Mantle Source Research Articles

Origin of the post-orogenic dyke swarms of Saharan Metacraton, at Qaret El-Maiyit-Bir Safsaf area, southwest Egypt: Constraints on the magmatic–tectonic processes at the end of the Precambrian

Deep in the Western Desert of Egypt, in the southernmost part, between Qaret El-Maiyit and Bir Safsaf, swarms of dykes cut through the Neoproterozoic rocks. This area lies halfway between the juvenile crust of the Arabian-Nubian Shield (ANS) and the Gebel Kamil terrains, close to the border with Libya. Acidic dykes include rhyolites and trachy-dacites. Intermediate dykes include trachy-andesite, basalt-trachy-andesite, and basalt-andesite, while basic dykes consist of basalt. Felsic dykes are more numerous and younger compared to the mafic (intermediate and basic) dykes. Felsic dykes trend mostly run to the northeast and northwest, while mafic dykes mainly run to the northwest, less frequently to the eastwest. Acidic and intermediate dykes show elevated REE concentrations (up to 164 and 203 ppm, respectively) with highly fractionated patterns (av. (La/Lu)N = 20.56 and 18) and moderately fractionated HREEs (av. (Gd/Lu)N = 2.5 and 3.2) and LREEs (av. (La/Sm)N = 5.7 and 3.6 respectively). The basic dyke samples exhibit modest REE concentrations (up to 112 ppm), weakly fractionated patterns (av. (La/Lu)N = 8), and mildly to weakly fractionated HREE (av. (Gd/Lu)N = 2.3) and LREEs (av. (La/Sm)N = 2.5) patterns. There are no recognizable Eu anomalies in the dyke samples. The magma ascended in an extensional setting and the geochemical features indicate a subduction mode, possibly originating from the Atmur-Delgo suture zone in northern Sudan or by mantle delamination during the early Neoproterozoic. Both the felsic and mafic dykes are derived from a mafic calc-alkaline melt and show fractionation on a single downward line, indicating a genetic relationship. The mafic dykes were formed by partial melting of an enriched mantle source (about 10%) that started at 2.7–3.0 GPa and a solidus temperature of about 1420 °C. The enrichment of the mantle melt beneath the Saharan metacraton, which is the origin of the studied melts, could be related to mantle delamination.

Geochemical and temporal evolution of Indian MORB mantle revealed by the Investigator Ridge in the NE Indian Ocean

The Investigator Ridge, located along the prominent Investigator Fracture Zone in the Wharton Basin, northeast Indian Ocean, was chosen to investigate the temporal and geochemical evolution of the Indian upper mantle. We present new 40Ar/39Ar age, as well as major and trace element and Sr-Nd-Pb-Hf isotope data, from a large part of this so far scarcely sampled ridge. The 40Ar/39Ar ages range from 100.0 Ma to 64.1 Ma, and except for the two youngest samples (65.5 and 64.1 Ma) of likely Christmas Island Seamount Province (CHRISP) origin, display decreasing ages along the ridge from south to north. This pattern is consistent with the ages derived from paleomagnetic anomalies on the local oceanic crust that also decrease in age northwards and suggests an origin of the studied rocks at or near the now extinct Wharton spreading center. These ages also provide information about the displacements along the multiple Investigator Fracture Zones indicating large left-lateral as well as right-lateral offsets of the paleo spreading axis. The fracture zone also shows signs of recent tectonic reactivation. The Investigator Ridge samples are derived from a DUPAL-like mantle source similar to present-day Indian MORB. This source contains a depleted mantle type component, an enriched, moderately HIMU-like, component such as common “C” or “FOZO”, and an additional enriched component of recycled subcontinental lithospheric mantle material. In both the Investigator Ridge and CHRISP samples, the trace element and isotope compositions systematically change with age implying a temporal geochemical evolution of the Indian Mantle Domain in this region. It contained higher proportions of subcontinental lithospheric material delaminated during breakup of Gondwana and mixed with depleted upper mantle material in its early phase. With time, the source evolved in the direction of “C” or “FOZO”, which then became the dominant enriched component and is observed in the youngest samples.

