Magma Source Research Articles

Late Neoproterozoic alkaline intrusives in Huqf sediments; a new reservoir target in southern Oman

Abstract Recent drilling in Block 56 on the southern Oman coast NE of Salalah has revealed the presence of intrusive rocks in Huqf sediments. The igneous bodies are found as both discrete and composite, decametre scale sills separated by screens of Huqf host rock. Compositionally they form a cogenetic suite of monzonites and syenites that can be related by fractional crystallization to a gabbro–diorite parental magma. Enrichment in alkalis and other large ion lithophile elements points to either an enriched mantle source or magma-crust interaction on ascent. Geochemical profiles match closely those of the latest Neoproterozoic alkaline eruptives now found as exotic volcanic blocks entrained in the salt diapirs of the Ghaba Salt Basin. Limited geochronological data indicate that the Block 56 intrusives are contemporaneous and derived from the same magma source. Porosities in excess of 10% have been recorded in the intrusives; it is a dissolution porosity created by volume loss during the conversion of primary ferromagnesian minerals to biotite and ferroan dolomite. Porosity creation took place during a short lived, fault-controlled Cretaceous hydrothermal event contemporaneous with uplift and alkaline volcanic activity in the Masirah Ophiolite. The intrusives contain a Huqf-sourced oil with light to medium viscosity appearance. Charging is believed to be coming from the cooler and shallower parts of the South Oman Salt Basin to the north and from beneath thick Cenozoic cover in the basin to the south.

Compositional signatures of clinopyroxenes from the mafic rocks of the Nagaland ophiolite (NE India): Inferences for melting of diverse source regions in a transitional arc-type setting

The Nagaland-Manipur ophiolite belt (NMOB) represents remnants of the Neo-Tethyan Ocean that evolved during the accretion of the Indian and Myanmar lithospheric plates. We studied clinopyroxenes of cumulate gabbro and basalt rocks from the northern section of the belt to decode the mechanism that controlled their petrogenesis and geotectonic setting. Clinopyroxenes (augite-diopside) exhibit compositional zoning and significant variations in their major and trace element contents. Zoned clinopyroxene from a single volcanic rock sample (2d-3-6-11) show higher concentrations of TiO2 (1.44 to 5.13 wt%) and Al (0.128–0.359 apfu) and lower Si (1.658–1.889 apfu) compared to clinopyroxenes from the other mafic rocks. The mg# [100 × Mg/(Mg + Fe2+)] values of clinopyroxenes range between 60.1 and 85.7. The trace element contents and the chondrite-normalised REE patterns of clinopyroxenes imply diverse geochemical affinities of both depleted mantle typical of tholeiite rocks and a less depleted LREE-enriched melt for the Nagaland mafic rocks. Pressure and temperature of crystallisation were estimated using single clinopyroxene geothermobarometry and were found to vary between 1–8 kbar and 840–1179 °C, respectively. In discrimination diagrams, the zoned clinopyroxenes show an alkaline composition consistent with a within-plate environment. On the other hand, clinopyroxenes from the rest of the mafic samples show a sub-alkaline composition typical of magmas produced in arc-related settings. Our study indicates the existence of diverse magma sources in an island arc-to-back arc setting during the formation of magmatic rocks in the NMOB.

Paleomagnetism of mafic dykes in South Rewa Basin, India: Constraints to extension of Deccan volcanism

The magma source for mafic dykes from South Rewa Basin (SRB) in central India is invariably linked to Rajmahal volcanism (117 Ma) for Damodar, Raniganj-Jharia, and Bengal basin (Permian - Quaternary) to the east, and Deccan volcanism (66 Ma) for Satpura Basin to the west of SRB. However, the magmatic events linked to the SRB are not explicit. To ascertain the dyke association with these volcanic events, we performed a comprehensive paleomagnetic study on the exposed dykes and basalts from Shahdol region in SRB. Rock magnetism indicate that magnetite or titano-magnetite is the main remanence carrier mineral in these dykes. The measured directions produce a mean declination (Dm) of 338° and mean inclination (Im) of - 35° (α95= 8.4°, k = 25.3, N = 13), is close to Deccan normal directions. The calculated Pole position (λp) is at 42.02°N, and (Lp) is at 289.33°E, suggesting that the studied dykes are emplaced simultaneously along with Deccan Traps (36.96°N/78.70°W) and not of Rajmahal Traps (11.37°N/297.58°E). These dykes can be the result of multiple Deccan magma intrusions along the Narmada-Tapti lineament and intra-basinal faults in the SRB of central India.

