %⁰) was found. The presence of high-grade gold (cr. Sukhoi), intergrowths of gold with bismuthite (cr. Zolotoy), as well as inclusions of maldonite (cr. Yagodny) gives opportunity for assuming that primary sources could be basic ores such presented in the P. Pinigin deposit. At the sites (cr. Elovyi, r. Evota), both medium-grade and very high-grade gold were found, the fineness range varied from 827 to 998 %⁰. The presence of gold with medium fineness and good roundness in studied watercourses probably indicates an additional supply of gold from primary sources formed as a result of the alkaline magmatism development of the Mesozoic age. Thus, for gold with high and very high fineness by admixture-elements, chemical composition and microinclusions, a genetic relationship with primary sources similar to the P. Pinigin deposit was found; for gold grains with medium fineness, formation in gold-ore mineralization characterized for the Mesozoic stage ore development such in deposits of Central-Aldan ore region is assumed.

Petrogenesis of Paleoproterozoic alkali-feldspar granites associated with alkaline rocks from the Trans-North China Orogen

Granitoids are the dominant component of the continental crust and can be used as probes for crustal formation and evolution. The origin of granites, especially ambiguous granites with alkaline affinities emplaced in orogenic settings, however, remains controversial. Here, we report the petrology, geochronology and geochemistry of Paleoproterozoic alkali-feldspar granites associated with carbonated alkaline rocks from the Trans-North China Orogen in the North China Craton. Corundum-bearing Al-rich gneisses are associated with the granites and occasionally contain granite pockets and veinlets. The rocks have high amounts of K2O (10–14 wt%) and low CaO (0.3–2.3 wt%) contents and show metaluminous to weakly peraluminous features. Moreover, they have low levels of total REE (18–100 ppm) and show strong LREE enrichment [(La/Yb)CN = 44–128] and positive Eu anomalies (δEu = 1.6–10.2). The zircons record a U–Pb age of 1809 ± 3 Ma and εHf(t) values ranging from −3.7 to −7.1, which are consistent with those in the gneisses (1827 ± 14 Ma, εHf(t) = −3.3 to −8.4). Inherited zircons occur in both rocks and show similar ages of ∼2.0 and ∼2.1 Ga. Considerably high δ18OSMOW ratios (9.9–10.9 ‰) in the zircons and low εNd(t) values (−4.0 to −6.2) of the granites were identified. The granite was generated by partial melting of pelitic rocks instead of differentiation of mantle-derived magmas, whilst the gneiss represents the residuum. The carbonated alkaline magmas arising from the mantle provided sufficient heat for anatexis of the crust. Granitoid rocks showing alkaline affinity in orogenic belts may not be the fractionation products of mantle-derived melts but may have formed from reworked sedimentary materials.

Geological factors controlling variations in the mineralogical and elemental compositions of Late Permian coals from the Zhijin-Nayong Coalfield, western Guizhou, China

Determination of the geochemical and mineralogical characteristics of coal can provide useful information on the geological setting and evolution of coal basins and assist in identifying factors that control the distribution of the minerals and elements of interest. The Late Permian coals of southwestern China have received much attention because some of these coals are usually considered to be potential sources of some critical metals and have high concentrations of potentially toxic elements.Mineralogical and geochemical analyses were carried-out on the Late Permian coals from the Kulishu and Sijichun mines of the Zhijin-Nayong Coalfield, Western Guizhou, China. The diagram of Al2O3/TiO2-Nb/Yb shows that the sediment source of the Kulishu and Sijichun coals is dominantly the Emeishan magmatic province with a smaller contribution from a Neoproterozoic metamorphic-granite complex, both of which are the two major components of the Kangdian Upland. The sediment source of the roof and floor rocks is dominantly basaltic volcanics in the Emeishan large igneous province (ELIP), while the partings were probably derived from eruptions of the Emeishan felsic-alkaline magmas and a mixture of ashes derived from a distal volcanic arc. Highly elevated total sulfur contents (dominated by pyritic and organic sulfur), abundant framboidal pyrite, as well as disseminated fine-grained pyrite provide evidence for input of seawater during the peat and coal formation. Paragonite mineralization with enrichment of Li, Nb (Ta), Zr (Hf), REY and Pb in the Sijichun coals and partings of the Kulishu coal are attributed to the intrusion of alkaline magma. The pyrite, quartz, albite, anatase and gypsum occurring as veins-, cleat-fillings and separate well-crystallized aggregates of epigenetic origin in the studied coals are indicative of the influence of hydrothermal fluids from a magmatic source.

