Melt Production Research Articles

Mantle Deformation Processes During the Rift‐To‐Drift Transition at Magma‐Poor Margins

AbstractThe rift‐to‐drift transition at rifted margins is an area of active investigation due to an incomplete understanding of the spatial and temporal evolution of physical and chemical processes at the ocean‐continent transition (OCT). Deep structures that characterize modern OCTs are often difficult to identify by seismic observations, while terrestrial exposures are preserved in fragments separated by tectonic discontinuities. Numerical modeling is a powerful method for contextualizing physical processes and observations relevant to rifted margin evolution. We synthesize results from geological observations of fossil OCTs preserved in ophiolites, a recent seismic experiment on the Ivorian margin, and numerical modeling to characterize mantle deformation and melt production for magma‐poor margins. Across varied surface heat fluxes, mantle potential temperatures, and extension rates, our model results show homologies with geological observations. We propose that the development of large shear zones in the mantle, melt infiltration, grain size reduction, and anastomosing detachment faults control the structure of OCTs. We also infer that a hot, upwelling, melt‐rich asthenosphere is an important control on the local stress environment. During the exhumation phase, continentward‐dipping shear zones couple with seaward‐dipping detachment faults to exhume the subcontinental and formerly asthenospheric mantle. The mantle forms core‐complex‐like domes of peridotite at or near the surface. The faults that exhume these peridotite bodies are largely anastomosing and exhibit magmatic accretion in their footwalls. A combination of magmatic accretion and volcanic activity derived from the shallow melt region constructs the oceanic lithosphere in the footwalls of the out‐of‐sequence continentward‐dipping detachment faults in the oceanic crust and subcontinental mantle.

Structural, lithostratigraphic and thermal features of a Permian lower crust from the Western Italian Alps (Valpelline Series, Valle d’Aosta)

AbstractThe Valpelline Series (Dent-Blanche Tectonic System, Western Italian Alps) is a sector of lower continental crust, which consists of Permian migmatitic metapelite with different mineral assemblages (i.e., garnet-, cordierite- and orthopyroxene-bearing), minor amphibolite and marble, intruded by aplite and pegmatite. Widespread melt production in metapelite and locally in amphibolite occurred during the development of the regional foliation. The P–T conditions during migmatisation, estimated using conventional geothermobarometers, range between 800–900 °C and 0.5–0.8 GPa, with a difference of up to ∼50 °C between cordierite- and orthopyroxene-bearing migmatites, the latter reaching higher temperatures. The Valpelline Series shows rock types, metamorphic assemblages, P–T conditions and published ages of high-temperature regional metamorphism like the archetypal lower crust section of the Ivrea-Verbano Zone in the Southern Western Alps. The latter likely represents an external portion of the same extending lower crust, at the onset of the Tethyan rifting due to lithospheric extension and asthenospheric rising.

Petrological models of magma evolution on the formation of mafic-felsic igneous complexes of the Emeishan large igneous province, SW China

Many Late Permian world-class Fe-Ti oxide-bearing layered intrusions in the Pan-Xi region, SW China are commonly considered to be fractionation products of the Emeishan mantle plume-related high-Ti basaltic melts. These layered intrusions coexist spatially and temporally with A-type granitoids, which consist of syenites and granites and are further classified into isotopically depleted metaluminous to peralkaline and isotopically enriched peraluminous to metaluminous groups. The petrogenesis of these mafic-felsic igneous complexes has been controversial. This study investigates geochemical correlations of the microgabbros at the margin of the Baima layered intrusions and the intruding isotopically enriched granitoids. The amphiboles at both sides of the contact zone are calcic (CaO = 10.4 to 12.5 wt.%) and display obvious mixing trends. Thermodynamical modeling using the MAGEMin program indicates that fractionation of basaltic melts compositionally similar to the Baima microgabbros seems incapable to crystallize olivine compositions comparable to those of the layered intrusions, but can be reconciled by early separation of 7.5 to 10 wt.% Fe-Ti-rich melts. Continued differentiation of the basaltic melts may lead to formation of the isotopically depleted granitoids, among which perthite accumulation in water-rich environments could result in the formation of metaluminous granitoids with high Na, K and Al contents and positive Eu anomalies. The isotopically enriched granitoids, on the other hand, can be produced by anatexis of the juvenile mafic lower crust together with the Yangtze ancient lower crust. Our study therefore provides a panoramic view on the complicated petrogenesis of the igneous complexes related to Emeishan mantle plume impingement through thermodynamic modeling.

