Subslab Asthenosphere Research Articles

Source variations in volatile contents of Bransfield Strait back-arc and Phoenix/West Scotia mid-ocean ridge lavas, northern Antarctic Peninsula

We present the first volatile contents (H2O, CO2, Cl, F, S) of young (< 6 Ma) submarine basaltic glasses from the Phoenix and West Scotia mid-ocean ridges and the Bransfield Strait back-arc of the South Shetland subduction zone in the Antarctic Peninsula.The volatile contents of the MORB glasses correspond well with those of published Pacific MORB and reflect covariations in source enrichment and extent of melting. Our results support the hypothesis that decreasing spreading rates at the Phoenix Ridge resulted in preferential melting of less abundant enriched MORB mantle, due to its greater fertility and higher volatile contents, relative to the more abundant depleted MORB mantle.The volatile contents of the Bransfield Strait back-arc glasses correlate with geochemical indicators of subduction processes and reveal an along-axis spatial distribution consistent with a toroidal inflow of sub-slab asthenosphere around the edges of the subducting plate into the mantle wedge. This inflow should be considered when assessing spatial and geochemical variability at subduction zones, particularly those with slab windows and tears.A small group of Bransfield Strait samples have volatile contents that do not correlate with geochemical signals of subduction influence. We speculate that these samples reflect flux melting of residual enriched mantle brought beneath the Bransfield Strait via corner flow following recent alkaline magmatism in the far eastern regions of the Antarctic Peninsula.Our new data on lavas from the W7 segment of the West Scotia Ridge reveal their source was significantly affected by subduction processes. Unexpectedly, these lavas have CO2-H2O pressures of vapor saturation that suggest they were collected in-situ and erupted relatively recently (∼6 Ma), at odds with previous interpretations of their origins. We suggest they originated from a subduction-modified mantle (lithosphere or asthenosphere) left behind by the eastward-migrating South Sandwich subduction zone sometime over the past ∼30 Myr. These lavas demonstrate the long-lasting effects of subduction processes on the upper mantle and their potential to influence melt compositions even in non-subduction environments today.We use the compositions of lavas from the Phoenix Ridge and Bransfield Strait to estimate mantle potential temperatures; our results agree well with global estimates for mid-ocean ridges and subduction zones, respectively.

Niobium-enriched basalts: Partial melting of a sediment-metasomatised mantle source in subduction zones?

Crust–mantle interaction and arc magmatism in subduction zones record the global recycling of elements from Earth's surface to its interior. Nb-enriched basalts are specific to arc settings and are generated through the partial melting of heterogeneous mantle sources or mantle wedge metasomatised by subducted slab-derived melts. However, it is unclear what role subducted sediments play in the formation of Nb-enriched basalts. We present the first report of Cambrian Nb-enriched basalts at A'qiang and Pulu in the Western Kunlun Orogen (NW China), which might have formed above a subducted ridge-related slab window. The A'qiang basalts (Nb = 6.7–7.7 ppm) exhibit E-MORB-like geochemical characteristics, with positive Nb and Ta anomalies, high Mo/Ce ratios (∼0.02), and positive εNd(t) (4.9–5.0) and low δ98/95Mo (−0.74‰ to −0.21‰) values. These basalts were generated through the partial melting of sub-slab asthenosphere modified by minor altered ocean crust and/or marly sediments. In contrast, the Pulu basalts (Nb = 10.9–13.7 ppm) are characterised by enrichment in light rare earth elements (LREEs), high Th/Ce (>0.1) and low Mo/Ce (∼0.001) ratios, negative εNd(t) (−1.5 to −1.1) and mantle-like zircon δ18O values (4.25‰–5.69‰), and high δ98/95Mo values (−0.03‰ to +1.17‰). The Pulu Nb-enriched basalts were derived from the partial melting of sub-slab asthenosphere modified by moderate siliciclastic sediments. Therefore, we propose that subducted sediments play a vital role in the transport of high field strength elements back into the mantle and their recycling into the crust, for example through the formation of Nb-enriched basalts.

