Oceanic Large Igneous Provinces Research Articles

Cretaceous large igneous provinces: from volcanic formation to environmental catastrophes and biological crises

Abstract The Cretaceous Period was marked by the formation of numerous large igneous provinces (LIPs), several of which were associated with geologically rapid climate, environmental and biosphere perturbations, including the early Aptian and latest Cenomanian oceanic anoxic events (OAEs 1a and 2, respectively). In most cases, magmatic CO 2 emissions are thought to have been the major driver of climate and biosphere degradation. This work summarizes the relationships between Cretaceous LIPs and environmental perturbations, focusing on how volcanism caused climate warming during OAE 1a using osmium-isotope and mercury concentration data. The new results support magmatic CO 2 output from submarine LIP activity as the primary trigger of climate warming and biosphere stress before/during OAE 1a. This submarine volcanic trigger of OAE 1a (and OAE 2), two of the most climatically/biotically severe Cretaceous events, highlights the capacity of oceanic LIPs to impact Earth's environment as profoundly as many continental provinces. Cretaceous magmatism (and likely output of CO 2 and trace-metal micronutrients) was apparently most intense during those OAEs; further studies are needed to better constrain the eruption histories of those oceanic plateaus. Another open question is why the Cretaceous Period overall featured a higher rate of magmatic activity and LIP formation compared with before and afterwards.

Geological history of the Atira Mons large shield volcano, Beta Regio, Venus.

Atira Mons is a large (∼300,000 km2) low-relief (1.8 km) shield volcano, with individual flows extending up to ∼700 km from the central caldera. It is located about 3000 km NW from the major plume center Beta Regio. Detailed mapping of the flows (at 1:500,000 scale, 10x more detailed than previous mapping) has identified fifty-three flow units which are grouped into eleven packages. Flow units are distinguished based on radar brightness, topography, morphology, continuity, structural modification, and sources, while flow packages group flows with clear stratigraphic relationship affinity.Cross-cutting relationships indicate a complex and multi-episodic eruption history with provisional identification of six mapped stages. The most voluminous flows are concentrated in the early stages, while the younger pulses, with a few exceptions, are shorter and less voluminous. A central caldera hosts the youngest volcanism with flows breaching its eastern side. Multiple stages of caldera collapse are indicated.The volume of the volcano is estimated using various methods and yields values range from ∼47,000 to ∼270,000 km3. The larger estimates are consistent with that of the magma volume of Large Igneous Provinces (LIP) on Earth. An appropriate terrestrial analogue is the Benham Rise Oceanic LIP in the western margin of the Philippine Sea, and particularly the Apolaki Caldera, which is the world's largest known basaltic caldera with a diameter of ∼150 km.

The role of subduction in the formation of Pangaean oceanic large igneous provinces

Abstract Large igneous provinces (LIPs) have been linked to both surface and deep mantle processes. During the formation, tenure and break-up of the supercontinent Pangaea, there is an increase in emplacement events for both continental and oceanic LIPs. There is currently no clear consensus on the origin of LIPs, but a hypothesis relates their formation to crustal emplacement of hot plume material originating in the deep mantle. The interaction of subducted slabs with the lowermost mantle thermal boundary and subsequent return flow is a key control on such plume generation. This mechanism has been explored for LIPs below the interior of a supercontinent (i.e. continental LIPs). However, a number of LIPs formed exterior to Pangaea (e.g. Ontong Java Plateau), with no consensus on their formation mechanism. Here, we consider the dynamics of supercontinent processes as predicted by numerical models of mantle convection and analyse whether circum-supercontinent subduction could generate both interior (continental) and exterior (oceanic) deep mantle plumes. Our numerical models show that subduction related to the supercontinent cycle can reproduce the location and timing of the Ontong Java Plateau, Caribbean LIP and potentially the Shatsky Rise by linking the origin of these LIPs to the return flow that generated deep mantle exterior plumes.

Limited sulfur degassing and muted environmental impact of Ontong Java Plateau lavas

The mainly deep-submarine Ontong Java Plateau (OJP) is the result of the largest outpouring of lava in the geologic record. Volcanic events of this magnitude can have dramatic environmental impacts due to volatile emissions. We report new S measurements in naturally glassy, olivine-hosted melt inclusions and pillow basalt glasses from the OJP. We combined these data with previous S measurements in OJP glasses to quantify S degassing in a suite of OJP glasses. Comparison with an S degassing model suggests OJP lavas that erupted at depths ~>1500 m did not degas S; OJP lavas that erupted at depths ~<1500 m degassed up to ~40% initial S, but these lavas likely made up a small fraction of OJP lavas. This result suggests that despite its large volume compared to continental large igneous provinces (LIPs), OJP lavas emitted less S, potentially contributing to its muted environmental impact. The OJP may provide a framework for the temporal evolution of S degassing at oceanic LIPs, with early eruptions at great water depths releasing limited to no S, and later eruptions at shallow water depths releasing larger, but still limited amounts of S. This framework may also have implications for continental LIP magmas, which may release significant amounts of CO2 but limited amounts of S during intrusive activity, with magmatic S emissions only becoming important during extrusive phases.

