Sub-arc Mantle Wedge Research Articles

Potassium isotope compositions of Mariana arc lavas and their sedimentary input

We apply the stable potassium isotope system (41K/39K) to well-studied Mariana arc lavas, in which inter-island geochemical variability has been interpreted to reflect near constant addition of an aqueous fluid flux, that dominantly samples the subducting mafic oceanic crust and variable amounts of sediment melt addition to the sub-arc mantle wedge. The nature of the sediment component in the Mariana arc lavas remains enigmatic, with a mixture of melts from all subducted sediment lithologies and the altered-mafic oceanic crust seeming likely, but some interpretations point towards a component from melting of the volcaniclastic sediment alone. We present K isotopic data on a set of well-characterised Mariana arc lavas and sediment samples (from Ocean Drilling Program site 801). Our data show that the majority of Mariana arc lavas are isotopically heavy (ẟ41K), by up to ∼0.2 ‰, relative to mid-ocean ridge basalt (MORB), but most of the subducting sediment samples are slightly isotopically lighter than, or indistinguishable from MORB. The volumetrically important volcaniclastic sedimentary unit however is significantly isotopically lighter than MORB, by ∼0.8 ‰, which reflects marine diagenetic processes. Thus, volcaniclastic material significantly influences the bulk sediment K isotope composition. We show that ẟ41K compositions of the Mariana arc lavas can be reproduced by the addition of an aqueous fluid with isotopically heavy K (relative to MORB) and an additional fraction of an incompatible-element-enriched isotopically light melt component. Modelling of K isotopes together with K/La and radiogenic Nd indicate that the melt component is best explained by a mix of melts from bulk sediment and altered-mafic oceanic crust. Our results show that distinctive K isotopic variations in subduction zone inputs and K isotopic fractionation during dehydration reactions makes K a useful tracer of subduction zone processes.

Sub-arc mantle fugacity shifted by sediment recycling across the Great Oxidation Event

The chemical exchange between the atmosphere, crust and mantle depends on sediment recycling via subduction. However, it remains unclear how atmospherically modified sediment may affect mantle oxygen fugacity through time. The Great Oxidation Event, among the most important atmospheric changes on Earth, offers an opportunity to investigate changes in magmatism related to surface–mantle interactions. Here we use sulfur K-edge micro X-ray absorption near-edge structure spectroscopy to measure the relative abundances of S6+, S4+ and S2− state in apatite inclusions hosted in 2.4–2.1-billion-year-old igneous zircons from the Mineiro Belt, Brazil. The host magmas record intracrustal melting of juvenile crust and the involvement of recycled sediments in the sub-arc mantle wedge. Unaltered apatite inclusions reveal a change from reduced to more oxidized magmas from pre- to post-Great Oxidation Event during the early Proterozoic. We argue that this change is a direct result of deep subduction of oxidized sediments and thus evidence of mantle–atmosphere interaction across the Great Oxidation Event. This suggests that the onset of sediment recycling in the Archaean provided atmospheric access to the mantle, and early ‘whiffs’ of oxygen may have already contributed to a localized increase of calc-alkaline magmatism and related ore deposits on Earth.

Potassium isotope evidence for slab-derived fluids in the sub-arc mantle

Subduction recycles water, other volatiles and incompatible elements into Earth's mantle and plays a major role in exchanges between its surface and interior. Potassium (K), a highly fluid-mobile and incompatible alkali element, is strongly affected by these processes. The emerging K stable isotope system has been proposed as a new tracer of subduction events, e.g. to distinguish the inputs to arc magmas from sediments, altered basalts and other near-surface materials. Yet, it remains unknown if and how the K isotopic signatures of these components are affected by extraction of K-bearing media from dehydrating slabs and their transfer to the mantle wedge. Fragments (xenoliths) of sub-arc mantle carried to the surface by arc magmas may retain these slab-derived components. We report high-precision K isotope data on mantle harzburgite xenoliths from Avacha, an active arc volcano in Kamchatka (Russia) to further investigate the nature of slab-derived fluids and their impact on arc mantle. We find a narrow δ41K range for amphibole-rich cumulates from host andesite magmas (−0.53 ± 0.09‰ to −0.34 ± 0.09‰) that overlaps those of the bulk silicate Earth (BSE) and the upper continental crust (UCC). In contrast, the Avacha peridotites derived from the uppermost lithospheric mantle (refractory melting residues metasomatized by subduction zone fluids) show an extremely broad K isotopic range (δ41K ∼ 3‰) and are commonly enriched in heavy K isotopes (up to +1.16 ± 0.08‰) relative to the BSE and UCC; some of the peridotites show the highest δ41K values for any reported mantle rocks. The new data on the Avacha xenoliths, along with available K isotope data on arc lavas and eclogites, suggest that slab-derived fluids may develop heavy K isotopic signatures, distinct from those in the original subducting materials (e.g., marine sediments and altered oceanic crust). These results provide direct evidence that (a) subduction dehydration and fluid percolation in the mantle lithosphere can significantly fractionate K isotopes, and (b) slab-derived fluids infiltrated the sub-arc mantle wedge. Due to the large K concentration difference between the depleted mantle and slab-derived fluids, K isotopes may be a sensitive tracer of fluid activity in sub-arc mantle.