Terminal stage of divergent double subduction: Insights from Early Cretaceous magmatic rocks in the Gerze area, central Tibet

Divergent double subduction (DDS) plays an important role in facilitating tectonic processes such as ocean closure, accretion and amalgamation of magmatic arcs, and continental growth. However, DDS geodynamics remain poorly constrained, especially its evolution and the accompanying magmatism at the terminal stage. According to geological and geophysical data, the Meso-Tethys Ocean (also known as the Bangong–Nujiang Tethyan Ocean) in central Tibet has been considered a Mesozoic example of DDS, which provides a rare opportunity to evaluate the geodynamic processes of the terminal stage of a DDS system. Here, we present new geochronological, geochemical, and isotopic data of volcanic rocks from the Gerze area in the western segment of the Bangong–Nujiang suture zone (BNSZ). Combined with previous data, we found that the ca. 105 Ma magmatic rocks in the Gerze area are distributed not only along the two sides of the BNSZ but also as manifold types. On the northern side, the magmatism ca. 105 Ma varied, including A-type rhyolites, bajaitic latites, high-Nb basalts, N-MORB-type basalts, adakites and bimodal volcanic rocks, which originated from sources including a sinking oceanic slab, asthenospheric mantle, metasomatized mantle and overlying continental crust. The lithology on the southern side only includes A-type rhyolite and magnesia-rich andesite and adakitic intrusions. Spatial and compositional variations in ca. 105 Ma magmatism should have been the response to slab breakoff, which may have occurred only in the northern branch of the Meso-Tethys oceanic slab after ocean closure. Our results not only verify the existence of slab breakoff in the classical DDS model but also emphasize that breakoff can cause upwelling of the subslab asthenosphere, which leads to a complex magmatic response. • Manifold magmatism from the Gerze area, central Tibet, is active at ca. 105 Ma. • The ca. 105 Ma Gerze magmatic rocks record a slab breakoff event. • Slab breakoff plays an important role in the divergent double subduction system.

Anisotropic tomography of the Cascadia subduction zone

The first P-wave tomography of 3-D azimuthal and radial anisotropy of the Cascadia subduction zone is determined by inverting local and teleseismic arrival-time data. Fast-velocity directions (FVDs) of azimuthal anisotropy in the crust are generally trench-parallel, reflecting N-S compression along the Cascadia margin. Radial anisotropy (RAN) is negative (i.e., Vpv > Vph) in the crust and upper-mantle wedge beneath the Cascadia volcanoes and back-arc area, reflecting hot and wet upwelling flows associated with fluids from dehydration reactions of the young and warm Juan de Fuca plate that is subducting toward the northeast. Trench-parallel FVDs occur in the subducting slab under the forearc, suggesting that the gently-dipping slab may still keep its original anisotropy produced at the mid-ocean ridge and modified at the outer-rise before subduction. The slab and subslab mantle exhibit the same RAN pattern: positive RAN in the Cascadia forearc whereas negative RAN under the Cascadia volcanoes and the back-arc. This feature suggests that the slab and the subslab asthenosphere are strongly coupled, and subslab mantle flow is formed by entrainment of the asthenosphere with the overriding slab. In northern Cascadia, NE-SW FVDs occur in a prominent subslab low-velocity zone that also exhibits negative RAN, reflecting thermally buoyant mantle materials derived from nearby oceanic hotspots, which flow toward the northeast and gradually accumulate under northern Cascadia, resulting in decompression melting.

Causes of the Occurrence of A-Type Volcanic Rocks in Active Continental Margins (Southern Sikhote-Alin, Russian Far East)

Abstract ––New isotope-geochemical data on the volcanic complexes of the South Yakut and Martel volcanic depressions in southern Primorye are presented. Their formation in the early Eocene (54.3 Ma) and Late Cretaceous (83.5 Ma), respectively, is evidenced by U–Pb zircon dating (LA-ICP-MS). Based on the geochemical characteristics, it is concluded that the volcanics are typical A-type igneous rocks. Their formation coincides with the sudden change in the vector of motion of the Pacific slab with respect to the continent in the Campanian and Paleocene–Eocene, which caused destruction of the slab with its probable discontinuity and the injection of the subslab asthenosphere. The effect of mantle fluids on the continental lithospheric-rock melting determined the generation of magmas with the specific geochemical features of A-type igneous rocks. The regularities of their composition are due to the deep-seated reduced F-rich fluids that caused the intense differentiation of magmas accumulating fluidized melts enriched in mobile components in the apical part.

