Sulfur belongs among H
O, CO
, and Cl as one of the key volatiles in Earth's chemical cycles. High oxygen fugacity, sulfur concentration, and δ
S values in volcanic arc rocks have been attributed to ...significant sulfate addition by slab fluids. However, sulfur speciation, flux, and isotope composition in slab-dehydrated fluids remain unclear. Here, we use high-pressure rocks and enclosed veins to provide direct constraints on subduction zone sulfur recycling for a typical oceanic lithosphere. Textural and thermodynamic evidence indicates the predominance of reduced sulfur species in slab fluids; those derived from metasediments, altered oceanic crust, and serpentinite have δ
S values of approximately -8‰, -1‰, and +8‰, respectively. Mass-balance calculations demonstrate that 6.4% (up to 20% maximum) of total subducted sulfur is released between 30-230 km depth, and the predominant sulfur loss takes place at 70-100 km with a net δ
S composition of -2.5 ± 3‰. We conclude that modest slab-to-wedge sulfur transport occurs, but that slab-derived fluids provide negligible sulfate to oxidize the sub-arc mantle and cannot deliver
S-enriched sulfur to produce the positive δ
S signature in arc settings. Most sulfur has negative δ
S and is subducted into the deep mantle, which could cause a long-term increase in the δ
S of Earth surface reservoirs.
•Garnet schists from the South Beishan Orogen experienced high-grade metamorphism at ca. 900Ma.•~900Ma metamorphism is coeval with extensive continental arc formation in the SBOB and CTA.•The SBOB ...and CTA occupied in the periphery of the Rodinia during the final assembly stage.
An early extensive Neoproterozoic (ca. 900Ma) continental magmatic arc system covering hundreds of kilometers has been reported to occur in the South Beishan Orogenic Belt (SBOB) and the Central Tianshan (CTA) in the southern Central Asian Orogenic Belt (CAOB). However, evidence for coeval high-grade metamorphism and thus the formation of an accretionary orogen in the framework of Rodinia is ambiguous or absent. This study provides new petrological, geochemical and geochronological data for garnet-bearing schists (quartz+garnet+biotite+plagioclase±muscovite) from the SBOB in order to constrain its Neoproterozoic metamorphic history. The metamorphic zircon rims are either unzoned or display sector zoning in CL-images and reveal REE patterns with flat HREE patterns and negative Eu anomalies, which are interpreted to be in chemical equilibrium with garnet and plagioclase. The zircon U-Pb dating yields concordant U-Pb ages of 900±3Ma, 897±2Ma and 898±4Ma for the metamorphic zircon rims. The inherited detrital zircon cores of one sample display a concordant U-Pb age of 1397±5Ma that is consistent with the timing of formation for the extensive Mesoproterozoic continental arc in the SBOB and CTA. Based on phase equilibrium geothermobarometry and average P-T thermobarometric calculations, minimum amphibolite-facies P-T conditions are estimated to be >600°C at pressure >0.6GPa, which is thought to have been overprinted by subsequent Paleozoic metamorphism. However, the Ti-in-zircon thermometer still reveals temperatures of up to 840°C using the composition of metamorphic zircon rims, suggesting former ca. 900Ma granulite-facies peak metamorphic temperatures. The combined petrological and geochronological evidence in conjunction with the continental affinity of the regional metamorphic rocks suggests that the SBOB and the eastern CTA experienced an early Neoproterozoic accretionary orogenesis during the final assembly stage of Rodinia.