Field and petrographic studies of Basement Complex rocks in Saigbe and Environs, North-Central, Nigeria

Geological mapping and petrographic studies of rocks provide useful information on texture and mineralogical composition of rocks which can be used in the exploration and exploitation of any associated mineralization. However, few regional studies have been conducted in Minna and environs. Here, preliminary field mapping and petrography of out crops in Saigbe environ were conducted to determine their identification, structural and mineralogical composition. Twenty-one rock samples were collected, eight of which were used for thin section analysis. Joint directions were measured and plotted on rose diagram. Results show that Saigbe is underlain by schist, amphibolite and granite with minor pegmatite intrusions. Joints, faults, fold and foliations constitute main structures. Joints in the schists trend NE-SW, while those in the granites trend NW-SE, suggesting different episodes of emplacement for both rocks. Petrography revealed that the schists comprised quartz, biotite, muscovite, plagioclase and opaque mineral, while the amphibolites are composed largely of quartz, hornblende, plagioclase feldspar and opaque minerals. The granites are composed of quartz, microcline, plagioclase feldspar, muscovite and opaque minerals. The pegmatite dykes in the granites comprised quartz, plagioclase feldspar, orthoclase feldspar, muscovite and gemstones, hence interpreted as product of residual melt. The pegmatites make interesting target for mineral exploration.

Sensitivity of the MAR regional climate model snowpack to the parameterization of the assimilation of satellite-derived wet-snow masks on the Antarctic Peninsula

Abstract. Both regional climate models (RCMs) and remote sensing (RS) data are essential tools in understanding the response of polar regions to climate change. RCMs can simulate how certain climate variables, such as surface melt, runoff and snowfall, are likely to change in response to different climate scenarios but are subject to biases and errors. RS data can assist in reducing and quantifying model uncertainties by providing indirect observations of the modeled variables on the present climate. In this work, we improve on an existing scheme to assimilate RS wet snow occurrence data with the “Modèle Atmosphérique Régional” (MAR) RCM and investigate the sensitivity of the RCM to the parameters of the scheme. The assimilation is performed by nudging the MAR snowpack temperature to match the presence of liquid water observed by satellites. The sensitivity of the assimilation method is tested by modifying parameters such as the depth to which the MAR snowpack is warmed or cooled, the quantity of water required to qualify a MAR pixel as “wet” (0.1 % or 0.2 % of the snowpack mass being water), and assimilating different RS datasets. Data assimilation is carried out on the Antarctic Peninsula for the 2019–2021 period. The results show an increase in meltwater production (+66.7 % on average, or +95 Gt), along with a small decrease in surface mass balance (SMB) (−4.5 % on average, or −20 Gt) for the 2019–2020 melt season after assimilation. The model is sensitive to the tested parameters, albeit with varying orders of magnitude. The prescribed warming depth has a larger impact on the resulting surface melt production than the liquid water content (LWC) threshold due to strong refreezing occurring within the top layers of the snowpack. The values tested for the LWC threshold are lower than the LWC for typical melt days (approximately 1.2 %) and impact results mainly at the beginning and end of the melting period. The assimilation method will allow for the estimation of uncertainty in MAR meltwater production and will enable the identification of potential issues in modeling near-surface snowpack processes, paving the way for more accurate simulations of snow processes in model projections.

