High MgO Contents Research Articles

Origin of the Oligocene–Miocene Sailipu ultrapotassic volcanic rocks in southern Tibet: Melting of Asian mantle pyroxenites triggered by eastward tearing of the subducting Indian continental slab

Widely distributed Oligocene−Miocene ultrapotassic volcanic rocks in the Lhasa terrane of southern Tibet have been associated with the melting of the lithospheric mantle, plateau uplift, and porphyry Cu-Au mineralization. This study presents the mineral chemistry of olivine and clinopyroxene phenocrysts, whole-rock major and trace element data, and zircon U-Pb geochronological and Hf isotopic data for the Sailipu primitive ultrapotassic volcanic rocks. The Sailipu volcanic rocks exhibit high MgO (5.6−11.4 wt%), Cr (386−981 ppm), Co (22−43 ppm), and Ni (95−423 ppm) concentrations and have highly fractionated rare earth elements [REEs; (La/Yb)N = 23−73] and high-Fo (89.1−90.8) olivine phenocrysts containing elevated NiO (up to 0.59 wt%), which suggests a pyroxenitic mantle source that partially melted in the garnet stability field. Their high K2O contents (4.8−8.0 wt%) and global subduction sediment-like trace element patterns suggest that the metasomatic agents, which reacted with mantle peridotites to form phlogopite-bearing pyroxenites, were dominantly derived from the melting of subducted continental sediments. Their high whole-rock Ba/La and Th/Nd ratios are consistent with this hypothesis. The Sailipu ultrapotassic volcanic rocks also exhibit low initial 176Hf/177Hf ratios that resemble those of Himalayan leucogranites, and high Ca contents in olivine phenocrysts, which is consistent with contributions from the subducted carbonate-rich sedimentary strata on top of the thinned Greater Indian continental crust. The zircon U-Pb chronological data yielded concordant ages of 24.33 ± 0.19 Ma, 21.20 ± 0.62 Ma, and 17.05 ± 0.31 Ma for different exposures of the Sailipu volcanic rocks, which establishes a maximum age of ca. 24 Ma for these rocks. The northwest−southeast spatial distribution and the southeastward decrease in age (80°E−90°E) suggest west-to-east tearing of the thinned Greater Indian slab, which caused asthenospheric upwelling and melting of the Tibetan lithospheric mantle. Geothermometric calculations show relatively high primary magma temperatures (∼1250 °C) that are consistent with asthenospheric upwelling. We propose a mechanism that could genetically link the coeval Cu-Au ore-forming granitoids with the ultrapotassic magmatism of the Gangdese belt. The ultrapotassic rocks supply a large-volume of external magmatic volatiles, particularly H2O, which could trigger melting of the Tibetan lower crust and lead to the generation of the ore-forming granitoids and the establishment of oxidizing conditions for porphyry deposits. The oxygen fugacity (log ƒO2 values of ΔFMQ) of the primitive Sailipu ultrapotassic volcanic rocks (ΔFMQ = 0.48 ± 0.51 based on the Dol/melt V oxybarometer and ΔFMQ = 0.33 ± 1.19 according to the magmatic zircon U-Ce-Ti oxybarometer) is slightly lower than that of porphyry Cu-Au ore-forming granitoids in the eastern Gangdese (ΔFMQ = +0.8 to +2.9), which suggests that the direct injection of ultrapotassic melts into ore-forming granitoids played a limited role in changing oxygen fugacity, but more oxidized fluids/volatiles exsolved from these ultrapotassic melts may have facilitated the remelting of sulfide-bearing lower crust and/or directly scavenged sulfides from the mush-state reservoirs of the ore-forming granitoids in the middle−upper crust.

Petrogenesis and tectonic setting of the proto-tethyan dachaidan ophiolite, North China: insights from clinopyroxene chemistry and whole-rock Sr–Nd–Pb and zircon Hf isotopes

IntroductionThe Dachaidan ophiolites outcrop within an ultrahigh-pressure metamorphic belt along the northern margin of the Qaidam Basin. However, their age, source, and tectonic setting remain still in debate.MethodIn this study, we investigated the geochemistry and geochronology of the Dachaidan ophiolitic gabbros.Results and DiscussionZircon U–Pb dating yielded a crystallization age of 510.0 ± 2.8 Ma and 510.0 ± 2.9 Ma for the gabbro. The gabbros have low SiO2 contents (47.15–50.10 wt.%) and high MgO contents (6.35–9.04 wt.%) and Mg# values (55–74). The total rare earth element (∑REE) contents are 8.35–28.07 ppm, lower than those of normal-type mid-ocean ridge basalts (MORBs), and the gabbros exhibit light REE depletion or flat REE patterns, with small positive Eu anomalies (Eu/Eu* = 1.06–1.40). Trace element patterns are depleted to enriched in Nb and Ta, similar to island arc rocks and MORB. Clinopyroxene thermobarometry indicates the parental magma of the gabbros formed by high-temperature (1,318°C–1,363°C) and medium-pressure (1.27–1.64 GPa) partial melting in a mantle wedge. The gabbros have depleted Sr–Nd–Pb-Hf isotopic compositions, with (87Sr/86Sr)i = 0.704586–0.707441, εNd(t) = 4.7–6.6, and zircon εHf(t) = 7.6–11.4. The age-corrected Pb isotope ratios of these volcanic rocks are variable, with 206Pb/204Pb(t) = 18.085–18.253, 207Pb/204Pb(t) = 15.595–15.614, and 208Pb/204Pb(t) = 37.880–38.148, which are similar to the isotopic compositions of typical Indian MORBs. The source of the Dachaidan ophiolite is inferred to have been depleted mantle. The Dachaidan ophiolite likely formed in a forearc oceanic setting along the northern margin of the Qaidam Basin, during the initial subduction of an oceanic plate.

