Subduction Zone Magmatism Research Articles

Episodic Fluid Action in Chinese Southwestern Tianshan HP/UHP Metamorphic Belt: Evidence from U–Pb Dating of Zircon in Vein and Host Eclogite

Fluid plays a key role in metamorphism and magmatism in subduction zones. Veins in high-pressure (HP) to ultrahigh-pressure (UHP) rocks are the products of fluid–rock interactions and can thus provide important constraints on fluid processes in subduction zones. In this study, we present an integrated study of zircon in situ U–Pb dating, trace element and mineral inclusion analysis for a complex vein and its host eclogite in the southwestern Tianshan UHP terrane, aiming to decipher the episodic fluid action during slab subduction and exhumation. Both zircon in eclogite and vein have euhedral, prismatic morphology similar to those crystallized from metamorphic fluid. Zircon in eclogite shows core–rim structures with distinct bounds and mineral inclusions. Zircon in the vein shows sector zoning or weak zoning, with bright rims around most zircon grains, which suggests recrystallization of the zircon crystals after their formation and multiple evolution of the vein. Eclogite zircon rims yield a weighted mean of 311 ± 3 Ma and cores yield a range from 413 ± 4 to 2326 ± 18 Ma, respectively. Vein zircon yields four groups of age (~355 Ma, ~337 Ma, ~315 Ma, and ~283 Ma), which date four episodes of fluid flow involving zircon growth. The first two groups of age may represent prograde epidote–amphibolite facies and amphibolite/blueschist facies metamorphism stage, respectively. The third group is similar to that of eclogite zircon rims, which is thought to date the eclogitic facie metamorphism (320–305 Ma), and the fourth group dates a later retrograde metamorphism after greenschist facies. The vein-forming fluid system was supposed to be an open system indicated by trace element of vein zircon and mineral assemblage of the vein. The coexistence of rutile, zircon, and garnet in prograde vein and the heavy rare earth elements (HREE) enrichment characteristic of vein zircon suggest that the vein-forming fluid are enriched in high field strength elements (HFSE) and HREE, and such fluid could be formed under low P–T conditions.

Tracing dehydration and melting of the subducted slab with tungsten isotopes in arc lavas

Tungsten is strongly incompatible during magmatic processes and is fluid mobile in subduction zones. Here we show that W isotope fractionation in arc lavas provide a powerful new tool for tracing slab dehydration and melting in subduction zones. Geochemically well characterized, representative arc-lavas from three subduction zones were chosen for this study to evaluate W isotope fractionation under different sub-arc conditions. Arc-lavas from SW Japan are produced by subducting a young, hot slab, and lavas from the volcanic front and rear arc of the Sangihe and Izu arcs are produced during subduction of a cold slab. The heaviest W isotope compositions (δ184W∼0.06‰) are observed in fluid-rich samples from the volcanic fronts of the Sangihe and Izu arcs. With increasing distance from the volcanic front, more melt-rich samples are characterized by progressively lighter W isotope compositions. Enriched alkali basalts from SW Japan, thought to be the product of mantle melting at a slab tear, and adjacent shoshonites have the lightest W isotope compositions (δ184W∼0‰). The correlation of W isotope fractionation with various indices of fluid release (e.g., Ce/Pb, Ba/Th) suggests that the heavy W isotope signatures record fluid recycling near the volcanic front due to dehydration of the subducted slab. Upon release of the heavy W, the residual slab preferentially retains isotopically light W, which is released during subsequent melting of drier lithologies in hot subduction zones, such as SW Japan. These data suggest that W isotopes can be used as a tracer of slab dehydration, potentially helping to determine the onset of cold subduction zone magmatism and hence, modern-style plate tectonics.

Experimental interaction of granitic melt and peridotite at 1.5 GPa: Implications for the origin of post-collisional K-rich magmatism in continental subduction zones

To decipher the genesis of post-collisional magmatism in continental subduction zones, we conducted a series of experiments on the interaction of granitic melt derived from continental crust with harzburgite at 1.5 GPa and temperatures ranging from 950 °C to 1250 °C. The two end-members were homogeneously mixed with different crustal melt/peridotite ratios and under water-saturated or water-undersaturated conditions. Interaction takes place in two stages: the initial reaction, followed by re-melting of the metasomatized product. In the first stage, crustal melt reacts with the harzburgite, consuming olivine and forming orthopyroxene and phlogopite. In the second stage, re-melting of the metasomatized product is dominated by the incongruent melting of both orthopyroxene and phlogopite, leading to increasing peritectic olivine production as melting progresses. Upon reaction with the peridotite and melting of the metasomatized product, initially granitic melts evolve in composition to trachytic and trachyandesitic. The crustal melt/peridotite ratio does not appear to affect the SiO2 content of melt that equilibrates with olivine and orthopyroxene, whereas the SiO2 content of the melt is higher when olivine is absent from the residue. K2O in the melt is buffered by coexisting phlogopite: this buffering effect of phlogopite weakens at higher crustal melt/peridotite ratio and in water-undersaturated conditions, leading to higher K2O in the melt. Trace element compositions of the hybrid melt generally inherit the characteristics of the crustal melt: most LILEs and transition elements are buffered by phlogopite, with Rb and V having the highest partition coefficients. Our experimental melts show compositional parallels with mantle-derived post-collisional magmas in the Variscan orogen, verifying the involvement of granitic continental crust melt in their source.

