Forearc Basalts Research Articles

Covariation of Slab Tracers, Volatiles, and Oxidation During Subduction Initiation

AbstractSubduction‐related lavas have higher Fe3+/∑Fe than midocean ridge basalts (MORB). Hypotheses for this offset include imprint from subducting slabs and differentiation in thickened crust. These ideas are readily tested through examination of the time‐dependent evolution of slab‐derived signatures, thickening crust of the overriding plate, and evolving redox during subduction initiation. Here, we present Fe3+/ΣFe and volatile element abundances of volcanic glasses recovered from International Ocean Discovery Program (IODP) Expedition 352 to the Izu‐Bonin‐Mariana (IBM) forearc. The samples include forearc basalts (FAB) that are stratigraphically overlain by low‐ and high‐silica boninite lavas. The FAB glasses have 0.18–0.85 wt% H2O, 75–233 ppm CO2, S contents controlled by saturation with a sulfide phase (602–1,386 ppm), Ba/La from 3.9‐10, and Fe3+/ΣFe ratios from 0.136 to 0.177. These compositions are similar to MORB and suggest that decompression melting of dry and reduced mantle dominates the earliest stages of subduction initiation. Low‐ and high‐silica boninite glasses have 1.51–3.19 wt% H2O, CO2 below detection, S contents below those required for sulfide saturation (5–235 ppm), Ba/La from 11 to 29, and Fe3+/∑Fe from 0.181 to 0.225. The compositions are broadly similar to modern arc lavas in the IBM arc. These data demonstrate that the establishment of fluid‐fluxed melting of the mantle, which occurs in just 0.6–1.2 my after subduction initiation, is synchronous with the production of oxidized, mantle‐derived magmas. The coherence of high Fe3+/∑Fe and Ba/La ratios with high H2O contents in Expedition 352 glasses and the modern IBM arc rocks strongly links the production of oxidized arc magmas to signatures of slab dehydration.

Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

Abstract The supra-subduction zone ophiolite or ophiolitic mélange formed in the forearc setting is generally considered to be a key geological record for subduction initiation (SI) with petrological characteristics comparable to the SI-related rock sequence from forearc basalt (FAB) to boninite in the Izu-Bonin-Mariana subduction zone. Nevertheless, the standard FAB and boninite are generally difficult to observe in the forearc rocks generated during SI. Yet, a typical rock sequence indicating the SI of the western Proto-Tethys Ocean is reported for the first time in the Qimanyute intra-oceanic forearc system in the western Kunlun Orogen, Northwest Tibetan Plateau. The magmatic compositions, which range from less to more high field strength element (HFSE)-depleted and large ion lithophile element (LILE)-enriched, are changing from oceanic plagiogranites (ca. 494 Ma) to forearc basalt-like gabbros (FAB-Gs, ca. 487 Ma), boninites, and subsequent Nb-enriched gabbros (NEGs, ca. 485 Ma), which are thus consistent with the Izu-Bonin-Mariana forearc rocks as well as the Troodos and Semail supra-subduction zone-type ophiolites. The geochemical data from the chemostratigraphic succession indicate a subduction initiation process from a depleted mid-oceanic-ridge (MORB)-type mantle source with no detectable subduction input to gradual increasing involvement of subduction-derived materials (fluid/melts and sediments). The new petrological, geochemical, and geochronological data, combined with the regional geology, indicate that the well-sustained FAB-like intrusive magmas with associated boninites could provide crucial evidence for SI and further reveal that the SI of the western Proto-Tethys Ocean occurred in the Late Cambrian (494–485 Ma).

The chromitites of the Neoproterozoic Bou Azzer ophiolite (central Anti-Atlas, Morocco) revisited

• Intermediate-Cr and high-Cr chromitites in the Bou Azzer ophiolite, Morocco. • Chromitites crystallized from fore-arc basalts (FAB) and boninites, respectively. • Fore-arc position of the ophiolite in a subduction-initiation geodynamic setting. • Super-reduced phases (moissanite and native Cu) formed during serpentinization. The Neoproterozoic Bou Azzer ophiolite in the Moroccan Anti-Atlas Panafrican belt hosts numerous chromitite orebodies within the peridotite section of the oceanic mantle. The chromitites are strongly affected by serpentinization and metamorphism, although they still preserve igneous relicts amenable for petrogenetic interpretation. The major, minor and trace element composition of unaltered chromite cores reveal two compositional groups: intermediate-Cr (Cr# = 0.60 – 0.74) and high-Cr (Cr# = 0.79 – 0.84) and estimates of parental melt compositions suggest crystallization from pulses of fore-arc basalts (FAB) and boninitic melts, respectively, that infiltrated the oceanic supra-subduction zone (SSZ) mantle. A platinum group elements (PGE) mineralization dominated by Ir-Ru-Os is recognized in the chromitites, which has its mineralogical expression in abundant inclusions of Os-Ir alloys and coexisting magmatic laurite (RuS 2 ) and their products of metamorphic alteration. Unusual mineral phases in chromite, not previously reported in this ophiolite, include super-reduced and/or nominally ultra-high pressure minerals moissanite (SiC), native Cu and silicates (oriented clinopyroxene lamellae), but “exotic” zircon and diaspore have also been identified. We interpret that clinopyroxene lamellae have a magmatic origin, whereas super-reduced phases originated during serpentinization processes and diaspore is linked to late circulation of low-silica fluids related to rodingitization. Zircon grains, on the other hand, with apatite and serpentine inclusions, could either have formed after the interaction of chromitite with mantle-derived melts or could represent subducted detrital sediments later incorporated into the chromitites. We offer a comparison of the Bou Azzer chromitites with other Precambrian ophiolitic chromitites worldwide, which are rather scarce in the geological record. The studied chromitites are very similar to the Neoproterozoic chromitites reported in the Arabian-Nubian shield, which are also related to the Panafrican orogeny. Thus, we conclude that the Bou Azzer chromitites formed in a subduction-initiation geodynamic setting with two-stages of evolution, with formation of FAB-derived intermediate-Cr chromitites in the early stage and formation of boninite-derived high-Cr chromitites in the late stage.

