Izu–Bonin–Mariana Forearc Research Articles

Chemostratigraphy of the extrusive sequence of a Late Cretaceous Neotethyan ophiolite in southern Iran and its significance for the mode and tempo of subduction initiation magmatism and melt evolution in forearc tectonic settings

Chemostratigraphy of lavas in extrusive sequences of modern and ancient forearc environments provides significant information on oceanic crust formation through progressive stages of melt evolution from subduction initiation magmatism (SIM) to arc infancy, and arc maturation-related magmatism. We present here new trace element and radiogenic Sr–Nd–Pb isotopic data from a Late Cretaceous ophiolite in the Outer Zagros Belt of southern Iran and discuss its magmatic development through SIM in a Neotethyan forearc setting. Initial volcanism produced the stratigraphically oldest boninitic (BON) lavas in the Haji-Abad ophiolite whose trace element ratios as well as Pb isotopic signatures indicate a refractory mantle source, which was contaminated by subducting slab-derived fluids. The BON lavas change structurally upwards into island-arc tholeiitic (IAT) lavas and their fractionated felsic derivatives that display isotopic evidence for the contribution of sediment-derived melts in their magmatic source(s). This melt evolution pattern of the Haji-Abad ophiolite marks a major difference from the progressive melt evolution trends reported from the Izu–Bonin–Mariana (IBM) forearc lava sequence, which starts at the bottom with forearc basalts (FAB) phasing up-section into BON and IAT affinity lavas. IAT lavas in the IBM are in turn overlain by calc-alkaline (CAL) lavas. Our geochemical modelling points to a probable existence of a less depleted, pre-subduction mantle whose partial melting probably generated primary magmas before the eruption of boninitic lavas, although the record of such magmas is missing in the Haji-Abad ophiolite. Our comparison of the chemostratigraphy of the Outer Zagros ophiolites with the documented record of subduction initiation generated volcanic rocks in the other Neotethyan ophiolites in the region and the IBM forearc setting strongly suggests that there is no single rule or template for SIM. Supplementary material: Details of analytical procedures and supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6837597 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Mineral and whole-rock geochemistry of high-Al podiform chromitites in the Fizh Massif of the Cretaceous Oman ophiolite: origin of hydrous N-MORB melts in a nascent forearc setting

The Oman ophiolite is one of the largest and best exposed ophiolites in the world and has >450 chromitite deposits. We report here a newly identified chromitite deposit in the Wadi Rajmi, Oman. This deposit occurs within a dunitic envelope, which is surrounded by harzburgite, and consists of both massive and disseminated chromitite types. The Rajmi peridotites represent depleted upper mantle rocks that underwent >20% partial melting and experienced metasomatism by melts and fluids derived from a subducting slab. They demonstrate geochemical affinities similar to those of the Izu–Bonin–Mariana forearc peridotites, supporting their formation in a forearc environment. The Rajmi chromitites have low Cr# values and are classified as high-Al chromitites. They have geochemical compositions comparable with those of chromitites crystallized from mid-ocean ridge basalt (MORB)-type melts. However, the chromites in these high-Al chromitites contain various silicate inclusions (e.g. amphiboles and micas), indicating the hydrous and atypical MORB nature of their parental magmas. Combined with the mineralogical and geochemical characteristics of the country rocks, we posit that the parental melts of the Rajmi high-Al chromitites have a MORB-like affinity derived from partial melting of a nascent forearc mantle. Supplementary material: The whole-rock and mineral data of the Rajmi harzburgites, dunites and chromitites are available at https://doi.org/10.6084/m9.figshare.c.6795689 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Evidence for early Pennsylvanian subduction initiation in the Mongol–Okhotsk Ocean from the Adaatsag ophiolite (Mongolia)