Submarine hydrothermal sulfide ores occur at mid-ocean ridges, intra-oceanic arcs and in back-arc basins associated with host rocks of highly variable composition. Pyrite is the dominant sulfide ...mineral in most samples presented within this study, and thus the trace element composition of pyrite may reflect the different metal sources and precipitation processes. Here, we report on a systematic study of minor and trace element contents in pyrite from active and inactive submarine hydrothermal vent fields at different plate-tectonic settings including the Indian and Mid-Atlantic Ridges, the Tonga–Kermadec intra-oceanic arc, the Lau back-arc and the central Okinawa Trough. Our results show that the trace element contents of pyrite from different locations vary significantly but for most elements without any systematic correlation to the concentration in the magmatic host rocks. Only As appears to be depleted in pyrite from ultramafic-hosted vent systems that are affected by serpentinization. These As depleted pyrites preferentially host Au 0 micro- or nano-particles. Bismuth is enriched in pyrite from hydrothermal systems that contain a sedimentary component and micro-inclusions of sphalerite are either due to fluid–sediment interaction or phase-separated fluid venting. Pyrites from individual locations have highly variable concentrations of elements like Au, Co, Cu, Se, Mo, Ag and Sb that are most likely related to fluid evolution and changes in fluid composition. Sub-seafloor hydrothermal fluid–seawater mixing influences the distribution of Au, Co, Cu, Se and Mo in pyrite. Elements like Au, Ag, Sb and Pb often have a characteristic affinity to As, while Cd correlates closely with Zn. A magmatic volatile contribution to the Hine Hina hydrothermal system may result in the precipitation of Cu-enriched pyrite. Our results show that the concentrations of most trace metals in pyrite are a function of the physicochemical parameters of the fluid phase rather than a reflection of the magmatic host rock composition.
•Global data base of trace element pyrite chemistry.•Most trace elements in hydrothermal pyrite are independent of the magmatic host rock composition.•Fluid–sediment interaction and serpentinization processes affect the composition of hydrothermal pyrite.•Fluid temperature variations control the trace element composition of pyrite.•Phase separation and magmatic volatiles also influence the pyrite composition.
The study of subduction-related high-pressure- and ultra-high-pressure metamorphic rocks exposed in orogenic belts is critical for unraveling the tectonothermal evolution of an orogen. The South ...Tianshan Orogen of the southwestern Central Asian Orogenic Belt, one of the world's largest accretionary orogens, contains several Paleozoic (ultra-)high-pressure ((U)HP) metamorphic terranes. (U)HP eclogites, blueschists and associated (U)HP metasedimentary rocks of continental and oceanic origin were reported from two localities in the Northern Tianshan (NTS) of Kazakhstan and Kyrgyzstan, namely from the Ordovician Aktyuz and Makbal metamorphic terranes. Furthermore Lower Carboniferous (U)HP eclogites and blueschists of oceanic origin are located in the Southern Tianshan Accretionary Complex (STAC) of southern Kyrgyzstan (Atbashi) and northwestern China (Akeyazi).
The continentally-derived Aktyuz eclogite is considered to have experienced a continuous clockwise prograde P–T path with peak metamorphic conditions of ca. 1.6–2.1GPa at 610–670°C, corresponding to a burial depth of about 70km. A peak metamorphic Lu–Hf garnet isochron age of 474±2Ma for the retrogressed eclogite of continental origin is in accordance with a Sm–Nd garnet isochron age of 462±7Ma interpreted as time of cooling and decompression of the metamorphic rock assemblage below 650 to 600°C. Quantitative P–T pseudosection modeling for peripheral Makbal HP garnet amphibolites of continental origin also revealed a continuous clockwise P–T path with peak-metamorphic conditions of ca. 1.4GPa at ca. 620°C followed by an isothermal uplift to ca. 0.7GPa. Furthermore, prograde P–T paths of oceanically-derived central Makbal high-pressure metabasic rocks revealed peak metamorphic conditions between 520°C and 560°C at 2.2GPa to 2.5GPa with subsequent isothermal decompressions. The metamorphic (P–T) evolution of a garnet–chloritoid–talc schist is exhibited by a P–T trajectory starting ca. 480°C at 2.5GPa, followed by peak pressures of ca. 2.85GPa at 525°C and peak temperatures of ca. 580°C at 2.4GPa. A Lu–Hf peak metamorphic age of 470.1±2.5Ma for a peripheral garnet amphibolite is in accordance with a garnet-growth Sm–Nd age of 475±4Ma of the UHP garnet–chloritoid–talc schist. Different and variable P–T paths show that former oceanic and continental crust at Makbal was subducted to different maximum depths (HP versus UHP