Magmatic Processes Research Articles

Petrogenesis of submarine volcanic arc rocks from Andaman subduction zone, Northeast Indian Ocean: Constraints from slab components and mantle wedge characteristics

The present study provides new petrological and geochemical data of the dredged rocks from submarine volcanoes along the Andaman arc and describes the petrogenetic evolution of the arc system in terms of mantle wedge characteristics, nature and quantitative input of subducted slab components, and fractionation processes of precursor magma. The studied rocks include basaltic andesite, andesite, dacite and rhyolite. These volcanic rocks exhibit LILE, LREE enrichments and HFSE depletion, corroborating their generation through subduction processes. High abundances of Th/Nd, La/Sm(N), LREE/HFSE than LILE/HFSE, LILE/LREE suggest a substantial contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids to the mantle wedge. The 87Sr/86Sr-Ba/La mixing model suggests 0.6–0.8% addition of slab fluid (90:10 AOC: sediment fluid) to account for the fluid signature, whereas the 143Nd/144Nd-La/Sm(N) mixing model envisages ∼3–4% addition of sediment melt to the mantle source, reconciling the sediment signature in Andaman submarine volcanic rocks. The presence of N-MORB type mantle is attributed to the absence and/or inefficient convection of asthenospheric material from the Andaman back-arc basin to the mantle wedge. This ineffective convection can be equated with the flat subduction of the Indian Plate, caused by the convergence of the aseismic Ninety East Ridge. The non-modal batch-melting model suggests that 13–24% partial melting of the spinel lherzolite mantle beneath the Andaman submarine volcanic arc formed the parent magma. The crystallization model invokes up to 60–70% of fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, sanidine and magnetite in all the rock types with subordinate proportions of amphibole, biotite, apatite, ilmenite, and sanidine in rhyolites. The basaltic andesites, andesites and dacites do not show upper crustal input, while rhyolites indicate crustal contamination from an upper crust and/or arc crust.

Partial melting mechanisms of peraluminous felsic magmatism in a collisional orogen: An example from the Khondalite belt, North China craton

AbstractSedimentary‐derived (S‐type) granites are an important product of orogenic metamorphism, and a range of subtypes can be recognized by differences in field occurrence, mineralogy and geochemistry. These subtypes can reflect variations of initial protolith composition, partial melting reactions, pressure and temperature of anatexis, or magmatic processes that occur during ascent through the crust (e.g. mineral fractional crystallization or crustal assimilation). Together, these diverse factors complicate geological interpretation of the history of peraluminous felsic melt fractions in orogenic settings. To assess the influence of these factors, we performed integrated field investigation, petrology, geochemistry, geochronology and phase equilibrium modelling on a series of leucosomes within migmatite associated with different S‐type granites within the Khondalite belt, North China craton (NCC), which is an archetypal collisional orogen. Three types of leucosome are recognized in the east Khondalite belt: leucogranitic leucosome, K‐feldspar (Kfs)‐rich granitic leucosome and garnet (Grt)‐rich granitic leucosome. Phase equilibrium modelling of partial melting and fractional crystallization processes indicate that the leucogranitic leucosomes were mostly produced through fluid‐present melting, Kfs‐rich granitic leucosomes are produced through muscovite dehydration melting with 3 vol.% garnet fractional crystallization, and Grt‐rich granitic leucosomes are produced through biotite dehydration melting with 20–40 vol.% K‐feldspar fractional crystallization and up to 20 vol.% peritectic garnet entrainment. Mineral fractional crystallization and peritectic mineral entrainment occur in the source during melting, and play equally important roles in partial melting mechanisms in terms of affecting the geochemical compositions of granitic melts. Thus, we suggest that peraluminous felsic magmas preserved in collisional orogens are dominantly produced by fluid‐absent melting in the middle to deep continental crust, although extraction of low‐volume melt fractions from an anatectic source region at shallower depths during fluid‐present melting can also generate small amounts of S‐type granites that subsequently crystallize at high structural levels in the crust.