Long-term Phanerozoic sea level change from solid Earth processes

The sedimentary rock record suggests that global sea levels may have fluctuated by hundreds of meters throughout Phanerozoic times. Long-term (10–80 Myr) sea level change can be inferred from paleogeographic reconstructions and stratigraphic methods can be used to estimate sea level change over 1–10 Myr in tectonically quiescent regions assumed to be stable. Plate tectonic reconstructions and mantle flow models make it possible to isolate, quantify and estimate the contribution of different solid Earth mechanisms to sea level change through time, including: the volume of water deeper than mid-oceanic ridges, mantle dynamic topography, marine sedimentation, oceanic large igneous province emplacement, deep-water cycle, volume above oceanic trenches and changes in continental area. Although these processes are intrinsically linked, their impact on sea level change is rarely studied in combination, and time-dependent models of long-term eustasy from tectonic and geodynamic processes are in their infancy. Here we couple plate tectonic reconstructions with time-dependent models of past mantle flow and develop a new holistic framework to model sea level change that accounts for the main solid Earth drivers of eustatic rise and fall. We present the first model of the effect of individual solid Earth mechanisms on eustatic change over the past 560 Myr, including time-dependent continental and oceanic dynamic topography, volume above oceanic trenches and changing continental slope. Our results are consistent with the Cretaceous highstand and Paleozoic and early Mesozoic lowstands deduced from stratigraphy, provided that the deep-water cycle contributed ∼240 m of sea level drop since 250 Myr. We find that changes in the volume of water below the depth of mid-ocean ridges are not balanced by changes in the volume of water above subduction zones or by global dynamic topography. Our results confirm that a young seafloor decreased the volume of water below the depth of ridges and contributed +280 m [+90/−0 m] to a sea level high at approximately 120 Ma, and show that changes in dynamic topography were the primary driver (−320 m [+80/−60 m]) of lowering sea level between ∼240–160 Ma when Pangea was assembled. Predicted sea level changes by up to −220/+470 m when Panthalassan mid-ocean ridge spreading rates are varied between 50 mm/yr and 200 mm/yr. We compute sea level estimates for two alternative global tectonic reconstructions and while the results differ (by ∼150 m) between 250-560 Ma, both suggest a sea level fall during Pangea assembly (between ∼380-320 Ma) that is consistent with published constraints.

A New Tectonic Model Between the Madagascar Ridge and Del Cano Rise in the Indian Ocean

AbstractThe southern Indian Ocean has several prominent aseismic ridges recognized as oceanic large igneous provinces (i.e., the Madagascar Ridge, Del Cano Rise, Crozet Plateau, and Conrad Rise) in the off‐axis areas of the Southwest Indian Ridge (SWIR). However, previously obtained magnetic survey lines are sparse and not correctly aligned with the seafloor spreading direction; thus, the detailed spreading history, including the formation of these aseismic ridges, remains an open question. We reconstructed the tectonic history of two segments between the Discovery II and Gallieni fracture zones in the SWIR using newly obtained magnetic data (total and vector magnetic field) and an open‐source magnetic data set. We revealed that the southern Madagascar Ridge and the Del Cano Rise once formed a single bathymetric high and separated by at least Chron 30y, which is quite different from the global age model. In addition, the rises may have formed before Chron 34y, assuming an extinct ridge south of the Del Cano Rise. The two rises have been recognized as having formed by Marion hotspot plume‐induced excess volcanism around the active spreading ridge of the SWIR, which can explain locally isostatically compensated thicker‐than‐normal crust. However, linear magnetic anomalies have not been observed across the main part of these rises, suggesting that magmatic activity controlled by seafloor spreading is unlikely. Like other aseismic ridges in the southern Indian Ocean, these two rises may possibly have been formed partly by continental fragments rather than plume‐induced excess volcanism.

