Enrichment In Incompatible Elements Research Articles

Tracing the Origin and Magmatic Evolution of the Rejuvenated Volcanism in Santa Clara Island, Juan Fernández Ridge, SE Pacific

Oceanic intraplate volcanoes sometimes experience late-stage eruptive activity known as rejuvenated volcanism, and contrasting interpretations for its petrogenesis depend on the compositional characteristics. In the Juan Fernández Ridge (JFR), a volcanic chain approximately 800 km in length emplaced on the Nazca Plate, some subaerial occurrences of rejuvenated volcanism have been recognized on the Robinson Crusoe and Santa Clara Islands, both part of the same deeply eroded shield volcano complex. This study aims to understand the origin and magmatic evolution of rejuvenated volcanism on Santa Clara Island, emplaced after ~2.15 Ma of quiescence above the shield sequence, mainly via the analysis of unpublished geochemical and isotopic data. Field reconnaissance identified two nearly coeval rejuvenated sequences on Santa Clara Island: Bahía W (BW) and Morro Spartan (MS), both formed by basanitic and picro-basaltic lava flows with brecciated levels and local intercalations of sedimentary and pyroclastic deposits. In comparison to the chemical signature of the preceding shield-building stage (comprised mainly of basalts and picrites), the two rejuvenated sequences exhibit a notable enrichment in incompatible elements, but the Sr, Nd, and Pb isotopes are very similar to the FOZO mantle endmember, with an apparent additional contribution of HIMU and EM1 components. The geochemistry of lavas revealed the involvement of various processes, including contamination by ultramafic xenoliths, high-pressure fractional crystallization of olivine and clinopyroxene, and potential partial assimilation of oceanic lithospheric components. While the oceanic lithosphere has been considered as a potential source, the isotopic data from Santa Clara lies outside of the mixing curve between depleted mantle (DM, here represented by the North Chile Rise and the East Pacific Rise) and the previous shield stage, suggesting that a lithospheric mantle is not the primary source for the rejuvenated stage volcanism. Therefore, we favor an origin of the rejuvenated volcanism from the mantle plume forming the JFR, supported by similarities in isotopic signatures with the shield stage and high values of 208Pb/204Pb (only comparable to San Félix—San Ambrosio in the vicinity of JFR), implying the presence of a regional source with radiogenic 208Pb/204Pb isotope ratios. In addition, isotopic variations are subparallel to the mixing line between HIMU and EM1 components, whose participation in different proportions might explain the observed trends. In conclusion, we propose that the source of the rejuvenated volcanism on Santa Clara Island is a heterogeneous mantle plume, the same one that fed the shield stage. The rejuvenated volcanism is derived from a secondary melting zone away from the main axis of the plume.

Nundorite: a geological oddity of critical minerals significance from the Mount Arrowsmith Volcanics, northwestern NSW

Nundorite is a unique and enigmatic rock type, known only from its type locality within the Koonenberry Belt of western NSW. The rock is highly anomalous in geochemical composition being peralkaline, with low silica and high alumina, but enriched in rare metals such as Zr, Nb and rare earth elements. This study presents a comprehensive mineralogical and geochemical study of nundorite to resolve its origins and its potential to inform on the geological processes that lead to critical metal concentration in the crust. Nundorite is found only at a single outcrop but shows a progression of alteration and deformation over ∼250 m from the northwest to southeast. The least deformed varieties consist of coarse aegirine grains within a matrix composed predominantly of natrolite and microcline with minor albite, nepheline and schizolite. With increasing deformation, the proportion of albite and aegirine decreases, the primary zeolite changes from natrolite to analcime, and the pyroxene and schizolite shift towards the aegirine and serandite (Mn-rich) endmember compositions, respectively. These transitions can be related to progressive hydration and recrystallisation of the rock in response to hydrothermal alteration associated with deformation. Nundorite also contains a diverse suite of accessory rare metal phases such as zircon, monazite, cerite-group minerals, fergusonite, fersmite, natroniobite, galgenbergite, epidote–allanite and a range of Zr alkali silicate minerals. Samarium–Nd isotopic compositions of nundorite (εNd ∼+5.5 at 585 Ma) are comparable with mafic volcanic rocks of the surrounding Mount Arrowsmith Volcanics and are consistent with derivation from a mantle source. These data, together with the uniform peralkaline composition, depletion in Ti and P, and enrichment in incompatible elements, are consistent with an origin as highly fractionated phonolite. Phonolites of similar composition are rare but have been reported from continental rift and intraplate alkaline provinces worldwide. Nundorite, therefore is not just a geological oddity, but also provides reaffirmation of the continental rift setting for the Mount Arrowsmith Volcanics and highlights the critical mineral resource potential of the region, as extensive crystal fractionation is a key mechanism to produce ore grade levels of rare metals in alkaline magmatic systems.