Petrogenesis and an Evaluation of the Melting Conditions of the Late Permian ELIP Picrites, SW China: Constraints Due to Primary Magma and Olivine Composition

The late Permian Emeishan large igneous province (ELIP) in SW China is a melting product of the Emeishan mantle plume. Recently, it has been debated whether peridotite or pyroxenite is the dominant lithology of the mantle source in the ELIP. To address this, systematic analyses of bulk-rock and coexisting spinel and olivine compositions were conducted on picrites from Lijiang–Yongsheng, Dali–Binchuan, Yumen, Muli, and Ertan. The ELIP picrites exhibit positive TiO2–CaO and negative MgO–CaO correlations, as well as low FC3MS values (−0.24–0.1), supporting a peridotite-dominated mantle source. This lithology of the mantle source is also supported by the high 100 × Mn–Fe (1.43–1.73) and Mn–Zn (13.6–18.4) values but low 10,000 × Zn–Fe (8.0–12.7) ratios of the olivine phenocrysts. The estimated mantle potential temperature for Lijiang, Yongsheng, Yumen–Ertan, Muli, and Dali–Binchuan picrites decreased away from Lijiang and Yongsheng, suggesting that the Lijiang and Yongsheng areas were the center of the ELIP. The Lijiang–Yongsheng primary magma shows similar SiO2 content but lower Al2O3 contents (average of 8.24 wt.%) and higher MgO contents (average of 21.42 wt.%) than those of Dali–Binchuan primary magma (Al2O3: 9.86 wt.%; MgO: 19.02 wt.%). Also considering the high Gd–Yb (average of 3.05) and La–Yb (average of 14.61) ratios and mantle potential temperature (average of 1599 °C), we proposed that Lijiang–Yongsheng lavas are produced via the melting of a garnet–peridotitic mantle. In contrast, the Dali–Binchuan lavas with low Gd–Yb (average of 1.91) and La–Yb (average of 5.88) ratios can be explained by their formation in the garnet–spinel transition zone of a peridotitic mantle. The Yumen–Ertan primary magma displays similar mantle potential temperature (average of 1600 °C), Al2O3 and FeO content, and Gd–Yb ratios to those of Lijiang–Yongsheng lavas, indicating that YumenvErtan primary magma may be attributed to the partial melting of garnet with minor peridotite. Therefore, heterogeneous plume-head mantle sources lead to the evaluation of melting conditions of the late Permian ELIP picrites.

Geochemistry and conditions of formation of Mesoarchean banded iron formations (BIF-1) from the Kostomuksha Greenstone belt, Karelian Craton

Three variably old groups of banded iron formation (BIF) are known in the Kostomuksha Greenstone belt (KGB) of the Karelian Craton. This paper deals with the earliest of them, Mesoarchean (2.87–2.81 Ga) – BIF-1. BIF-1 occurs among the komatiite-basalt unit of the KGB. BIF-1 consists mainly of quartz and magnetite, with varying amounts of amphibole, biotite, and garnet; they contain 48.3-58.6 SiO2 and 21.34–33.82 wt. %, Fe2O3T, suggesting that the rocks are BIF. BIF-1 of the KGB, as well as most Archean BIFs, contain high Fe2O3T, concentration, display a contrasting positive Eu anomaly, lost of Ce anomaly, the depletion of LREE relative to HREE. However, they stand out among other BIFs with high Al2O3, TiO2, MgO, K2O, Cr, Ni, Zr, Ba, Cu and Zn concentrations. BIF-1 was formed in a marine basin in an anoxic atmosphere due to hydrothermal fluids, the proportion of which varies from 20 to 80 %, and a terrigenous component derived mainly from basalts, komatiites, and dacites in host rocks. Mesoarchean BIF-1 of the KGB s was formed in a small rift within an oceanic volcanic plateau, the formation of which is associated with the influence of a mantle plume on the oceanic lithosphere.