The Analysis Static of Chassis Robot Arm as Design Modification Induction Melting Furnace Machine Using FEA Method

Induction furnace is a metal melting furnace that uses heater. The high temperatures can cause work accidents if not carefully. The cause of work accidents, especially machines is had mistake when initially machine designed. An alternative solution is to make improvements the machine. The researcher made a robot arm design that functions to transfer melting products and reduce work accidents like being exposed to melted metal. The researcher’s purpose is to analyze the design static of robot arm as modification induction melting furnace machine. The method is to study previous research literature related to static simulation. Researchers use the SolidWork premium 2018 software which consists of stress (von mises), displacement, and FOS (factor of safety) calculations. Based on the static simulation results, it can be concluded that the highest values ​​of simulated stress (von mises) at 5N, 10N, and 15N loads were 13.926Mpa, 27.728Mpa, and 41.531Mpa which categorized as feasible because they were below yield strength value of 64.205Mpa. Simulation displacement, the design experienced the most significant deformation at 15N load with value of 0.337 mm. The FOS (factor of safety) value each loading were 4.610; 2.315; and 1.474 which fulfills the static load safety requirement 1.25.

Effect of Heat Treatment of Beverage Cans Remelting on Mechanical Properties

This study compared the mechanical properties of the repeated melting of beverage cans. From the effects of the first melting, a chemical composition test was held out, which turned out that the aluminum produced was not explicitly admitted in the aluminum alloy series 2xxx, 3xxx, 4xxx or 6xxx. The semelting process I have increased the element Fe, as well as the addition of elements of Pb and Sn. Part of smelting I and II were annealed and quenched at 400?C with a holding time of 2 hours. The highest tensile strength is held from the first smelting product that has received quenching heat treatment, 9.67 kg/mm2. After remelting, it decreased to 9.07 kg/mm2. Products that have received quenching heat treatment have the most significant impact value, both for the first and second melting products, namely 74.34 BHN and 101.86 BHN. The most significant impact value was obtained from the first smelting product that received annealing heat treatment, which was 0.0388 J/mm2, and after receiving quenching heat treatment, the impact value became 0.0379 J/mm2.

Constraints on subglacial melt fluxes from observations of active subglacial lake recharge

AbstractActive subglacial lakes provide a rare glimpse of the subglacial environment and hydrological processes at play. Several studies contributed to establishing active subglacial lake inventories and document lake drainage and connection, but few focused on the period between lake drainage when the melt production and transport contribute to the refilling of these lakes. In this study, we employ high-resolution CryoSat-2 altimetry data from 2010 to 2021 to compile an inventory of recharging lakes across Antarctica. We extract recharge rates from these lakes, which serve as a lower limit on subglacial melt production. These recharge rates are compared against predictions obtained by routing modelled subglacial meltwater at the ice-sheet's base. Our findings indicate that modelled recharge rates are consistent with observations in all but one of the investigated lakes, providing a lower bound on geothermal heat fluxes. Lake Cook E2 displays recharge rates far exceeding predictions, indicating that processes are taking place that are currently unaccounted for. Considering recharge in hydrologically connected lake networks instead of individually provides a stricter constraint on melt production. Recharge rates extracted from the Thwaites Lake system suggest that subglacial melt production has been underestimated.

Analysis of the possibility of replacing carbon with hydrogen in the conditions of smelting cast iron from vanadium-containing titanomagnetites

75 % of the total carbon dioxide emissions by ferrous metallurgy enterprises is generated in the blast furnace process. One of the directions of CO2 emission reduction in pig iron production is partial replacement of carbon monoxide with hydrogen as a reducing agent. It is shown that such a replacement can lead to a decrease in the total carbon consumption due to reduction of heat consumption for the direct reduction of iron oxides. Using a mathematical model of the blast furnace process, the efficiency of partial replacement of process fuel (coke, natural gas, pulverized coal) with a hydrogen additive was evaluated. Calculations were performed for the operating conditions of blast furnaces of EVRAZ NTMK JSC, which melt vanadium-containing titanomagnetites. The coefficients of process fuel replacement with hydrogen and the coefficients of the influence of the replacement of process fuel with hydrogen on the change in CO2 emissions are calculated. The dependence of the change in the productivity of the furnace on the consumption of hydrogen at a constant minute flow rate of the blast and its adjustment to maintain the pressure drop has been established. It is shown that in the absence of gas dynamics reserves, the replacement of process fuel with hydrogen will be accompanied by a decrease in furnace productivity. The smelting of pig iron from titanomagnetites with the replacement of technological fuel with hydrogen will complicate the refining of melting products due to an increase in the formation of titanium carbides and carbonitrides. The reduction in CO2 formation at hydrogen entering the blast furnace with natural gas is 0.35 kg/m3 compared to pure hydrogen replacement of 0.73 kg/m3, not taking into account CO2 emission during hydrogen production

