Subduction Cycle Research Articles

Halogen Cycling in the Solid Earth

Each of the halogens constrains a different aspect of volatile cycling in the solid Earth. F is moderately incompatible in the mantle and has a low mobility at Earth's surface, meaning that it is preferentially retained in the mantle and continental crust. In contrast, Cl, Br, and I are strongly incompatible and highly soluble. Chloride is the dominant anion in seawater and many geofluids and a major component of evaporite minerals. Br and I are essential for life and significantly incorporated into organic matter that accumulates in marine sediments. Surficial fluids circulated into continental and oceanic crust incorporate surface-derived halogens into alteration minerals. As a result, subducting slabs and arc lavas are weakly enriched in F and strongly enriched in Cl, Br, and I. Subduction has maintained mantle Cl and Br concentrations at relatively constant levels since Earth's early differentiation, but mantle I/Cl has decreased over time. ▪Halogen abundances on the early Earth were affected by I partitioning into Earth's core and possible loss of hydrophilic Cl, Br, and I in an early formed ocean.▪Halogens are powerful tracers of subduction zone processes on the modern Earth, with Cl, Br, and I having a dominantly subducted origin in Earth's mantle.▪The deep subduction cycles of Cl, Br, and I are more similar to that of H2O than they are to F, but the geochemical cycle of each halogen differs in detail.▪Halogen abundance ratios and stable isotope ratios vary systematically in Earth's surface reservoirs, meaning that halogens are powerful tracers of geological fluids and melts.

Heavy Mo isotope enrichment in the Pitcairn plume: Implications for the subduction cycle of anoxic sediments

Subduction redistributes elements between Earth's principal geochemical reservoirs, modifying the chemical composition of Earth's mantle, crust, atmosphere, and hydrosphere, and consequently having an impact on the evolution of life itself. Subduction of surface material that has been geochemically modified by low-temperature processes leads to mineralogical and chemical heterogeneities in mantle reservoirs over time and is recorded in modern ocean island basalts. One of the principal geochemical end members of the heterogeneous deep mantle, the enriched mantle 1 (EM-1) source of Pitcairn Island, has been attributed to the contribution of crustal material with vastly different chemical compositions and ages. The Mo isotope composition of lavas from Pitcairn Island constrains the nature of this recycled crustal component. Pitcairn lavas have elevated δ98/95Mo relative to the depleted mantle. The high δ98/95Mo is associated with high time-integrated 232Th/238U and 87Rb/86Sr, and low time-integrated 147Sm/144Nd and 238U/204Pb. These characteristics can be attributed to the recycling of nearly pristine pelagic sediments that were deposited in a Proterozoic anoxic deep-ocean into the sources of the Pitcairn Island lavas. The isotope composition of these lavas is similar to that of EM-1 hotspots from the South Atlantic, indicating the addition of reduced sediments in both of Earth's large low shear wave velocity provinces (LLSVPs). Consistent data from both locations imply that the subduction cycling of sedimentary redox-sensitive elements such as Mo, S, Se, and U into arc magmas was in these cases inefficient in the Precambrian and the chemical and isotopic signature of reduced sediments is preserved in the source of ocean island basalts bearing the EM-1 characteristics.

Neotectonic faults in the Southern Chile intra-arc (38°S–40.5°S): Insights about their seismic potential and the link with the megathrust earthquake cycle

Megathrust earthquakes are a primary geological hazard in subduction zones. In oblique subduction margins, however, seismic threat must be also considered concerning crustal strike-slip faults driving the parallel-to-the-trench component of the convergent vector. These faults have proven their capacity to produce moderate-to-large shallow earthquakes, causing severe damage for surrounding areas. The Southern Chile Subduction Zone (SCSZ) is characterized by the oblique convergence between the Nazca and the South America plates. Between 37°S and 46°S, slip partitioning is significantly led by intra-arc faults like the Liquiñe-Ofqui Fault System (LOFS) and subsidiary NW to NE striking structures. Despite it has been demonstrated that the behavior of these faults is seismogenic, the inland evidence of Holocene deformation along them is scarce. This has concealed to accurately define individual active faults, discussing their seismic potential, and addressing their link with the megathrust earthquake cycle. In this contribution, we first present field evidence for Holocene deformation on intra-arc faults in the area. Based on this, we define 5 active faults and discuss their seismic capacity. To assess the link with the megathrust earthquake cycle, we calculate the Coulomb stress changes induced on these faults by the Mw 9.5 Valdivia Earthquake slip distribution and an interseismic period of 300 years. We conclude that 1) the identified neotectonic faults have the potential for producing moderate-to-large earthquakes (Mw 5.5 to 6.8) and 2) their occurrence would be enhanced by the coseismic and the interseismic stages of the subduction cycle. With this, we propose that the intra-arc faults addressed in this paper, and certainly others in the SCSZ, must be considered as significant sources of seismic hazard.