Igneous Diversity of the Early Martian Crust

Mars missions and Martian meteorites revealed how complex the Martian crust is. The occurrence of both alkaline and sub-alkaline igneous rocks of Noachian age (>3.7 Ga) in Gale crater indicates diverse magmatic processes, with sub-alkaline rocks likely formed through the partial melting of hydrous mafic rocks, as commonly observed on Earth. The orbital discovery of excavated evolved igneous rocks scattered in Noachian terrains raise questions about the petrology of the ancient Martian crust, long thought to be basaltic. A possibly evolved crust beneath a mafic cover is supported by geophysical and seismic measurements from the Insight lander that indicate the bulk crust has a lower density than expected if it were homogeneously basaltic. If localized magmatic processes could form evolved terrains, the detection of abundant intermediate to felsic Noachian crustal exposures through remote sensing suggest regional- to global-scale processes that produced evolved crustal component(s) that are now buried below mafic materials. Due to the lack of centimetric to millimetric textural imaging and compositional measurements, the petrology of such crust is ambiguous. Future orbiter, rover, and aerial missions should focus on Noachian exposed regions exhibiting evolved crustal characteristics to unfold the petrology of the Martian crust and its formation.

Carbon-rich polyphasic inclusions in postcollisional mafic magmatic rocks from the Dabie Shan, China: Implications for the carbon cycle in continental subduction zones

Subduction is a fundamental geodynamic process that transfers carbon from Earth’s surface into the mantle. However, current understanding of the migration mechanisms, final storage region, and species involved in carbon recycling from continental crust remains limited. Here, we investigated the compositions of polyphasic inclusions and Mg isotopes in postcollisional mafic magmatic rocks from the Dabie Shan region of China. The main rock-forming minerals contained two distinct types of polyphasic inclusions, which displayed systematic differences in daughter mineral/gaseous phase assemblages, including host-like silicates ± carbonates (magnesite, dolomite, and calcite) + CH4 and carbonates + talc ± SiO2 (aqueous) + CH4, respectively. These inclusions indicate that carbon-rich silicate melts and carbon-rich magmatic fluids were trapped by host minerals during magmatic processes. The abundant carbonates and CH4 in both types of inclusions suggest that the mantle source of these postcollisional mafic magmatic rocks was rich in carbon, most likely existing in the forms of CO2 and CH4. Moreover, the studied postcollisional mafic magmatic rocks have mantle-like Mg isotope compositions, with δ26Mg values ranging from −0.23‰ to −0.16‰. The combined observations of polyphase inclusions and Mg isotopes indicate that a substantial carbon-rich mantle domain arose from the metasomatism of silicate melts derived from subducted continental slabs that had dissolved a certain quantity of CO2 and CH4. We proposed that continental subduction is an efficient pathway for transporting crustal carbon into an orogenic subcontinental lithospheric mantle wedge, where the recycled carbon can be stored for >100 m.y. and eventually released to the surface during postcollisional magmatism.

Remobilization and enrichment of Nb during magmatic and hydrothermal processes: insights from titanite in Nb-rich dyke swarms of South Qinling, China

Remobilization and enrichment of Nb during magmatic and hydrothermal processes: insights from titanite in Nb-rich dyke swarms of South Qinling, China

About this title - Supercontinents, Orogenesis and Magmatism

A tribute to the career of J. Brendan Murphy, this volume covers topics that encompass the three main fields of his influence: (i) supercontinents and the supercontinent cycle; (ii) orogenesis and terranes; and (iii) magmatism and magmatic processes. Papers range from strongly field-based studies to conceptual analyses, and focus on clarifying some crucial geological processes.

Mineralizations of Nb-Ta-Rb-Zr and rare-earth elements in Boziguoer, South Tianshan, NW China: Geochronology and geochemistry of monazite and bastnäsite

