Subduction Zone Setting Research Articles

A strength inversion origin for non-volcanic tremor

Non-volcanic tremor is a particularly enigmatic form of seismic activity. In its most studied subduction zone setting, tremor typically occurs within the plate interface at or near the shallow and deep edges of the interseismically locked zone. Detailed seismic observations have shown that tremor is composed of repeating small low-frequency earthquakes, often accompanied by very-low-frequency earthquakes, all involving shear failure and slip. However, low-frequency earthquakes and very-low-frequency earthquakes within each cluster show nearly constant source durations for all observed magnitudes, which implies characteristic tremor sub-event sources of near-constant size. Here we integrate geological observations and geomechanical lab measurements on heterogeneous rock assemblages representative of the shallow tremor region offshore the Middle America Trench with numerical simulations to demonstrate that these tremor events are consistent with the seismic failure of relatively weaker blocks within a stronger matrix. In these subducting rocks, hydrothermalism has led to a strength-inversion from a weak matrix with relatively stronger blocks to a stronger matrix with embedded relatively weaker blocks. Tremor naturally occurs as the now-weaker blocks fail seismically while their surrounding matrix has not yet reached a state of general seismic failure.

Anomalous elasticity of talc at high pressures: Implications for subduction systems

Talc is a layered hydrous silicate mineral that plays a vital role in transporting water into Earth’s interior and is crucial for explaining geophysical observations in subduction zone settings. In this study, we explored the structure, equation of state, and elasticity of both triclinic and monoclinic talc under high pressures up to 18 GPa using first principles simulations based on density functional theory corrected for dispersive forces. Our results indicate that principal components of the full elastic constant tensor C11 and C22, shear components C66, and several off-diagonal components show anomalous pressure dependence. This non-monotonic pressure dependence of elastic constant components is likely related to the structural changes and is often manifested in a polytypic transition from a low-pressure polytype talc-I to a high-pressure polytype talc-II. The polytypic transition of talc occurs at pressures within its thermodynamic stability. However, the bulk and shear elastic moduli show no anomalous softening. Our study also shows that talc has low velocity, extremely high anisotropy, and anomalously high VP/VS ratio, thus making it a potential candidate mineral phase that could readily explain unusually high VP/VS ratio and large shear wave splitting delays as observed from seismological studies in many subduction systems.

Chlorine isotope behavior in subduction zone settings revealed by olivine-hosted melt inclusions from the Central America Volcanic Arc

The isotopic composition of Cl, a highly hydrophilic and incompatible element, can provide new insights into the processes of element recycling in subduction zone settings. Samples from 13 localities in Guatemala, El Salvador, Nicaragua and Costa Rica, representing a ca. 1000 km long NW-SE segment along the Central American Volcanic Arc (CAVA), were selected. Ninety-seven melt inclusions, hosted by olivine Fo90−70, were measured for Cl isotope ratios and trace element concentrations. Melt inclusions from samples from Guatemala to northwest Nicaragua have a restricted range of δ37Cl values (range < 1‰ within a sample) with values decreasing from Santa Maria (Guatemala) to San Miguel (El Salvador), whereas melt inclusions from Nicaragua and Costa Rica display larger variation within a sample (δ37Cl value range >1‰, up to 3.8‰) and do not show any systematic variation along the arc. For some samples, the δ37Cl in the melt inclusions is shifted by up to 2‰ to higher values compared to bulk rock data from the same volcanic center, for which the extent of Cl degassing is not known. The combination of δ37Cl values in melt inclusions with trace elements and the existing knowledge about the slab contributions along the arc allows us to elucidate the Cl isotope composition of different endmembers in this subduction zone. From Guatemala to northwest Nicaragua, a fluid component, originating from serpentinite, has a δ37Cl value close to +0.6‰. This value, similar to lithospheric serpentinites, confirms that despite the aqueous fluid migration through the entire slab, Cl isotopes do not fractionate significantly during transport. A melt-like component, present in the southern part of the arc, has negative δ37Cl, possibly down to −2.5‰. This component has lower δ37Cl than values of the oceanic crust but similar to sediments currently subducting beneath CAVA. Finally, a common component, most likely amphibole-bearing metasomatized mantle, is identified in samples with the highest δ37Cl values (up to +3.0‰). The melting of amphibole, a mineral concentrating 37Cl over 35Cl, could explain the high δ37Cl values. The difference between melt inclusions and bulk rock δ37Cl in some volcanic centers probably results from late-stage processes such as mixing of different batches of magma at shallower levels after melt inclusions entrapment. Melt inclusions thus give a more comprehensive picture of Cl isotope systematics along the CAVA and in primitive subduction-related magmas.