Seismic signature of subduction termination from teleseismic P- and S-wave arrival-time tomography: The case of northern Borneo

Studies attempting to gain new insights into the last stage of the subduction cycle are typically challenged by limited direct observations owing to a lack of recent post-subduction settings around the world. Central to unravelling how the subduction cycle ends is an understanding of crust and mantle processes that take place after subduction termination. Northern Borneo (Malaysia) represents a unique natural laboratory because it has been the site of two sequential subduction episodes of opposite polarity since the mid-Paleogene. The region exhibits several enigmatic post-subduction (after ∼ 10 Ma) features, including: subsidence followed by rapid uplift, localised intraplate volcanism, possible orogen collapse, and a pluton that emerged to become the third highest peak in southeast Asia, Mt Kinabalu (4095 m). Arrival-time residuals from distant earthquake data recorded by the nBOSS seismic network have been used to investigate P- and S-wavespeed variations in the crust and underlying upper mantle beneath northern Borneo. Our 3-D tomographic images consistently show a high-velocity perturbation in western Sabah that we associate with an upper-mantle remnant of the Proto South-China Sea slab, thus providing important constraints for tectonic reconstructions of SE Asia. The tomographic models, combined with other seismological and geological information, reveal evidence for lithospheric removal in eastern Sabah via a drip instability. Our results suggest that lithospheric drips can be smaller than previously thought, yet their effects on the post-subduction evolution of continental lithosphere can be significant.

Linking Upper‐Plate Fault Reactivation With the Megathrust Earthquake Cycle: The Case of the Northern Chile Outer Forearc (19°S–23°S)

AbstractCrustal faults above subduction megathrusts have been shown to have the capacity to produce moderate‐to‐large earthquakes (fault reactivation), causing severe damage for inhabited areas in the vicinity. Fault reactivation has been dynamically related to the interseismic stage of the subduction cycle and to megathrust earthquakes. In the northern Chile outer forearc (19°S–23°S), published works have demonstrated that six faults of different orientation and kinematics produced M ∼ 7 paleoearthquakes during the Late Quaternary; these lie above the interplate rupture of the 1877 M8.7 Iquique Earthquake. Here, we assess the potential of these and similar faults to experience reactivation due to stress perturbations induced by the interseismic and coseismic phases of the megathrust earthquake cycle. To do so, we compute Coulomb Failure Stress changes (ΔCFS) produced by different megathrust locking/earthquakes (real and synthetic) scenarios. Our results show that fault reactivation is enhanced by both interseismic and coseismic stages. Furthermore, they suggest that reverse slip on EW and NS faults is favored during the interseismic stage; in turn, faults of different orientations and kinematics are favored following megathrust earthquakes. The easiest way to promote the reactivation of most of the considered faults is with interplate ruptures slipping within (but not necessarily restricted to) the seismogenic zone between 20°S and 23°S. These ruptures would augment the hazard from crustal faulting in the area. These results highlight the need to continue studying the paleoseismology of these faults and their dynamic link with the megathrust seismic cycle, in this and other subduction margins.

Constraints on the sulfur subduction cycle in Central America from sulfur isotope compositions of volcanic gases

The sulfur cycle at convergent margins remains poorly constrained yet is fundamentally important for understanding the redox state of Earth's reservoirs and the formation of ore deposits. In this study we investigate the sulfur isotope composition of high temperature volcanic gases emitted from the Nicaraguan (average of +4.8 ± 1.3‰) and Costa Rican (average of +2.3 ± 1.3‰) arc segments contributing to emissions from the Southern Central American Volcanic Arc (SCAVA; average of +3.8 ± 1.7‰). Along-arc variations in geochemical tracers at SCAVA are widely accepted to reflect variations in subduction parameters and deep fluid sources and correlations between these parameters and gas S isotope compositions are observed. These correlations suggest that gas emissions are sourced from a mixture of mantle S with δ34S ~ 0‰ and isotopically heavy slab-derived sulfur with δ34S ≥ ~ +8‰. We employ Monte Carlo mass balance modeling to constrain S inputs to the subduction zone and relative contributions from mantle and slab to arc sulfur emissions. The models indicate that bulk subduction input in Nicaragua has a S isotope composition of +1.4 ± 0.5‰ compared to −0.2 ± 0.4‰ in Costa Rica, requiring preferential release of isotopically heavy oxidized S from the slab to explain the relatively high δ34S observed in arc outputs. We show that the flux of S from the slab is sufficient to oxidize the entire mantle wedge within the lifetime of the arc, indicating that S is a primary oxidizing agent in subduction zones. Furthermore, the preferential removal of heavy S from the slab requires retention of isotopically light S in the residual slab. Subduction-scale fractionation of S isotopes is fundamentally important in explaining why Earth's bulk surface reservoirs are isotopically positive.