The Boziguoer alkaline intrusion is located in the South Tianshan Orogenic Belt on the northern margin of the Tarim Craton. The intrusion is entirely mineralized and forms a super-large rare metal (RM) and rare-earth element (REE) deposit dominated by Nb-Ta-Rb, accompanied by Zr-REE. The primary RM minerals include pyrochlore, astrophyllite, and zircon, while the main REE minerals consist of fluocerite, monazite, xenotime, and bastnäsite, which are commonly present as granular aggregates or singularly filling the interstices between gangue minerals. Through a combination of geochronological and geochemical analyses of different types of monazite and bastnäsite in the mineralized alkaline rocks, this study elucidates the role of magmatic-hydrothermal evolution on the Boziguoer RM-REE mineralization and reconstructs the geochronological framework of alkalic magmatic-hydrothermal evolution and mineralization processes, as well as establishing mechanisms responsible for RM-REE enrichment. Petrographic observations and back-scattered electron (BSE) imaging revealed several types of monazite and bastnäsite with different characteristics as follows: (1) type Ia monazite (Mnz-Ia) experienced intense hydrothermal alteration, forming residual cores of monazite mantled by apatite and allanite coronas; (2) type Ib monazite (Mnz-Ib) was partially eroded, forming concentric zoning patterns with a core of monazite, a mantle of apatite, and an outer rim of allanite coronas; (3) type II monazite (Mnz-II) is slightly modified, and commonly associated with fluorite; (4) bastnäsite and fluocerite exhibit a core-rim structure, with the core of fluocerite being brighter than the rim of bastnäsite in BSE images. According to the paragenetic relationships and compositional variations, all two types of monazite are of primary magmatic origin, while bastnäsite is of hydrothermal origin. In addition, principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) results indicate that Mnz-Ia and Mnz-Ib belong to the same group, different from Mnz-II. The latter has higher (Ce/Gd)N and (Th/U)N ratios, indicating relatively low-temperature crystallization of Mnz-II at late stages. In-situ LA-ICP-MS U-Th-Pb dating of monazite and bastnäsite show that Mnz-Ia and Mnz-Ib have ages of ca. 290 Ma, consistent with the zircon age of the intrusion, whereas the Mnz-II and bastnäsite have younger ages of ca. 280 Ma, representing a post-magmatic hydrothermal mineralization event at Boziguoer. These new ages indicate that both magmatic and subsequent hydrothermal processes played critical roles in the RM-REE mineralization in this deposit. The latest findings also support the model that the formation of the alkaline belt where the Boziguoer ore-bearing intrusion was temporally and spatially linked to the Tarim Large Igneous Province that is genetically related to a mantle plume.

Magmatic–hydrothermal stage recorded by mica zonation and Nb–Ta–W–Sn mineralization: New insight into the genesis of highly evolved Songshugang Nb–Ta deposit (Jiangxi, SE China)

The role of magmatic and hydrothermal processes in niobium-tantalum mineralization is still controversial and has been limited by a lack of mineralogical records in the evolving magmatic–hydrothermal system. Micas as essential rock-forming minerals and comprise the amount of elements (especially rare-metal elements), become one of the ideal indicator. The Songshugang rare-metal granite (RMG) located in the southern part of Jiangxi Province, China, is renowned as the largest tantalum reservoir in South China. The albite–topaz–K-feldspar granite is the major granite, with layers of pegmatite, topaz-K-feldspar granite, and greisen overlay on it. It is the typical highly evolved peraluminous with low phosphorous RMGs (PLP-type) according to geochemistry. The Songshugang pluton preserves analogical characters in topaz-albite-K feldspar granites and topaz-K feldspar granite, and partly reserves in greisen. Detailed petrographic, mineralogy, and chemical composition study of zoned mica and accessory Nb–Ta minerals suggest that zinnwaldite and Nb-Ta oxides have experienced magmatic fractionation with gradually decrease of K/Rb and Nb/Ta ratio in prismatic zinnwaldite and columbite-(Fe). The leap of Rb (Rb2O up to 4.08 wt%) and Ta (Ta# up to 0.29) at the rim of zinnwaldite and columbite is likely attributed to supersaturation. Afterwards the overgrowth of Rb-deficit zinnwaldite (0.56–0.72 wt% Rb2O) is coeval with the Ta-highest wodginite, taking place during the magmatic-hydrothermal transition associated with a fluid-melt interaction in hydrosaline melt. The ascent of fluxing elements contributes to the formation of hydrothermal zinnwaldite and cassiterite through greisenization. The systemic coupling mineralization of zinnwaldite and Nb–Ta oxides altogether record the both the melt-driven and fluid-driven processes in highly evolved Songshugang deposit that is critical for rare-metal mineralization. The Songshugang displays a great similarity to the typical PLP RMGs Cínovec cupola in geochemistry, which are metaluminous to peraluminous, containing low phosphorous, intermediate REE, Zr, Hf, and Th content. But Songshugang has experienced strong supersaturation; the latter is more akin to highly evolved melts alike Yichun or Beauvoir. The strong supersaturation and crystallization demonstrate its discrepant mineralization mechanism in Songshugang RMGs.

