Arc Basalts Research Articles

Temporal covariation of island arc Sr isotopes and seawater chemistry over the past 2 billion years

The chemical compositions of island arc basalts (IAB) reflect contributions from the mantle as well as fluids and melts from the subducting slab. Addition of radiogenic seawater Sr to oceanic crust through hydrothermal alteration and subsequent subduction is often invoked to explain elevated 87Sr/86Sr signatures in modern IAB. However, changes in the 87Sr/86Sr of island arc magmatic rocks through time has not been investigated, limiting our understanding of the factors influencing the Sr budgets of arcs throughout Earth's history. To address this, we compiled 87Sr/86Sr values from island arc magmatic rocks ranging in age from modern to Paleoproterozoic, only including data from island arc localities that best preserve initial magmatic 87Sr/86Sr. Median initial 87Sr/86Sr values are consistently elevated compared to depleted mantle 87Sr/86Sr over this period, indicating persistent enrichment in radiogenic Sr in island arcs. Moreover, the elevation in island arc 87Sr/86Sr relative to the depleted mantle is variable. A notable rise in island arc 87Sr/86Sr during the late Neoproterozoic coincides with a steep increase in seawater 87Sr/86Sr and Sr concentration. To investigate this potential connectivity, we modeled the 87Sr/86Sr of island arc magmas between 0 and 830 Ma with inputs of depleted mantle 87Sr/86Sr, seawater 87Sr/86Sr, and seawater Sr concentration. The model reproduces the overall trajectory of the compiled data. We interpret the observed temporal variation in island arc 87Sr/86Sr values and its close association with fluctuations in seawater chemistry as evidence that changes in marine geochemistry have strongly influenced the Sr isotopic record of island arc magmas over time.

The fate of ultramafic-rich mélanges in cold to hot subduction zones: Implications for diapirism (or not) and chemical geodynamics

Buoyant ultramafic-rich (serpentine- or chlorite-rich) mélange diapirs in sediment-starved subduction zones can transport slab material to arc sources. While the buoyancy of chlorite-rich mélanges was previously investigated, serpentine-rich mélanges were never explored. Thus, the overall contribution of ultramafic-rich mélanges to buoyancy, the conditions for diapir formation, and their fate in subduction zones are not well constrained. Here, we investigate the partial melting behavior and the associated density transformations of a serpentine-rich matrix (5–10 wt.% H2O) with minor sediments (9:1 ratio) at fore-arc (∼65 km) to sub-arc (∼95 km) depths (2–3 GPa and 800–1250 °C) and compare to that of chlorite-rich mélanges from the literature. Our results show that the solidus of serpentine-rich matrices is between 1050 and 1100 °C and requires either diapiric rise of the mélange into the hotter mantle wedge or interactions with a hotter asthenosphere through slab tears to partially melt and produce basaltic melts, whether in hot or cold slab channels. Chlorite-rich mélanges may account for the sources of some arc lavas, but partial melting of serpentine-rich mélanges produce melts depleted in CaO, TiO2, alkalis, and are highly enriched in MgO compared to basaltic arc lavas. Both serpentine-rich and chlorite-rich matrices dehydrate to form denser peridotite and lose buoyancy at ∼800 °C and ≥1000 °C, respectively. Even if diapirism initiates in such mélanges near the slab-mantle interface, they would likely lose buoyancy upon ascent into the hotter mantle wedge resulting in stalled or failed diapirs. Diapir growth (τa) is controlled by the interplay of density, thickness and viscosity of the mélange, as well as the timescale of slab subduction (τs) and thermal structure of the subduction zone. We observe that the onset of diapirs in cold subduction zones requires mélanges that may sometimes be thicker than that observed by field and geophysical studies, while hot subduction zones overall require thinner mélanges. Thus, ultramafic-rich mélange diapirs may occur but only under specific conditions and when the diapiric ascent timescale is faster than the thermal equilibration barrier of ∼800–1000 °C (especially at the core of the mélange). Dehydration or partial melting of ultramafic-rich mélanges can affect the large ion lithophile element (LILE), volatiles, and high-field strength element (HFSE) budgets in the mantle wedge. Partial melting (caused by a diapiric rise or slab tear) does not fractionate LILEs from HFSEs at T ≥ 1100 °C and if the mélange has a lower LILE/HFSE to begin with, that signature is transferred to arc sources. Dehydration releases aqueous fluids rich in fluid-mobile elements (LILE and volatiles) relative to HFSE. Thus, the characteristic high LILE/HFSE signature of aqueous fluids is transferred to arc magma sources. Given high LILE/HFSE ratio is a ubiquitous arc magma signature, but slab tears are not, and diapirism in ultramafic-rich mélanges is highly conditional, this study corroborates that aqueous fluids released from sediment-starved mélanges are the predominant mass transfer agents rather than diapirs.