Trace element fingerprints of Ni–Fe–S–As minerals in subduction channel serpentinites

A variety of base-metal minerals (BMM) may form during hydration-dehydration of ultramafic rocks within subduction zones. However, the trace element fingerprints of these minerals and their relation to different stages of the subduction cycle are still unexplored. Here, we present the first comprehensive in situ analysis by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) of (semi)-metals (Ni, Fe, As, Sb, Co, Bi, Te, Pb, Cd, Se, Cu, Zn, and Mn) and precious metals (Os, Ir, Rh, Pt, Pd, Au, and Ag) for Ni–Fe–S–As minerals in subducted serpentinites from the La Cabaña area (South-Central Chile). The targeted rocks are medium- and high-pressure serpentinites recording the entire cycle of burial-exhumation within a subduction zone. A first stage of hydration of upper mantle peridotites within the mantle wedge led to formation of lizardite after magmatic olivine at ~300 °C. This stage was followed by prograde hydration (i.e., antigoritization at ~320–400 °C; <1GPa) and subsequent partial dehydration (formation of prograde olivine at ~600 °C and 1.1 GPa) within the subduction channel, and final exhumation of the serpentinites and incorporation in the accretionary prism still in the stability field of antigorite (>300 °C). The Ni–Fe–S–As minerals in these serpentinites include Ni-Fe-rich sulfides [pentlandite (Ni,Fe)9S8), smythite (Fe9S11), heazlewoodite (Ni3S2), millerite (NiS)], arsenides [orcelite (Ni5-xAs2), nickeline (NiAs) and maucherite (Ni11As8)] and, to a lesser extent, alloy (awaruite, Ni3Fe) and sulfarsenides (gerdorffite, NiAsS). Their abundance, morphology and chemistry strongly vary with the degree of serpentinization and style of deformation of the host rock. Thus, euhedral grains of heazlewoodite ± awaruite ± magnetite formed in equilibrium with lizardite when magmatic olivine was hydrated within the mantle wedge. Once the lizardite-olivine serpentinites were incorporated into the subduction serpentinite channel, the infiltration of hotter (S-As-Sb)-bearing fluids promoted antigoritization under static regime and precipitation of orcelite and pentlandite in equilibrium with antigorite. Channelling of fluids in zones of focussed strain enhanced further antigoritization in some schistose serpentinites at decreasing fO2 and fS2, resulting in the transformation of pentlandite into a second generation of heazlewoodite–awaruite–magnetite. After relic olivine exhaustion, fO2 and fS2 increased promoting the replacement of the secondary heazlewoodite to millerite. No significant variations in terms of trace elements were observed during these mineral replacements associated to hydration upon prograde metamorphism and/or increasing of strain. In contrast, partial dehydration of serpentinites under high-pressure conditions (> 1GPa) generated Ni-rich awaruite in equilibrium with the prograde assemblage antigorite-metamorphic olivine. These awaruites are depleted in trace elements, indicating substantial (semi)-metal and precious metal mobility during high P-T metamorphism within the subduction channel. During the final stage of deformation linked to exhumation inside the accretionary prism some orcelite grains formed during early antigoritization recrystallized without substantial change in trace element concentration. At this stage, nickeline singularly enriched in gold formed in equilibrium with the recrystallizing orcelite. These results demonstrate that Ni–Fe–S–As minerals formed or modified during the entire subduction cycle of upper mantle rocks have their own characteristic trace element signature (i.e., depletion in precious metals and enrichment in As, Sb, Te, Bi and Pb) that distinguish them from magmatic counterparts.

Geochemistry and U-Pb-Hf detrital zircon geochronology of metamorphic rocks in terranes of the West Kunlun Orogen: Protracted subduction in the northernmost Proto-Tethys Ocean

The West Kunlun Orogenic Belt (WKOB) is a key area for evaluating the evolution of the Proto-Tethys Ocean. To constrain this history, we present geochemical and zircon U-Pb geochronological data from a suite of metasedimentary rocks within this orogenic belt. Detrital zircons ages are clustered between 1000 and 400 Ma, with major peaks at ca. 510 Ma and 460 Ma, and minor peaks at ca. 840 Ma and 650 Ma. The age groups younger than 1000 Ma broadly overlap magmatic activity at ca. 540–420 Ma and minor magmatism at 920–540 Ma in the WKOB. The age data also reveal Early Paleozoic metamorphic basement rocks in the WKOB that were probably sourced from Kunlun terranes and blocks of Gondwanan affinity rather than the Tarim Craton. Compiled geochemical data suggest a protracted subduction setting in the WKOB in the Early Paleozoic. εHf(t) values show increasing trends at 900–740 Ma and 740–600 Ma and a distinct decreasing trend at 540–420 Ma, consistent with retreating subduction during the Neoproterozoic and advancing subduction in the Early Paleozoic. The retreating-advancing subduction cycle in the WKOB was possibly related to the opening and closure of the Proto-Tethys Ocean along the margin of Gondwana.