1.38 Ga magmatism and the extension tectonics in East Kunlun, northern Tibetan Plateau

Numerous Paleo- to Mesoproterozoic magmatic rocks were emplaced during the assembly, accretion and break-up of the Columbia supercontinent, and are keys to illustrating the supercontinent circle. The geological, petrological, geochronological and geochemical data of the newly discovered meta-felsic and meta-mafic magmatic rocks from the Wulonggou area are presented in this study to shed light on the Mesoproterozoic geodynamic setting of the Central Kunlun Belt and the Qaidam Block. Zircon U-Pb geochronological results indicate that the crystallization ages of the meta-felsic samples are 1385–1376 Ma, and the meta-mafic is 1379 ± 25 Ma (MSWD=0.17). Samples from the meta-felsic unit have SiO2 contents of 68.37–73.03 wt% and belong to the high-K calc-alkaline series. They exhibit similar REE distribution patterns and display enrichment in light rare earth elements (LREES) and negative Eu anomalies. Enrichments in Rb, Ba, Th, U and K, depletion in Nb, Ta, Sr, P and Ti are seen in the primitive mantle-normalized trace element pattern diagram. Magmatic zircons from different meta-felsic samples yield variable εHf(t) values of −8.14 to + 9.37 corresponding to ca. 1.7–2.0 Ga two-stage Hf model ages. Samples from the meta-mafic unit have low SiO2 concentrations of 49.87–50.43 wt%, high contents of Fe2O3T, MgO, CaO and TiO2, and Mg# values of 52–58. They show low total REE concentrations of 19.8–30.4 µg/g, and depletion in LREES, flat HREES distribution patterns with (La/Yb)N of 0.27–0.53 and insignificant Eu anomalies. Flat distribution pattern of high field strength elements (HFSEs) is observed in the primitive mantle-normalized trace element pattern diagram. They display similar immobile elements’ concentrations and distribution patterns, low Ti/Y, Nb/Y, La/Yb, and high Nb/La ratios, comparable with the present-day normal middle ocean ridge basalt. Zircons from the meta-mafic sample have mostly positive εHf(t) values ranging from −0.10 to + 4.10 with a single stage Hf model ages of ca. 1.7–2.0 Ga. The geochemical result implies that the meta-felsic unit was generated by partial melting of the Paleoproterozoic juvenile mafic lower-crustal material with mantle attributions, and the meta-mafic unit was probably from partial melting of the lithospheric mantle. Synthesizing the above evidences, Wulonggou Mesoproterozoic meta-magmatic rocks are a bimodal suite formed in a continental extensional tectonic setting which is probably related to the break-up of the Columbia supercontinent.

A comprehensive evaluation of crumb rubber modified asphalt-aggregate interfacial behavior and the effect of water: Molecular dynamics simulation and experimental investigation

The incorporation of crumb rubber (CR) makes the asphalt-aggregate interfacial properties highly complex and uncertain, while water is a key environmental factor causing interfacial failure. This paper aims to evaluate the characteristics of crumb rubber modified asphalt (CRMA)-aggregate interaction and the interfacial failure mechanism under the influence of different material properties and water at ambient temperature. Firstly, the CRMA-aggregate interfacial behavior before and after water intrusion is investigated using molecular dynamics simulations at the molecular scale. Next, the failure process and characteristics of the CRMA-aggregate interface were analyzed using the disk-shaped compact tension (DCT) test at the macro scale. Finally, the effect of water intrusion on the interface failure mode was analyzed through image processing. The results show that CR enhances binder cohesion and significantly affects its adhesion to aggregates with high CaO and MgO contents regardless of the presence of water. The asphalt concentration distribution on the CaO and MgO surfaces is more concentrated with higher peak concentration, leading to superior interfacial adhesion and water damage resistance with binder, while Al2O3 and SiO2 were inferior. The use of asphalt with higher creep stiffness, high-strength alkaline aggregates, and smaller CR particle size is beneficial for enhancing the CRMA binder-aggregate interfacial fracture performance. Water intrusion severely affected the interfacial adhesion, resulting in a significant increase in the proportion of adhesion failure, which is the main cause of the deterioration of the interface properties. The findings of this study provide insights into the factors affecting the CRMA-aggregate interfacial behavior, which facilitate the accurate design and application of high-strength durable CRMA mixtures.