The Late Paleozoic–Mesozoic Mongol–Okhotsk Ocean was the latest ocean basin witnessing the final amalgamation of East Asia. However, the kinematic evolution and geodynamics of Mongol–Okhotsk's subduction initiation remain enigmatic mainly due to the lack of appropriate studies of its ophiolitic records. Here, we report for the first time a subduction initiation record from the Adaatsag ophiolite in the central Mongolia. This ophiolite presents a magmatic sequence akin to that in the Izu–Bonin–Mariana forearc, considered archetypical of subduction initiation. The gabbroic and basaltic rocks of the ophiolite display geochemical signatures similar to the forearc basalts. Zircon UPb geochronology of the gabbro shows a mean age of 319 ± 2 Ma, which we interpret as crystallization age. The high-Mg diorite intruding the gabbros of the ophiolite displays a transitional affinity between sanukite and boninite. Zircon UPb geochronology of the high-Mg diorite shows a mean age of 317 ± 2 Ma. Together with the neighbor Khuhu Davaa suprasubduction zone ophiolite (∼321 Ma), we interpret that the subduction of the southwestern Mongol–Okhotsk Ocean initiated during the Early Pennsylvanian (∼320 Ma). The structurally underlying gneissic granite (∼534 Ma) indicates the Adaatsag ophiolite was emplaced southward onto the Ereendavaa cratonic terrane, and the overlying molassic sequences (youngest detrital zircon peak age of ∼252 Ma) constrain this emplacement occurred before ∼252 Ma. The older (499–870 Ma) inherited zircons of the ophiolites and negative Hf isotope data of the high-Mg diorite indicate that a continental fragment existed in the newly formed intra-oceanic arc, which probably facilitated the Early Pennsylvanian subduction initiation in the Mongol–Okhotsk Ocean.

Forearc lava stratigraphy of the Beila Ophiolite, north-central Tibetan Plateau: Magmatic response to initiation of subduction of the Bangong-Nujiang Meso-Tethys Ocean

It has been suggested that boninites and associated volcanic rocks found in supra-subduction zone (SSZ) ophiolites form in a forearc setting, meaning that these rocks could provide crucial constraints on understanding the magmatic response to subduction initiation. Here, we report newly documented forearc basalts (FAB) and boninites from the Beila Ophiolite of the Bangong–Nujiang Suture Zone, north-central Tibetan Plateau. The lava stratigraphy, from bottom to up, includes FAB, basaltic boninites, and boninites, similar to lavas from the Izu–Bonin–Mariana (IBM) forearc. The FAB has geochemical characteristics similar to those of IBM forearc basalts, with nearly flat REE patterns, and shows enrichment in Rb, Ba, Pb, and Sr and depletion in Nb, Ta, and Zr, suggesting an origin by decompression melting of a depleted mantle source with minor input of subducting plate materials. The basaltic boninites and boninites show similar petrological features, as well as porphyritic textures with phenocrysts of olivine, clinopyroxene, and minor orthopyroxene. The basaltic boninites and boninites display uniform major- and trace-element characteristics that are similar to those of typical boninites, with high MgO (>10.78 wt%), Cr, and Ni contents, extremely low TiO2 (<0.21 wt%), rare earth element (REE), and high-field-strength element (HFSE) contents, and U-shaped REE patterns. In addition, the basaltic boninites and boninites have lower ɛNd(t) values than those of the FAB and display variable degrees of influence by slab melts and fluids. Geochemical modeling further suggests that the boninites were generated later during residual highly depleted mantle fluxing by the subducting plate. Together with contemporary boninitic and arc-related magmas along the Bangong-Nujiang Suture Zone and adjacent terranes (the North Lhasa and South Qiangtang), it suggests that the whole Meso-Tethys Ocean was subducted since the Middle Jurassic. Moreover, the Beila lava stratigraphy with FAB overlain by boninite is similar to that found in the IBM forearc and in many ophiolites (e.g., Troodos, Oman, and Xigaze), and represents a continuous geochemical transition from MORB-like to boninitic affinities. This similarity suggests that initiation of subduction might be commonly involved in the formation of ophiolites.

The Fuchuan Ophiolite in South China: Evidence for Modern‐Style Plate Tectonics During Rodinia Breakup

AbstractSubduction initiation processes, such as those evidenced in the Izu‐Bonin‐Mariana (IBM) forearc crust are a key feature of modern‐style plate tectonics. However, the evidence for this process in the Precambrian is limited, due to the scarcity of ophiolite suites and in particular a lack of typical complete ophiolitic sequences similar to those observed in Phanerozoic ophiolites. Mineral analyses of olivine, orthopyroxene, and chrome spinel from the harzburgite and two types of clinopyroxene from the cumulate gabbro of the Fuchuan ophiolite, South China suggest formation in a Neoproterozoic subduction system similar to the IBM. High temperature (1,000–1,200°C), low pressure along with a high degree of melt extraction (&gt;18%) have been recorded in the harzburgite. This evidence, in conjunction with magma evolution from forearc basalt to boninite to hydrous calc‐alkaline magmatism, recorded in the crustal section of the Fuchuan ophiolite indicates a transition from initial to mature subduction in a forearc setting. Statistical analysis of global ophiolites through time suggests that IBM‐like subduction systems may have successfully operated since at least the latest Neoproterozoic (ca. 1,000 Ma) and be pervasive on Earth from ca. 850 Ma. Additional evidence for Neoproterozoic ophiolites from India, Siberia, and Arabian‐Nubian Shield, which are also similar to the IBM forearc ophiolite, suggests that cold and deep subduction became much more common in the Neoproterozoic (ca. 850 Ma) around the periphery of the Rodinia supercontinent. This IBM‐like subduction system may have acted as the geodynamic trigger for the break‐up of Rodinia.