Magmatic–Hydrothermal Transport of Metals at Arc Plutonic Roots: Insights from the Ildeus Mafic–Ultramafic Complex, Stanovoy Suture Zone (Russian Far East)

The Ildeus mafic–ultramafic complex represents plutonic roots of a Triassic magmatic arc tectonically emplaced into the thickened uppermost crust beneath the Mesozoic Stanovoy collided margin. The mafic–ultramafic complex cumulates host Ni-Co-Cu-Pt-Ag-Au sulfide-native metal-alloy mineralization produced through magmatic differentiation of subduction-related primary mafic melt. This melt was sourced in the metal-rich sub-arc mantle wedge hybridized by reduced high-temperature H-S-Cl fluids and slab/sediment-derived siliceous melts carrying significant amounts of Pt, W, Au, Ag, Cu and Zn. Plutonic rocks experienced a pervasive later-stage metasomatic upgrade of the primary sulfide–native metal–alloy assemblage in the presence of oxidized hydrothermal fluid enriched in sulfate and chlorine. The new metasomatic assemblage formed in a shallow epithermal environment in the collided crust includes native gold, Ag-Au, Cu-Ag and Cu-Ag-Au alloys, heazlewoodite, digenite, chalcocite, cassiterite, galena, sphalerite, acanthite, composite Cu-Zn-Pb-Fe sulfides, Sb-As-Se sulfosalts and Pb-Ag tellurides. A two-stage model for magmatic–hydrothermal transport of some siderophile (W, Pt, Au) and chalcophile (Cu, Zn, Ag) metals in subduction–collision environments is proposed.

Triassic Appinite from the Qinling Orogen (Central China): Hydrous Melting of Depleted Mantle Wedge in Post-Collision Stage

Mantle wedge melting and the formation of hydrous mafic melts in collision orogenic belts have great significance for crustal melting and the related granitic magmatism, which can provide key messages about the crustal–mantle interaction in the convergent margin. This paper reported Late-Triassic appinite (212 ± 2 Ma), which is closely associated with Late-Triassic granites. The large euhedral hornblende crystals in the appinite indicate a water-rich feature. This appinite displays low SiO2 (46.55% to 50.44%) contents, high MgO (7.62 to 10.76%) and Cr and Ni contents, with high Mg# values of 61 to 75. It also displays insignificant Eu anomalies (Eu*/Eu = 0.91 to 0.93), high Sr (596 to 676 ppm) contents and moderate Sr/Y (34 to 40) ratios; these features are similar to those of Sanukite formed by the hydrous melting of the mantle wedge. Its depleted zircon Lu-Hf isotopic composition (εHf(t) = +0.97 to +18.21) indicates a depleted mantle source. Zircons in the appinite display extremely high Ti-in zircon temperatures (>1000 °C) and high oxygen fugacity, indicating a high-temperature hydrous condition. In combination with its typical arc-like trace element geochemistry (depletion in Nb, Ta and Ti), it is proposed that this appinite represents hydrous mafic melts that derived from the melting of the depleted sub-arc mantle wedge. The occurrence of this appinite has great significance for the further understanding of Triassic granitic magmatism and potential magmatic metal ore deposits in the Qinling orogenic belt.

Genesis and source affinities of heterogeneous ultramafic rocks in the North Kongling Complex, Yangtze craton: Architecture of a Paleoproterozoic accretionary orogen