Pre-eruptive magmatic processes at Taranaki volcano from an amphibole perspective

Amphibole phenocrysts in post-1 ka lava domes deposits of Taranaki volcano, an andesite arc volcano in New Zealand, provide an opportunity to explore late-stage changes in melt composition and volatiles in the run up to eruption. Amphibole is a subordinate phase (0–7% of modal phenocrysts) and the phenocrysts display three types of growth history. Type 1 are large compositionally and texturally uniform phenocrysts that have low MgO (<13 wt%) and high K2O (∼1–1.4 wt%) contents. In contrast, Type 2 have similar cores but have been partially resorbed and overgrown by a mafic rim distinguished by high MgO (up to 15 wt%) and low K2O (<1.0 wt%) contents. Type 3 are the least common, and have either normal zoned or concentrically zoned interiors with respect to MgO and FeO. Overall, elemental substitutions and zonation patterns are related to periods of resorption and regrowth that are best explained by changes in melt composition. Based on calculated equilibrium melt compositions, the amphibole crystallisation is consistent with an incrementally grown andesitic to dacitic (SiO2 58–67 wt%) crystal mush that was periodically recharged with more mafic magma (SiO2 ∼ 55 wt%). The melts span a compositional gap (SiO2 ∼ 60–65 wt%) that is not represented in eruption products from the last 8 ka. Such melts were likely brief stages during ongoing fractional crystallisation and magma mixing. Each recharge event disrupted an evolving system and mixed crystals from different parts of it. This concept supports triggering of eruptions by recharge of mafic melts, as also inferred from plagioclase zonation for some eruptions from Taranaki. The outermost rims (0–5 μm) of many amphibole phenocrysts are enriched in fluorine (up to 1.9 wt%) and surrounded by a thick opacitic decomposition growths. We interpret this as the result of slow magmatic ascent and extensive crystallisation that produced a late-stage halogen-rich interstitial melt. This likely extended the conditions for amphibole stability to lower pressure. If F enrichment was accompanied by comparable enrichments in other halogens that preferentially partition into an aqueous phase, then extensive degassing would have occurred during the dome extrusions.

New data and insights on the secondary glass workshop of Comacchio (Italy): MgO contents, steatite crucibles and alternatives to recycling

This study introduces a collection of 33 glass samples, encompassing production indicators (blocks, fluidity tests, drops, cuts and wastes) and finished products (mainly goblets and probably a lamp) dating to the second half of the 7th century, except for a single more recent specimen (12th-14th). Additionally, a fragment was taken from a crucible bearing a thin layer of glass inside it. This new collection complements the investigation of glass materials from the Comacchio workshop previously analysed by Bertini et al. (2020).Measurements were performed using scanning electron microscopy, electron microprobe, laser ablation inductively coupled plasma mass spectroscopy on all samples and Sr–Nd isotopic analyses on 5 blocks.The results showed how the entire collection can be classified as natron-based silica-soda-lime glass and that the high MgO contents frequently observed are due to contamination with the steatite crucible. Contextually, the hypothesis of using plant ash-based glass mixed with natron-based glass formulated in the previous literature seems to have run out, along with the use of plant ash-based glass itself, further weakened by the very low representativeness of this latter type of glass on the site.The technological investigation further elucidated that recycling may not singularly account for the Comacchio glass technology. Discernible correlations may suggest the introduction of different types of metals, indicating a specialised control over the production process. Notably, the preference for green–blue glass emerges as a distinctive hallmark, underscoring the deliberate pursuit of a specific aesthetic taste.Lastly, the provenance analysis showed that over three-quarters of production was based on semi-finished products from Egypt, while only the remaining quarter came from the Levantine coast.

Early Jurassic A-type granite and monzodiorite from the Baoji batholith: Implication for tectonic transition from post-collision to post-orogenic extension in the Qinling Orogenic Belt, China

Introduction: The early Jurassic granitoids in the Qinling Orogenic Belt (QOB) play a crucial role in understanding the tectonic implications for the geological evolution of China. To elucidate the early Jurassic tectonic setting of QOB, we performed a comprehensive analysis of zircon U-Pb ages, whole-rock geochemistry, and in situ zircon Lu-Hf isotopes from early Jurassic monzodiorite and Kfeldspar granite within the Baoji batholith in western QOB.Geochronology Method and Results: The intrusions yielded zircon U-Pb ages of 186 ± 2 Ma and 188 ± 2 Ma, respectively.Geochemistry Results: The monzodiorites are characterized by relatively high MgO, Rb, Th, U, and LREE contents, as well as low P, Ti, and HREE contents. They also exhibit high Nb/Ta ratios (20.6–23.4). The zircon εHf(t) values for the monzodiorite sample range from −4.36 to 6.47, indicating significant contributions from a fertile continental lithospheric mantle with the involvement of crustal components. The K-feldspar granites are enriched in K2O+ Na2O, Rb, Zr, Hf, and Nb, and lower Ba, Sr, Ti, and P. They exhibit high Nb/Ta and Ga/Al ratios but low Y/Nb and Yb/Ta ratios. Their geochemical characteristics reveal an A-type granite affinity with elevated zircon saturation temperatures (848°C–900 °C). Additionally, the K-feldspar granite exhibits REE and trace element patterns similar to those observed in the monzodiorite. However, a wide range of zircon εHf(t) values (−4.72 to 3.98), differing from those of the monzodiorite, indicate that the parental magma of the K-feldspar granite experienced magma mixing between a monzodioritic magma and a crustal-derived felsic magma.Discussion: These findings suggest that both A-type K-feldspar granite and monzodiorite likely formed during post-orogenic processes. Additionally, the QOB commenced its postorogenic evolution as an extensional tectonic environment during the early Jurassic period.