Recycling of terrigenous sediments recorded in late Permian-early Triassic Fe-rich intrusions from Yunkai Massif, South China

The origin of Fe-rich intrusion remains debatable regarding the respective role of magmatic evolution and Fe enrichment in the source. Here, we report detailed mineral and bulk-rock geochemistry of Permian-Triassic (251–250 Ma) mafic intrusions from the Tengxian area in Yunkai Massif, South China. These rocks consist of hornblende norites, show Fe-rich affinities with high FeO*(FeO* = FeOt / (FeOt + MgO) in weight ratio, >0.8, based on total iron oxide in the rock), and exhibit significant enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but depletion in Sr and high field strength elements (HFSEs), resembling subduction-related magmas. In addition, they show remarkable features that include very high δ18O values in zircon (δ18O = 8.8–10.6 ‰) and apatite (δ18O = 8.7–10.8 ‰), and extremely enriched SrPb [e.g. 87Sr/86Sr(i) = 0.7181–0.7196, 207Pb/204Pb(i) = 15.77–15.78, 208Pb/204Pb(i) = 38.95–39.05] and nonradiogenic NdHf isotopic compositions [e.g. εNd(t) = −11.1 ~ − 10.1, εHf(t) = −8.8 ~ −7.3] in bulk rocks. These characteristics distinguish the Tengxian norites from modern arc basalts and subduction-related magmas in the Paleo-Pacific Tectonic Domain. Instead, they are isotopically similar to Cenozoic Tibetan and Mediterranean potassic to ultrapotassic rocks in the Tethys Tectonic Domain. Regardless of variable influence by orthopyroxene and plagioclase accumulation, the intrinsically low SiO2 and high FeOt and Fe/Mn ratios in these rocks were likely attributed to significant contribution of a Fe-rich mantle component such as Si-poor pyroxenite, which might have formed through crystal accumulation of mafic magmas at mantle conditions. The highly evolved Sr-Nd-Pb-Hf isotopic signatures and very high δ18O values in zircon and apatite required substantial (10–20 %) involvement of recycled crust in the mantle source. The combined mineral and bulk-rock geochemical data suggest that the parental magmas for the Tengxian norites originated from a metasomatized mantle wedge through addition of terrigenous sediment-derived melt following the subduction of Paleo-Tethys Ocean beneath the Yunkai Massif.

Disentangling the Roles of Subducted Volatile Contributions and Mantle Source Heterogeneity in the Production of Magmas Beneath the Washington Cascades

AbstractThe compositional diversity of primitive arc basalts has long inspired questions regarding the drivers of magmatism in subduction zones, including the roles of decompression melting, mantle heterogeneity, and the amount and composition of slab‐derived materials. This contribution presents the volatile (H2O, Cl, and S), major, and trace element compositions of melt inclusions from basaltic magmas erupted at three volcanic centers in the Washington Cascades: Mount St. Helens (two basaltic tephras, 2.0–1.7 ka), Indian Heaven Volcanic Field (two <600 ka basaltic hyaloclastite tuffs), and Glacier Peak (late Pleistocene to Holocene basaltic tephra from Whitechuck and Indian Pass cones). Compositions corrected to be in equilibrium with mantle olivine display variability in Nb and trace element ratios indicative of mantle source variability that impressively spans nearly the entire range of arc magmas globally. All volcanic centers have magmas with H2O and Cl contributions from the downgoing plate that overlap with other Cascade Arc segments. Volatile abundances and trace element ratios support a model of melting of a highly variably mantle wedge driven by a subduction component of variably saline fluid and/or slab partial melt. Magmas from Glacier Peak in northern Washington have unusually high Th/Yb ratios that are similar to Lassen region basalts, indicating possible contributions of “subcreted” metasediments that geophysical data suggest are not present beneath central Oregon and southern Washington. This data set adds to the growing inventory of primitive magma volatile concentrations and provides insight into spatial distributions of mantle heterogeneity and the role of slab components in the petrogenesis of arc magmas.

Why are oceanic arc basalts Ca-rich and Ni-poor? Insights from olivine-hosted melt inclusions from Kibblewhite Volcano in the Kermadec arc

Ankaramites, which are clinopyroxene-rich basalts with primitive whole-rock compositions (Mg# >65), are common in oceanic arcs and are characterized by high whole-rock CaO/Al2O3 (>1.0) ratios and olivine crystals with anomalously low nickel contents (<0.2 wt% NiO). These geochemical characteristics cannot be explained by the melting of ordinary mantle peridotite. However, their origin is critical for understanding the formation of primary magmas in oceanic arcs. Here, we investigated olivine-hosted melt inclusions (MIs) from ankaramites and magnesian andesites of the Kibblewhite Volcano in the Kermadec arc. The MIs from the ankaramites have similar major and trace element characteristics to the host rocks, indicating that the ankaramites did not result from an accumulation of mafic minerals but rather represent the primary magma in the Kibblewhite Volcano. The MIs from the magnesian andesites were hosted in forsteritic olivine xenocrysts with a wide range of NiO contents (Fo90–92; 0.13–0.39 wt% NiO) and have similar major element compositions to the ankaramites but exhibit a wide range of CaO/Al2O3 (0.85–1.54). The trace element characteristics of the MIs from the magnesian andesites do not match those of the host rocks, indicating that they are not primary melts of the magnesian andesites but primitive basaltic melts generated before the magnesian andesites formed.Interestingly, the CaO/Al2O3 ratio of MIs from the magnesian andesites was negatively correlated with the NiO content of their host olivines. This correlation suggests that the composition of the primary basaltic magmas of the Kibblewhite Volcano changed continuously from peridotite-derived to ankaramitic. This correlation could not be explained by grain-scale process, crustal anatexis, or contribution of slab-derived carbonate-rich fluids. Instead, we propose that this correlation can be explained by the interaction of the ascending primary basaltic melts with the lithospheric mantle. During melt-mantle interaction, the assimilation of clinopyroxene and fractionation of olivine and orthopyroxene caused the CaO/Al2O3 ratio to increase in the melt and the Ni content to decrease. Furthermore, because the magnesian andesites have low CaO/Al2O3 ratios and could be derived from a clinopyroxene-poor mantle lithology, the interaction between the melt and mantle may also be closely related to the origin of the magnesian andesites at Kibblewhite Volcano. This interpretation provides a new perspective on the origin of the oceanic arc ankaramites and why primary andesitic and basaltic magmas coexist in the Kibblewhite Volcano.