Ultramafic massifs are minor but meaningful components of orogens because they record the mantle properties and the processes of plate generation, subduction, accretion, and collision, thus providing key information about crust-mantle interaction processes and the evolution of orogens. Here, we present an integrated study of field observations, whole-rock and mineral geochemistry, zircon U-Pb ages, and whole-rock Sm-Nd and Re-Os isotopes for serpentinized harzburgites and olivine pyroxenites from the Paleoproterozoic Shuiyuesi mélange belt, North Kongling Complex, to constrain the tectonic evolution of the Yangtze craton, South China. The serpentinized harzburgites are characterized by enrichment of light rare earth elements (LREEs) and negative εNd(t) (−4.3 to −3.9) values, but they have high olivine Mg# (91.7−93.1) and very low Re contents (<0.09 ppb) and negative γOs(t) (−5.1 to −4.0) values. The Archean Re-Os model ages (TRD = 2.82 Ga) of the harzburgites that have the most depleted Re-Os isotope composition represent estimates for the age of the initial melt extraction. Metasomatic zircons from the serpentinized harzburgites yield a weighted average age of ca. 2.04 Ga, which is interpreted to represent the age of metasomatism during a subduction event. These geochemical and isotopic features and geochronologic results suggest that the protolith of the serpentinized harzburgites in the North Kongling Complex originated as fragments of late Mesoarchean (ca. 2.82 Ga) subcontinental lithospheric mantle and experienced melt/fluid metasomatism in a subarc mantle wedge at 2.04 Ga. The olivine pyroxenites are cumulates, and they are characterized by high MgO, Ni, and Cr contents and depleted to slightly enriched LREEs but depletion in high field strength elements (HFSEs). They have positive εNd(t) (+2.0 to +6.6) values and Paleoproterozoic Nd model ages (2.41−2.05 Ga), suggesting that the olivine pyroxenites were derived from a Paleoproterozoic depleted mantle source. Metamorphic zircons from the olivine pyroxenites indicate subsequent metamorphism at 2.0 Ga, in response to a collision. The olivine pyroxenites together with the metabasites in the Shuiyuesi mélange belt represent the dismembered fragments of oceanic lithosphere that formed in a suprasubduction zone during the Paleoproterozoic. Combined with the Archean and Paleoproterozoic rocks in the North Kongling Complex, it appears that the Mesoarchean subcontinental lithospheric mantle peridotites and Paleoproterozoic ophiolitic rocks were simultaneously accreted during a Paleoproterozoic orogenic event. We further propose that a subduction channel may have played an important role in the architecture of the accretionary orogen, in which both oceanic lithospheric materials and subcontinental lithospheric mantle wedge materials would have been simultaneously scraped off and accreted during slab subduction-accretion.

Subduction of the paleo-Pacific plate recorded by arc volcanism in the South China Sea margin

Accretionary prisms along the East Asia continental margin have been interpreted as having formed in response to subduction of the paleo-Pacific plate in the Paleozoic–Mesozoic. However, early arc magmatic records in the East Asia margin have been partly destroyed owing to episodic rifting. Widespread arc magmatism and fold and thrust belts in the South China block have recorded Jurassic and Cretaceous paleo-Pacific plate subduction. In addition, the exterior of the South China continental margin was rifted during the Cenozoic to form the South China Sea, and it is presumed that a relic arc is preserved in the northern and southern South China Sea. In this study, we obtained samples of volcanic rocks through drilling >2000 m in the southern South China Sea. The drill core consisted of interbedded basalts, trachytes, and minor trachy-andesites, with arc-type trace-element patterns. Zircon U–Pb dating of trachyte samples from the drill core yielded clustered ages with an overall weighted mean age of 211.2 ± 1.6 Ma, providing robust evidence for the existence of a volcanic arc in the East Asia margin during the Late Triassic. Primitive basalts with MgO > 8.5 wt% have the highest values of εNd(i) and εHf(i) and indicate derivation from Indian-type sub-arc mantle wedge. The increasing Th/Nb with decreasing Nd−Hf isotope values of the basalt samples indicates the incorporation of sediment into the melts. The trachyte samples have enriched K2O and isotopic compositions and are consistent with a sediment-dominated source. On the basis of a comparison with regional magmatic rocks related to plate subduction during Triassic–Jurassic time, the arc volcanic rocks in the southern South China Sea are interpreted to mark a temporal compositional transition in the mantle source, representing steepening subduction of the paleo-Pacific plate during the Late Triassic.

Metals in Avachinsky peridotite xenoliths with implications for redox heterogeneity and metal enrichment in the Kamchatka mantle wedge

Peridotite xenoliths from the Avachinsky volcano in southern Kamchatka contain diverse assemblages of native metals and intermetallic compounds associated with primary olivine and orthopyroxene and metasomatic amphibole. Mineral compositions in Avachinsky xenoliths (Mg-rich olivine and low-Al enstatite associated with Cr-spinel) coupled with common absence of primary clinopyroxene and low bulk Al2O3 and CaO contents, suggest a depleted mantle wedge beneath the southern segment of the Kamchatka volcanic arc.Metallic phases in Avachinsky xenoliths occur as euhedral to anhedral grains in fractures andirregularly shaped voids in Fe-Mg silicates as well as micro-particles localized along boundaries between oivine and orthopyroxene. Siderophile metal assemblage is composed of platinum, iron and gold along with Cr-Fe and W-Fe-Mn alloys. Chalcophile metals include native copper, bismuth, lead and zinc along with Cu-Ag, Cu-Sn, Cu-Sn-Au, Ag-Au, Cu-Ag-Au and Ni-Cu-Ag-Au intermetallic compounds. Metals in Avachinsky xenoliths are associated, both spatially and texturally, with magnetite, rutile, galena, cassiterite, Cu-Ag-bearing Sb-Bi-Pb sulfide phase, AgS and non-stoichiometric Cl-bearing silver compound. Siderophile metals were introduced by primitive metasomatizing melts into low fO2 domains in the mantle wedge below the Kamchatka arc. Phase relations in the Cu-Ag-Au system suggest high-temperature (>900 °C) origin for Cu-Ag-Au intermetallic compounds in Avachinsky xenoliths. While Cu-Ag-Au alloys precipitated from metal-rich, mantle-derived silicate melts, low-melting point chalcophile metals and their minerals (Ag, Bi, Sb, Zn, Pb, Sn) were deposited in the sub-Kamchatkan mantle from hydrous slab fluids under reduced conditions. Metal assemblages in Avachinsky xenoliths further attest to reducing conditions in some subduction systems and potentially indicate existence of extreme redox gradients at sufficiently small scales in sub-arc mantle wedge environments.