Neotectonic evidence for Late Quaternary reverse faulting in the northern Chile outer forearc (22.5°S-23°S): Implications for seismic hazard

In the northern Chile outer forearc (19°S to 23°S), extension is the dominant deformation style conducted by crustal faults during the Neogene-Quaternary. Most of this extension has been produced by normal faulting along submeridian branches of the Atacama Fault System (AFS). During the Late Quaternary, several of these faults have built conspicuous metric-scale normal faulting scarps in alluvial deposits; the construction of these scarps has been related, in some cases, to M~7 normal faulting paleoearthquakes. On the contrary, shortening during this time span has only been locally reported. Until now, no metric-scale reverse faulting scarps have been documented along submeridian strands of the AFS. This study presents novel evidence corresponding to reverse fault scarps produced during the Late Quaternary by three NNE and NNW trending faults in the outer forearc between 22.5°S and 23°S: the El Toro (ETF), Caliche (CF), and Sierra Valenzuela (SVF) faults. These scarps, with up to 2.5 m height, developed in basement rocks, pre-Quaternary, and/or Quaternary alluvial deposits. Based on their characteristics and the evidence suggesting that similar fault scarps in the Andean forearc have a seismogenic origin, we interpret that these neotectonic landforms were produced by single reverse faulting paleoearthquakes. Considering the maximum scarp height as a proxy for the coseismic slip and fault length as a proxy for surface rupture length, we have estimated magnitudes between M6.2 and M6.9 for the causative rupture events. To discuss how reverse crustal faulting may be related to the interseismic and/or coseismic stages of the subduction cycle, we perform elastic dislocation models to calculate the horizontal displacement fields (at 0 and 20 km depth) induced by the interplate locking distribution in the area (over a seismic cycle) and three megathrust earthquakes (Antofagasta, 1995, Tocopilla, 2007, and a synthetic rupture). Modeling results suggest that the studied faults may experience reverse slip during both stages of the subduction cycle. We propose that the ETF, CF and SVF are faults capable of producing moderate-to-large earthquakes in the future. Finally, we reinforce the need to address the earthquake potential of the studied faults, and others similar to them, to acquire a complete assessment of the seismic hazard in the northern Chile outer forearc.

Switching from advancing to retreating subduction in the Neoproterozoic Tarim Craton, NW China: Implications for Rodinia breakup

Geodynamic drivers for the supercontinent cycle are generally attributed to either top-down (subduction-related) or bottom-up (mantle-related) processes. Compiled geochemical data and U–Pb ages and Hf isotopic signatures for magmatic and detrital zircons from the Tarim Craton reveal a distinct change in subduction style during the Neoproterozoic. The subduction cycle is recorded in increasing and decreasing intensity of subduction-related magmatic rocks and time-equivalent sedimentary successions, and converse trends of εHf(t) values and corresponding changes in crustal incubation time. These trends are consistent with a switch from advancing to retreating subduction. The switch likely occurred at ca. 760 Ma when zircon εHf(t) values increase and crustal incubation times decrease following a transitional shift between 800 ​Ma and 760 ​Ma. A switch at this time is consistent with Rodinia breakup and may have resulted in the late Neoproterozoic Tarim rift basin. The long-lived (ca. 500 Ma) subduction recorded in the Tarim Craton suggests the predominance of a top-down process for Rodinia breakup on this part of its margin.

Tracing Highly Siderophile Elements through Subduction: Insights from High-pressure Serpentinites and ‘Hybrid’ Rocks from Alpine Corsica