Lower crustal control in the iron isotope systematics of plutonic xenoliths from Adak Island, Central Aleutians, with implications for arc magma geochemistry

We present bulk-rock and mineral Fe isotope data of ultramafic to mafic xenoliths and basaltic to andesitic lavas from Adagdak Volcano (Adak Island, Central Aleutians) to study the effects of early differentiation on the Fe isotopic evolution of island arc basalts and their crystallization products. The Fe isotope composition of ultramafic cumulate xenoliths increases from dunite (δ56Fe = –0.09 to –0.02 ‰) to clinopyroxenite (δ56Fe = +0.06 to +0.09 ‰), consistent with higher modal proportions of clinopyroxene (δ56Fe = –0.05 to +0.11 ‰) relative to olivine (δ56Fe = –0.10 to +0.06 ‰) in the latter. Mid-crustal cumulate amphibole gabbro and hornblendite cumulates also record heavier Fe isotope compositions (δ56Fe = +0.04 to +0.08 ‰) due to the abundance of isotopically heavy amphibole (δ56Fe = +0.07 to +0.09 ‰) and magnetite (δ56Fe = +0.11 to +0.13 ‰) in these rocks. High inter-mineral fractionations observed in spinel-olivine and spinel-clinopyroxene pairs (Δ56Fespl-ol = +0.12 to +0.28 and Δ56Fespl-cpx = +0.06 to +0.19) suggest that spinel is not recording equilibrium crystallization conditions for the ultramafic assemblages, likely due to subsolidus Fe-Mg exchange. Our data also include Fe isotope measurements of one mantle dunite (δ56Fe = +0.03 ± 0.05 ‰). Five Adagdak lavas, spanning from basalts to andesites, yield a narrow range of δ56Fe between +0.03 and +0.06 ‰. Our results highlight the potential of amphibole in driving the Fe isotope depletion trends observed in many erupted arc lavas, as amphibole hosts 28–99 % of the FeOT budget in the amphibole gabbro and hornblendite cumulates. This is also supported by single-crystal synchrotron Mössbauer spectroscopy of two amphibole grains, the first from an amphibole gabbro and the second from a hornblendite, which yield Fe3+/ΣFe ratios of 0.55 ± 0.06 and 0.58 ± 0.02, respectively. Water content and hydrogen isotope compositions determined by secondary-ion mass spectrometry from the same amphibole grains indicate partial dehydrogenation. Using Rayleigh fractionation modeling to account for oxidation during post-crystallization dehydrogenation, we calculate magmatic Fe3+/ΣFe ratios of 0.41 ± 0.04 for the amphibole gabbro and 0.30 ± 0.05 for the hornblendite. These data are then used to estimate an appropriate Fe force constant for Adagdak amphibole and quantitatively evaluate the effects of amphibole fractionation. Through a fractional crystallization model, we show how arc melts may experience periods of increasing δ56Fe during olivine-dominated fractionation, followed by decreasing δ56Fe once magnetite and amphibole saturate as cumulate phases. Notably, this dichotomy between fractionation of isotopically light versus heavy cumulate assemblages and its effects on the Fe isotope evolution of arc magmas is not captured by the Adagdak lava record, highlighting the utility of cumulates in chronicling the early isotopic evolution of magmatic systems.

Back-arc magmatism of the Proto-Tethys Ocean in the West Kunlun Orogenic Belt: evidence from the Ordovician Nanpingxueshan Nb-enriched gabbros

ABSTRACT The West Kunlun Orogenic Belt (WKOB) in the northwestern Tibetan Plateau records key information about the tectonic evolution of the Proto-Tethys Ocean. To further constrain the Early Palaeozoic tectonic evolution of the WKOB, we report petrography, geochronology, whole-rock geochemistry, and Sr-Nd-Hf-O isotope data of the newly identified Ordovician Nanpingxueshan gabbros in the southern WKOB. Laser ablation-inductively coupled plasma-mass spectroscopy (LA-MC-ICP-MS) U-Pb zircon dating reveals that the gabbroic intrusion was emplaced during 476–471 Ma. Geochemically, the gabbroic rocks exhibit a tholeiitic signature and can be subdivided into two groups, i.e. normal arc-like gabbros (AGs) and Nb-enriched gabbros (NEGs). The AGs have TiO2 (1.20–1.64%), P2O5 (0.18–0.25%), and Nb (4.60–7.64 ppm) contents, fractionated REE patterns ((La/Yb)N = 2.64–4.11), and Nb-Ta depletion (Nb/La = 0.37–0.50). Compared with AGs, the NEGs have higher TiO2 (1.74–2.68%), P2O5 (0.34–0.55%), and Nb (10.79–14.08 ppm) contents and Nb/La ratios of 0.45–0.53, which are similar to typical Nb-enriched arc basalts. Isotopically, the AGs have depleted whole-rock εNd(t) values of 3.22 to 4.04 and negative to positive zircon εHf(t) values of −1.8 to 7.8, while the NEGs exhibit slightly lower whole-rock εNd(t) values of 0.69 to 3.18, zircon εHf(t) values of −7.8 to 4.6, and mostly δ18O values greater than those of normal mantle zircon. Integrated analyses reveal that the AGs and NEGs can be attributed to a similar mantle source metasomatized by different proportions of subducted sediment-derived melt during the subduction process. Our new data, combined with the results of previous studies, suggest that the southward subduction of the Proto-Tethys Ocean under the WKOB initiated at ca. 530 Ma. The long-term southward subduction led to the formation of the massive Early Palaeozoic accretionary complex in the South Kunlun Terrane, while trench retreat led to migration of the arc magmatism northward from the Mazar – Tianshuihai Terrane to the South Kunlun Terrane, as well as back-arc extension in the Mazar – Tianshuihai Terrane.