Lamprophyres from the Chhota Udaipur alkaline–carbonatite sub-province, Deccan Large Igneous Province, India: implication for petrogenesis and crustal structure

This paper discusses the petrogenesis of lamprophyre dykes occurring in the Chhota Udaipur Alkaline–Carbonatite Sub-province (CUACS). Both calc-alkaline as well as alkaline varieties occur in the area. The major and trace elements geochemistry of the CUACS lamprophyres display good correlation with each other and other Deccan-related lamprophyres. Trace element ratios reveal a minor role of crustal contamination and support their derivation from partial melting of a lithospheric mantle source that is significantly modified by interaction with asthenosphere-derived melts emanating from an impinging mantle plume. The restricted initial 87 Sr/ 86 Sr (0.70599–0.70658) and 143 Nd/ 144 Nd (0.512501–0.512513) ratios of the CUACS lamprophyres further attest to their derivation from an ocean island basalts (OIB)-type parental melt. Their negative εNdi values (−1 to −6) are comparable with other lamprophyres of the Deccan Large Igneous Province (LIP). The lamprophyres from the present study share similar geochemical characteristics to Mt Girnar lamprophyres, which are derived from the spinel–garnet transition zone at a depth of ∼85 km, distinct from the previously considered depth (∼100 km) of the post-Deccan lithosphere–asthenosphere boundary (LAB) beneath NW India. Such geochemical characteristics of the Chhota Udaipur lamprophyres indicate a vital role of plume–lithosphere interaction at the thermal boundary layer at about ∼66 Ma.

Apatite and pyroxene as records of magmatic–hydrothermal processes and platinum-group mineral (PGM) formation in the Wengeqi mafic–ultramafic intrusion, Inner Mongolia, China: The role of oxidized, H2O-rich magmas

The Devonian Wengeqi mafic–ultramafic intrusion, situated on the northern margin of the North China Craton (NCC) — an Andean-style convergent margin during the Paleozoic — hosts many platinum-group minerals (PGMs), especially within the S-deficient, magnetite-rich clinopyroxenite zones. These PGMs occur in two distinct associations: Pt-rich PGMs (e.g., sperrylite) closely associated with primary magnetite and Pd-rich PGMs (e.g., sudburyite, kotuskite, arsenopalladinite) associated with secondary minerals (e.g., actinolite, pyrite). The mechanisms underlying the formation of PGMs in S-deficient rocks and the spatial decoupling of various PGM types remain elusive. The relationship of PGM formation and characteristics of magmas at convergent margins is also not well understood. In this contribution, we utilized apatite and clinopyroxene textures and chemistry alongside whole-rock SrNd isotopes to characterize i) the nature of the parental magma from which the Wengeqi intrusion crystallized, and ii) the magmatic–hydrothermal processes that operated to generate the Pt- and Pd-rich PGM. Based on the composition of clinopyroxene and clinopyroxene–melt partition coefficients, the parental magma of the Wengeqi intrusion is estimated to have an arc-like affinity on a primitive mantle-normalized trace-element diagram. This, together with the EMI-like SrNd isotope signature, implies that the Wengeqi magma was sourced from metasomatized SCLM beneath the NCC with an EMI-like composition. Three types of apatite (Ap1, Ap2, and Ap3) were identified within the intrusion. Ap1 and Ap2 are magmatic in origin, and appear as isolated grains, whereas Ap3 occurs as stringers or heterogeneous domains within Ap1 and Ap2, and likely resulted from hydrothermal modification of magmatic apatite. Notably, the abundant Ap2 exhibits higher VS contents and Eu/Eu* ratios than the less common Ap1, and contains S predominantly in the form of S6+, suggesting that the Wengeqi magma was relatively oxidized, with fO2 > FMQ + 1.2. Based on the chemistry of magmatic apatite, the Wengeqi magma had ∼5 wt% H2O, which, together with its high fO2, facilitated crystallization of magnetite, causing reduction of the magma by removal of Fe3+. This reduction process promoted PGM nucleation by decreasing PGE solubility in the magma, leading to the association of PGM with magnetite. Additionally, the oxidized, H2O-rich magma likely released oxidizing fluids, selectively mobilizing Pd rather than Pt, separating Pt-rich PGMs from Pd-rich PGMs. The budget of PGE in the parental magma could have been increased by i) metasomatism of the SCLM source (e.g., by carbonatitic fluids), which would have elevated the concentration of PGE in the mantle source, and ii) the high fO2 and H2O levels of the magma, which would have delayed sulfide saturation. Accordingly, mantle metasomatism and high fO2–H2O are deemed favorable for enrichment of PGE in magmas within convergent margins.