Evidence for basalt enrichment in the mantle transition zone from inversion of triplicated P- and S-waveforms

Recycling of chemically stratified oceanic lithosphere at subduction zones and mantle mixing is the main source for the production and long-term maintenance of heterogeneities as evidenced through mantle samples and the presence of seismic scatterers. The mantle transition zone (MTZ), which is bound by seismic discontinuities around 410 and 660 km depth that reflect mantle mineral phase transformations in the olivine system, is considered to play a significant role in reprocessing and remixing of subducted oceanic plates. However, the compositional nature and mixing state of the MTZ is generally unconstrained with models ranging from a fully-equilibrated pyrolitic to a mechanically-mixed MTZ enriched in basalt. Here, we consider regional P- and S-waveforms from different regions that have interacted extensively with the MTZ and are triplicated at the seismic discontinuities. Because of the strong interaction with the MTZ and the sensitivity to mantle temperature and composition, triplicated waveforms can be used as probes of the thermochemical conditions in and around the MTZ. Here, we model the MTZ by considering two-endmember models: a fully-equilibrated and a mechanical mixture of depleted (harzburgite) and fertile (basalt) lithologies, whose properties are computed self-consistently using petrologic phase equilibria in combination with equation-of-state modeling as a function of pressure, temperature, and composition. Based on this method, we invert carefully selected triplicated P- and S-waveforms for local one-dimensional profiles of MTZ structure. In some of the studied regions, we find both radial and lateral heterogeneities in composition that show a trend from more fertile lithologies in the mid-MTZ to more pyrolitic compositions below the MTZ. This is in line with evidence provided by global geodynamic models that support the segregation and accumulation of basalt toward the mid-MTZ. Because a number of the regions studied here encompass subduction zone settings, we also observe lower-than-average mantle temperatures (∼1200∘C) and, as a consequence, thicker-than-average MTZ (∼240–280 km). While some areas appear to be better fit with a particular mixing model, rigorous statistical analysis of the datafit shows that on average, and at the level of the resolving power of the data, an equilibrated mantle across a large part of the studied subduction zone settings is favored.

Sedimentary response of a structural estuary to Holocene coseismic subsidence

AbstractStratigraphic evidence for coseismic subsidence has been documented in active-margin estuaries throughout the world. Most of these studies have been conducted in subduction zone or strike-slip settings; however, the stratigraphic response to coseismic subsidence in other tectonic settings would benefit from further study. Here we show evidence of late Holocene coseismic subsidence in a structural estuary in southern California. Below the modern marsh surface, an organic-rich mud containing marsh gastropods, foraminifera, and geochemical signatures indicative of terrestrial influence (mud facies) is sharply overlain by a blue-gray sand containing intertidal and subtidal bivalves and geochemical signatures of marine influence (gray sand facies). We use well-established criteria to interpret this contact as representing an abrupt 1.3 ± 1.1 m rise in relative sea level (RSL) generated by coseismic subsidence with some contribution from sediment compaction and/or erosion. The contact dates to 1.0 ± 0.3 ka and is the only event indicative of rapid RSL rise in the 7 k.y. sedimentary record studied. Consistent with observations made in previous coseismic subsidence studies, an acceleration in tidal-flat sedimentation followed this abrupt increase in accommodation; however, the recovery of the estuary to its pre-subsidence elevations was spatially variable and required 500–900 years, which is longer than the recovery time estimated for estuaries with larger tidal ranges and wetter climates.

Mobilization and fractionation of Ti-Nb-Ta during exhumation of deeply subducted continental crust