Allanite in Mantle Eclogite Xenoliths

Abstract Rare-Earth Elements (REE) are key geochemical tracers of crust–mantle differentiation, but there are few direct data on REE-rich minerals in mantle rocks. Here, we report the combined petrography and comprehensive chemical and isotopic characterization of three coesite- and kyanite-bearing eclogite xenoliths from the Udachnaya kimberlite pipe (Siberian craton), which are unusual in that two xenoliths (one with diamond and graphite) contain discrete, idiomorphic crystals of allanite at the grain boundaries of garnet and omphacite. Another xenolith contains allanite as part of a complex aggregate of calcite, apatite, barite, and celestine hosted by serpentine, which is a low-temperature secondary minerals likely result from metasomatic reaction at shallower depths during the transport of eclogite by the erupting kimberlite melt. The bulk rock composition reconstructed from the trace element composition of garnet and omphacite show marked depletion in LREE, precluding equilibration with kimberlite melt, whereas the measured bulk compositions show chondrite-normalized REE patterns with conspicuous depletions of Ce–Pr–Nd relative to La and Sm. The presence of 0.005 to 0.008 wt % of allanite, texturally and chemically out of equilibrium with the rock-forming minerals, allows balancing the LREE and Sm–Nd budget of the rock, whereas Th and U require additional hosts. This not only highlights the utility of measuring bulk eclogite xenoliths in bringing this unusual component to light, but also demonstrates that the long-known incompatible element enrichment in bulk eclogites reflects the deposition of discrete phases rather than merely bulk kimberlite melt addition. Although allanite is stable in metabasalts at the pressure–temperature conditions of 1025°C to 1080°C and 3.6 to 4.8 GPa recorded by the eclogite xenoliths, its association with Ba-Sr minerals suggests its formation via reaction of the host eclogites with kimberlite melt. This is supported by the similarity in 143Nd/144Nd ratios between bulk eclogite (0.51227–0.51249) and the host kimberlite at eruption, whereas clinopyroxene in part retains unradiogenic Sr (87Sr/86Sr = 0.70205 ± 0.00011) related to ancient depletion. The discovery of allanite in the Udachnaya eclogites demonstrates that this REE mineral can form when omphacite and grossular-rich garnet in eclogite breakdown in contact with REE- and alkali-rich carbonatite/kimberlite melt, and may be more common than hitherto recognized. Crystallization of allanite in the cratonic mantle eclogite reservoir may also help explain the difference in LREE abundances between the more strongly enriched carbonatite/kimberlite at depth and the final erupted product. It is likely that allanite is overlooked at eclogites xenoliths, while it is common accessory mineral, hosting REE in orogenic UHP/HP eclogites. Further studies are required to deciphered the peculiarities in metamorphic history recorded in eclogites xenoliths and orogenic eclogites, as well as the differences ancient (Archean/Proterozoic) and Phanerozoic subduction processes.

Mineral carbonation of peridotite fueled by magmatic degassing and melt impregnation in an oceanic transform fault

Most of the geologic CO2 entering Earth's atmosphere and oceans is emitted along plate margins. While C-cycling at mid-ocean ridges and subduction zones has been studied for decades, little attention has been paid to degassing of magmatic CO2 and mineral carbonation of mantle rocks in oceanic transform faults. We studied the formation of soapstone (magnesite-talc rock) and other magnesite-bearing assemblages during mineral carbonation of mantle peridotite in the St. Paul's transform fault, equatorial Atlantic. Clumped carbonate thermometry of soapstone yields a formation (or equilibration) temperature of 147 ± 13 °C which, based on thermodynamic constraints, suggests that CO2(aq) concentrations of the hydrothermal fluid were at least an order of magnitude higher than in seawater. The association of magnesite with apatite in veins, magnesite with a δ13C of -3.40 ± 0.04‰, and the enrichment of CO2 in hydrothermal fluids point to magmatic degassing and melt-impregnation as the main source of CO2. Melt-rock interaction related to gas-rich alkali olivine basalt volcanism near the St. Paul's Rocks archipelago is manifested in systematic changes in peridotite compositions, notably a strong enrichment in incompatible elements with decreasing MgO/SiO2. These findings reveal a previously undocumented aspect of the geologic carbon cycle in one of the largest oceanic transform faults: Fueled by magmatism in or below the root zone of the transform fault and subsequent degassing, the fault constitutes a conduit for CO2-rich hydrothermal fluids, while carbonation of peridotite represents a vast sink for the emitted CO2.

Petrology of the Cretaceous Riacho do Cordeiro dike swarm: Unraveling the giant low-Ti tholeiitic plumbing system of the Equatorial Atlantic Magmatic Province