Geochemical characteristics of Neoproterozoic metavolcanic rocks of Ariab Auriferous Volcanogenic Massive Sulfide deposit, Red Sea hills, North-East Sudan

The Ariab volcanogenic massive sulfide (VMS, ∼118.3 Mt with >2 Moz Au recovered) district is part of the Ariab-Arbaat belt located in the Haya terrane, Red Sea Hills, Northeast Sudan. The gold-bearing gossan and VMS deposits are mainly hosted by Neoproterozoic bimodal volcanic succession. The petrogenesis, age dating, metallogenic evolution, and tectonic environment as well as the relationship between the metals occurrences and hosting materials remain poorly constrained and controversial. The study aims to investigate the geochemical characteristics of metavolcanic rocks of Ariab Auriferous Volcanogenic Massive Sulfide. XRF and ICP-MS techniques were employed to analyze major, trace, and rare earth elements content. The metavolcanics composition of Ariab VMS hosting rocks ranging from basalt to rhyolitic lava with variable pyroclastics. The chemical affinity varies from sub-alkaline tholeiitic to calc-alkaline nature, raising an argument against the possibility of fractional crystallization. The Ariab VMS metavolcanics were likely formed in the intra-oceanic arc tectonic environment. Ariab mafic lava is characterized notably by high Cr and Ni concentrations. The negative and positive anomalies in trace element profiles are typically ocean island basalt patterns, implying minimal crustal contamination. The geochemical results of dacitic and rhyolitic rocks reveal an assimilation-fractional-crystallization process, including the possibility that plagioclase-rich restite was formed in the parent magma source with a significant contribution from the old crust. The mobility of elements such as Zn, Cu, Pb, Ag, Au, As, and Co complicates the determination of their primary abundances. The partitioning behavior of REEs, the geochemical signatures of chalcophile elements, and their associations with VMS deposits, underscores the complex interaction between igneous processes and subsequent hydrothermal alteration.

Mineral Textures and Chemistry Trace the Origin and Transcrustal Evolution of the Sanyuangou Syenite in Southern North China Craton

Abstract Alkaline rocks are widely distributed in various geological environments and are important carriers for exploring the formation and compositional diversity of continental crust. Extensive studies have investigated the processes of mantle melting and crustal differentiation that produced such rocks. However, the potential interaction processes between mantle-derived magma and crust during their formation are poorly constrained. In this study, we focus on a Paleoproterozoic garnet-bearing syenite in the Xiong'er large igneous province (LIP) on the southern margin of the North China Craton through detailed whole-rock and mineral analyses. The high K2O (7.4–8.8 wt %) syenite emplaced at ~1772 Ma with ancient inherited zircon (1800–2800 Ma). The dominant mineral assemblage is composed of clinopyroxene, garnet, and K-feldspar with a small amount of titanite. Complex compositional oscillatory zoning of clinopyroxene and garnet indicates that magma mixing played a significant role in the formation of the syenite. We estimated the major elements composition for melts in equilibrium with clinopyroxene and calculated the clinopyroxene/melt partition coefficients by crystal lattice strain model, thus calculating the trace elements of the equilibrated melts. The equilibrium melts of high Mg# (>85) clinopyroxene have high CaO/Al2O3 (>0.6) and low Hf/Sm (<0.4), which suggests a role for carbonatitic metasomatism of the mantle. The variables La/Yb (24.4–56.1), Dy/Yb (0.8–5.9), and Yb (0.6–10.3 μg/g) indicate that the initial melts were formed by 1% to 2% partial melting of spinel-garnet phlogopite lherzolite. An REE-based model for melts in equilibrium with low Mg# (<85) clinopyroxene indicates that 10% to 30% felsic magma from ancient crust participated in hybridization. In addition to well-documented magma mixing, the oscillatory zoning of garnet is also related to competition with titanite. The initial alkaline magmas have a high water content (~4 wt %), which delays the crystallization of K-feldspar, leading to the enrichment of K2O, until the K-feldspar accumulates in the shallow crust to form the syenite. Magma mixing under an open system further leads to alkalinity enrichment. Magma source and crustal evolution jointly determine the potassium-rich characteristics of syenite. Multiple episodes of magma mixing and fractional crystallization occur in the crust (700–300 MPa), suggesting complex and vertically extensive magma chambers. This study represents the first identification of carbonatitic metasomatism as a mantle source in the Xiong'er LIP. Furthermore, it offers a new perspective on magma mixing between the mantle and crust in transcrustal magmatic systems, contributing to the formation of alkaline rocks.