The behavior of Ti-Nb-Ta is crucial to reveal the genesis of island arc magmatism. However, mobilization and fractionation of Ti-Nb-Ta in subduction zone settings remain poorly understood. The discovery of felsic veins rich in coarse-grained rutile within retrograde eclogite of the North Qaidam UHP metamorphic belt provides a unique and novel opportunity to study age variation during rutile formation and alteration, as well as Ti-Nb-Ta mobility and fractionation during fluid/melt-rock interaction. Rutile high-resolution elemental mapping, and U-Pb bulk grain (ID-TIMS), and in-situ U-Pb geochronology have been utilized to focus on the properties of rutile in both, felsic vein and retrograde eclogite host to gain insight into possible similarities and differences. Three groups of rutile were distinguished according to its host rock, trace elements signature, and genetical connection to ilmenite: eclogite-hosted rutile (Rt-1), felsic vein-hosted rutile not associated with ilmenite (Rt-2a), and associated with ilmenite (Rt-2b). Field evidence and rutile trace elements characteristics document the source of vein-hosted rutile to be mainly derived from the eclogite during fluid/melt-rock interaction. Principal Component Analysis reveals that Nb, Ta, Sn, and W are more enriched in Rt-2a compared to Rt-1; Rt-2b has higher Nb, U, and Hf than Rt-2a. High-resolution mapping across large rutile grains shows the enrichment of high field strength elements (HFSEs) in rutile near to ilmenite, which indicates a HFSEs back diffusion from the rutile–ilmenite boundary during the replacement of rutile by ilmenite. The Nb/Ta ratios of Rt-2a are lower than those of Rt-1, which result from different partition coefficients of Nb and Ta during fluid/melt-rock interaction. The diffusion-influenced rutile exhibits suprachondritic Nb/Ta ratios and demonstrates that diffusion of Nb in rutile is higher than that of Ta under identical P-T conditions. Rutiles Rt-1 and Rt-2a yield consistent 206Pb/238U ages of 426–423 Ma, which is similar to the 433 ± 3 Ma determined by ID-TIMS results of bulk rutile grains. This indicates that Ti-Nb-Ta must have been mobilized during the exhumation of deeply subducted continental crust. However, the diffusion-influenced rutile shows a large variation of ages compared to the rutile not associated with ilmenite, demonstrating that the back-diffusion may affect the U-Pb system in rutile. Therefore, when rutile is partially altered into ilmenite or titanite, its dating should be used with caution. Thus, this study demonstrates volume diffusion is a very important geological process to result in extreme HFSEs fractionation and age variation of rutile on the mineral scale. The rutile aggerates that occur in the felsic veins in 3–5 m distance to the adjacent retrograde eclogite suggest that Ti-Nb-Ta-rich melts/fluids were transported over a distance of at least several meters and that rutile does not represent a residual phase of the Na-Si-Al-, F- and CH4-bearing fluid/melt environment that formed during anatexis of the subducted continental crust. The formation of rutile-rich aggregates during the generation, transport, and crystallization of subducted continental crust-derived melts/fluids in the deep roots of orogenic belts may be a critical trigger for the depletion of HFSEs in arc magmatic rocks during the formation of the continental crust.

Petrogenesis of mantle peridotite and cumulate peridotite rocks from the Nagaland Ophiolite Complex, NE India

Here, we report new geochemical data from mantle peridotite (spinel lherzolite, dunite, and harzburgite) and cumulate peridotite (websterite) rocks from the northernmost portion of the Nagaland Ophiolite Complex. Based on detailed textural, mineral (major element) chemistry, and whole‐rock (major and trace element) chemistry, we reveal the origin of these rocks in different tectonic settings and their modification via the melt–mantle interaction process. The spinel lherzolite rocks with low Cr# (0.16–0.25) chrome spinel are mantle residues formed after low degrees (~10–15%) of partial melting in a mid‐oceanic ridge setting, which has been subsequently modified in a subduction zone setting. However, the dunites and harzburgite samples are interpreted to be depleted mantle residue formed in a supra‐subduction zone (SSZ) setting. Additionally, the dunites are inferred to be of replacive origin representing former harzburgites. Based on Cr# of chrome spinel, the dunites are classified into low Cr# (0.53–0.57), medium Cr# (0.73–0.79), and high Cr# (0.89–0.90) dunites. High Cr# dunite with low‐TiO2 (0.01–0.06 wt%) possibly formed by interaction with a boninitic melt, whereas low‐medium Cr# dunite with high TiO2 (0.14–0.34 wt%) formed by interaction with a high‐Ti melt in an SSZ setting. Harzburgite samples with their highly resorbed orthopyroxene grain boundaries contain chrome spinels (Cr# = 0.84–0.88; TiO2 = 0.00–0.06 wt%) similar to the high Cr# dunite samples and are also interpreted to be formed by interaction with a boninitic melt. The websterite samples with their low Mg# (0.79) olivine and low Ni (345–413 ppm) content are interpreted to be of cumulate origin formed in an SSZ setting. However, the interstitial chrome spinel blebs with low Mg# (0.18–0.19), high Cr# (0.60–0.61), Fe3+ (0.20–0.21), and very high TiO2 (2.03–2.04 wt%) are interpreted to be a result of post‐cumulus interaction of interstitial chrome spinel with high‐Ti intercumulus melt. These mantle peridotites and cumulates of different tectonic origins were obducted and juxtaposed during the final stages of the India‐Asia collision.