Geochronology and geochemical properties of the large-scale graphite mineralization associated with the Huangyangshan alkaline pluton, Eastern Junggar, Xinjiang, NW China

The Huangyangshan alkaline pluton is located within the southern part of the Eastern Junggar orogenic belt in Xinjiang Province, and forms part of the Kalamaili alkaline granite belt. The pluton hosts the Huangyangshan super-large graphite deposit, which develops unique spherical structure and coexists with metal sulfides. This study examines the genetic relationship between the alkaline magmatism that formed the pluton and the graphite mineralization using zircon LA–ICP–MS U–Pb dating, geochemical analysis for representative rock types in the Huangyangshan pluton, and new Re–Os isotope dating for the graphite in the Huangyangshan graphite deposit. Zircons from medium-grained arfvedsonite granite, medium–fine-grained amphibole granite, medium-grained biotite granite, and fine-grained biotite granite phases of the Huangyangshan pluton yield weighted mean U–Pb ages of 322.7 ± 4.5, 318.3 ± 4.0, 303.9 ± 2.1, and 301.1 ± 3.6 Ma, respectively, indicating that all of the granite phases were emplaced during the Late Carboniferous over a period of around 20 Myr. Six graphite samples from the deposit yield a Re–Os isochron age of 332 ± 53 Ma. Combining these ages with the genetic relationship between the graphite mineralization and magmatism in the study area and the relatively large uncertainty on the Re–Os isochron age for the graphite suggests that the mineralization formed at ca. 320 Ma. The graphite samples yield an initial 187Os/188Os value of 0.38 ± 0.2, indicative of carbon derived from a mixture of organic and mantle-derived sources. The different granite phases in the Huangyangshan pluton are geochemically similar with relatively high SiO2 (75.6–78.2 wt%) and Na2O + K2O (8.01–9.04 wt%) and relatively low CaO (0.18–0.7 wt%), MgO (0.06%–0.13 wt%) and Fe2O3 (TFe2O3 = 1.08–2.06 wt%) contents. The granites are enriched in light rare earth elements (LREE), large-ion lithophile elements (LILEs) (e.g. Rb, Th, and K), and high field strength elements (HFSEs) (e.g. Zr and Hf), depleted in heavy rare earth elements (HREEs), and have negative Ba, Sr, P, Ti, and Eu anomalies. These geochemical characteristics are indicative of derivation from juvenile basaltic oceanic crustal materials in the lower crust. This suggests that the Huangyangshan pluton formed from magmas generated by partial melting caused by mantle-derived magma underplating, with the magmas then undergoing mixing, separation, and significant fractional crystallization. Diorite enclaves within the granites have weaker trace element anomalies that are indicative of magma mixing. In addition, the mantle-derived intermediate–basic end-member involved in the magma mixing is likely one of the important carriers of carbon and metal. In summary, the Late Carboniferous Huangyangshan pluton and its associated graphite mineralization formed in a post-collision extensional tectonic setting in the Kalamaili area.

Post-spreading volcanism triggered by CO2 along the South China Sea fossil spreading axis

The post-spreading seamount chain formed along the fossil ridge of the South China Sea (SCS) consists of carbonated silicate melts and alkaline basalts. These temporally and spatially linked samples give an intriguing case where CO2-rich magmas and associated basaltic rocks occurred upon the ultra-thin oceanic lithosphere, and thus they could carry robust information on the generation of alkaline magma via melting of carbonated mantle source. Here we present magnesium isotopic data (δ26Mg), and elemental and radiogenic isotope data for the lavas from the on-axis SCS seamount chain. Both the early- and late-stage samples classified based on their formation ages exhibit remarkable δ26Mg variations (−0.53‰ to −0.11‰ and − 0.38‰ to −0.23‰, respectively). The low-δ26Mg signatures found in two groups of the SCS samples indicate a recycled origin for the source carbon inventory. However, the low-δ26Mg endmember of the early-stage samples is characterized by low SiO2 and enrichment in rare earth elements (REEs), whereas the late-stage samples display a low-δ26Mg endmember that produced high-SiO2 and REE-depleted melts. The Mg-Nd isotope systematics suggest that the early-stage lavas were yielded by CO2-enhanced melting of sediment-rich oceanic crust (εNd = 4.2), whereas the late-stage lavas have sourced the residual and carbon-deficient oceanic crust (εNd = 8.7). Our model demonstrates that the melting of carbonated component can produce compositionally distinct carbonatitic and silicate melts in sequence due to different solidus temperatures. Owe to the relatively thin lithosphere, these two types of melt could individually appear along the fossil ridge of the SCS.