Basalt derived from highly refractory mantle sources during early Izu-Bonin-Mariana arc development

The magmatic character of early subduction zone and arc development is unlike mature systems. Low-Ti-K tholeiitic basalts and boninites dominate the early Izu-Bonin-Mariana (IBM) system. Basalts recovered from the Amami Sankaku Basin (ASB), underlying and located west of the IBM’s oldest remnant arc, erupted at ~49 Ma. This was 3 million years after subduction inception (51-52 Ma) represented by forearc basalt (FAB), at the tipping point between FAB-boninite and typical arc magmatism. We show ASB basalts are low-Ti-K, aluminous spinel-bearing tholeiites, distinct compared to mid-ocean ridge (MOR), backarc basin, island arc or ocean island basalts. Their upper mantle source was hot, reduced, refractory peridotite, indicating prior melt extraction. ASB basalts transferred rapidly from pressures (~0.7-2 GPa) at the plagioclase-spinel peridotite facies boundary to the surface. Vestiges of a polybaric-polythermal mineralogy are preserved in this basalt, and were not obliterated during persistent recharge-mix-tap-fractionate regimes typical of MOR or mature arcs.

Forearc magmatic evolution during subduction initiation: Insights from an Early Cretaceous Tibetan ophiolite and comparison with the Izu-Bonin-Mariana forearc

Abstract Subduction initiation is a key process in the operation of plate tectonics. Our understanding of melting processes and magmatic evolution during subduction initiation has largely been developed from studies of the Izu-Bonin-Mariana forearc. Many suprasubduction zone ophiolites are analogous to the Izu-Bonin-Mariana forearc sequence. However, whether there are differences between Izu-Bonin-Mariana subduction initiation sequences and suprasubduction zone ophiolites remains unclear. Here, we report field geological, geochemical, and geochronological data from mafic and felsic rocks in the Xigaze ophiolite (southern Tibet) mantle and crustal section; the same types of published data from both this ophiolite and the Izu-Bonin-Mariana forearc are compiled for comparison. The ophiolite section is intruded by various late-stage dikes, including gabbroic pegmatite, diabase, basalt, and plagiogranite. The compositions of clinopyroxene and amphibole suggest that gabbroic pegmatite formed from hydrous high-SiO2 depleted melts, while whole-rock compositions of basaltic and diabase dikes show negative Nb and Ta anomalies, suggesting flux melting of depleted mantle. Along with the mafic rocks, plagiogranite has a roughly constant content of La and Yb with increasing SiO2 contents, implying hydrous melting of mafic amphibolite. Early-stage pillow basalts exhibit geochemical affinities with Izu-Bonin-Mariana forearc basalts, but they are slightly enriched. Synthesized with the regional geological setting and compared with Izu-Bonin-Mariana forearc magmatism, we propose that the transition from mid-ocean ridge basalt–like lavas to subduction-related mafic and felsic dikes records an Early Cretaceous subduction initiation event on the southern flank of the Lhasa terrane. However, the mantle sources and the magmatic evolution in the Xigaze ophiolite are more variable than those for the Izu-Bonin-Mariana forearc.

Magma Source Evolution Following Subduction Initiation: Evidence From the Element Concentrations, Stable Isotope Ratios, and Water Contents of Volcanic Glasses From the Bonin Forearc (IODP Expedition 352)

AbstractInternational Ocean Discovery Program Expedition 352 to the Bonin forearc drilled the sequence of volcanic rocks erupted in the immediate aftermath of subduction initiation along the western margin of the Pacific Plate. Pristine volcanic glasses collected during this expedition were analyzed for major and trace elements, halogens, sulfur, and H and O isotopes with goals of characterizing the fluids and melts of subducted materials that were involved in generating the nascent upper plate crust. Incompatible trace element compositions of the oldest lavas (forearc basalts [FAB]) are similar to those of the most depleted mid‐ocean ridge basalts globally. Most FAB were generated by decompression melting during seafloor spreading in a near‐trench, supra‐subduction zone environment with only minor involvement of diverse and generally dilute water‐rich fluids from the subducting plate. Boninite series glasses are enriched in incompatible trace elements mobilized from the subducting plate, but strongly depleted in other elements, such as the middle‐heavy rare‐earth elements. These traits are attributed to generation of boninites largely by flux melting involving water‐rich melts first derived from the leading edge of subducted basaltic crust and then from both subducted crust and sediment. These melts were generated at low pressures as the shallow, embryonic slab extracted heat from hot asthenosphere near the trench. The progressive depletion of the mantle source for the FAB‐through‐boninite sequence suggests that the asthenospheric mantle remained trapped above the nascent subducting plate for the first several million years of subduction beneath the Philippine Sea Plate.