Abstract The highly siderophile elements (HSE) include the economically critical platinum group elements (PGE; Os, Ir, Ru, Rh, Pt, Pd, Au and Re), gold and rhenium. The HSE are redox sensitive in mantle and seafloor environments and have a strong affinity to iron and sulphur, therefore their distribution within the subducted mantle lithosphere record changes to oxidation state and sulphidation. The mobility of the HSE during subduction has important implications for Re–Os isotopic signatures in the mantle, and the formation of Cu–Au arc-related ore deposits. In this study, subducted rock samples from Alpine Corsica are used to track the HSE in serpentinites and hybrid ultramafic–mafic rocks through the subduction cycle. A comparison of bulk-rock HSE concentrations with those in pre-subduction analogues provides insights into the transfer of the HSE throughout the subduction cycle. Serpentinites subducted to blueschist–eclogite-facies conditions have similar HSE concentrations to primitive upper mantle (PUM) concentrations, therefore it is concluded that the HSE are not mobilized from serpentinites on the scale of the whole-rock or greater. Therefore, as suggested in previous studies, crustal lithologies may be more important contributors of the HSE to the sub-arc mantle, particularly Pt, Pd and Re. In contrast, HSE concentrations in hybrid rocks (talc schist and chlorite schist) deviate from protolith concentrations. Rhenium is higher in the talc schist, and Ir and Ru are lower in the chlorite schist than in the PUM, or possible mafic protoliths. Mineral parageneses place temporal constraints on the growth of hosts to the HSE (sulphides, oxides and metal alloys), and record changes to the activities of oxygen and sulphur (aO2–aS2), and hence redox conditions, from pre-subduction to exhumation. Laser ablation inductively coupled plasma mass spectrometry was used to determine the HSE concentrations in sulphides and oxides, and the detection of small (∼2–25 µm2) platinum group minerals utilized high-resolution SEM mapping techniques. The prograde and retrograde sulphides have lower HSE concentrations compared with sulphides from pre-subduction settings. Therefore, the redistribution of the HSE on a mineral scale from sulphides to alloys and/or other sulphides has occurred within the serpentinites, which may reflect more reducing conditions during serpentinization or subduction, consistent with the results of thermodynamic modelling. In contrast, the mineral assemblages in the hybrid rocks imply an increase in the extent of sulphidation and oxidation, and higher fluid:rock ratios during exhumation, coincident with Re enrichment in the talc schist, and a decrease in the concentrations of Ir and Ru in the chlorite schist, at length scales greater than those of the rock samples. Therefore, hybridization of lithologies at the slab–mantle interface may enhance the transfer of the HSE to the sub-arc mantle. If Re transfer from the slab to the sub-arc mantle is possible, this questions the robustness of Re–Os isotope signatures as tracers of crustal recycling.

Post-melting oxidation of highly primitive basalts from the southern Andes

The oxygen fugacity (fO2) of the Earth’s upper mantle and its melting products is an important parameter in the geochemical evolution of arc magmas and their connection with the continental crustal construction and growth. Several works have focused on the fO2 of peridotite xenoliths, primitive melts in relatively young arc settings, and mid-ocean ridge basalts (MORB) but few studies have attempted to examine the early redox history of primitive magmas in mature arcs. Hence, our understanding of the nature and evolution of fO2 during the subduction cycle remains limited. Here, we investigate the basaltic tephra from the Los Hornitos monogenetic cones in central-southern Chile, which are among the most primitive materials reported in the Southern Andes (olivine Mg# ≤ 92.5, and Ni ≤ 5000 µg·g−1). These features offer a unique opportunity to explore the fO2 conditions below the Andean arc by studying olivine phenocrysts and their contained crystal and melt inclusions. We integrated EPMA, LA-ICP-MS, and µ-XANES analyses to constrain the redox conditions recorded in the basaltic tephra by three different and self-reliant methods. First, we determined the fO2 based on the olivine-spinel equilibrium, yielding average values ΔFMQ + 1.3 ± 0.4 (1σ). Second, we constrained the fO2 conditions of melt inclusions using Fe µ-XANES data and the redox dependent olivine-melt vanadium partitioning. After correcting for post-entrapment crystallization and diffusive iron loss, the Fe µ-XANES data indicate that the melt inclusions were trapped in average at ΔFMQ +2.5 ± 0.5 (1σ). Results using the olivine-melt vanadium partitioning oxybarometer in melt inclusions are in agreement with Fe µ-XANES data, yielding average ΔFMQ values of +2.6 ± 0.3 (1σ). In order to test the potential effects of other post-entrapment modifications of the melt inclusions that could have affected the fO2 prior to eruption, we assessed the residence time of these magmas using Mg-Fe interdiffusion modelling in olivine. The short residence times (<200 days) compared to vanadium re-equilibration models strongly suggest that the melt inclusions preserve the prevailing fO2 conditions during their entrapment. Correlations between melt inclusions major element composition and their fO2 determined by Fe µ-XANES, as well as V/Sc modelling reveal a case of post-melting oxidation of the LHC magmas. We argue that primitive arc magmas behave as an open system with respect to fO2 during their early geochemical evolution. Our data indicate a complex fO2 early history of primitive melts in the southern Andes and provide a cautionary note on the direct extrapolation of primitive melts fO2 values to that of their mantle source.