conditions). The (U)HP metamorphism of oceanic and continental mafic rocks from the Aktyuz and Makbal metamorphic terranes is assumed to have occurred in a similar time frame of between 500 and 460Ma. The proposed geodynamic model for the HP Aktyuz metamorphic terrane incorporates the deep northeast-directed subduction of NTS continental crust under the Ankrakhai block of Paleo-Kazakhstan subsequent to the closure of the Djalair–Naiman Basin. At Makbal the final consumption of the oceanic crust of the Terskey Ocean to the northeast was immediately followed by the deep subduction of a (thinned) passive continental margin of the Central Tianshan Arc Terrane (CTA). Both scenarios are in accordance with continental-type subduction geodynamics producing the subduction-related (U)HP rocks during continental–continental collision processes. The exhumation of detached oceanic material by channel flow may have been initiated by the transient subduction of oceanic and subsequent continental crust. The Lower Carboniferous (U)HP metamorphic terranes (Atbashi and Akeyazi) in the Southern Tianshan Accretionary Complex are mainly composed of metasediments which host some lenses and boudins of oceanically-derived blueschists and eclogites. Most eclogites, blueschists and metasediments have experienced peak high-pressure metamorphism (480–580°C at 1.4–2.1GPa), while some slices display UHP-conditions (2.7–3.3GPa at 570–630°C). Multi-point Lu–Hf and Sm–Nd isochrons from several eclogite and blueschist samples from the Atbashi and Akeyazi metamorphic terranes revealed consistent ages of ca. 320–315Ma for the peak of metamorphism, thereby confirming that (U)HP metamorphism occurred during a single subduction event. The highly variable peak metamorphic conditions displayed by the (U)HP rocks imply that metamorphism of the metasediments and the various metabasic rocks occurred in different depths of the subduction zone. (U)HP metabasites and metasediments exhibit different P–T paths, which is consistent with the chaotic incorporation in a subduction channel resulting in the eventual exhumation as a mélange. This scenario is much in agreement with initial subduction of oceanic crust and accretionary wedge sediment removal by subduction erosion and thus is similar to accretionary-type subduction terranes, although, with higher peak P–T conditions than previously recorded. The absence of significant melting, in combination with the variable peak pressure metamorphic conditions and the intimate interlayering of high- and ultrahigh-pressure rocks, suggests that the (U)HP rocks were derived from varying depths within the subduction zone and then juxtaposed during exhumation in the subduction channel. This is in accordance with exhumation of the (U)HP rocks by channel flow.
Display omitted
•Metamorphic evolution of (U)HP subduction-related transient crust in the South Tianshan•Shallow-level juxtaposition of blueschists, eclogites and paragneisses•Exhumation of the (U)HP rocks by channel flow•Geochronology of the prograde and retrograde P–T evolution of the (U)HP rocks•Accretionary-type subduction and continental-type subduction
Results of more than 400
in situ U–Th–Pb and Lu–Hf zircon isotope analyses combined with cathodoluminescence images and field relationships reveal that the oldest magmatic rocks of the Limpopo Belt ...(Musina area) formed during the Palaeoarchaean between 3.40 and 3.27
Ga, experienced a first anatexis at 3.24–3.07
Ga (M1) and were overlain by sediments mainly between 3.24 and 3.0
Ga. Subsequently, the infra- and supracrustal rocks together underwent two high-grade metamorphic overprints, a first during the Neoarchaean at 2.65
Ga (M2), and a second at 2.025
Ga (M3). Hafnium isotope data additionally reveal that the Palaeoarchaean magmatic rocks result from substantial recycling of much older crust. This is well reflected by nearly chondritic ɛHf
int obtained from primary magmatic zircon domains of four Sand River TTG-gneiss samples (ɛHf
3.27–3.28
Ga
=
−1.3 to −1.8), from leucosomes within the Sand River Gneisses (ɛHf
3.40
Ga
=
−2.0; ɛHf
3.28
Ga
=
−1.5), and from two meta-anorthosite samples of the Messina layered intrusion (ɛHf
3.35
Ga
=
0.1–1.4). Crustal recycling is also supported by hafnium isotope data of detrital zircon grains from two quartzite samples. These data reflect a general increase of the ɛHf
int between 3.65 and 3.2
Ga from ca. −4.0 to 0.0, indicating that an Eo- to Palaeoarchaean crust was mixed with juvenile mantel material in the hinterland of the Limpopo Belt. The present study also demonstrates that Hf isotope data provide a powerful tool, which makes it possible to distinguish zircon domains formed during distinct magmatic or metamorphic events, from such affected by a pseudomorphic alteration, even if the investigated zircon grain underwent multiple alteration processes.
