Mafic Composition Research Articles

Neoproterozoic arc-magmatism of the Silet Terrane, Western Hoggar (Algeria), new constraints from zircon U-Pb-Hf isotope analyses and whole rock geochemistry

This study presents results of zircon U-Pb-Hf isotope analyses and whole-rock geochemistry applied to magmatic rocks of the Silet terrane in the Western Hoggar, forming a part of the Neoproterozoic arc exposed along the boundary between the LATEA metacraton and the Paleoproterozoic In Ouzzal terrane. According to zircon U-Pb geochronology four distinct pulses of Neoproterozoic granitoids magmatism are recognized in the Silet terrane: Tonian (c. 870–840 Ma), Late Tonian (c. 770 Ma), Latest Tonian (c. 730 Ma) and Late Cryogenian (c. 675 Ma).Geochemical and Hf isotope data indicate a progressive shift in geotectonic settings. The c. 870–840 Ma Tonian TTG pulses (Tonalite-Trondhjemite-Granodiorite) in the North Silet batholith, with superchondritic εHf values (+5.8 to + 7.2), suggest formation in a subduction zone. The subsequent c. 770 Ma Late Tonian magmatism, characterized by mafic compositions, reflects a back-arc tectonic setting, as evidenced by high Ti/V ratios, low V contents, and εHf values of + 6.12 to + 8.95. The c. 730 Ma Latest Tonian magmatism, with more evolved acidic pulses, indicates hybrid mantle–crustal involvement (εHf + 6.53 to + 8.08). The Late Cryogenian intrusions (c. 675 Ma), featuring the lowest εHf values (+1.84 to + 4.09) and inherited c. 870 Ma zircons, suggest formation from the melting of Tonian TTGs during continental collision.Integrating U-Pb dating with geochemical data provides a comprehensive view of the Silet terrane’s tectonic evolution, supporting a continental arc origin. Disparities in magmatism ages between the northern (c. 870–675 Ma) and southern (c. 740–640 Ma) segments of the Silet arc correlate with the diachronous collision/subduction of a V-shaped “Silet Ocean”, forming an introverted oceanic system (oceans that have formed by rifted continental blocks that were subsequently re‑joined in approximately the same position) within the broader West Gondwana context. Structural analysis indicates that the regional structures of the Silet arc were shaped in a WNW-ESE collisional transpressive regime during the Pan-African orogeny, offering new insights into the Neoproterozoic Silet arc’s geodynamic evolution and its paleogeographical location in the West Gondwana orogeny.

Geochemistry of fine-grained sedimentary rocks: Implications for source area, paleoweathering, paleoclimate, and provenance of the Santa Maria Formation (Triassic, Paraná Basin, Brazil)

The integration of ICP-OES, XRD, chemical analysis, and scanning electron microscopy was used to investigate the source area, weathering, paleoclimate conditions, and provenance of the siltstones of the Santa Maria Formation (Middle to Upper Triassic, Southern Brazil). This unit is regionally subdivided, from base to top, into the Passo das Tropas Member (Late Anisian–Middle Ladinian) and the Alemoa Member (Middle Ladinian–Carnian). The Chemical Index of Alteration (CIA) was calculated for both members to highlight the weathering processes of primary minerals. The CIA results indicate moderate to extreme weathering of the source rocks for the siltstones. The extreme chemical alteration observed in the Passo das Tropas siltstones (CIA = 76.97–90.83) and in the mid-section of the Alemoa Member (CIA = 87.96–89.63) suggests weathering under more humid conditions, related to an increase in paleorainfall. Paleoclimate was interpreted as semi-arid to arid, with alternating wet and dry seasons. These results align with the geochemical behavior of uranium and thorium. Samples with higher CIA values show higher Th/U ratios, indicating a shift toward a more humid climate. X-ray diffraction analysis of the fine fraction (FF < 2 μm) of the samples identified mixed-layered illite-smectite (I/S) clay minerals with minor amounts of quartz. Regarding provenance, Th/U ratios coupled with Th vs. Sc and La-Th-Sc plots, suggest an upper crustal protolith for the southern Santa Maria siltstones, consisting of uplifted rocks from the crystalline basement of the Sul-Rio-Grandense shield and sedimentary rocks of the Camaquã Basin. The Th/Sc, Sc/Th, Co/Th, and La/Sc ratios, along with the contents of Al₂O₃, SiO₂, TiO₂, Co, V, Ni, and Sc, indicate a more felsic composition for the Passo das Tropas Member. Meanwhile, the source rocks of the Alemoa Member sediments demonstrate a more mafic composition in the mid-section, transitioning to a felsic composition in the uppermost succession.