The formation of the Early Cretaceous Equatorial Atlantic Magmatic Province (EQUAMP), northeastern Brazil, is related to the fragmentation of the West Gondwana supercontinent that culminated in the separation of South America and Africa, during the Lower Cretaceous. The EQUAMP is characterized by its intrusive nature, where mafic dike swarms intrude the Proterozoic basement of the Borborema Province. The other components of this province are represented by sill complexes hosted between the Paleozoic sediments in the Parnaíba Basin, described as the Sardinha Formation. The magmatism related to the EQUAMP is tholeiitic, with high-Ti, and low-Ti magmas compositions that with similar features that are observed in other Mesozoic igneous provinces of Gondwana. The Riacho do Cordeiro dike swarm (RCo) represents the southernmost manifestation of the EQUAMP, consisting of a series of igneous bodies aligned along the N45E trend, arranged in an en echelon pattern. This structure extends for about 350 km, cutting the Precambrian basement of the southern segment of the Borborema Province. The dikes have a composition similar to tholeiitic basalts and basaltic andesites, but they are distinguished from the other components of the EQUAMP by their exclusively low-Ti content (TiO2 = 1.7–1.2 wt%). Magmatic differentiation is the main petrogenetic mechanism of the low-Ti magmas, typified by the systematic fractionation of major elements with respect to the variation of MgO (6.4–2.4% wt%). Trace (including rare earth) element patterns show enrichment in incompatible elements and negative anomalies in NbTa, a pattern recognized in continental basalts derived from (or contaminated with) lithospheric mantle. The Sr (87Sr/86Sr(i) 0.7072 to 0.7091) and Pb (206Pb/204Pb 18.360 to 18.785) isotopic signatures, combined with negative values for εNd(i) (−4.8 to −6.1) and TDM model ages >1.2 Ga, confirm the involvement of reservoir(s) in the upper mantle possibly modified by ancient subduction processes. The first reported 40Ar/39Ar ages for the low-Ti tholeiites of the RCo indicate ages from 134.53 ± 3.10 to 132.19 ± 1.28 Ma. Geochemically, the low-Ti tholeiites of the RCo are equivalent to low-Ti dikes that constitute part of the Rio Ceará Mirim dike swarm, the main component of the EQUAMP. Besides that, in a scenario external to this province, the same compositional correspondence is found within the Cretaceous dikes of the Vitória-Colatina swarm, southeastern Brazil, and the low-Ti type lavas of the Paraná-Etendeka Province. This compositional and geochronological connection between the tholeiitic magmatism located north and south of the São Francisco craton supports a possible genetic connection between these two Early Cretaceous LIPs and serve as base to future investigations that deal with what would have been the (global) mechanisms that led to the rifting of the Atlantic, from the southern segment to the equatorial margin.

Heterogeneous Archean oceanic protoliths in Neoproterozoic retrogressed eclogites from Northeast Brazil: Petrological, geochemical and Sr–Nd–Pb–Hf isotopic constraints

Eclogites are reliable indicators of high to ultrahigh-pressure (HP-UHP) conditions for oceanic and continental crust and are essential rocks to understand the lithospheric convergence from subduction to post-collisional setting. Furthermore, the origin and nature of eclogite precursors may reveal crucial constraints on the crustal history before HP-UHP metamorphism. Here, we provide petrological, geochemical and Pb-Sr-Nd-Hf isotopic constraints from the Campo Grande retrogressed eclogites, Northeast Brazil, to determine the source and evolution of their Archean protoliths. Eclogites were formed under HP conditions (18 ± 1.0 kbar and 680 ± 20 °C) followed by retrogression in the amphibolite facies, which is characterized by Mg-rich garnet porphyroblasts surrounded by clinopyroxene + plagioclase + amphibole + orthopyroxene symplectites (stable at <6.0 kbar). These retrogressed eclogites have chemical compositions corresponding to enriched-type tholeiites such as E-MORB protoliths (G1 eclogitic group). A second group of retrogressed eclogites with higher amphibole content (G2 eclogitic group) shows similar chemical patterns to G1, however with higher enrichment in LILE, REE and incompatible elements. The G1 eclogite is characterized by higher modal concentration of garnet and clinopyroxene, positive εNd(t) (+2.1 to +5.8) with low 87Sr/86Sr(i) ratios (0.7024 to 0.7131), whereas G2 displays dominant coarse-grained amphibole, positive and negative εNd(t) (−7.5 to +8.5) with high radiogenic 87Sr/86Sr(i) ratios (0.7109 to 0.7255). The chemical and isotopic G1 values show typical E-MORB signatures, whereas G2 indicate an E-MORB enriched by crustal components in a supra-subduction zone setting. A LuHf isochron provided the age of 2667.9 ± 30.7 Ma, which is close to the published ∼2.65 Ga crystallization age for the G1 and G2 oceanic protoliths. Therefore, Hf isotopes in whole rock and cogenetic minerals were useful for constraining the crystallization time of metamafic protoliths. In contrast, SmNd isochron age of 606.9 ± 24.8 Ma falls within the range of 623 to 590 Ma HP metamorphism recorded in the G1 and G2 rocks. Lastly, a third type of eclogite group (G3 eclogitic group) is represented by a banded Mn-rich-garnet (spessartite) and Al-rich clinopyroxene with detrital zircon cores from 3.42 to 2.15 Ga and recrystallized rims at 607.7 ± 5.4 Ma. Mineral texture and composition, geochemistry, strongly negative εNd(t) (−4.3 to −1.5) and higher radiogenic 87Sr/86Sr(i) ratios (0.7434–0.7501) suggest a different Mn-rich protolith for the G3 eclogites. Integrated data reveal that retrogressed eclogites retain the memory of the original chemical and isotopic patterns of Archean oceanic protoliths and its diachronous reworking processes at deep crustal levels.

