Crustal Accretion Research Articles

A Latest Jurassic to Early Cretaceous Syn‐Collisional Trench Sequence in Central Tibet Recorded the Role of Microcontinents in Lhasa–Qiangtang Collision

AbstractAs the most recent collisional event prior to the early Cenozoic India‐Asia collision, the suturing of the Lhasa‐Qiangtang terranes resulted in crustal accretion and the formation of lithospheric structures that greatly influenced subsequent Cenozoic rock uplift in central Tibet. The discovery of several microcontinents within the Bangong‐Nujiang suture zone between the Lhasa and Qiangtang terranes potentially implies multiple suturing stages, however, how and when the suturing of these terranes occurred are highly disputed. Here, we present a newly discovered latest Jurassic to Early Cretaceous deep‐marine gravity‐fan succession that is over 660 m thick in the Jienu Mountain region, south to the Cenozoic Lunpola basin. By employing multi‐proxy, single‐grain provenance analysis and forward modeling of sediment mixtures, we find the detritus was deposited in a syn‐collisional trench and was sourced from the Dongkaco microcontinent, the obducted ophiolite—subduction complex, the accreted seamounts, and the volcanic arc rocks. We interpret that this trench deposition occurred due to the latest Jurassic to Early Cretaceous collision of the Lhasa terrane beneath the Dongkaco microcontinent and other accreted components. The sutured Dongkaco microcontinent forms the basement to the Lunpola basin and is inferred not present beneath the narrower Nima, Gaize, and Awenco basins along strike to the west where these basins are either narrower or exhibit abrupt facies changes with time. This study highlights the importance of Mesozoic tectonic inheritance on subsequent Cenozoic uplift of the Tibetan Plateau.

Fine‐Scale Crustal Velocity Structure at the Lucky Strike Segment of Mid‐Atlantic Ridge From Full Waveform Inversion of Wide‐Angle Seismic Data

AbstractThe Lucky Strike segment at the Mid‐Atlantic ridge, characterized by a well‐defined median valley with a central volcano, is an archetypical slow‐spreading ridge segment and hence an ideal site for studying magmatic and tectonic processes at slow‐spreading ridges. Here we present fine‐scale velocity models of this segment, by applying full waveform inversion to wide‐angle seismic data, that allows characterization of crustal accretion processes along the entire segment. Along ridge axis, the crust thins from ∼8.4 km at the center of the segment to ∼3.7–4.1 km at the segment ends. This large variation in crustal thickness is mainly accommodated by lower crustal thinning toward the segment ends. The ratio of the lower/upper crust thickness varies from 2.2 at the segment center to 0.1 at the segment ends, so upper crust at segment ends accounts for ∼90% of the crustal thickness, suggesting that the lateral dyking is the primary crustal accretion mechanism. The reduction of lower crustal velocity at the segment center indicates the presence there of melt within the lower crust, which is the source of melt delivery for dyke propagation. The upper crustal velocity gradually decreases from the segment center to segment ends, consistent with an increase in faulting and the presence of more evolved magma toward the segment ends. These observations demonstrate the presence of focused magma supply to the segment center. Off‐axis, the upper crustal thickness shows little variation over ∼30 km on both flanks, suggesting the current magmatic accretion mode could have been active for 3 Myr.

Deep crustal composition and Late Paleozoic geotectonic evolution in West Junggar, China