Copper recycling and redox evolution through progressive stages of oceanic subduction: Insights from the Izu-Bonin-Mariana forearc

Addition of subducted materials from the slab to the mantle wedge is often thought to elevate the oxygen fugacity of arc magmas and also to fertilize the mantle wedge in metals for subduction-related Cu-Au deposits. However, it remains controversial if slab-driven metal addition is effective and whether it occurs at all stages of subduction. The Izu-Bonin-Mariana (IBM) forearc preserves a full record of arc development from the initiation of subduction of the Pacific plate to mature arc volcanism. Here, we study the ore-forming and redox-sensitive metal Cu and its isotopes (δ65Cu) in the type-locality forearc basalts (FABs), boninites and high-Mg andesites from the IBM forearc, as well as MORBs from the East Pacific rise. Overall, the FABs display variably high Cu contents and MORB-like δ65Cu, consistent with limited Cu isotopic fractionation during partial melting of typical MORB mantle sources. Beginning with the boninites, the magma products from the IBM record strong slab signals, high water contents and high fO2 (ΔFMQ > +1). However, the oxidized, hydrous boninites and subsequent high-Mg andesites display low Cu contents, and mantle-like Cu/Sc ratios and δ65Cu, with no clear indication of the addition of Cu from the slab. The boninites show noticeably lower TiO2, Yb and Cu contents than the FABs, consistent with refractory mantle sources. Combined with available data from boninites and arc basalts worldwide, our results lead to a general conclusion that subducted oceanic slabs from initial to mature subduction contribute little Cu to the mantle wedge. This is further supported by the compiled Cu contents of arc peridotites. Adding Cu-poor, water-rich slab melts to the mantle wedge at any stage of Pacific oceanic subduction causes limited release of Cu, which remains trapped predominantly by reduced sulfides in the subducting slab. This likely reflects the overall reducing nature of slab materials added to the mantle wedge. However, we propose that the subsequent reaction of such reducing, hydrous slab melts with peridotites and flux melting produce oxidized primitive arc magmas deep in the mantle wedge. The reactive process promotes sulfide dissolution and metal release from the mantle wedge itself to oxidized arc magmas with high sulfur solubility, which explains the inheritance of mantle-like δ65Cu and the sulfide-undersaturated early-stage evolution in boninites and arc basalts. This study elucidates the role of subducted oceanic slabs in metal transfer and redox evolution and implies no significant Cu enrichment in the metasomatized mantle sources of magmatic arcs.

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.

Petrogenesis of boninitic lavas from the Troodos Ophiolite, and comparison with Izu–Bonin–Mariana fore-arc crust

Supra-subduction zone (SSZ) ophiolites such as the Troodos Ophiolite of Cyprus are thought to have formed at spreading centres close to subduction zones. Similarities in the geochemistry between lavas from SSZ ophiolites and fore-arc lavas from active subduction zones, and the presence of boninites in both, have led to the suggestion that SSZ ophiolites represent fragments of fore-arc crust and mantle, formed during subduction initiation. Here we present major and trace element and Sr, Nd and Pb isotope data for fresh volcanic glasses from a section through the lava pile on the southern margin of the Troodos Ophiolite, and compare these to lavas from the Izu–Bonin–Mariana fore-arc. In Troodos, boninites and tholeiitic basalts are interbedded and were derived from a highly depleted mantle source that was later enriched by both fluids and melts derived largely from subducted sediment, before melting beneath the spreading axis. Troodos boninites differ from Izu–Bonin–Mariana boninites by their greater source depletion, enrichment in Nb by small degree melts, and lack of Zr enrichment relative to Sm. Together with the lack of fore-arc basalts in Troodos, our data imply that the Troodos Ophiolite was formed in a fore-arc location at a back-arc spreading centre that propagated into arc crust. The Troodos Ophiolite was thus not formed during subduction initiation and thus may not be used as analogue for the formation of fore-arc basalts.

