We establish the ‘subduction initiation rule’ (SIR) which predicts that most ophiolites form during subduction initiation (SI) and that the
diagnostic
magmatic chemostratigraphic progression for SIR ...ophiolites is from less to more HFSE-depleted and LILE-enriched compositions. This chemostratigraphic evolution reflects formation of what ultimately becomes forearc lithosphere as a result of mantle melting that is progressively influenced by subduction zone enrichment during SI. The magmatic chemostratigraphic progression for the Izu–Bonin–Mariana (IBM) forearc and most Tethyan ophiolites is specifically from MORB-like to arc-like (volcanic arc basalts or VAB ± boninites or BON) because SI progressed until establishment of a mature subduction zone. MORB-like lavas result from decompression melting of upwelling asthenosphere and are the first magmatic expression of SI. The contribution of fluids from dehydrating oceanic crust and sediments on the sinking slab is negligible in early SI, but continued melting results in a depleted, harzburgitic residue that is progressively metasomatized by fluids from the sinking slab; subsequent partial melting of this residue yields ‘typical’ SSZ-like lavas in the latter stages of SI. If SI is arrested early, e.g., as a result of collision, ‘MORB-only’ ophiolites might be expected. Consequently, MORB- and SSZ-only ophiolites may represent end-members of the SI ophiolite spectrum. The chemostratigraphic similarity of the Mariana forearc with that of ophiolites that follow the SIR intimates that a model linking such ophiolites, oceanic forearcs, and SI is globally applicable.
Data collated from subalkaline lavas and dikes of Palaeogene ophiolites of the SW Pacific in Papua, New Guinea, New Caledonia, and the North Island, New Zealand interpreted as having formed first ...upon subduction initiation, are investigated to determine whether these are MORB like, similar to other subduction initiation ophiolites and the IBM forearc. SW Pacific ophiolite subalkaline lavas and dikes are mostly tholeiitic basalt; however, calc-alkaline lavas comprise ~ 25% of New Zealand samples. Major element abundances of first-formed subalkaline lavas and dikes of SW Pacific ophiolites are broadly similar to MORB, however incompatible element abundances range from MORB-like to more depleted than MORB and similar to the most enriched IBM forearc basalt (FAB). The reason for the more enriched incompatible-element nature of first-formed lavas and dikes of SW Pacific ophiolites relative to the IBM FAB may be that the sources of the former, unlike the latter, based on recent studies were not subjected to a pre-subduction initiation melt extraction event. Partial melting estimates for first-formed lavas and dikes of SW Pacific ophiolites range from 12-23% of an initial MORB-like source to a maximum of 18% partial melting of this source after 20% melt extraction. Partial melting was due to decompression of a MORB-like source that received little to no slab-derived contributions. Thermobarometry calculations of primitive lavas and dikes with ≥ 8 wt.% MgO and Mg# ≥ 60 show that the mantle source of SW Pacific ophiolites records potential temperatures and pressures of 1341-1431°C and 1.1-1.8 GPa, similar to primitive MORB. These temperatures are at least 30-70°C lower than those required for Central American and IBM first-formed lavas supporting previous studies suggesting a plume-induced subduction initiation origin for Central America and the IBM.
Boninites (BONs), high-Mg andesites (HMAs), and related lavas and dikes of Palaeogene ophiolites of the SW Pacific in Papua, New Guinea, New Caledonia, and the North Island, New Zealand, interpreted ...as having formed subsequent to forearc basalt (FAB) during subduction initiation (SI) are investigated to determine whether these are similar to second-stage lavas and dikes of other SI ophiolites and the IBM forearc. SW Pacific second-stage lavas and dikes are mostly high-Si BONs, but range to HMAs with the latter appearing to have preceded the formation of the former. Partial melting was due to flux-melting of a depleted source with the addition of hydrous and LREE-, Sr-, and Zr-enriched sediment melts to the mantle wedge subsequent to extraction of first-formed FAB. New Caledonia second-stage BONs are mostly consistent with ~5-15% partial melting of a source, which had undergone two previous melt extraction events and in New Zealand, second-stage HMA dikes are consistent as the result of 10-20% partial melting of a depleted source having undergone initial FAB extraction followed by a maximum of ~25% partial melting of a source produced subsequent to the first extraction event. Thermobarometry calculations of primitive second-stage BON and HMA lavas and dikes record potential temperatures generally similar to but pressures lower than MORB. Although most PUB ophiolite and both New Zealand HMA samples plot within the field of MORB with T
P
of about 1280-1350°C and pressures of ~0.8-1.80 GPa, all remaining second-stage BONs and HMA exhibit T
P
and pressures lower than MORB. A clear demarcation exists between New Caledonia BONs and PUB HMA, the latter of which records much higher T
P
and lower pressures than the former and range from ~1330 to 1420°C and from 0.7 to 1.40 GPa. These anomalously high temperatures for the PUB may infer a plume-induced SI event.