Magmatic Response to Subduction Initiation, Part II: Boninites and Related Rocks of the Izu‐Bonin Arc From IODP Expedition 352

AbstractInternational Ocean Discovery Program Expedition 352 to the Izu‐Bonin forearc cored over 800 m of basement comprising boninite and boninite‐series lavas. This is the most extensive, well‐constrained suite of boninite series lavas ever obtained from in situ oceanic crust. The boninites are characterized as high‐silica boninite (HSB), low‐silica boninite (LSB), or basaltic boninite based on their SiO2‐MgO‐TiO2 relations. The principal fractionation products of all three series are high‐Mg andesites (HMA). Lavas recovered >250 meters below seafloor (mbsf) erupted at a forearc spreading axis and are dominated by LSB and HMA. Lavas recovered from <250 mbsf erupted off‐axis and are dominated by HSB. The axial and off‐axis lavas are characterized by distinct chemostratigraphic trends in their major, trace, and isotopic compositions. The off‐axis lavas are chemically similar to boninite from the type locality at Chichijima, with concave‐upward rare earth elements patterns. In contrast, the more abundant axial lavas have distinctly light rare earth element‐depleted patterns and represent a new, previously unsampled precursor to the Chichijima‐type boninite lavas. Petrogenetic modeling suggests that the axial lavas formed by fluxing of refractory mantle (likely the residue from forearc basalt extraction), with amphibolite‐facies melt derived from subducting altered oceanic crust. The upper, off‐axis lavas require an additional component of sediment‐derived melt in addition. Both models are consistent with previously published isotopic data.

Formation mechanisms of macroscopic globules in andesitic glasses from the Izu\u2013Bonin\u2013Mariana forearc (IODP Expedition 352)

The Izu–Bonin–Mariana volcanic arc is situated at a convergent plate margin where subduction initiation triggered the formation of MORB-like forearc basalts as a result of decompression melting and near-trench spreading. International Ocean Discovery Program (IODP) Expedition 352 recovered samples within the forearc basalt stratigraphy that contained unusual macroscopic globular textures hosted in andesitic glass (Unit 6, Hole 1440B). It is unclear how these andesites, which are unique in a stratigraphic sequence dominated by forearc basalts, and the globular textures therein may have formed. Here, we present detailed textural evidence, major and trace element analysis, as well as B and Sr isotope compositions, to investigate the genesis of these globular andesites. Samples consist of hbox {K}_2hbox {O}-rich basaltic globules set in a glassy groundmass of andesitic composition. Between these two textural domains a likely hydrated interface of devitrified glass occurs, which, based on textural evidence, seems to be genetically linked to the formation of the globules. The andesitic groundmass is Cl rich (ca. 3000, mu hbox {g/g}), whereas globules and the interface are Cl poor (ca. 300, mu hbox {g/g}). Concentrations of fluid-mobile trace elements also appear to be fractionated in that globules and show enrichments in B, K, Rb, Cs, and Tl, but not in Ba and W relative to the andesitic groundmass, whereas the interface shows depletions in the latter, but is enriched in the former. Interestingly, globules and andesitic groundmass have identical Sr isotopic composition within analytical uncertainty (^{87}hbox {Sr}/^{86}hbox {Sr} of 0.70580 pm 10), indicating that they likely formed from the same source. However, globules show high delta ^{11}B (ca. + 7permille), whereas their host andesites are isotopically lighter (ca. – 1 permille), potentially indicating that whatever process led to their formation either introduced heavier B isotopes to the globules, or induced stable isotope fractionation of B between globules and their groundmass. Based on the bulk of the textural information and geochemical data obtained from these samples, we conclude that these andesites likely formed as a result of the assimilation of shallowly altered oceanic crust (AOC) during forearc basaltic magmatism. Assimilation likely introduced radiogenic Sr, as well as heavier B isotopes to comparatively unradiogenic and low delta ^{11}hbox {B} forearc basalt parental magmas (average ^{87}hbox {Sr}/^{86}hbox {Sr} of 0.703284). Moreover, the globular textures are consistent with their formation being the result of fluid-melt immiscibility that was potentially induced by the rapid release of water from assimilated AOC whose escape likely formed the interface. If the globular textures present in these samples are indeed the result of fluid-melt immiscibility, then this process led to significant trace element and stable isotope fractionation. The textures and chemical compositions of the globules highlight the need for future experimental studies aimed at investigating the exsolution process with respect to potential trace element and isotopic fractionation in arc magmas that have perhaps not been previously considered.

Early Cambrian oceanic island-arc magmatism at the paleo-Pacific margin of East Gondwana: Evidence from northern Victoria Land (Antarctica)