Emplacement of the Argyle diamond deposit into an ancient rift zone triggered by supercontinent breakup

Argyle is the world’s largest source of natural diamonds, yet one of only a few economic deposits hosted in a Paleoproterozoic orogen. The geodynamic triggers responsible for its alkaline ultramafic volcanic host are unknown. Here we show, using U-Pb and (U-Th)/He geochronology of detrital apatite and detrital zircon, and U-Pb dating of hydrothermal titanite, that emplacement of the Argyle lamproite is bracketed between 1311 ± 9 Ma and 1257 ± 15 Ma (2σ), older than previously known. To form the Argyle lamproite diatreme complex, emplacement was likely driven by lithospheric extension related to the breakup of the supercontinent Nuna. Extension facilitated production of low-degree partial melts and their migration through transcrustal corridors in the Paleoproterozoic Halls Creek Orogen, a rheologically-weak rift zone adjacent to the Kimberley Craton. Diamondiferous diatreme emplacement during (super)continental breakup may be prevalent but hitherto under-recognized in rift zones at the edges of ancient continental blocks.

Investigation of microstructural evolution and mechanical properties Al13Fe4 produced by casting and spark-plasma sintering

In this paper, the preparation and evolution of the polycrystalline Al13Fe4 microstructure were investigated during the casting and spark plasma sintering process (SPS). Experimental results showed that an increase in the cooling rate (90–100 °C/min) for both methods of synthesis led to the formation of cracks and chips in the bulk of the material. Cracks during casting are formed by the passage of a peritectic reaction. The high cooling rate does not allow contact melting products to fill the pores that coalesce into cracks. With SPS, the main driving force for the formation of cracks is the effect of the thermal expansion coefficient. The analysis of XRD and results of mechanical testing revealed the maximum tensile micro stresses (425–430 MPa) in samples obtained at a high cooling rate. A decrease in cooling rate promotes the formation of high-density polycrystalline Al13Fe4. It is noted that the material prepared by SPS has a fine-grained structure by using Al13Fe4 powder as the initial material. The highest indicators of flexural strength are 60 ± 10 MPa, Vickers hardness HV = 8.2 ± 0.8 GPa, fracture toughness K1c = 1.5 ± 0.1MPa × m−0.5, modulus of elasticity 180 ± 10 GPa. The fractography of the surfaces after the flexural test has common features of a quasi-brittle trans crystalline fracture with a “river” structure.

Role of crustal radiogenic heating in ultra-high temperature metamorphism of the Greater Himalayan Crystalline complex: Phase equilibrium and numerical modelling

Ultra-high temperature (UHT) metamorphism is a significant feature of the Greater Himalayan Crystalline complex (GHC), which constrains the dynamics of continental subduction and subsequent continental collision. However, the mechanism of such kind of UHT metamorphism is widely debated, including slab tearing-induced mantle upwelling, mechanical heating, radiogenic heating, and others. To better understand the UHT metamorphism and the thermal budget of collisional orogens, we first conduct a forward phase equilibrium modelling coupled with geochemical modelling to explore the variation of radiogenic heat production (RHP) of felsic melt, indicating that it may range from 4 μW/m3 to 9 μW/m3. Then, we incorporate the variable RHP into a thermo-mechanical model to study its effect on the thermal evolution of the collision zone. The numerical models reveal that a reasonably increased radiogenic heat production of 4 μW/m3 for the partially molten felsic crust contributes to UHT metamorphism after the continental collision for 30–35 million years. The model results are consistent with the structural observations, geochemical signatures, and P-T paths of the GHC. After the comparison with other mechanisms (e.g., the effects of mechanical heating and anomalous mantle heating), we propose that radiogenic heating could be a predominant heat source for the rapid UHT metamorphism of the GHC, whereas mechanical heating and heat flux from the mantle play secondary roles.