Magmatic–hydrothermal evolutionary processes in highly evolved granitic systems: Insights from zircons of the Baishitouquan pluton, NW China

Abstract As a robust accessory mineral in igneous rocks, the mineralogical and geochemical characteristics of zircon can record the lithological differentiation and magmatic–hydrothermal evolution of highly evolved granitic systems. The F-Rb–rich, highly evolved Baishitouquan pluton of NW China exhibits gradual lithological changes from leucogranite, amazonite-bearing granite, and amazonite granite in the lower levels of the pluton to topaz-bearing amazonite granite, topaz albite granite, and pegmatite in the upper levels. In this study, three types of zircon grains were identified in five lithological zones based on textural and chemical characteristics. Type I zircon, which mostly occurs in leucogranite and amazonite-bearing granite, exhibits oscillatory zoning in cathodoluminescence images and experienced low degrees of radiation damage (0.21–0.68 × 1015 α-decay events/mg), which is indicative of its magmatic origin. Type II zircon, which mostly occurs in amazonite granite and amazonite pegmatite, exhibits textures that are indicative of hydrothermal alteration (e.g., spongy texture, porosity, and microcracks), and has elevated concentrations of some cations, such as Ca and Al. Type II zircon contains a higher concentration of non-formula elements, including rare earth elements (REEs), and Hf, Th, and U, than Type I and III zircons. Additionally, Type II zircon exhibits a significant M-type lanthanide tetrad effect and experienced varying levels of radiation damage (3.75–11.72 × 1015 α-decay events/mg). These characteristics suggest that Type II zircon has a hydrothermally altered origin. Type III zircon, which is restricted to the topaz-albite granite, has the smallest crystal size among all types of zircon grains, shows a euhedral to anhedral mottled appearance, and is characterized by patchy, cloudy, or irregular zoning, with numerous fluid inclusions. This type of zircon contains higher concentrations of Ti (110–1030 μg/g) than other types of zircon grains. Additionally, this type of zircon experienced limited radiation damage (2.18–3.69 × 1015 α-decay events/mg), and has a smooth surface and homogeneous internal textures. These characteristics suggest that Type III zircon is the product of fluid interaction with hydrothermally altered Type II zircon. Accordingly, this type of zircon crystallized directly from a Zr-saturated hydrothermal fluid during the later stages of magmatic–hydrothermal evolution. These contrasting textural and compositional features of the three types of zircon grains are indicative of three stages of magmatic–hydrothermal evolution of the Baishitouquan pluton: magmatic, magmatic–hydrothermal transition, and hydrothermal. These magmatic and hydrothermal processes were involved in the enrichment, transport, and precipitation of rare metals, such as Rb. Accordingly, this contribution demonstrates that the textures and chemistry of zircon grains can serve as petrogenetic indicators for assessing magmatic–hydrothermal evolution and rare-metal mineralization in highly evolved granitic systems. Furthermore, this study presents a model of the magmatic–hydrothermal evolution of F-rich, highly evolved granitic systems through the lens of zircon.

The evolution of Archean crust undisturbed for three billion years: A case study from the Bahalda Pluton, Singhbhum Craton, eastern India

The Archean Bahalda Pluton (Singhbhum Craton, eastern India) is an I-type mafic granodiorite, surrounded by Paleoarchean TTG and TTG-derived granites of the Singhbhum Suite. The Singhbhum Suite is highly unusual amongst Eo- and Paleo-Archean terranes because most of the Singhbhum Craton escaped post-Archean deformation and metamorphism. The Bahalda granodiorite contains accessory zircon, titanite, and fluorapatite, which are rarely studied together but are a powerful tool for exploring magmatic and post-magmatic processes. U-Pb zircon and titanite dating indicate a ca. 3.35 Ga emplacement age of the Bahalda granodiorite. Subsequent ca. 3.27–3.05 Ga U-Pb fluorapatite ages are interpreted as recording low-grade metamorphic or hydrothermal event(s). The Bahalda fluorapatite and titanite are LREE-enriched compared to the HREE. The complex zonation of fluorapatite suggests that primary igneous fluorapatite was recrystallised with the removal of REE and overgrown by REE-depleted rims. This is in agreement with the apatite trace element systematics which imply fluorapatite cores are of mafic I-type granitoid affinity, with rims of low- and medium-grade metamorphic / metasomatic affinity. No textural evidence of titanite alteration was detected, but some domains have minor REE depletion compared to crystal interiors. The titanite and fluorapatite data display similar trends in LREE vs Sr/Y space due to metamorphic/hydrothermal alteration. The evolution of the Bahalda Pluton, which did not exceed greenschist facies for 3 Gyr, invites comparison with other stabilized cratons from the Mesoarchean.