How many subductions in the Variscan orogeny? Insights from numerical models

We developed a 2D numerical model to simulate the evolution of two superposed ocean-continent-ocean subduction cycles with opposite vergence, both followed by continental collision, aiming to better understand the evolution of the Variscan belt. Three models with different velocities of the first oceanic subduction have been implemented. Striking differences in the thermo-mechanical evolution between the first subduction, which activates in an unperturbed system, and the second subduction, characterised by an opposite vergence, have been enlighten, in particular regarding the temperature in the mantle wedge and in the interior of the slab. Pressure and temperature (P-T) conditions predicted by one cycle and two cycles models have been compared with natural P-T estimates of the Variscan metamorphism from the Alps and from the French Massif Central (FMC). The comparative analysis supports that a slow and hot subduction well reproduces the P-T conditions compatible with data from the FMC, while P-T conditions compatible with data of Variscan metamorphism from the Alps can be reproduced by either a cold or hot oceanic subduction models. Analysing the agreement of both double and single subduction models with natural P-T estimates, we observed that polycyclic models better describe the evolution of the Variscan orogeny.

From magmatic generation to UHP metamorphic overprint and subsequent exhumation: A rapid cycle of plate movement recorded by the supra-subduction zone ophiolite from the North Qaidam orogen

The rapid subduction and exhumation of high-pressure (HP) and ultrahigh-pressure (UHP) rocks exposed in collisional belts play a critical role in understanding the evolution of geodynamics and plate tectonics in subduction zone environments. Herein, ophiolitic fragments, eclogites, and amphibolites, from a subduction channel exposed in the Dulan terrane, North Qaidam orogen were investigated. Garnet compositional zoning from the studied eclogites records only a single cycle of subduction and exhumation. New uranium–lead (U–Pb) zircon ages of the current study combined with previous rutile U–Pb ages from eclogites indicate a rapid cycle of events, starting with the generation of the magmatic protolith at ca. 446 Ma, its subsequent subduction reaching a depth of ca. 100–110 km at ca. 435 Ma, and its exhumation into shallow portions of the crust which occurred at ca. 416 Ma. Thus a rapid 30 million years (Myr) protolith generation–subduction–exhumation cycle could be recorded. According to an ophiolitic rock association with island arc basalt (IAB)-like and mid-ocean ridge basalt (MORB)- to ocean island basalt (OIB)-like trace element compositions, the slab-derived components determined by Sr–Nd–Hf isotopes and the short time span of only about 11 Myr between photolith generation and UHP metamorphism in eclogites, the protolith of eclogite is best explained as a supra-subduction zone ophiolite. This finding likely records the closure of the South Qilian ocean and the initiation of a continent-arc collision at ca. 446 Ma. Furthermore, starting with subduction erosion during continent-arc collision in the early Palaeozoic, ophiolitic and arc-related materials become subducted in the North Qaidam. It was inferred that such a rapid succession of events from protolith formation to UHP overprint and final exhumation, could be explained by movements of rock material in a subduction channel. Thus, this study provides a new view in understanding the mechanics of different episodes of subduction systems in North Qaidam UHP metamorphic belt.

Insights into sulfur cycling at subduction zones from in-situ isotopic analysis of sulfides in high-pressure serpentinites and ‘hybrid’ samples from Alpine Corsica

Devolatilisation of serpentinites at depth in subduction zones contributes significant quantities of sulfur and other redox sensitive elements to the sub-arc mantle. However, the fate of sulfur in subducted serpentinites is poorly constrained. Textures of sulfur-bearing phases in subducted serpentinites are rarely studied, yet provide important information on the changes to sulfur distribution throughout the subduction cycle and as a result of fluid infiltration. δ34S values of sulfides provide constraints on sulfur sources, the redox state of sulfur in the host mineral, and on processes that have occurred subsequent to sulfide crystallisation, including interaction with oxidised or reduced fluids. Therefore, it is possible to use δ34S values in subducted serpentinites to constrain the redox state of sulfur in sulfides and subduction zone fluids. Furthermore, the proximity of serpentinites to ocean crust and metasediments may influence enrichment or depletion of 34S during subduction relative to serpentinites distal to such lithologies.This study investigates the redox state, the likelihood of sulfur addition to the sub-arc mantle from serpentinite dehydration, and the distribution of sulfur within subducted serpentinites and ‘hybrid’ mafic/ultramafic rocks from Alpine Corsica. The techniques utilised include petrographic analysis, in-situ sulfur isotopic analysis and trace element analysis of sulfides hosted in these rocks. All sulfides investigated have high δ34S values of 1.9–15.5‰, which suggests that mantle-derived sulfur (δ34S ~0.1‰), was not the sole source of sulfur. The highest δ34S values are recorded in pyrites of a hybrid mafic/ultramafic sample. High δ34S values are preserved in sulfides attributed to prograde metamorphism, and is most consistent with the retention of sulfur derived from hydrothermal sulfate reduction on the seafloor. However, a shift towards higher δ34S values in sulfides associated with the advanced stages of exhumation suggests that late stage exhumation enables enhanced access to slab-derived fluids bearing oxidised sulfur (SO42− or SO2). Such fluids may have been derived from the devolatilisation of serpentinite at greater depth, or from other lithologies.