Unveiling spatial variability within the Dotson Melt Channel through high-resolution basal melt rates from the Reference Elevation Model of Antarctica

Abstract. The intrusion of Circumpolar Deep Water in the Amundsen and Bellingshausen Sea embayments of Antarctica causes ice shelves in the region to melt from below, potentially putting their stability at risk. Earlier studies have shown how digital elevation models can be used to obtain ice shelf basal melt rates at a high spatial resolution. However, there has been limited availability of high-resolution elevation data, a gap the Reference Elevation Model of Antarctica (REMA) has filled. In this study we use a novel combination of REMA and CryoSat-2 elevation data to obtain high-resolution basal melt rates of the Dotson Ice Shelf in a Lagrangian framework, at a 50 m spatial posting on a 3-yearly temporal resolution. We present a novel method: Basal melt rates Using REMA and Google Earth Engine (BURGEE). The high resolution of BURGEE is supported through a sensitivity study of the Lagrangian displacement. The high-resolution basal melt rates show a good agreement with an earlier basal melt product based on CryoSat-2. Both products show a wide melt channel extending from the grounding line to the ice front, but our high-resolution product indicates that the pathway and spatial variability of this channel is influenced by a pinning point on the ice shelf. This result emphasizes the importance of high-resolution basal melt rates to expand our understanding of channel formation and melt patterns. BURGEE can be expanded to a pan-Antarctic study of high-resolution basal melt rates. This will provide a better picture of the (in)stability of Antarctic ice shelves.

Speed function effects on properties of Ti-5Al-5Mo-5 V-1Cr-1Fe alloy manufactured by electron beam melting

The speed function (SF) parameter, which is a unique combination of beam speed and beam current, is one of the most important parameters of the electron beam melting (PBF-EB) production process. It allows the same process conditions (size of the melt pool, process temperature, etc.) to be maintained for parts with different geometries and sizes. The aim of this research was to understand the effect of SF on the Ti-5Al-5Mo-5V-1Cr-1Fe alloy produced by electron beam melting technology. The results showed that increasing the SF from 86 to 116 resulted in a decrease in the average process temperature, a decrease in the length of the α phase plates, and change in the proportion of α and β phases from 53% (SF 86) to 50% (SF 116) of the β phases. Differences in microstructure (column diameters, thickness, and length of α-phase plates), phase composition, microhardness, porosity at the bottom, and top of the samples were observed on all samples. The mechanical property analysis did not show a significant effect of SF on elongation, but an increase in SF from 86 to 96 resulted in an increase in ultimate tensile strength (UTS) of approximately 7%, whilst an additional increase to 116 resulted in a decrease in UTS of approximately 8.5% compared to SF 96. From the results, it can be seen that the microstructural and mechanical properties of the material strongly depend on the SF. Based on it, SF 86 can be considered as the recommended value for the manufacture of Ti-55511 alloy parts.

Syn‐Rift Magmatism and Sequential Melting of Fertile Lithologies in the Lithosphere and Asthenosphere

AbstractThe timing and rate of decompression melting of a compositionally heterogeneous mantle during continental rifting are assessed with a new one‐dimensional geodynamic code, MELT1D. MELT1D computes pressure and temperature in the extending lithosphere and rising asthenosphere and calculates the resulting melt fraction and eruption rates for different lithologies. A series of models simulate syn‐rift melt production from (a) dry and wet depleted lherzolite similar to the mantle that underlies most mid‐ocean ridges, (b) dry and wet relatively fertile ultramafic compositions representing plume or primitive mantle material, (c) pyroxenite representing recycled ultramafic oceanic crust or magmatic metasomes, and (d) basalt representing recycled mafic crust or metasomes. The models predict sequential melting of the different compositions that is broadly consistent with basalt eruption histories in many Phanerozoic rifts. Results show a progressive transition in magma sources as the lithosphere thins, beginning with melting of wet mantle and compositionally fertile mafic components near the lithosphere‐asthenosphere boundary during the earliest stages of extension. This transitions to magmatism dominated by melting of relatively fertile ultramafic components (pyrolitic and pyroxenitic compositions) as extension progresses, and finally to melting of ambient lherzolite asthenosphere as lithosphere thinning approaches breakup. Mantle composition, pre‐rift lithosphere thickness, and mantle temperature exert the greatest controls on the timing and volumes of magmas produced from each lithology. In general, a cool or thick lithosphere has a greater capacity to sequester fertile lithologies than thin or warm lithosphere, and thus has a greater capacity to produce early syn‐rift magmas without requiring a hot mantle plume.