New precise age constraints of the Hatu gold belt, west Junggar, NW China: Implications for a 300 Ma magmatic hydrothermal event in post-collisional setting

Abstract A 300 Ma magmatic hydrothermal ore-forming process is identified in the Hatu gold belt in west Junggar, northwest China, based on precise age constraint by secondary ion mass spectroscopy U-Pb dating of hydrothermal zircon and Ar-Ar dating of muscovite. The gold deposits in the Hatu belt (Hatu, Qi-V, Huilvshan, etc.) are similar in geology, with NE- and NW-trending orebodies composed of gold-bearing quartz veins and auriferous altered tuffaceous wall-rocks. Hydrothermal zircon grains separated from gold-bearing quartz veins in the Qi-V gold deposit provide a concordia age of 299.6 Ma and a nearly identical weighted mean 206Pb/238U age of 300 ± 2 Ma. Muscovite samples also from the gold-bearing quartz veins yield 40Ar/39Ar plateau ages of 299.6 ± 1.7 Ma, 299.9 ± 1.8 Ma, and 300.6 ± 1.9 Ma. By comprehensive compilation of geological and geochemical characteristics, tectonic evolution, and geochronology of the gold deposits in the Hatu gold belt, our new precise age data confirm that the gold deposits in the Hatu belt formed simultaneously at ca. 300 Ma, and likely formed during a post-collisional extensional setting by magmatic hydrothermal fluid from cooling magmatic sources.

Xenon compatibility in magmatic processes: Hadean to current contexts

Xenon compatibility in magmatic processes: Hadean to current contexts

Insights on the origin of oldhamite in enstatite meteorites from Ca stable isotopes

In order to understand the origin of oldhamite (CaS) in enstatite meteorites, we report Ca isotopic compositions (δ44/40Ca) of oldhamite (obtained from water leachate of bulk chondrites and aubrites and mineral separates from the Norton County aubrite) and silicate minerals from different types of enstatite chondrites and aubrite. The δ44/40Ca of the bulk enstatite chondrites range from 1.05 ‰ to 1.24 ‰, with an average of 1.13 ± 0.12 ‰, higher than that of the estimate of the bulk silicate earth (∼0.94 ‰). Major and trace element analyses show that the water leachates of enstatite chondrites are mainly composed of oldhamite, and they take over 20.6–68.5 % Ca of the bulk meteorite Ca budget. The Ca isotope fractionation between oldhamite and residual silicate (Δ44/40Caoldhamite-silicate) for the studied enstatite chondrites is minimum (−0.44 ‰) for Abee (impact-melt breccia) and maximum (+0.16) for St.Marks (EH5). The Ca isotope fractionation between oldhamite (individual mineral grains and leachate) and silicates in Norton County varies from −0.47 ‰ to −0.31 ‰ with an average of −0.41 ‰. These Δ44/40Caoldhamite-silicate correlate well with previous theoretical calculation and suggests that the oldhamites in Norton County are in isotopic equilibrium with co-existing silicates, and therefore were formed during magmatic processes. However, in enstatite chondrites, the large variation on Δ44/40Caoldhamite-silicate and its negative correlation with metamorphic temperature reflects the redistribution and equilibration of Ca isotopes during metamorphism. The variable Δ44/40Caoldhamite-silicate found in unequilibrated chondrites reflect kinetic Ca isotope fractionation between oldhamite and nebular gas and therefore is evidence for the formation of oldhamite by condensation in the solar nebula.

Deciphering Clues Regarding Magma Composition Encoded in Quartz‐Hosted Embayments and Melt Inclusions Through Direct Numerical Simulations

AbstractCrystal‐hosted melt embayments and melt inclusions partially record magmatic processes at depth, but it is not always obvious how to interpret this record. One impediment is our incomplete understanding of how embayments and melt inclusions form. In this study, we investigate the formation mechanism of embayments and melt inclusions during quartz growth to quantify the relationship between the compositions of the entrapped and average melt. We study the growth of embayments and inclusions through direct numerical simulations that couple the growth of a crystal surface with the evolution of the concentrations of incompatible components in the surrounding melt. We find that H2O is more enriched in the interior of defects on crystal surface compared to the exterior. The resultant lower disequilibrium in the defect interior causes lower growth rate than in the exterior, elongating the defect into an embayment. If crystal growth stops, the composition in the embayment equilibrates with the average melt within days to months. If crystal growth continues until the embayment neck closes, a melt inclusion forms. The melt entrapped by both embayments and melt inclusions is enriched in incompatible components, such as H2O and CO2. In addition to inclusion size, the enrichment of incompatible components in melt inclusions also depends on component diffusivity and the crystal growth regime. High‐diffusivity components like H2O have similar enrichment levels in all scenarios, while lower‐diffusivity components like CO2 are more enriched in melt inclusions with smaller sizes or formed in continuous crystal growth.