The Cretaceous Yunshan caldera complex in SE China consists of an unusual coexisting assemblage of peraluminous and peralkaline rhyolites and a resurgent intra-caldera porphyritic quartz monzonite. ...In this study, we use zircon trace element data to study the compositional differences of zircons from cogenetic magmas and to track the evolution of the entire magmatic system. Our results indicate that the zircons from the peraluminous and peralkaline rhyolites formed from highly evolved compositions with high Hf concentrations and low Ti contents, and low Th/U and Zr/Hf ratios, which are distinct from those of the intrusive porphyritic quartz monzonite. Zircons from the peraluminous and peralkaline rhyolites display overlapping Zr/Hf and Hf, but the zircons from the peralkaline rhyolites have extremely low Eu/Eu* ratios (<0.1) and Ti contents (2.26–13.3 ppm). The lack of overlapping zircon trace element compositions between the volcanic and intrusive caldera units is interpreted to represent crystal–melt segregation processes. In addition, zircon grains from the porphyritic quartz monzonite and a few zircon grains from the peraluminous rhyolite display distinctly bright rims and whole grains in cathodoluminescence imaging, which have high Ti, Zr/Hf, and Eu/Eu*, and are similar to those of zircons from the mafic microgranular enclave within the porphyritic quartz monzonite. We interpret these signatures to reflect crystallization from a relatively hot and less evolved magma indicating a magma chamber recharge event. We further developed a model in which the magmas of the peraluminous and peralkaline rhyolites were successively extracted from a primitive crystal mush by crystal–melt segregation with a rejuvenation of the crystal mush after the extraction of the peraluminous rhyolitic melt, leaving behind residual mushes solidified as porphyritic quartz monzonite. Our study shows that trace element analyses of zircons can effectively be used to pinpoint multiple crystal–melt segregation and melt extraction events as well as magma recharge processes in silicic magmatic systems.
•Volcanic-plutonic link was revealed by zircon trace elements from caldera complex.•Peralkaline rhyolite extracted from rejuvenated mush after peraluminous rhyolite extraction.•Contrasting compositions of volcanic and plutonic zircons imply crystal–melt segregation.•Multistage zircon crystallization reflects magma recharge processes.
Field evidence from the western Tianshan subduction complex in northwestern China indicates that the high field strength elements Ti, Nb, and Ta were mobilized and thereby fractionated from Zr and Hf ...during the dehydration process that transformed blueschist into eclogite. Both a segregation with a depletion halo, thought to represent initial mobilization during dehydration, and a transport vein, indicative of the long distance transport were investigated. In each case, centimeter-sized rutile grains grew as needle-like crystals in the segregation and as prismatic crystals in the vein. Within the host rock of the segregation, the Ti contents of garnet and omphacite, the modal abundances of rutile and titanite and the bulk rock Ti, Nb, and Ta contents decrease towards the segregation. These observations are consistent with transport of Ti, Nb, and Ta from the host rock into the segregation. Textural and geochemical data for the eclogite-facies vein minerals indicate that Ti–Nb–Ta-rich fluids were transported over long-distances (at minimum meter-scale) during fracture-controlled fluid flow. Complex forming ligands (e.g., Na–Si–Al polymers and F
−) may have enhanced the solubility of Ti, Nb, and Ta in the fluid. Changes in fluid composition (e.g.,
X
CO
2
) may both precipitate rutile and fractionate Ti, Nb, and Ta from LILE and REE.