Petrology and Geochemistry of Mafic Intrusive Rocks from the Sapi-Shergol Ophiolitic Mélange, Indus Suture Zone, Western Ladakh: Constraints on Petrogenesis and Tectonic Setting

This article reports the existence of subduction-related tholeiitic, normal mid-ocean ridge basalt (N-MORB)-type mafic intrusives emplaced within the Sapi-Shergol ophiolitic mélange of the Indus Suture Zone, western Ladakh. The Shergol mafic intrusives show Fe enrichment with basalt to basaltic-andesite composition. Based on their mineralogy and textures, these mafic rocks can be identified as fine- to medium-grained gabbros that have undergone greenschist-grade metamorphism. These rocks have N-MORB-type geochemical characteristics, exhibiting nearly flat to depleted light rare earth element patterns ((La/Sm)N=0.66–1.05). Petrogenetic modeling suggests <20% partial melting of a depleted MORB-type mantle source, within the spinel peridotite stability. The presence of slightly negative anomalies of high field strength elements like Nb, Zr, and Ti in multielement patterns reflect the influence of subduction zone magmatism. The presence of low-Ti clinopyroxene (En38–50Fe11–25Wo31–43; enstatite-ferrosilite-wollastonite), Ca-rich plagioclase (An2–36; andesine), and pargasitic amphibole also reflects their subduction-related depleted-mantle origin. The MORB–island arc tholeiite signature displayed by the Shergol gabbros intrusive in Shergol peridotites reflect their generation in a mantle wedge associated with the Early Cretaceous intraoceanic subduction within the Neo-Tethys Ocean. They are similar to Spongtang ophiolite gabbros intrusive in the Spongtang ophiolite mantle peridotites from south Ladakh, ophiolitic mélange gabbros from southern Tibet, and Muslim Bagh ophiolite gabbros from Pakistan. Based on this study, we offer a geodynamic model suggesting that the Sapi-Shergol ophiolitic slice was intruded by mafic intrusive rocks that represent the relict of the intraoceanic substratum of the Cretaceous Dras arc complex.

Crustal maturation through chemical weathering and crustal recycling revealed by Hf–O–B isotopes

Juvenile continental crust is dominantly formed at intra-oceanic arcs via subduction zone magmatism. However, it remains unclear how basaltic oceanic arcs convert to granodioritic continental compositions. Here we present zircon U–Pb and Hf–O isotope data, whole-rock B isotope compositions, combined with a synthesis of over 1100 geochemical analyses from magmatic rocks that span a wide range of emplacement ages (∼490–270 Ma), from the Junggar intra-oceanic arc of the southern Central Asian Orogenic Belt (CAOB). Geochemical data show that the Junggar evolved from a juvenile oceanic arc composition (low SiO2, K2O and Rb, and high MgO contents) to one closer to continental crust (high SiO2, K2O and Rb, and low MgO contents) at ca. 300 Ma. All samples show very high and uniform Hf isotope ratios with εHf(t) values from +10.6 to +14.4. Zircon δ18O values and whole-rock δ11B values, however, are highly variable. The Silurian – Carboniferous (pre-300 Ma) rocks have distinctly lower zircon δ18O values ranging from 5.16 to 6.72‰ with an average of 5.78‰, and high δ11B values (−7.5–+12.2‰), suggesting that they were derived from the asthenospheric mantle wedge, which supports the genesis of primitive intra-oceanic crust during that time interval. In contrast, the Early Permian (post-300 Ma) rocks display much higher zircon δ18O values (8.24 to 10.29‰) and lower δ11B values (−9.0 to −12.2‰), combined with the presence of Carboniferous inherited zircons, which requires a source component comprising young, weathered, volcanogenic sediments from the Carboniferous Junggar intra-oceanic arc. The Early Permian rocks were produced by low degrees of partial melting of all sources (ca. 10%) and element and Hf–O–B isotope mixing calculations indicate a contribution of more than ∼50% from weathered sediments in those sources. Thus, the evolution of the Junggar segment from a primitive basaltic intra-oceanic arc toward a young continent was related to the recycling of crustal residues of chemical weathering and the younger magmatism was generated by crustal melting of these weathering products after they were buried to lower crustal depths. Our study highlights that an intra-oceanic arc's own chemical weathering history promotes its transformation into continental crust during collisional events and clarifies the relationship between continental crust formation and intra-oceanic arcs through time.