Origin of depleted basalts during subduction initiation and early development of the Izu-Bonin-Mariana island arc: Evidence from IODP expedition 351 site U1438, Amami-Sankaku basin

The Izu-Bonin-Mariana (IBM) island arc formed following initiation of subduction of the Pacific plate beneath the Philippine Sea plate at about 52 Ma. Site U1438 of IODP Expedition 351 was drilled to sample the oceanic basement on which the IBM arc was constructed, to better understand magmatism prior to and during the subduction initiation event. Site U1438 igneous basement Unit 1 (150 m) was drilled beneath 1460 m of primarily volcaniclastic sediments and sedimentary rock. Basement basalts are microcrystalline to fine-grained flows and form several distinct subunits (1a-1f), all relatively mafic (MgO = 6.5–13.8%; Mg# = 52–83), with Cr = 71–506 ppm and Ni = 62–342 ppm. All subunits are depleted in non-fluid mobile incompatible trace elements. Ratios such as Sm/Nd (0.35–0.44), Lu/Hf (0.19–0.37), and Zr/Nb (55–106) reach the highest values found in MORB, while La/Yb (0.31–0.92), La/Sm (0.43–0.91) and Nb/La (0.39–0.59) reach the lowest values. Abundances of fluid-mobile incompatible elements, K, Rb, Cs and U, vary with rock physical properties, indicating control by post-eruptive seawater alteration, but lowest abundances are typical of fresh, highly depleted MORBs. Mantle sources for the different subunits define a trend of progressive incompatible element depletion. Inferred pressures of magma segregation are 0.6–2.1 GPa with temperatures of 1280–1470 °C.New 40Ar/39Ar dates for Site U1438 basalts averaging 48.7 Ma (Ishizuka et al., 2018) are younger that the inferred age of IBM subduction initiation based on the oldest ages (52 Ma) of IBM forearc basalts (FAB) from the eastern margin of the Philippine Sea plate. FAB are hypothesized to be the first magma type erupted as the Pacific plate subsided, followed by boninites, and ultimately typical arc magmas over a period of about 10 Ma. Site U1438 basalts and IBM FABs are similar, but Site U1438 basalts have lower V contents, higher Ti/V and little geochemical evidence for involvement of slab-derived fluids. We hypothesize that the asthenospheric upwelling and extension expected during subduction initiation occurred over a broad expanse of the upper plate, even as hydrous fluids were introduced near the plate edge to produce FABs and boninites. Site U1438 basalts formed by decompression melting during the first 3 Ma of subduction initiation, and were stranded behind the early IBM arc as mantle conditions shifted to flux melting beneath a well-defined volcanic front.

Age of Izu–Bonin–Mariana arc basement

Documenting the early tectonic and magmatic evolution of the Izu–Bonin–Mariana (IBM) arc system in the Western Pacific is critical for understanding the process and cause of subduction initiation along the current convergent margin between the Pacific and Philippine Sea plates. Forearc igneous sections provide firm evidence for seafloor spreading at the time of subduction initiation (52 Ma) and production of “forearc basalt”. Ocean floor drilling (International Ocean Discovery Program Expedition 351) recovered basement-forming, low-Ti tholeiitic basalt crust formed shortly after subduction initiation but distal from the convergent margin (nominally reararc) of the future IBM arc (Amami Sankaku Basin: ASB). Radiometric dating of this basement gives an age range (49.3–46.8 Ma with a weighted average of 48.7 Ma) that overlaps that of basalt in the present-day IBM forearc, but up to 3.3 m.y. younger than the onset of forearc basalt activity. Similarity in age range and geochemical character between the reararc and forearc basalts implies that the ocean crust newly formed by seafloor spreading during subduction initiation extends from fore- to reararc of the present-day IBM arc. Given the age difference between the oldest forearc basalt and the ASB crust, asymmetric spreading caused by ridge migration might have taken place. This scenario for the formation of the ASB implies that the Mesozoic remnant arc terrane of the Daito Ridges comprised the overriding plate at subduction initiation. The juxtaposition of a relatively buoyant remnant arc terrane adjacent to an oceanic plate was more favourable for subduction initiation than would have been the case if both downgoing and overriding plates had been oceanic.