The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four ...sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO2, P2O5, Zr) and Ni compared to MORB, with Na2O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle.
Plain Language Summary
This projects looks at how subduction zones form and evolve before island arc volcanism becomes established. Subduction zones are important because they are the primary sites for recycling chemically enriched crustal materials and because they form some of Earth's most important ore deposits. We drilled two deep core holes on the inner trench wall of the Izu‐Bonin subduction zone to recover samples from its earliest history, before formation of Izu‐Bonin island arc volcanoes. Samples from these cores were analyzed chemically to establish how lava compositions varied through time and to understand the processes that control their chemistry. We found a diverse set of lavas, with chemical compositions that are low in elements that are normally enriched in arc lavas. Calculations also show that these lavas were hotter than normal mid‐ocean ridge lavas. We found that the first lavas in a new subduction zone form by the upwelling of hot material from deeper in the Earth, which partially melts as it rises toward the surface. Small amounts of melt formed in equilibrium with garnet deeper in the Earth, while later melts formed in equilibrium with spinel at shallower depths in the Earth. The high extent of melting required, and the high calculated temperatures of these lavas, suggests the involvement of the Manus hot spot, which was located above the newly formed subduction zone some 52 million years ago. The lavas found in these drill holes are similar to lavas found in ocean crust that has been emplaced into mountain belts throughout the world and supports the proposition that in most areas, on‐land ocean crust formed above subduction zones, not at mid‐ocean ridges.
Key Points
Fore‐arc basalts (FABs) formed by decompression melting in response to subduction initiation and differ from mid‐ocean ridge basalts
FABs form by two‐stage melting, with early garnet field melts extracted prior to spinel field melting, resulting in LREE/HREE depletion
Highly depleted FAB may reflect both an older depletion event and higher ambient temperatures related to the Manus plume
Display omitted
•Most Iranian basement rocks have radiometric ages ranging from 570 to 530 Ma.•The bimodality of igneous rocks infers an extensional regime for the magma sources.•Inherited zircons ...(2500–650 Ma) infer the existence of older basement beneath Iran.•A plume triggered formation of oldest Iranian basement rocks far from a subduction zone.•Iranian basement has more similarity with east Asia than Cadomian terranes.
Late Neoproterozoic–Early Paleozoic basement in Iran mainly comprises granites and mafic rocks locally affected by high-grade metamorphism. In this contribution, we collate whole rock chemical data for more than 600 samples; Sr–Nd isotope data for 200 samples; and zircon U-Pb ages of > 2500 grains. On the basis of zircon U-Pb ages, Iranian basement spans 570–530 Ma. The mafic rocks are tholeiitic and the felsic (granitic) rocks show affinities for predominantly I-and S-type granite with minor A-type granite. Sr-Nd isotope ratios show two discrete sources for the basement rocks. The mafic rocks yield younger Nd model ages (TDM < 1.0 Ga) and lack old zircon grains (mainly less than 800 Ma) confirming a juvenile mantle source. Conversely, the felsic rocks have older TDM ages (2.5 to 1.6 Ga) similar to inherited zircon 238U-206Pb ages (2.5 Ga to 650 Ma), which reflects recycling of old (Late Archean-Early Proterozoic) continental crust in the generation of felsic melts in the Late Neoproterozoic. Based in part on a recent study of Late Neoproterozoic Gondwana evolution, long-lived Proto-Tethys subduction beneath northern Gondwana (Hun terranes) may have caused: (1) Proto-Tethys slab avalanche in the lower mantle; (2) subsequent triggering of mantle plume emplacement; and (3) the calving off of a ribbon of Hun terranes from northern Gondwana. That Iran was far from the Cadomian orogeny is supported by the fact that ‘Cadomian’ terranes of Iran show evidence of having formed in an extensional tectonic regime as opposed to that of a subduction zone as for Cadomian basement in Europe. Hence, the common-use of Cadomian basement to explain Ediacaran–Cambrian magmatism in Iran is a misinterpretation. This misconception stems largely from an erroneous correlation of Iran with North America and Europe, as opposed to correlation with Central and Eastern Asia such as microcontinents in the Tibetan Plateau.