Metallogenic Difference between the Late Aptian Nansu and Aishan Pluton in Jiaodong: Constraints from in Situ Apatite Elemental and Nd Isotopic Composition

A series of Mo-polymetallic deposits have been developed in the Jiaodong Peninsula. Notably, these Mo-dominant deposits formed essentially during the same period as the well-known world-class Au deposits in this area, hinting at a potentially unique geological correlation between them. Therefore, conducting thorough research on Mo deposits in Jiaodong holds significant importance in exploring the area’s controlling factors of Mesozoic metal endowments. To reveal the petrogenesis and metallogenic potentials of Mo-fertile and ore-barren granitoid, apatite grains from the Late Aptian Nansu granodiorite and Aishan monzogranite are investigated in this study. Detailed petrographical observations, combined with in situ analysis of electron probe micro-analyzer (EPMA) and Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), have been conducted on apatite grains from the Nansu and Aishan plutons. This comprehensive analysis, encompassing both major and trace elements as well as isotopic characteristics of apatite, aims to elucidate the metallogenic differences within the late Early Cretaceous granitoids of Jiaodong. The results reveal that the apatite grains across all samples belong to fluorapatites, suggesting their magmatic origin. Additionally, chondrite-normalized rare earth element (REE) patterns of apatites in ore-fertile and ore-barren granitoids exhibit a “right-leaning” trend, characterized by relative enrichments in light REEs and depletions in heavy REEs. Both the Nansu and Aishan plutons exhibit moderately negative Eu anomalies (with averages δEu values of 0.44 and 0.51, respectively), along with slightly positive Ce anomalies (averaging δCe values of 1.08 and 1.11, respectively). A negative correlation is observed between their δEu and δCe values, indicating that the parental magmas of ore-fertile and ore-barren granitoids were formed in a relatively oxidizing environment. The calculated apatite OH contents for the Nansu pluton range from 0.26 to 1.38, while those for the Aishan pluton vary between 0.24 and 1.51, indicating comparable melt H2O abundances. Consequently, the results suggest that neither the oxygen fugacities nor the water contents of the parental magma can account for the metallogenic differences between Nansu and Aishan plutons. The apatite in the Nansu pluton exhibits a higher Ce/Pb ratio and a relatively lower Th/U ratio, indicating the involvement of a greater volume of fluids in the magmatic evolution process of this ore-bearing granitoid. Apatite grains sourced from the Nansu and Aishan plutons exhibit εNd(t) values ranging from −16.63 to −17.61 (t = 115.7 Ma) and −17.86 to −20.86 (t = 116.8 Ma), respectively. These results suggest that their parental magmas primarily originated from the partial melting of Precambrian metamorphic basement rocks within the North China Craton, with a minor contribution from mantle-derived materials. Additionally, the presence of mafic microgranular enclaves in both the Nansu and Aishan plutons indicates that both have undergone magma mixing processes. The binary diagrams plotting the ratios of Ba/Th, Sr/Th, and U/Th against La/Sm demonstrate that apatite grains of ore-fertile granitoid exhibit a distinct trend towards sediment melting. This suggests the potential incorporation of sedimentary materials, particularly those rich in molybdenum, into the magmatic source of the Nansu pluton, ultimately leading to the occurrence of molybdenum mineralization.