ПОЗДНЕМИОЦЕН-ПЛИОЦЕНОВАЯ ТРАНСФОРМНАЯ ОКРАИНА КАМЧАТКИ

Testing of the geochemical compositions of the Late Cenozoic volcanites of Kamchatka on new discriminant diagrams confirmed the idea of the existence of different geodynamic regimes at this time. It is shown that the Late Miocene (~6 Ma)-Pliocene volcanites of Eastern Kamchatka and the Central Kamchatka Depression, as well as the Late Pliocene (~3.5 Ma)-Holocene alkaline, calcareous-alkaline, and adakite volcanites of the central part of the Middle Ridge are similar to the volcanites of the transform margins of the Pacific type. At the same time, the Miocene–Holocene volcanites of Southern Kamchatka, the Miocene-Early Pliocene volcanites of the Middle Ridge, and the Pleistocene–Holocene volcanites of Eastern Kamchatka are similar to the volcanites of the convergent margins. In the central part of Kamchatka (from the coast to the Middle Ridge), at the end of the Miocene-Pliocene, during the collision of the Kronotsky terrane of the island arc and the slip of the Pacific plate, magmatic complexes typical of the transform margin were formed in this area. The geochemistry of the transform margin volcanites is due to the upwelling of the subslab asthenosphere both into the collision zone and into the zone of the volcanic arc of the Middle Ridge, after the rupture and subsequent separation of the Komandor-Kronotsky microplate slab.

Distinct slab interfaces imaged within the mantle transition zone

Oceanic lithosphere descends into Earth’s mantle at subduction zones and drives material exchange between Earth’s surface and its deep interior. The subduction process creates chemical and thermal heterogeneities in the mantle, with the strongest gradients located at the interfaces between subducted slabs and the surrounding mantle. Seismic imaging of slab interfaces is key to understanding slab compositional layering, deep-water cycling and melting, yet the existence of slab interfaces below 200 km remains unconfirmed. Here, we observe two sharp and slightly dipping seismic discontinuities within the mantle transition zone beneath the western Pacific subduction zone that coincide spatially with the upper and lower bounds of the high-velocity slab. Based on a multi-frequency receiver function waveform modelling, we found the upper discontinuity to be consistent with the Mohorovicic discontinuity of the subducted oceanic lithosphere in the mantle transition zone. The lower discontinuity could be caused by partial melting of sub-slab asthenosphere under hydrous conditions in the seaward portion of the slab. Our observations show distinct slab–mantle boundaries at depths between 410 and 660 km, deeper than previously observed, suggesting a compositionally layered slab and high water contents beneath the slab. Two seismic discontinuities in the mantle transition zone beneath the western Pacific represent subducted slab interfaces that could be the slab Moho and partially molten sub-slab asthenosphere, according to an analysis of seismic data.

The Ore and Petroleum Regions of the South Okhotsk Province and Deep Geodynamics

The South Okhotsk Sea province, which includes the Sakhalin, Kunashir, Iturup, Urup islands and surrounding sea areas, comprises numerous occurrences of rare, noble metals and other mineralization, as well as petroleum fields, gas hydrate accumulations, and, locally, active emission of water–hydrocarbon gases. The occurrences and deposits of solid, liquid, and gaseous mineral resources are controlled by hidden deep-seated transform-type fault zones: Nosappu (Tuscarora), Iturup, and Urup. These long-lived extended (over 1000 km) zones are distinguished on the NW Pacific megaplate margin, near the SE flank of the Kuril–Kamchatka trough. Using seismotomographic methods we have established their extension to the west from the seismic focal zone in the oceanic slab subsided into mantle transition zone. In the areas of pull-apart extension, the faults provided active formation of drainage channels for seawater penetration in the lithosphere with subsequent serpentinization of its ultramafic rocks, and for decompressional generation of ascending mantle-derived abiogenic fluid flows. The latter penetrated from the subslab asthenosphere into the suprasubduction mantle wedge and sublithospheric mantle and caused metasomatism. The subsequent migration of the flows led to the generation of the primary magma reservoirs in the lower parts of the continental lithosphere, and intermediate and peripheral chambers in the Earth’s crust. The injection of melts from the chambers in the consolidated crust led to the formation of abyssal and hypabyssal intrusive massifs, arch-dome uplifts, and magmatogenic-ore systems predominantly among the rocks of the pre-Pliocene basement. The concentration of oil and gas accumulations from the mantle-derived abiogenic hydrocarbons bearing mercury, gold, rhenium, and PGE in reservoirs beneath fluid-impermeable beds among Cenozoic sedimentary basins also was related to deep sub- and supraslab fluid flows.