Mineralogy and Geochemistry Variation of Igneous Rocks from Ungaran and Muria Volcano and Its Processes related to Subduction Zone Magmatism of Sunda Arc

Sunda Arc represent Quaternary magmatism as one of the most significantly active volcanic arc in Indonesia resulted from subduction of oceanic plate beneath continental plate. Along this subduction system, there are systematic variations in subduction angle, depth, crust thickness, and composition contribute to geochemical characteristic of igneous rocks along and across the arc. This study compared mineralogy and geochemistry characteristics from two volcanoes which represent across-arc variation. Petrographic analysis have been conducted on rocks’ thin section samples from Ungaran and Muria Volcano to characterize mineralogy of rocks. Selected samples were analyzed using X-Ray Fluorescence (XRF) for geochemistry of major and trace elements. Mineralogy of Ungaran rocks are dominated by plagioclase, clinopyroxene, and hornblende phenocrysts with composition range from andesite to basaltic andesite of high-K calc alkaline series. Samples from Muria Volcano are from alkaline series characterized by feldspathoid and alkali feldspar rich rocks with calcic plagioclase and pyroxene phenocrysts. This mineral assemblages, supported by major elements data suggest fractionation process in magma. Geochemical trend is systematically consistent with other island arc setting, where there is significant increase of total alkali from Ungaran to Muria. Trace element data shows decreasing in LILE/HFSE ratio further from arc. This suggests that Ungaran has more crustal and subducted slab influence, while Muria has higher degree of partial melting from deeper Benioff Zone.

Geochemical characteristics of submarine rhyolitic pumice from Andaman subduction zone: Inferences on magmatism and tectonics of north‐eastern Indian Ocean

This paper reports new geochemical data for submarine volcanic pumice sampled from two volcanoes (SM‐1 and SV‐1) which are part of the Andaman subduction system. SM‐1 volcano is located closer to the Andaman back‐arc spreading axis while SV‐1 volcano is part of a linear chain of arc volcanoes in the southern Andaman Basin. The pumice are products of phreatomagmatic eruptions and are highly vesicular in nature; however, immediately after eruption of these lava froth, sea water entering into the vesicles made them sink to the sea floor. A sheet of dull grey to white‐coloured pumice observed on top of the SM‐1 volcano indicates a nonexplosive eruption referred to as Tangoroan type. Pumice from these two spatially apart volcanoes share uniform physical properties but display minor variations in mineralogical compositions and distinct characteristics of selected major and trace elements. Pumice from SM‐1 comprised plagioclase, hornblende, and spinel while those from SV‐1 are composed of plagioclase and quartz as main crystalline phases. Geochemically, the pumice samples from SM‐1 volcano are classified as medium‐K calc‐alkaline rhyolites, while those from SV‐1 volcano show low‐K tholeiitic compositions. Trace and REE compositions for these rhyolitic pumice collectively reflect LILE‐LREE‐enriched, HFSE‐depleted signatures of subduction zone magmatism and implicate an oceanic arc tectonic affinity. Selective enrichment in LILE and LREE with relative HFSE depletion is attributed to infiltration of fluid‐mobile LILE and LREE into the mantle via fluids released from slab dehydration with retention of fluid‐immobile HFSE in the subducted slab. Elevated abundances of LILE/HFSE, LREE/HFSE, and LREE/HREE for the medium‐K calc‐alkaline rhyolitic pumice relative to their low‐K tholeiitic counterparts invoke increased fluid flux into the mantle with progressive maturation of oceanic slab subduction. The precursor magmas parental to the SM‐1 and SV‐1 volcanic pumice were derived by partial melting of a mantle wedge metasomatized by variable slab–mantle interactions, and influx of slab‐dehydrated fluids and sediments. Pronounced REE fractionation trends suggest evolution of resultant melts in a cold, hydrous, oxidizing regime of intraoceanic arc setting that eventually reheated, remelted, remobilized, and assimilated lower oceanic arc crust and erupted as rhyolites. The volcanic pumice from Andaman are geochemically and tectonically analogous to those from Mariana arc and Okinawa Trough of Pacific Ocean.