Effect of MgO content and CaO/Al2O3 ratio on melting temperature and viscosity of CaF2–CaO–Al2O3–MgO slag for electroslag remelting

During electroslag remelting (ESR), high MgO contents are typically added to CaF2–CaO–Al2O3 slag to control magnesium content in iron- and nickel-based alloys. The melting temperature and viscosity of ESR slag are pivotal for energy consumption, production efficiency, smooth operation, and ingot quality. However, there is currently a notable scarcity of research on the melting temperature and viscosity of CaF2–CaO–Al2O3–MgO slag with high MgO levels. Thus, the melting temperatures and viscosity of CaF2–CaO–Al2O3–MgO slags with varying MgO contents and CaO/Al2O3 (C/A) ratios were investigated. The results show that as the MgO content increases from 7.83 % to 13.18 %, the final precipitation content of the MgO phase significantly increases, resulting in an increase in the melting temperature from 1306 °C to 1319 °C. With the increase in the C/A ratio from 0.86 to 1.16, the precipitation completion temperatures of the Ca12Al14F2O32 and MgO phases significantly decrease and the MgAl2O4 phase transforms into the CaO phase. Hence, the melting temperature decreases from 1329 °C to 1314 °C. Further increasing the C/A ratio to 1.40, the final precipitation content of the CaO phase increases, resulting in a decrease in the melting temperature from 1314 °C to 1282 °C. Moreover, as the MgO content and C/A ratio increase, the slag viscosity exhibits different change trends within various temperature ranges. This depends on which of the depolymerization of the slag structure and the precipitation and clustering of the MgO phase has a greater effect on the slag viscosity.

Physical and chemical properties of CaF2−CaO−Al2O3−MgO slag for electroslag remelting of Mg-containing GH3128 superalloy

During electroslag remelting, Mg in the electrode reacts with some components in the slag, resulting in its serve oxidation loss. The CaF2−CaO−Al2O3−MgO slags with high MgO content are generally used for remelting Mg-containing Fe- and Ni-based alloys. However, there is a scarcity of foundational data available on the physical and chemical properties of the slags, posing a challenge in developing a suitable slag for remelting Mg-containing GH3128 superalloy. Hence, this paper conducted a comprehensive study on the melting temperature, viscosity, and reactivity of four known slags. The results show that D2 slag (50%CaF2–20%CaO–20%Al2O3–10%MgO) has the lowest melting temperature of 1588 K due to containing fewer high-melting-point phases and possesses lower viscosity because of simpler aluminate structures. Additionally, the D2 slag has the highest activity ratio, enabling the superalloy to achieve the highest equilibrium Mg content of 0.0174%. The D2 slag exhibits a heightened MgO activity and facilitates the release of more Mg2+ ions, thereby exerting a pronounced inhibitory effect on the diffusion of Mg2+ ions from the slag–metal interface to the slag. The superalloy attains a highest Mg yield of about 30%. Therefore, the D2 slag emerges as the most suitable choice for remelting the Mg-containing GH3128 superalloy.

Investigation of Slagging-Fouling and Mineral Contents in Low-Rank Coals for Improved Combustion Performance

ABSTRACT Despite the problems associated with low-rank coal, this type of coal is still widely used in various applications including the power generation sector. In this study, three low-rank coals (LRC) with high mineral content of SiO2, Al2O3, and MgO were selected and comprehensively investigated. Investigations include ash fusion temperature (AFT), theoretical calculation, combustion experiment with drop tube furnace (DTF), and ash analysis using scanning electron microscope (SEM) and x-ray diffraction (XRD). The investigation revealed that T-Coal with high SiO2 and P-Coal with high Al2O3 mineral produce ash deposition which did not adhere on the probe surface of either slagging or fouling area. T-Coal ash consists of fine and solid particles, but the presence of the dominant albite mineral, 29.09% in slagging probe and 28.71% in fouling probe, may have a negative impact on ash deposition. P-Coal ash is dominated by fine particles with a predominance of quartz and other high-melting minerals. A-Coal with high MgO mineral has good results in slagging aspects. However, the high Fe2O3 on A-Coal results in low ash fusion and produces more hematite mineral (17.70%) in the fouling probe. This study shows that several types of LRC have good evaluation on ash-related problems which potentially can be used as coal blending for further utilization in power generation.