We present new geochemical, isotopic and geochronological data for two Cambrian magmatic sequences exposed in northern Victoria Land (Antarctica) at the eastern margin of the Ross-orogenic Wilson Terrane: (i) the metavolcanic and metaplutonic rocks from the Dessent tectono-metamorphic complex, and (ii) the boninite-derived mafic-ultramafic Niagara Icefalls igneous complex.The Dessent metaigneous rocks are subdivided into three main types. Type I metavolcanites show LREE-depleted chondrite-normalized patterns and high initial ɛNd (mostly +6 to +7). Metavolcanic and metaplutonic type II rocks show low MREE-HREE concentrations and initial ɛNd ranging from +2 to +4. Type III metavolcanites mostly differ from type II rocks in the somewhat elevated MREE-HREE abundances. Type I, II and III rocks geochemically resemble forearc basalts, boninites s.l. and firstarc volcanites, respectively, from the Izu-Bonin-Mariana island arc system. Zircon separated from two metaplutonic rocks documents a crystallization age of ~516 Ma.The Niagara Icefalls rocks display a wide range of Nd–Sr isotopic compositions (e.g., initial ɛNd + 5 to −8). Zircons separated from a gabbronorite and from a granitoid vein allowed us to date the building of the Niagara Icefalls igneous complex at ~537 Ma. The melts forming the Niagara Icefalls igneous complex were most likely sourced in a supra-subduction zone mantle wedge, which included both depleted mantle and slab-derived sedimentary components. We propose that the mafic-ultramafic Niagara Icefalls igneous complex and the metaigneous rocks of the Dessent tectono-metamorphic complex are fossil analogues of two distinct oceanic island arcs, which formed in the earliest and the late early Cambrian, respectively.

Geochemical evidence for forearc metasomatism of peridotite in the Xigaze ophiolite during subduction initiation in Neo-Tethyan Ocean, south to Tibet

Subduction initiation is one of the key processes in the operation of plate tectonics. Our understanding of this process is largely derived from geochemical studies of basaltic rocks in ophiolites at continental margins. Because of the petrological inheritance in forearc basalts, it is important to acquire geochemical signatures for fluid metasomatism of the nascent mantle wedge at forearc depths. This may be provided by the study of peridotite in ophiolites, which is least affected by tectonic emplacement of the ophiolites onto continental margins. For this purpose, we have carried out a comprehensive study of whole-rock and mineral geochemical compositions for harzburgites from the Xigaze ophiolites in the Yarlung Zangbo suture zone, southern Tibet. The results provide insights into the forearc metasomatism during subduction initiation before the closure of Neo-Tethyan Ocean for the India-Asia continental collision in the Cenozoic. The harzburgites show high spinel Cr# values and strong whole-rock depletion in CaO, Al2O3 and REE, suggesting high degrees of melt extraction (~10–20%) during seafloor spreading to form the Neo-Tethyan mid-ocean ridge basalts. They exhibit positive LILE anomalies in trace element distribution patterns, indicating metasomatism by subduction zone fluids. This is supported by the occurrence of tremolite and dolomite in some of the harzburgites. Based on the presence or absence of carbonate minerals, two types of metasomatic fluids are distinguished. One is the aqueous solutions, leading to the formation of tremolite and the enrichment of LILE in clinopyroxene. The other is the carbonate-bearing fluids, resulting in the formation of dolomites, tremolite and low Mg# (79–82) olivine. The metasomatism also results in the high contents of structural hydroxyl in pyroxenes and elevated δ18O values for the harzburgites. It is inferred that the fluid metasomatism was induced by incipient subduction of the juvenile Neo-Tethyan oceanic slab to forearc depths shortly after conversion of the Neo-Tethyan mid-ocean ridge into the trench. During this period of subduction initiation, the fluids were produced by heating dehydration of the sinking ridge crust to the forearc depths. It is these fluids that would metasomatize the overlying nascent mantle wedge, leading to superimposition of the geochemically enriched signature on the lithochemically depleted harzburgites in the ophiolites. Therefore, the forearc metasomatism of peridotite in ophiolite can be used as a geochemical proxy for subduction initiation. Furthermore, the Xigaze ophiolites were produced by the ridge-trench conversion for subduction initiation in the Mesozoic and they were emplaced onto the southern margin of the Asian continent in the Cenozoic.

Tectono-magmatic evolution of Tethyan oceanic lithosphere in supra subduction zone fore arc regime: Geochemical fingerprints from crust-mantle sections of Naga Hills Ophiolite

The Naga Hills Ophiolite (NHO) belt in the Indo-Myanmar range (IMR) represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Burmese Plate during the Late Cretaceous-Eocene. Here, we present a detailed petrological and geochemical account for the mantle and crustal sections of NHO, northeastern India to address (i) the mantle processes and tectonic regimes involved in their genesis and (ii) their coherence in terms of the thermo-tectonic evolution of Tethyan oceanic crust and upper mantle. The NHO suite comprises well preserved crustal and mantle sections discretely exposed at Moki, Ziphu, Molen, Washelo and Lacham areas. The ultramafic-mafic lithologies of NHO are mineralogically composed of variable proportions of olivine, orthopyroxene, clinopyroxene and plagioclase. The primary igneous textures for the mantle peridotites have been overprinted by extensive serpentinisation whereas the crustal section rocks reflect crystal cumulation in a magma chamber. Chondrite normalised REE profiles for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to depleted LREE patterns consistent with their generation from depleted MORB-type precursor melt in an extensional tectonic setting, while the mantle peridotites depict U-shaped REE patterns marked by relative enrichment of LREE and HREE over MREE. These features collectively imply a dual role of depleted MORB-type and enriched arc-type mantle components for their genesis with imprints of melt-rock and fluid-rock interactions. Tectonically, studied lithologies from NHO correspond to a boninitic to slab-proximal Island Arc Tholeiite affinity thereby conforming to an intraoceanic supra subduction zone (SSZ) fore-arc regime coherent with the subduction initiation process. The geochemical attributes for the crustal and mantle sections of NHO as mirrored by Zr/Hf, Zr/Sm, Nb/Ta, Zr/Nb, Nb/U, Ba/Nb, Ba/Th, Ba/La and Nd/Hf ratios propound a two-stage petrogenetic process: (i) a depleted fore arc basalt (FAB) type tholeiitic melt parental to the crustal lithologies was extracted from the upwelling asthenospheric mantle at SSZ fore-arc extensional regime thereby rendering a refractory residual upper mantle; (ii) the crust and upper mantle of the SSZ fore arc were progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration. The vestiges of Tethyan oceanic lithosphere preserved in NHO represent an accreted intra-oceanic fore arc crust and upper mantle section which records a transitional geodynamic evolution in a SSZ regime marked by subduction initiation, fore arc extension and arc-continent accretion.