Crustal structure of the Bushveld complex, South Africa from 1D shear wave velocity models: Evidence for complex-wide crustal modification

Thirty-nine 1D shear wave velocity profiles, obtained by jointly inverting receiver functions and Rayleigh wave group velocities, are used to investigate the crustal structure of the Bushveld Complex in northern South Africa. Data from teleseismic earthquakes recorded on broadband seismic stations between 1997 and 1999 and 2015–2020 were used to compute P-wave receiver functions. Rayleigh wave group velocities between 5 and 30 s period were obtained from an ambient noise tomography and combined with group velocities between 30 and 60 s period from a published continental-scale surface wave tomography model. Moho depths of 45–47.5 km are found under the center of the complex compared to 40 km thick crust, on average, surrounding the complex, indicating ∼5–7 of crustal thickening. The bottom ∼10 km or more of the lower crust across much of the Bushveld Complex has a Vs ≥ 4.0 km/s, consistent with a mafic composition, whereas in most areas around the margins of the complex the thickness of the mafic lower crust is much less than 10 km. In the upper crust higher velocity structure (Vs > 3.6 km/s) above 15 km depth underlain by lower velocity structure is seen in many locations, suggesting the presence of mafic/ultramafic layering. These results favor the continuous-sheet model for the structure of the Bushveld Complex because the ensemble of 1D models is characterized by three diagnostic features consistent with that model: (1) thicker crust under the center of the complex than away from the complex; (2) a greater thickness of high-velocity (i.e., mafic) layering in the lower crust under the complex compared to away from the complex; (3) high-velocity (i.e., mafic/ultramafic) layering in the upper crust beneath much of the complex. The lack of upper crustal mafic/ultramafic layering beneath some parts of the complex is consistent with the post-emplacement tectonic and magmatic history of the complex.

Observations of Igneous Subsurface Stratigraphy during the Jezero Crater Floor Rapid Traverse from the RIMFAX Ground-penetrating Radar

Perseverance traversed the eastern, northern, and western margins of the Séítah formation inlier on the rover’s western fan front approach. Mapping the stratigraphy and extent of the Máaz and Séítah formations is key to understanding the depositional history and timing of crater floor resurfacing events. Perseverance's rapid progress across the Jezero crater floor between the Octavia E. Butler landing site and the western fan front resulted in limited contextual images of the deposits from the Navigation Camera and Mast Camera Zoom. By combining the limited surface images with continuous subsurface sounding by the Radar Imager for Mars’ Subsurface Experiment (RIMFAX) ground-penetrating radar, Jezero crater floor stratigraphy was inferred along this rapid traverse. We produced the first subsurface map of the Máaz formation thickness and elevation of the buried Séítah formation for 2.3 km of the rapid traverse. Three distinct reflector packets were observed in RIMFAX profiles interspersed with regions of low-radar reflectivity. We interpret these reflector packets with increasing depth to be the Roubion member of the Máaz formation (covered in places with regolith), the Rochette member, and the Séítah formation. We found a median permittivity of 9.0 and bulk density of 3.2 g cm−3 from hyperbola fits to RIMFAX profiles, which suggests a mafic composition for Máaz and Séítah. The low-radar reflectivity regions within each reflector packet could indicate potential depositional hiatuses where low-density material like sediment or regolith could have accumulated between successive Máaz formation lava flows and the Séítah formation at depth.

An Orbital Comparison of a Late Mantling Unit on Aeolis Mons With Other Erosion‐Resistant Strata Explored by MSL in Gale Crater, Mars

AbstractPrevious investigations along Curiosity's traverse in Gale crater have explored the relationship between orbital and in situ observations. This work aims to better understand the geologic environment of units only observable from orbit and compare them to the properties of units examined by Curiosity. Here, we map an erosion‐resistant dark‐toned mantling unit that overlies the modern topography of Aeolis Mons (informally known as Mt. Sharp) and compare this unit to two other previously mapped dark‐toned resistant units, the marker band and the mound skirting unit (MSU), that have been inferred to represent different geologic environments (lacustrine and aeolian, respectively). Visible to short wave infrared spectra from the Compact Reconnaissance Imaging Spectrometer for Mars and visual images from the High Resolution Imaging Science Experiment and Context Cameras aboard the Mars Reconnaissance Orbiter are used for this comparison. Spectral data suggest a mafic composition with minor alteration, although the composition varies more with location around Mt. Sharp rather than between units. Morphologically, the mantling unit has strong similarities to the marker band based on their consistent low‐albedo, erosion‐resistance, and smooth appearance, contrasting with the highly variable surface texture of the MSU. We hypothesize that all three units had a similar sediment source but experienced aqueous alteration at different times: early ubiquitous cementation in a surface aqueous environment in the mantling unit and marker band versus patchy late diagenesis in the MSU. If true, these results suggest that water activity continued within the Gale crater long after the erosion of Mt. Sharp.