Isotopic (Nd, Sr, Pb) Composition of the Permo–Triassic and Late Cretaceous Basaltoids in the Minusa Depression (Southern Siberia, Kop’evo Uplift): Heterogeneity of Mantle Sources of Mafic Magmas

Isotopic (Nd, Sr, Pb) Composition of the Permo–Triassic and Late Cretaceous Basaltoids in the Minusa Depression (Southern Siberia, Kop’evo Uplift): Heterogeneity of Mantle Sources of Mafic Magmas

Improved artificial bee colony algorithm for pressure source parameter inversion of Sakurajima volcano from InSAR data

A novel artificial bee colony algorithm was introduced for the eruption event of the Sakurajima volcano on August 9, 2020, to invert the magma source characteristics below the volcano based on the point source Mogi model. Considering that the Sakurajima volcano is surrounded by sea, all the deformation data are used to obtain the location and magma eruption volume of the volcano. In response to the weak local search capability of the artificial swarm algorithm, the difference between the global optimal individual and the un-roulette screened individual is introduced as the variance component in the onlooker stage. Detailed simulation experiments verify the improvement of the algorithm in terms of convergence speed. In real experiments, the Sakurajima volcano inversion shows closer fitting results and smaller residuals compared to the existing literature. Meanwhile, the convergence speed of the algorithm echoes with the simulation experiments.

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

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

Placing constraints on the age and origin of basaltic dykes on the Ghaap Plateau, Griqualand West, South Africa

Abstract Well-preserved strata of the Ghaap Plateau along the western margin of the proto-Kalahari Craton have been intruded by a large number of mafic dykes. Owing to the scarcity of outcrops, these dykes are generally not well documented and intrusion ages remain poorly understood. In order to address this lack of knowledge, a total of seventeen Ghaap Plateau dykes that included seven north-northeast to northeast-trending dykes, eight northwest-trending dykes and two east to east-northeast-trending dykes were sampled for this study. Here, we report on the petrography and whole rock geochemistry of these dykes. We also report three new U-Pb baddeleyite ages along with some palaeomagnetic data. The mineralogy of the dykes is dominated by clinopyroxene and plagioclase showing varying degrees of alteration. Whole rock geochemical data identify the dykes as basalts with MORB, E-MORB or within plate-like signatures, suggesting an asthenospheric magma source. Our study indicates that the Ghaap Plateau dykes represent several magmatic events of markedly different age. Based on their whole rock geochemical composition, all but one of the dyke samples are tentatively arranged into four distinct groups (Groups A to D). Group A dykes (four dykes) are compositionally similar to the Umkondo Large Igneous Province (LIP). Two north-northeast-trending dykes of this Group yielded overlapping U-Pb baddeleyite ages of 1 103 ± 84 Ma and 1 112 ± 55 Ma, thereby adding to the known footprint of the Umkondo LIP along the western margin of the proto-Kalahari craton. In Group B (four dykes), one northeast-trending dyke yielded a U-Pb baddeleyite age of 1 929 ± 17 Ma. Its palaeomagnetic signature (Lat. 24.5°, Long. 317.6°, A95= 14.20°) is supported by a positive baked contact test and agrees well with the ~1.93 Ga Hartley LIP palaeopole. Group B dykes are thus interpreted to be related to the Hartley LIP, with our new 1 929 ± 17 Ma age supporting a wider extent for the Hartley magmatism than previously known. A new palaeopole for the Hartley LIP (Lat. 22.3°, Long. 327.4°, A 95 = 9.25°) was calculated by combining the data of our 1 929 Ma dyke with data reported previously from other sites. Whole rock geochemical data for the remaining dykes are consistent with the Karoo magmatism (two east to east-northeast-trending dykes of Group D) or the ~2.4 Ga Ongeluk LIP (north-trending dyke). The final four dykes (Group C) are difficult to match with other known mafic units. Precise geochronology is needed to confirm the exact age of these unassigned dykes.

