Hot Mantle Wedge Research Articles

Pathway from subducting slab to surface for melt and fluids beneath Mount Rainier.

Convergent margin volcanism originates with partial melting, primarily of the upper mantle, into which the subducting slab descends. Melting of this material can occur in one of two ways. The flow induced in the mantle by the slab can result in upwelling and melting through adiabatic decompression. Alternatively, fluids released from the descending slab through dehydration reactions can migrate into the hot mantle wedge, inducing melting by lowering the solidus temperature. The two mechanisms are not mutually exclusive. In either case, the buoyant melts make their way towards the surface to reside in the crust or to be extruded as lava. Here we use magnetotelluric data collected across the central state of Washington, USA, to image the complete pathway for the fluid-melt phase. By incorporating constraints from a collocated seismic study into the magnetotelluric inversion process, we obtain superior constraints on the fluids and melt in a subduction setting. Specifically, we are able to identify and connect fluid release at or near the top of the slab, migration of fluids into the overlying mantle wedge, melting in the wedge, and transport of the melt/fluid phase to a reservoir in the crust beneath Mt Rainier.

The role of magmatically driven lithospheric thickening on arc front migration

AbstractVolcanic activity at convergent plate margins is localized along lineaments of active volcanoes that focus rising magma generated within the mantle below. In many arcs worldwide, particularly continental arcs, the volcanic front migrates away from the interface of subduction (the trench) over millions of years, reflecting coevolving surface forcing, tectonics, crustal magma transport, and mantle flow. Here we show that extraction of melt from arc mantle and subsequent magmatic thickening of overlying crust and lithosphere can drive volcanic front migration. These processes are consistent with geochemical trends, such as increasing La/Yb, which show that increasing depths of differentiation correlate with arc front migration in continental arcs. Such thickening truncates the underlying mantle flow field, squeezing hot mantle wedge and the melting focus away from the trench while progressively decreasing the volume of melt generated. However, if magmatic thickening is balanced by tectonic extension in the upper plate, a steady crustal thickness is achieved that results in a more stationary arc front with long‐lived mantle melting. This appears to be the case for some island arcs. Thus, in combination with tectonic modulation of crustal thickness, magmatic thickening provides a self consistent model for volcanic arc front migration and the composition of arc magmas.

東京直下に沈み込む2枚のプレートと首都圏下の特異な地震活動

Recent investigations based on seismic tomography, hypocenter determinations and focal mechanism analyses using dense seismic network data reveal the precise configurations of the Pacific (PAC) and Philippine Sea (PHS) plates subducting beneath the Tokyo metropolitan area. Estimated geometry shows a broad contact zone between the two plates located directly beneath the Kanto plain. The overlap with the PHS plate subducting above it provides the PAC plate with protection from being heated by the hot mantle wedge. Moreover, the fore-arc portion of the PHS plate, before its subduction beneath Kanto, had been cooled by the subduction of the PAC plate from the Izu-Bonin trench. These cause lower-temperature conditions within the two oceanic plates and the upper continental plate beneath the Tokyo metropolitan area. As a result, depth limits of seismic activities within the plates and along their boundaries are anomalously deep. Seismic tomography studies show that the easternmost portion of the PHS slab mantle is serpentinized. The PHS slab may have been torn in two along the western boundary of this serpentinized mantle, with the eastern portion being left behind relative to subduction of the western portion. This is accompanied by the generation of large intraslab earthquakes along the boundary. We need to take these observations into consideration to understand the mechanism generating M7-class earthquakes, which are anticipated to occur in the southern Kanto region with a high probability.

Origin of Late Oligocene to Middle Miocene Adakitic Andesites, High Magnesian Andesites and Basalts from the Back-arc Margin of the SW and NE Japan Arcs