Compositional Characterization of Glassy Volcanic Material From VSWIR and MIR Spectra Using Partial Least Squares Regression Models

AbstractThe glass phase in volcanic rocks presents a challenge to obtaining compositional data from visible and short‐wave‐infrared (VSWIR) and mid‐infrared (MIR) spectral data of remote surfaces due to its amorphous structure and variable composition. Nonetheless, glass is a common phase in volcanic materials because it forms via the rapid quench of magma and can constitute up to the entirety of a volcanic deposit. Use of partial least squares regression (PLS) to predict glass contents creates models that are insensitive to viewing geometry and sample conditions such as grain size and spectrally inactive compositional variables, enhancing the ability to detect glasses with remote sensing. PLS models are used here to predict crystallinity and oxide composition of samples from VSWIR and MIR spectral data using training spectra from natural volcanic rocks and geologically relevant synthetic samples. Three spectral resolutions of VSWIR and MIR spectra (1, 10, and 100 nm/band, and 1.9, 19, and 190 cm−1/band, respectively) were tested to assess the effects of collection configuration on different spectrometers. PLS models trained on 1 nm and 1.9 cm−1 data sets have the lowest uncertainties of glass modal abundance for VSWIR and MIR, respectively. MIR models predicting sample wt. % SiO2 and FeO, and VSWIR models of wt. % FeO provide accurate estimates (e.g., RMSE‐P of 3.4 wt. % FeO) at all spectral resolutions. Results are based on training data sets skewed to mafic compositions, which affects model accuracies.

3D shear wave velocity imaging of the subsurface structure of granite rocks in the arid climate of Pan de Azúcar, Chile, revealed by Bayesian inversion of HVSR curves

Abstract. Seismic methods are emerging as efficient tools for imaging the subsurface to investigate the weathering zone. The structure of the weathering zone can be identified by differing shear wave velocities as various weathering processes will alter the properties of rocks. Currently, 3D subsurface modelling of the weathering zone is gaining increasing importance as results allow the identification of the weathering imprint in the subsurface not only from top to bottom but also in three dimensions. We investigated the 3D weathering structure of monzogranite bedrock near the Pan de Azúcar National Park (Atacama Desert, northern Chile), where the weathering is weak due to the arid climate conditions. We set up an array measurement that records seismic ambient noise, which we used to extract the horizontal-to-vertical spectral ratio (HVSR) curves. The curves were then used to invert for 1D shear wave velocity (Vs) models, which we then used to compile a pseudo-3D model of the subsurface structure in our study area. To invert the 1D Vs model, we applied a transdimensional hierarchical Bayesian inversion scheme, allowing us to invert the HVSR curve with minimal prior information. The resulting 3D model allowed us to image the granite gradient from the surface down to ca. 50 m depth and confirmed the presence of dikes of mafic composition intruding the granite. We identified three main zones of fractured granite, altered granite, and the granite bedrock in addition to the mafic dikes with relatively higher Vs. The fractured granite layer was identified with Vs of 1.4 km s−1 at 30–40 m depth, while the granite bedrock was delineated with Vs of 2.5 km s−1 and a depth range between 10 and 50 m depth. We compared the resulting subsurface structure to other sites in the Chilean coastal cordillera located in various climatic conditions and found that the weathering depth and structure at a given location depend on a complex interaction between surface processes such as precipitation rate, tectonic uplift and fracturing, and erosion. Moreover, these local geological features such as the intrusion of mafic dikes can create significant spatial variations to the weathering structure and therefore emphasize the importance of 3D imaging of the weathering structure. The imaged structure of the subsurface in Pan de Azúcar provides the unique opportunity to image the heterogeneities of a rock preconditioned for weathering but one that has never experienced extensive weathering given the absence of precipitation.

Significant variations in the structure and composition of the crust beneath northwestern India: Imprints of magmatism