Ar-Ar ages and geochemistry of Late Cretaceous basalts in the Nanxiong Basin, SE China: Constraints on the subduction and rollback of the Paleo-Pacific Plate

The Cretaceous arc-related and intraplate mafic rocks are widely distributed across Southeast (SE) China, and provide important information for investigating the Cretaceous dynamical processes of the Paleo-Pacific Plate. This study presents new petrological, geochronological, and geochemical analyses for the Late Cretaceous basalts from the Nanxiong Basin in SE China. The Nanxiong basalts yield the 40Ar/39Ar plateaus ages of 76.2-69.9 Ma, they belong to alkaline-calc-alkaline basalts and have OIB-like geochemical patterns with enrichment on LILEs and LREEs. These basalts have initial 87Sr/86Sr ratios of 0.7055-0.7062 and εNd (t) values of +2.5-+3.4. Such signatures suggest that the Nanxiong basalts were the products of partial melting of asthenospheric mantle, which might be metasomatized by slab-derived melts with a proportion of sediments. Our new results, along with the regional reported ages and geological observations, suggest that the Nanxiong OIB-like basalts were formed in an intraplate extensional setting, in response to the rollback of the subducted Paleo-Pacific slab during the Late Cretaceous.

Heterogeneous mantle source compositions for boninite from Bonin and Troodos, evidence from iron isotope variations

Boninite has been widely accepted to be the melting product of highly refractory mantle with the addition of slab-derived fluids. Previous studies have found that boninite, like other subduction-related magmas (e.g., mature arc basalts), has generally consistent chondrite-like iron isotope composition (c. 0.03‰), which is on average lighter than those of oceanic basalts. However, it remains unclear whether the lighter iron isotope composition of subduction-related magmas is inherited from the depleted mantle source or caused by the addition of slab-derived oxidized fluids. Moreover, boninite has highly varied element patterns (e.g., Yb, Zr), reflecting varying degrees of mantle depletion and different contributions of slab-derived fluids. Given the limited data for iron isotope composition of boninite so far available, it is thus required to systematically study iron isotope composition of boninite with different compositions to further understand the petrogenesis of boninite and the iron isotope variation of subduction-related magmas. Here we report iron isotope compositions of boninite from two classic suites on Earth, i.e., those from Bonin islands and submarine forearc (low-Ca boninite; CaO/Al2O3 mainly <0.75) and Troodos ophiolite complex, Cyprus (high-Ca boninite: CaO/Al2O3 = c. 0.85). The δ56Fe values of the Bonin boninite vary from −0.046 ± 0.003‰ to 0.078 ± 0.031‰ (2SD, SD = standard deviation of 4 times repeated analysis; with an average of 0.03 ± 0.03‰, n = 14). The δ56Fe value of the Troodos boninite is relatively constant, i.e., from 0.046 ± 0.010‰ to 0.091 ± 0.016‰ with an average of 0.06 ± 0.01‰ (n = 7). Bonin forearc basalts (FABs) were also analyzed for comparison with δ56Fe of ~0.10‰, except for those FABs with higher Th/U ratio and Cu contents (δ56Fe = 0.01–0.05‰). Together with previously reported δ56Fe data for boninite from New Caledonia (low-Ca boninite with CaO/Al2O3 = ~ 0.4, δ56Fe = ~ +0.03‰), the lower δ56Fe values of the Bonin boninite than Bonin FAB and Troodos boninite reflect the lighter iron isotope composition of the more depleted mantle source. Based on relationships between different indicators for various fluids (e.g., Ba/La, Zr/Sm) and modeling, Bonin boninite is likely produced by partial melting of the refractory mantle with an addition of ~5–10% melts from sediments and oceanic crust, while the formation of Troodos boninite can be ascribed to the melting of less refractory mantle, which has been previously metasomatized by ~5% fluids derived from oceanic crust and sediments.