Paleoproterozoic ophiolitic mélanges and orogenesis in the northern Yangtze Craton: Evidence for the operation of modern-style plate tectonics

Ophiolitic mélange that forms along convergent plate margins plays an important role in reconstructing the tectonic evolution of ancient orogenic belts and associated geodynamic settings. However, ophiolitic mélanges have rarely been documented in the Archean and Paleoproterozoic geological records, which led to the controversy surrounding the onset or not of plate tectonics at that time. Here we present an integrated study of petrography, geochemistry, zircon U-Pb-Lu-Hf isotope, and metamorphic P–T estimations on the newly-recognized 2.12–2.03 Ga ophiolitic mélange that contains amphibolite and serpentinite blocks in the North Kongling Complex (NKC), northern Yangtze Craton to better constrain the tectonic evolution of the northern Yangtze Craton and prevailing geodynamic backgrounds. The amphibolites and serpentinites in the mélange exhibit slightly-enriched light rare earth elements (LREEs), large-ion lithophile elements (LILEs, e.g., Rb, Ba) and negative high field strength elements (HFSEs, e.g., Nb, Ta, Zr, Hf, and Ti) anomalies, suggesting their derivation from a subarc mantle wedge metasomatized by slab-derived fluids/melts. The mafic–ultramafic components likely represent the dismembered fragments of a ca. 2.12 Ga suprasubduction zone (SSZ) arc/forearc complex. The garnet amphibolite underwent intermediate T/P metamorphism at ca. 2.03 Ga, with a clockwise P–T path. The estimated (near) peak P–T conditions of ∼10.1–10.6 kbar/660–670 °C correspond to thermobaric ratios of ∼625–635 °C/GPa, consistent with subduction-collision-related geothermal gradients. Collectively, the mélange recorded an intra-oceanic subduction-accretion event at ca. 2.12 Ga and subsequent collision at ca. 2.03 Ga. In combination with regional geological data, the accretionary-to-collisional orogenic events in the NKC of the northern Yangtze Craton could be a response to global-scale convergence and assembly of the Paleoproterozoic supercontinent Nuna/Columbia. The Paleoproterozoic (2.12–2.03 Ga) ophiolitic mélanges and orogenesis in the northern Yangtze Craton thus support the operation of the modern-style plate tectonic regime at that time.

Experimental evidence for decompression melting of metasomatized mantle beneath Colima Graben, Mexico

Primitive subduction zone magmas provide information about the composition and thermal structure of the underlying mantle wedge. In the Colima Graben, Mexico, primitive lavas erupted from cinder cones range from high magnesium calc-alkaline basalts to high-K trachybasalts. This chemical diversity suggests that the sub-arc mantle wedge from which they derive is heterogeneous. To explore the conditions of magma generation in the wedge beneath Colima we used an inverse experimental approach to constrain multiple saturation points on the liquidus surface of a primitive high-K basanite (COM-1). Equilibrium piston-cylinder experiments were carried out between 1.0 and 2.4 GPa under hydrous (1.8–3.8 wt% H2O) and oxidizing (ƒO2 = − 0.5 to 4.3 log units relative to NNO) conditions. COM-1 + 3.8 wt% H2O is shown to be multiply-saturated with a phlogopite-bearing spinel pyroxenite assemblage (cpx + opx + phl + sp) close to its liquidus at 1.9–2.4 GPa and 1300 ºC. Experimental mapping of the liquidus surface reveals a multiple saturation point (MSP) where a lherzolitic phase assemblage of ol + cpx + opx + sp + phl coexist. The topology of the MSP indicates a peritectic of the form cpx + opx + phl + sp = liquid + ol. Four bracketing experiments define the MSP of COM-1 as 1300 ± 10 °C, 1.7 ± 0.1 GPa, ∆NNO = 3.4 ± 0.5 log units, for melt containing 3.6 ± 0.4 wt% H2O. The MSP olivine is too forsterite-rich (Fo92-94) to be in equilibrium with mantle lherzolite, but matches phenocryst core compositions in the natural basanite. Thus, experimental results indicate that COM-1 was produced by incongruent melting of an olivine-free, phlogopite-pyroxenite source that itself is the result of metasomatism of mantle wedge by slab-derived fluids. These conditions provide a valuable constraint on the thermal structure and chemical composition of the mantle wedge beneath Colima.