The arc arises: The links between volcanic output, arc evolution and melt composition

Subduction initiation is a key process for global plate tectonics. Individual lithologies developed during subduction initiation and arc inception have been identified in the trench wall of the Izu–Bonin–Mariana (IBM) island arc but a continuous record of this process has not previously been described. Here, we present results from International Ocean Discovery Program Expedition 351 that drilled a single site west of the Kyushu–Palau Ridge (KPR), a chain of extinct stratovolcanoes that represents the proto-IBM island arc, active for ∼25 Ma following subduction initiation. Site U1438 recovered 150 m of oceanic igneous basement and ∼1450 m of overlying sediments. The lower 1300 m of these sediments comprise volcaniclastic gravity-flow deposits shed from the evolving KPR arc front. We separated fresh magmatic minerals from Site U1438 sediments, and analyzed 304 glass (formerly melt) inclusions, hosted by clinopyroxene and plagioclase.Compositions of glass inclusions preserve a temporal magmatic record of the juvenile island arc, complementary to the predominant mid-Miocene to recent activity determined from tephra layers recovered by drilling in the IBM forearc. The glass inclusions record the progressive transition of melt compositions dominated by an early ‘calc-alkalic’, high-Mg andesitic stage to a younger tholeiitic stage over a time period of 11 Ma. High-precision trace element analytical data record a simultaneously increasing influence of a deep subduction component (e.g., increase in Th vs. Nb, light rare earth element enrichment) and a more fertile mantle source (reflected in increased high field strength element abundances). This compositional change is accompanied by increased deposition rates of volcaniclastic sediments reflecting magmatic output and maturity of the arc. We conclude the ‘calc-alkalic’ stage of arc evolution may endure as long as mantle wedge sources are not mostly advected away from the zones of arc magma generation, or the rate of wedge replenishment by corner flow does not overwhelm the rate of magma extraction.

Physical properties and seismic structure of Izu‐Bonin‐Mariana fore‐arc crust: Results from IODP Expedition 352 and comparison with oceanic crust

AbstractMost of the well‐preserved ophiolite complexes are believed to form in suprasubduction zone (SSZ) settings. We compare physical properties and seismic structure of SSZ crust at the Izu‐Bonin‐Mariana (IBM) fore arc with oceanic crust drilled at Holes 504B and 1256D to evaluate the similarities of SSZ and oceanic crust. Expedition 352 basement consists of fore‐arc basalt (FAB) and boninite lavas and dikes. P‐wave sonic log velocities are substantially lower for the IBM fore arc (mean values 3.1–3.4 km/s) compared to Holes 504B and 1256D (mean values 5.0–5.2 km/s) at depths of 0–300 m below the sediment‐basement interface. For similar porosities, lower P‐wave sonic log velocities are observed at the IBM fore arc than at Holes 504B and 1256D. We use a theoretical asperity compression model to calculate the fractional area of asperity contact Af across cracks. Af values are 0.021–0.025 at the IBM fore arc and 0.074–0.080 at Holes 504B and 1256D for similar depth intervals (0–300 m within basement). The Af values indicate more open (but not necessarily wider) cracks in the IBM fore arc than for the oceanic crust at Holes 504B and 1256D, which is consistent with observations of fracturing and alteration at the Expedition 352 sites. Seismic refraction data constrain a crustal thickness of 10–15 km along the IBM fore arc. Implications and inferences are that crust‐composing ophiolites formed at SSZ settings could be thick and modified after accretion, and these processes should be considered when using ophiolites as an analog for oceanic crust.

A record of spontaneous subduction initiation in the Izu–Bonin–Mariana arc

The initiation of tectonic plate subduction into the mantle is poorly understood. If subduction is induced by the push of a distant mid-ocean ridge or subducted slab pull, we expect compression and uplift of the overriding plate. In contrast, spontaneous subduction initiation, driven by subsidence of dense lithosphere along faults adjacent to buoyant lithosphere, would result in extension and magmatism. The rock record of subduction initiation is typically obscured by younger deposits, so evaluating these possibilities has proved elusive. Here we analyse the geochemical characteristics of igneous basement rocks and overlying sediments, sampled from the Amami Sankaku Basin in the northwest Philippine Sea. The uppermost basement rocks are areally widespread and supplied via dykes. They are similar in composition and age—as constrained by the biostratigraphy of the overlying sediments—to the 52–48-million-year-old basalts in the adjacent Izu–Bonin–Mariana fore-arc. The geochemical characteristics of the basement lavas indicate that a component of subducted lithosphere was involved in their genesis, and the lavas were derived from mantle source rocks that were more melt-depleted than those tapped at mid-ocean ridges. We propose that the basement lavas formed during the inception of Izu–Bonin–Mariana subduction in a mode consistent with the spontaneous initiation of subduction.