The tectonic evolution of Laxmi basin, presently located along western Indian passive margin, remains debated. Prevailing geodynamic models of Laxmi basin include two mutually competing hypotheses, ...culminating in either a hyper-stretched continental crust or an oceanic crust overlying an extinct spreading centre. The longstanding conundrum surrounding its precise crustal affinity precludes a complete understanding of the early opening of the Indian Ocean. Here, we present distinct geochemical and geophysical imprints from the igneous crust in Laxmi basin obtained through International Ocean Discovery Program Expedition 355. The geochemical and isotopic signatures of the Laxmi basin crust exhibit uncanny similarities with forearc tectonic settings. Our observations imply a relict subduction initiation event occurred in the Laxmi basin in the Late Cretaceous-Early Cenozoic that marks a significant Cenozoic plate reorganisation record in the northwest Indian Ocean. New findings therefore warrant re-evaluation of the Gondwana breakup to account for the nascent subduction in the northwest Indian Ocean.
IODP Expedition 357 used two seabed drills to core 17 shallow holes at 9 sites across Atlantis Massif ocean core complex (Mid-Atlantic Ridge 30°N). The goals of this expedition were to investigate ...serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration in one borehole. The cores show highly heterogeneous rock types and alteration associated with changes in bulk rock chemistry that reflect multiple phases of magmatism, fluid-rock interaction and mass transfer within the detachment fault zone. Recovered ultramafic rocks are dominated by pervasively serpentinized harzburgite with intervals of serpentinized dunite and minor pyroxenite veins; gabbroic rocks occur as melt impregnations and veins. Dolerite intrusions and basaltic rocks represent the latest magmatic activity. The proportion of mafic rocks is volumetrically less than the amount of mafic rocks recovered previously by drilling the central dome of Atlantis Massif at IODP Site U1309. This suggests a different mode of melt accumulation in the mantle peridotites at the ridge-transform intersection and/or a tectonic transposition of rock types within a complex detachment fault zone. The cores revealed a high degree of serpentinization and metasomatic alteration dominated by talc-amphibole-chlorite overprinting. Metasomatism is most prevalent at contacts between ultramafic and mafic domains (gabbroic and/or doleritic intrusions) and points to channeled fluid flow and silica mobility during exhumation along the detachment fault. The presence of the mafic lenses within the serpentinites and their alteration to mechanically weak talc, serpentine and chlorite may also be critical in the development of the detachment fault zone and may aid in continued unroofing of the upper mantle peridotite/gabbro sequences.
New technologies were also developed for the seabed drills to enable biogeochemical and microbiological characterization of the environment. An in situ sensor package and water sampling system recorded real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential (Eh), and temperature and during drilling and sampled bottom water after drilling. Systematic excursions in these parameters together with elevated hydrogen and methane concentrations in post-drilling fluids provide evidence for active serpentinization at all sites. In addition, chemical tracers were delivered into the drilling fluids for contamination testing, and a borehole plug system was successfully deployed at some sites for future fluid sampling. A major achievement of IODP Expedition 357 was to obtain microbiological samples along a west–east profile, which will provide a better understanding of how microbial communities evolve as ultramafic and mafic rocks are altered and emplaced on the seafloor. Strict sampling handling protocols allowed for very low limits of microbial cell detection, and our results show that the Atlantis Massif subsurface contains a relatively low density of microbial life.