U-Pb detrital zircon age determination and provenance of the lower Karoo succession from the Karoo Research Initiative (KARIN) borehole KWV-1, Eastern Cape Province, South Africa

Abstract In recent years, the maximum age of sedimentation and possible sources of detritus to the sedimentary successions of the Karoo Supergroup has received a lot of interest, with various authors presenting age data from across the main Karoo Basin (MKB). Our paper aims to contribute to this ever-growing geochronological database as we report weighted mean ages for the youngest zircons as well as detrital zircon age distributions for the lower Karoo Supergroup succession sampled from the Centre of Excellence for Integrated Mineral and Energy Resource Analysis (CIMERA)-Karoo Research Initiative (KARIN) drill core KWV-1. A total of 880 near concordant zircon ages were determined using Laser Ablation Quadrupole Inductively Coupled Plasma Mass Spectrometry (LA-Q-ICP-MS) from fifteen samples representing the Dwyka Group, the Collingham-, Ripon-, Fort Brown- and Waterford formations of the Ecca Group and the Koonap Formation of the Beaufort Group as intersected by this ~2 352 m deep research borehole drilled in the southeastern sector of the basin. The weighted mean ages for the youngest zircons calculated for the formations of the Ecca- and Beaufort groups compares with findings reported by other authors and with the ages reported for tuffaceous layers associated with these units. The weighted mean ages for the youngest zircons of the Dwyka Group reflect the age of the youngest source area and do not aid in constraining the maximum age of deposition. A significant change in detrital zircon age distribution was noted between the Dwyka Group and the overlying formations of the Ecca and Beaufort groups, which we interpret as a change in provenance. The major Meso-to Neoproterozoic detrital zircon age fractions present in the Dwyka Group are attributed to source areas located towards the north and northeast of the MKB, in line with regional ice flow directions. A significant late Carboniferous to Permian detrital zircon age fraction is however observed throughout the formations of the Ecca- and Beaufort groups, with a lesser fraction of older zircon ages. The prevailing youngest age fraction reflects ages associated with magmatic source areas in the Gondwanide Orogeny located towards the south of the MKB at the time of deposition, while the recycling of deformed sedimentary strata of the Cape Supergroup contained in the Cape Fold Belt is considered the source of older detritus. The source of detritus to the MKB remained unchanged during the deposition of the respective formations of the Ecca- and Beaufort groups, despite changes in the depositional environment brough on by regional tectonics.

Oligocene magmatism in the eastern Sakarya Zone, NE Turkey: Geodynamic implications

During the Cenozoic, the eastern Sakarya Zone, northeastern Turkey, has witnessed voluminous Eocene and subsidiary Miocene magmatism. By contrast, except in one previous study, Oligocene magmatism has not been reported. Here, we present new data about an Oligocene dyke swarm from the Artvin district in the easternmost part of the Sakarya Zone. Our study improves our understanding of the Oligocene magmatic and geodynamic evolution of the central Tethyan orogenic belt.The Oligocene dykes consist of two distinct rock types: quartz-bearing trachytic and adakitic trachyandesite/andesite, dated at 31.0 ± 0.2 Ma and 31.1 ± 0.2 Ma to 29.2 ± 0.3 Ma by U-Pb zircon geochronology, respectively. Geochemical analysis reveals enrichment in LREE and MREE for quartz-bearing trachytic dykes, whereas adakitic trachyandesite/andesite dykes show depletion in MREE and HREE. Radiogenic isotopic ratios (87Sr/86Sr(i) and 143Nd/144Nd(i)) and trace element data suggest that the source of Oligocene magmatism was linked to low degree (≈10–15 %) partial melting of a metasomatised lithospheric mantle previously enriched by slab-derived melts and fluids. During the magma ascent in the crust, parental magmas of the trachyandesite/andesite dykes have assimilated crustal rocks and undergone fractional crystallization dominated by amphibole. By contrast, quartz-bearing trachytic dykes have recorded plagioclase fractionation.Local extension, in conjunction with continuous foundering of delaminated thickened lower crust beneath the eastern Sakarya Zone, which likely triggered asthenospheric mantle upwelling and resulted in sporadic Oligocene magmatism.Our study highlights the distinct tectonic and magmatic setting of the rare Oligocene magmatism in the eastern Sakarya Zone compared to Oligocene magmatism in the Tauride-Anatolide and South Armenian blocks.