Zircon U-Pb dating and stable isotopic compositions for constraining the genesis of the Chagangnuoer magnetite deposit in western Tianshan, NW China

The Chagangnuoer magnetite deposit in western Tianshan (NW China) is hosted by submarine volcanic rocks and marble lens, with a resource of >220 Mt Fe at 38 wt%. Two major orebodies FeI and FeII are current in production. Pervasive garnet skarn, magnetite, actinolite–epidote, and marble zones are developed at Chagangnuoer. Paragenetic sequences are divided into garnet–pyroxene (I), magnetite–pyrite ± chalcopyrite (II), actinolite–epidote–K-feldspar (III), garnet ± tremolite ± scapolite (IV), magnetite ± hematite (V), actinolite–epidote (VI), and sulfide–calcite–quartz (VII) stages. A diorite beneath the FeII and a granodiorite with skarn alteration have zircon U-Pb ages of 329 ± 2 and 317 ± 4 Ma, respectively. These intrusions have high potassic to shoshonitic, metaluminous affinities, and significant Nb, Ta, Ti and Sr anomalies. Stage II magnetite has δ18OV-SMOW compositions of 1.9–3.5‰, higher than Stage V magnetite (0.5–2.1‰), indicating a hydrothermal fluid source of magmatic origin. The δ18O values of garnet, actinolite, epidote and quartz range from 4.1 to 12.2‰. Stage VII calcite has δ13CV-PDB of −2.0 to −0.8‰ and δ18O of 8.9 to 9.9‰, lower than for the marble (average δ13C + 2.3‰, δ18O 12.8‰). δ34SV-CDT compositions for pyrite and chalcopyrite from Stages II and VII vary between 0.8 and 13.1‰. The isotopic compositions imply that minor non-magmatic sources (e.g., marine carbonate, seawater, and meteoric water) have been circulated into hydrothermal fluids producing the skarn alterations and associated iron mineralization. The early-stage skarn alteration and main Fe mineralization relate to the emplacement of diorite stock at depth, whereas the late-stage skarn-magnetite assemblages are intimately associated with the granodiorite. The Chagangnuoer deposit is concluded to be magnetite skarns formed by magmatic-hydrothermal fluids that emanated from the dioritic and granodioritic magmas intruding above a sub-arc mantle wedge and that metasomatized intensely with the Carboniferous andesitic-rhyolitic volcanic rocks and carbonate.

Evidence for Neoarchean hydrous arc magmatism, the anorthosite-bearing Mayville Intrusion, western Superior Province, Canada

The Neoarchean (2742.8 ± 0.8 Ma) megacrystic anorthosite-bearing Mayville layered intrusion of the Bird River greenstone belt in the Superior Province of Canada is well preserved and well exposed. Given its good preservation, we studied the Mayville Intrusion through a combination of extensive field and petrographic observations, mineral chemistry, and whole-rock major and trace element and Sm-Nd and U-Th-Pb isotopic analyses to unravel its petrogenesis and geodynamic setting, and determine whether its amphibole is of igneous origin and indicative of hydrous magmatism. The Mayville Intrusion intrudes pillow basalts and gabbros of the Mayville Assemblage, which was erupted onto and emplaced into the Mesoarchean phase of the Maskwa Lake tonalite-trondhjemite-granodiorite (TTG) Batholith. Some of the anorthosites and leucogabbros of the Mayville Intrusion have undergone extensive shearing and hydrothermal alteration and had their Sm-Nd and U-Th-Pb isotope systems reset. Despite metamorphism and deformation affecting the Mayville Intrusion, it has largely retained its primary igneous minerals and textures, and trace element and Sm-Nd and Pb isotope compositions. Abundant interstitial magmatic amphibole oikocrysts in the anorthosites and leucogabbros exhibit whole-grain optical continuity and well-preserved igneous grain boundaries with magmatic plagioclase, indicating that the Mayville Intrusion crystallized from a hydrous magma.The Mayville Intrusion has variably positive εNd (2743 Ma) values (+0.7 to +3.9) that are indicative of its derivation from a depleted mantle source. The anorthosites and leucogabbros of the Mayville Intrusion crystallized from a hydrous Ca- and Al-rich tholeiitic parental magma that fractionated from a hydrous primitive arc tholeiitic parental magma. These lithologies were derived by partial melting of a depleted, shallow hydrous sub-arc mantle wedge source. The parental magmas to the gabbros, leucogabbros and anorthosites of the Anorthosite to Leucogabbro Zone subsequently experienced minor contamination by Mesoarchean TTGs (Maskwa Lake TTG Batholith). The field relationships and trace element geochemistry of the Mayville Intrusion and its proximity to the continental arc Maskwa Lake TTG Batholith indicate that it formed in a Chilean-style continental back-arc geodynamic setting. The findings of this study demonstrate that hydrous arc magmatism and modern-style plate tectonic processes have been operating since at least the Late Archean.