Slab-controlled progressive evolution of the Kudi back-arc ophiolite in response to the rollback of the Proto-Tethys oceanic slab, in Western Kunlun, NW Tibetan Plateau

Slab-derived components can significantly diversify subduction-related magmatism, for example, fore-arc ophiolites evolving from normal mid-ocean ridge basalts to fore-arc basalts and then to boninites. Here we show a progressive evolution of back-arc ophiolites controlled by slab-derived components but different from fore-arc ophiolites through studying the Kudi back-arc ophiolite in Western Kunlun, NW Tibetan Plateau. Our results reveal that the Kudi ophiolite was formed at 516–512 Ma in a back-arc setting in response to the rollback of the Proto-Tethys oceanic slab. The back-arc setting is supported by geochemistry of basalts (e.g., low Th/Nb and V/Ti ratios), low melting pressure (<10 kbar) revealed by amphibole geobarometer and low δ11B values (−19.1 to −8.1‰) of the mantle units. In such a scenario, the early-stage depleted Kudi basalts are characterized by depletions in light rare earth elements and enrichments in fluid-mobile elements, probably resulting from decompression melting of the primary back-arc mantle slightly modified by slab-derived fluids. Subsequently, partial melting of the back-arc mantle enriched by progressive addition of slab-derived sediments resulted in the formation of residual harzburgites and corresponding primary magmas that evolved into dunites, gabbros, quartz diorites and enriched pillow lavas. Additionally, both the residual harzburgites and cumulated dunites experienced minor impregnation of melts, without significant interstitial melt-crystal reactions. Compared with the early-stage depleted Kudi basalts, the late-stage enriched pillow lavas are characterized by enrichments in light rare earth elements and depletions in high field strength elements, typical arc affinities, confirming the addition of slab-derived sediments. Compared with typical fore-arc ophiolitic series, it is possible that there is a progressive evolution in back-arc basins in which a “new” enriched mantle source (corresponding arc basalts) modified by slab-derived sediments displaces an “old” depleted mantle source (corresponding basalts characterized by enrichments in fluid-mobile elements) modified by slab-derived fluids.

Mineral compositions and thermobarometry of basalts and boninites recovered during IODP Expedition 352 to the Bonin forearc

AbstractCentral aims of IODP Expedition 352 were to delineate and characterize the magmatic stratigraphy in the Bonin forearc to define key magmatic processes associated with subduction initiation and their potential links to ophiolites. Expedition 352 penetrated 1.2 km of magmatic basement at four sites and recovered three principal lithologies: tholeiitic forearc basalt (FAB), high-Mg andesite, and boninite, with subordinate andesite. Boninites are subdivided into basaltic, low-Si, and high-Si varieties. The purpose of this study is to determine conditions of crystal growth and differentiation for Expedition 352 lavas and compare and contrast these conditions with those recorded in lavas from mid-ocean ridges, forearcs, and ophiolites. Cr# (cationic Cr/Cr+Al) vs. TiO2 relations in spinel and clinopyroxene demonstrate a trend of source depletion with time for the Expedition 352 forearc basalt to boninite sequence that is similar to sequences in the Oman and other suprasubduction zone ophiolites. Clinopyroxene thermobarometry results indicate that FAB crystallized at temperatures (1142–1190 °C) within the range of MORB (1133–1240 °C). When taking into consideration liquid lines of descent of boninite, orthopyroxene barometry and olivine thermometry of Expedition 352 boninites demonstrate that they crystallized at temperatures marginally lower than those of FAB, between ~1119 and ~1202 °C and at relatively lower pressure (~0.2–0.4 vs. 0.5–4.6 kbar for FAB). Elevated temperatures of boninite orthopyroxene (~1214 °C for low-Si boninite and 1231–1264 °C for high-Si boninite) may suggest latent heat produced by the rapid crystallization of orthopyroxene. The lower pressure of crystallization of the boninite may be explained by their lower density and hence higher ascent rate, and shorter distance of travel from place of magma formation to site of crystallization, which allowed the more buoyant and faster ascending boninites to rise to shallower levels before crystallizing, thus preserving their high temperatures.