Teleseismic Indication of Magmatic and Tectonic Activities at Slow- and Ultraslow-Spreading Ridges

Magmatic and tectonic processes in the formation of oceanic lithosphere at slow–ultraslow-spreading mid-ocean ridges (MORs) are more complicated relative to faster-spreading ridges, as their melt flux is overall low, with highly spatial and temporal variations. Here, we use the teleseismic catalog of magnitudes over 4 between 1995 and 2020 from the International Seismological Center to investigate the characteristics of magmatic and tectonic activities at the ultraslow-spreading Southwest Indian Ridge and Arctic Gakkel Ridge and the slow-spreading North Mid-Atlantic Ridge and Carlsberg Ridge (total length of 14,300 km). Using the single-link cluster analysis technique, we identify 78 seismic swarms (≥8 events), 877 sequences (2–7 events), and 3543 single events. Seismic swarms often occur near the volcanic center of second-order segments, presumably relating to relatively robust magmatism. By comparing the patterns of seismicity between ultraslow- and slow-spreading ridges, and between melt-rich and melt-poor regions of the Southwest Indian Ridge with distinct seafloor morphologies, we demonstrate that a lower spreading rate and a lower melt supply correspond to a higher seismicity rate and a higher potential of large volcano-induced seismic swarms, probably due to a thicker and colder lithosphere with a higher degree of along-axis melt focusing there.

The Zircon U-Pb Age, Hf Isotopes, and Lithogeochemistry of Ore-Bearing Rocks from the Archean Hongtoushan Volcanogenic Massive Sulfide Deposit in the North China Craton: Implications for Tectonic Setting

Volcanogenic massive sulfide (VMS) deposits are globally significant sources of metals. The Hongtoushan VMS deposit is the only large Archean Cu-Zn VMS deposit in the North China Craton, carrying substantial economic value. Significant deformation and metamorphism have made the tectonic setting of the Hongtoushan VMS deposit the subject of extensive debate. This study investigates the petrogenesis and chronology of the ore-bearing host rocks from the Hongtoushan Cu-Zn VMS deposit in the North China Craton. By utilizing whole-rock geochemical analyses and zircon dating, this research sheds light on the origin and evolution of the ore-bearing rocks within the deposit. The whole-rock geochemical analysis data indicate that the Hongtoushan ore-bearing rock series is mainly composed of amphibole plagioclase gneiss (basalt protolith) and biotite plagioclase gneiss (andesite and rhyolite protolith), suggesting a complete volcanic cycle from basic to medium-acidic volcanic rocks. The amphibole plagioclase gneiss has slight LREE enrichment patterns with unremarkable depletions of Nb, Ta, and Ti and belongs to contaminated ocean plateau basalt (OPB) in terms of composition, which is generally interpreted as being generated from the mantle plume head. Meanwhile, the biotite plagioclase gneiss has relatively steep LREE enrichment distribution patterns with remarkable negative Ta, Nb, and Ti anomalies and a wide range of Zr/Y ratios, indicating their classification as FI- and FII-type felsic rocks; they were likely formed through the fractional crystallization of basic magma combined with crustal assimilation. When combined with the zircon dating results, the ore-bearing host rocks of the Hongtoushan VMS deposit were generated via a continuous magmatic evolution process. The zircon dating of the host rocks indicates a formation age of between 2609 and 2503 Ma, with metamorphic events between 2540 and 2466 Ma, which is consistent with the 2.5 Ga-related global mantle plume event. Further research shows that the ore-bearing host rocks are more likely to have been formed in a mantle plume-related stretching environment, possibly a margin rift.

Numerical Modeling of Melting Processes During Slab Break-off: Insights Into Tectonic Setting for Massif-Type Anorthosites

Abstract The concept that lithosphere detachment or break-off has long been conceived as a viable mechanism to explain prominent geological phenomena in Earth’s crust and the surface. One of the strengths of slab delamination mechanism is that it can account for the extensive magmatism in active orogenic belts due to the upwelling of the asthenosphere after the slab break-off. However, in the last 20 years, geodynamic simulations show that the inflow of the asthenosphere upon slab break-off is insufficient to cause significant melting of the overriding lithosphere adjacent to the slab window. The primary reasons include the occurrence of slab break-off at a location that is too deep to effectively heat the overriding lithospheric mantle. Another factor is the presence of a thin film of crustal material that is retained during the slab break- off, inhibiting a significant thermal perturbation within the lithosphere. In this work, we couple petrological–thermomechanical simulations with magmatic melting processes to examine the lithospheric melting and surface lithological expression associated with slab break-off. Our work shows that in the early Earth when the mantle temperature is relatively higher, shallow slab break-off can give rise to significant lithospheric melting during the development of slab break-off. Moreover, because the slab becomes weaker in the earlier hotter mantle, it may break-off prior to the stage of continental collision, thus the magmatism it induced may not give a direct constraint on the time of continental collision. Our study has implications for the interpretation of geological and tomography studies in orogenic belts. It also provides insights into reconciling conflicts between geodynamic and geological studies regarding slab break off-induced melting and magmatism.