Highly refractory dunite formation at Gibbs Island and Bruce Bank, and its role in the evolution of the circum-Antarctic continent

ABSTRACT The continental margin is of profound importance as it records continental growth by accretion of orogenic magmas and following continental rifting. A high degree of mantle melting due to hydrous fluid input is expected to simultaneously stimulate continental growth and lower the intrinsic density of the mantle than more fertile mantle, which in turn isolates the continental lithosphere from the convective mantle. The mantle peridotites from Gibbs Island (South Shetland Islands) and Bruce Bank in the Drake Passage provide us an insight into the tectonic history in the circum-Antarctic region. To elucidate the continental growth of Antarctica, we present geochemical data of eight dunites from Gibbs Island and one dunite from Bruce Bank, including Re–Os isotope and highly siderophile element compositions. The dunites are severely affected by serpentinization as evidenced by antigorite + brucite or lizardite (loss on ignition = LOI ranging from 3 to 34 wt.%) but contain primary euhedral to subhedral chromites with or without spherical inclusions. The chromites rarely form lens-shaped aggregates. A dunite from Gibbs Island contains fresh olivine grains filling a fracture in the chromite with low LOI (3 wt.%), indicating a deserpentinization origin from a precursor serpentinized dunite. The dunites show highly depleted bulk-rock major element compositions (Mg/Si = 1.4–1.6 and Al/Si = 0.004–0.01 for Gibbs Island dunites, Mg/Si = 0.66 and Al/Si = 0.008 for Bruce Bank dunite), overlapping a compositional field defined by forearc peridotites. The positive correlation in Re/Ir–LOI space corroborates Re input during the later serpentinization process. The 187Os/188Os ratios of the dunites range from 0.11907 to 0.14493. Phanerozoic Re-depletion (melt depletion) ages of ca. 535–129 Ma are recorded in the Gibbs Island dunites, except for one with a Mesoproterozoic Re-depletion age of ca. 1.2 Ga. Since there exists serpentinization-related perturbation of Re, the ages provide minimum time estimates for melt depletion events. The early Paleozoic melt depletion is inferred to have occurred at a very early stage of Antarctic Peninsula formation in response to plate convergence along the margin of Gondwana, whereas the Mesoproterozoic Re-depletion age reflects convecting mantle heterogeneity unrelated to any nearby crust-forming events. The petrographic characteristics of the chromites and highly depleted nature of the dunites are attributed to melt–peridotite reaction in a subduction zone setting. A feasible interpretation for the dunite formation is that the mantle had experienced two stages of melting with the final stage occurring along the Gondwana continental margin in the subduction zone setting. Resultant highly refractory lithospheric mantle was later displaced and dispersed during the Gondwana breakup. Widespread existence of the dunite may be attributed to multi-stage melt depletion along the continental margin.

The upper Cretaceous Ermioni VMS deposit, Argolis Peninsula, Peloponnese, Greece: Type, genesis, and geotectonic setting

The age, type and geotectonic setting of the Ermioni VMS are explored. Although obscured by post-ore deformation and metamorphism, the Ermioni VMS is genetically and spatially related to hydrothermally altered volcaniclastic rocks and arkoses at the footwall, and turbidites at the hangingwall. The Ermioni VMS deposits occur at Karakasi, Roro and Cambrorosso mine sites, and in close proximity to each other. At Karakasi, the identified hydrothermal alteration zones include a lower and innermost grunerite zone, succeded by an intermediate silica-chlorite zone which shows the largest spatial development and surrounded by a silica-epidote-chlorite zone at the flanks, whereas an albite-prehnite zone is developed at the top of the VMS system. The Ermioni VMS comprises nearly monomineralic pyrite with elevated Cu and Co content, and Cu/(Cu + Zn) and Co/Ni ratios typical for “Mafic-Pelitic”-type VMS deposit. Two ore stages are recognized, Stage I euhedral and coarse-grained pyrite (major Co carrier) with minor chalcopyrite, magnetite and pyrrhotite was followed by Stage II pyrite and minor chalcopyrite and sphalerite. Rubidium-Sr and Os-Re radiogenic pairs on pyrite from the Roro massive and Karakasi stringer ores define an upper Cretaceous age of VMS formation (65.58 ± 0.9 to 66.02 ± 0.1 Ma and 64.96 ± 0.9 to 65.12 ± 0.1 Ma, respectively). This age debunks the previously proposed classification as Cyprus-type. The stable isotope compositions of the hydrothermal fluids, including δ18OΗ2Ο, δDΗ2Ο, δ13CCO2, δ30SiNBS, point to mixing between subduction related hydrothermal fluids, seawater and seafloor turbidites, which accords well with the proposed classification. The δ34SH2S and δ57Fe values confirm a magmatic origin of sulfur and leaching of the footwall volcaniclastic rocks with contributions from seawater and turbidites. Radiogenic isotope and trace element geochemistry indicate that the Ermioni “Mafic-Pelitic” VMS was formed at a partly sediment-covered, upper Cretaceous back-arc spreading center above a retreating subduction zone setting within the “Adheres Melange unit”. The Ermioni VMS is the only known upper Cretaceous sulfide ore identified in Greece. The findings of this work may be used in expanding future research on the metallogenic potential of Greece.