Subduction initiation of the Neo-Tethys oceanic lithosphere by collision‐induced subduction transference

Collision-induced subduction transference may have triggered subduction of the Neo-Tethys oceanic lithosphere. To verify this proposition, we present an integrated geochemical and geochronological studies of new and published data of the Permian-Triassic magmatism in the Lhasa Terrane. The newly identified Chongni mafic intrusions (gabbros and diorites) in the North Lhasa Terrane (NLT) were emplaced during the Early Permian (ca. 275 Ma). The mafic rocks have (87Sr/86Sr)t ratios between 0.7037 and 0.7042, and εNd(t) values between + 5.2 and + 5.8. The zircons of the Chongni mafic rock samples have εHf(t) values from + 9.1 to + 12.9. The isotopic data in conjunction with trace element features suggest that the Chongni mafic intrusions have been derived mainly by partial melting of a metasomatized mantle wedge source. The newly identified Quxu granodiorites and hornblende gabbros in the South Lhasa Terrane (SLT) were emplaced in the Late Triassic (ca. 228–203 Ma). The granodiorites are characterized by (87Sr/86Sr)t ratios of 0.7036–0.7037 and εNd(t) values of +6.6 to +6.7, while the hornblende gabbro samples have (87Sr/86Sr)t ratios of 0.7033–0.7035 and εNd(t) values of +7.3 to +7.4. The zircons of the granodiorite and hornblende gabbro samples revealed εHf(t) values of +10.3 to +14.0 and +10.8 to +16.4, respectively. The isotopic data in combination with trace element signatures suggest that the granodiorites were derived from a juvenile basaltic lower crust with a minor input of mantle components, while the hornblende gabbros are interpreted to have formed during partial melting of a formerly metasomatized depleted mantle wedge source. The Early Permian and Late Triassic magmatic rocks consistently show arc-related signatures, which probably record the northward subduction of the Sumdo Paleo-Tethys and the Neo-Tethys oceanic slabs, respectively. This evidence, in combination with that of other studies on the Permian-Triassic magmatism across the Lhasa Terrane, indicates that arc magmatism within the NLT switched to collisional magmatism at ca. 260 Ma due to the collision of oceanic islands/plateaus and, further on, the collision of the SLT with the NLT. Furthermore, the change from ca. 265–255 Ma alkaline magmatism to ca. 240–200 Ma calc‐alkaline magmatism within the SLT may reflect the geodynamic transition from lithospheric extension to Andean-style orogenesis. Therefore, we propose that subduction initiation of the Neo-Tethys oceanic lithosphere in the Tibetan Plateau resulted from the southward transference of the Sumdo Paleo-Tethys subduction zone, involving the collision of oceanic islands/plateaus and, subsequently, the collision of the SLT with the NLT. Additionally, the geological records of the Tethyan orogenic system suggest that the subduction initiation of the Neo-Tethys oceanic lithosphere is a uniform response to the collision between Cimmeria and Eurasia.

An inorganic origin of the “oil-source” rocks carbon substance

On the basis of an inorganic concept of the petroleum origin, the phase relationships of crystalline kerogens of black shales and liquid oil at the physicochemical conditions of a typical geobarotherm on the Texas Gulf Coast are considered. At the conditions of the carbon dioxide (CO2) high fluid pressure, the process of oil transformation into kerogens of varying degrees of “maturity” (retrograde metamorphism) takes place with decreasing temperature and hydrogen pressure. Kerogen generation in black shale rocks occurs by the sequential transition through metastable equilibria of liquid oil and crystalline kerogens (phase “freezing” of oil). The upward migration of hydrocarbons (HC) of oil fluids, clearly recorded in the processes of oil deposit replenishment in oil fields, shifts the oil ↔ kerogen equilibrium towards the formation of kerogen. In addition, with decreasing of the hydrogen chemical potential as a result of the process of high-temperature carboxylation and low-temperature hydration of oil hydrocarbons, the “mature” and “immature” kerogens are formed, respectively. The phase relationships of crystalline black shale kerogens and liquid oil under hypothetical conditions of high fluid pressure of the HC generated in the regime of geodynamic compression of silicate shells of the Earth in the result of the deep alkaline magmatism development. It is substantiated that a falling of hydrogen pressure in rising HC fluids will lead to the transformation of fluid hydrocarbons into liquid oil, and as the HC fluids rise to the surface, the HC ↔oil ↔ kerogen equilibrium will shift towards the formation of oil and kerogen. It is round that both in the geodynamic regime of compression and in the regime of expansion of the mantle and crust, carboxylation and hydration are the main geochemical pathways for the transformation of oil hydrocarbons into kerogen and, therefore, the most powerful geological mechanism for the black shale formations.