The importance and difficulties of identifying mantle plumes in orogenic belts: An example based on the fragmented large igneous province (LIP) record in the Ural fold belt

Mantle plumes can be recognized by their magmatic expression as large igneous provinces (LIPs). However, identification of plumes in old, structurally complicated fold belts is particularly difficult due to deformation, which obscures the LIP record. On the other hand, fold belt regions are particularly important in the search for LIPs for at least three reasons: 1) they can represent prior plate margins associated with plume-generated continental breakup and LIP magmatism; 2) the deformation may expose basement rocks (containing LIP units) covered by younger sedimentary rocks elsewhere in the continental block; and 3) they preserve deformed remnants of oceanic LIPs and hot spot chains accreted during ocean closure.Herein we provide an initial survey of the plume /LIP record of one of the world’s great orogenic belts, the Ural fold belt. The following events are identified: The 1750 Ma Navysh event is coeval with units in Sarmatia and Karelia (other parts of Baltica) and on other crustal blocks. The 1385 Ma Mashak event is associated with a range of ore deposit types, is part of Nuna supercontinent breakup, and is postulated to have had a global environmental impact linked to the Calymmian-Ectasian boundary. The ca. 720 Ma Igonino event can be approximately matched with 720 Ma LIPs in northern Laurentia, and elsewhere, which can be linked to the onset of the Sturtian glaciation (Tonian-Cryogenian boundary). The ca. 480 Ma Kidryasovo and 450 Ma Ushat events have age matches in Siberia and other crustal blocks; the ages approximately match the end-Cambrian and end-Ordovician periods, respectively. The 370 Ma Timaiz event belongs to the c. 370 Ma Kola-Dniepr LIP which is widespread in Baltica, has an age match in Siberia and collectively can be linked with the end Devonian period. An Early Carboniferous (350–320 Ma) event follows island-arc/continent collision and slab break-up in the Magnitogorsk zone. Three orogenic/postorogenic plume intraplate episodes are also described, the ca. 285 Ma Stepninsky monzogabbro-granosyenite-granite complex, the 308–304 Ma Kalymbaevsky lamproite complex and units that are coeval with the 251 Ma Siberian Traps LIP, linked to the Ural-Siberian superplume and with the end Permian mass extinction.

Coupled supercontinent–mantle plume events evidenced by oceanic plume record

AbstractThe most dominant features in the present-day lower mantle are the two antipodal African and Pacific large low-shear-velocity provinces (LLSVPs). How and when these two structures formed, and whether they are fixed and long lived through Earth history or dynamic and linked to the supercontinent cycles, remain first-order geodynamic questions. Hotspots and large igneous provinces (LIPs) are mostly generated above LLSVPs, and it is widely accepted that the African LLSVP existed by at least ca. 200 Ma beneath the supercontinent Pangea. Whereas the continental LIP record has been used to decipher the spatial and temporal variations of plume activity under the continents, plume records of the oceanic realm before ca. 170 Ma are mostly missing due to oceanic subduction. Here, we present the first compilation of an Oceanic Large Igneous Provinces database (O-LIPdb), which represents the preserved oceanic LIP and oceanic island basalt occurrences preserved in ophiolites. Using this database, we are able to reconstruct and compare the record of mantle plume activity in both the continental and oceanic realms for the past 2 b.y., spanning three supercontinent cycles. Time-series analysis reveals hints of similar cyclicity of the plume activity in the continent and oceanic realms, both exhibiting a periodicity of ∼500 m.y. that is comparable to the supercontinent cycle, albeit with a slight phase delay. Our results argue for dynamic LLSVPs where the supercontinent cycle and global subduction geometry control the formation and locations of the plumes.

Benham Rise unveiled: Morphology and structure of an Eocene large igneous province in the West Philippine Basin