Multiphase solid inclusions reveal the origin and fate of carbonate-silicate melts in metasomatised peridotite

Orogenic peridotite in ultrahigh-pressure ultrahigh-temperature terranes provides increasingly strong evidence of material transfer in a subduction-zone setting, causing the metasomatism and melting of mantle rocks. Multiphase solid inclusions (MSI) trapped in garnet can provide clues to these processes occurring at great depth. The MSI enclosed in garnet in pyroxenite, lherzolite and harzburgite from a drillcore in the Saxothuringian basement in the Variscan Bohemian Massif are dominated by hornblende, barian mica and carbonate (dolomite and magnesite), and they contain a large variety of other phases, including spinel, clinopyroxene, orthopyroxene, garnet II, apatite, monazite, thorianite, graphite, norsethite and barian feldspar, scheelite and sulfides (e.g., pentlandite). We demonstrate, that the bulk chemical composition of the MSI evolves from the MSI trapped in garnet pyroxenite, which was the source of the metasomatising liquids, to the MSI in the host peridotite. Garnet pyroxenite has a carbonate-silicate composition comparable to kimberlite, and represents a rare example of a low-degree supercritical melt derived from a mixed crust-mantle source frozen in the mantle. Its high-pressure fractional crystallization produced a residual melt, enriched in alkalies (Na, K), highly incompatible elements (LILE – Ba, Sr; Th, U), LREE, Ti, W and volatiles (CO2, Cl, F, P), which is now trapped in the MSI in pyroxenite garnet. The MSI in peridotite reflect changes in the physical and chemical characteristics of the metasomatising medium during its reactive infiltration into peridotite through silicate crystallization and interaction with mantle minerals reflecting the type of rock (lherzolite vs. harzburgite). A change from a more silicic, solute-rich melt to a more diluted carbonate-rich liquid gradually enriched in LILE (K, Ba) and volatiles (CO2, Cl) and LREE fractionation with an increasing degree of fluid evolution coincides with the evolution of kimberlitic melts to carbonatitic melts. The low-viscosity carbonate-rich liquids can migrate far and metasomatise a considerable volume of the rock. Our work demonstrates that the MSI trapped in garnet represent a unique tool to trace the chemical and physical characteristics of the liquids that metasomatise the mantle wedge and their evolution.

Porphyry Cu-Au prospectivity modelling using semi-supervised learning algorithm in Dehsalm district, eastern Iran

Porphyry Cu-Au prospectivity modelling using semi-supervised learning algorithm in Dehsalm district, eastern Iran

High sulfur solubility in subducted sediment melt under both reduced and oxidized conditions: With implications for S recycling in subduction zone settings

The relative enrichment of sulfur (S) observed in arc magmas when compared to MORB, reflects the addition of slab-derived S to the mantle wedge source region. However, the mechanisms and efficiency of such S recycling remain poorly constrained. In this study, sediment melting experiments have been conducted using a synthetic pelite starting composition containing ∼7 wt% H2O and ∼1.9 wt% S, at 3 GPa, 1050 °C and variable oxygen fugacity (fO2), to investigate the effect of fO2 on S solubility in sediment melts. To assess temperature and concentration effects, selected experiments were repeated either at lower temperatures of 950 °C and 1000 °C, or with a higher bulk S content of ∼4 wt%. All experiments produced hydrous rhyolitic melts, saturated with either pyrrhotite under reduced conditions or anhydrite under oxidized conditions. For 3 GPa, 1050 °C experiments, the sulfur content at sulfide saturation (SCSS) in melt is found to increase with decreasing fO2, from ∼200 ppm at FMQ-0.5 to ∼1900 ppm at FMQ-7.5. The highest S solubility is achieved at FMQ + 1.6 where melt is saturated with both anhydrite and pyrrhotite. The sulfur content at sulfate saturation (SCAS) decreases from ∼2600 ppm at FMQ + 1.6 to ∼2000 ppm at FMQ + 7. Increasing either bulk S content or temperature produces a positive effect on SCSS and SCAS. Raman spectra of our experimental melts show that S exists as H2S/HS− under reduced conditions and as SO42− under oxidized conditions. The solubility minimum, i.e., the onset of transition from S2− to S6+ is estimated to occur at ∼FMQ, with full transition to S6+ by ∼FMQ + 2. While the SCAS values are in good agreement with previous reports, the distinct increase of SCSS with decreasing fO2 (when fO2 < FMQ) has not been observed in previous slab melting experiments. Furthermore, we report for the first time that dissolution of H2S and SO2 in hydrous rhyolitic melt follows the Fincham-Richardson relationship; and propose the definition of a hydro-sulfide capacity as CHS = [HS]*(fO2/fS2)1/2; where [HS] is the concentration of S in melt (in ppm) dissolved as HS− and H2S. SCSS for H2S dissolution can therefore be modeled using CHS in an analogous fashion to modeling SCSS for anhydrous melts using the sulfide capacity (CS2−), with the relation ln[HS]SCSS=-ΔGFes - FeO°/RT+lnCHS-lnaFeOmelt+lnaFeSsulfide. As predicted by such a model framework, we indeed observe a linear correlation between logSCSS and logXFeO (the mole fraction of FeO in melt) with a slope close to −1, i.e., SCSS experiences a sharp increase when FeO in melt falls below 1 wt%. Therefore, both our experimental results and model predictions suggest hydrous low-Fe rhyolitic melt produced by sediment melting under reduced conditions has the required S solubility to account for the relative enrichment of S observed in arc magmas.