From subduction initiation to arc–polarity reversal: Life cycle of an Archean subduction zone from the Zunhua ophiolitic mélange, North China Craton

Subduction initiation and arc–polarity reversal have rarely been recognized in the Archean rock record. We document Neoarchean subduction initiation, fore-arc magmatism, and an arc–polarity reversal event from the Zunhua ophiolitic mélange along the eastern margin of the Central Orogenic Belt (COB) of the North China Craton (NCC). The Zunhua ophiolitic mélange is a mappable unit characterized by blocks of metamorphosed harzburgite/lherzolite, podiform chromite–bearing dunite, pyroxenite, amphibolite, metabasites (basalt and diabase) with rare intermediate volcanics, chert, and tectonic lenses of banded iron formation in a strongly sheared metapelitic matrix. New geochronological and geochemical analyses of magmatic blocks within the ophiolitic mélange show that the crustal magmatic rocks were produced in a fore-arc region at 2.55–2.52 Ga from depleted harzburgitic–lherzolitic mantle tectonites. Chemical, petrological, and temporal links between the depleted mantle blocks, and the suite of magmatic blocks derived from partial melting and metasomatism of these depleted mantle blocks, unequivocally shows that they represent part of the same original Neoarchean fore-arc ophiolite suite. After formation and accretion in the oceanic realm, the mélange was emplaced on the continental margin of the Eastern Block between 2.52 and 2.50 Ga, and underwent two stages of metamorphism at ca. 2.48–2.46 Ga and 1.81 Ga. Metamorphosed intermediate–mafic volcanic blocks exhibit systematic successive geochemical variations, from MORB-like to volcanic arc-like, and the N-MORB-like meta-basalts show remarkable similarity with the subduction initiation-related Izu–Bonin–Mariana (IBM) fore-arc basalts. We suggest that the Zunhua fore-arc complex records continuous geodynamic processes from subduction initiation to arc magmatism. The Zunhua ophiolitic mélange is part of a ca. 2.5 Ga suture between an oceanic arc of the COB and Eastern Block of the NCC. After the arc–continent collision, an arc–polarity reversal event initiated a new eastward–dipping subduction zone on the western side of the COB. This arc–polarity reversal is traced for more than 1600 km along the length of the orogen, similar in scale, geometry, and duration between collision and polarity flip to the present-day arc–polarity reversal of the Sunda–Banda arc during its ongoing collision with the Australia continent. This indicates that a life cycle of an Archean subduction zone, including birth (subduction initiation), maturity (arc magmatism), death (arc-continent collision) and re-birth (arc–polarity reversal), is recorded in the Zunhua ophiolitic mélange, and the geodynamics of plate tectonics at the end of the Archean was similar to those of today.

Geochemical and chronological evidence for collision of proto-Yap arc/Caroline plateau and rejuvenated plate subduction at Yap trench

The Yap Arc was considered to have derived from the proto-Yap-Izu Bonin-Mariana arc system. It is a rare example of subduction of an oceanic plateau with short arc–trench distance and consists mainly of metamorphic rocks, which are distinct from the Izu Bonin-Mariana arc system. However, it is not known on the origin and formation time of the basement rocks of the Yap Arc. For the first time, in this study we provide the results of radiometric age-dating and whole-rock major element, trace element and Sr–Nd–Pb–Hf isotope analyses for the metamorphic basement rocks (amphibolites) of the Yap Arc. These rock samples can be classified into three groups based on their major and trace element compositions. Group 1 samples have strong depletion of LREEs and a Dupal-type isotopic signature, and they represent forearc basalts (FABs) of the nascent Yap forearc. Group 2 samples have an alkali basalt-like geochemistry and oceanic island basalts (OIBs) type trace element patterns, and they represent the basalts of the Caroline Plateau. Group 3 samples have island-arc-type negative anomalies of Nb–Ta–Zr–Hf, and they may represent the volcanic rocks of the proto-Yap Arc. These amphibolites were formed at 21 Ma according to 40Ar/39Ar amphibole dating and in situ SIMS UPb titanite dating, and they record the timing of the collisional event between the Caroline Plateau and the proto-Yap Arc. We propose that the Yap arc has been evolving distinctly from the Izu-Bonin-Mariana Arc since this collision time. These rocks were metamorphosed and brought onto the present Yap Arc as a result of the collision between the proto-Yap Arc and the Caroline Plateau. Based on the new results of age-dating and geochemistry of this study, we propose three stages of tectonic evolution for the Yap Arc as a result of collision from the Caroline plateau: an early stage (proto-Yap Arc) of subduction of the Caroline Plate, collision of the proto-Yap Arc with the in-coming young Caroline Plateau, and rejuvenated plate subduction at the Yap Trench with limited volcanism.

Missing Sr[sbnd]Nd isotopic decoupling in subduction zone: Decoding the multi-stage dehydration and melting of subducted slab in the Chinese Altai