Garnet stability during crustal melting: Implications for chemical mohometry and secular change in arc magmatism and continent formation

Understanding how new felsic crust is formed and subsequently evolves through time is critical to identifying the geodynamic regimes that have dominated various parts of Earth history, and have important implications for feedbacks between the lithosphere and biosphere, such as controlling the influx of continental detritus into the oceans. In recent years, several trace element-based geochemical proxies have been proposed to allow determination of paleo-crustal thicknesses, which have been calibrated primarily using data collected from modern-day arcs. The application of these proxies through deep time has revealed surprising results, including the suggestion that the mid-Proterozoic continents were atypically thin compared to those in the Archean and the Phanerozoic; however, a range of factors may influence commonly cited trace element ratios (e.g. Sr/Y) rather than just crustal depth, leading to additional and unexpected magnitudes of uncertainty. Here we perform geochemical modelling to deduce the effect of variable bulk-rock composition and geothermal gradient on the trace element signature of felsic melts generated in arc systems. Using a range of protoliths representative of deep arc crust, the results show that considerable care must be taken when analysing simple trace element ratios of granitoid melts and making direct interpretations of the pressure of crystallisation. In particular, changes in geothermal gradients and differences in arc basalt composition impart strong controls on the relative stability of garnet and plagioclase during metamorphism and partial melting, and wide ranges of Sr/Y and La/Yb may be produced in derivative felsic melts produced at the same crustal depth. The interpretation of mid-Proterozoic continental arcs being atypically thin may instead be an artefact of underestimation of the active geothermal gradient at the time of magma formation, which acts to reduce Sr/Y and La/Yb ratios, even in normal thickness (∼35–40 km) crust. Furthermore, we argue that the potentially garnet-free residua during the formation of mid-Proterozoic felsic magmas points to crust formation without lower crustal foundering, and thus, that this commonly invoked paradigm for formation of the continental crust may only be applicable to certain periods of Earth history.

Secular Changes in the Occurrence of Subduction During the Archean

AbstractSubduction processes play a pivotal role in facilitating material exchange between the crust and mantle, contributing to the growth of continents. However, the onset and evolution of subduction remain hotly debated. Here, we developed a high‐dimensional machine learning (ML) model to use multiple compositional data (e.g., Nb/La, Nb, Ti, Nb/U, Pb/Nd, and Nb/Th) to distinguish arc‐type from non‐arc basalts worldwide, then applied this well‐trained ML model to identify and delineate the secular occurrence of Archean arc‐type basalts. Our findings indicate that basaltic arc magmatism can be traced back to only a few early Archean cratons. A noticeable increase in the prevalence of arc‐type basalts during the Mesoarchean and Neoarchean suggests the synchronous formation of Archean subduction across different cratons. This observation hints at transitioning from the predominantly stagnant geodynamic regime to the early, more mobile tectonic regime.

The role of intra-oceanic arc in the growth of continental crust: Evidence from the Early Paleozoic Bainaimiao arc

The Early Palaeozoic arc-related formations in the northern margin of the North China Craton (NCC) belonging to the Bainaimiao arc belt is an important part of the eastern Central Asian Orogenic Belt (CAOB). The tectonic evolution of this belt remains controversial. Here we present detailed data on the geochronology and geochemistry on a newly identified Early Palaeozoic igneous suite from the Faku area in northern Liaoning. Zircon U–Pb dating of the Faku volcanic and intrusive rocks yielded weighted mean 206Pb/238U ages of ca. 458 Ma and 433–405 Ma, respectively, which represent their crystallization (eruption) times. Zircon Hf isotopic analyses show a low negative to positive εHf (t) values of − 0.33 to + 12.4, implying depleted characteristics of the source. The geochemical features of Laolingshan basaltic andesites display higher TiO2 and lower P2O5 contents, similar to that of mid-ocean ridge basalts, and relatively high Zr/Y and low Ta/Yb ratios, suggesting oceanic island arc basalt affinity. Therefore, we suggest that the Laolingshan basaltic andesites were formed in an intra-oceanic environment. The Bachagou granodiorites, with adakitic geochemical signatures, belong to the calc-alkaline and high-K calc-alkaline series, implying formation in an active continental margin tectonic setting. The Baerhushan granodiorites, which geochemically belong to S-type granites, were probably formed in a syn-collisional environment influenced by arc–continent collision. The Donggou granitoids, which have higher SiO2 and alkali and lower MgO and P2O5, show A-type granite features. Geochemical characteristics of the Donggou A-type granite suggest formation in an extensional setting caused by orogenic intermittent extension. The Baerhushan gabbro diorites, which display properties of both mid-ocean ridge and continental rift basalts, were produced under post-orogenic extensional tectonic setting after the arc–continent collision. Our study traces the Early Palaeozoic tectonic evolution of the south-eastern segment of the CAOB and the related accretionary tectonics.