Northwestern India was affected by the Neoproterozoic Malani and Erinpura magmatism, Cretaceous rifting, and magmatism associated with the Deccan precursors. Receiver function analysis was done using 1704 RFs from 18 stations to image and decipher the imprints of the magmatic events and comprehend the crustal modifications in terms of variations in structure and composition. The H-κ grid search and neighbourhood inversion techniques are used to retrieve the crustal structure and Vp/Vs ratios (κ). The study reveals significant variations in the crustal thickness (H = 34 km to 43 km), composition (κ=1.74 to 1.92) and shear wave velocity structure across northwestern India. The terrains encompassing the Erinpura granite and the Malani igneous suite in the vicinity of the Barmer rift are characterized by a thick (H ≥ 41 km) crust with felsic to intermediate composition (κ ≤ 1.81). The crust beneath the southern part of the Marwar basin is ≈ 38–40 km thick with mafic composition (κ> 1.81). The region around the Barmer rift has a thin crust (H = 34–36 km) with intermediate to mafic composition. The region hosting the Early Cretaceous to Paleogene alkaline complexes exhibits a high Vp/Vs ratio (κ= 1.91) that may be associated with mafic cumulates emplaced by magmatic events that overprint the signatures of the Malani event. The crust is heterogeneous with low/high velocity intracrustal layers that reflect the fractionation of magma at different depths. The mafic residue, together with magmatic intrusions, results in a mafic lowermost crust with high shear velocities of 3.9–4.0 km/s beneath most stations. Overall, northwestern India is characterized by a thick crust with intermediate crustal composition and intracrustal layering resulting from large scale magmatic events linked to the Neoproterozoic reorganization of plates and younger magmatic events.

Crustal structure beneath Central Kamchatka inferred from ambient noise tomography

In this study, we investigated the crustal structure beneath the area of Central Kamchatka (Kamchatka Peninsula, Russian Far East) limited by latitudes 52.8 and 54.1 degrees. This area comprises parts of the active Eastern Volcanic Front and two mountain ridges Sredinny and Ganalsky, as well as several backarc volcanoes. We used the continuous data from the permanent stations and the temporary network installed in 2019–2020 to perform ambient noise tomography. We implemented a two-step tomographic inversion algorithm that initially generated the two-dimensional maps of group velocities for periods ranging from 1 s to 30 s, then found the optimal 1D model of shear wave velocities and derived the three-dimensional S-wave velocity distribution. In the resulting model, we observe a prominent low-velocity anomaly below the active volcanoes of Avacha and Koryaksky, which is traced to depths of ∼40 km and may be associated with a zone of heated rocks suitable for petrothermal energy extraction. We propose that this anomaly represents a common magma pathway through the crust feeding both volcanoes. Beneath another active Zhupanovsky volcano, we observe three low-velocity layers at depths of 2 km, 8 km and 20 km indicative of a multilevel magma feeding system. We believe our findings will further the understanding of the complex processes occurring within these active volcanoes, which pose a tangible hazard to the relatively populated Petropavlovsk-Kamchatsky urban district. Beneath the Sredinny Ridge we reveal a high-velocity pattern at shallow depths, likely associated with consolidated magmatic structures. The low-velocity in the lower crust is interpreted as a high-temperature zone conserved here since the time when this area hosted an active volcanic arc. Upon further exploration, this high-temperature zone could prove to be a prospective source of geothermal energy. Below the Ganalsky Ridge, a prominent high-velocity anomaly extending to a depth of ∼15 km can be attributed to the Precambrian to Carboniferous metamorphic rocks of predominantly mafic composition.

Devonian intrusions of southwestern New Brunswick, Canada: U-Pb geochronology, petrogenesis and genetic constraints on gold mineralization

ABSTRACT The Clarence Stream intrusion-related gold deposit is hosted within metasedimentary and metavolcanic sequences of the Ordovician Cookson and Silurian Mascarene groups that were intruded by several phases of the Late Silurian to Early Devonian Saint George Plutonic Suite. Based on geochemical and geochronological data, the intrusive rocks in the deposit are categorized into three distinct suites, including gabbroic, granitoid, and Whittier Ridge Granite (WRG). The gabbroic suite is the same as the East Branch Brook metagabbro intrusion, which exhibits a subalkaline mafic composition fractionated from a depleted mantle in a suprasubduction zone setting. The granitoid suite shows the same geochemical characteristics and age as the metaluminous to slightly peraluminous Magaguadavic Granite and its fractionated phases. The WRG suite reflects characteristics of calc-alkaline, metaluminous to peraluminous I-type granites in the area, such as Sorrel Ridge and Mount Douglas granitic plutons. In situ U-Pb zircon geochronology yielded an age of 419.5 ± 3.8 Ma for a monzodiorite from the gabbroic suite. Porphyritic granite and alkali granite from granitoid suite yielded U-Pb zircon age of 402.4 ± 1.8 Ma and 394.3 ± 3.8 Ma, respectively. Geochronologic data from the WRG suite reflect two different crystallization ages of 383.8 Ma (382.4 ± 3.1 to 385.1 ± 4) and 372.4 Ma (367.4 ± 1.3 to 377.1 ± 1.9). Several pieces of evidence demonstrate that the gold mineralizing events occurred after the emplacement of the East Branch Brook metagabbroic dykes and are linked to the various intrusive phases of the nearby syntectonic Early Devonian Magaguadavic Granite. The absence of gold and sulphide mineralization in felsic dykes related to the WRG confirms that this plutonic body formed adjacent to the gold zones after the main gold mineralizing event.