Influences of subduction derived fluids and melt in the genesis of Nidar ophiolite peridotites, Ladakh Himalaya, India: Evidence from mineralogy, PGE and Nd isotopic compositions

The Nidar ophiolite is one of the well-preserved and almost complete ophiolite sections of the Neo-Tethyan oceanic lithosphere, obducted along the continental margin between the Indian and the Eurasian plate. This ophiolite sequence is mostly dominated by ultramafic rocks, consisting of forearc-related refractory, mainly harzburgite, dunite, and serpentinite, with minor intrusions of lherzolite, chromitites, and pyroxenites. In this present study, detailed mineralogical, whole rock geochemistry (major oxides, trace elements, PGE), and Nd isotopic composition of mantle-derived peridotites have been carried out to constrain the petrogenesis and melt evolution. These peridotites are depleted in nature due to the low modal composition of clinopyroxene, high forsterite content in olivine, and wide variation in Cr# and bulk rock chemistry, indicating variable degree of partial melting. The spoon-shaped rare earth element (REE) patterns indicate metasomatism by fluids derived from a subducting slab enriched in light REEs. Geochemical composition of the studied peridotites rocks is marked by high ratio of Al2O3/TiO2, LILE-LREE enrichment, HFSE depletion, and spoon-shaped chondrite-normalized REE patterns and (La/Sm)N > 1 and (Gd/Yb)N < 1, indicates some involvement of boninitic mantle melts and validate a subduction initiation process. The total PGE of the peridotites (ΣPGE = 33–337 ppb) is much more enriched than that of the primitive mantle and other ophiolite peridotites. The PGE distribution displays a concave upward pattern with higher PPGE/IPGE ratios (i.e., 0.11–1.45), suggesting that partial melting is not the only process for the evolution of the Nidar ophiolite peridotites. Enrichment of PPGE and incompatible elements (like LREE) and higher Pd/Ir ratio (0.69–8.26) indicates that these peridotites have undergone fluid/melt interaction in a supra-subduction zone (SSZ) tectonic setting. PGE concentrations of these depleted harzburgites and dunites, formed by partial melting of cpx–harzburgites in an SSZ that produced the boninitic-like melt. The enrichment of incompatible elements like the PPGE is mainly due to the circulation of fluids in the subduction zone, which leads to the PGE fractionation in mantle peridotites. Also, these peridotites have 143Nd/144Nd ratios (0.51148–0.51262) and εNd(t) (t = 140 Ma) values (i.e., +0.97 to −21.3), indicating derivation from depleted mantle sources within an intra-oceanic arc setting. The geochemical behavior exhibited by the Nidar ophiolite peridotites suggests the evolution of a highly depleted fore-arc mantle wedge significantly modified by various fluids and melts during subduction. The mineralogical, geochemical, and Nd isotopic composition of these peridotites and dunites mutually depict the diverse mantle compositions, suggesting insights into the interactions between the oceanic crust and mantle as well as associated geochemical cycling in an SSZ environment.

Geochemistry, petrogenesis, and tectonic setting of the Cúcamo mafic and intermediate volcanic rocks from the Ahualulco Volcanic Complex, San Luis Potosí, Mexico

The Ahualulco Volcanic Complex (AVC) is situated in the north-central part of the San Luis Potosí Volcanic Field (SLPVF) that is found in the southern portion of the Mesa Central (MC). The Cúcamo, AVC is mainly composed of mafic and intermediate volcanic rocks. The present study focuses on understanding the evolution, origin, and magmatic processes and petrogenesis of mafic and intermediate rocks in the Cúcamo, AVC. The Quaternary mafic rocks have porphyritic textures with the mineral assemblage of olivine, and clinopyroxene. These volcanic rocks display high K calc-alkaline basaltic compositions with enrichment in light rare earth elements (LREEs) and incompatible elements. Geochemical modeling reveals that mafic magmas were derived through a partial melting process of a spinel lherzolite source at low degrees of melting (~2 to 15 %) in an extensional regime. The intermediate volcanic rocks show porphyritic and glomeroporphyritic textures with matrix formed by randomly oriented microlites. The main mineral assemblage consists of plagioclase, K-feldspar, and clinopyroxene. These volcanic rocks are characterized by calc-alkaline basaltic andesitic and andesite compositions with enrichment in light rare earth elements and incompatible elements. Geochemical modeling suggests that intermediate rocks were derived from high ratios of assimilation and fractional crystallization processes between mafic melts and continental crust in an extensional environment.