High-Mg andesites and Nb-enriched basalts in the Dulate arc, East Junggar (NW China): Evidence of ocean ridge subduction

Abstract The Dulate arc, located in East Junggar (NW China) in the southern Central Asian orogenic belt, records a Devonian magmatic arc evolution, offering a window to understanding the orogenic processes of the Central Asian orogenic belt. Here we present new geochemical and isotopic data for Late Devonian high-Mg andesite (HMA) and Nb-enriched basalt (NEB) suites from the Qiakuerte area, East Junggar. The HMA samples are typical subduction-related volcanic rocks. They have SiO2 contents ranging from 53.30 to 54.59 wt%, high MgO (5.0–5.26 wt%), and high Mg# values (~55) and show enrichments in large ion lithophile elements (LILEs) and depletions in high field strength elements (HFSEs). The HMA samples have high (La/Yb)N ratios and Sr/Y (~6.5 and 50–59, respectively) with no Eu anomalies. The HMA samples have high Na2O (~3.3 wt%) and low K2O (~2.5 wt%) and Th (~2.4 ppm) contents, combined with positive εNd(t) and low (87Sr/86Sr)i values. These characteristics suggest that the samples were formed mainly through interactions between subducted oceanic melts and mantle peridotites. Compared to normal arc basalts, the NEB samples have higher concentrations of Nb (~20 ppm), higher primitive mantle–normalized Nb/La (0.50–0.58), and higher ratios of Nb/U (9.4–14.6). The NEB samples also have positive εNd(t) and low (87Sr/86Sr)i values, indicating that their source was mantle wedge that had been metasomatized by slab melt. Considering the widespread presence of A-type granites, the abnormally high heat flow, and the tectonic characteristics of East Junggar, we conclude that a slab window created by the subduction of an ocean ridge was responsible for the melting of slab and the formation of the NEB-HMA suites. These processes may have also played a key role in the tectonic evolution processes of East Junggar during the Late Devonian.

Petrogenesis and tectonic implications of the Bloubergstrand basaltic andesites, Tygerberg Formation, Malmesbury Group, Pan-African Saldania Belt, southwestern Africa

Abstract The Bloubergstrand Member, consisting of mafic to intermediate tuffs and flows within the Tygerberg Formation of the Malmesbury Group, is a crucial but understudied component of the western Saldania Belt in southwestern Africa. With a U-Pb zircon age of 555 ± 5 Ma, these volcanic rocks provide valuable insights into the origins and, more broadly, the tectonic setting of the Saldania Belt during the final stages of Malmesbury Group sedimentation in the context of the construction of southwestern Gondwana at approximately 560 to 540 Ma. The Bloubergstrand Member amygdaloidal volcanic rocks vary from basaltic andesitic to andesitic and have experienced varying degrees of alteration. The volcanic rocks are enriched in the large ion lithophile elements (LILE) relative to the high field strength elements (HFSE). When normalised to primitive mantle values, they show enrichment in Cs, Rb, K and Pb and mild enrichment in Th, U, Zr and Hf, with the light rare earth elements (LREE) enriched relative to the heavy rare earth elements (HREE). They are depleted in Ba, Nb, Ta, Sr, Eu, P and Ti. εNd(t) values are mildly negative ranging from -3.60 to -2.39, and Nd TDM model ages range from 1.4 to 1.7 Ga. Initial 87Sr/86Sr ratios vary from 0.70478 to 0.70620, with one higher value of 0.72270, the latter likely due to extensive alteration. The Bloubergstrand Member volcanic rocks exhibit characteristics suggesting their origin from partial melting of lithospheric to transitional asthenospheric upper mantle, influenced by sediment-derived melts and variable degrees of crustal contamination. With continental arc basalt compositions and similarities to shoshonites, the samples reflect variable degrees of partial melting and source heterogeneities. Fractional crystallisation of pyroxenes, hornblende, and plagioclase contributes to compositional variability. Erupting in a continental arc margin, likely part of the Arachania block of the Kalahari craton, separated during the Tonian break-up of Rodinia, the volcanics were associated with the Marmora basin formed by eastward-directed subduction below the western Kaapvaal Craton margin. Extruded in a distal position relative to the Cuchilla Dionisio Arc in present-day southern Brazil and Uruguay, these volcanic rocks preceded the closure of the southern Marmora basin. The mantle melting, possibly a result of slab roll-back, break-off, ridge subduction, or a combination, served as a precursor to the lower crustal heating that generated the granitic magmas of the Cape Granite Suite.

Geochronology, geochemistry, and tectonic setting of the Neoproterozoic magmatic rocks in Pan-African basement, West Ethiopia