Seismic Anisotropy in Subduction Zones: Evaluating the Role of Chloritoid

Subduction zones are often characterized by the presence of strong trench-parallel seismic anisotropy and large delay times. Hydrous minerals, owing to their large elastic anisotropy and strong lattice preferred orientations (LPOs), are often invoked to explain these observations. However, the elasticity and the LPO of chloritoid, which is one of such hydrous phases relevant in subduction zone settings, are poorly understood. In this study, we measured the LPO of polycrystalline chloritoid in natural rock samples, obtained the LPO-induced seismic anisotropy, and evaluated the thermodynamic stability field of chloritoid in subduction zones. The LPO of chloritoid aggregates displayed a strong alignment of the [001] axes subnormal to the rock foliation, with a girdle distribution of the [100] axes and the (010) poles subparallel to the foliation. New elasticity data of single-crystal chloritoid showed a strong elastic anisotropy of chloritoid with 47% for S-waves (VS) and 22% for P-waves (VP), respectively. The combination of the LPO and the elastic anisotropy of the chloritoid aggregates produced a strong S-wave anisotropy with a maximum AVS of 18% and a P-wave anisotropy with an AVP of 10%. The role of chloritoid LPO in seismic anisotropy was evaluated in natural rock samples and a hypothetical blueschist. Our results indicate that the strong LPO of chloritoid along the subduction interface and in subducting slabs can influence the trench-parallel seismic anisotropy in subduction zones with “cold” geotherms.

PETROLOGY OF THE CRUSTAL PLUTONIC ROCKS OF NAWEOBA BLOCK, ZHOB OPHIOLITE, BALOCHISTAN, PAKISTAN

Zhob Ophiolite complex is composed of three detached blocks named Omzha, Ali Khanzai and Naweoba blocks. The crustal plutonic section of the Naweoba block is mapped and divided into gabbro and granite. Based on petrographical studies, the gabbros fell in the domain of gabbro, gabbronorite, and hornblende gabbro while granitic rocks fell in the vicinity of quartz-rich granitic rocks, granodiorite, plagiogranite and tonalite. Gabbroic rocks cover the maximum area of the crustal plutonic section and are usually medium-grained while at many places the grain size is quite large to be seen with naked eyes. Minerlogically gabbroic rocks consist of orthopyroxene, clinopyroxene, amphibole and plagioclase. These rocks maybe the fragments of main crustal plutonic section of the Zhob ophiolite. The granitic rocks having mafic minerals dominted in the eastern portion, while the felsic minerals dominted ones are in the west. The eastern side of the granitic body is compact and massive compared to western portion which is quite altered and shattered. Granitic rocks are composed of plagioclase, alkali feldspar and quartz where rutile and Cr-spinel exist in trace amounts. The gabbros of Naweoba block may have formed in a magma chamber as a result of fractional crystallization. While the granites maybe a late magmatic differentiate from the same magma chamber. The close correlation of gabbroic and granitic rocks of Naweoba block with Muslim Bagh, Khanozai and Bela ophiolites suggests their formation in supra subduction zone setting.

Imaging of a magma system beneath the Merapi Volcano complex, Indonesia, using ambient seismic noise tomography

SUMMARYMt Merapi, which lies just north of the city of Yogyakarta in Java, Indonesia, is one of the most active and dangerous volcanoes in the world. Thanks to its subduction zone setting, Mt Merapi is a stratovolcano, and rises to an elevation of 2968 m above sea level. It stands at the intersection of two volcanic lineaments, Ungaran–Telomoyo–Merbabu–Merapi (UTMM) and Lawu–Merapi–Sumbing–Sindoro–Slamet, which are oriented north–south and west–east, respectively. Although it has been the subject of many geophysical studies, Mt Merapi's underlying magmatic plumbing system is still not well understood. Here, we present the results of an ambient seismic noise tomography study, which comprise of a series of Rayleigh wave group velocity maps and a 3-D shear wave velocity model of the Merapi–Merbabu complex. A total of 10 months of continuous data (October 2013–July 2014) recorded by a network of 46 broad-band seismometers were used. We computed and stacked daily cross-correlations from every pair of simultaneously recording stations to obtain the corresponding inter-station empirical Green's functions. Surface wave dispersion information was extracted from the cross-correlations using the multiple filtering technique, which provided us with an estimate of Rayleigh wave group velocity as a function of period. The group velocity maps for periods 3–12 s were then inverted to obtain shear wave velocity structure using the neighbourhood algorithm. From these results, we observe a dominant high velocity anomaly underlying Mt Merapi and Mt Merbabu with a strike of 152°N, which we suggest is evidence of old lava dating from the UTMM double-chain volcanic arc which formed Merbabu and Old Merapi. We also identify a low velocity anomaly on the southwest flank of Merapi which we interpret to be an active magmatic intrusion.