Rhenium–Os isotopic systematics of late Cenozoic intraplate basaltic rocks from Korea: Implications for recycled slab materials in their mantle source

We present, for the first time, comprehensive Re-Os isotopic data for 23 late Cenozoic intraplate basaltic rocks from seven locations (Mount Baekdu, Jeongok, Baengnyeong Island, Ganseong, Ullueng Island, Dok Island, and Jeju Island) on the Korean Peninsula. The Re-Os isotopic system serves as an important tracer of processes such as the reworking of old continental lithosphere or the deep crustal recycling. We use these data to investigate the role of the lithospheric mantle, which is currently debated as a source of the basaltic rocks, and to constrain the source lithology. The initial 187Os/188Os ratios of the Korean basalts range from 0.1321 to 0.6455, with γOs values of 4.0–408.3. The γOs values do not show a simple correlation with Sr-Nd-Hf-Pb isotope compositions. There are no significant correlations between MgO contents and 187Os/188Os ratios or Os contents and 187Os/188Os ratios, which demonstrate that crustal assimilation did not produce the supra-chondritic Os isotopic compositions. The γOs values of the Korean basalts are much higher than previously published γOs of spinel peridotite xenoliths (−9.1 to +1.2) hosted in the basalts, indicating the subcontinental lithospheric mantle was not the main source of the basaltic magmatism. The combination of the Os and Sr-Nd-Hf-Pb isotopic compositions suggests that recycled oceanic crust (potentially Pacific oceanic lithosphere), along with pelagic sediments, which possibly reside in the mantle transition zone, is the enriched component with the supra-chondritic Os isotopic composition in the mantle source of the Korean basalts.

Deep recycling of volatile elements in the mantle: Evidence from the heterogeneous B isotope in intra-plate basalts

Volatiles in the mantle are crucial for Earth’s geodynamic and geochemical evolution. Understanding the deep recycling of volatiles is key for grasping mantle chemical heterogeneity, plate tectonics, and long-term planetary evolution. While subduction transfers abundant volatile elements from the Earth’s surface into the mantle, the fate of hydrous portions within subducted slabs during intensive dehydration processes remains uncertain. Boron isotopes, only efficiently fractionating near the Earth’s surface, are valuable for tracing volatile recycling signals. In this study, we document a notably large variation in δ11B values (−14.3‰ to +8.2‰) in Cenozoic basalts from the South China Block. These basalts, associated with a high-velocity zone beneath East China, are suggested to originate from the mantle transition zone. While the majority exhibit δ11B values (−10‰ to −5‰) resembling the normal mantle, their enriched Sr-Nd-Pb isotope compositions and fluid-mobile elements imply hydrous components in their source, including altered oceanic crust and sediments. The normal δ11B values are attributed to the dehydration processes. Remarkably high δ11B values in the basalts indicate the presence of subducted serpentinites in their mantle source. A small subset of samples with low δ11B values and radiogenic isotope enrichments suggests a contribution from recycled detrital sediments, though retaining minimal volatile elements after extensive dehydration. These findings provide compelling evidence that serpentinites within subducted slabs predominantly maintain their hydrous nature during dehydration processes in subduction zones. They may transport a considerable amount of water into deep mantle reservoirs, such as the mantle transition zone.

Forearc magmatism in the Pannonian basin recorded by metasomatised skarnoid xenoliths in Pliocene basalt (Novohrad-Gemer Volcanic Field, Slovakia)

Anorthite-diopside-spinel-forsterite xenoliths occur in basaltic lava flow in northern part of the Pannonian basin in the Novohrad-Gemer Volcanic Field (NGVF). Several lines of geochemical evidence suggest that the typical skarn assemblage originated by multiple carbonatitic and alkaline silicate metasomatism of pristine basalt to picrobasalt protolith rather than by the modification of crustal carbonates around shallow magma chambers. Interaction with mantle-derived magnesite or dolomite carbonatite melt triggered the crystallization of neoformed forsterite at the expense of primary clinopyroxene. The alkaline silicate metasomatism resulted in the replacement of primary diopside and augite by neoformed aegirine and aegirine-augite accompanied by amphiboles (richterite, katophorite, arfvedsonite). Weakly metasomatised Group 1 xenoliths retained their fore-arc magmatic geochemical signatures: flat REE patterns, low concentrations of high-field strength elements (Ti, P, Zr) and Ni compared to MORB, low ratios of highly-to-less incompatible elements (Ti/V, Zr/Y) relative to MORB, low Na2O, low total REE, and supra-subduction affinity of primary clinopyroxenes, being thus analogues to fore-arc basalts (FAB). In contrast, strongly metasomatised Group 2 xenoliths exhibit LREE-enriched patterns sub-parallel with those of host trachybasalts. Some Group 1 xenoliths also exhibit large positive Eu anomalies and strongly depleted signatures diagnostic of low-pressure plagioclase-pyroxene cumulates. One xenolith with complementary negative Eu, Sr and positive Rb anomalies has been identified as high-Mg basaltic andesite melt residuum. Sr and Nd isotopes plot along the OIB-MORB array. The Group 1 converges at initial 87Sr/86Sr and 143Nd/144Nd ratios of 0.7038–0.7041 and 0.5127–0.5128, respectively, thus recording a 140–190 Ma old mantle source. Thermobarometry data on metasomatically unmodified pyroxenes have been arranged along decompression cooling trend, propagating from ∼1230 °C and ∼ 1 GPa and ending at ∼1160 °C around atmospheric pressure. The maximum pressure indicates the melt generation zone ∼35 km deep, reflecting thin lithospheric mantle. The FABs identified for the first time in the AlCaPa megaunit corroborate the affinity of the NGVF lithospheric segment to Dinarides.