The West Ethiopian plateau lies in the transition zone between the Arabian-Nubian shield and the Mozambique belt of the Pan-African orogen, underlain by Precambrian to Tertiary rocks in the Gimbi-Nejo area. The Precambrian basement consists mainly of intrusive-meta-intrusive and meta-sedimentary rocks with ice-rafting deposits. Based on the field survey, unique material records, deformation features, magmatism, and metamorphism indicate that the Gimbi-Nejo area likely underwent four tectonic stages during the Neoproterozoic era.The sedimentary formation of 980 ± 3.9 Ma rift valleys is represented by meta-clastic rocks, calc-silicate rocks, meta-basic rocks bearing marble in Chochi, and the Kata domain. Geochemically, meta-peridotite and meta-gabbros show tholeiitic features, while meta-granitoids are medium-K calc-alkaline peraluminous to metaluminous A-type (Ce/Nb 2.1–11.9; Y/Nb 1.2–8; Yb/Ta 2.6–14). Meta-gabbro samples with immobile HFSE (Th, Ta, Hf) and REE (La, Sm, Yb) fall in the within-plate alkaline basalt, oceanic island basalt (OIB), and continental rift basalt field. Meta-granitoid samples also fall in the within-plate granite field. The lithospheric subduction stage magmatic arc, composed of gabbros and granitoids, was emplaced around 827 ± 3.2 Ma. The granitoids include calcic tonalite and trondhjemite showing an adakitic signature with high Sr/Y (14.2–48.2) ratios. Gabbro samples fall in the island arc basalt and continental arc basalt field. Meta-peridotites and meta-gabbros to gabbros originated from the partial melting of garnet-spinel lherzolite to spinel lherzolite (<10 %) and garnet lherzolite to garnet-spinel lherzolite (<30 %) mantle source, respectively. The continental collisional area contains thrust folds and faults trending in a north–south direction. The ca. 797 ± 3.7 Ma calc-alkalic granitoid rocks along the suture zone are high-K peraluminous I-type granitoids (ASI < 1.1). The strike-slippage area is represented by sinistral ductile shearing deformation. The alkali-calcic granitoid rocks associated with shearing and decollements are low to high-K calc-alkaline peraluminous-metaluminous I-S type granitoids (ASI = 0.95–1.17). The U-Pb age of alkali-calcic granitoid is 564 ± 3.4 Ma. Syn-collisional calc-alkalic and late-post orogenic alkali-calcic granitoid samples both fall within the category of within the plate granite.The εHf (t) values of meta-granitoid (+9.4 to + 15.5), calcic granitoid (+7.6 to + 13.2), calc-alkalic granitoid (+7.4 to + 15.5), and alkali-calcic granitoid (+7.11 to + 17.12) suggest that these Neoproterozoic granitoids have been derived from a Meso-Neoproterozoic juvenile crust that formed through depleted mantle source. Depleted mantle magma likely ascended to the juvenile crust and remained there for a certain period (mean time = 122 Ma, 257 Ma, 248 Ma, and 256 Ma, respectively).

The role of high oxygen fugacity on genesis of the late Cenozoic Abaga basalts in Xilin Gol League, Inner Mongolia, China

ABSTRACT The Abaga basalts, which erupted within an intraplate background in the late Cenozoic, are situated in eastern central Inner Mongolia. Previous studies have explored the genesis of these basalts, which were influenced by the subduction of slab. Although trace element ratios demonstrate that altered oceanic crust (AOC) and sediments have contributed to the genesis of the basalts, limited information about the detailed percentages of mixing among sediments, AOC, and depleted mantle (DM) based on Sr‒Nd‒Pb isotopic values has been reported. In this study, we modelled the possible mixing percentages among AOC, DM, and sediments. The calculated results illustrate that a proportion of 80% DM, 16% AOC, and 4% sediments is appropriate for interpreting the Sr‒Nd‒Pb isotopic variation of the basalts. In addition, the associated major and trace elements of olivine and the partition coefficient of V between olivine and melt are used to estimate the crystallization temperatures of the olivine phenocrysts and oxygen fugacity of the basalts. The calculated results display that the olivine mainly crystallized between 1248 and 1293°C (standard error estimate: 51°C), and the fO2 values of the Abaga basalts range from FMQ + 0.14 to FMQ + 1.46 (FMQ, fayalite–magnetite–quartz buffer). The estimated oxygen fugacity values plot in the range of island arc basalts (from FMQ to FMQ + 2) but are higher than those of both mid-ocean ridge basalt (MORB) (from FMQ-0.4 to FMQ + 0.5) and Abaga xenoliths (from FMQ-1.55 to FMQ + 0.53), which indicates that the basaltic melts were more oxidized than those of MORB and the Abaga lithospheric mantle. Furthermore, the primary magma and the corresponding mantle source were more oxidized than MORB. The Ba/Th and Ce/Pb ratios of the basalt present positive correlations with fO2 values, demonstrating that the addition of sediments and oceanic crust into the mantle source may have contributed to increasing the fO2 values of the basalts.

Zirconium stable isotope fractionation during intra-crustal magmatic differentiation in an active continental arc

Zirconium (Zr) stable isotope variations occur among co-existing Zr-rich accessory phases as well as at the bulk-rock scale, but the petrologic mechanism(s) responsible for Zr isotope fractionation during magmatic differentiation remain unclear. Juvenile magma generation and intra-crustal differentiation in convergent continental margins may play a crucial role in developing Zr isotope variations, and the Northern Volcanic Zone of the Andes is an ideal setting to test this hypothesis. To investigate the influence of these processes on Zr stable isotope compositions, we report δ94/90ZrNIST of whole rock samples from: 1) juvenile arc basalts from the Quaternary Granatifera Tuff, Colombia; 2) lower crust-derived garnet pyroxenites (i.e., arclogites), hornblendites, and gabbroic cumulates found in the same unit; and 3) felsic volcanic products from the Doña Juana Volcanic Complex, a dacitic composite volcano in close proximity to and partially covering the Granatifera Tuff. The basalts have δ94/90ZrNIST values ranging from −0.025 ± 0.018 ‰ to +0.003 ± 0.015 ‰ (n = 8), within the range of mid-ocean ridge basalts. The dacites have δ94/90ZrNIST values ranging from +0.008 ± 0.013 ‰ to +0.043 ± 0.015 ‰ (n = 14), slightly positive relative to the Granatifera and mid-ocean ridge basalts. In contrast, the (ultra)mafic cumulates have highly variable, predominantly positive δ94/90ZrNIST values, ranging from −0.134 ± 0.012 ‰ to +0.428 ± 0.012 ‰ (n = 15). Individual grains and mineral fractions of major rock-forming phases, including garnet (n = 21), amphibole (n = 9), and clinopyroxene (n = 18), were analyzed from 8 (ultra)mafic cumulates. The mineral fractions record highly variable Zr isotopic compositions, with inter-mineral fractionation (Δ94/90Zrgarnet-amphibole) up to 2.067 ‰. Recent ab initio calculations of Zr–O bond force constants in rock-forming phases predict limited inter-mineral Zr isotope fractionation in high-temperature environments, suggesting that the large fractionations we observe are not the product of vibrational equilibrium processes. Instead, we propose a scenario in which large Zr isotopic fractionations develop kinetically, induced by sub-solidus Zr diffusion between coexisting phases via changes in Zr distribution coefficients that arise from changes in temperature. Altogether, Zr isotope variability in this calc-alkaline continental arc setting exhibits no correlation with indices of magmatic differentiation (e.g., Mg#, SiO2), and is not a simple function of fractional crystallization. Furthermore, the garnet clinopyroxenite cumulates studied here represent density-unstable lower arc crust material; consequently, material with isotopically variable δ94/90Zr can be recycled into the mantle as a consequence of lower crustal foundering.