The West Junggar area in the southwestern part of the Central Asian Orogenic Belt (CAOB), is one of the largest areas of growth of the Phanerozoic crust in the world that has experienced intense Late Paleozoic magmatic activity, where crust-mantle interaction is significant. The issue of crustal growth has long been regarded as one of the most fundamental in earth sciences. In light of the challenges posed by the composition of deep materials and the Late Paleozoic crustal growth in the West Junggar area, there is a continued need to systematically determine the spatial distribution characteristics of deep materials in the crust, analysis the growth pattern and growth volume of the crust, and enhance the Late Paleozoic tectonic evolution of the region. Focusing on granite type Sr-Nd-Pb-Hf isotopic mapping, this study found that the West Junggar area has the isotopic characteristics of high positive εHf (t) and εNd (t), low (87 Sr/86 Sr)i, and young crustal mode age, there is almost no old crystalline basement in the deep crust. During the Late Paleozoic, about 85% of West Junggar had 75%–95% crustal growth, dominated by lateral crustal growth and material recirculation; about 15% of the area (connected to the Jietebudiao area) had 50%–75% crustal growth, dominated by vertical crustal growth. The West Junggar area mainly experienced four orogenic stages in the Late Paleozoic. In the Early Carboniferous period (360–320 Ma), there was significant intra-oceanic subduction, involving a substantial amount of juvenile material in lateral crustal accretion. The Late Carboniferous-Early Permian period (320–294 Ma) is the post-orogenic extension stage, during which a massive amount of juvenile mantle source was added. This resulted in the most intense magmatism and crustal growth, which could have the growth of the crust potentially more than 75%. In the Early Permian period (294–272 Ma), there was an intracontinental evolution stage and a decrease in the participation of juvenile material. During the Early Permian-Early Triassic period (272–250 Ma), magmatic activity decreased significantly, where the southwestern region experienced high-temperature, low-pressure, crustal thinning extension. Despite this, the crust received juvenile material, and plutonic magma intrusion occurred.

Sm-Nd Isotope Data Compilation from Geoscientific Literature Using an Automated Tabular Extraction Method

The rare earth elements Sm and Nd significantly address fundamental questions about crustal growth, such as its spatiotemporal evolution and the interplay between orogenesis and crustal accretion. Their relative immobility during high-grade metamorphism makes the Sm-Nd isotopic system crucial for inferring crustal formation times. Historically, data have been disseminated sporadically in the scientific literature due to complicated and costly sampling procedures, resulting in a fragmented knowledge base. However, the scattering of critical geoscience data across multiple publications poses significant challenges regarding human capital and time. In response, we present an automated tabular extraction method for harvesting tabular geoscience data. We collect 10,624 Sm-Nd data entries from 9,138 tables in over 20,000 geoscience publications using this method. We manually selected 2,118 data points from it to supplement the previously constructed global Sm-Nd dataset, increasing its sample count by over 20%. Our automatic data collection methodology enhances the efficiency of data acquisition processes spanning various scientific domains.

A geochemical and isotopic outline on the Alto Moxotó Terrane: a record of the best-preserved Paleoproterozoic crust in the Central Borborema Province, NE Brazil

The Alto Moxotó Terrane is one of the best-preserved Paleoproterozoic crustal segments of the Borborema Province and shows a close correlation with basement domains of the Central African Fold Belt. Here, we review geochemical and isotopic data of Neoarchean and Paleoproterozoic rocks within the terrane and their implications for the crustal evolution of western Gondwana. The lithospheric framework of the terrane consists of three main tectono-magmatic stages: i) Neoarchean (ca. 2.6 Ga) crustal accretion with TTG juvenile injections, consisting of calcic, magnesian, meta- to peraluminous magmas derived from a basaltic oceanic crust that likely experienced high-pressure conditions; ii) Rhyacian-Orosirian (ca. 2.1 – 1.9 Ga) tholeiitic mafic-ultramafic and calc-alkaline series Cordilleran-related granitic and gneissic rocks; and iii) Statherian-Calymmian within-plate magmatism (ca. 1.6 Ga), with granitic rocks that correspond to A-type ferroan magmas, generated via partial melting of a continental source. The presence of older events within the Alto Moxotó Terrane enables correlation with basement domains within the Borborema Province and other Neoproterozoic orogens of Gondwana. These correlations strengthen the role of the province Paleoproterozoic domains in sspaleogeographic reconstructions of the Nuna/Columbia supercontinents.