Laboratory Emissivity Spectra of Sulphide-Bearing Samples, New Constraints for the Surface of Mercury: Oldhamite in Mafic Aggregates

Exploration of Mercury will continue in the near future with ESA/JAXA’s BepiColombo mission, which will increase the number and the type of datasets, and it will take advantage of the results from NASA’s MESSENGER (MErcury Surface, Space ENviroment, GEochemistry and Ranging) mission. One of the main discoveries from MESSENGER was the finding of a relatively high abundance of volatiles, and in particular of sulphur, on the surface. This discovery correlates well with the morphological evidence of pyroclastic activity and with features attributable to degassing processes like the hollows. BepiColombo will return compositional results from different spectral ranges and instruments, and, in particular, among them the first results from the orbit of emissivity in the thermal infrared. Here, we investigate the results from the emissivity spectra of different samples between a binary mixture of a volcanic regolith-like for Mercury and oldhamite (CaS). The acquisitions are taken at different temperatures in order to highlight potential shifts due to both mineral variation and temperature dependence on these materials that potentially could be present in hollows. Different absorption features are present for the two endmembers, making it possible to distinguish the oldhamite with respect to the regolith bulk analogue. We show how, in the mixtures, the Christiansen feature is strongly driven by the oldhamite, whereas the Reststrahlen minima are mainly dominated by mafic composition. The spectral contrast is strongly reduced in the mixtures with respect to the endmembers. The variations of spectral features are strong enough to be measured via MERTIS, and the spectral variations are stronger in relation to the mineralogy with respect to temperature dependence.

Geological Implications of Crustal Thickness, Vp/Vs, and Poisson's Ratio Beneath the Western Mesopotamian Plain Edge, Iraq

To know the crustal structure, composition, deformation, and evolution, it is crucial to appreciate the ratio between the velocity of the primary wave and the velocity of the secondary wave, Poisson's ratio, and crustal thickness. The receiver function inversion method was used to determine the (h), the primary wave and velocity of the secondary wave, and the Poisson's ratio. The chosen earthquakes' receiver functions were examined after being recorded at the two seismic stations in Anbar ANB1 and Karbala KAR2. According to the research, the westernmost part of the Mesopotamian plain has a crustal thickness between 44 and 46 km. The calculated crustal thickness value agrees with data from platform regions. The study area's crustal primary wave and velocity of the secondary wave is 1.79. For the platform region and andesitic rocks, this value is typical. The crustal Poisson's ratio was reported to have an intermediate value (0.27). It demonstrates that there are not many differences between the felsic and mafic compositions of the crust.

Contemporaneous alkaline and subalkaline intraplate magmatism in the Dunedin Volcanic Group, NZ, caused by mantle heterogeneity

ABSTRACT Monogenetic volcanism in the Maniototo Basin in the Dunedin Volcanic Group was unusual because the eroded lavas, plugs and dikes comprise spatially and temporally restricted silica-saturated and silica-undersaturated magmas. 40Ar/39Ar dating coupled with existing data and field relationships indicate that volcanism was focused at ∼11 Ma. Major and trace element data show the volcanic rocks to have mafic compositions and little evidence for fractionation and/or crustal contamination, but to have been variably altered. The volcanic rocks comprise an alkaline group and an alkaline to subalkaline transitional group, with one intermediate unit. With the exception of one flow, the transitional group has more radiogenic 87Sr/86Sr and less radiogenic 143Nd/144Nd, 206Pb/204Pb and 208Pb/204Pb than the alkaline group, as well as smaller Ti and K primitive mantle-normalised anomalies. Overall, the different magma compositions cannot be related to varying degrees of melting of equivalent mantle sources. Trace element ratios indicate that residual Ti and K-bearing mantle phases (amphibole, phlogopite) probably played a major role in the source of alkaline group but a smaller role in the transitional group. Therefore, the varying compositions of the Maniototo volcanic rocks relates to melting of a heterogeneous lithospheric mantle, or/and varying degrees of interaction with mantle hydrous metasomes as the magmas ascended.