Peperites

Peperites are volcanosedimentary materials generated by the mingling of magma and unconsolidated wet sediments. They have unique insights into submarine volcanisms and the tectonic environments where they form. For the 1st time, the authors identified two types of peperites (blocky and fluidal) hosted by micritic limestone rocks in the Walash Volcanosedimentary Group of the Mawat area, Kurdistan Region-Iraq. They are designated as peperitic facies one and two (PF1 and PF2) and consist of black basaltic rocks mixed with chocolate-brown micritic limestone rocks. Their abundance demonstrates the contemporaneity of deep marine sediment deposition and submarine volcanism during Walash’s nascent arc. Despite hydrothermal alteration, the basaltic rocks retained their magmatic textures. Basaltic rocks comprise mainly albite, anorthite, diopside, hematite, and alkali-feldspar. Calcite dominates micritic limestone rocks, while quartz is minor. Based on geochemical data, igneous sections are basaltic rocks with tholeiitic series that are strongly enriched in Light Rare Earth Elements with low concentration ratios of (La/Yb) and (Sr/Y), indicating geochemical affinity to normal island arc basalt with a primitive arc signature. Furthermore, their formation is thought to be caused by partial melting of subducted slabs deep within 30 km and the associated derived fluids above the subducted slab. Thirteen species of planktonic foraminifera (Morozovella) are identified through paleontological research and biostratigraphy. Using these various tools lead the authors to illustrate the tectonic setting of the formation of peperitic rocks in arc fronts of the subducted Walash arc during the Middle to Late Paleocene (60 Ma).

Petrogenesis of Andesite Rocks in Datae Area, Sidenreng Rappang Regency, South Sulawesi Province

The research area is in the Datae Area, Watangpulu District, Sidenreng Rappang Regency, South Sulawesi Province. This study aims to determine the distribution of volcanic rocks, determine the crystallization phase based on petrographic analysis, and determine the type, magma affinity and tectonic environment based on geochemical data. The method used in this study was field data collection and rock sampling for analysis through petrographic analysis and geochemical analysis using the X-Ray Fluorescence (XRF) method by analysing the main elements, trace elements and rare earth elements (REE). The results of the petrographic analysis show that the rocks found in the field are volcanic breccia and ignimbrite. Volcanic breccia showed coarse-grained texture composed of angular to rounded andesite fragments and pyroclastic material fused together in a matrix. Meanwhile, ignimbrite showed fine grained texture with lapilli to boulder-sized fragments, poor sorting, open-packed and non-layered structure. Based on the Total Alkali Silika (TAS) diagram, AFM diagram, and binary diagram, the results of the geochemical analysis showed that the rocks found in the study area were andesite and trachy-andesite, while the magma affinity area is high-K calc-alkaline and shoshonitic. High-K calc-alkaline magmas are associated with subduction zones and are characterized by elevated levels of potassium and aluminum, while shoshonitic magmas are typically found in intraplate or back-arc settings, characterized by their distinctive potassium, sodium, and barium-rich compositions. The results from ternary diagram and geochemical Spider plots proved that the magma tectonic environment is island arc—continental arc basalt, indicating that the rock was formed in a subduction area. This research supports previous research regarding the tectonics of the western arm of Sulawesi, which stated that this area was formed by subduction.

Variations of Sr–Nd–Hf–O isotopes and redox conditions across a Neoproterozoic arc magmatic belt in the western margin of the Yangtze craton