Variable controlling factors lead to contrasting patterns of volcanism in the Changbaishan volcanic area (Tianchi-Longgang), China-North Korea: Insights from morphometry and spatial-temporal analyses

The coexistence of monogenetic and polygenetic volcanoes is a common phenomenon in volcanic areas. However, the genetic relationship between monogenetic and polygenetic systems and the factors controlling their distinct eruptive styles are not well understood. In active volcanic areas, analysing the clustering and vent alignment of monogenetic volcanoes, as well as examining the geomorphology and relative ages of scoria cones, offers quantitative insights into magma supply rates, volcano type distribution, and volcanic development trends. Our study presents geomorphological and spatio-temporal analyses of the co-existing monogenetic volcanoes in the Longgang Volcanic Field (LVF) and those associated with a polygenetic volcano (Tianchi) in the Changbaishan Volcanic Area, China. The distance between the two volcanic areas is around 150 km. Monogenetic vents in the LVF exhibit greater density compared to the dispersed system associated with Tianchi. The LVF vents also show better alignment, particularly in the direction of pre-existing basement faults (NE-SW, NW-SE and EW). By using scoria cone morphometric parameters and features, we estimated the relative ages and erupted volumes of monogenetic volcanoes in the LVF and the Tianchi area. We classified the cones of the two volcanic systems into five eruptive periods and found that, despite similar magma sources and output rates over approximately 870 kyr, differing numbers of scoria cones across age classes suggest that Tianchi's magma system influences its associated monogenetic volcanic plumbing. Furthermore, the continuous rise in output rates of monogenetic volcanoes in the Tianchi area highlights the increasing magma supply sustaining Tianchi volcano. Together, these interpretations are consistent with the two systems being controlled by different factors: the Tianchi monogenetic volcanic system is more controlled by magmatism, whereas the LVF is more strongly controlled by local tectonic structures, alongside an increasing magma supply causing the formation of progressively larger individual volcanoes. In volcanic areas, analysing monogenetic volcanoes' spatial-temporal distribution, volumes and recurrence rate provides a framework to evaluate magma supply rates and tectonic associations, which are key to the development of different volcano types.

Chemical constraints and tectonic setting of the Ginebra ophiolite complex

The occurrence and types of ophiolites in the Central Cordillera of the Colombian Andes have not been identified in terms of their tectonic settings and geochemical features. Therefore, the study of the tectonic setting and subduction zones in the ophiolites’ magmatic evolution becomes a significant issue. The Ginebra Ophiolitic Complex (GOC) is located on the western flank of the Central Cordillera in the Valle del Cauca Department, southwest of Colombia, assigned to the Lower Cretaceous period, as indicated by the contact of the GOC with the Buga Batholith. The GOC is formed by three main lithologies: amphibolite, gabbro, and ultramafic rocks (pyroxenite and peridotite), according to Ossa (2007). The rocks in the Puente Piedra section, located in the northeast of Ginebra municipality, are part of the gabbroic cumulates within the ophiolitic sequence. This sequence comprises interspersed layers of gabbroic cumulates, gabbrodiorites, and diorites; interpreted as a result of crystal accumulation and fractional crystallization processes. The mafic rocks of the GOC are sub-alkaline and correspond to the low potassium (K2O wt% 0.03-0.06) tholeiitic series. Geochemically, they have SiO2 wt% ranging from 49 to 61 and an aluminum oxide saturation index of approximately ~0.4 and ~0.8, indicating a metaluminous type. The geochemistry of the studied rocks from Puente Piedra section indicates that the GOC formed through fractional crystallization and accumulation processes from a single magma source. Cluster analysis, used to compare the geochemistry of GOC rocks and the Amaime Complex basalts, suggests a similar magma source, possibly linked to multiple recharge events that underwent fractional crystallization, melt extraction, and accumulation processes. The geochemical parameters are indicative of a suprasubduction zone ophiolite, characterized by a low potassium tholeiitic series affinity, TiO2 values typically <1.2 wt%, Th enrichment typical of subduction zones, high Pb content, and low values of Ti, Y, Yb, Ta, Nb, Zr, and Hf.

Magmatic and hydrothermal evolution of the superlarge Dashui goldfield in the West Qinling Orogen, China