Petrogenesis of the Ulungur Intrusive Complex, NW China, and Implications for Crustal Generation and Reworking in Accretionary Orogens

Abstract Accretionary orogens are characterized by voluminous juvenile components (recently derived from the mantle) and knowing the origin(s) of such components is vital for understanding crustal generation. Here we present field and petrological observations, along with mineral chemistry, zircon U–Pb age and Hf–O isotope data, and whole rock geochemical and Sr–Nd isotopic data for the c.320 Ma Ulungur intrusive complex from the Central Asian Orogenic Belt. The complex consists of two different magmatic series: one is characterized by medium- to high-K calc-alkaline gabbro to monzogranite; the other is defined by peralkaline aegirine–arfvedsonite granitoids. The calc-alkaline and peralkaline series granitoids have similar depleted mantle-like Sr–Nd–Hf isotopic compositions, but they have different zircon δ18O values: the calc-alkaline series have mantle-like δ18O values with mean compositions ranging from 5·2 ± 0·5‰ to 6·0 ± 0·9‰ (2SD), and the peralkaline granitoids have low δ18O values ranging from 3·3 ± 0·5‰ to 3·9 ± 0·4‰ (2SD). The calc-alkaline series were derived from a hydrous sub-arc mantle wedge, based on the isotope and geochemical compositions, under garnet peridotite facies conditions. This study suggests that the magmas underwent substantial differentiation, ranging from high pressure crystallization of ultramafic cumulates in the lower crust to lower pressure crystallization dominated by amphibole, plagioclase and minor biotite in the upper crust. The peralkaline series rocks are characterized by δ18O values lower than the mantle and enrichment of high field strength elements (HFSEs) and heavy rare earth elements (HREEs). They likely originated from melting of preexisting hydrothermally altered residual oceanic crust in the lower crust of the Junggar intra-oceanic arc. Early crystallization of clinopyroxene and amphibole was inhibited owing to their low melting temperature, leading to HFSEs and HREEs enrichment in residual peralkaline melts during crystallization of a feldspar-dominated mineral assemblage. Thus, the calc-alkaline and peralkaline series represent episodes of crust generation and reworking, respectively, demonstrating that the juvenile isotopic signature in accretionary orogens can be derived from diverse source rocks. Our results show that reworking of residual oceanic crust also plays an important role in continental crust formation for accretionary orogens, which has not previously been widely recognized.

Geotectonic significance of the Neoproterozoic ophiolitic metagabbros of Muiswirab area, South Eastern Desert, Egypt: constraints from their mineralogical and geochemical characteristics

Petrological and geochemical studies of Neoproterozoic metagabbros were carried out in the Muiswirab area, South Eastern Desert of Egypt. The Muiswirab area comprises of ophiolitic metagabbroic rocks (MOM), which are tectonically thrusted over a thick pile of metavolcanic rocks and intruded by syn- and post-tectonic granitoid rocks. The whole-rock geochemical variations coupled with chemical compositions of mineral constituents are used to attain the genesis and tectonic evolution of the studied metagabbros. The geothermobarometric investigation of the analyzed amphiboles from (MOM) revealed that these metagabbros underwent regional metamorphism under lower to upper greenschist facies (biotite zone) conditions (at a temperature of 450 to 500 °C and pressure of 1–3 kbar). Geochemically, the metagabbros (MOM) show tholeiitic affinity and exhibiting both arc- and MORB- like characters as evidenced by their clinopyroxene compositions and the Ti/V ratios (11.84–31.65), which considered as prominent features of forearc tectonic regime. The geochemical features suggest a probable fractionation of olivine ± clinopyroxene ± plagioclase as well as insignificant crustal contamination. The parental magma of the investigated MOM rocks seems to be developed in a sub-arc mantle wedge setting due to the enrichments of LILE (e.g., Rb, Ba, Sr, Pb) over HFSE (e.g., Ti, Nb, Y, Zr, Hf, Ta). The studied MOM rocks have lower values of Nb/U relative to MORB and OIB indicating that their geochemical variation produced due to the enrichment of a lithosphere mantle by OIB-like components. The ratios of Zn/Fet, La/Sm, Sm/Yb, Th/Yb and Nb/Yb indicate that the MOM rocks represent a fragment of oceanic crust originated at a supra-subduction zone environment and their parental magma developed by 5–30 % partial melting of a spinel lherzolite mantle rather than pyroxenite in an island arc setting and conformable with most of the Egyptian ophiolitic metagabbros.