Proterozoic Magnesian Feldspathic Magmatism in the Nallamalai Fold Belt, Southern India – Petrogenesis of Vellaturu-Ipuru-Nakerikallu (VIN) Arc Magmatic Granites, Eastern Dharwar Craton

ABSTRACT Through extensive field studies substantiated by detailed petrographic, geochemical and mineral chemistry studies, a significant event of “Proterozoic magnesian feldspathic magmatism” is reported from the SE part of the Indian subcontinent. Three major granitoids i.e. Vellaturu, Ipuru and Nakerikallu (VIN) are emplaced along an NNE-SSW axis in the Proterozoic Nallamalai fold belt (NFB) in the Eastern Dharwar craton (EDC). The axis of emplacement of VIN granites represents a tectonic contact that is juxtaposed along the NE fringes of NFB and adjoining Precambrian accretionary terrain comprising of gneisses and amphibolites with intrusive granites in the northern part of the Nellore schist belt (NSB). Petrographically the VIN granitoids are biotite ± hornblende bearing with apatite and zircon as accessory phases. Exsolution texture is observed in the form of microcline microperthite intergrowth. In IUGS, QAP diagram the VIN granitoids essentially fall in granite - granodiorite field. Presence of mafic rich microgranitoid enclaves (MMEs) are prominently noticed in the VIN granitoids indicating magma mingling. These granitoids are deformed along their margins. Enclaves of quartzite of NFB and older amphibolites indicate the intrusive nature of VIN granitoids. Major oxide analyses indicate that the Mg# ranges from 43 to 56. Rare earth element studies reveal a general enrichment of LREE relative to HREE. Incompatible HFSE element studies indicate a significant Nb and Ti negative anomaly; characteristic of subduction zone magmatism. In Y + Nb vs Rb trace element tectonic discrimination diagram the granitic rocks of Nakerikallu, Ipuru and Vellaturu essentially fall in post-collisional granites (post-COLG) field. Based on the remarkable magnesian feldspathic character coupled with their emplacement in an arc magmatic tectonic setting, it is concluded that the VIN granitoids in the Proterozoic NFB, SE India; mark a significant event of “Proterozoic magnesian feldspathic magmatism” in the EDC.

Identifying the ingredients of hydrous arc magmas: insights from Mt Lamington, Papua New Guinea.

Volcanism is the surface expression of magma intrusion, crystallization, assimilation and hybridization processes operating throughout the crust over a range of time periods. Many magmas, including those erupted at subduction zones, have complex textures that reflect these processes. Here, we use textural and geochemical characteristics of calcic amphiboles to help identify multiple ingredients of subduction zone magmatism at Mt Lamington volcano, Papua New Guinea. Our approach uses existing trace element partitioning schemes to calculate the compositions of amphibole equilibrium melts (AEMs). The AEM compositions show that Mt Lamington andesites and plutonic enclaves are dominated by fractionation of amphibole + plagioclase + biotite, with assimilation of plagioclase and zircon. Magnesiohastingsite crystals in the andesite and diktytaxitic mafic enclaves reflect multiple episodes of recharge by more primitive, geochemically variable melts. The andesite also contains clots with rounded grains and melt on grain boundaries. These features indicate slow crystallization, and the retention of melt films could significantly enhance the potential for remobilization of crystals by infiltrating melts or during magma mixing. Variations in crystallization conditions could thus significantly affect the mush microstructure. We suggest that this could result in a significant bias of the volcanic record towards the preferential incorporation of more slowly cooled plutonic material from the lower crust or from more thermally mature plumbing systems. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Petrogenesis of carbonatites in the Luliangshan region, North Qaidam, northern Tibet, China: Evidence for recycling of sedimentary carbonate and mantle metasomatism within a subduction zone

Carbonatitic magmatism in subduction zones provides extremely valuable information on the cycling, behavior and storage of deep carbon within the Earth. It may also shed light on insights into crust-mantle interaction and mantle metasomatism within subduction zones. Origin of carbonatite has long been debated: all hypotheses need to reflect the different mineral assemblages and geochemical compositions of carbonatites and their diverse tectonic settings. Here we present a petrological, geochronological, geochemical and isotopic study of carbonatite bodies associated with orogenic peridotites, which occur as stocks or dykes with widths of tens to hundreds of meters in the Luliangshan region, North Qaidam, northern Tibet, China. On the basis of modal olivine (Ol) content, the studied samples were subdivided into two groups: Ol-poor carbonatite and Ol-rich carbonatite. Zircon grains from the Ol-poor carbonatite show detrital features, and yield a wide age spectrum between 400 Ma and 1000 Ma with a pronounced peak at ca. 410–430 Ma. By contrast, oscillatory zoned zircons and inherited cores show two relatively small Neoproterozoic age peaks at ca. 920 and 830 Ma. Zircon grains from the Ol-rich carbonatite sample are also distributed in a wide spectrum between 400 and 1000 Ma, with a pronounced peak at ca. 440 Ma and a slightly inferior peak at ca. 410 Ma. The oscillatory zoned zircons and inherited cores exhibit a smaller Neoproterozoic age peak at ca. 740 Ma. The pronounced peaks ranging from 430 to 410 Ma are consistent with the deep subduction and mantle metasomatic events recorded in associated ultramafic rocks.Both groups of carbonatites are characterized by enrichment of light rare earth elements (LREEs) with high (La/Yb)N values and pronounced negative Eu anomalies. They show high 87Sr/86Sr values (0.708156–0.709004), low 143Nd/144Nd values (0.511932–0.512013) and high δ18OV-SMOW values (+17.9 to +21.3‰). This geochemical and isotopic evidence suggests that these carbonatites were derived from remobilized sedimentary carbonate rocks. We propose that the primary carbonatite magma was formed by partial melting of sedimentary carbonates with mantle contributions. Sedimentary carbonates were subducted into the shallow upper mantle where they melted and formed diapirs that moved upwards through the hot mantle wedge. The case presented provides a rare example of carbonatite originating from sedimentary carbonates with mantle contributions and relevant information on the mantle metasomatism within a subduction zone.