Tectono‐Magmatic Evolution of the Southern Reykjanes Ridge, North Atlantic, From ∼11 M.y. to Present

AbstractOur understanding of the geological evolution of mid‐ocean ridges in response to tectonic reconfigurations and associated mantle processes is hampered by a lack of exploration in off‐axis areas. A notable exception is the Reykjanes Ridge, where multibeam bathymetry, magnetics, and gravity surveys have been conducted up to ∼150 km from the ridge axis. Previous work shows that the ridge has undergone a major reorganization following changes in spreading direction, resulting in the progressive formation and then elimination of transform faults from north to south under the influence of the regional mantle melting anomaly. Notably, this process is incomplete near the southern termination of the ridge, providing a window into the processes of crustal accretion and segmentation prior to and immediately following this reorganization. Here, we employ remote‐predictive geological and structural mapping methods linked to chrono‐magnetic data to elucidate changes in segment morphology, magma supply, and structural fabrics along the southern ∼200 km of the ridge over the past ∼11 M.y. We identify two new fracture zones and three new non‐transform discontinuities, with elimination of transform motion occurring between ∼9.7 and 4.2 Ma, which is later than previously thought. Transform elimination coincides with rift propagation and the emergence of a new magmatically robust segment at ∼58°N at ∼9.7–8.2 Ma. This transition is also associated with a reorientation of seafloor fabric from dominantly N‐trending to NE‐trending, associated with the dissection of axial volcanic ridges by the oblique (NE‐trending) plate boundary, resulting in more crustal accretion to the North American plate overall.

A New Geological Map of the Marginal Basins of Eastern Papua New Guinea: Implications for Crustal Accretion and Mineral Endowment at Arc–Continent Collisions

Abstract Accretion of island arc terranes is a fundamental process of crustal growth and the formation of new continents. Convergent margin tectonics, both compressional and extensional, in accretionary orogens also control the origin and distribution of their contained mineral resources, including many of the world’s important Cu and Au deposits. However, the details of crustal growth and accretion are often lost because of deformation and selective preservation during subduction. The Melanesian Borderland, which includes the offshore regions of eastern Papua New Guinea and the Solomon Islands, contains several active and relict arc and backarc systems that have formed in response to more than 50 Ma of subduction and complex plate tectonic adjustments. The composite terrane is a region of some of the fastest growing crust on Earth and also spectacular mineral endowment, including three of the top ten porphyry Cu and epithermal Au deposits in the world. However, more than 80% of the belt is submerged, and so little is known about its geological evolution and makeup. Here, we present the first detailed geological map of the region in one map sheet, including the marginal deep ocean basins. The map identifies and groups the key lithostratigraphic formations and correlates associated tectonic events across the belt. The final compilation is presented at 1:1,000,000 scale, which is sufficient to allow quantitative analysis of crustal growth and accretion during ocean–continent collision throughout the region. The map shows the diversity of assemblages in accreting terranes that may eventually become part of a growing continent and highlights their complex formation and structural relationships. Because so much of that history has occurred offshore, the new map presents the first complete picture of the geology of the region in the critical period leading up to its eventual incorporation in the Australian continent.

Interaction of Kerguelen and Amsterdam-St. Paul dual hotspots with Southeast Indian Ridge

Interaction of Kerguelen and Amsterdam-St. Paul dual hotspots with Southeast Indian Ridge