•Seabed rock drills and real-time fluid monitoring for first time in ocean drilling•First time recovery of continuous sequences along oceanic detachment fault zone•Highly heterogeneous rock type and alteration in shallow detachment fault zone•High methane and hydrogen concentrations in Atlantis Massif shallow basement•Oceanic serpentinites potentially provide important niches for microbial life
The “subduction initiation rule” (SIR) (Whattam and Stern, 2011) advocates that proto-arc and forearc complexes preserved in ophiolites and forearcs follow a predictable chemotemporal and/or ...chemostratigraphic vertical progression. This chemotemporal evolution is defined by a progression from bottom to top, from less to more depleted and slab-metasomatized sources. This progression has been recently documented for other igneous suites associated with subduction initiation. The Sona-Azuero forearc complex of southern Panama represents the earliest magmatic arc activity at the Central American Volcanic Arc system. Comparison of new and existing geochemical data for the circa 82-40 Ma Sona-Azuero Proto-Arc/Arc, its underlying 89-85 Ma “oceanic plateau” of SW Panama and the 72-69 Ma Golfito Proto-Arc of southern Costa Rica with the 70-39 Ma Chagres-Bayano Arc of eastern Panama exhibits a chemotemporal progression as described above and which follows the SIR. Sona-Azuero lavas are predominantly MORB-like, whereas those of the younger Chagres-Bayano complex are mostly VAB-like; lavas of the Golfito Proto-Arc typically show characteristics intermediate to that of the Sona-Azuero and Chagres-Bayano proto-arc/arc complexes. On the basis of isotope evidence as shown in other studies, lava types of all three complexes are clearly derived from a source contaminated by the Caribbean Large Igneous Province plume; we term these “plume-contaminated” forearc basalts and volcanic arc basalts, respectively. Apart from a plume-induced subduction initiation origin for the Panamanian forearc, these insights suggest otherwise similar petrogenetic origins and tectonic setting to lavas comprising earliest-formed forearc crust, and most ophiolites, which follow the SIR.
Display omitted
•Older Sona-Azuero complex is MORB-like.•Younger Chagres-Bayano complex is arc-like.•Early Central American forearc adheres to SIR.
Abstract
Miocene magmatic rocks are exposed as lava flows (OKV group), subvolcanic rocks (SRG group), and as lavas interbedded with shallow-basin sedimentary layers (TVN group) in the Takab area, NW ...Iran. Zircon U–Pb dating yields ages of 18 to 15 Ma. Whole-rock chemistry shows that most of the Early Miocene magmatic rocks are andesite with subordinate dacite. The magmatic rocks have low contents of MgO, Ni, Cr, Ti, Nb and Ta, and high concentrations of Li, large ion lithophile elements such as Rb, K and Ba, and light rare earth elements. The OKV and SRG groups have similar initial 87Sr/86Sr ratios (0·70557–0·70768) and εNd(t) values (+1·0 to +2·2). The TVN group show larger variations of 87Sr/86Sr(i) ratios from 0·70628 to 0·71033 and εNd(t) values from –3·8 to +1·6. This implies a greater role of involvement of supra-crustal domains in the evolution of the TVN group relative to the SRG and OKV groups. Early Miocene magmatic rocks in the Takab area are situated between the Sanandaj–Saqqez Cretaceous calc-alkaline andesite in the SW and the Late Eocene–Oligocene (35–27 Ma) ocean island basalt-like Mianeh–Hashtrood magmatic belt of possible back-arc affinity in the NE. In addition, Late Eocene (40–37 Ma) syn-collision granites in the Baneh–Marivan area along the Zagros suture zone along the west side of the Sanandaj–Saqqez Cretaceous calc-alkaline andesite body indicate that collision of the Arabian Plate and the NW Iran Block occurred in the Late Eocene. These observations support the idea that Early Miocene andesites (18–15 Ma) in the Takab area were generated after collision, which was also associated with doubling of the thickness of the continental crust in the Zagros suture zone, thinning of continental crust far from the Zagros suture zone, and development of shallow-basin sedimentary rocks in NW Iran. Partial melting of mafic calc-alkaline bodies at depth or highly metasomatized fossil mantle owing to thinning of continental crust and asthenospheric upwelling may represent possible sources for the Late Miocene andesite. We conclude that andesitic rocks, even with typical arc signatures, are not always generated in an active margin and that some were probably generated in a post-collision tectonic regime. Misinterpretation of the arc signature can result in erroneous assumptions as to the geodynamic regime, and in the particular case of NW Iran, the timing of collision of the Arabian and Iranian plates.
International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located ...in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.
PDF