Geochronology and geochemistry of the Paleoproterozoic Yinyugou Group in the southern North China Craton: Implications for provenance and tectonic evolution

The Paleoproterozoic Yinyugou Group in the southern North China Craton is a littoral facies sedimentary sequence which is mainly composed of feldspar quartzites and mica schists with minor carbonate rocks. U-Pb isotopic dating of detrital zircons bracket the depositional age of the Yinyugou Group at ∼2.43–2.30Ga. Chemical Index of Alteration (CIA) values of most feldspar quartzite and schist samples are lower than 75, indicating generally weak to moderate source weathering conditions for the Yinyugou rocks. The majority of the studied Yinyugou samples give a large range of Index of Compositional Variability (ICV) values of 0.93–2.59, revealing a chemically immature source which deposited in an active continental margin setting. Chondrite-normalized REE patterns and discrimination diagrams involving La, Th, Hf, Co and Sc reveal that the Yinyugou Group dominantly received detritus from felsic sources with minor contribution from mafic igneous rocks. U-Pb ages of detrital zircons from the Yinyugou Group can be divided into two groups: 2.90–2.40Ga and 3.50–3.20Ga, with a predominant age peak at ∼2.50Ga. The 3.50–3.20Ga detrital zircons have negative εHf(t) values ranging from −11.2 to −3.8 and old crustal model ages (TDMC) of 4246 to 3909Ma, indicating a mature crustal source. Most ∼2.50Ga zircons show sub-depleted mantle positive εHf(t) values and TDMC ages mainly ranging from 3.10 to 2.70Ga, representing the time when juvenile materials were extracted from the depleted mantle. Taking all these data into account, the Yinyugou Group was most likely deposited in a back-arc setting. In combination with previous studies on other Paleoproterozoic volcanic-sedimentary sequences and magmatic rocks in the NCC, a cycle of subduction (collision)-extension (rift)-subduction processes is proposed to explain the tectonic evolution of the NCC during Paleoproterozoic.

Boron isotope fractionation in magma via crustal carbonate dissolution.

Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ11B values down to −41.5‰, reflecting preferential partitioning of 10B into the assimilating melt. Loss of 11B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports 11B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ11B melt values in arc magmas could flag shallow-level additions to the subduction cycle.

Micro‐sampling Lu–Hf geochronology reveals episodic garnet growth and multiple high‐P metamorphic events

AbstractWe present Lu–Hf dates from multiple growth zones within two large garnet porphyroblasts by a micro‐sampling method. A single eclogite sample, taken from the Huwan shear zone in the Hong'an orogen, was investigated with Lu–Hf geochronology by micro‐sampling and traditional bulk separation methods. The sample contains a few large garnet porphyroblasts up to several centimetres in diameter and a second major population of smaller (0.1–2.0 mm) garnet porphyroblasts, comprised of dark cores and pale rims. Elemental compositions and mineral inclusions in the garnet appear consistent with two garnet generations. Lu–Hf dates (c. 400–264 Ma) were determined from twelve micro‐sawed garnet sections from two large garnet porphyroblasts. These Lu–Hf dates overlap with age peaks defined by 115 SIMS zircon U–Pb analyses from the same eclogite sample. Bulk analyses of smaller garnet separates define a minimum date of c. 306 Ma and a maximum date of c. 252 Ma for the dark cores and pale rims respectively. These Lu–Hf dates were interpreted to bracket the period of garnet growth and the broad interval from c. 400 to 264 Ma is best explained by protracted and episodic garnet growth, which may require that these rocks experienced two subduction cycles that were initiated during the Devonian and terminated in the Triassic.

Detrital zircon U–Pb, Lu–Hf, and O isotopes of the Wufoshan Group: Implications for episodic crustal growth and reworking of the southern North China craton