Seismic structure of Iceland revealed by ambient noise Rayleigh wave tomography

Seismic structure of Iceland revealed by ambient noise Rayleigh wave tomography

Highly variable magmatic accretion at the ultraslow-spreading Gakkel Ridge

Crustal accretion at mid-ocean ridges governs the creation and evolution of the oceanic lithosphere. Generally accepted models1–4 of passive mantle upwelling and melting predict notably decreased crustal thickness at a spreading rate of less than 20 mm year−1. We conducted the first, to our knowledge, high-resolution ocean-bottom seismometer (OBS) experiment at the Gakkel Ridge in the Arctic Ocean and imaged the crustal structure of the slowest-spreading ridge on the Earth. Unexpectedly, we find that crustal thickness ranges between 3.3 km and 8.9 km along the ridge axis and it increased from about 4.5 km to about 7.5 km over the past 5 Myr in an across-axis profile. The highly variable crustal thickness and relatively large average value does not align with the prediction of passive mantle upwelling models. Instead, it can be explained by a model of buoyant active mantle flow driven by thermal and compositional density changes owing to melt extraction. The influence of active versus passive upwelling is predicted to increase with decreasing spreading rate. The process of active mantle upwelling is anticipated to be primarily influenced by mantle temperature and composition. This implies that the observed variability in crustal accretion, which includes notably varied crustal thickness, is probably an inherent characteristic of ultraslow-spreading ridges.

Successful subduction of oceanic plate after failed attempts in the Late Archean: Petrological and geochemical constraints

Successful subduction of oceanic plate after failed attempts in the Late Archean: Petrological and geochemical constraints

Earthquake Seismicity Reveals the Location and Significance of the Shona Mantle Plume in the South Atlantic Ocean

AbstractThe South Atlantic Ocean hosts several well‐studied volcanic ridges and seamount chains, but the origin of their associated mantle plumes is debated. Reduced seismicity on the southern Mid‐Atlantic Ridge (MAR) suggests anomalously ductile thermomechanical conditions at 52°S and 47.5°S. These low seismicity patches extend 120–560 km along‐axis, and correspond with axial high spreading ridge morphology, geochemical anomalies, and mantle wave speed patterns likely associated with the Shona and Discovery plumes. Bathymetric data show that the northern extent of the Shona swell is associated with increased volcanism, elevated axial bathymetry, and a series of northward‐propagating rifts, with the overall swell geometry suggesting a buoyancy flux of 0.4–0.5 Mg s−1. The nearby Bouvet Island may be a product of a branch of the larger Shona plume swell, which has influenced crustal accretion on the southern MAR for the past 24 million years.

Reinterpretation of Marine Magnetic Anomalies in the Bay of Bengal: Implications for Plate Tectonics During the Mid‐Cretaceous

AbstractThe breakup of India from Antarctica in the Early Cretaceous and subsequent seafloor spreading in the NW‐SE direction initiated the formation of the Bay of Bengal and its conjugate Enderby Basin. Two hypotheses are proposed to explain the evolutionary tectonics between India and Antarctica. One involves a single breakup, while the second invokes another breakup episode. Further, a major reorganization of the plate boundaries in the mid‐Cretaceous caused the spreading center to align E‐W by Late Cretaceous. The tectonics involved in this spreading direction change is unknown and difficult to understand as it occurred in the mid‐Cretaceous, coincident with the Cretaceous Normal Superchron. In the present study, we have generated a revised magnetic isochron map of the Bay of Bengal and used the modulus of its analytic signal to understand the evolutionary tectonics between the two plates, India and Antarctica. Mesozoic magnetic anomalies M12n to M0, and mid‐Cretaceous incursion Q2 (108 Ma) are identified in the western basin of the Bay of Bengal, while Q2, Q1 (92 Ma) and Late Cretaceous isochron A34 are inferred in its eastern basin. Gravity grid reconstructions demonstrate asymmetrical crustal accretion in the conjugate corridors. Fan‐shaped spreading and a southward ridge jump between 108 and 92 Ma in the eastern basin, aided the ∼40° clockwise rotation of the spreading center. The presence of volcanic basement beneath the additional crust suggests on‐axis mode of formation for the Ninetyeast Ridge by 100 Ma. The present study favors the two‐phase breakup model for the evolution of these plates.