The Benham Rise is an oceanic large igneous province at the western margin of the Philippine Sea. It has an ocean island basalt geochemistry, a surface area of 0.11 Mkm2 and a volume of ~0.13 Mkm3. A crustal thickness of ~15 km is estimated from gravity anomalies. Volcanism was most active in the early stages of its formation, during the Lutetian from ~47.9 Ma to 41.3 Ma, although volcanic activity extended to ~26 Ma.Multibeam bathymetry with almost 100% coverage has revealed the morphology of Benham Rise for the first time. It consists of a main body (~310 km by 330 km) with the Narra, Loro and Molave Spurs extending 100–200 km from its eastern side. The main body is built on a shield platform from ~5200 m to ~3800 m sub-sea. The platform flanks consist of ~3–15 km wide terraces with scarps as high as 100 m to 300 m. The platform is surmounted by a crest exhibiting caldera morphology at an average depth of ~2500 m. The crest is named Apolaki Caldera and may be the world's largest known caldera with a diameter of ~150 km. Features like a breached rim, intra-caldera benches, and a resurgent dome indicate a multi-phase volcanic history consisting of both quiet and explosive eruptions.The bathymetry and publicly available geological and geophysical data record three main stages in the tectonic history of Benham Rise – shield-building, caldera formation, and post-caldera/late stage volcanism. The platform base is interpreted to have formed over a hotspot beneath the Central Basin Spreading Center during the shield-building phase between ~47.9 Ma to ~42.5 Ma. The arrowhead shaped Molave Spur is interpreted to have formed late in this stage along a ridge that propagated away from the hotspot. The caldera formation followed and is interpreted to have occurred before ~41.3–41.5 Ma when Benham Rise was still joined with Urdaneta Plateau. The Narra Spur was formed by late-stage hotspot related volcanism, as Benham Rise and Urdaneta Plateau separated, and ended with the eruptions of the Vinogradov Seamount at ~26 Ma.

Contributions of Scientific Ocean Drilling to Understanding the Emplacement of Submarine Large Igneous Provinces and Their Effects on the Environment

The Ontong Java Plateau (OJP), Shatsky Rise (SR), and Kerguelen Plateau/Broken Ridge (KP/BR) represent three large igneous provinces (LIPs) located in oceanic settings. The basement lavas have been investigated through scientific ocean drilling and, in the case of the OJP, fieldwork on the emergent obducted portions of the plateau in the Solomon Islands. Such studies show that these three LIPs have very different characteristics. For example, the KP/BR still has an active hotspot, whereas the OJP and the SR do not. The OJP is remarkable in its compositional monotony across the plateau (the Kwaimbaita geochemical type), with minor compositional variation found at the margins (the Kroenke, Singgalo, and Wairahito types). Shatsky Rise shows more compositional variation and, like the OJP, has a dominant lava type (termed the type) in the early stages (Tamu Massif), but subsequent eruptions at the Ori and Shirshov massifs comprise isotopically and trace element enriched lavas, likely reflecting a change in mantle source over time. The KP/BR has highly variable basement lava compositions, ranging from lavas slightly enriched above that of normal mid-ocean ridge basalt in the northern portion (close to the South East Indian Ridge) to more enriched varieties to the south and on Broken Ridge, with a continental crust signature present in lavas from the southern and central KP/BR. The OJP and the KP/BR appear to have formed through punctuated magmatic events, whereas the SR was formed by one relatively long, drawn out event. The formation of oceanic LIPs has in many (but not all) cases been synchronous with oceanic anoxic events. This paper focuses on three oceanic plateaus to emphasize the debate surrounding the environmental impact such LIPs may have had, and also highlights the contributions of scientific ocean drilling to our knowledge of oceanic LIP formation and evolution. This new knowledge allows planning for future oceanic LIP drilling.

Second-stage Caribbean Large Igneous Province volcanism: The depleted Icing on the enriched Cake

A long-standing question has been whether the Lower Nicaraguan Rise in the western Caribbean Sea represents thinned continental crust or is part of the Caribbean Large Igneous Province (CLIP). During the R/V Meteor cruise M81/2, the northeastern part of the Lower Nicaraguan Rise including the Hess Escarpment was extensively sampled. Only volcanic and carbonate rocks were recovered and no metamorphic rocks or rocks with continental crustal affinities, indicating that at least the uppermost basement is volcanic and belongs to a volcanic province, probably the CLIP. We provide a comprehensive geochemical dataset, including major and trace elements and Sr-Nd-Hf-Pb double spike isotope ratios, for the volcanic rocks. The basalts are geochemically subdivided into two groups: 1) depleted and 2) enriched. The majority belong to the depleted group with 6–9 wt% MgO and highly depleted incompatible element (e.g. (La/Yb)N = 0.15–0.76, (La/Sm)N = 0.21–0.65) compositions and depleted radiogenic isotope (e.g. εNdi = 8.9–11.0, εHfi = 13.6–16.5, 206Pb/204Pbi = 18.28–18.98) ratios. The subordinate enriched group basalts have 5–8 wt% MgO and were solely recovered from the lower part of the central Hess Escarpment. These rocks possess enriched incompatible element ((La/Sm)N = 1.15–1.60, (La/Yb)N = 1.40–2.57) and isotopic (εNdi = 7.52–8.34, εHfi = 12.6–12.9, 206Pb/204Pbi = 18.87–19.07) ratios resembling E-MORB. The depleted group is distinct from Pacific and Atlantic MORB based on combined 206Pb/204Pb, 143Nd/144Nd and 176Hf/177Hf data. It is similar, but not identical to the highly depleted Gorgona komatiites, thought to be generated during the CLIP event, and depleted Galápagos hotspot track lavas accreted to the Pacific margin of Central America. Ages of ~81 Ma from rocks drilled on the Hess Escarpment postdate the main CLIP phase (95-83 Ma), and indicate that at least the northeastern part of the Nicaraguan Rise formed during a later volcanic event. Lithospheric thinning after the main CLIP phase is suggested to have triggered upwelling of still anomalously hot, partially-melted or unmelted CLIP mantle material to form the younger depleted group of lavas. Our investigations imply that second-stage melting to generate depleted compositions during formation of oceanic large igneous provinces, as has been recently proposed for the Manihiki Plateau, is a more common process than originally thought.