Asthenospheric buoyancy and the origin of high-relief topography along the Cascadia forearc

The forearc topography of the Cascadia subduction zone varies systematically along-strike, with high-relief in the north and south (the Olympic and Klamath ranges) separated by relatively low relief in its central region. This systematic topographic variability reflects the long-term pattern of uplift and erosion, however, the underlying cause of uplift and the mechanisms by which current topography is supported are unclear. Here, we synthesize results from seismic imaging, geodetic (decadal) and geomorphic (millennial) uplift rates, erosion rates, topographic analysis, and characteristics of the megathrust interface, with a mechanical model to infer that buoyancy in the subslab asthenosphere influences the development and longevity of Cascadia's forearc topography. The Cascadia margin can be divided into three broad segments, with the northern and southern segments characterized by rapid geodetic and geomorphic uplift rates, rapid erosion rates, high coseismic subsidence of great megathrust ruptures, shallower slab dip angles, and increased plate locking compared to the central segment. Tomographic images show low-velocity anomalies beneath the slab in the northern and southern segments, which are inferred to be regions of partial melt, resulting from localized upwellings, and regions of positive mantle buoyancy. Modeling suggests that these buoyant regions can locally increase the total shear force at the megathrust by either shallowing the slab dip — thereby increasing the area of the seismogenic zone — or increasing mechanical plate coupling along the megathrust, by increasing normal stress and/or the effective coefficient of friction. We propose that: 1) sub-slab buoyancy influences topographic development by modulating along-strike patterns of strain within the over-riding plate during the seismic cycle, 2) Permanent development of surface topography occurs due to unrecovered strain over thousands of seismic cycles, and 3) variations in Cascadia's forearc topography are laterally supported by changes in the total shear force at the megathrust interface. Using independent estimates for slab dip and plate locking, we predict the first-order variations in Cascadia forearc topography, with local maxima in the north and south as observed. Given our results, variations in subslab buoyancy may be critical to explaining forearc topography in other subduction systems.

Slab break-off triggered lithosphere - asthenosphere interaction at a convergent margin: The Neoproterozoic bimodal magmatism in NW India

The Neoproterozoic Malani Igneous Suite (MIS) is described as the largest felsic igneous province in India. The linearly distributed Sindreth and Punagarh basins located along eastern margin of this province represent the only site of bimodal volcanism and associated clastic sediments within the MIS. The in-situ zircon U-Pb dating by LA-ICPMS reveals that the Sindreth rhyolites were erupted at 769–762Ma. Basaltic rocks from both the basins show distinct geochemical signatures that suggest an E-MORB source for Punagarh basalts (low Ti/V ratios of 40.9–28.2) and an OIB source (high Ti/V ratios of 285–47.6) for Sindreth basalts. In the absence of any evidence of notable crustal contamination, these features indicate heterogeneous mantle sources for them. The low (La/Yb)CN (9.34–2.10) and Sm/Yb (2.88–1.08) ratios of Punagarh basalts suggest a spinel facies, relatively shallow level mantle source as compared to a deeper source for Sindreth basalts, as suggested by high (La/Yb)CN (7.24–5.24) and Sm/Yb (2.79–2.13) ratios. Decompression melting of an upwelling sub-slab asthenosphere through slab window seems to be the most plausible mechanism to explain the geochemical characteristics. Besides, the associated felsic volcanics show A2-type granite signatures, such as high Y/Nb (5.97–1.55) and Yb/Ta (9.36–2.57) ratios, consistent with magma derived from continental crust that has been through a cycle of continent-continent collision or an island-arc setting. A localized extension within an overall convergent scenario is interpreted for Sindreth and Punagarh volcanics. This general convergent setting is consistent with the previously proposed Andean-type continental margin for NW Indian block, the Seychelles and Madagascar, all of which lay either at the periphery of Rodinia supercontinent or slightly off the Supercontinent.

Slab-derived adakites and subslab asthenosphere-derived OIB-type rocks at 156 ± 2 Ma from the north of Gerze, central Tibet: Records of the Bangong–Nujiang oceanic ridge subduction during the Late Jurassic