Unexpectedly High Magma Productivity Inferred From Crustal Roughness and Residual Bathymetry on the Eastern Part of the Ultra‐Slow Spreading Gakkel Ridge Since ∼45 Ma, Eurasian Basin, Arctic Ocean

AbstractThe Gakkel Ridge in the Eurasian Basin has the slowest seafloor spreading worldwide. The western Gakkel Ridge (3°W–85°E; 14–11 mm/a) alternate between magmatic and sparsely magmatic zones, while the eastern Gakkel Ridge (85–126°E; 11–6 mm/a) appears to be dominated by magmatic zones despite ultraslow spreading. Little is known about the seafloor spreading conditions in the past along the entire ridge. Here, we exploit the residual bathymetry and basement roughness to assess the crustal accretion process of the Gakkel Ridge over time using 23 published regional multichannel seismic reflection profiles. Full seafloor spreading rates were faster (20–24 mm/a) up to ∼45 Ma, and residual bathymetry for the older crust is deeper than the world average in the entire Eurasian Basin. There is a sharp transition to 300–400 m shallower residual bathymetry for seafloor <45 Ma in the eastern Eurasian Basin. The crustal roughness versus spreading rate of the western Eurasian Basin is on the global trend, while that of the eastern is significantly below. Both low roughness and shallow residual bathymetry of the eastern Eurasian Basin is close to that of oceanic crust for spreading rates above 30 mm/a, demonstrating increased magmatic production of the eastern Gakkel Ridge since ∼45 Ma. A recent mantle tomography model predicts partial melting in the upper mantle based on the low Vs anomaly underneath. The sedimentary pattern toward the Lomonosov Ridge indicates that this hot mantle anomaly started to cause dynamic uplift of the area at ∼45 Ma.

Accretion of the Lower Oceanic Crust at Fast-Spreading Ridges: Insights from Hess Deep (East Pacific Rise, IODP Expedition 345)

Abstract At mid-ocean ridges, melts that formed during adiabatic melting of a heterogeneous mantle migrate upwards and ultimately crystallize the oceanic crust. The lower crustal gabbros represent the first crystallization products of these melts and the processes involved in the accretion of the lowermost crust drive the chemical evolution of the magmas forming two thirds of Earth’s surface. At fast-spreading ridges, elevated melt supply leads to the formation of a ⁓6-km-thick layered oceanic crust. Here, we provide a detailed petrochemical characterization of the lower portion of the fast-spread oceanic crust drilled during IODP Expedition 345 at the East Pacific Rise (IODP Holes U1415), together with the processes involved in crustal accretion. The recovered gabbroic rocks are primitive in composition and range from troctolites to olivine gabbros, olivine gabbronorites and gabbros. Although textural evidence of dissolution-precipitation processes is widespread within this gabbroic section, only the most interstitial phases record chemical compositions driven by melt-mush interaction processes during closure of the magmatic system. Comparing mineral compositions from this lower crustal section with its slow-spreading counterparts, we propose that the impact of reactive processes on the chemical evolution of the parental melts is dampened in the lower gabbros from magmatically productive spreading centres. Oceanic accretion thereby seems driven by fractional crystallization in the lower gabbroic layers, followed by upward reactive percolation of melts towards shallower sections. Using the composition of clinopyroxene from these primitive, nearly unmodified gabbros, we estimate the parental melt trace element compositions of Hess Deep, showing that the primary melts of the East Pacific Rise are more depleted in incompatible trace elements compared to those formed at slower spreading rates, as a result of higher melting degrees of the underlying mantle.

Mesozoic rifting in SW Gondwana and break-up of the Southern South Atlantic Ocean