In the subduction complex of the Tianshan mountains, western China, massive blueschist is cross-cut by an eclogite-facies major fluid conduit surrounded by a reaction zone which is mainly composed of ...omphacite and garnet. Petrological as well as geochemical evidence suggest that formation of the vein and the eclogitic selvage around the vein was caused by fluid infiltration under peak metamorphic conditions of 21
±
1.5
kbar and 510
±
30
°C. The combination of whole-rock with mineral trace-element data as well as mass-balance calculations indicate that substantial differences exist between the unaltered host rock and the part of the system which was altered by fluid–rock interaction. These differences include: (1) depletion of mainly large-ion lithophile elements (LILE) and Li of up to 60% relative to their concentrations in the unaltered host rock; (2) an extreme enrichment of CaO (∼115%), Sr and Pb (>300%) in the altered parts of the vein-wall-rock system; (3) redistribution of heavy rare earth elements (HREE) from partly replaced rutile and recrystallized titanite in the blueschist–eclogite transition zone into newly grown garnet rims in the eclogitic selvage around the vein; (4) transformation of high Nb/Ta rutile into low Nb/Ta titanite which is associated with preferred mobilization of Nb over Ta; and (5) decoupling of Zr and Hf from Nb and Ta; the latter are depleted by ∼30% relative to the unaltered blueschist host rock whereas the former are depleted by only ∼10%. The prerequisite for the transformation of Ca-poor blueschist (6–7
wt.% CaO) into Ca-rich eclogite (up to 13
wt.% CaO) was the infiltration of a Ca-rich fluid. The release of trace elements can be attributed to partitioning of these elements into the passing fluid phase during dissolution–reprecipitation processes in the course of eclogitization. The reactivity of the precursor mineral assemblage and the chemical gradients between the reacting and passing fluid of the conduit are mainly responsible for trace-element mobilization in the studied samples. The suite of trace elements released upon fluid-induced eclogitization of the reactive wall-rock resembles that in island arc magmas showing strong enrichment of LIL elements, strong depletions in HFS elements and intermediate concentrations of REE.
•The Neoproterozoic gneissic granitoids in the CTA represent a continental arc setting.•The ca. 730Ma bimodal intrusive complex was generated in a continental rift setting.•A record of assembly and ...breakup of Rodinia exists in the Southwestern Altaids.
Neoproterozoic gneissic granitoid rocks and a bimodal intrusive complex exposed in the Chinese Western Tianshan Orogen were geochemically and geochronologically studied in order to evaluate the assembly and breakup of Rodinia in the Southwestern Altaids. Zircon U–Pb ages of 903.5±2.2Ma and 933.6±1.2Ma were obtained for the Huoshaoqiao and the Wulasutanwutuaiken gneissic granitoid plutons, respectively. Both intrusions have a calcic to calci-alkalic composition and display an enrichment of LREE and LILE and a depletion of HREE and HFSE resembling the geochemical characteristics of Andean arc granites. Their in-situ zircon εHf(t) values of −1.7 to +1.9 and −2.2 to +4.5 further suggest a Late Paleoproterozoic crustal source with limited input of juvenile materials. The Wuwamen bimodal intrusive complex consists of meta-gabbro host rocks and crosscutting granitic dykes. Zircon U–Pb ages of 733±5Ma and 730±5Ma were determined for the meta-gabbros and the granitic dykes, respectively. The relative depletion of Nb, Ta and Ti, the Nd isotopic value (εNd(t)=−4.3) of the meta-gabbros, and the Hf isotopic values of gabbroic zircon grains (εHf(t)=+0.4 to +2.8) suggest that the protolithic mafic magma originated from a sub-continental fertilized lithospheric mantle source. Furthermore, the Nd isotopic value (εNd(t)=−11.2) of the granitic dykes and the Hf isotopic values of granitic zircon grains (εHf(t)=−14.3 to −5.6) indicate that the acidic magma originated from lower crustal melting. The dataset presented here in conjunction with previously published data suggest that the ∼900Ma plutons originated from remelting of older crust in an Andean-type active continental arc setting associated with the assembly of Rodinia, whereas the development of the ∼730Ma bimodal intrusive magmatism is interpreted as a result of the Rodinia breakup. Thus, the Neoproterozoic magmatism is believed to be a direct consequence of the geodynamic process related to the assembly and breakup of Rodinia in the Southwestern Altaids.