Petrogenesis of K-rich and high-mg adakitic rocks in the North Dabie terrane, China: products of early cretaceous orogenic collapse

ABSTRACT Understanding orogenic collapse is key to comprehending the evolution of continental collision orogens. The Dabie orogen is a collapsed continental collision orogen that provides an ideal window for revealing orogenic collapse. Here, we conduct fieldwork, whole-rock geochemistry, and zircon U-Pb dating of the Daoshichong pluton in the North Dabie terrane to elucidate the collapse mechanism of the Dabie orogen. The Daoshichong pluton is composed of quartz monzodiorite and mafic microgranular enclaves (MMEs) with an Early Cretaceous (~130 Ma) crystallization age. The Daoshichong pluton is strongly deformed and ductile, displaying a gneissic structure. The quartz monzodiorites are K-rich and high-Mg adakitic rocks. They contain 60.2–63.3 wt.% SiO2 and have high K2O (2.88–3.42 wt.%) and MgO (2.08–3.04 wt.%) contents with relatively high Mg# values (44.4–49.3). They exhibit high Sr (673–819 ppm) and low Y (11.4–20.9 ppm) and heavy rare earth elements (e.g. Yb: 0.94–1.87 ppm) contents, resulting in high Sr/Y (35.1–72.0) and (La/Yb)N (15.7–36.1) ratios. The MMEs exhibit geochemical characteristics similar to those of the Early Cretaceous mafic – ultramafic rocks of the Dabie orogen. They have low SiO2 contents (48.1–51.9 wt.%) and high MgO contents (3.27–5.62 wt.%) with high Mg# values (47.3–51.2). They also exhibit relatively high Ni (16.6–63.6 ppm), Cr (30.5–164.6 ppm), Y (18.2–25.8 ppm) and Yb (1.50–1.94 ppm) contents and high Sr/Y ratios (53.7–109.5). Evidence from fieldwork and geochemistry distinctly indicates magma mixing during magma evolution between the MMEs and the quartz monzodioritic host. Thus, the Daoshichong quartz monzodioritic host and the MMEs can be interpreted as a mixture of mantle-derived magma and over-thickened lower continental crust-derived adakitic melt. Previous studies suggest that the Early Cretaceous (~130 Ma) high-Mg adakite-like rocks are distributed only along the eastern margin of the Dabie orogen (i.e. along the Tan – Lu Fault). This study is the first to report a homochronous (~130 Ma) K-rich and high-Mg adakite-like pluton in the core of the Dabie orogen. Our results suggest that a whole delamination of the over-thickened orogenic root caused the orogenic collapse, subsequently triggering crust – mantle interactions, dome structure, and magmatism in the Dabie orogen.

Late Cenozoic high and low temperature magma generation from primordial and age-modified mantle materials beneath Dariganga in Southeast Mongolia: Factors of mantle degassing and adiabatic upwelling

Representative sampling of the Dariganga volcanic field was conducted to decipher its inner structure in terms of its deep magma sources. Rocks with a high La/Yb ratio (40–54) and a high MgO content (11–15.8 wt%) are identified among the predominantly moderate La/Yb ratio (7–40) and a moderate MgO content (5–11 wt%) rocks. These rock markers, traced along linear volcanic zones, are considered as indicators of high and low temperature magma generation processes. A general agreement exists that partial melting predominated in the transitional asthenosphere–lithosphere region; however, between 10 and 5 Ma, these processes were complicated by melts that either adiabatically ascended from a residual slab source in the deep mantle at a high potential temperature (Тр = 1489°С) or were generated due to mantle fluid degassing at a low temperature or both these processes, simultaneously. Magmas were subsequently adiabatically upraised from an OIB-like source in the deep mantle with potential temperatures (Tp) of up to 1423°C and were also generated by mantle fluids at low temperature. The rock markers yield Pb-isotope age estimates of the proto-mantle (at 4.47 and 4.45 Ga) and the age-modified mantle (at 3.11 and 2.74 Ga) beneath Dariganga. In the last 16 Ma, encompassing the late geodynamic epoch of Earth's mantle, similar high and low temperature magmas have erupted across vast sections of the Japan-Baikal Geodynamic Corridor; these magmas are not reported from the adjacent Abaga and Dalinuoer areas, with the exception of the final (Holocene) fluid-derived compositions.

Early Triassic Silicic Volcanics of South China: Petrogenesis and Constraints on the Geodynamic Evolution of the Paleo‐Tethys Ocean Region

AbstractThe only occurrence of Lower Triassic silicic volcanic rocks within the South China Block is in the Qinzhou Bay area of Guangxi Province. LA‐ICP‐MS zircon U‐Pb dating reveals that volcanic rocks of the Beisi and Banba formations formed between 248.8 ± 1.6 and 246.5 ± 1.3 Ma, coeval with peraluminous granites of the Qinzhou Bay Granitic Complex. The studied rhyolites and dacites are characterized by high SiO2, K2O, and Al2O3, and low MgO, CaO, and P2O5 contents and are classified as high‐K calc‐alkaline S‐type rocks, with A/CNK = 0.98–1.19. The volcanic rocks are depleted in high field strength elements, e.g., Nb, Ta, Ti, and P, and enriched in large ion lithophile elements, e.g., Rb, K, Sr, and Ba. Although the analyzed volcanic rocks have extremely enriched zircon Hf isotopic compositions (εHf(t) = –29.1 to –6.9), source discrimination indicators and high calculated Ti‐in‐zircon temperatures (798–835°C) reveal that magma derived from enriched lithospheric mantle not only provided a heat source for anatectic melting of the metasedimentary protoliths but was also an endmember component of the S‐type silicic magma. The studied early Triassic volcanics are inferred to have formed immediately before closure of the Paleo‐Tethys Ocean in this region, as the associated subduction would have generated an extensional setting in which the mantle‐derived upwelling and volcanic activity occurred.