Some island arc basalts (IAB) show NdHf isotopic decoupling, which can be attributed to higher mobility of Nd over Hf in melts released from subducted slab and subsequent metasomatism in the mantle wedge. If the same mechanism applies, SrNd isotopic decoupling should be common in typical IAB, but in fact it is not the case. In order to investigate this problem, we carried out a systematic study on the Habahe mafic dykes at ~360 Ma, which emplaced in the south margin of the Chinese Altai due to the northward subduction of the Junggar Ocean. These dykes can be subdivided into four types based on distinct geochemical compositions and spatial distribution. Type-I mafic dykes in the southern area are enriched in light rare earth elements (LREE) and Th with positive Pb anomaly. They have high initial 87Sr/86Sr (0.7088–0.7095), 206Pb/204Pb (17.971–18.034), 207Pb/204Pb (15.526–15.551) and 208Pb/204Pb (37.860–37.991) ratios and exhibit NdHf isotopic decoupling with high εHf(t) (+9.5 − +12.1) and low εNd(t) values (+3.5 − +3.9). Thus Type-I mafic dykes are interpreted as originating from a depleted mantle source metasomatized by melts from subducted sediments. Type-II mafic dykes in the western segment of central area are depleted in LREE and Th but enriched in Ba, Sr and Pb. They show high positive εNd(t) (+6.5 − +8.1) and εHf(t) (+13.4 − +15.4) values with relatively low initial 87Sr/86Sr (0.7040–0.7049), 206Pb/204Pb (17.715–17.839), 207Pb/204Pb (15.484–15.495) and 208Pb/204Pb (37.739–37.797) ratios, which would be derived from the partial melting of a depleted mantle source with input of fluids from subducted sediments. Type-III mafic dykes in the eastern segment of central area are enriched in LREE with weakly positive Pb anomaly. They show low initial 87Sr/86Sr (0.7036–0.7038), 206Pb/204Pb (17.871–18.001), 207Pb/204Pb (15.485–15.491) and 208Pb/204Pb (37.610–37.642) ratios and have high positive εNd(t) (+6.7 − +6.9) and εHf(t) (+14.6 − +14.8) values, which are interpreted as products of partial melting of a depleted mantle source refertilized by melts from the subducted oceanic crust. Type-IV mafic dykes in the northern area are also enriched in LREE but show significantly positive Pb anomaly with intermediate (87Sr/86Sr)i ratios (0.7068–0.7070), low εNd(t) values (+0.7 − +1.0), high εHf(t) values (+5.5 − +7.8), and high initial 206Pb/204Pb (18.001–18.402), 207Pb/204Pb (15.522–15.553) and 208Pb/204Pb (38.163–38.299) ratios, thus interpreted as originating from a depleted mantle source with the involvement of melts released from subducted sediments. Type-I mafic dykes have similar Nd-Hf-Pb isotopic compositions with Type-IV mafic dykes but show higher (87Sr/86Sr)i ratios, indicating that the melts and fluids from subducted sediments at shallower depth possess high Sr/Nd ratios because of breakdown of plagioclase. Involvement of such melts and fluids can explain SrNd isotopic decoupling for the southern mafic dykes of this study, as the modern analogue of fore-arc basalts and boninites in the Izu-Bonin-Mariana arc and Western Java, which have higher 87Sr/86Sr ratios than those in typical IAB. Consequently, melting of the residual subducted sediments at the sub-arc depth will generate low Sr/Nd fluids and melts, which can account for the SrNd isotopic coupling of the typical IAB and Type-IV mafic dykes of this study.

Effects of reactive dissolution of orthopyroxene in producing incompatible element depleted melts and refractory mantle residues during early fore-arc spreading: constraints from ophiolites in eastern Mediterranean

Compositional variations of peridotites from the Cretaceous ophiolites in southern Turkey and Northern Cyprus are presented to document the nature of partial melting and possible effects of reactive dissolution of primary mantle phases during fore-arc spreading. The peridotites overall exhibit a range of 187Os/188Os ratios from 0.1171 to 0.1266 and appear to represent a mantle region that preserves a record of ancient melt depletion. The samples are depleted in 187Os/188Os compared to the ambient oceanic upper mantle (187Os/188Os ~0.127), suggesting that they are representatives of a shallow fore-arc mantle where transport of radiogenic 187Os during slab dehydration was limited. Chemical variations of primary mantle minerals indicate that the peridotites are the residues of moderate to high degrees (>16%) of partial melting and have experienced significant modal and chemical compositional modification through interaction with oxidizing hydrous basaltic melts. Interacting melts, which appear to be similar in composition to primitive arc tholeiite, are likely to have originated from sub-lithospheric lower part of the mantle wedge during early stages of fore-arc spreading and migrated upward to react with variably depleted harzburgites to induce further melting in the overlying lithospheric mantle through open-system reactive flow. This second stage melting resulted in (1) common occurrence of reactive harzburgites and dunites by incongruent melting of orthopyroxene and crystallization of olivine through interaction with olivine saturated melt; and (2) local development of refertilized peridotites by shallower melt impregnation that involves interaction with olivine + clinopyroxene saturated melt. The dissolution of orthopyroxene caused the reacting melt to be enriched in silica and diluted in incompatible elements which led to the production of the final melts similar in composition to fore arc basalt and boninite. Involvement of compositionally variable mantle and melt components with different rates of melt influx therefore appears to explain the generation of fore-arc crust with a range of diverse rock suites including temporally and spatially associated arc tholeiites and boninites with significant depletion in incompatible elements.

Geochemistry and Microtextures of Vein Calcites Pervading the Izu-Bonin Forearc and RearArc Crust: New Insights From IODP Expeditions 352 and 351.