Volatiles and Redox Along the East African Rift

AbstractThe upper mantle under the Afar Depression in the East African Rift displays some of the slowest seismic wave speeds observed globally. Despite the extreme nature of the geophysical anomaly, lavas that erupted along the East African Rift record modest thermal anomalies. We present measurements of major elements, H2O, S, and CO2, and Fe3+/ΣFe and S6+/ΣS in submarine glasses from the Gulf of Aden seafloor spreading center and olivine‐, plagioclase‐, and pyroxene‐hosted melt inclusions from Erta Ale volcano in the Afar Depression. We combine these measurements with literature data to place constraints on the temperature, H2O, and fO2 of the mantle sources of these lavas as well as the initial and final pressures of melting. The Afar mantle plume is C/FOZO/PHEM in isotopic composition, and we suggest that this mantle component is damp, with 852 ± 167 ppm H2O, not elevated in fO2 compared to the depleted MORB mantle, and has temperatures of ∼1401–1458°C. This is similar in fO2 and H2O to the estimates of C/FOZO/PHEM in other locations. Using the moderate H2O contents of the mantle together with the moderate thermal anomaly, we find that melting begins at around 93 km depth and ceases at around 63 km depth under the Afar Depression and at around 37 km depth under the Gulf of Aden, and that ∼1%–29% partial melts of the mantle can be generated under these conditions. We speculate that the presence of melt, and not elevated temperatures or high H2O contents, are the cause for the prominent geophysical anomaly observed in this region.

Alkaline Devonian magmatism of the Menorca Island: Tracking the mantle isotopic sources in the realm of the western Paleo-Tethys Ocean

Several locations with alkaline magmatism are recognised in Silurian-Devonian basins along the southern variscan autochthon units (e.g. Central Iberian Zone) of the northern Gondwana margin. The origin of the Devonian basins and their magmatism has not been studied in the context of the passive margin of Gondwana. The basement of Menorca, Balearic Islands, consists of a deep Devonian-Carboniferous basin with mafic igneous rocks, the Tramuntana Gabbros. In this study, we trace the geodynamic setting and isotopic sources of the Tramuntana Gabbros through elemental geochemistry, isotopic geochemistry (SrNd) and UPb geochronology in zircons. These gabbros are the product of an intraplate alkaline magmatism with immobile trace element and REE contents similar to those of Ocean Island Basalts. Average 87Sr/86Sr(370) of 0.708456 and εNd(370) of +4.0 indicate a source similar to a Type-2 enriched mantle with average TDM of 665 Ma, suggesting a relatively old metasomatized mantle. Concordant UPb ages of c. 597 Ma (Ediacaran, radiometric age) from a single population of 31 zircons separated from the Tramuntana Gabbros (Devonian, biostratigraphic age) reinforce the presence of older units in the corresponding lithospheric mantle. The Tramuntana Gabbros and the Devonian-Carboniferous sequences of Menorca limit the westward extension of the Paleo-Tethys Ocean, whose development never reached these westernmost regions. Assuming a common sublithospheric mantle source for the peri-Gondwanic Devonian alkaline rocks and considering the previous Cadomian (Neoproterozoic) subduction to be the most favourable origin of the separated zircons, the bulk rock TDM and zircon UPb data obtained from the Tramuntana Gabbros track the mixing, recycling, and mantle accretion in this peri-Gondwanic section from Precambrian to Devonian times.

Geochemistry of Olivine Melt Inclusion Reveals Interactions Between Deeply Derived Carbonated Melts From the Big Mantle Wedge and Pyroxenite in the Lithospheric Mantle Beneath Eastern Asia

AbstractOur current knowledge regarding the distribution patterns of ancient and recent recycled materials, as well as the origin of intraplate basalts in the eastern Asia Big Mantle Wedge (BMW), is limited. To address this, we conducted a detailed geochemical analysis of olivine melt inclusions (OLMI) in nephelinite samples. The normal OLMIs detected in our investigation exhibit geochemical features that closely resemble those of the hosting nephelinite, indicating a consistent association with a carbonated mantle source. Additionally, we identified a distinct group of anomalous OLMIs that displayed different geochemical characteristics from the hosting nephelinite but showed similarities to regional alkali basalts sourced from pyroxenite. The observed geochemical diversity in the nephelinite OLMIs suggests an interaction between deeply derived carbonated melts originating from the flattened Pacific slab in the mantle transition zone and pyroxenite in the lithospheric mantle. Carbonated fluid‐fluxed melting is key to basalt formation in the BMW.