Subducting serpentinites release reduced, not oxidized, aqueous fluids

The observation that primitive arc magmas are more oxidized than mid-ocean-ridge basalts has led to the paradigm that slab-derived fluids carry SO2 and CO2 that metasomatize and oxidize the sub-arc mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen fugacity (fO2) and speciation of the fluids generated by serpentinite dehydration during subduction. Silicate-magnetite assemblages maintain fO2 conditions similar to the quartz-fayalite-magnetite (QFM) buffer at fore-arc conditions. Sulphides are stable under such conditions and aqueous fluids contain minor S. At sub-arc depth, dehydration occurs under more reducing conditions producing aqueous fluids carrying H2S. This finding brings into question current models in which serpentinite-derived fluids are the cause of oxidized arc magmatism and has major implications for the global volatile cycle, as well as for redox processes controlling subduction zone geodynamics.

Petrology and geochemistry of Alkaline Basalts and Gabbroic xenoliths from Utila Island (Bay Islands, Honduras): Insights into back - Arc processes in the Central American Volcanic Arc

The island of Utila (Bay islands, Honduras), is situated in a back-arc environment, behind the Central American Volcanic Arc (CAVA). The Utila Volcanic Center (UVC) is Quaternary in age, based on geologic and stratigraphic relations with underlying carbonate rocks and is made up of three distinct, volcanic units: palagonite lapilli tuff, main lava flow and vent facies (in stratigraphic order). The UVC contains gabbroic xenoliths up to 5 cm in size embedded in lapilli and ash tuff. Alkaline basaltic (trachybasalt) flows are porphyritic extrusive rocks with phenocrysts composed of olivine, clinopyroxene, plagioclase and rare kaersutite. Gabbros are composed of amphibole, plagioclase, clinopyroxene, olivine, and Fe-Ti oxides. Primitive mantle normalized trace element patterns show enrichment in Ba, Nb, Ta and light rare earth elements (LREEs) similar to ocean island basalts (OIB) and share trace and major element characteristics similar to high-Nb basalt (HNB). Both basalts and gabbros Sr, Nd and Pb isotopes are distinct from OIB mantle sources in the Central American-Southern Caribbean region but similar to the Pacific MORB (Cocos Plate). A combination of OIB-like trace element characteristics and mid-ocean ridge basalt (MORB) -like isotopic ratios suggest that the UVC basalts originate from an enriched mantle source created by the hybridization of a depleted sub-arc mantle wedge in an earlier subduction stage of the CAVA.

Evolution and maturation of the nascent Mariana arc

Long-standing debate remains regarding the processes by which nascent island arcs evolve into thicker-crust, mature volcanic edifices, long before being accreted into continental crust. How mature island arcs maintain their location for millions of years is still unclear. Arc location is currently believed to be controlled by slab dehydration or by the thermal state of the sub-arc mantle. However, most studies have focused on modern island arcs that have developed mature and stable volcanic edifices. Using published geochemical datasets, here we show that the southern Mariana arc is an incipient island arc that evolves into stable, thicker-crust stratovolcanoes to the north. Because it shares similar geochemical features with the early arc that formed during subduction infancy, it can place new important constraints onto the processes that occurred during island arc infancy. Examination of the Mariana arc along a S-N transect suggests that the growth of the serpentinized fore-arc mantle has the potential to modulate the depth of arc magma generation, which allows, in return, stabilization of the volcanic edifices during arc maturation. Arc stabilization further enables the deeply-sourced, slab fluids to infiltrate the sub-arc mantle wedge, which results in magmas with a typical arc fingerprint to the north. Melt focusing along magmatic conduits further promotes magma differentiation, magma-crust assimilation, and subsequent crustal thickening. We further propose that transient magmatic activity may also occur during island arc infancy in the Izu-Bonin-Mariana convergent margin, until serpentinization of the fore-arc mantle develops and stabilizes.