The timescales of subduction initiation and subsequent evolution of an oceanic island arc

The Bonin Ridge and trench slope preserves the geological record of subduction initiation and subsequent evolution of the Izu–Bonin–Mariana (IBM) arc. Diving and dredging in this region has revealed a bottom to top stratigraphy of: 1) mantle peridotite, 2) gabbroic rocks, 3) a sheeted dyke complex, 4) basaltic pillow lavas, 5) boninites and magnesian andesites, 6) tholeiites and calcalkaline arc lavas. This forearc stratigraphy is remarkably similar to that found in other IBM forearc localities and many ophiolites. Zircon U–Pb ages obtained here for gabbros are 51.6–51.7 Ma. The overlying basalts have 40Ar/ 39Ar ages of 48–52 Ma. A forearc basalt from the Mariana forearc near Guam produced a similar 40Ar/ 39Ar age of 51.1 Ma. The collective geochronology of igneous rocks from throughout the IBM system now indicates that the first basaltic magmatism at subduction initiation was produced by decompression melting of the mantle and took place at 51–52 Ma. The change to flux melting and boninitic volcanism took 2–4 m.y., and the change to flux melting in counterflowing mantle and “Normal” arc magmatism took 7–8 m.y. This evolution from subduction initiation to arc normalcy occurred nearly simultaneously along the entire length of the IBM subduction system. Mesozoic rocks found in the deep Bonin forearc suggest that the overriding plate at subduction initiation consisted of Mesozoic terranes and subduction preceded the opening of most or all of the Philippine Sea basins. The contemporaneousness of IBM forearc magmatism with the major change in plate motion in Western Pacific at ca. 50 Ma suggests that the two events are intimately linked.

Making and breaking an island arc: A new perspective from the Oligocene Kyushu‐Palau arc, Philippine Sea

The Kyushu‐Palau Ridge (KPR) is a 2600 km long remnant island arc that is separated from the active Izu‐Bonin‐Mariana (IBM) arc by a series of spreading and rift basins. We present 40Ar/39Ar ages and geochemical data for the entire length of the Kyushu‐Palau arc as well as for the conjugate arc which is stranded within the IBM fore arc. New 40Ar/39Ar ages indicate that the KPR was active between 25 and 48 Ma, but the majority of the exposed volcanism occurred in the final phase, between 25 and 28 Ma. Rifting of the Kyushu‐Palau arc to form the Shikoku and Parece Vela basins occurred simultaneously along the length of the arc (circa 25 Ma), and at a similar distance from the trench. Unlike the IBM, the KPR has only limited systematic along‐arc geochemical trends. Two geochemical components within the KPR indicate an origin in the suprasubduction mantle. First, EM‐1‐like lavas are identified in a restricted section of the arc, suggesting a localized heterogeneity. Second, EM‐2‐like arc volcanoes formed on juvenile West Philippine Basin crust, potentially reflecting ingress of mantle from the then active EM‐2 province which lies in the west. Another geochemical heterogeneity is found at the KPR–Daito Ridge intersection where the arc developed on preexisting Cretaceous Daito Ridge crust. The geochemical characteristics at this intersection likely result from the involvement of sub–Daito Ridge lithospheric mantle. Subduction flux beneath the KPR generally matches post–45 Ma Eocene/Oligocene lavas in the IBM fore arc, involving fluids and melts derived from altered igneous crust.

Insight into the uppermost mantle section of a maturing arc: The Eastern Mirdita ophiolite, Albania