Deep crustal assimilation during the 2021 Fagradalsfjall Fires, Iceland.

Active basaltic eruptions enable time-series analysis of geochemical and geophysical properties, providing constraints on mantle composition and eruption processes1-4. The continuing Fagradalsfjall and Sundhnúkur fires on Iceland's Reykjanes Peninsula, beginning in 2021, enable such an approach5,6. Earliest lavas of this volcanic episode have been interpreted to exclusively reflect a change from shallow to deeper mantle source processes7. Here we show using osmium (Os) isotopes that the 2021 Fagradalsfjall lavas are both fractionally crystallized and strongly crustally contaminated, probably by mid-ocean-ridge gabbros and older basalts underlying the Reykjanes Peninsula. Earliest eruptive products (187Os/188Os ≤ 0.188, platinum (Pt)/iridium (Ir) ≤ 76) are highly anomalous for Icelandic lavas or global oceanic basalts and Os isotope ratios remain elevated throughout the 2021 eruption, indicating a continued but diluted presence of contaminants. The 2022 lavas show no evidence for contamination (187Os/188Os = 0.131, Pt/Ir = 30), being typical of Icelandic basalts (0.132 ± 0.007). Initiation of the Fagradalsfjall Fires in 2021 involved pre-eruptive stalling, fractional crystallization and crustal assimilation of earliest lavas. An established magmatic conduit system in 2022 enabled efficient magma transit to the surface without crustal assimilation.

Neoproterozoic tectonic shift along the western margin of the Yangtze block, South China craton: Implications for its paleogeographic position during the Rodinia−Gondwana transition

Neoproterozoic igneous rocks in the Panxi region from the western margin of the Yangtze block, South China craton, record major changes in tectonic evolution and crustal thickness, and provide important constraints on the paleogeographic position and climate changes of the block in Rodinia and Gondwana reconstructions. Our new dating results show that the Pengguan complex from the Panxi region comprises ca. 743 Ma adakitic granodiorites and ca. 741 Ma A-type granites. The former is plagioclase-rich and has relatively low SiO2 content (65.22−67.36 wt%), high Mg# values (44.40−52.33), Sr/Y (23.22−61.94) and (La/Yb)N ratios (normalized to chondritic values; 13.41−24.00), with positive whole-rock εNd(t) values (+1.17 to +5.15), and high zircon εHf(t) (+5.53 to +8.83) and low zircon δ18O (4.21‰ to 5.51‰) values, indicating derivation from the partial melting of thickened juvenile mafic lower crust. By contrast, the granites are rich in K-feldspar and have high SiO2 contents (78.71−84.62 wt%), 10,000 × Ga/Al ratios (>2.6), and Fe/Mg ratios, with negative εNd(t) values (−1.47 to −0.02), high zircon εHf(t) (+2.94 to +9.77), and high saturation temperatures (828−920 °C). These features indicate fractional crystallization and partial melting of continental crust in a low-pressure, high-temperature, extensional environment. Integrated with the zircon Eu anomalies (Eu/Eu*) data and Hf-O isotopic data on pre-740 Ma magmatism in the Panxi region, we find that a rapid increase of crustal thickening (820−740 Ma) was synchronous with a gradual decrease of zircon δ18O and an increase of zircon εHf(t) values. These variations indicate an increase in the degree of hybridization between mantle sources and subducting slab-derived materials, thus generating a great number of low-δ18O melts, and continuous crustal thickening through the underplating of juvenile magmas. In addition, thinning of the crust since ca. 740 Ma, along with the gradual increase in zircon δ18O and εHf(t), suggests a shift from regional compression to extension. This documented shift, together with regional data from along the western margin of the Yangtze block, indicates a change from the well-developed and long-established Panxi convergent plate margin arc system (at least ca. 870−750 Ma) to passive-margin development and Cryogenian (720−635 Ma) glaciation. The long-lived subduction zone requires a peripheral location of South China that was linked to northern India during the Rodinia breakup. The subsequent and contemporaneous passive-margin development of the Yangtze block and India, along with the similar tectonic affinities of the Cathaysia block with India, indicate that South China lay outboard of the northern Gondwana margin during the assembly of this supercontinent.