This paper reports zircon U–Pb age and Hf isotope, whole-rock major and trace element, and whole-rock Sr–Nd–Hf isotope data of the dacites from Rena Tso and mafic rocks (diabases and basalts) from Duobuza, north of Gerze, central Tibet. These data reveal the presence of a distinct rock association of slab-derived adakites (154±1Ma) and subslab asthenosphere-derived OIB-type (oceanic island basalt) mafic rocks (157.6±1.4Ma). The medium-K calc-alkaline dacites (SiO2=66–69wt.%) from Rena Tso are enriched in Sr (520–1083ppm) and depleted in heavy rare earth elements (HREEs) and Y (9.8–10.8ppm), resembling adakites. These adakitic dacites have low whole-rock initial 87Sr/86Sr ratios of 0.7043–0.7046, positive εNd(t) (+1.0 to +3.4), εHf(t) (+6.4 to +7.0), and zircon εHf(t) (+1.9 to +7.6) values, indicating an oceanic slab origin (crust and sediment). Considering the low Mg# (32–53) and (La/Yb)N (19–23), the adakitic dacites are most likely derived from the partial melting of the subducting slab at shallow depths and the subsequent interaction with peridotite in a thin mantle wedge during magma ascent. The diabases and basalts (SiO2=49–53wt.%) from Duobuza show an alkali signature with enrichment of high field strength elements (HFSEs) (e.g., Zr=213–285ppm) and exhibit positive Nb–Ta–Ti anomalies that are geochemically comparable to those of OIB. These samples show positive whole-rock εNd(t) values of +3.3 to +3.7, εHf(t) values of +4.7 to +5.7, and negative to positive zircon εHf(t) values of −1.5 to +5.2. These OIB-type mafic samples are interpreted as the products of low-degree decompression melting of the upwelling subslab asthenosphere with a minor contribution from the sub-continental lithospheric mantle (SCLM). Our new data indicate the presence of a distinct rock association of coeval slab-derived adakites and subslab asthenosphere-derived OIB-type rocks. Such an association along with the normal arc rocks further to the north of the Bangong-Nujiang suture zones (BNSZ) records the northward ridge subduction of the Tethyan Bangong-Nujiang Ocean at ca. 156±2Ma.

Sub-slab mantle anisotropy beneath south-central Chile

Knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones. By analysing shear-wave splitting in both teleseismic and local arrivals, we can determine the relative contribution from different parts of the subduction zone to the total observed SKS splitting, providing us with a depth constraint on anisotropy. We use this methodology to determine the location, orientation and strength of seismic anisotropy in the south-central Chile subduction zone. Data come from the TIPTEQ network, deployed on the forearc during 2004–2005. We obtain 110 teleseismic SKS and 116 local good-quality shear-wave splitting measurements. SKS average delay times are 1.3s and local S delay times are only 0.2s. Weak shear-wave splitting from local phases is consistent with a shape preferred orientation (SPO) source in the upper crust. We infer that the bulk of shear-wave splitting is sourced either within or below the subducting Nazca slab. SKS splitting measurements exhibit an average north-easterly fast direction, with a strong degree of variation. Further investigation suggests a relationship between the measurement's fast direction and the incoming ray's back-azimuth. Finite-element geodynamic modelling is used to investigate the strain rate field and predicted LPO characteristics in the subduction zone. These models highlight a thick region of high strain rate and strong S-wave anisotropy, with plunging olivine a-axes, in the sub-slab asthenosphere. We forward model the sub-slab sourced splitting with a strongly anisotropic layer of thick asthenosphere, comprising an olivine a-axis oriented parallel to the direction of subduction. The subducting lithosphere is not thick enough to cause 1.2s of splitting, therefore our results and subsequent models show that the Nazca slab is entraining the underlying asthenosphere; its flow causes it to be strongly anisotropic. Our observation has important implications for the controlling factors on sub-slab mantle flow and the movement of asthenospheric material within the Earth.

Subduction of the South Chile active spreading ridge: A 17 Ma to 3 Ma magmatic record in central Patagonia (western edge of Meseta del Lago Buenos Aires, Argentina)

The Chile Triple Junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent. The MLBA plateau (Meseta del Lago Buenos Aires) is one of the Neogene alkali basaltic plateaus located in the back-arc region of the Andean Cordillera at the latitude of the current Chile Triple Junction. The genesis of MLBA can be related with successive opening of slabs windows beneath Patagonia: within the subducting Nazca Plate itself and between the Nazca and Antarctic plates. Detailed 40Ar/ 39Ar dating and geochemical analysis of bimodal magmatism from the western flank of the MLBA show major changes in the back-arc magmatism which occurred between 14.5 Ma and 12.5 Ma with the transition from calc-alkaline lavas (Cerro Plomo) to alkaline lavas (MLBA) in relation with slab window opening. In a second step, at 4–3 Ma, alkaline felsic intrusions were emplaced in the western flank of the MLBA coevally with the MLBA basalts with which they are genetically related. These late OIB-like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting Nazca plate during the opening of the South Chile spreading ridge-related slab window. These basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones. The close association of bimodal magmatism with extensional tectonic features in the western MLBA is a strong support to the model of Patagonian collapse event proposed to have taken place between 5 and 3 Ma as a consequence of the presence of the asthenospheric window (SCR-1 segment of South Chile Ridge) below the MLBA area.