International Ocean Discovery Program Expeditions 352 and 351 drilled into the Western Pacific Izu‐Bonin forearc and rear arc. The drill cores revealed that the forearc is composed of forearc basalts (FAB) and boninites and the rear arc consists of FAB‐like rocks. These rocks are pervaded by calcite veins. Blocky vein microtextures enclosing host rock fragments dominate in all locations and suggest hydrofracturing and advective fluid flow. Significant diffusion‐fed and crystallization pressure‐driven antitaxial veining is restricted to the rear arc. The lack of faults and presence of an Eocene sedimentary cover in the rear arc facilitated antitaxial veining. Rare earth element and isotopic (δ18O, δ13C, 87Sr/86Sr, and Δ47) tracers indicate varying parental fluid compositions ranging from pristine to variably modified seawater. The most pristine seawater signatures are recorded by FAB‐hosted low‐T (<30 °C) vein calcites. Their 87Sr/86Sr ratios intersect the 87Sr/86Sr seawater curve at ~35–33 and ~22 Ma. These intersections are interpreted as precipitation ages, which concur with Pacific slab rollback. Boninite‐hosted low‐T (<30 °C) vein calcites precipitated from seawater that was modified by fluid‐rock interactions. Mixing calculations yield a mixture of >95% seawater and <5% basaltic 87Sr/86Sr. In the rear arc, low‐T rock alteration lowered the circulating seawater in δ18O and 87Sr/86Sr. Thus, vein calcites precipitated from modified seawater with up to 20–30% basaltic 87Sr/86Sr at temperatures up to 74 ± 12 °C. These results show how the local geology and vein growth dynamics affect microtextures and geochemical compositions of vein precipitates.

Subduction initiation of the SE Paleo-Asian Ocean: Evidence from a well preserved intra-oceanic forearc ophiolite fragment in central Inner Mongolia, North China

Subduction of oceanic lithosphere is the primary driver of plate tectonics on Earth. However, one of the least understood aspects of plate tectonics is how subduction zones initiate, due to the lack of recognition of subduction initiation records in many ophiolites. Here we report a newly recognized complete magmatic record including Early Carboniferous fore-arc basalts (FABs), transitional lavas and boninites in the Diyanmiao ophiolite zone in central Inner Mongolia, North China. The stratigraphy and chemo-temporal relations of the suite are identical to those of the Izu–Bonin–Mariana (IBM) forearc as well as the ophiolite suites associated with subduction initiation such as Semail in Oman. In the Diyanmiao suite, the FABs are exposed over a large area, overlying gabbroic rocks of the ophiolite, and underlain by transitional lavas and boninites from bottom to top. Zircon U–Pb geochronology of the FABs shows a mean age of 335.6±2.6 Ma. The Diyanmiao FABs are mainly aphyric, with variolitic groundmass texture, and pillow structure. The boninites (∼328 Ma) are strongly porphyritic with skeletal olivine crystal, clinopyroxene and orthopyroxene phenocrysts. The major and trace element and Sr–Nd isotopic characteristics of FABs, transitional lavas and boninites in the Diyanmiao ophiolite are comparable to those of corresponding rocks in the IBM forearc. The FABs display compositions similar to those of the most depleted mid-ocean ridge basalts (MORB), but with lower Ti/V ratios, suggesting slightly higher concentrations of fluid-soluble elements than MORB. The Diyanmiao transitional lavas have transitional compositions between FABs and boninites, and have higher large ion lithophile elements (LILEs) and lower high field strength elements (HFSEs) than FABs. The boninites are characterized by significant depletion of Ti, heavy rare earth element (HREE), and HFSE and are more LILE-enriched. This volcanic sequence is akin to that in the Mariana forearc, suggesting that the Diyanmiao FABs might record initial volcanism associated with subduction initiation in the southeastern Paleo-Asian Ocean during Early Carboniferous (∼335.6 Ma). We infer that the primary magmas of the Diyanmiao FABs were generated by partial melting of upwelling mantle associated with decompression during forearc spreading with little or no slab input. Progressive subduction of the Paleo-Asian oceanic slab (ca. 328 Ma), and flushing by slab-derived hydrous fluids resulted in melting of the more depleted, residual mantle at (probably) shallower levels generating the boninites.

Geochemistry of volcanic glass from Oligocene detrital sediments at DSDP Site 296, Kyushu Palau Ridge: Interpreting the magmatic evolution of the early northern Izu–Bonin–Mariana Island Arc

AbstractUnderstanding the petrologic and geochemical evolution of island arcs is important for interpreting the timing and impacts of subduction and processes leading to the formation of a continental crust. The Izu–Bonin–Mariana (IBM) Arc, western Pacific, is an outstanding location to study arc evolution. The IBM first arc (45–25 Ma) followed a period of forearc basalt and boninite formation associated with subduction initiation (52–45 Ma). In this study, we present new major and trace element data for the IBM first arc from detrital glass shards and clasts from DSDP Site 296, located on the northernmost Kyushu Palau Ridge (KPR). We synthesize these data with published literature for contemporaneous airfall ash and tephra from the Izu–Bonin forearc, dredge and piston core samples from the KPR, and plutonic rocks from the rifted eastern KPR escarpment, locations which lie within or correlate with KPR Segment 1 of Ishizuka, Taylor, Yuasa, and Ohara (2011). Our objective is to test ways in which petrologic and chemical data for diverse igneous materials can be used to construct a complete picture of this section of the Oligocene first arc and to draw conclusions about its evolution. Important findings reveal that widely varying primary magmas formed and differentiated at various depths at this location during this period. Changes in key trace element ratios such as La/Sm, Nb/Yb, and Ba/Th show that mantle sources varied in fertility and in the inputs of subducted sediment and fluids over time and space. Plutonic rocks appear to be related to early K‐poor dacitic liquids represented by glasses sampled both in the forearc and volcanic fronts. An interesting observation is that the variation in magma compositions in this relatively small segment encompasses that inferred for the IBM Arc as a whole, suggesting that sampling is a key factor in inferring temporal, across‐arc, and along‐strike geochemical trends.