The relationship between δ18O values and incompatible element and radiogenic isotope enrichment in the Mashikiri nephelinites, of the Karoo LIP, southern Africa

The Mashikiri nephelinites (MN) are the earliest and most incompatible element-enriched magmas of the Karoo LIP. Their Zr/Nb ratios of ∼1.8 are distinct from most Karoo LIP mafic rocks, which have Zr/Nb from 10 to 18 (average ∼ 16). Low εNd (−6 to −22) with initial 87Sr/86Sr ratios of 0.7046 to 0.7073 have previously been explained as due to their derivation from ancient metasomatized mantle lithosphere by low degrees of partial melting. Olivine and pyroxene phenocrysts from the MN have δ18O values ranging from 5.30 to 5.96‰, and 5.50 to 6.61‰, respectively, and crystallization was from magmas having δ18O values that vary from 6.0 to 6.6‰, up to 0.9‰ higher than magmas derived from a depleted mantle source. There is no correlation between δ18O value and indications of either crustal contamination, such as SiO2 and Pb content, or fractional crystallization, such as Mg-number and Ni and Zr content. High δ18O values are, therefore, a feature of the mantle source. There is no relationship between δ18O value and indicators of mantle metasomatism such as εNd, alkalinity, or incompatible trace element concentrations and processes that enriched the magma in 18O are, therefore, unrelated to mantle metasomatism.High δ18O values in the mantle are most likely to be due to the presence of recycled crust e.g. eclogite or pyroxenite. A mixed peridotite-eclogite mantle source could have been formed by emplacement of oceanic crust into the base of the craton during Archaean subduction. Subsequent metasomatic veining that variably infiltrated the mantle, including the high-δ18O component, before eruption of the lavas would allow extremely negative εNd values to develop.

Constraints on near-ridge magmatism using 40Ar/39Ar geochronology of enriched MORB from the 8°20' N seamount chain

Our understanding of the spatial-temporal-compositional relationships between off-axis magmatism and mid-ocean ridge spreading centers is limited. Determining the 40Ar/39Ar ages of mid-ocean ridge basalt (MORB) lavas erupting near mid-ocean ridges (MOR) has been a challenge due to the characteristically low K2O contents in incompatible element-depleted normal MORB (NMORB). High-precision 40Ar/39Ar geochronology is used here to determine ages of young, basaltic lavas erupted along the 8°20' N seamount chain west of the East Pacific Rise (EPR) axis that have a range of incompatible element enrichments (EMORB) suitable for 40Ar/39Ar geochronology (e.g., K2O contents > 0.3 wt%). 40Ar/39Ar ages were determined in 29 well-characterized basalts sampled using HOV Alvin and dredging. Detailed geochronology and geochemical analyses provide important constraints on the timing, distribution, and origins of lavas that constructed this extensive volcanic lineament relative to magmatism beneath the adjacent EPR axis. Seamount eruption ages are up to ∼1.6 Ma younger than the underlying lithosphere, supporting a model of prolonged off-axis magmatism for at least 2 Myrs at distances as great as ∼90 km from the ridge axis. Increasing geochemical heterogeneity with eruption distance reflects the diminishing effect of sub-ridge melt focusing. The range of geochemically distinct lavas erupted at given distances from the ridge highlights the dynamic nature of the near-ridge magmatic environment over Myr timescales. Linear ridge-like (EPR-parallel) morphotectonic features erupt the youngest and most incompatible element-enriched lavas of the entire seamount chain, indicating there is a recent change in the influence of mantle heterogeneity and off-axis melt metasomatism on the near-ridge lithospheric mantle. Changes in seamount morphologies are attributed to counter-clockwise rotation and southward migration of the nearby Siqueiros transform over the last few million years.

Cryptic crustal growth identified through Variscan post-collisional lamprophyre-granite composite dykes, French Massif Central

The processes that control crustal growth by addition of new mantle-derived material to the crust and the recycling of crustal components that were introduced into the mantle during subduction remain poorly understood in collisional orogens. This is largely due to the fact that the sub-orogenic mantle has, in many instances, been fertilized by the introduction of crustal components during previous subduction events, and lacks the typical depleted mantle signature. Here, we present a study of lamprophyric–granitic composite dykes from the northern margin of the post-collisional Variscan Aigoual granite, French Massif Central (FMC). At Aigoual, lamprophyres and granites have gradational contacts and similar geochemical characteristics which indicate that the two were co-magmatic. UPb dating of zircon yields crystallization ages between 311 ± 3 Ma and 313 ± 3 Ma for dykes, identical within uncertainty with the ages of 312 ± 3 Ma and 313 ± 3 Ma obtained for the emplacement of the Aigoual granite. The lamprophyres are metaluminous to slightly peraluminous and display enrichment in both compatible (Fe, Mg, Ni, Cr) and incompatible elements (K, LILE, LREE) and have crustal isotopic signatures in both radiogenic (Sr, Nd and Hf) and stable (O) isotope systems. This dual crust/mantle signature of the lamprophyres can be interpreted to reflect partial melting, within the spinel stability field, of an enriched mantle that was metasomatised by the addition of subducted sediments. The granites are peraluminous, high-K and have shoshonitic affinities. Their primitive mantle-normalized trace-element patterns are similar to the lamprophyres and the isotopic compositions of the two rock types overlap. The high Cr, Ni, Fe, and Mg contents of the granites are consistent with a mantle component in the magmas. Our results suggest that between 65 and 80 wt% of material in the lamprophyres and granites was derived from the depleted mantle, and the rest from recycled crustal material. Although blurred from an (conventional) isotopic point of view, this magmatism contributed to the crustal growth via the post-collisional magmatism of the FMC. Coupled with their high preservation potential, post-collisional sites might thus represent significant sites of crustal growth.