An age-integrated geochemical and computational phase-equilibria study from the Wangtu Gneissic Complex, N-W Himalaya, and its paleogeographic implications in Columbia assembly

The Paleoproterozoic thermal evolution of the north-Indian continental margin is debatable as the Lesser Himalaya granites are interpreted either as a subduction-zone volcanic arc or rift-related magmatism during Columbia assembly or disintegration processes, respectively. Integrated mineralogical, geochemical investigations, temperature estimates from Ti solubility in biotite and zircon, and computational phase equilibria modeling from the Wangtu Gneissic complex (WGC), Himachal Himalaya reveal peraluminous nature for most of the WGC rocks that crystallize at ~650 °C and ~ 1.0–1.1 GPa. The U-Pb ages from magmatic zircons from the WGC exhibit two prominent age clusters at 1867 Ma and 2487 Ma. The U-Pb zircon data and modeled phase equilibria for metasedimentary rock reveal the generation of S-type peraluminous magma parental to the WGC, through melting of preexisting supracrustal rocks at ~1800 Ma, within ~850–900 °C and 1.1–1.2 GPa, similar to P-T conditions observed in modern-day subduction zone settings. The results obtained from this study reveal the north Indian continental margin was an active subduction zone during the Paleoproterozoic Columbia supercontinent assembly that extends across the Himalayas.

Halogen Bearing Amphiboles, Aqueous Fluids, and Melts in Subduction Zones: Insights on Halogen Cycle From Electrical Conductivity

AbstractAmphiboles are hydrous minerals that are formed in the oceanic crust via hydrothermal alteration. The partial substitution of halogens for OH− makes amphibole one of the principal hosts of Cl and F in the subducting slab. In this study, we investigated the electrical conductivity of a suite of halogen bearing amphibole minerals at 1.5 GPa up to 1,400 K. The discontinuous electrical behavior indicates dehydration of amphibole at ∼915 K. This is followed by dehydration induced hydrous melting at temperatures above 1,070 K. We find that the released aqueous fluids have an electrical conductivity of ∼0.1 S/m. This high electrical conductivity is likely to explain anomalously high electrical conductivity observed in certain subduction zone settings. This high electrical conductivity of an order of magnitude greater than the electrical conductivity of pure aqueous fluids at similar conditions is likely due to the partitioning of the F and Cl into the aqueous fluids. We also noted that subsequent to the dehydration, secondary phases form due to the breakdown of the primary halogen bearing amphibole. Chemical analyses of these secondary phases indicate that they are repositories of F and Cl. Hence, we infer that upon dehydration of the primary halogen bearing amphibole, first the F and Cl are partitioned into the aqueous fluids and then the halogens are partitioned back to the secondary mineral phases. These secondary minerals are likely to transport the halogen to the deep Earth and may in part explain the halogen concentration observed in ocean island basalt.

On the cratonization of the Arabian-Nubian Shield: Constraints from gneissic granitoids in south Eastern Desert, Egypt