We examined peridotite massifs in the eastern part of the Mirdita ophiolite (EMO), Albania, where arc-related magmas are abundant in the upper volcanic sequences. Structurally, clinopyroxene porphyroclast-bearing harzburgites (Cpx-harzburgite hereafter) occur in the lower parts of the peridotite massifs, whereas harzburgites and dunites are more abundant towards the upper parts. Dunite is commonly associated with chromitite layers. Orthopyroxenite occurs as dikes and/or networks at all structural levels, although it is more abundant in the uppermost sections. Orthopyroxenite commonly crosscuts the foliation of peridotites and the lithological boundaries between dunites (chromitite) and harzburgites, suggesting that it was formed in the late stage. Major and trace element compositions of minerals in the Cpx-harzburgites indicate that they were formed as the residue of less-flux partial melting, and are similar to those in abyssal peridotites from mid-ocean ridge systems. Harzburgites have more depleted major element compositions than the Cpx-harzburgites. Light rare earth element (LREE)-enrichment in clinopyroxene coupled with hydrous silicate mineral inclusions in spinels in harzburgites indicate that harzburgites were produced as a result of enhanced partial melting of depleted peridotites due to infiltration of hydrous LREE-enriched fluids/melts. Based on olivine and spinel chemistries, dunites are classified into two types: high-Cr# (= Cr/(Cr + Al) atomic ratio) spinel-bearing dunite and medium-Cr# spinel-bearing dunite. Orthopyroxenites formed at the expense of the pre-existing peridotite by reaction with hydrous orthopyroxene-saturated melts, which were produced by assimilation of dissolved pyroxene during the formation of the dunite. Refractory harzburgite, high-Cr# spinel-bearing dunite, and orthopyroxenite may have a genetic link to the late stage boninitic magmas in the crustal section of the EMO. In contrast, the Cpx-harzburgite was a residue related to mid-ocean ridge basalts (MORBs) or the “MORB-like” fore-arc basalt recently proposed by Reagan et al. (2010) from the Izu–Bonin–Mariana fore-arc. The medium-Cr# spinel-bearing dunite can be caused by interaction with a melt transitional between MORB-like and boninitic melts. The lithological variations and their relationships in the upper mantle section of the EMO were caused by changes in magmatic compositions from mid-ocean ridge signatures to boninitic magmas, due to an increasing contribution of slab-derived fluids in an island arc setting.

Early stages in the evolution of Izu–Bonin arc volcanism: New age, chemical, and isotopic constraints

A remarkable record of early arc volcanism in the Izu–Bonin–Mariana (IBM) arc is exposed in and around the Bonin Islands, an uplifted segment of the IBM forearc. New 40Ar/ 39Ar dating results imply that the boninitic volcanism on Chichijima Island occurred in a brief period during Eocene time, between 46–48 Ma. A slightly younger volcanic succession is identified along the Bonin Ridge, including 44.74 ± 0.23 Ma high-Mg andesite from the Mikazukiyama Formation, the youngest volcanic sequence on Chichijima, 44.0 ± 0.3 Ma tholeiitic to calcalkaline andesite from Hahajima Island, and 3 samples of andesite collected by the submersible SHINKAI 6500 from the Bonin Ridge Escarpment (BRE) that range in age from 41.84 ± 0.14 to 43.88 ± 0.21 Ma. Four SHINKAI 6500 dives (YK 04–05) on the BRE mapped an elongated constructional volcanic ridge atop the escarpment; we observed steeply west-dipping volcaniclastic debris flows shed from the summit of this ridge into the Ogasawara Trough to the west. These dives recovered fresh andesitic clasts from debris flows along the northern segment of the ridge, and high-Mg andesite lava blocks and Nummulitic limestone of middle Eocene age from the escarpment northwest of Chichijima. Our results also confirm previous inferences that melting of depleted mantle at shallow levels beneath the length of the arc with the aid of hydrous fluids from newly subducted slab to produce boninitic volcanism occurred nearly simultaneously along the entire length of the IBM arc system during the earliest stage of arc evolution. BRE–Mikazukiyama Formation–Hahajima andesites represent a transitional stage from forearc spreading (represented by ODP site 786-Chichijima boninites) and the stable, mature arc that developed in the Oligocene. These OPX-bearing high-Mg or tholeiitic to calcalkaline andesites were erupted along the BRE, as the arc magmatic axis localized and retreated from the trench.

Late Neoproterozoic proto-arc ocean crust in the Dariv Range, Western Mongolia: a supra-subduction zone end-member ophiolite

An unusual late Neoproterozoic ( c . 572 Ma) ophiolite is exposed in the Dariv Range (western Mongolia), which contains intermediate to acidic lavas and sheeted dykes, and an igneous layered complex consisting of gabbro–norites, websterites, orthopyroxenites and dunites underlain by serpentinized mantle harzburgites. Based on the compositions of the crustal units and the crystallization sequences in the mafic and ultramafic cumulates we conclude that the entire oceanic crust, including the cumulates, was made from arc magmas with boninitic characteristics. The Dariv rocks bear a strong resemblance to rocks recovered from the modern Izu–Bonin–Mariana fore-arc, a fragment of proto-arc oceanic basement, and we propose that the Dariv Ophiolite originated in a similar tectonic setting. A metamorphic complex consisting of amphibolite- to granulite-facies metasedimentary and meta-igneous rocks was thrust over the ophiolite. This metamorphic complex probably represents a Cambrian arc. Thrusting started before 514.7 ± 7.6 Ma as constrained by new sensitive high-resolution ion microprobe U–Pb zircon analyses from a syn- to post-tectonic diorite. The Dariv Ophiolite is a type-example of a proto-arc ophiolite, a special class of supra-subduction zone ophiolites.