Plume–lithosphere interactions and LIP-triggered climate crises constrained by the origin of Karoo lamproites

We identified a ca. 180 Ma diamondiferous lamproite event in Zambia, establishing a link between ultrapotassic volcanism and the early Jurassic Karoo flood basalt province of sub-Saharan Africa. The cratonic lamproites erupted through the Permo–Triassic Luangwa Rift structure, but MgO-rich ultrapotassic magma formation was unrelated to rifting and triggered by plume–lithosphere interactions during the Karoo LIP event. Elevated Li–Zn–Ti concentrations in magmatic olivine (up to 18.5 ppm Li at 86–90 mol.% forsterite) and strong Sr–Nd–Hf–Pb isotopic enrichment of the host lamproites (87Sr/86Sr = 0.70701–0.70855, εNd = −10.8 to −10, εHf = −20.3 to −19.1, 206Pb/204Pb = 16.8–17.5) suggest partial melting of phlogopite-metasomatized lithospheric mantle domains, at approximately 180–200 km depth. The mantle-like δ7Li values (+2.8 to +5.7‰) of the most pristine lamproite samples are compatible with source enrichment by asthenosphere-derived melts, without significant involvement of recycled sedimentary components. This geochemical fingerprint stands in sharp contrast to the negative δ7Li compositions of primitive K-rich volcanic rocks from collision zone settings, where the shallow mantle sources contain recycled sediment.Isotope modelling demonstrates that the sub-Saharan lamproites originate from a MARID-style metasomatized peridotitic mantle source that underwent incompatible element enrichment at ca. 1 Ga, during tectonic activity associated with Rodinia supercontinent formation. Plume-sourced basaltic and picritic magmas of the 180 Ma Karoo LIP interacted with such K-rich hydrous lithospheric mantle domains, thereby attaining enriched incompatible element and radiogenic isotope compositions. Nd–Hf isotope mass balance suggests that up to 25% of MARID-sourced lamproite melt component contributed to some of the high-Ti flood volcanic units.Although large quantities of volatiles can be transferred from Earth’s mantle to the atmosphere via plume–lithosphere interactions, it is unlikely that outgassing of mantle-sourced sulphur can exceed the climatic impact caused by the release of much more abundant carbon from thick continental roots. Thus, the excess SO2 required to account for transient atmospheric cooling during the early Jurassic, coincident with the Karoo LIP event, must have had a thermogenic origin near the surface of Earth.

Ca. 830 Ma alkaline mafic dykes in the southeastern Guizhou Province, South China: New constraints on the Neoproterozoic evolution of the Yangtze Block

The Neoproterozoic alkaline mafic rocks in the Yangtze Block are critical to investigate the magmatic and tectonic evolution of South China. However, these rocks are rarely exposed. We here present a detailed study of ca. 830 Ma alkaline mafic dykes newly identified from a drilled borehole at the Nage area in the southern Yangtze Block. The dykes have SIMS zircon U-Pb ages of 827 ± 8 Ma. They show coherent variations in major and trace elements, characterized by enrichment in LREE and incompatible elements and no significant Nb-Ta and Zr-Hf anomalies, suggesting that their parental magmas have experienced varying degrees of fractionation crystallization with negligible crustal contamination. Modeling calculations based on their εNd(T) values (-0.55–3.23), SiO2 contents (48.38–53.61 wt%) and elemental ratios (e.g., La/Ta and Nb/La) suggest that the Nage mafic dykes were probably generated from an asthenospheric mantle with previously input of a plume-derived enrich component occurred within the source region rather than during ascent. Rocks from the Nage dykes exhibit OIB-like trace element and REE patterns as well as trace elemental compositions, which are different from arc-related igneous rocks but similar to alkaline basalts and mafic dykes formed in mantle plume related rifting systems around the world. In combination with tectonostratigraphic evidences, the Nage dykes in this study are suggested to have been generated in an intraplate rift which was probably induced by a mantle plume beneath the Yangtze Block.

Petrogenesis of potassic granite suites along the southern margin of the Zimbabwe Craton