Geochemistry, U-Pb geochronology and Sr-Nd isotopes of the Neogene igneous rocks, at the Iju porphyry copper deposit, NW Shahr-e-Babak, Iran

The Iju porphyry copper deposit is located in the Kerman copper belt in the northwest of Shahr-e-Babak city within the southern extension of the Urumieh-Dokhtar magmatic arc. Based on a cut-off grade of 0.15%, the deposit contains 24 million tonnes (Mt) of ore with an average grade of 0.4 wt% Cu. The igneous rocks at the Iju deposit are primarily made up of Upper Miocene sub-volcanic and volcanic sequences that intruded Eocene volcanic rocks. Sub-volcanic rocks comprise barren quartz-monzonite porphyry and syn-mineralized granodiorite porphyry along with andesitic volcanic rocks. Geochemical evidences show that they are co-genetic, medium- to high-potassium calc-alkaline, and likely part of a subduction zone I-type granitoid magma. These rocks are characterized by enrichment in LREEs relative to HREEs with negative Nb–Ta anomalies similar to subduction zone magmatism. All samples exhibit enriched uniform chondrite-normalized REE patterns without noticeable Eu anomalies similar to magnetite-series granitoids. The igneous rocks of the Iju deposit exhibit geochemical features similar to adakites. The U-Pb zircon dating shows 9.33 ± 0.19 Ma (Tortonian stage) for quartz-monzonite (oldest unit), 9.01 ± 0.20 Ma for granodiorite and 8.83 ± 0.19 Ma for andesite (youngest unit) unit. The initial 87Sr/86Sr and 143Nd/144Nd ratios and εNd(t) values for the quartz-monzonites are 0.704234, 0.512819 and 3.75, respectively, similar to the values for the granodiorite, which are 0.704278, 0.512813 and 3.64, respectively. In most discrimination diagrams, the Iju samples show characteristics of both oceanic slab-derived and delaminated/collisional thickened lower crust-derived adakites. The geochronological, geochemical and isotopic data for the igneous rocks of the Iju deposit indicate that after long term subduction of the Neo-Tethys oceanic crust beneath the central Iranian plate, continental collision between the Arabian and Eurasian plates during the Miocene-Pliocene led to crustal thickening, breaking-off of a subducted slab and development of an asthenospheric window, thereby facilitating partial melting of the subduction-contaminated sub-continental lithospheric mantle and thickened lower crust and generating the Cu-bearing adakites in the Iju region.

Thermal impact of magmatism in subduction zones

Magmatism in subduction zones builds continental crust and causes most of Earth's subaerial volcanism. The production rate and composition of magmas are controlled by the thermal structure of subduction zones. A range of geochemical and heat flow evidence has recently converged to indicate that subduction zones are hotter at lithospheric depths beneath the arc than predicted by canonical thermomechanical models, which neglect magmatism. We show that this discrepancy can be resolved by consideration of the heat transported by magma. In our one- and two-dimensional numerical models and scaling analysis, magmatic transport of sensible and latent heat locally alters the thermal structure of canonical models by ∼300 K, increasing predicted surface heat flow and mid-lithospheric temperatures to observed values. We find the advection of sensible heat to be larger than the deposition of latent heat. Based on these results we conclude that thermal transport by magma migration affects the chemistry and the location of arc volcanoes.