Late Oligocene to Middle Miocene adakitic andesites are found in the southern part of Okushiri Island, the northern Noto Peninsula and in the Toyama region in the present-day back-arc margin of the SW and NE Japan arcs. On Okushiri Island, adakitic andesite is accompanied by moderately alkaline basalt, whereas on the Noto Peninsula, adakitic andesite has been erupted along with high magnesian andesite (HMA), bronzite andesite and tholeiitic basalt. Adakitic andesites from all three locations are characterized by high Sr/Y and low Y, and have higher MgO contents than adakitic melts generated by experimental melting of metabasalt and amphibolite. They also have higher Ni and Cr contents than either Archaean tonalite–trondhjemite–granodiorite (TTG) suites or Early Cretaceous adakitic granites, which have been attributed to partial melting of subducted oceanic crust. The Noto Peninsula adakitic andesite has Sr and Nd isotopic compositions identical to normal mid-ocean ridge basalt (N-MORB), whereas the Okushiri Island and Toyama adakitic andesites are more isotopically primitive than N-MORB. The Noto Peninsula primary adakitic melt was derived from subducted oceanic N-MORB crust, whereas the Okushiri Island and Toyama primary adakites are interpreted as melts of subducted N-MORB and sediment that have subsequently interacted with the overlying mantle wedge peridotite. To explain the comagmatism of adakite, HMA and basalt, the following model is proposed. A hydrated adakitic diapir ascends from the subducting slab and is heated because it enters the overlying hot mantle wedge. The subsequent establishment of thermal and H2O gradients in the adakitic diapir and surrounding mantle wedge peridotite results in concurrent generation of adakitic andesite magma in the inner adakitic diapir region (low temperature and high H2O content), HMA and bronzite andesite magmas in the intermediate peridotite region (intermediate temperature and H2O content), and tholeiitic basalt magma in the outer peridotite region (high temperature and lower H2O content). Comagmatic adakite and mildly alkaline basalt are found in cooler and wetter adakitic diapirs and hotter and drier peridotite regions respectively. The most likely tectono-magmatic situation for the genesis of adakitic magmas in this example of a cool subduction zone involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath the back-arc side of the NE Japan arc and northern end of the SW Japan arc, during the period spanning the pre-Japan Sea opening to syn-opening stages. The unusually high temperature conditions established in the mantle wedge owing to upwelling of hot asthenosphere caused partial melting of the relatively cool subducting Pacific plate, resulting in the generation of adakitic magmas.

Seismotectonics beneath the Tokyo metropolitan area, Japan: Effect of slab‐slab contact and overlap on seismicity

We first determine the configuration of the upper surface of the Pacific (PAC) slab beneath Kanto, Japan, from the distribution of interplate earthquakes relocated by an appropriate 1‐D velocity model. Then, traveltime tomography is carried out to estimate three‐dimensional seismic velocity structures around Kanto using 735,520 P wave and 444,049 S wave arrival times from 6508 local earthquakes. The obtained results suggest that the Philippine Sea (PHS) slab is subducting to depths of 130–140 km without a gap, even to the northwest of the Izu collision zone. We subsequently define the lateral extent of the contact zone between the bottom of the PHS slab and the upper surface of the PAC slab (PHS‐PAC interface) and reveal that the slab contact zone underlies a wider area beneath Kanto in harmony with the Kanto plain. The downdip limit of interplate (thrust‐type) earthquakes on the PAC slab is deepened by ∼30 km locally under the slab contact zone. This deepening is probably caused by a lower‐temperature environment in the PAC slab, resulting from the overlap with the PHS slab subducting above and consequent thermal shielding by the PHS slab from the hot mantle wedge. We detect an extremely low‐velocity anomaly in the easternmost portion of the PHS slab, which is probably attributable to serpentinization of mantle peridotite. Interplate earthquakes are almost absent along the PHS‐PAC interface overlain by the serpentinized mantle in the PHS slab, suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine.

Dynamic effects of aseismic ridge subduction: numerical modelling

The subduction of oceanic aseismic ridges, oceanic plateaus and seamount chains is a common process that takes place in a variety of tectonic settings and seems to coincide spatially and temporally with a gap of volcanic activity, shallow or even horizontal slab angles, enhanced seismic activity and various topographic features. In the present study we focus on these dynamic effects on the basis of 2D thermomechanical modelling incorporating effects of slab dehydration, mantle-wedge melting and surface topography development. In order to ascertain the impact of a moderate-size (200 × 18 km) aseismic ridge, 12 pairs of experiments (one for the case with a ridge, the other without) were carried out varying slab density and subducting- and overriding-plate velocities. By analysing pairs of experiments we conclude that subduction of a moderate-sized ridge does not typically result in strong slab flattening and related decrease of magmatic activity. This, in turn, suggests that, when slab flattening is indeed associated with the ridge subduction in nature, the slab itself should be in a nearly critical ( i.e ., transient from inclined to flat) state so that any local addition of positive buoyancy may strongly affect overall slab dynamics. Therefore, subducting ridges may serve as indicators of transient slab states in nature. Another important result from our study is the numerical quantification of strongly decreased magma production associated with flat slabs that may explain gaps in recent active volcanism at low-angle subduction margins. Lowering of magmatic rock production is caused by the absence of a hot mantle wedge above the flat slabs and does not directly depend on the mechanism responsible for the triggering of slab flattening. Finally we document several very distinct surface effects associated with the moderate-size ridge subduction such as local increase in elevation of overriding margin, enhancement of subduction erosion and landward trench displacement. Surface uplift may exceed the original ridge height due to additional uplift resulting from the overriding plate shortening. Topographic perturbations within the accretionary wedge domain are transient and have a tendency to relax after the ridge passes the trench. In contrast, the topographic high created in the continental portion of the overriding plate relaxes more slowly and may even be sustained for several millions of year after the ridge subduction.