AbstractAn integrated approach embracing field studies, petrographic and geochemical investigations together with zircon U-Pb-Hf data was used to investigate the petrogenesis of potassic granite suites along the southern margin of the Zimbabwe Craton. Zircon U-Pb geochronology identifies age relationships, revealing coeval magmatism of the ca. 2 635 ± 5 to 2 625 ± 3 Ma Chilimanzi Suite, and the ca. 2 627 ± 7 Ma Razi Suite. Both suites represent syn- to late-tectonic, high-K, calc-alkaline, and metaluminous to weakly peraluminous granites and granodiorites with I-type affinity. The granite suites contain xenocrystic zircons, with the Chikwanda Pluton of the Chilimanzi Suite yielding a grain of up to 3 206 Ma old. Both granite suites exhibit eHf values of between -5.6 ± 1.3 and -7.3 ± 1.6 and TDM model ages of ca. 3.4 to 3.5 Ga which suggests a similar crustal source. The unradiogenic zircon Hf isotopic compositions are consistent with formation of the granite suites through partial melting of pre-existing crustal protoliths, including Palaeoarchaean tonalite-trondhjemite-granodiorites (TTGs) of the Zimbabwe proto-craton. Partial melting of lower crust gave rise to granitic melts that became emplaced over a relatively short time interval from 2 635 to 2 625 Ma and heralded the stabilisation of the Zimbabwe Craton.In addition to virtually identical ages, the Razi and Chilimanzi suites have similar geochemistry. Small geochemical differences between the Chilimanzi and the Razi suites are attributed to the crustal level at which they are preserved, the modal mineralogy and the extent to which the melts are evolved. The Razi Suite melts were generated from lower crust partial melting of thickened charnockite-enderbite source rocks rich in heat producing elements. The partial melting occurred under fluid-absent conditions and magmas were emplaced at lower to mid crustal levels. The Chilimanzi Suite magmas were similarly derived by the partial melting of TTG lower crust and were emplaced at upper crustal levels. Accordingly, the Chilimanzi Suite exhibits more evolved magmatic fractionation indices indicated by high Rb/Sr, as well as low K/Rb ratios relative to the Razi Suite. Both suites reveal varying degrees of enrichment in incompatible elements including Rb, Th, and U, as well simultaneous depletions in Ba, Sr, and Hf which underscores the role of fractional crystallisation in the evolution of the granitic magmas.

Paleoproterozoic Shoshonite Mafic Associations of the Irkut Block (Sharyzhalgai Uplift, Southwest Siberian Craton): U–Pb Age and Conditions of Zircon Crystallization

Abstract —The paper presents data on the composition and age of mafic rocks of the shoshonitic series in the Irkut block of the Sharyzhalgai uplift (southwest of the Siberian Сraton). According to the U–Pb dating of magmatic zircon, the formation of monzodiorites of the Poludennyi massif and gabbro-dolerites in the endo- and exocontact zones of the Toisuk pluton occurred at 1.87 and 1.86–1.85 Ga, respectively. The intrusion of mafic magmas and their underplating into the basement of the crust under postcollisional extension resulted in the near-coeval mafic and granitoid magmatism in the Irkut block between 1.87 and 1.84 Ga. The Paleoproterozoic mafic associations belong to the shoshonitic series and are characterized by enrichment in incompatible elements, including Zr, and low negative εNd(T) values. These geochemical and isotopic characteristics point to the derivation of magma from a long-lived enriched-mantle source, such as the subcontinental lithospheric mantle. The crystallization of zircon from the last portions of the evolved mafic melt is evidenced by low zirconium saturation temperatures (710–965 °C) and zircon enrichment in U and Th with increasing Th/U, reflecting the growth of concentrations of highly incompatible elements in the residual melt.

Enrichment of Incompatible Elements in Alkaline Syenites in Large Igneous Provinces Due to Magma Replenishment and Reactive Porous Flow in a Mush Reservoir

AbstractUnderstanding the petrogenesis of alkaline syenites is important for constraining the mechanisms of rare earth element (REE) and rare-metal mineralization. Here we report a detailed petrological and geochemical study of early Permian syenitic rocks from the southwestern Tarim large igneous province (TLIP) in the Wajilitag area (China). We use these data to investigate the complex magmatic processes responsible for the enrichment of incompatible elements (e.g. REEs and rare metals) in these rocks. The Wajilitag syenitic rocks comprise early hornblende syenite (281 Ma) and later nepheline syenite (278 Ma), both of which are spatially and temporally associated with mafic intrusions (i.e. gabbro and diabase). These syenitic rocks show continuous variations in major elements with the mafic rocks. They also have similar Sr–Nd–Hf isotopic compositions with the nearby mafic rocks, denoting that the Wajilitag syenitic rocks should be derived from the melts represented by these mafic rocks. However, these syenitic rocks, particularly the nepheline syenites, are typically characterized by extreme enrichment of incompatible elements, which is hard to be accounted by simple fractional crystallization of mafic magmas. Some clinopyroxene phenocrysts in the Wajilitag nepheline syenites show oscillatory zoning with strikingly increasing and then gradually decreasing MgO contents, which recorded replenishment of mafic magma at the interval. Low-MgO clinopyroxene mantles and/or rims have much higher incompatible element contents (e.g. Nb, Ce, and Zr) and ratios (e.g. Ce/Y) than their cores, which could reflect melt injection controlled by reactive porous flow in a mush reservoir of a crustal magma chamber. Such processes may also cause the high Ce and/or Nb contents of the mantles and/or rims of zoned titanite and apatite phenocrysts. The injection of reactive porous flow melts is the key process that produces the extreme enrichment of incompatible elements in the alkaline syenites from the western TLIP, as well as other plume-related alkaline syenites that host world-class REE and rare-metal deposits.