The importance of mantle wedge heterogeneity to subduction zone magmatism and the origin of EM1

The composition of the convecting asthenospheric mantle that feeds the mantle wedge can be investigated via rear-arc lavas that have minimal slab influence. This “ambient mantle wedge” composition (the composition of the wedge prior to the addition of a slab component) varies substantially both worldwide and within individual arcs. 143Nd/144Nd measurements of rear-arc samples that have minimal slab influence are similar to 143Nd/144Nd in the stratovolcanoes of the adjacent volcanic fronts, suggesting that 143Nd/144Nd of arc-front volcanics are largely inherited from the ambient mantle composition. 143Nd/144Nd correlates with ratios such as Th/U, Zr/Nb, and La/Sm, indicating that these ratios also are strongly influenced by ambient wedge heterogeneity. The same phenomenon is observed among individual volcanoes from the Chilean Southern Volcanic Zone (SVZ), where along-strike variability of the volcanic front tracks that of rear-arc monogenetic volcanics. Depleted mantle wedges are more strongly influenced by slab-derived components than are enriched wedges. This leads to surprising trace element correlations in the global dataset, such as between Pb/Nb and Zr/Nb, which are not explicable by variable compositions or fluxes of slab components. Depleted ambient mantle is present beneath arcs with back-arc spreading; relatively enriched mantle is present adjacent to continents. Ambient mantle wedge heterogeneity both globally and regionally forms isotope mixing trajectories for Sr, Nd and Hf between depleted mantle and EM1-type enriched compositions as represented by Gough Island basalts. Making use of this relationship permits a quantitative match with the SVZ data. It has been suggested that EM1-type mantle reservoirs are the result of recycled lower continental crust, though such models do not account for certain trace element ratios such as Ce/Pb and Nb/U or the surprisingly homogeneous trace element compositions of EM1 volcanics. A model in which the EM1 end-member found in continental arcs is produced by low-degree melt-metasomatism of the sub-continental lithospheric mantle may be more plausible. The 143Nd/144Nd maximum along the SVZ may be a consequence of either rifting and collision of two ancient lithospheric domains or a slab tear. The correspondence of mantle wedge variations with EM1 suggests a potential role for metasomatized sub-continental lithosphere in creating EM1 sources globally.

Crustal basement controls granitoid magmatism, and implications for generation of continental crust in subduction zones: A Sr–Nd–Hf–O isotopic study from the Paleozoic Tongbai orogen, central China

Ascertaining the petrogenesis of granitoid rocks in subduction zones holds the key for understanding the processes of how continental crust is produced. The synchronous Taoyuan and Huanggang plutons occur in two different geological units of the Paleozoic Tongbai orogen of central China. They provide an optimal opportunity for a study to address the role of the crustal basement in generating voluminous granitoid magmatism in subduction zones. The Taoyuan and Huanggang plutons have identical U–Pb zircon crystallization ages of 440–444Ma, which are temporally related to northward subduction of the Paleotethyan Ocean. The Taoyuan samples show high SiO2 (73.36–79.16%) and low Al2O3 (12.00–13.45%) contents, Mg numbers (20.6–38.2), and Sr/Y (2.04–10.1) and (La/Yb)N (2.34–7.32) ratios with negative Eu anomalies (Eu/Eu*=0.33–0.93). They yielded positive εNd(t) (+3.0 to +6.7) and εHf(t) (+11.8 to +13.2) values, elevated initial Sr isotopic ratios (0.7040–0.7057) and relatively low zircon δ18O values of 4.62–5.39‰. These suggest that they were produced through partial melting of hydrothermally altered lower crust of the accreted Erlangping oceanic arc. In contrast, the Huanggang samples exhibit variable whole-rock geochemical and isotopic compositions with SiO2 contents of 57.01–64.42wt.%, initial Sr isotopic ratios of 0.7065–0.7078, and εNd(t) values of −5.7 to −9.4. Additionally, they have high zircon δ18O values of 7.57–8.45‰ and strongly negative zircon εHf(t) values of −14.4 to −10.5. They were suggested to have been mainly derived from ancient continental crust of the Kuanping crustal unit with the addition of 20–40% juvenile, mantle-derived material. Accordingly, the granitoids in both oceanic and continental arcs are likely to be mainly derived from intracrustal melting of their crustal basement. It is revealed by the Huanggang pluton that little net continental crust growth occurs in continental arcs, and addition of new volume of continental crust may be balanced by the loss of old mafic lower continental crust. Continental growth was likely achieved through the accretion and subsequent differentiation of oceanic arcs, as manifested by the Taoyuan pluton.