Features of Using the Lithogeochemical Indicators for the Reconstruction of Paleoclimate and Composition of Demolition Sources in the Late Jurassic-Lower Cretaceous West Siberian Sedimentary Basin

For the rocks of the Upper Jurassic-Lower Cretaceous Bazhenov formation, a significant positive linear dependence of the Th, Hf, Sc, La content on the Al2O3 content was revealed, and their terrigenous genesis was confirmed. It has been determined that the samples in which the distribution of the values of the Sc/Al2O3 and La/Al2O3 ratios does not satisfy the linear dependence are mixed clayey-siliceous rocks with a P2O5 content 1 mas. % or substantially pyritized (in which the content of pyrite exceeds the content of S and C/S ≤ 1), as well as siliceous mudstones with a SiO2 content 70 mas. % by weight. It is concluded that before analyzing geochemical indicators for reconstructing the conditions of formation of the Bazhenov formation, in addition to carbonated rocks, rocks of the above types, as well as rocks that have undergone late diagenetic kaolinization, should be excluded from the analysis. The conditions of formation of the studied deposits were reconstructed based on the analysis of the values of a number of geochemical modules and indicators. As a result of the study of CIA, CIW variations, it was confirmed that the climate in the Late Jurassic-Early Cretaceous period in the West Siberian sedimentary basin was warm, semiarid. It was revealed that during the entire period under review, it did not change significantly. For the deposits of the Bazhenov formation, a number of indicators such as (La/Yb)N, Eu/Eu*, as well as the distribution of trace element content values on the triangular diagram Th‒La‒Sc, suggest that in the central and southeastern regions of the formation distribution area, the sources of demolition of the mafic composition prevailed.

First Ce-Nd isotope measurements of middle and lower continental crust samples support massive lower crust recycling over Earth's history

Long-lived radioactive isotopes provide valuable information on the evolution of Earth's geological reservoirs. Coupled measurements of the 138La-138Ce and 147Sm-143Nd systems have attracted much interest, but some critical reservoirs, such as the deep continental crust, have yet to be investigated. To address this gap, we report Ce-Nd isotope measurements of 35 crustal samples from Canada, western Europe, and Siberia, with Archean to Phanerozoic ages. Most samples from western Europe and Siberia are interpreted to represent the deepest continental crust because of low SiO2 content (< 55 wt%) and generally mafic compositions. In contrast, Precambrian composites from the Canadian shield have highly felsic compositions (61–71 wt% SiO2) and represent the old upper continental crust. Xenoliths from the Massif Central (France) and samples from uplifted massifs in the Ivrea-Verbano zone (Italy) plot on the Ce-Nd mantle array. Precambrian Canadian composites and xenoliths from Udachnaya (Siberian Craton) plot to the left of the mantle array, with the Siberian samples farthest from the array. The La-Ce system yields a ca. 1.8 Ga age for the Siberian xenoliths, distinct from the ca. 2.7 Ga Sm-Nd errorchron age. The La-Ce system was most likely reset during a large-scale episode of delamination and rejuvenation of the Archean lower lithosphere established in Siberia. A change in the La/Ce ratio during this event explains an increasing deviation with time of the Ce-Nd isotopic composition of the Siberian deep crust from the mantle array. We introduce a new parameter (θ) to quantify the likelihood of any rock evolving away from the mantle array. It represents the angle between the evolution vector of a sample and the mantle array, and it is calculated from measured La/Ce and Sm/Nd ratios. Samples that plot to the right of the mantle array have θ > 0, whereas samples that plot to the left of the mantle array have θ < 0. The angle θ was used to compare our samples to a worldwide granulite compilation (1581 samples) because it is independent of the age and the initial isotopic composition of the samples. The majority of lower crust rocks have negative θ, suggesting they are prone to evolve to the left of the mantle array, but to a lesser extent than the Siberian samples that are characterized by the most negative θ values. Thus, the Siberian samples with the most unusual Ce-Nd isotopic compositions are the only group to plot close to the deep crust end-member calculated from classical mass-balance budget of the bulk silicate Earth. These results suggest that the lower crust sampled by Siberian xenoliths is only a minor component in the terrestrial deep crust. We use this database to estimate new parent/daughter ratios for the lower continental crust: 138La/142Ce = 0.00380 ±0.00007 and 147Sm/144Nd = 0.128 ±0.007. Measurements and models are finally reconciled when considering massive recycling of lower continental crust through Earth's history, totaling 3 to 4 present-day, continental crust masses.