Tectonic setting of Eocene boninite magmatism in the Izu–Bonin–Mariana forearc

Middle Eocene boninites were simultaneously generated over a large region during the early history of the Izu–Bonin–Mariana (IBM) arc. However, widespread boninite magmatism is not recognised in younger subduction zones of similar dimensions. This suggests that an additional tectonic or thermal factor influenced the generation of the IBM boninite suite. Regional uplift, ocean island basalt-style magmatism and high heat-flow also characterised the northern Philippine Sea plate (PSP) at the start of the Middle Eocene. These features are similar to those observed in large volume basaltic volcanic provinces such as the early Tertiary North Atlantic and suggest that the IBM boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the PSP. The reconstructed Middle Eocene location of the IBM arc and West Philippine Basin lies close to the present day Manus Basin where petrological and geochemical evidence indicate the presence of a mantle plume. A calculated plume track linking these locations through time also passes close to the Eauripik Rise, an aseismic ridge on the Caroline Plate, during the Oligocene and Miocene. Therefore, we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western Pacific from the Middle Eocene until the present day.

Trace and REE content of clinopyroxenes from supra-subduction zone peridotites. Implications for melting and enrichment processes in island arcs

We measured trace element and Rare Earth Element (REE) contents of clinopyroxenes (cpx) in peridotites from ophiolite complexes from the Hellenic Peninsula: Vourinos, Pindos, Othris (Greece), and Bulqiza (Albania). Compared to abyssal peridotites the Hellenic peridotites have a highly depleted mineralogy (<1% modal cpx) and the cpxs have extremely low concentrations in Ti (30–150 ppm) and heavy REE. The light and middle REE, Zr and Sr contents of the cpxs show enrichments compared to cpx from abyssal peridotites. Our cpx data falls in the field of previous studies on modern arc peridotites ([Parkinson, I.J., Pearce, J.A., Thirwall, M.F., Johnson, K.T.M., Ingram, G., 1992. Trace element geochemistry of peridotites from the Izu–Bonin–Mariana forearc, Leg 125. In: Fryer, P., Pearce, J.A., Stokking, L.B. (Eds.), Proc. Ocean Drilling Program. pp. 487–506; Bonatti, unpubl.]) suggesting that these complexes originated above a subduction zone. Dry melting of an upper mantle source similar to a mid-ocean ridge basalt (MORB) source (e.g., [Johnson, K.T.M., Dick, H.J.B., Shimizu, N., 1990. Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites. J Geophys. Res. 95, 2661–2678.]) cannot generate the extreme depletions in Ti and heavy rare earth element (HREE) seen in the cpxs from these supra-subduction zone (SSZ) peridotites. We propose that these high degrees of depletion can be achieved during hydrous melting of a MORB-depleted peridotite above a subduction zone. The melting rate of cpx during hydrous melting is expected to be less than in dry melting, while the rate of orthopyroxene (opx) consumption increases. These changes in melting modes allow for larger degrees of total melting and larger depletions in Ti and HREE in the cpx before cpx is exhausted from the residual peridotite. The enrichments in light rare earth element (LREE), MREE, Zr and Sr are best modeled by a constant flux of a slab-derived fluid component to the mantle wedge during melting. Ti and the HREE are not mobile in this fluid and are not enriched in the mantle peridotite. The continuous flux is necessary to sustain hydrous melting in the sub-arc mantle during the complete melting range. The calculated fluid composition matches well previous estimates based on the arc basalt chemistry ([McCulloch, M.T., Gamble, J.A., 1991. Geochemical and geodynamical constraints on subduction zone magmatism. Earth Planet. Sci. Lett. 102, 358–374; Stolper, E., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett. 121, 293–325.]) and experiments on fluids ([Ayers, J.C., Dittmer, S.K., Graham, D.L., 1997. Partitioning of elements between peridotite and H 2O at 2.0–3.0 GPa and 900–1100°C, and application to models of subduction zone processes. Earth Planet. Sci. Lett. 150, 381–398; Ayers, J.C., 1998. Trace element modeling of aqueous fluid–peridotite interaction in the mantle wedge of subduction zones. Contrib. Mineral. Petrol. 132, 390–404.]). The resulting melts from our model closely resemble melts from an arc environment (boninites) further arguing for the plausibility of this model.