Miocene to Late Quaternary Patagonian basalts (46–47°S): Geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction

Miocene to Quaternary large basaltic plateaus occur in the back-arc domain of the Andean chain in Patagonia. They are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the South Chile Ridge (SCR). We have investigated three volcanic centres from the Lago General Carrera–Buenos Aires area (46–47°S) located above the inferred position of the slab window corresponding to a segment subducted 6 Ma ago. (1) The Quaternary Río Murta transitional basalts display major, trace elements, and Sr and Nd isotopic features similar to those of oceanic basalts from the SCR and from the Chile Triple Junction near Taitao Peninsula (e.g., ( 87Sr/ 86Sr) o = 0.70396–0.70346 and εNd = + 5.5 − + 3.0). We consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction component. (2) The Plio-Quaternary (< 3.3 Ma) post-plateau basanites from Meseta del Lago Buenos Aires (MLBA), Argentina, likely derive from small degrees of melting of OIB-type mantle sources involving the subslab asthenosphere and the enriched subcontinental lithospheric mantle. (3) The main plateau basaltic volcanism in this region is represented by the 12.4–3.3-Ma-old MLBA basalts and the 8.2–4.4-Ma-old basalts from Meseta Chile Chico (MCC), Chile. Two groups can be distinguished among these main plateau basalts. The first group includes alkali basalts and trachybasalts displaying typical OIB signatures and thought to derive from predominantly asthenospheric mantle sources similar to those of the post-plateau MLBA basalts, but through slightly larger degrees of melting. The second one, although still dominantly alkalic, displays incompatible element signatures intermediate between those of OIB and arc magmas (e.g., La/Nb > 1 and TiO 2 < 2 wt.%). These intermediate basalts differ from their strictly alkalic equivalents by having lower High Field Strength Element (HFSE) and higher εNd (up to + 5.4). These features are consistent with their derivation from an enriched mantle source contaminated by ca. 10% rutile-bearing restite of altered oceanic crust. The petrogenesis of the studied Mio-Pliocene basalts from MLBA and MCC is consistent with contributions of the subslab asthenosphere, the South American subcontinental lithospheric mantle and the subducted Pacific oceanic crust to their sources. However, their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment SCR-1, which entered the trench at 6 Ma. Indeed, magmatic activity was already important between 12 and 8 Ma in MLBA and MCC as well as in southernmost plateaus, i.e., 6 Ma before the subduction of the SCR-1 segment. We propose a geodynamic model in which OIB and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear-in-the-slab, which formed when the southernmost segments of the SCR collided with the Chile Trench around 15 Ma. During their ascent, they interacted with the Patagonian supraslab mantle and, locally, with slivers of subducted Pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts.

δ11B as tracer of slab dehydration and mantle evolution in Western Anatolia Cenozoic Magmatism

AbstractBoron isotope data are presented for Cenozoic Western Anatolia rocks, which define two main associations: (i) calc‐alkaline, shoshonitic and ultra‐potassic rocks (Early to Middle Miocene); and (ii) Late Miocene–Quaternary intraplate alkali basalts. Boron data, together with Sr–Nd isotope and other trace elements, are consistent with a progressive dehydration of the slab, producing fluid phases gradually depleted in B (and 11B). These fluids were added to the supraslab mantle, triggering a partial melting that gave rise to orogenic magmatism. The stretching and tearing of the slab caused by the faster convergence of Greece over Africa with respect to Anatolia facilitated an interaction of the upwelling subslab asthenosphere with residual slab‐fluids during the Late Miocene followed by production of typical intraplate magmas during the Pleistocene–Holocene, whose relatively high δ11B (approximately −2‰) is considered representative of the local asthenosphere not affected by subduction contamination.