Multiphase magmatic and tectonic evolution of a large igneous province - Evidence from the crustal structure of the Manihiki Plateau, western Pacific

The Manihiki Plateau, a Large Igneous Province (LIP) in the western Pacific, has been proposed to be emplaced as part of the “Greater Ontong Java Event” during the early Cretaceous. Shortly after its formation, the Manihiki Plateau fragmented into multiple sub-provinces. Plate tectonic reconstructions ignore this fragmentation, treating the Manihiki Plateau as a single crustal block. By analyzing two seismic refraction/wide-angle reflection profiles crossing the two largest sub-provinces of the Manihiki Plateau, we provide new insight into their deep crustal structure and magmatic evolution. Our data indicate that the High Plateau and the Western Plateaus were emplaced as a single unit during an initial phase of massive magmatic activity, but later magmatic stages altered the individual sub-provinces considerably.The High Plateau has a crustal thickness of 20 km and its P-wave velocity distribution is comparable to previously surveyed oceanic LIPs. Strong secondary magmatic phases are visible by eruptive centers and former magmatic pathways in the middle and lower crust. The Western Plateaus, which have a crustal thickness decreasing gradually from 17.3 km (East) to 9.2 km (West), experienced smaller amounts of magmatism mainly along fault zones and on local seamounts.Therefore, we propose a distinct development of the two main sub-provinces of the Manihiki Plateau after their initial joint emplacement. The High Plateau experienced voluminous multi-phase magmatic accretion and extrusion, whereas the thinner Western Plateaus exhibit only relatively minor magmatic growth. Such a large difference in the evolution process has not been reported from any other oceanic LIP so far.

3D imaging of the crust and upper mantle beneath the Arctic Ocean from Rayleigh‐wave group‐velocity analysis

A 3D S‐velocity model for the crust and upper mantle beneath the Arctic Ocean is determined from the analysis of fundamental‐mode Rayleigh‐wave group velocity. This new model allows the characterization of the principal features present beneath the Arctic Ocean, in terms of S‐velocity, for a depth range from 0 to 400 km. In the depth range from 0 to 5 km, the basins with thicker sediments are imaged by the lowest S‐velocity values (2.2–2.3 km/s), whereas the highest S‐velocity values (3.2–3.3 km/s) are associated with a thin sedimentary cover. In the depth range from 5 to 10 km, the lowest S‐velocity values (2.6–2.7 km/s) are also shown in areas with a thick sedimentary cover (>10 km). The higher S‐velocity values (4.0–4.2 km/s) are associated with the areas in which the Moho depth is overcome. In the depth range from 10 to 35 km, also the S‐velocity values are higher (>4 km/s) for the zones in which the Moho depth is overcome. The S‐velocities determined in this study allows to consider the Alpha‐Mendeleev Ridge (formed by the Alpha Ridge and the Mendeleev Ridge) as a continuous feature, because the Alpha Ridge and the Mendeleev Ridge are characterized by similar S‐velocities (e.g., ~3.6 km/s for depths from 10 to 15 km). This region also can be considered as an oceanic large igneous province, in terms of S‐velocity, because the determined S‐velocity pattern is characteristic of these oceanic regions. In general, for the lithosphere (from 35 to 180 km depth), the S‐velocity can be easily correlated with the different age of the Arctic Ocean regions and with the surface tectonic features, whereas for the asthenosphere and the mantle below it (from 180 to 400 km depth), this correlation is not clear. In general, a small S‐velocity contrast (0.1–0.2 km/s) is found between the lithosphere and asthenosphere, which makes it difficult to determine this boundary. Opposite to this, the high S‐velocity contrast (0.4–0.5 km/s) found at the asthenosphere base allows the easy determination of this boundary.