Trace elements in zircon record changing magmatic processes and the multi-stage build-up of Archean proto-continental crust

Zircon trace element geochemistry has become an increasingly popular tool to track crustal evolution through time. This has been especially important in early-Earth settings where most of the crust has been lost, but in some fortuitous instances detrital zircons derived from that lost crust have been preserved in younger sediments. To study the formation and geochemical evolution of continental crust from the Hadean to the Paleoarchean, the 3.6 to 3.2 Ga Barberton Greenstone Belt in southern Africa is an excellent target due to its outstanding preservation and presence of detrital zircons that span almost a billion years. Here, we use trace elements, in combination with hafnium and oxygen isotopes, of 3.65 to 3.22 Ga detrital and tuffaceous zircons of the Moodies and Fig Tree groups and compare their geochemistry to previously studied 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed of the Onverwacht Group. The major detrital zircon age clusters in the former at 3.55 Ga, 3.46 Ga, and 3.26–3.23 Ga overlap with episodes of TTG emplacement and felsic volcanism in the Barberton area, suggesting a local provenance. In contrast, age clusters at 3.65 Ga and 3.29 Ga of the Moodies and Fig Tree groups as well as 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed do not have known intrusive sources and were likely derived from outside the present-day Barberton belt. This indicates that more than half of the felsic igneous events in the detrital zircon record do not have a whole-rock representation that can be directly studied. The similar compositions and inferred crustal evolution histories recorded in zircons from the Fig Tree and Moodies groups, as well as from the Green Sandstone Bed, suggest that they were derived from connected terranes experiencing similar crustal processes diachronously. Together, they show three phases of felsic continent formation, reflecting different crustal processes: (1) long-lived protocrust formed in the Hadean from undepleted mantle sources. These zircons are vastly different from younger zircons and, hence, Barberton TTGs are not good analogues of Hadean crust formation. (2) At 3.8 Ga, onset of significant crustal growth though cyclic juvenile additions and hydrous melting, possibly within a volcanic plateau setting but an arc-like setting cannot be excluded based on this data. (3) Between 3.4 and 3.3 Ga, felsic crust is generated through a previously unrecognized episode of crustal growth by shallow melting of mafic, mantle-derived sources. This is immediately followed by the onset of crustal thickening through the transport of surface-altered, hydrated materials to deep crustal levels. Since there is geological evidence for extension and shortening at that time this may reflect the onset of horizontal movement. Whether this last geodynamic setting reflects modern-style plate tectonics or not, continent formation and the onset of plate tectonics in the Barberton area occurred through complex multi-stage processes spanning almost a billion years, most of which is only accessible through the detrital zircon record.

A Multi-Method Approach to Geophysical Imaging of a Composite Pluton in North Portugal

Potassium (K), thorium (Th), and uranium (U) are good markers of magmatic or alteration processes and the surface concentrations of these radioelements can be mapped at the regional or local scale through radiometric (gamma) surveys. In this study, a radiometric survey was performed in a post-orogenic pluton located in North Portugal, namely the Lamas de Olo Pluton, composed by three granitic facies. This pluton has already been intensively studied, including magnetic susceptibility, gravimetric, geochemical, and petrographic studies. The main objective of this work is to evaluate the radiometric data and combine them with other characteristics, such as magnetic susceptibility, and gravimetry, as well as to elucidate structures such as faults and fractures, outline geological boundaries, and identify alteration zones within various granites of the pluton. The radiometric approach reveals the spatial distribution of radioelements, offering a more distinct portrayal of the geology in the studied area. The radioactive heat production rate was calculated for the studied pluton, showing that the mean value is 4.09 µW m−3, surpassing the known mean values for granites. Our study highlights that radiometric measurements unveil compositional variations within granitic pluton and aid in identifying feeder zones. Furthermore, these measurements can be correlated with each type of granites, demonstrating associations with surface concentrations of K-Th-U. Our findings indicate a spatial alignment between the NE feeder root and a U-rich granite (Barragem granite), as evidenced by its elevated concentration of this radioelement. Conversely, the other root displays a notable relative concentration of Th, consistent with the Th-rich characteristics observed of the two other granites (Lamas de Olo and Alto dos Cabeços granites).