Chalcophile element processing beneath a continental arc stratovolcano

The chalcophile elements are important both in terms of their economic value and as potential tracers of magmatic processes at convergent margins. However, because of analytical difficulties, comprehensive datasets of chalcophile element concentrations for volcanic rocks are rare. Here, we present analyses of a near complete suite of chalcophile elements (S, Cu, Ag, Se, As, Sb, Sn, W, Mo, Pb, Bi, Tl, Zn, Ga, Co) for volcanic rock samples collected from a typical continental arc stratovolcano in southern Chile (Antuco). Enrichment in Pb, Bi, W, Tl, Sb and As relative to Parental-MORB indicates that these elements have been mobilised from the subducting slab into the sub-arc mantle wedge, in contrast to Cu and Ag. Very low Se concentrations suggest that Se, like S, was lost during co-eruptive degassing of the Antuco magmas. Previous studies on oceanic arcs have demonstrated that as higher fO2 subduction-related magmas ascend through the overlying lithosphere, magnetite fractionation may trigger sulfide fractionation during crystallisation. If such a process is extensive and has a sharp onset, this would result in a plummet in the Cu, Se and Ag contents of the residual melt. At Antuco, although a decrease in the Fe2O3(T) and TiO2 concentrations at ∼55 wt.% SiO2 (∼3 wt.% MgO) indicates magnetite fractionation, this is not associated with a corresponding drop in Cu contents. Instead, we observe a general decrease in Cu and a decrease in Cu/Ag with increasing SiO2 and decreasing MgO. Furthermore, Cu/Ag in the most primitive Antuco rocks are lower than the global MORB array, indicating that the melts were sulfide saturated at an early stage in their crustal evolution. Through modelling fractional crystallisation, we show that only a minor volume (0.5–0.6 vol.%) of fractionating sulfide is needed to produce divergent trends in Cu and Ag, as observed in the Antuco samples. Our results show that sulfide fractionation occurred from an early stage during the crustal evolution of Antuco's magmas. We infer that this was promoted by stalling in the lower crust, which for oxidised magmas at depths >20 km is within the sulfide stability field. However, elevated DyN/YbN of the Antuco magmas compared to oceanic island arc magmas provides an additional, or alternate mechanism to inducing sulfide fractionation in the lower crust prior to ascent, through initial garnet fractionation. Fractional crystallisation within this depth range meant that later magnetite fractionation had only a minor impact on the partitioning behaviour of the chalcophile elements. In contrast, arc magmas transiting thinner crust may not experience sulfide saturation until a later stage in their evolution, induced by magnetite fractionation. Our results imply that convergent margin crustal thickness, and therefore the depth range of magmatic differentiation, determines the dominant control on initial magmatic sulfide saturation and therefore the primary distribution of chalcophile elements. This implies that secondary processes are required to explain the transport and concentration of sulfides and chalcophile elements at shallower crustal levels.

Hf isotopic fingerprinting of geodynamic settings: Integrating isotopes and numerical models

Hf isotopes have proven invaluable in understanding the evolution of Earth's crust-mantle system, but their use in reconstructing tectonic environments, in many cases, remains equivocal. In this study, we introduce a new approach to predict the Hf isotopic evolutionary pattern for rifting and collision based on the integration of numerical models and 176Hf/177Hf isotopes. The geodynamic numerical models allow us to estimate the proportion of juvenile material added to the crust through time. On the basis of this proportion, we calculate changing 176Hf/177Hf ratios using mixing models. Predicted Hf isotopic patterns generated through this numerical approach imply that juvenile signals are observed during back-arc extension, whereas evolved signatures dominate collisional settings. We use this novel modeling approach in the case study region of the Halls Creek Orogen to elucidate its tectonic setting through time. In addition, the geochemical features of magmatic rocks in the case study region imply partial melting of a sub-arc mantle wedge with magma-crust interaction on ascent in a convergent margin setting. The links between predicted Hf isotopic evolution, geodynamic numerical models, whole rock geochemistry and measured zircon Hf isotopic evolution trend resolve three discrete stages in the tectonomagmatic development of the Halls Creek Orogen: (1) oceanic crust subduction; (2) back-arc formation with addition of juvenile mantle input; and (3) docking of the North Australian and Kimberley cratons resulting in the development of mixed-source magmatism formed in a collisional setting. We provide a new method to validate geodynamic models with isotopic datasets, which should lead to more rigorous understanding of crustal evolution.

A Ca. 2.8‐Ga Plume‐Induced Intraoceanic Arc System in the Eastern North China Craton

AbstractThe conversion of Archean tectonic regimes from mantle plume to plate tectonic and the mechanism of subduction initiation are two crucial issues in the current Earth sciences. A typical komatiite‐tholeiite sequence consisting of ~2,850–2,800‐Ma komatiites and ~2,780–2,710‐Ma basaltic and andesitic to dacitic volcanic rocks is well preserved in the Archean Western Shandong terrane of the eastern North China Craton. These volcanic rocks were derived from the partial melting of a depleted mantle source with subsequent fractional crystallization, a metasomatized subarc mantle wedge, and a descending oceanic slab. Structural and kinematic observations suggest that the Qixingtai‐Yanlinguan‐Panchegou granite‐greenstone belt experienced ~2.85–2.80‐Ga extensional deformation (D1) by upwelling of a mantle plume and NE‐SW‐oriented shortening (D2) during ~2.78–2.71 Ga. The petrogenesis of the Mesoarchean to early Neoarchean metamorphosed volcanic rock series and regional deformation indicate that a plume‐induced self‐sustaining intraoceanic arc system with SW‐dipping subduction polarity operated ~2.8 Ga, followed by the evolution from initial to mature subduction. An increasing pull caused by the descending slab generated gravitational instability in the surrounding oceanic lithosphere and formation of a new initial subduction zone adjacent to the preexisting arc system. Similar to the process in the Western Shandong terrane, plume activity generally culminated in subduction events in many other Archean greenstone belts, revealing that plume‐arc interaction may have been a crucial mechanism for global Archean tectonic regime conversion from mantle plume to initial plate subduction in the crust of the early Earth.