Paleomagnetic and gravimetrical reconnaissance of Cretaceous volcanic rocks from the Western Colombian Andes: paleogeographic connections with the Caribbean Plate

The reconstruction of the tectonic evolution of the oceanic crust, including the recognition of ancient oceanic plumes and the differentiation between multiple and single oceanic arcs, relies on the paleogeographic analysis of accreted oceanic fragments found in orogenic belts. Here we present paleomagnetic and gravity data from Cretaceous oceanic basaltic and gabbroic rocks, the continental metamorphic basement, and their associated cover from northwestern Colombia. Based on regional scale tectonic reconstructions and geochemical constraints, such rocks have been interpreted as remnants of an oceanic large igneous province formed in southern latitudes, which was accreted to the sialic continental margin during the Late Cretaceous. Gravity analyses suggest the existence of a coherent high density segment separated by major suture zones from a lower density material related to the continental crust and/or thick sedimentary sequences trapped during collision. A characteristic paleomagnetic direction in Early and Late Cretaceous oceanic volcano-plutonic rocks, revealing a southeastern declination (D) and a negative inclination (I), may be interpreted in two different ways: (1a primary magnetization (tilt-corrected direction D = 130.3°, I = -23.3°, k = 23.4, α95 = 26.4°), suggesting clockwise rotation around 130°, and magnetization acquired in southern latitudes (range of 4°S to 21°S); or (2) a remagnetization event during a reverse interval of the Earth’s magnetic field in the Cenozoic (in situ direction D = 128.7°, I = -6.2°, k = 23.1, α95 = 26.1°), suggesting a counter-clockwise rotation around 50°. The first scenario seems more plausible, as it is consistent with previous paleomagnetic studies at other localities; it is compatible with a southern paleogeography for this block, and when integrated with other regional geological and paleomagnetic studies, supports a southern Pacific origin of a major oceanic block, formed as a part of a broader Cretaceous plateau that may have extended south or southwest of Galapagos. After its initial accretion, this block was subsequently fragmented due to the oblique SW-NE approach to the continental margin during the Late Cretaceous.

Trench-perpendicular Geochemical Variation Between two Adjacent Kermadec Arc Volcanoes Rumble II East and West: the Role of the Subducted Hikurangi Plateau in Element Recycling in Arc Magmas

Little is known about the effects that subducting an oceanic large igneous province (LIP) has on the petrogenesis of submarine arc volcanoes and their geochemical composition. The southern Kermadec arc represents a rare example where an LIP—the Hikurangi Plateau—is currently subducting and where its effect on mantle composition, element recycling and arc volcanism can be studied. We present mineral chemistry and whole-rock major and trace element, and Sr–Nd–Pb isotope data from samples recovered from the southern Kermadec arc volcanoes Rumble II East and Rumble II West, together with shipboard gravity and magnetic measurements. The Rumble II volcanoes (including a volcanic cone ∼10 km further west) form an ∼23 km long arc–backarc transect located ∼250 km north of New Zealand above the subducting Hikurangi Plateau. Although only a short distance apart, rocks from the two volcanoes have different mineral and whole-rock geochemical compositions. Lavas from Rumble II East are predominantly basaltic and contain primitive olivine phenocrysts (≤Fo91), high-Mg# clinopyroxene (≤96) and anorthitic plagioclase (≤An97). Geochemically these lavas are very diverse and cover a spectrum from low Th/Yb ( 1014) to higher Th/Yb (>0·15) at lower Ba/Th (<844). This spectrum, together with 206Pb/204Pb and 143Nd/144Nd in the range of 18·74–18·83 and 0·51309–0·51298 respectively (at similar to slightly elevated 87Sr/86Sr), suggests a mantle wedge that has undergone previous melt extraction and significant fluid addition from the subducting Pacific Plate and that contains sediment and HIMU-type Hikurangi Plateau components. The geochemistry of the sediment–HIMU-type components is exemplified in an olivine pyroxenite (e.g. 206Pb/204Pb = 20·02; 87Sr/86Sr = 0·70516; 143Nd/144Nd = 0·5126). We propose that the olivine pyroxenite formed through melt or fluid–rock metasomatism and represents the first direct evidence of a near Moho arc mantle rock that shows the imprint from a subducting HIMU-type (Hikurangi) seamount. Conversely, lavas from Rumble II West and the cone ∼10 km to the west are generally more silica rich than Rumble II East lavas and mainly contain plagioclase with less ortho- and clinopyroxene + olivine phenocrysts. The low Ba/Th (<470) and 206Pb/204Pb (<18·74), a range of 143Nd/144Nd (0·51297–0·51307) and elevated Th/Yb (0·13–0·39) in these lavas can best be explained by minor sediment input into a less depleted mantle wedge. In addition, the geochemical composition of the Rumble II West lavas does not require involvement of a Hikurangi component, placing a spatial limit on Hikurangi material influencing regional melt generation beneath the backarc. Supported by a gravity model requiring two distinct magma chambers, the different geochemical compositions of Rumble II East and West lavas are inconsistent with a shared magma plumbing system. The different geochemical compositions of lavas from the two Rumble II volcanoes furthermore demonstrate that across-arc geochemical heterogeneities can occur within a few kilometres and may originate from both a geochemically heterogeneous mantle wedge and Moho transition layer, recording inherited geochemical heterogeneities beneath the volcanoes.