Gold (Au) is a precious metal that has played an important role in the development of the global economy. It is therefore important to better define the genesis of Au mineralisation as an aid to focus exploration as the consumption of Au increases. The Dashui goldfield contains a super large Au resource of over 120 t (tonnes) in the West Qinling Orogen, central China. Despite several studies have focused on the genesis of the Dashui goldfield, the original physico–chemical natures of magmatic and hydrothermal evolution remain unclear. Biotite and apatite in the mineralised diorite porphyry of the goldfield are sensitive to the changes in the physico–chemical conditions and the evolution of magmatic and hydrothermal fluids, and thus provide important keys to address these issues. Hence, we conducted detailed in–situ major and trace element studies of biotite and apatite from the ore–bearing diorite porphyry in the Dashui goldfield. Our aim is to elucidate the physico–chemical conditions, magma source, and magmatic and hydrothermal evolution processes during the deposition of the gold, and to enhance the understanding of the Au genesis in the West Qinling Au province. Using the geochemistry of magmatic apatite, and magmatic and hydrothermal biotite, we have found that the crystallisation temperature and pressure along with emplacement depth of diorite porphyry were constrained at ∼ 737–791 °C, 1.08–1.73 kbar, and 3.94–6.30 km. The diorite porphyry has the geochemical affinities with calcium alkaline orogenic suites derived from a mixed crustal and mantle source. During the early emplacement of diorite porphyry, the melt had a high oxygen fugacity and was highly enriched in Au. During the late magma crystallisation of diorite porphyry, the magma was enriched continuously in most elements with the Ba/Rb ratios in the biotite varying from 10 to 5, and most of the Au was hosted by pyrite. Additionally, elements were continuously dissolved into fluids generating Cl–rich hydrothermal fluids during the evolution of the magma. During hydrothermal evolution, Au is migrated with As and Sb in the vapour phases of fluids, whereas Au migrated with Cl in the liquid phase. When temperatures dropped to ∼ 200–400 °C, Au started to combine with S and Si in the liquid phase of fluids. Subsequently, hydrothermal fluids interacted with host carbonate to form the Dashui goldfield, Au during this stage was also transported and hosted in magnetite, hematite, and quartz within the carbonate rocks.

Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A1-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications

In the northern part of the Marrakech High Atlas (MHA), along the southern Variscan segment of the Western Meseta, a Variscan granitic intrusion crops out, intruding metasediments and meta-volcanosedimentary rocks of Early Cambrian to Ordovician age. A new whole-rock Rb-Sr isochron age of 268 ± 9 Ma for the granite, combined with a previously published whole-rock Rb-Sr radiometric dating (271 ± 3 Ma), reveals a post-kinematic (tectonic) character with regard to the main Variscan deformational event, belonging within the tectonic context of the Moroccan Variscan orogenic belt. Geochemically, the Azegour intrusion is metaluminous to peraluminous and exhibits a calc-alkaline affinity with a ferruginous composition. The massif shows an extremely differentiated character (SiO2 = 77.53–78.14 per cent), K2O and high total alkali contents, FeOt/(FeOt + MgO) and Ga/Al ratios, which have typical characteristics of an A-type granite. In addition, the granite contains high concentrations of LREE (LaN/SmN= 7.9–13.67) relative to HREE (LaN/YbN= 4.81–11.61) and a well-defined Eu negative anomaly (Eu/Eu* = 0.44–0.75). The granitic samples exhibit a strong enrichment of the most incompatible elements (RbN/YbN = 69.84–159.98) and a strong depletion of Ba, Sr, Eu, Nb, P and Ti. These characteristics are similar to those of A1-type granites. The absence of mineralogy typical of an S-type granite, combined with its weakly peraluminous character [A/CNK (molar Al2O3/CaO+Na2O+K2O) = 1,013–1,045], suggest that there is little or no significant involvement of supracrustal sources in the petrogenesis of the intrusion studied. Despite the strongly differentiated character of Azegour granitic rocks samples, their multi-element patterns shows many similarities to those of I-type granitoids, which has led to postulate that the parental liquids of A1-type were derived from partial melting of mafic magmas. The representative samples studied show less depleted εNd(t = 270 Ma)values of –0.94 to –4.85 and lower positive to slightly negative εSr(t = 270 Ma) values of –1.45 to 9.32. The isotopic data suggest that the Azegour granite was emplaced 270 myr ago, apparently generated by partial melting of a mafic/intermediate magma source in the lower crust as a result of the underplating of the asthenosphere mantle-derived Oceanic Island Basalt-like magmas. Alternatively, their isotopic signatures also can be attributed to the interaction and/or hybridisation of basaltic liquids derived from the mantle with these lower crust materials. The generated parental magma probably occurred at deep structural levels and involved fractional crystallisation processes by the separation of a mineralogical association composed of plagioclase + potassium feldspar ± biotite ± amphibole ± sphene ± apatite. The whole-rock Rb-Sr age of 268 ± 9 Ma, whole-rock geochemistry and Sr-Nd isotopic compositions of εNd(t = 270 Ma) and εSr(t = 270 Ma), combined with fieldwork data, suggest that the Azegour granite was emplaced.