Abstract The opening of the South Atlantic Ocean in the Early Cretaceous was only the final stage of the complex rifting process of SW Gondwana. In this contribution, we reassess the chronology of Mesozoic basin formation in southern South America and Africa and integrate it in the long-term rifting and break-up history of SW Gondwana. During the Triassic, after the Gondwanides orogeny, plate-scale instabilities produced intracontinental rifting in Africa, and retro-arc extension on the SW-margin of Gondwana. This process was followed and accentuated by the impingement of the Karoo plume in the Early Jurassic, which triggered rifting in East Africa and ultimately produced the break-up of Eastern from Western Gondwana in the Middle Jurassic. Retro-arc extension continued to affect the palaeo-Pacific margin, with emplacement of the Chon Aike magmatic province in the Patagonian retro-arc during the Early–Middle Jurassic. By the Late Jurassic, retro-arc rifting reached a point of oceanic crust accretion, with the establishment of the Rocas Verdes back-arc basin in southern Patagonia, together with the formation of the Weddell Sea further south, between the South American plate and Antarctica. The core of the Late Paleozoic Gondwanides orogen, between southern South America and Africa, was subjected to oblique rifting at this time and produced the Outeniqua and Rawson/Valdés basins. This area was the locus of extension and oceanization in the Early Cretaceous associated with a rotation of the stress field from NE–SW to east–west extension. The formation of the South Atlantic Ocean resulted from lithospheric extension and was accompanied by extensive intrusive magmatism and extrusive flood basalts identified as seaward dipping reflectors, which were emplaced diachronically from south to north, along different segments along both conjugate margins. These volcanic rocks form the South Atlantic Large Igneous Province. The chronology of the South Atlantic opening and the magmatic sources and processes associated with the formation of seaward dipping reflectors remain interpretative as they have only been studied on seismic data but are still undrilled; hence, scientific drilling will be key to unravel many of these unknowns.

On the Relative Importance of Buoyancy and Thickening of Aging Lithosphere in Mantle Upwelling and Crustal Production Beneath Global Mid‐Ocean Ridge System

AbstractBeneath mid‐ocean ridges, mantle upwelling and decompression melting are fundamental processes contributing to the formation of oceanic crust. Previous geodynamic models have suggested that mantle upwelling driven by separating plates can be intensified by thermochemical buoyancy and the thickening of aging lithosphere, resulting in thicker crust. However, the relative contributions of these factors to crustal accretion remain uncertain. We conducted numerical modeling in three scenarios to investigate this: (a) buoyant flow models incorporating age‐dependent lithospheric thickening as a reference; (b) passive flow models incorporating age‐dependent lithospheric thickening to isolate the effect of buoyancy; and (c) passive flow models that neglect age‐dependent lithospheric thickening to isolate its effect. Models are performed under varying potential mantle temperatures (Tp), viscosity structures, and spreading rates. The model‐predicted crustal thickness suggests that both buoyancy and thickening of aging lithosphere increasingly contribute to crustal production as spreading rate decreases. However, given the range of reference mantle viscosities examined here (1019–1020 Pa s), except in ultraslow spreading rates with lower reference viscosity and warmer Tp, the impact of the thickening of aging lithosphere predominates over buoyancy. Furthermore, the relative importance of buoyancy‐induced crustal production increases as spreading rates decrease. Variations in Tp further amplify the variability in crustal thickness compared to that at fast‐spreading ridges where passive upwelling dominates, when combined with 3‐D effects of buoyant flow on altering axial crustal production at ultraslow‐spreading ridge segments. This 4‐D effects of buoyant flow on melt production may explain the observed more variable crustal thickness at ultraslow‐spreading ridges.

Constraints on the Timing and Lower Crustal Accretion at the Schulz Massif, Mohns Ridge, Arctic Mid Ocean Ridges

AbstractThe ultraslow‐spreading Mohns Ridge is a key supersegment of the Arctic Mid‐Ocean system, where it represents a boundary between the Jan Mayen hotspot in the south and the highly anomalous Knipovich Ridge to the north. Understanding the timing and mode of Plio‐Pleistocene seafloor spreading along this ridge segment is critical for establishing the recent geodynamic evolution of the Norwegian‐Greenland Sea. To investigate magmatic accretion at this ultraslow spreading ridge, we collected samples from the Schulz Massif, which is located off‐axis at 73.4°N and exposes gabbroic intrusives and mantle peridotite. The petrology and petrography of these samples indicate that the exposed crustal section underwent multiple episodes of magmatism, which are characterized by distinct crystal sizes and geochemistry. To calibrate the age of the seafloor, we combined high‐resolution and high‐precision single zircon U‐Pb geochronology. Our data suggest that seafloor spreading has been nearly symmetrical for the last ∼4.6 Myr with a time‐averaged half‐spreading rate of ∼7.4 mm yr−1. Crystal size analysis of olivine in porphyric intrusions suggests that the crustal section was fed crystal‐laden melts with recurrence rates predicted to stabilize fault‐dominated seafloor spreading. Our combined geochronological and crystal size approach gives a critical perspective on the mode of seafloor spreading in the Mohns Ridge and allows insights into accretionary mechanisms and crustal structures during symmetric seafloor spreading.