Tectonic transition during the Jurassic-Cretaceous in the Jiaodong Peninsula, North China: insights from asynchronous adakitic and A-type granitic plutons

ABSTRACT Jurassic-Cretaceous magmatic rocks are widely distributed in the Jiaodong area, North China. These magmatic rocks are closely associated with gold deposits, but their petrogenesis and geodynamic background are still controversial. The Late Jurassic-Early Cretaceous granitic rocks exposed in the Guocheng area, Jiaodong Peninsula are the focus of this paper. Zircon U-Pb dating, Hf-O isotope and whole rock geochemical analysis are used to determine the petrogenesis of these rocks. Combined with previous data, the geodynamic processes of granite formation in different time intervals are investigated which provide new petrological evidence for understanding the magmatic-mineralization and tectonic background of the Jiaodong area. Samples were collected from the Queshan and Shazibu plutons. The Queshan pluton is a mylonite granite, one sample of which yields a 206U-238Pb zircon age of 156 Ma, while the Shazibu pluton is a quartz monzonite, one sample of which records a 206U-238Pb zircon age of 117 Ma. The two plutons have variable zircon Hf-O isotopic compositions: zircon εHf(t) values of the Queshan pluton are −29.2 to −22.8, and δ18O values are 7.30–7.84 ‰, while the Shazibu pluton has relatively high zircon εHf(t) values (−21.6 to −21.0) and δ18O values of 7.48–8.43 ‰. Whole-rock geochemical data shows that the Queshan pluton has high Sr/Y (60.7–169) and (La/Yb)N (34.6–65.9) values and low Y (3.11–10.3 ppm) and Yb (0.29–0.85 ppm) contents, akin to adakites, whereas granites of the Shazibu pluton are A-type with high Zr+Nb+Ce+Y contents (354–420 ppm) and zircon saturation temperatures (752–795°C). Based on the geochemical and isotopic data, the Late Jurassic Queshan pluton is the product of partial melting of thickened lower crust of the North China Block, while the Early Cretaceous Shazibu pluton formed via fractional crystallization of mafic rocks derived from partial melting of the lower crust of the North China Block. Although both plutons were derived from the lower crust of the North China Block, the nature of the lower crust was modified by involvement of more depleted mantle-derived materials during the Early Cretaceous. From the Late Jurassic to the Early Cretaceous, the rock types (from the adakite to A-type granite) and the nature of the magma source suggest a shift in geodynamic processes. Combined with previous research, the Late Jurassic Queshan pluton is proposed to have formed under low-angle subduction of the ancient Pacific Plate, while the Early Cretaceous Shazibu pluton formed subsequently during Pacific Plate rollback and extension. The Early Cretaceous transformation of tectonomagmatic environments provided favourable conditions for the formation of large-scale gold deposits in the Jiaodong region.

Late Neoarchean charnockite in Daqingshan terrane, North China Craton: petrogenesis and implications for continental crustal evolution