The REE-Zr-U-Th Minerals of the Maronia Monzodiorite, N. Greece: Implications on the Saturation and Segregation Mechanisms of Critical Metals in Intermediate–Mafic Compositions

This work delves into the presence of REE-Ti-Zr-U-Th minerals, in the mafic–intermediate rocks of the Maronia pluton, Greece, an Oligocene intrusion formed through arc-magmatism during subduction. In Maronia monzodiorite, critical metals are contained in three principal mineral groups, namely, the REE-Ti-Zr, REE-Ca-P, and U-Th assemblages. The REE-Ti-Zr group includes REE-ilmenite, chevkinite-like phases, zirconolite, and baddeleyite. The REE-Ca-P assemblage is represented by allanite-(Ce), monazite-(Ce), and huttonitic monazite-(Ce). The U-Th assemblage comprises thorite–coffinite and uraninite–thorianite solid solutions. The paragenetic sequencing of these minerals offers insights into their formation conditions and correlation with the pluton’s magmatic evolution. In the REE-Ti-Zr group, mineral formation progresses from REE-ilmenite to baddeleyite through chevkinite-like phases and zirconolite under oxidizing conditions. The REE-Ca-P sequence involves allanite-(Ce), followed by monazite-(Ce), late allanite-(Ce), and huttonitic monazite-(Ce). In the U-Th group, earlier thorite–coffinite phases are succeeded by uraninite–thorianite solid solutions, indicating Si-undersaturation at late magmatic stages. Fluctuations in Ca-activity induce alternating formations of allanite-(Ce) and monazite-(Ce). These mineral variations are attributed to early-stage interactions between high-K calc-alkaline and shoshonitic gabbroic melts, influencing critical metal enrichment and mineral speciation. The study’s insights into paragenesis and geological processes offer implications for mineral exploration in analogous geological settings.

Mineral Chemistry of Pyroxene Gneiss in Obudu, SE Nigeria, and Its Petrological Significance

Abstract The pyroxene gneiss which forms part of the basement cover in southeast Nigeria is a coarse-grained weakly foliated rock that has experienced high-grade metamorphism and anatexis. Electron microprobe data obtained from samples of this pyroxene-bearing gneiss confirm that the essential minerals are plagioclase (andesine, An30 – 37), orthopyroxene (hypersthene, En55.3 – 61.2, Wo1.0 – 2.6, Fs36.3 – 43.7), and clinopyroxene (augite, En39.7 –42.3, Wo42.0 – 45.1, Fs14.2 – 17.0). This assemblage is a typical granulite facies mineralogy produced by igneous rocks with intermediate to mafic composition that have been metamorphosed at medium pressure. Other minerals are calcic amphibole (XMg = 0.56–0.59), biotite (XMg = 0.58–0.69), orthoclase, and quartz. Orthoclase occurs mainly in leucocratic bands and clinopyroxene absent samples and may have resulted from dehydration reaction and thus dissolved in the melt phase. Fe-Ti oxides of ilmenite, hematite, and magnetite occur as accessory minerals, giving the imprint of metamorphism under oxidizing conditions. The presence of exsolved titanohematite in ilmenite indicates retrogressive metamorphism.

Seismically constrained gravity inversions reveal magmatic and metamorphic processes at a major lithospheric boundary in northwestern Australia

Magmatic and metamorphic processes modifying the lower crust and uppermost mantle are key to understanding continental growth and evolution. However, our ability to identify and quantify such processes is challenged by inaccessibility, data sparsity, and uncertainties in defining properties at depth. As a result, knowledge of composition, metamorphic grade, and the nature of the continental crust-mantle transition is often limited. Here, we model the structure and properties of the crust and uppermost mantle between the West and North Australian Cratons to infer tectonic processes at this major lithospheric boundary. Seismically constrained 3D gravity inversions were used to estimate crustal and mantle densities and Moho depths. The application of isostatic principles and the joint interpretation of gravity, active and passive seismic models, well data, and outcrop data are used to identify and characterize anomalous masses that are otherwise unresolvable using supracrustal datasets. The results suggest a thick, dense crust with a mafic bulk composition, a high-density, reflective lower crust, and a low-density uppermost mantle beneath the southern Canning Basin. We interpret these crustal and mantle features as reflecting mafic mantle-derived magmatism and crustal anatexis related to Proterozoic-Cambrian Large Igneous Provinces. In contrast, the Proterozoic Paterson Orogen exhibits a thick, dense, high-grade metamorphic bulk crust with a felsic to intermediate composition, a high-density, low-reflectivity lower crust, and low-density uppermost mantle, inferred to result from convergent tectonics. The northern Canning Basin displays a thinner, less dense crust with an intermediate to mafic composition, and mafic, high-density, reflective lower crust, associated with mafic magma emplacement during rifting and passive margin formation. These variations in crustal structure reveal a complex interplay of Proterozoic terrane amalgamation to the cratonic margin, repeated intraplate high-volume mafic magmatic events, and rifting of the offshore basin, which have collectively shaped the deep crust and uppermost mantle in this region.