Magnetite geochemistry of giant alkalic-type epithermal gold deposits: Insights into the magmatic evolution of mineralized alkaline systems

Magnetite, a common mineral in alkaline magmatic complexes hosting giant epithermal Au deposits, is examined to obtain insights into the magmatic evolution and ore fertility of alkaline systems. Textural and compositional analyses of magmatic magnetite from Cripple Creek, USA, and Lihir Island, Papua New Guinea, reveal distinct magma sources and evolution paths. High HFSE concentrations and high La/Yb ratios in magnetite from Cripple Creek suggest relatively low-degree partial melting of lithospheric and asthenospheric mantle sources, whereas magnetite from Lihir Island indicates higher degree partial melting from subduction-modified mantle. Chromium concentrations track magma evolution, while V systematics record decreasing oxygen fugacity with magma differentiation. Sulfide saturation is observed at Cripple Creek, potentially affecting ore fertility. Magnetite itself bears the capability to scavenge Au from melts as indicated by Au concentrations of up to 40 ng/g. This might point towards elevated Au concentrations in the parental magmas, although experimental data on Au partitioning is lacking. Magnetite compositions overlap across different mineralization styles, challenging its reliability as an exploration tool but analysis of noble metals in magnetite and its inclusion cargo could offer promising alternative approaches. Overall, magnetite serves as a valuable tool for investigating ore deposit petrogenesis and holds potential for exploration.

Pristine helium from the Karoo mantle plume within the shallow asthenosphere beneath Patagonia

Mantle xenoliths usually represent fragments derived from the depleted and degassed lithospheric mantle with 3He/4He isotope ratios (6 ± 1 RA) lower than those of mid-ocean ridge basalts (8 ± 1 RA). Otherwise, basalts from oceanic islands related to hotspots often have high 3He/4He ratios (>10 RA), suggesting a deep and pristine undegassed mantle source. Here we present a striking high-3He/4He component (up to 27.68 RA) recorded by spinel-facies mantle xenoliths from Patagonia. Remarkably, the highest ratios were found in a long-lived trans-lithospheric suture zone related to the Carboniferous-Permian collision of two continental blocks: the Deseado and the North Patagonian massifs. The mantle xenoliths with notably high-3He/4He ratios are inferred to be fragments of the shallow asthenosphere rising through the eroded and rejuvenated thin lithosphere. The pristine helium component is derived from the western margin of the Karoo mantle plume, related to the initial stages of the Gondwana fragmentation.

Age and Petrogenesis of Paleoproterozoic Diorites from Dikes of the Baikal Uplift of the Siberian Craton

Abstract ––We carried out a detailed geological, geochronological, geochemical, and isotope study of diorites from a dike located in the central part of the Baikal uplift of the Siberian craton. The geochemical and isotope data obtained for diorites of the studied intrusion were compared with coeval mafic and intermediate igneous rocks of the southern part of the Siberian craton. The U–Pb (ID-TIMS) baddeleyite age of 1862 ± 7 Ma has been estimated for diorite from a dike located in the area of the Onguren Village. The obtained data are the first reliable age determination for the Paleoproterozoic mafic and intermediate igneous rocks of the Baikal uplift, which are part of the South Siberian postcollisional magmatic belt. The dike is of NE strike. The dike rocks correspond in chemical composition to diorites and are highly differentiated varieties (mg# = 36.5–37.4). There are no significant variations in the composition of diorites in the marginal and central parts of the dike. The rocks are characterized by low contents of TiO2, P2O5, and Nb, high contents of Th, Zr, Ba, and LREE, and low negative values of εNd(T) (–5.9 to –6.2). We assume that the diorites formed from the enriched subcontinental lithospheric mantle. The obtained geochemical and isotope data show similar negative values of εNd(T) from –4.3 to –11.6 for most of Paleoproterozoic mafic and intermediate igneous rocks of the South Siberian postcollisional magmatic belt within the Aldan Shield, the Baikal uplift, and the Irkut block of the Sharyzhalgai uplift. These rocks correspond in geochemical features to rocks resulted from the melting of subduction-modified lithospheric mantle sources in the postcollisional extension setting at the final Paleoproterozoic stage of formation of the Siberian craton.