ABSTRACT Modern analogues of Archaean charnockite are important because they facilitate investigations into processes of early crustal formation and tectonic transition. Herein, we present the results of the petrography, geochemistry, and isotope geochronology of Late Neoarchean charnockites in the Daqingshan terrane at the northern margin of the North China Craton. The sources, petrogenesis, and tectonic implications of the samples obtained are determined based on their geochemical characteristics. Charnockites mainly exhibit magnesian and calc–alkaline characteristics; furthermore, they contain high amounts of light rare-earth elements, with slightly positive Eu anomalies. Analysis of zircon U–Pb dating using a sensitive high-resolution ion microprobe reveals that Hademengou–Xiwulanbulang (HDMG–XWLBL) charnockites were emplaced during the (i) Late Neoarchean (ca. 2500 Ma), with zircon εHf(t) values ranging from 1.57 to 7.81; (ii) single-stage model ages (TDM1) from 2529 to 2637 Ma; and (iii) two-stage model ages (TDM2) from 2710 to 2820 Ma. A comparative study has performed on the petrography, geochemistry, and geochronology of charnockite and granulite shown that the parent magma of the charnockites is the product of the partial melting of granulite. Combining our results with geological data pertaining to the Archaean structural evolution of the Daqingshan terrane, we speculate that the Late Neoarchean HDMG–XWLBL charnockite was formed during a subduction-related thermo-tectonic event in the northern margin of the North China Craton.

Primitive high-Mg andesites from the Western Alps, Italy: Products of interaction of sediment diapir derived melts with mantle-wedge peridotite in a continental collision zone

Primitive (high-Cr, Ni), high-Mg andesites (HMA) at convergent plate margins are considered primary, mantle-derived melts, and are not part of the classical tholeiitic or so-called calc-alkaline igneous differentiation series. Their genesis is often considered the product of melting of lower crustal sequences or the subducting slab, though their genesis is far from being understood. Alternatively, they may represent differentiated rocks from boninitic precursors. Here we present isotope and geochemical data for a rare suite of high-Mg andesites that were preserved as decimetre-sized boulders in molasse-type sedimentary rocks of the Oligocene-Miocene Gonfolite Lombarda (Como Formation; Italy), as part of the post-orogenic alpine sedimentary succession. The HMA have moderate to high MgO (1.9–6.4 wt%), high Al2O3 (16.3–18.8 wt%), along with high Cr (30–306 ppm) and Ni (13–92 ppm), and moderate Yb (1.2–2.5 ppm) and Y (12–26 ppm) abundances, matching those of primitive HMA. These HMA are LILE- and LREE-enriched and most samples have negligible negative Eu anomalies. Primitive mantle-normalized compositions show depletions in Nb, Ta, P, Ti and Y. Most samples have unradiogenic Nd (εNd: −6.2 to −7.8) and radiogenic 87Sr/86Sr isotope compositions (0.708 to 0.710), elevated δ18O values (+7.6 ‰ to +9.5 ‰) and radiogenic Pb isotope compositions, with a pronounced variation in 207Pb/204Pb and 208Pb/204Pb at rather constant 206Pb/204Pb. Initial εHf isotope values range from −0.4 to −2.3, some of which deviate from the HfNd crust-mantle isotope array. One sample is unevolved (87Sr/86Sr: 0.708; εNd: −3.3, εHf: +1.8, δ18O: 8.5 ‰) whereas another sample is strongly evolved (87Sr/86Sr: 0.720; εNd: −9.3, εHf: −5.8, δ18O: 9.3 ‰); the latter probably representing a crustal melt. The remaining samples have intermediate compositions (87Sr/86Sr: 0.709–0.713; εNd: −6.2 to −8.5, εHf: −0.4 to −4.3, δ18O: 7.6–10.6 ‰) that either result from limited AFC processes or represent source heterogeneities. The sum of these features cannot be the result of a simple slab or mantle wedge melting process. To account for the observed geochemical signatures of the high-Mg andesites, a three-stage model is suggested that invokes melting of slab-derived sediment diapirs, followed by melt-rock-reaction in the mantle wedge and subsequent crustal assimilation-fractional crystallization (AFC). The diapirs that rise into the mantle wedge are composed of subducted wet, Alpine sediments. Hydrous dacitic (siliceous) partial melts derived from them react with the ambient mantle wedge and cause the formation of boninitic melts with elevated MgO, Ni and Cr; the archetype features of primitive high-Mg andesites. Subsequent interaction with the overlying arc crust is evidenced by crustal xenoliths, which may account for some isotope features of the rock suite. We consider this scenario most plausible because it is relatively simple, and explain most if not all geochemical features, based on modelling and experimental evidence.