The Shaitian granite complex (SGC) spans more than 80 Ma of crustal growth in the Arabian–Nubian Shield in southeast Egypt. It is a voluminous composite intrusion (60 km2) comprising a host tonalite massif intruded by subordinate dyke-like masses of trondhjemite, granodiorite and monzogranite. The host tonalite, in turn, encloses several, fine-grained amphibolite enclaves. U-Pb zircon dating indicates a wide range of crystallization ages within the SGC (800 ± 18 Ma for tonalites; 754 ± 3.9 Ma for trondhjemite; 738 ± 3.8 Ma for granodiorite; and 717 ± 3.2 Ma for monzogranite), suggesting crystallization of independent magma pulses. The high positive εNdi (+6–+8) indicate that the melting sources were dominated by juvenile material without any significant input from older crust. Application of zircon saturation geothermometry indicates increasing temperatures during the generation of melts from 745 ± 31 °C for tonalite to 810 ± 25 °C for trondhjemite; 840 ± 10 °C for granodiorite; and 868 ± 10 °C for monzogranite. The pressure of partial melting is loosely constrained to be below the stability of residual garnet (<10 kbar) as inferred from the almost flat HREE pattern ((Gd/Lu)N = 0.9–1.1), but >3 kbar for the stability of residual amphibole as inferred from the significantly lower NbN and TaN compared with LREEN and the sub-chondrite Nb/Ta ratios exhibited by the granitic phases. The inverse relation between the generation temperatures and the ages estimates of the granitoid lithologies argue against a significant role of fractional crystallization. The major and trace element contents indicate the emplacement of the SGC within a subduction zone setting. It lacks distinctive features for melt derived from a subducted slab (e.g. high Sr/Y and high (La/Yb)N ratios), and the relatively low MgO and Ni contents in all granite phases within the SGC suggest melting within the lower crust of an island arc overlying a mantle wedge. Comparison with melts produced during melting experiments indicates an amphibolite of basaltic composition is the best candidate as source for the tonalite, trondhjemite and granodiorite magmas whereas the monzogranite magma is most consistent with fusion of a tonalite protolith. Given the overlapping Sm-Nd isotope ratios as well as several trace element ratios between monzogranite and tonalite samples, it is reasonable to suggest that the renewed basaltic underplating may have caused partial melting of tonalite and the emplacement of monzogranite melt within the SGC. The emplacement of potassic granite (monzogranite) melts subsequent to the emplacement of Na-rich granites (tonalite-trondhjemite-granodiorite) most likely suggests major crustal thickening prior arc collision and amalgamation into the over thickened proto-crust of the Arabian-Nubian shield. Eventually, after complete consolidation, the whole SGC was subjected to regional deformation, most probably during accretion to the Saharan Metacraton (arc–continent collisions) in the late Cryogenian -Ediacaran times (650–542 Ma).

Petrology and geochronology of the Yushigou nephrite jade from the North Qilian Orogen, NW China: Implications for subduction-related processes

The North Qilian Orogen in NW China hosts several commercial quality serpentine jade deposits, although nephrite jade has not so far been reported from this region. Here, we investigate a green nephrite (tremolite) jade from Yushigou ophiolite suite in the North Qilian orogen. We present results from petrographic, scanning electron microscopy (SEM), X-ray diffraction, Electron microprobe analyses (EMPA) and zircon U-Pb geochronological studies to characterize the nephrite jade and its genetic setting. Our results suggest that the high-quality green nephrite jade is predominately composed of tremolite and actinolite, with transformation to actinolite with increasing of Fe and Cr, together with minor chlorite, calcite, magnetite, and titanite. In contrast, the serpentine jade consists mainly of antigorite with minor magnesite, talc, calcite, magnetite, chromite, and apatite. Zircon U-Pb dating yields a lower intercept age of 500 ± 12 Ma, representing the formation age of the Yushigou ophiolite. A few young zircon grains yield a weighted mean age of 227.9 ± 5.3 Ma, which is interpreted to represent the crystallization age of the tremolite, in response to the tectonics associated with late Paleo-Tethys to early Neo-Tethys. Accordingly, we propose that the host serpentinites in this region formed within a Late Cambrian supra-subduction zone through water-rock interaction, involving Ca-rich and COH-bearing fluid systems. The Ca- or COH-bearing fluids released from mantle-derived magma, and then migrated to higher levels during late metamorphic-metasomatic processes in the Mesozoic, finally generating the nephrite jade. Moreover, our study also provides insights into the tectonic processes associated with the formation of jade within subduction zone settings.

Magmatic and Tectonic Setting of Archean Granitoids in the Southeastern Singhbhum Craton, India: Developing constraints with major and trace element geochemistry and geochronology

The Singhbhum craton is one of five Archean nuclei comprising Peninsular India. It is a composite Archean block that includes the Older Metamorphic Group, the Older Metamorphic Tonalite Gneisses, the Singhbhum Granite, and the Iron Ore Group as its major units. The ages of these components range from ~3.5 to ~3.1 Ga, although overlapping ages and similar rock types confound their genetic relationships. Plutonic felsic rocks from the southeastern Singhbhum craton (BK1: a foliated tonalite, KP1: a non-foliated granite, and SG14: a non-foliated granite) yield U-Pb (zircon) ages of 3321 ± 2 Ma (BK1), 3301 ± 1 Ma (KP1), and 3261 ± 1 Ma (SG14) that coincide with a pulse of Singhbhum Granite emplacement at 3.27 to 3.33 Ga. REE patterns and tectonic discrimination diagrams based on major and trace element ratios suggest a subduction zone setting for these rocks. We report major and trace element data for and compare them to previous works in order to characterize the Archean felsic plutonic history of the craton.