The Paleoproterozoic orogens of the North China Craton (NCC) provide important windows to evaluate the crustal evolution history during early Precambrian. Here we report results from SIMS zircon geochronology and oxygen isotopes, as well as LA-ICPMS zircon Hf isotopic analyses on detrital zircons from Mesoproterozoic-Neoproterozoic sandstones of the Wufoshan Group from the southern NCC to probe the Paleoarchean-Paleoproterozoic crustal evolution. Our data show episodic magmatism during the Paleoarchean-Neoarchean and Paleoproterozoic as inferred from the 207Pb/206Pb age range of 3514–2751Ma and the well-defined 207Pb/206Pb weighted mean ages of 2669±15Ma, 2510±8Ma, 2402±39Ma, 2298±36Ma, 2179±10Ma, 2058±16Ma, 1940±6Ma, 1874±7Ma and 1819±5Ma. The similarity in U–Pb geochronology and Hf isotopes of the Archean detrital zircons from the Wufoshan Group to those from the Paleoproterozoic Songshan Group reveal that the 2669Ma zircons record a major phase of Neoarchean crustal growth, and that 2510Ma zircons correspond to the latest Neoarchean crustal reworking. The 18O-depleted isotopic features displayed by some detrital zircons are absent in those from Songshan Group, and together with the rarity of core-rim textures which were widespread in Archean zircons from the Songshan Group, we suggest that the source materials for the Mesoproterozoic-Neoproterozoic Wufoshan Group are different from the Archean cratonic source for the Paleoproterozoic Songshan Group. We propose that the zircons were probably sourced from the newly-formed Trans-North China Orogen (TNCO). Our Paleoproterozoic zircon data provide robust evidence for continuous and complex reworking of the Mesoarchean to early Neoarchean crust in the southern NCC during Early Paleoproterozoic at ca. 2402Ma, 2298Ma, and 2179Ma. The Middle Paleoproterozoic ages (2058Ma, 1940Ma, and 1874Ma) mark a partial cycle of oceanic crust subduction to continental collision during the formation of the TNCO. The extreme oxygen isotopic variation with the fairly restricted range in Hf isotopes demonstrates that the components from both high-temperature and the low-temperature rock–water interaction induced by the release of hydrothermal fluids from the subducted altered oceanic crust must have played a significant role during the growth of the zircons. The Paleoproterozoic thermal event at 1819Ma records post-collisional uplifting, as evidenced by an apparent negative correlation in the ɛHf(t) vs. δ18O plot, possibly inherited from the mixing of the ancient crust and the juvenile input during the oceanic subduction and continental collision. Integrated with the information from granulite facies metamorphic rocks in the orogenic root of the TNCO, our data suggest that the episodic Paleoproterozoic magmatism was mainly induced by the tectonics of plate subduction and subsequent continental collision, leading to the final cratonization of the NCC. Following the Mesoproterozoic-Neoproterozoic sedimentation of the Paleoproterozoic zircons, except for the Pb loss during the subsequent thermal events, there was no major alteration in the Hf and oxygen isotopic compositions. Thus, the remarkably large variation of oxygen isotopes from the lower than normal mantle to the supracrustal value (δ18O=2.28–10.09‰) in the Paleoproterozoic detrital zircons further demonstrates that these zircons came from the uplifted and exposed upper part of the TNCO. Our detrital zircon data provide convincing evidence for the repeated crustal reworking in Early Paleoproterozoic and the onset of modern-style plate tectonics in the NCC in Middle Paleoproterozoic.

Subduction initiation in the Neo‐Tethys: constraints from counterclockwise P–T paths in amphibolite rocks of the Nagaland Ophiolite Complex, India

AbstractHigh‐P metamorphic rocks that are formed at the onset of oceanic subduction usually record a single cycle of subduction and exhumation along counterclockwise (CCW) P–T paths. Conceptual and thermo‐mechanical models, however, predict multiple burial–exhumation cycles, but direct observations of these from natural rocks are rare. In this study, we provide a new insight into this complexity of subduction channel dynamics from a fragment of Middle‐Late Jurassic Neo‐Tethys in the Nagaland Ophiolite Complex, northeastern India. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of a medium‐grade amphibolite tectonic unit within a serpentinite mélange, we establish two overprinting metamorphic cycles (M1–M2). These cycles with CCW P–T trajectories are part of a single tectonothermal event. We relate the M1 metamorphic sequence to prograde burial and heating through greenschist and epidote blueschist facies to peak metamorphism, transitional between amphibolite and hornblende‐eclogite facies at 13.8 ± 2.6 kbar, 625 ± 45 °C (error 2σ values) and subsequent cooling and partial exhumation to greenschist facies. The M2 metamorphic cycle reflects epidote blueschist facies prograde re‐burial of the partially exhumed M1 cycle rocks to peak metamorphism at 14.4 ± 2 kbar, 540 ± 35 °C and their final exhumation to greenschist facies along a relatively cooler exhumation path. We interpret the M1 metamorphism as the first evidence for initiation of subduction of the Neo‐Tethys from the eastern segment of the Indus‐Tsangpo suture zone. Reburial and final exhumation during M2 are explained in terms of material transport in a large‐scale convective circulation system in the subduction channel as the latter evolves from a warm nascent to a cold and more mature stage of subduction. This Neo‐Tethys example suggests that multiple burial and exhumation cycles involving the first subducted oceanic crust may be more common than presently known.