High-pressure (HP) veins in eclogites provide insight into element mobility during fluid–rock interaction in subduction zones. Here, we present a petrological–geochemical study of a sulfide-bearing ...HP vein and its massive lawsonite eclogite host rock from the Chinese Tianshan (ultra-)high-pressure/low-temperature metamorphic belt. The omphacite-dominated vein is enveloped by a garnet-poor, sulfide-bearing eclogite-facies reaction selvage. Lawsonite, garnet, omphacite, glaucophane and other HP minerals occur as inclusions in pyrite porphyroblasts of the selvage rock, indicating that the selvage formed prograde under eclogite-facies conditions. Relicts of wall-rock garnet (Grt_I) cores in recrystallized selvage garnet (Grt_II) close to the wall rock and the Ca distribution in Grt_II, which images the overgrown selvage matrix, indicate that the selvage formed due to dissolution–precipitation processes as a consequence of fluid–rock interaction of the wall rock eclogite with the vein-forming fluid. The peak metamorphic P–T conditions of the wall-rock eclogite are estimated at ca. 590°C and 23kbar. Mass-balance calculations indicate that the reaction selvage experienced: (1) a depletion of the large-ion lithophile elements (K–Rb–Ba) of up to 100% relative to their concentrations in the wall-rock eclogite; (2) a moderate depletion of the HREE and some transition metal elements including Fe, Cu, Ni, Zn, Co, Cr, and Mn (10–40%); (3) a significant enrichment of CaO (up to 50–80%), Sr (up to ∼200%), Pb (up to ∼170%) and S (up to ∼210%); (4) a slight to moderate enrichment of the LREE (10–20%) and MREE (0–40%); whereas (5) the HFSE show no significant variations.
The chemical changes in the selvage suggest that the fluid, which caused the dissolution of the wall-rock and the precipitation of the selvage assemblage while the vein formed, was probably a mixture of an “internal” fluid derived from the prograde dehydration (e.g., lawsonite breakdown) of the wall rock and an “external” fluid most likely derived from dehydrating altered oceanic crust located in stratigraphically lower units of the subducting slab. The external fluid introduced Ca, S, Sr, Pb and at least parts of the LREE and the MREE into the selvage, whereas some elements, such as the remaining LREE and MREE, may have been derived from the wall rock eclogite via diffusional transport into the selvage. The enrichment of Ca and L-MREE is displayed by the abundant growth of selvage epidote. In contrast, the dissolution of garnet and phengite released significant amounts of HREE and LILE (K–Rb–Ba) into the passing fluid, because the chemical changes within the selvage prevented the formation of a mineral assemblage with sufficiently high bulk-fluid partition coefficients for these elements. Significant amounts of transitional metal elements were released into the fluid during the dissolution of white mica and the dissolution–precipitation behavior of garnet, omphacite, dolomite and sulfides. Thus the LILE and HREE along with some transition metal elements (e.g., Fe, Cu, Ni and Zn) were mobilized during the dissolution–precipitation processes that led to the selvage formation. Accordingly the slab fluids are not only strongly enriched in LILE and depleted in HFSE, but also carry significant amounts of transition metals. It is most likely that slab fluids strongly contribute to the metal flux into the arc magma systems finally resulting in giant arc-related ore deposits.
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