Petrogenesis and Metallogenesis of Late Cretaceous Adakites in the Nuri Large Cu-W-Mo Deposit, Tibet, China: Constraints from Geochronology, Geochemistry, and Hf Isotopes

The Gangdese metallogenic belt in Tibet is an important polymetallic metallogenic belt formed during the subduction of the Neo-Tethys Ocean and subsequent India–Asia collision. Adakitic rocks are widely distributed in this belt and are considered to be closely related to porphyry–skarn Cu-Mo polymetallic mineralization. However, the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt remain controversial. In this study, we focus on the quartz diorite in the Nuri Cu-W-Mo deposit along the southern margin of the eastern Gangdese belt. LA-ICP-MS zircon U-Pb dating yields a Late Cretaceous age of 93.6 ± 0.4 Ma for the quartz diorite. Whole-rock geochemistry shows that the quartz diorite possesses typical adakitic signatures, with high SiO2, Al2O3, and Sr contents, but low Y and Yb contents. The relatively low K2O content and high MgO, Cr, and Ni contents, as well as the positive zircon εHf(t) values (+6.58 to +14.52), suggest that the adakites were derived from the partial melting of the subducted Neo-Tethys oceanic slab, with subsequent interaction with the overlying mantle wedge. The Late Cretaceous magmatic flare-up and coeval high-temperature granulite-facies metamorphism in the Gangdese belt were likely triggered by Neo-Tethys mid-ocean ridge subduction. The widespread occurrence of Late Cretaceous adakitic intrusions and associated Cu mineralization in the Nuri ore district indicate a strong tectono-magmatic-metallogenic event related to the Neo-Tethys subduction during this period. This study provides new insights into the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt, and has important implications for Cu polymetallic deposit exploration in this region.

Geochemistry and geochronology of the TTG-sanukitoid suite in the Zhulagou area: Constraints on the Neoarchean crustal evolution of the western North China Craton

The Archean basement in the Yinshan Block of the western North China Craton consists of a variety rock of assemblage, including TTG (tonalite-trondhjemite-granodiorite) rocks and some dioritic gneiss (sanukitoid). Understanding the petrogenesis of these rocks is crucial for comprehending the late Neoarchean crustal evolution of the western North China Craton. This study provides a new geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic data for the TTG-sanukitoid suit that was newly identified in the Zhulagou area. The late Neoarchean TTG gneisses were emplaced around 2.54–2.50 Ga, while dioritic gneiss was emplaced between 2.53 and 2.50 Ga. The TTG gneisses exhibit high SiO2 (64.46–72.24 wt%) and Al2O3 (13.37–16.48 wt%) contents, relatively elevated (La/Yb)N and Sr/Y ratios, as well as low Y and Yb contents. They display variable εHf(t) (+0.94 to +3.57) and εNd(t) (−1.02 to +0.82) values, with relatively low Pb isotopic compositions, suggesting that they were derived from the partial melting of a thickened mafic lower crust. On the other hand, dioritic gneiss shows moderate SiO2 (58.05–61.87 wt%) content, high MgO (2.62–4.06 wt%), Al2O3 (14.42–18.44 wt%) and Cr contents, but relatively low (La/Yb)N values, showing geochemical affinity to the Archean sanukitoids. The dioritic gneiss exhibits variable εHf(t) values (−2.29 to +5.56) and εNd(t) (−2.05 to +2.02), with relatively low Pb isotopic compositions. Furthermore, the presence of amphibolite xenoliths, along with the high Mg# contents (36.84–53.49), indicating the addition of mantle-derived components to its source region. Therefore, the formation of the dioritic gneiss is primarily attributed to partial melting of the thickened (mafic) lower crust and the addition of mantle material. By combining previous studies with the new data presented in this study, we propose that a late Neoarchean tectonic regime involving a mantle plume is the most favorable explanation for the two major magmatic events in the Yinshan Block. Firstly, the earlier pulse/stage of mantle plume resulted in the ∼2.7 Ga thickened (mafic) lower crust and a limited amount of TTG rocks. Subsequent pulse/stage of mantle plume triggered partial melting of pre-existing thickened (mafic) lower crust and generated 2.54–2.50 Ga TTG rocks and dioritic gneiss.