Subduction Zone Magmatism at Depths to 2900km : an Overview

Subduction Zone Magmatism at Depths to 2900km : an Overview

Continental basalts record the crust-mantle interaction in oceanic subduction channel: A geochemical case study from eastern China

Continental basalts, erupted in either flood or rift mode, usually show oceanic island basalts (OIB)-like geochemical compositions. Although their depletion in Sr-Nd isotope compositions is normally ascribed to contributions from the asthenospheric mantle, their enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) is generally associated with variable enrichments in the Sr-Nd isotope compositions. This indicates significant contributions from crustal components such as igneous oceanic crust, lower continental crust and seafloor sediment. Nevertheless, these crustal components were not incorporated into the mantle sources of continental basalts in the form of solidus rocks. Instead they were processed into metasomatic agents through low-degree partial melting in order to have the geochemical fractionation of the largest extent to achieve the enrichment of LILE and LREE in the metasomatic agents. Therefore, the mantle sources of continental basalts were generated by metasomatic reaction of the depleted mid-ocean ridge basalts (MORB) mantle with hydrous felsic melts. Nevertheless, mass balance considerations indicate differential contributions from the mantle and crustal components to the basalts. While the depleted MORB mantle predominates the budget of major elements, the crustal components predominate the budget of melt-mobile incompatible trace elements and their pertinent radiogenic isotopes. These considerations are verified by model calculations that are composed of four steps in an ancient oceanic subduction channel: (1) dehydration of the subducting crustal rocks at subarc depths, (2) anataxis of the dehydrated rocks at postarc depths, (3) metasomatic reaction of the depleted MORB mantle peridotite with the felsic melts to generate ultramafic metasomatites in the lower part of the mantle wedge, and (4) partial melting of the metasomatites for basaltic magmatism. The composition of metasomatites is quantitatively dictated by the crustal metasomatism through melt-peridotite reaction at the slab-mantle interface in oceanic subduction channels. Continental basalts of Mesozoic to Cenozoic ages from eastern China are used as a case example to illustrate the above petrogenetic mechanism. Subduction of the paleo-Pacific oceanic slab beneath the eastern edge of Eurasian continent in the Early Mesozoic would have transferred the crustal signatures into the mantle sources of these basalts. This process would be associated with rollback of the subducting slab at that time, whereas the partial melting of metasomatites takes place mainly in the Late Mesozoic to Cenozoic to produce the continental basalts. Therefore, OIB-like continental basalts are also the product of subduction-zone magmatism though they occur in intraplate settings.

Effects of decarbonation on elemental behaviors during subduction-zone metamorphism: Evidence from a titanite-rich contact between eclogite-facies marble and omphacitite

In this paper, we show the effects of subducted carbonates on geochemical processes during subduction-zone metamorphism (SZM) through the study of an eclogite-facies marble coexisting with metabasite from the ultrahigh pressure metamorphic belt of the Chinese Western Tianshan orogen. Between the marble and metabasite is a titanite-rich contact resulting from fluid-facilitated metamorphic reactions between the two lithologies, and recording elemental changes of geodynamic significance. Because this titanite-rich contact is dominated by titanite (an important host for high field strength elements, HFSEs) without white micas (an important host for large ion lithophile elements, LILEs), HFSEs are largely conserved in titanite whereas LILEs are moved away. This observation emphasizes the potential significance of subducting carbonate in retaining HFSEs in the slab through the formation and stabilization of titanite, contributing to the characteristic “arc signature” unique to subduction-zone magmatism (i.e., high LILEs, low HFSEs). The implicit assumption in this interpretation is that the observed lithological assemblage represents residues of subducting oceanic crust that has undergone major episodes of dehydration. Subducted carbonates also have significant implications for the origin of mantle isotopic heterogeneity as revealed from oceanic basalts.

A Calcium-in-Olivine Geohygrometer and its Application to Subduction Zone Magmatism

High-precision electron microprobe analyses were obtained on olivine grains from Klyuchevskoy, Shiveluch and Gorely volcanoes in the Kamchatka Arc; Irazú, Platanar and Barva volcanoes of the Central American Arc; and mid-ocean ridge basalt (MORB) from the Siqueiros Transform. Calcium contents of these subduction zone olivines are lower than those for olivines from modern MORB, Archean komatiite and Hawaii. A role for magmatic H2O is likely for subduction zone olivines, and we have explored the suggestion of earlier workers that it has affected the partitioning of CaO between olivine and silicate melt. We provide a provisional calibration of DCaOOl/L as a function of magmatic MgO and H2O, based on nominally anhydrous experiments and minimally degassed H2O contents of olivine-hosted melt inclusions. Application of our geohygrometer typically yields 3–4 wt % magmatic H2O at the Kamchatka and Central American arcs for olivines having ∼1000 ppm Ca, which agrees with H2O maxima from melt inclusion studies; Cerro Negro and Shiveluch volcanoes are exceptions, with about 6% H2O. High-precision electron microprobe analyses with 10–20 μm spatial resolution on some olivine grains from Klyuchevskoy and Shiveluch show a decrease in Ca content from the core centers to the rim contacts, and a sharp increase in Ca in olivine rims. We suggest that the zoning of Ca in olivine from subduction zone lavas may provide the first petrological record of temporal changes that occur during hydration of the mantle wedge and dehydration during ascent, and we predict olivine H2O contents that can be tested by secondary ionization mass spectrometry analysis.