Slab break-off of the Kalamaili oceanic slab revealed by the latest Carboniferous mafic–ultramafic rocks in eastern North Tianshan (NW China)

Identifying the tectonic transition from oceanic subduction to collision is crucial for tracking the final stage evolution of ancient orogenic belts. In this study, we present new geochronological and geochemical data for the Mozbaysay mafic–ultramafic complex in the Balikun area, eastern North Tianshan of the southern Central Asian Orogenic Belt. This complex had intruded into the late Carboniferous volcano-sedimentary rocks and is comprised mainly of hornblende gabbro and lherzolite. Zircon U-Pb ages of the hornblende-gabbros reveal that this complex was emplaced at ca. 305 Ma. Geochemical analyses suggest these mafic–ultramafic rocks are characterized by slight enrichment of light rare earth elements (LREEs) and relatively depleted heavy rare earth elements (HREEs), resembling enriched mid-ocean ridge basalt (E-MORB). They also exhibit restricted (87Sr/86Sr)i ratios (0.702396–0.704295) and εNd(t) values (+7.0 to +9.1), indicative of a depleted mantle source with minimal crustal contamination. Incompatible element ratios (i.e., Nb/Ta, Zr/Hf, Rb/Nb, and Ba/Nb) suggest the involvement of subducted slab-derived aqueous fluids in their mantle source. These collectively indicate that the parental magmas of the Mozbaysay mafic–ultramafic rocks may have been generated by a mixed mantle source consisting of the E-MORB-like asthenospheric mantle, subcontinental lithospheric mantle (SCLM), and hydrous fluids from subducted slab. Furthermore, a slab break-off model is proposed to explain the generation of these latest Carboniferous mafic–ultramafic rocks. Integrating these findings with regional geological data, we propose that the tectonic transition from subduction (slab roll-back) to collision (slab break-off) along the Kalamaili suture zone occurred at ca. 305–300 Ma.

Lithology, U[sbnd]Pb zircon geochronology, and geochemistry of the Austroalpine Sieggraben Complex: Pre-Alpine vs. Alpine history

The Sieggraben Complex (SC) in Eastern Austria provides comprehensive information on the pre-Permian, Permian, and Alpine (Cretaceous) evolutional stages of the crustal and mantle sections included into the Austroalpine basement HP/UHP belt. The SC in the Sieggraben–Schwarzenbach area consists of three, top-to-bottom tectono-stratigraphic units: (1) the Paragneiss unit, (2) the Marble–Eclogite unit, which contains MORB and OIB types of mafic rocks, and (3) the Micaschist–Calcschist unit. The N-MORB type mafic rocks yield a UPb age of 369.7 ± 2.0 Ma, have εNd(370) = +8.5 to +7.4 and TDM(2st) of 0.42–0.50 Ga, while the E-MORB types (UPb ages around 400 Ma) show εNd(400) = +9.4 to +5.6 and TDM(2st) of 0.37–0.67 Ga, both indicating rather juvenile mantle sources during the formation of the inferred oceanic Devonian basin. The rocks of these units revealed Cambrian, Proterozoic, and Archean zircon sources. The ages of 285.8 ± 9.2 and 265.7 ± 2.1 Ma from eclogite and amphibolite, respectively, and 265.5 ± 1.4 Ma from paragneiss indicate an important Permian metamorphic overprint. Permian granitic dykes dated from ∼265 to 260 Ma, exceptionally at 273 and 253 Ma, crosscut the lithological units. A clinopyroxenite dyke, which crosscuts harzburgite of the underlying mantle fragment, yielded a magmatic age of 253.3 ± 2.9 Ma. Overall, these ages suggest a strong Permian extension, overheating, and melting of the crustal and mantle rocks. The eclogite ages from ∼100 Ma, but mainly between 92 and 90 Ma, constrain the Late Cretaceous metamorphic event terminated by an 88.4 ± 0.8 Ma pegmatite intrusion in the Paragneiss unit. The Permian and Cretaceous metamorphic events often caused resetting of the relatively older zircon ages, together with a decrease of zircon εHf(t) values, and an increase of whole-rock 87Sr/86Sr values. A Carboniferous metamorphic age of ∼340 Ma was rarely detected. The SC finally represents a part of an early Late Cretaceous intra-continental subduction zone, which formed within a strongly-thinned crystalline basement due to the Devonian and Permian extensions. A similar Cretaceous suture zone with HP amphibolite, dated at ∼130–100 Ma by white mica ArAr ages (100–90 Ma exhumation), has been described around the Lubeník line between the Gemeric and Veporic tectonic units of the Inner Western Carpathians in the framework of the Cenozoic AL–CA–PA (Alpine–Carpathian–Pannonian) microplate.