Origin of Late Cretaceous, enclave-bearing granitoids in southern Tibet: Implications for magma recharge and crustal thickening

Exposures of enclave-bearing granitoids can provide rare opportunities to directly evaluate the connection between compositional variability and the depth of origin of arc magmatic rocks. The ∼1000 km long Gangdese batholith is a composite batholith with composition from mafic to felsic; SiO2 ranges from 51 wt% to 70 wt%. New zircon U−Pb dating of the Nyemo plutons, Renbu plutons, and Xigaze plutons in the Gangdese batholith is consistent with their emplacement and crystallization in the Late Cretaceous (ca. 90−85 Ma). Mafic magmatic enclaves (MMEs) in the plutons are characterized by low SiO2 (50.9−56.0 wt%) and Nb/U, Ce/Pb, and Nb/La ratios coupled with enrichment in light rare earth elements and large ion lithophile elements and depletion in high field strength elements. These geochemical features, combined with depleted whole-rock εNd(t) (+4.2 to +4.7) and zircon εHf(t) (+9.0 to +13.8), suggest that they were derived by partial melting of a depleted mantle source associated with subduction-related fluids. The granitoids with high SiO2 (55.6−66.9 wt%) display adakitic geochemical characteristics, such as low Y and Yb contents, and high Sr/Y and La/Yb ratios. Their positive whole-rock εNd(t) (+4.0 to +5.5) and zircon εHf(t) (+6.9 to +14.3) values, as well as enrichment of incompatible elements, indicate that the granitoids were derived from partial melting of the juvenile lower crust. Geochemical modeling suggests that the compositional diversities of MMEs and adakitic granitoids were inherited from heterogeneous sources. This genetic relationship indicates that the underplated basaltic magmas could have supplied sufficient heat to trigger the melting of the thickened crust and thus formation of the enclave-bearing granitoid. In this regard, the origin of arc rocks can mirror the evolution of crustal thickness. Our results reveal that the crust was thickened to ∼50 km during the Late Cretaceous (90−85 Ma) and provide a magmatic record of crustal thickening prior to the Cenozoic Indo-Asia collision.

Europium anomalies in detrital zircons record major transitions in Earth geodynamics at 2.5 Ga and 0.9 Ga

Abstract Trace elements in zircon are a promising proxy with which to quantitatively study Earth’s long-term lithospheric processes and its geodynamic regimes. The zircon Eu anomaly reflects the crystallization environment of its felsic or intermediate parental magma. In particular, it provides insight into the water content, magmatic redox conditions, and the extent of plagioclase fractionation in the source rock or its occurrence as a cogenetic crystallizing phase from the magma. We performed a statistical analysis of Eu anomalies from a compilation of detrital zircons over geologic time and found a major decrease in Eu anomaly ca. 2.5 Ga and an important increase ca. 0.9 Ga. Coupled with thermodynamic modeling, we suggest that these variations could be due to long-term change in the chemical system of the mafic source from which the intermediate to felsic melt and derived zircons were produced. The 2.5 Ga drop was likely associated with an enrichment in incompatible elements in the mafic source, which extended the pressure-temperature field of plagioclase stability as a cogenetic melt phase. We interpret the 0.9 Ga rise to record increasing hydration of magmagenetic sites due to the general development of cold subduction systems, which would delay and/or suppress the saturation of plagioclase in hydrous magmagenetic sites.

Europium anomalies in detrital zircons record major transitions in Earth geodynamics at 2.5 Ga and 0.9 Ga

Trace elements in zircon are a promising proxy with which to quantitatively study Earth’s long-term lithospheric processes and its geodynamic regimes. The zircon Eu anomaly reflects the crystallization environment of its felsic or intermediate parental magma. In particular, it provides insight into the water content, magmatic redox conditions, and the extent of plagioclase fractionation in the source rock or its occurrence as a cogenetic crystallizing phase from the magma. We performed a statistical analysis of Eu anomalies from a compilation of detrital zircons over geologic time and found a major decrease in Eu anomaly ca. 2.5 Ga and an important increase ca. 0.9 Ga. Coupled with thermodynamic modeling, we suggest that these variations could be due to long-term change in the chemical system of the mafic source from which the intermediate to felsic melt and derived zircons were produced. The 2.5 Ga drop was likely associated with an enrichment in incompatible elements in the mafic source, which extended the pressure-temperature field of plagioclase stability as a cogenetic melt phase. We interpret the 0.9 Ga rise to record increasing hydration of magmagenetic sites due to the general development of cold subduction systems, which would delay and/or suppress the saturation of plagioclase in hydrous magmagenetic sites.

Multistage in situ fractional crystallization of magma produced a unique rare metal enriched quartz-zinnwaldite-topaz rock

Highly fractionated granites and pegmatites have special petrogenesis and potential economic significance due to the enrichment of incompatible elements (Li, Rb, Cs, etc.) and flux elements (F, B, Li, etc.) in the magmatic volatile phase (MVP). Here we decipher the origin of the giant Zhengchong Rb-dominated deposit associated with highly fractionated A-type granite in South China. An extreme fractional crystallization (up to 98.3%) was required to generate super enrichment of incompatible elements and flux elements by the model of single-step in situ crystallization. An alternative model incorporates episodic remobilization and multistage in situ crystallization of a long-lived mush. The MVP and incompatible elements migrate via reactive flow at intermediate degree of crystallization (70–80%) in each intrusion fits with the observed geochronological and geochemistry data. This geochemical model can help us understand the evolution of magma reservoirs and the concept of magmatic distillation columns.