Paleoproterozoic arc basalt-boninite-high magnesian andesite-Nb enriched basalt association from the Malangtoli volcanic suite, Singhbhum Craton, eastern India: Geochemical record for subduction initiation to arc maturation continuum

The Singhbhum Craton of eastern India preserves distinct signatures of ultramafic-mafic-intermediate-felsic magmatism of diverse geodynamic affiliations spanning from Paleo-Mesoarchean to Proterozoic. Here we investigate the 2.25Ga Malangtoli volcanic rocks that are predominantly clinopyroxene- and plagioclase-phyric, calc-alkaline in nature, display basalt-basaltic andesite compositions, and preserve geochemical signatures of subduction zone magmatism. Major, trace and rare earth element characteristics classify the Malangtoli volcanic rocks as arc basalts, boninites, high magnesian andesites (HMA) and Nb enriched basalts (NEB). The typical LILE enriched-HFSE depleted geochemical attributes of the arc basalts corroborate a subduction-related origin. The boninitic rocks have high Mg# (0.8), MgO (>25wt.%), Ni and Cr contents, high Al2O3/TiO2 (>20), Zr/Hf and (La/Sm)N (>1) ratios with low (Gd/Yb)N (<1) ratio, TiO2, and Zr concentrations. The HMA samples are marked by moderate SiO2 (>54wt.%), MgO (>6wt.%), Mg# (0.47) with elevated Cr, Co, Ni and Th contents, depleted (Nb/Th)N, (Nb/La)N, high (Th/La)N and La/Yb (<9) ratio, moderate depletion in HREE and Y with low Sr/Y. The NEBs have higher Nb contents (6.3–24ppm), lower magnitude of negative Nb anomalies with high (Nb/Th)pm=0.28–0.59 and (Nb/La)pm=0.40–0.69 and Nb/U=2.8–34.4 compared to normal arc basalts [Nb=<2ppm; (Nb/Th)pm=0.10–1.19; (Nb/La)pm 0.17–0.99 and Nb/U=2.2–44 respectively] and HMA. Arc basalts and boninites are interpreted to be the products of juvenile subduction processes involving shallow level partial melting of mantle wedge under hydrous conditions triggered by slab-dehydrated fluid flux. The HMA resulted through partial melting of mantle wedge metasomatized by slab-dehydrated fluids and sediments during the intermediate stage of subduction. Slab-melting and mantle wedge hybridization processes at matured stages of subduction account for the generation of NEB. Thus, the arc basalt-boninite-HMA-NEB association from Malangtoli volcanic suite in Singhbhum Craton preserves the signature of a complete spectrum of Paleoproterozoic active convergent margin processes spanning from subduction initiation to arc maturation.

Fluid flow and water–rock interaction across the active Nankai Trough subduction zone forearc revealed by boron isotope geochemistry

Compositional changes, dehydration reactions and fluid flow in subducted sediments influence seismogenesis and arc magmatism in subduction zones. To identify fluid flow and water–rock interaction processes in the western Nankai Trough subduction zone (SW Japan) we analyzed boron concentration and boron isotope composition (δ11B) of pore fluids sampled across the subduction zone forearc from depths of up to ∼922m below seafloor during four Integrated Ocean Drilling Program (IODP) Expeditions. The major structural regimes that were sampled by coring include: (1) sedimentary inputs, (2) the frontal thrust zone, (3) the megasplay fault zone, and (4) the forearc basin. From mass balance consideration we find that consumption of boron (B) by ash alteration and desorption of B from the solid phase, mediated by organic matter degradation, produces a net decrease in B concentrations with depth down to ∼120μM and variable δ11B values in the range of ∼+20‰ and +49‰. Interstitial water in sediments on the incoming oceanic plate are influenced by more efficient mobilization of exchangeable B from the solid phase due to higher temperatures and alteration of the oceanic crust that acts as a sink for 10B. At the tip of the megasplay fault zone, elevated B concentration and B isotopic composition suggest that underthrust coarse-grained slope sediments provide a pathway for fluids out of the upper (<2km) accretionary prism. Silt and sand layers in the underthrust section of the downgoing plate favor fluid escape in seaward direction from depths equivalent to the temperature range of 60–150°C. At both locations the δ11B signature evolves during updip migration through re-adsorption. Mass balance considerations suggest a shallower fluid source depth compared to pore fluids sampled previously near the décollement zone along the central portion of the Nankai margin.