Selective metasomatism of ultramafic cumulates within Archean supracrustal sequences

The Neoarchean Storø Supracrustal Belt in SW Greenland comprises a sequence of mature quartzite, metapelite, amphibolite, and ultramafic rocks that underwent amphibolite facies metamorphism during the amalgamation of the Mesoarchean Akia Terrane and the Eoarchean Færingehavn Terrane. In this belt, tourmaline is found in a transition zone between ultramafic and metapelitic rocks, but also occurs as orbicules within the ultramafic rocks. These tourmaline orbicules hosted by ultramafic rocks are reported for the first time in the North Atlantic craton, thus indicating a unique formation mechanism. We conducted a comprehensive examination of the petrology, whole-rock and mineral chemistry, and oxygen isotope compositions from borehole samples in the Storø Supracrustal Belt, to elucidate the metasomatic events associated with the formation of the orbicular tourmalines. The Storø ultramafic rocks have high MgO, Cr, and Ni contents, with low abundances of REE and HFSE, and preserve a typical cumulate texture. These features are similar to those of ultramafic cumulates found in other Archean supracrustal belts, suggesting a cumulate origin for the Storø ultramafic rocks. Furthermore, the morphology and composition of the tourmaline orbicules within these cumulates indicate that they originated from melts with high boron and high water concentrations that infiltrated the ultramafic rocks. The main factor influencing the morphology of the tourmaline orbicules is the silicification of the ultramafic rocks, driven by their lower chemical potential of SiO2 compared to the surrounding rocks. This silicification process, in combination with compositional variations of cumulates during fractional crystallization, has contributed to the geochemical diversity observed in Archean ultramafic rocks. Thus, it is crucial to understand the effects of such selective metasomatism on Archean ultramafic rocks, as this will facilitate the extraction of original information preserved in the early rock record.

Corrosion of MgO–C refractories by vanadium slag with high MgO content

Due to the limitation of the subsequent process for producing V2O5, the converter of vanadium extraction needs to use the slag with high MgO content to splash slag. This paper focus on the corrosion behavior of vanadium slag with high MgO content on MgO–C refractory. Corrosion experiment, SEM-EDS analysis, XRF analysis, and theoretical calculation are utilized to study the interface and mass transfer between slags and refractory. The results indicate that the average width of the reaction interface decreases from 9.3 μm to 3 μm with the increase of MgO content from 2 wt% to 8 wt%. When the corrosion time increases from 15 min to 60 min, the average width of the reaction interface decreases from 9 μm to 5 μm, and stabilizes at 5 μm as the corrosion continues. The mass transfer coefficient of MgO rises to 0.663 × 10−2 cm s−1 in the first 60 min of corrosion time, and then falls to 0.592 × 10−2 cm s−1 as the corrosion continues to 120 min. The mass transfer coefficient of MgO also shows an increasing trend with the increase of temperature and rotation speed. When the temperature increases from 1445 °C to 1490 °C, the mass transfer coefficient of MgO increases from 0.1313 × 10−2 cm‧s−1 to 1.4556 × 10−2 cm‧s−1. When the rotation speed increases from 10 rpm to 30 rpm, the mass transfer coefficient of MgO increases from 0.411 × 10−2 cm s−1 to 0.818 × 10−2 cm s−1.

Geochemical Evidence of Plume Sources for High-MgO Lavas in the Western Kunlun Orogenic Belt

Abstract Plume-derived high-MgO lavas provide important information on the lithological, thermal and chemical variations of Earth’s deep mantle. Here we present results from detailed field, mineralogical and geochemical studies of Late Permian–Late Triassic high-MgO lavas near the Chalukou area in the Western Kunlun (WK) orogenic belt, NW China. The major element compositions of the lavas show extremely high MgO contents (26.6–33.8 wt %) in accordance with olivine accumulation. The parental magma is inferred to be picritic in composition with MgO of 17.2 ± 0.9 wt %. Olivine Zn/Fe and Mn/Zn ratios suggest a peridotite-dominated source with a minor fraction of pyroxenite. The temperature and oxygen fugacity estimates based on multi-methods including olivine-melt Mg–Fe equilibria, Al-in-olivine and olivine–spinel thermometry and oxybarometer yield a mantle potential temperature of 1522–1556 °C and high oxygen fugacity of FMQ (fayalite-magnetite-quartz) + 0.93. The H2O contents in the picrite flows are estimated as 3.67 ± 1.0 wt %, indicating the volatile-rich nature of parental magma and its mantle source. The immobile trace element features show that the WK picrites are OIB (oceanic island basalt)-like, with the enrichment in light rare earth elements and positive Nb, Ta, Zr and Hf anomalies. Furthermore, the Nd–O–Os isotopes display typical mantle values without involvement of recycled materials. Our results suggest the high-MgO volcanism in the WK orogenic belt originated from a volatile-rich plume source.