Plio–Pleistocene basalts from the Meseta del Lago Buenos Aires, Argentina: evidence for asthenosphere–lithosphere interactions during slab window magmatism

Plio–Pleistocene (3.4–0.125 Ma) post-plateau magmatism in the Meseta del Lago Buenos Aires (MLBA; 46.7°S) in southern Patagonia is linked with the formation of asthenospheric slab windows due to ridge collision along the Andean margin ∼6 Ma ago. MLBA post-plateau lavas are highly alkaline (43–49% SiO 2; 5–8% Na 2O+K 2O), relatively primitive (6–10% MgO) mafic volcanics that have strong OIB-like geochemical signatures. Their relatively enriched Sr–Nd isotope ratios ( 87Sr/ 86Sr=0.7041–0.7049; 143Nd/ 144Nd=0.51264–0.51279), low 206Pb/ 204Pb (18.13–18.45), steep REE patterns (La/Yb=11–54), and low LILE/LREE and LILE/HFSE ratios (Ba/La<15, La/Ta<15, Ba/Ta<180; Sr/La=15–22; Th/La<0.13; Ce/Pb>15) are distinctive from most other Neogene Patagonian slab window lavas. These data are interpreted to indicate contamination of OIB-like asthenosphere-derived slab window magmas with an EM1-type component derived from the Patagonian continental lithospheric mantle (CLM). The EM1-type signature in Patagonian slab window lavas are geographically associated with the Deseado Massif and indicate important regional differences in lithospheric mantle chemistry beneath southern Patagonia. We propose that hot, upwelling subslab asthenosphere in slab window tectonic settings can cause significant thermo-mechanical erosion and thinning of the continental lithospheric mantle and, thus, may be an important process in slab window magma petrogenesis.

The Pali Aike Volcanic Field, Patagonia: slab-window magmatism near the tip of South America

The Pali Aike Volcanic Field (PAVF) represents the southernmost occurrence of the Cenozoic back-arc Patagonian Plateau Lavas. Its activity (Pliocene–Recent) started forming tabular lavas followed by the growth of about 470 essentially monogenetic volcanic centers (tuff-rings, maars, spatter and scoria cones). Azimuths of cone alignment, cone elongation and morphologic lineations show prevailing ENE–WSW and NW–SE trends. Erupted products consist mainly of alkaline basalt and basanite, with minor olivine basalt. PAVF rocks are quite primitive in composition (average Mg#=66, Ni=220 ppm and Cr=313 ppm) with relatively high TiO 2 (average 3.0 wt.%). Ultramafic garnet- and/or spinel-bearing xenoliths are found within PAVF volcanics. Chondrite-normalized REE patterns are significantly LREE-enriched and almost rectilinear [(La/Yb) N=10.9–21.0]. Primordial mantle-normalized distributions of incompatible trace elements, as well as Sr and Nd isotope ratios ( 87Sr/ 86Sr=0.70317–0.70339, 143Nd/ 144Nd=0.51290–0.51294), show values typical of intra-plate basalts, despite the fact that these rocks occur only 200 km east of the Andean Cordillera. Primary magmas were generated from a fertile garnet-bearing asthenospheric source at P=1.9–2.9 GPa and T=1420–1470°C. The data suggest a geodynamic model that implies sub-slab asthenosphere flow through a slab window, which started opening below this sector of South America 14 m.y. ago as a consequence of the collision of the Chile Ridge with the Chile Trench. The trailing edge of the Nazca Plate crossed below the Pali Aike area at 9–10 Ma, that is 6–5 m.y. before the onset of the volcanic activity. We hypothesize that this time delay resulted from changes in the kinematics of the South America–Scotia transform plate boundary which only allowed the Pali Aike magmas to rise after about 4 m.y.

Late Miocene volcanism and intra-arc tectonics during the early development of the Trans-Mexican Volcanic Belt