Jurassic plume-origin ophiolites in Japan: accreted fragments of oceanic plateaus

The Mikabu and Sorachi–Yezo belts comprise Jurassic ophiolitic complexes in Japan, where abundant basaltic to picritic rocks occur as lavas and hyaloclastite blocks. In the studied northern Hamamatsu and Dodaira areas of the Mikabu belt, these rocks are divided into two geochemical types, namely depleted (D-) and enriched (E-) types. In addition, highly enriched (HE-) type has been reported from other areas in literature. The D-type picrites contain highly magnesian relic olivine phenocrysts up to Fo93.5, and their Fo–NiO trend indicates fractional crystallization from a high-MgO primary magma. The MgO content is calculated as high as 25 wt%, indicating mantle melting at unusually high potential temperature (T p) up to 1,650 °C. The E-type rocks represent the enrichment in Fe and LREE and the depletion in Mg, Al and HREE relative to the D-type rocks. These chemical characteristics are in good accordance with those of melts from garnet pyroxenite melting. Volcanics in the Sorachi–Yezo belts can be divided into the same types as the Mikabu belt, and the D-type picrites with magnesian olivines also show lines of evidence for production from high T p mantle. Evidence for the high T p mantle and geochemical similarities with high-Mg picrites and komatiites from oceanic and continental large igneous provinces (LIPs) indicate that the Mikabu and Sorachi–Yezo belts are accreted oceanic LIPs that were formed from hot large mantle plumes in the Late Jurassic Pacific Ocean. The E- and D-type rocks were formed as magmas generated by garnet pyroxenite melting at an early stage of LIP magmatism and by depleted peridotite melting at the later stage, respectively. The Mikabu belt characteristically bears abundant ultramafic cumulates, which could have been formed by crystal accumulation from a primary magma generated from Fe-rich peridotite mantle source, and the HE-type magma were produced by low degrees partial melting of garnet pyroxenite source. They should have been formed later and in lower temperatures than the E- and D-type rocks. The Mikabu and Sorachi Plateaus were formed in a low-latitude region of the Late Jurassic Pacific Ocean possibly near a subduction zone, partially experienced high P/T metamorphism during subduction, and then uplifted in association with (or without, in case of Mikabu) the supra-subduction zone ophiolite. The Mikabu and Sorachi Plateaus may be the Late Jurassic oceanic LIPs that could have been formed in brotherhood with the Shatsky Rise.

Picrites in central Hokkaido: Evidence of extremely high temperature magmatism in the Late Jurassic ocean recorded in an accreted oceanic plateau

The Sorachi-Yezo belt, central Hokkaido, Japan, is composed of voluminous tholeiitic basaltic volcanics, and has been thought to be accreted fragments of an oceanic plateau formed in the Late Jurassic Pacific Ocean. Picrites have been reported as pillow lava and hyaloclastite from the Sorachi-Yezo belt. These picrites are characterized by very magnesian olivine phenocrysts (up to Fo [=100* Mg/(Mg+Fe)] = 94.1), which indicate that the primary magma was unusually Mg rich. The estimated MgO of the primary magma and the mantle potential temperature are as high as 29 wt% and 1700 °C, respectively, comparable to those of the Neoarchean komatiites, and higher than those of the Gorgona komatiites and picrites. The rare earth element patterns of the Sorachi-Yezo picrites are divided into two groups that are chemically akin to the Neoarchean komatiites and Gorgona komatiites and picrites, indicating different melting regimes in an extremely hot mantle plume. The Sorachi-Yezo picrites provide evidence for extremely high temperature magmatism, like that of Archean komatiite, caused by melting of the hottest mantle plume among the Phanerozoic oceanic large igneous provinces.