Deciphering the sources and processes feeding young monogenetic volcanoes from the Michoacán Guanajuato Volcanic Field (Mexico): A study case of El Astillero and El Pedregal

El Astillero and El Pedregal monogenetic volcanoes formed ∼500–700 CE in the Tancítaro region in the southern part of the Michoacán-Guanajuato volcanic field, only 25 km to the SW of the historic Paricutin volcano. The ∼6-year-long eruption was characterized by a change from explosive to effusive activity, accompanied by a shift in the location of the active vents. Initial activity was Strombolian-explosive and first formed the El Astillero cone before turning effusive with the emission of several lava flows. Then, a new vent located 2 km to the ENE opened and produced the purely effusive (non-explosive) El Pedregal lava flow field. As the eruption progressed, the bulk magma composition (major and trace elements) changed from basaltic andesite to andesite (SiO2 = 52–59 wt%), which is also reflected in a successive change in the petrography of the erupted tephras and lavas. However, the El Pedregal lava sequence shows small Mg# reversals followed by a marked final reversal to more mafic compositions. Likewise, 87Sr/86Sr (0.70388–0.70403), 143Nd/144Nd (0.512836–0.512742), 206Pb/204Pb (18.632–18.671), 207Pb/204Pb (15.583–15.598), 208Pb/204Pb (38.376–38.450), 176Hf/177Hf (0.28301–0.28290), and 187Os/188Os (0.1258–0.1865) isotope ratios changed systematically as the eruption progressed, and record a final shift to a distinct isotopic signature. The spatio-temporal proximity of both vents and the petrographic and geochemical characteristics of their magmas suggest a comagmatic evolution that can be explained by a combination of variable degrees of magma recharge, magma mixing, and fractional crystallization of subduction-modified mantle melts. A similar combination of magmatic processes for the genesis and evolution of these magmas has also been proposed for other young monogenetic volcanoes in the Michoacán-Guanajuato volcanic field (e.g., Paricutin, Jorullo, and the Tacámbaro cluster). Accordingly, primitive magmas in the Trans-Mexican Volcanic Belt are subduction-modified mantle melts that evolve largely by crystal fractionation and pass through the crust without significant assimilation.

Revisiting the genesis of the adakite-like granitoids in collisional zones: Water-fluxed melting of intermediate to felsic rocks with dilution by low Sr/Y phases

Abstract High-Sr/Y granitoids in continental settings are sometimes erroneously regarded as the products derived from partial melting of thickened/delaminated mafic lower curst under relatively higher pressures (&amp;gt;1.5 GPa) in a collisional orogenic setting. In fact, multiple magmatic processes in the trans-crustal magma system, such as recycling of antecrysts, crustal assimilation, and fractional crystallization, can create or modify the primary “adakitic” signature. As a result, the generation of adakitic magmas in continental settings remains controversial from a bulk-rock perspective. Here, we address the origin of adakitic plutonic rocks through geochemical and textural characterization of rock-forming minerals in the pyroxene-bearing Zhuyuan granodiorite, West Qinling, China. The Zhuyuan granodiorite formed in a post-collisional setting and primarily consists of resorbed orthopyroxene, three types of clinopyroxene, amphibole, two types of plagioclases, K-feldspar, biotite, and quartz. Type-1 Cpx has high XMg (70.0–81.7). Type-2 Cpx displays normal zoning and decreasing XMg (80.9 to 71.5) from the core to rim. Type-3 Cpx is reversely zoned, where the rims have higher XMg (75.5–86.9), Ni, Cr, suggesting a recharge event. Orthopyroxene has high-Ni and -Cr contents, as well as high XMg (80.9–82.8), indicative of antecrysts that grew in mafic magma reservoirs. The injection of magmas from different sources is supported by sieve-textured plagioclase and crystal size distributions of non-poikilitic amphibole. Finally, non-sieve textured plagioclase, biotite, K-feldspar, and quartz are late-crystallized phases, indicative of an orthocrystic origin. The melts in equilibrium with these orthocrysts display significantly higher Sr/Y values than the magma batches that crystallized other mafic phases (i.e., amphibole, clinopyroxene, and orthopyroxene). Thus, we propose that the system involved an initial high-Sr/Y melts in equilibrium with the orthocryst assemblage was generated by water-fluxed melting of intermediate to felsic sources. The addition of low Sr/Y non-orthocrysts (e.g., amphibole and pyroxene) and associated melt diluted the original “adakitic signal” in the magma reservoir and drove the bulk composition to more mafic values. Consequently, the Zhuyuan pyroxene-bearing granodiorite represents a mixture of crystals with diverse origins and distinct magma batches of various compositions (from felsic to mafic compositions). Our study emphasizes that the origin of adakitic granitoids cannot be clearly deciphered without geochemical analysis of the constituent minerals. We also suggest that Sr/Y values in plutons should be cautiously used in paleo-crustal thickness estimates in collisional settings because of possible open system scenarios as described here.