Evidence from adakitic rocks for a switch in magmatic source at ca. 460 Ma in the Karamaili Ocean of East Junggar, NW China

Early Paleozoic magmatism can preserve critical geological records related to subduction-accretion in the Central Asian Orogenic Belt (CAOB). Here, we report new geochemical, geochronological and isotopic results for diorites-granitoids in the Karamaili region of East Junggar, Northwest China. These rocks record high Sr, but low Y and Yb contents, with high Sr/Y ratios but low La/Yb ratios, and can be divided into low-silica adakite and high-silica adakite. All samples are with typical features of subduction-related magmas and show relatively juvenile Sr, Nd and Hf isotopic ratios. Zircon LA-ICP-MS UPb ages are between 468 ± 4 Ma and 441 ± 5 Ma. The low-silica adakite has relatively high Mg# and Cr, Co and Ni contents, implying a possible mantle origin. The Hongliuxia diorite (low-silica adakite) has relatively high Th contents and Th/Yb ratios, but records lower ɛNd(t) (+4.1) and ɛHf(t) (+8.9 − +12.2) values than those of the granitoids (high-silica adakite), implying the low-silica adakite was influenced by the subduction of sediments. The low-silica adakite likely originated from partial melting of a mantle wedge, metasomatized by subduction-related fluids and sediments, whereas the high-silica adakite was likely derived from partial melting of juvenile mafic lower crust. There is little involvement of garnet in the magma source, consistent with fluid-flux melting at pressures between 10 and 12.5 kbar (corresponding to 30–40 km depth). We suggest that the increase in Nd and Hf isotopic ratios at ∼460 Ma may imply that subduction began to retreat at this time in the Karamaili Ocean during evolution of the CAOB.

Diverse apatite geochemical compositions in early Paleozoic granitoids of the North Qinling orogen, China: Insights into their petrogenesis and magma sources

Apatite geochemistry in granitoids is controlled primarily by the original melt compositions and partition coefficients. However, additional factors (e.g., post-crystallization effects, mineral crystallization sequences, and crustal assimilation) can also contribute to variations in apatite compositions. In this study, we collected three groups of early Paleozoic granitoids from the North Qinling Orogen and obtained in situ geochemical data for apatite. The granitoids were formed by partial melting of oceanic arc crust (group A), recycling of ancient crust (group B), and melting of the lithospheric mantle and extensive fractionation (group C). Trace element and Nd isotope data show the apatite from the Taiping and Manziying plutons (group A) were affected by post-crystallization processes and crustal assimilation, respectively. In addition, crystallization of plagioclase/allanite and apatite resulted in the variable Eu anomalies and rare earth element contents of apatite in the group A granitoids. In contrast, apatite compositions in group C were controlled primarily by the fractionation of hornblende. However, the decoupling of Eu anomalies of apatite and their host rocks, particularly for group B, cannot be easily explained by the aforementioned factors. As such, the redox state may also have been an important factor. We used a ternary diagram (10 × [Eu/Eu*]N–V–Ga) to constrain the redox state from the apatite data, and provide insights into the types and petrogenesis of the host granitoids. Combined with the whole-rock and zircon geochemical data, we found that the apatite from the rocks derived from continental crust (group B and cratonic adakites) is generally reduced, whereas apatite from the mafic rocks and high BaSr granitoids is highly oxidized. The highest degree of oxidation was found for the altered apatite from the group A granitoids and unaltered apatite in the orogenic adakites, possibly due to interactions with fluid derived from oceanic crust. Moreover, unaltered group A granitoids exhibit a transitional trend from an oxidized to a reduced state. Apatite geochemistry can be used to constrain the redox state of adakitic rocks and whether they have a continental or oceanic crustal origin. The high Sr contents of high BaSr granitoids are inherited from their mantle source.