The Caribbean: an oroclinal basin?

Abstract Tightly curved mountain belts are prominent features of global topography. Typically, these ‘oroclines’ occur in areas of regional compression but enclose basins where extension has been contemporaneous with outward directed thrusting in the orogens. Examples of such basin–orogen pairs include the Alboran Sea–Gibraltar Arc, Tyrrhenian Sea–Aeolian Arc, Aegean Sea–Hellenic Arc and Pannonian Basin–Carpathian Arc, all in the western Tethys but matched in the eastern Tethys by the Banda Sea and Outer Banda Arc. The development of the basins has been variously explained by gravitational collapse of rapidly elevated mountain blocks and by extrusion prompted by asthenopheric flows, but it is not even universally agreed that similar processes have operated in all cases. Critics have cited gross differences in volcanic activity (absent from the Gibraltar Arc, modest in the Carpathians but intense in other examples) and in the presence or absence of recognizable Wadati–Benioff Zones. The superficial similarities between the Caribbean Sea–Antilles Arcs and typical oroclinal basin–orocline pairs have recently been invoked in support of an in situ Caribbean evolutionary model, even though the disputed origins of oroclines limit their reliability as analogues. The Caribbean's considerably greater area further emphasizes the need for caution, while the most obvious objection to identifying it as a member of the oroclinal group is its very long history. Oroclinal basins typically pass from initiation to effective stabilization in a few tens of millions of years, whereas the original Caribbean oceanic crust, which is now bounded to the east and west by active subduction zones, was probably formed in the Jurassic. Rather than invoking an overall common origin for the Caribbean and the Tethyan basins, it is more useful to look for shared causes of specific individual similarities. The impact of a rigid block might be as effective in imposing curvature on a mountain belt as rapid expansion in an adjacent area. However, it does seem that the case for the crust of the Caribbean being typical of oceanic large igneous provinces (LIPs) may have been overstated and, in the light of oroclinal analogues, that some features of the still poorly understood Beata Ridge and Lower Nicaragua Rise may be most easily explained by east–west extension promoted by the convergence between North and South America.

The Agulhas Plateau: structure and evolution of a Large Igneous Province

SUMMARY Large Igneous Provinces (LIP) are of great interest due to their role in crustal generation, magmatic processes and environmental impact. The Agulhas Plateau in the southwest Indian Ocean off South Africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity. With new geophysical data from seismic refraction and reflection profiling, we are able to present improved evidence for its crustal structure and composition. The velocity‐depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km, where three major units can be identified in the crust. In our seismic reflection records, evidence for volcanic flows on the Agulhas Plateau can be observed. The middle crust is thickened by magmatic intrusions. The up to 10 km thick lower crustal body is characterized by high seismic velocities of 7.0‐7.6 km s −1 . The velocity‐depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic LIPs such as the Ontong-Java Plateau or the northern Kerguelen Plateau. The total volume of the Agulhas Plateau was estimated to be 4 × 10 6 km 3 of which about 10 per cent consists of extruded igneous material. We use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material. The Agulhas Plateau was formed as part of a larger LIP consisting of the Agulhas Plateau itself, Northeast Georgia Rise and Maud Rise. The formation time of this LIP can be estimated between 100 and 94 (± 5) Ma.

Anomalous uplift and subsidence of the Ontong Java Plateau inferred from CO2 contents of submarine basaltic glasses

The Ontong Java Plateau in the western Pacific is anomalous compared to other oceanic large igneous provinces in that it appears to have never formed a large subaerial plateau. Paleoeruption depths (at 122 Ma) estimated from dissolved H2O and CO2 in submarine basaltic glass pillow rims vary from 1100 m below sea level (mbsl) on the central part of the plateau to 2200–3000 mbsl on the northeastern edge. Our results suggest maximum initial uplift for the plateau of 2500–3600 m above the surrounding seafloor and 1500 ± 400 m of postemplacement subsidence since 122 Ma. Our estimates of uplift and subsidence for the plateau are significantly less than predictions from thermal models of oceanic lithosphere, and thus our results are inconsistent with formation of the plateau by a high-temperature mantle plume. Two controversial possibilities to explain the anomalous uplift and subsidence are that the plateau (1) formed as a result of a giant bolide impact, or (2) formed from a mantle plume but has a lower crust of dense garnet granulite and/or eclogite; neither of these possibilities is fully consistent with all available geological, geophysical, and geochemical data. The origin of the largest magmatic event on Earth in the past 200 m.y. thus remains an enigma.