Geochemistry of Cenozoic mafic potassic and sodic volcanic rocks in southwestern Madagascar: Long-lived lithospheric mantle heterogeneities in an extensional tectonic setting

Cenozoic (≥ 43 Ma) silica undersaturated (potassic) trachybasalts and trachyandesites in southwestern Madagascar (Tsianihy-Manja, southern Morondava Basin) form a small monogenetic volcanic field emplaced above Paleogene detritic sedimentary rocks, along a NE-SW-trending fault system. These olivine-chromite±clinopyroxene-phyric primitive lavas (Mg# = 69; MgO = 10–11 wt%; Cr = 450 ppm, Ni = 200 ppm; K2O = 3–4 wt%) have highly peculiar trace element and isotopic composition (e.g., Ba/Nb = 18.4; 87Sr/86Sri = 0.70529–0.70555, 143Nd/144Ndi = 0.51262–0.51263, 206Pb/204Pbm = 18.415–18.424, 207Pb/204Pbm = 15.576–15.579, 208Pb/204Pbm = 38.799–38.813). A hitherto undescribed plug of primitive (sodic) basanite of the 11–12 Ma-old Ankililoaka district south of Tsianihy-Manja (hosting spinel lherzolite mantle xenoliths) has noticeable different geochemistry (Ba/Nb = 8–9.2; 87Sr/86Sri = 0.70346–0.7036, 143Nd/144Ndi = 0.51281–0.51282, 206Pb/204Pbm = 19.079–19.374, 207Pb/204Pbm = 15.621–15.645, 208Pb/204Pbm = 39.115–39.424).The relatively low CaO, Sc, V, Fe2O3t, MnO at high MgO, Cr and Ni, and the potassic affinity of the Tsianihy-Manja trachybasalts, all indicate that the mantle source is relatively clinopyroxene-poor (i.e., depleted by previous melt extractions), in the same way as the source of lamproitic (or boninitic) magmas, but the primitive nature, the concentration of high field strength elements, the incompatible element patterns and their isotopic ratios indicate their unequivocal within-plate setting and indicate a derivation by low-degree partial melting of an incompatible element-enriched mantle and insignificant role for crustal contamination. In terms of incompatible element concentrations, and thus also Sr-Nd-Pb-isotopic composition, we find no evidence in favour of a mid-ocean ridge basalt (MORB)-mantle component, or for a MORB-mantle strongly enriched by ocean island basalt-like components, to form the mantle source regions of the Tsianihy-Manja and Ankililoaka mafic alkaline rocks. The significant isotopic change from the northernmost Cenozoic volcanic rocks of Madagascar and those in the central and southern part of the island (which range in composition from sodic to potassic, and from tholeiitic basalt to olivine melilitite) implicates a distinct source heterogeneity, and ultimately assess the role of the old continental lithospheric mantle as source region.

High-temperature boron partitioning and isotope fractionation between basaltic melt and fluid

In the last two decades, boron has gained significance as a geochemical tracer in mantle studies, particularly related to fluid-mediated processes. In our investigation, we explore how boron and its stable isotopes distribute between basaltic melt and hydrous fluid under conditions relevant to magmatic degassing in the shallow crust (1000–1250 °C, 150–250 MPa). We utilized a synthetic MORB-like composition with added boric-acid isotope standard (NIST-SRM951a) and additional trace elements, subjecting it to varying pressure, temperature, and melt-fluid ratios using an internally heated pressure vessel. The B isotope composition in the quenched glasses were determined through femtosecond laser ablation coupled to a multi-collector inductively-coupled-plasma mass spectrometer. Our experiments revealed that, even at the highest temperatures, boron strongly partitions into the fluid phase, accompanied by significant B isotope fractionation. This leads to an enrichment of the heavy B isotope in the fluid, with a constrained Δ11Bmelt-fluid range of -1.7 ± 0.9‰, consistent with ab-initio modeling results. These findings highlight the potential of B isotopes to trace geochemical processes at elevated temperatures with \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\Delta}^{11}{{B}}_{melt-fluid}=2.913-9.693\\frac{{10}^{6}}{{{T}}^{2}}$$\\end{document}. Our results have implications for predicting the δ11B of degassed, water-bearing basaltic magmas and estimating the B isotope composition of their mantle source.

Slab steepening and rapid mantle wedge replacement during back-arc rifting in the New Hebrides

The effects of the composition and angle of the subducting slab and mantle wedge flow on tectonic and magmatic processes in island arcs and associated back-arcs are poorly understood. Here we analyse the ages and compositions of submarine lavas from the flanks and the floor of the back-arc Futuna Trough some 50 km east of Tanna Island in the New Hebrides arc front. Whereas >2.5 Ma-old back-arc lavas formed from an enriched mantle source strongly metasomatized by a slab component, the younger lavas show less slab input into a depleted mantle wedge. The input of the slab component decreased over the past 2.5 million years while the enriched mantle was replaced by depleted peridotite. The change of Futuna Trough lava compositions indicates rapid (10 s of km/million years) replacement of the mantle wedge by corner flow and slab steepening due to rollback, causing extensional stress and back-arc rifting in the past 2.5 million years.