We use zircon trace elements and HfO isotopes, plus whole-rock chemical and SrNd isotope compositions to investigate the controls on the chemistry of Neoproterozoic arc magmas. The samples are from the gabbrodiorite intrusion of the Gaojiacun mafic-ultramafic complex and from the Tongde diorite batholith of the Panxi Neoproterozoic magmatic belt in the western margin of the Yangtze craton, western China. Zircon UPb ages reveal that both intrusions are coeval, emplaced at ∼820 Ma. Major element compositions of whole rocks and major silicate minerals indicate that the Tongde batholith formed from more evolved magma than the parental magma for the Gaojiacun intrusion. The mantle-normalized trace element patterns of whole rocks from both intrusions are similar, showing light REE enrichments and pronounced negative NbTa anomalies. The SrNd isotopic compositions of the whole rocks are also similar, plotting within or very close to the mantle array, indicating no to minor contamination with the upper crust. Zircon HfO isotopes indicate that the Gaojiacun magma has much higher ƐHf (6.26 ± 0.52) and δ18O (7.0 ± 0.3 ‰) than the Tongde magma (ƐHf = 4.81 ± 0.39; δ18O = 5.7 ± 0.2 ‰). The δ18O values of the Gaojiacun zircons are also higher than the normal mantle (δ18O = 5.3 ± 0.6 ‰). Zircon trace element compositions show that, despite their close spatial association (<10 km), the coeval intrusions have contrasting redox states, with the estimated fO2 values of ΔFMQ+1.0 and ΔFMQ-0.7 for Tongde and Gaojiacun, respectively. The former is within the range of modern arc basalts, whereas the latter is lower than the average value of MORBs. The very low fO2 of the Gaojiacun magma could be due to the presence of subducted oceanic sediments enriched in organic matter (OM) in the source or due to contamination with OM-rich sedimentary rocks in the upper crust. Since such crustal rocks have not been reported for the region, we prefer the former explanation. The preferred model is also supported by Sr–Nd–Hf–O isotopes. The results from this study reveal that both reduced and oxidized magma could be produced simultaneously in a subduction zone.

Iron isotope heterogeneity in magmas of subduction zones and its origin

Iron isotope heterogeneity of subduction-related primitive magmas was traditionally thought to be predominantly attributed to the addition of slab-derived components. However, the specific subducted material responsible for inducing Fe isotope heterogeneity in the subarc-backarc mantle remains poorly constrained. Here we report Fe isotopic compositions of a series of backarc basin basalts (BABBs) from the Woodlark Basin (an intra-arc rift basin), Vate Trough (a nascent rear-rift basin), and Lau Basin (a relatively mature basin) in the southwestern Pacific. Even though these BABBs have various geochemical compositions ranging from mid-ocean ridge basalts (MORB)-like to arc-like, which should be affected by various amounts of the slab-derived fluids and/or sediment melts, their δ56Fe values (0.05–0.18‰) are within the range of global MORB (0.05–0.17‰). After correction for crystal fractionation, the primitive δ56Fe values (δ56Fe-prim) displayed no co-variation with indicators of subduction contributions (e.g., Ce/Pb, Nb/U, Ba/La, and Th/Yb) or source redox conditions caused by slab inputs (e.g., V/Yb, V/Ti), suggesting that the δ56Fe-prim values of these BABBs are rarely affected by subduction metasomatism and the change in oxygen fugacity caused by subducted input. Therefore, we proposed that the MORB-like δ56Fe values of the studied BABBs were primarily caused by mantle melting processes. Furthermore, we presented a compilation of available Fe isotopic data for BABBs and island arc basalts (IABs) and used these data to evaluate Fe isotope heterogeneity in subduction settings. In particular, compared with δ56Fe-prim of MORB, the δ56Fe-prim of BABBs from the Central Lau Spreading Centre in the Lau Basin and the IABs have lower values, whereas BABBs from the Rochambeau Ridges in the Lau Basin have higher δ56Fe-prim values. By comprehensively considering the factors affecting the variation in Fe isotopic compositions in magmatic rocks in subduction zones (e.g., crystal fractionation, partial melting of the mantle, redox conditions, and subduction-related metasomatism), we proposed that the varied Fe isotopic compositions in subduction settings could reflect their heterogeneous sources, which may be related to plate subduction. Specifically, during plate subduction, a melt-depleted refractory forearc mantle peridotite (with low δ56Fe values) could be dragged by subducted slab into subarc or backarc depths, resulting in light Fe isotopic enrichment in island arc magmas or BABBs; additionally, serpentinites (both in the slab and mantle wedge) can generate light Fe isotopic enrichment in island arc magmas. High δ56Fe values in the BABB (e.g., from the Rochambeau Ridges in the Lau Basin) may result from the toroidal influx of nearby upwelling plume components or the assimilation of hydrothermally altered oceanic crust.

Coexisting Nb-enriched basalts and arc volcanic rocks in the northern Qiangtang Terrane, China: Implications for the effects of ambient mantle on subduction zone magmatism

The petrogenesis of Nb-enriched basalts (NEBs) is controversial. NEBs are generally considered to form by melting of either a mixed enriched and depleted mantle source or a mantle wedge metasomatized by adakitic melts. Here we present geochronological, petrological, geochemical, and Sr-Nd-Os isotope data for Early Permian volcanic rocks from the Tuotuohe area in the northern Qiangtang Terrane, China. The studied volcanic rocks can be divided into two groups: arc basalts and basaltic andesites in the lower sequence and NEBs in the upper sequence. Zircon grains from one basaltic andesite sample in the lower sequence yielded a concordia age of 302.0 ± 1.1 Ma. The geochemical and isotopic characteristics suggest that the NEBs were derived from a mixed source consisting of enriched mid-oceanic ridge basalts (MORB)- to oceanic-island basalt-like ambient mantle and slab-derived fluids, whereas the arc basalts and basaltic andesites originated from a source made of the MORB-type ambient mantle modified by slab-derived melts. We propose for the first time that the Nb enrichment of the NEBs was most likely inherited from high-Nb ambient mantle and is independent of the addition of slab-derived components. In addition to slab-derived components, the nature of the ambient mantle is also an important factor controlling the diversity of arc magmatism. Our new findings show that the Late Carboniferous−Early Permian magmatism in the northern Qiangtang Terrane was related to northward subduction of the Paleo-Tethys Ocean, suggesting that subduction might have occurred in at least the Early Permian rather than in the previously thought Middle Permian.