Kinematics of the Reykjanes Ridge: Influence of the Iceland Hotspot on Plate Boundary Evolution

AbstractThe slow spreading Reykjanes Ridge overlies the Iceland hotspot and has undergone well ordered changes in crustal segmentation. Previous studies have attributed these changes to varying mantle plume thermal effects, rendering the lithosphere ductile or brittle. Here we use seafloor spreading magnetic anomalies to show that crustal accretion has been focused throughout its spreading history and to determine the detailed evolution of Reykjanes Ridge segments. By ∼53 Ma, organized spreading had developed on an orthogonally spreading linear axis following continental breakup. After a plate motion change at ∼38 Ma, orthogonally spreading offset ridge segments formed by ridge propagation forming varying length fracture zones. From then to the present, the offset segments diachronously migrated back to the original linear geometry from north to south replacing orthogonal with oblique spreading as the axis became linear again. Fracture zones were not terminated, however, simply by reducing segment offsets even to zero. Their termination involved the axial propagation of buoyant upwelling instabilities across the discontinuities, correspondingly extending V‐shaped crustal ridges southward. This evolution was guided by a persistent linear deep damp mantle melting interval maintained by the episodic propagation of buoyant upwelling instabilities. Our study indicates that at slow spreading ridges, where buoyant upwelling instabilities govern crustal segmentation, spatial gradients in mantle melting properties may direct the behavior of the instabilities. Where ridges overlie regional hotspot gradients in mantle melting, buoyant instabilities may propagate systematically, and plate boundary evolution may follow an organized pattern.

Relationship Between D‐MORB and E‐MORB Magmatism During Crustal Accretion at Mid‐Ocean Ridges: Evidence From the Masirah Ophiolite (Oman)

AbstractEnriched mid‐ocean ridge basalts (E‐MORB) commonly erupt at mid‐ocean ridges (MOR) and seamounts, but their relationship to “depleted” MORB (D‐MORB) and the processes controlling their magmatic evolution at MORs are not fully understood, hence raising more general questions about magma generation in the mantle. We here explore this conundrum through an investigation of the Masirah ophiolite (southeast Oman), a near‐unique “true” MOR ophiolite. Unlike most (e.g., Tethyan) ophiolites, it was not affected by subduction and is therefore potentially able to provide valuable geological insights into the magmatic evolution of a full section of oceanic crust. Previous work has shown that the igneous crust at Masirah was thin (1.5–2.0 km) and constructed from both D‐ and E‐MORB magmas, concluding that it formed at a slow‐spreading ridge at ∼150 Ma followed by an episode of “Nb‐enriched” magmatism with trace‐element enrichments exceeding E‐MORB during intraplate rifting ∼20 Ma later. We reinvestigate the geology of Masirah and present new field observations, geochemical data and high‐precision U‐Pb ages to constrain the magmatic history of seafloor spreading and off‐axis magmatism. We found that D‐MORB and E‐MORB magmatism at Masirah was synchronous and overlapped in both composition and time with the Nb‐enriched magmatism (no older than 135 Ma). Both types of magmatism were therefore integral in the formation of the Masirah ocean crust. The relationship between D‐MORB and E‐MORB magmatism described here may be applicable to modern MORs more broadly, but is especially prominent at Masirah due to reduced magmatism and hence a weaker crustal filter.