The early stage of the Trans-Mexican Volcanic Belt (hereafter TMVB) is marked by widespread, mafic to intermediate, volcanism emplaced between 11 and 7 Ma from the Pacific coast to the longitude of Mexico City, to the north of the modern volcanic arc. Petrological and geochronological data support the hypothesis that this volcanism made up a unique late Miocenic central Mexican comagmatic province. Mafic lavas at the mouth of the Gulf of California and along the northwestern sector of the TMVB made up the Nayarit district, which includes calc-alkaline to transitional varieties. The central sector of the TMVB is characterized by two basaltic districts: the Jalisco–Guanajuato and the Queretaro–Hidalgo, which are distinguished from the westernmost ones by their lower Nb/La and generally lower HFSE/LILE values, as well as by spider diagrams characterized by larger negative spikes at Th, Ta, Nb, and Ti. The surface occurrence of the late Miocene basalts appears to be controlled by pre-existing zones of crustal weakness that channeled the mafic magmas. Field observations suggest that these structures have been reactivated in a transtensional fashion induced by differential tectonic motion of crustal blocks to the south and to the north of the TMVB. Starting from ∼12 Ma the TMVB separates a northern tectonic domain, subject to the developing divergent Pacific–North America plate boundary, from a southern tectonic domain, characterized by oblique subduction of the Rivera and Cocos plates. Apparently, far field stresses related to these complex plate boundaries reactivated older suture zones, allowing rapid uprise of mantle-derived magmas. The subduction-related signature shown by Miocene mafic lavas of the Jalisco–Guanajuato district argues against the existence of mantle plumes beneath this sector of the North America plate. On the other hand, the occurrence in the western TMVB and in the Guadalajara region of a large volume of mafic magmas, which sometimes show characteristics transitional to Ocean Island Basalts, could be due to passive upwelling of the subslab asthenosphere that might have interacted with subduction-related magmas and continental lithosphere to produce the observed basaltic varieties. Subslab magmas may have flowed through slab-free areas along the northern and eastern edges of the subducting Rivera plate, which in the late Miocene was already detached from the Farallon plate remnants and diverging from the Cocos plate.

Neogene Patagonian plateau lavas: Continental magmas associated with ridge collision at the Chile Triple Junction

Extensive Neogene Patagonian plateau lavas (46.5° to 49.5°S) southeast of the modern Chile Triple Junction can be related to opening of asthenospheric “slab windows” associated with collisions of Chile Rise segments with the Chile Trench at ≈ 12 Ma and 6 Ma. Support comes from 26 new total‐fusion, whole rock 40Ar/39Ar ages and geochemical data from back arc plateau lavas. In most localities, plateau lava sequences consist of voluminous, tholeiitic main‐plateau flows overlain by less voluminous, 2 to 5 million year younger, alkalic postplateau flows. Northeast of where the ridge collided at ≈12 Ma, most lavas are syncollisional or postcollisional in age, with eruptions of both sequences migrating northeastward at 50 to 70 km/Ma. Plateau lavas have ages from 12 to 7 Ma in the western back arc and from 5 to 2 Ma farther to the northeast. Trace element and isotopic data indicate main‐plateau lavas formed as larger percentage melts of a garnet‐bearing, oceanic island basalt (OIB) ‐like mantle than postplateau lavas. The highest percentage melts erupted in the western and central plateaus. In a migrating slab window model, main‐plateau lavas can be explained as melts that formed as upwelling, subslab asthenosphere which flowed around the trailing edge of the descending Nazca Plate and then interacted with subduction‐altered asthenospheric wedge and continental lithosphere. Alkaline, postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window. Thermal problems of high‐pressure melt generation of anhydrous mantle can be explained by volatiles (H2O and CO2) introduced by the subduction process into slab window source region(s). An OIB‐like, rather than a mid‐ocean ridge basalt (MORB) ‐like source region, and the lack of magmatism northeast of where ridge collision occurred at ≈13 to 14 Ma can be explained by entrainment of “weak” plume(s) or regional variations in an ambient, OIB‐like asthenosphere.

Anisotropic loci in the mantle beneath central Peru

Seismic anisotropy of the upper mantle beneath the central part of Peru is examined by analyzing shear waves observed at broad-band stations and a temporary array of short-period stations. Shear-wave splitting is seen on various shear phases, such as direct S waves from local intermediate to deep earthquakes, ScS waves from regional deep earthquakes, and SKS waves from teleseismic earthquakes. It is inferred that the shear-wave anisotropy in the uppermost 100 km of the subcontinental mantle overlying the subducted Nazca plate is 0.5% at most, while the anisotropy in the subslab asthenosphere (depth range of about 150–350 km) is 2% or greater. The fast shear-wave polarization direction in this depth range, as observed at two broad-band stations, is 30°–40° different from the absolute motion of the Nazca plate. This does not fit simple two-dimensional (2-D) models of olivine alignment caused by slab-induced mantle flow, and implies either the existence of a complex flow pattern in the asthenosphere underneath the Nazca plate or the presence of an unknown mechanism for the anisotropy formation. The lower mantle beneath central Peru is found to be effectively isotropic for nearly vertically propagated shear waves.