Hydrochemical evolution and source mechanisms governing the unusual lithium and boron enrichment in salt lakes of northern Tibet

The large-scale salt lakes widely distributed in the Tibetan Plateau provide unique and potential resources for lithium (Li) and boron (B). The concentration and characteristics of elements in these salt lakes resemble those found in geothermal water in northern Tibet, which highlights both as crucial sources of rare elements. This study presents comprehensive analyses of the hydrochemical composition and isotopes of B, strontium (Sr), hydrogen (H), and oxygen (O) in typical salt lakes, along with samples from surrounding springs and rivers in the Bangong-Nujiang suture zone of northern Tibet. The results reveal an extremely negative and anomalous distribution pattern of B isotopes in Zabuye Salt Lake that is closely associated with geothermal water. The enrichment of these elements in other salt lakes in the region is attributed to concentration of evaporation and sediment adsorption. Given the very high elevation of the recharge for geothermal water, the infiltration of salt lakes obviously cannot feed geothermal springs. On the contrary, we correlate the unusual enrichment of Li and B and other resources in salt lakes to geothermal spring discharge. The ultimate origin of these elements lies in magmatic sources, with later water-rock interaction leading to significant enrichment of incompatible elements such as Li, rubidium (Rb), cesium (Cs), and B in the geothermal system. The geothermal springs directly or indirectly fed the salt lakes, and with further evaporation, they became super-scale brine deposits.

Neoarchean granitoid magmatism and geodynamic process in the northeastern North China craton

The composition of Archean granitoid rocks changed from predominantly tonalite-trondhjemite-granodiorite (TTG) gneisses in the early Archean (4−3 Ga) to diversified granitoid rock assemblages in the late Archean (3.0−2.5 Ga), marking a crucial transformation in the geodynamic processes of early Earth. However, the reason for this major transition remains enigmatic because the petrogenetic features of different granitoid assemblages and their crust-mantle interactions during different periods are poorly understood. We use variations in the spatial-temporal distribution, lithological association, chemical composition, and petrogenesis of Neoarchean (2.7−2.5 Ga) granitoids and inferred correlative crust-mantle interactions in the Eastern Liaoning Range (ELR) of the northeastern North China craton to explore this geodynamic transition. The early Neoarchean (ca. 2.7 Ga) ELR granitoids were dominated by TTG gneisses, and the late Neoarchean (2.6−2.5 Ga) ELR granitoid typology and compositions became more complex, changing into TTGs and more K2O-rich granitoid rocks. The TTGs can be subdivided into a high-Ca group and a low-Ca group: The 2.71−2.68 Ga high-Ca group TTG magma originated from partial melting of subducted juvenile oceanic crust, and the low-Ca group TTG magma was derived from fractionation crystallization of the high-Ca group TTG magma. The chemical composition of the magmatic sources played a dominant role on the 2.60−2.50 Ga TTG magmatism: the high-Ca and low-Ca group TTG magmas came from low-K mafic rocks and tonalites, respectively. The 2.58−2.49 Ga K2O-rich granitoids can be divided into three petrogenetic series: (1) The high-Ca-Mg group K2O-rich granitoid magma originated from partial melting of high-K mafic rocks, (2) the low-Ca-Mg group K2O-rich granitoid magma was derived from partial melting of sedimentary rocks, and (3) the transition group K2O-rich granitoid magma was sourced from metagreywackes. The 2.71−2.68 Ga TTGs were generated in an island arc belt, and subducted slab melting and subsequent magmatic differentiation were the dominant mechanisms of the TTG magmatism. The 2.60−2.50 Ga diversified granitoids were formed in the oceanic-continental subduction process under the active continental margin; the complicated oceanic slab subduction and arc-arc and arc-continent collisions contributed to the diversity of late Neoarchean granitoid magmatism.

Limitations of provenance diagnoses and maximum‐depositional‐age constraints based on detrital‐zircon geochronology: the fertility bias

AbstractDetrital zircon U–Pb geochronology (DZG) is widely used in the provenance analysis and calculating maximum depositional ages (MDAs) of strata. To assess the geologic limitations of this approach, we conducted DZG coupled with bulk‐petrology and heavy‐mineral analyses of Miocene volcaniclastic and non‐volcanic siliciclastic sandstones from the SW Tarim Basin. Although these two sandstone types display greatly different bulk‐petrography and heavy‐mineral signatures, they exhibit similar detrital‐zircon‐age spectra, and thus represent a less common case in which interpretations based on DZG alone may misalign with bulk‐sediment provenance. Most zircon‐based MDAs of volcaniclastic sandstones range from 12.3 to 14.8 Ma, deviating from their ca. 11 Ma true depositional age constrained previously. The similarity of zircon‐age spectra in volcaniclastic and siliciclastic sandstones and the 1–4 M.y. the error of the zircon‐based MDAs is ascribed to the low zircon fertility of coeval alkaline magmatic sources. This study emphasizes the importance of an integrated approach to provenance analysis and chronostratigraphy.