Mantle wedge asymmetries and geochemical signatures along W- and E–NE-directed subduction zones

Subduction zone kinematics predict that, assuming a fixed lower plate, the velocity of the subduction equals the velocity of the subduction hinge (Vs = − Vh). In all subduction zones the subduction hinge migrates toward the lower plate. However, two main types of subduction zones can be distinguished: 1) those where the upper plate converges toward the lower plate slower than the subduction hinge (mostly W-directed), and 2) those in which the upper plate converges faster than the subduction hinge (generally E- or NE-directed). Along the first type, there generally is an upward flow of the asthenosphere in the hanging wall of the slab, whereas along the opposite second type, the mantle is pushed down due to the thickening of the lithosphere. The kinematics of W-directed subduction zones predict a much thicker asthenospheric mantle wedge, larger volumes and faster rates of subduction with respect to the opposite slabs. Moreover, the larger volumes of lithospheric recycling, the thicker column of fluids-rich, hotter mantle wedge, all should favour greater volumes of magmatism per unit time. The opposite, E–NE-directed subduction zones show a thinner, if any, asthenospheric mantle wedge due to a thicker upper plate and shallower slab. Along these settings, the mantle wedge, where the percolation of slab-delivered fluids generates melting, mostly involves the cooler lithospheric mantle. The subduction rate is smaller, andesites are generally dominant, and the lithosphere thickens, there appears to be a greater contribution to the growth of the continental lithosphere. Another relevant asymmetry that can be inferred is the slab-induced corner flow in the mantle along W-directed subduction zones, and an upward suction of the mantle along the opposite E- or NNE-directed slabs. The upward suction of the mantle inferred at depth along E–NE-directed subduction zones provides a mechanism for syn-subduction alkaline magmatism in the upper plate, with or without contemporaneous rifting in the backarc. Positive δ 11B and high 143Nd/ 144Nd characterize W-directed subduction zones where a thicker and hotter mantle wedge is present in the hanging wall of the slab. However, this observation disappears where large amounts of crustal rocks are subducted as along the W-directed Apennines subduction zone.

Multistage metamorphism of orogenic garnet–lherzolite from Zhimafang, Sulu UHP terrane, E. China: Implications for mantle wedge convection during progressive oceanic and continental subduction

Seven stages of mineral assemblage were recognized in the Zhimafang orogenic garnet lherzolite from the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China, which suggest that it experienced progressive mantle wedge convection during subduction of previous oceanic and subsequent continental slabs. M 1 is recorded by inclusions of high-Mg olivine, high-Mg–Al–Cr–Ca orthopyroxene and high-Mg clinopyroxene in the high Ca–Cr cores of the garnet porphyroclasts, suggesting that the Zhimafang peridotite was probably a garnet lherzolite originated from a deep and hot mantle wedge above the previously subducted oceanic slab. M 2 is represented by inclusions of high-Mg–Al chromite, high-Mg olivine and high-Mg orthopyroxene in the low-Cr mantles of the garnet porphyroclasts and core of the matrix garnet, suggesting that the rock was convected to shallow and hot mantle wedge, and was transformed to refractory spinel harzburgite or spinel dunite due to high degree of decompressional partial melting. M 3 is manifested by the origin of metasomatic clinopyroxene and orthopyroxene porphyroblasts with inclusions of high-Fe chromite, tremolitic or edenitic amphibole, phlogopite, barite and pyrite, indicating that the rock was convected to shallow, cold and wet corner of the mantle wedge, and was transformed to a fertile amphibole-bearing spinel lherzolite during continental subduction. The lherzolites was subsequently transformed to high-pressure (HP) amphibole-bearing garnet lherzolite ( M 4) and UHP garnet lherzolite ( M 5), indicating its convection to deep mantle wedge. Finally, the UHP garnet–lherzolite exhumed to the Earth’s surface, together with the UHP terrane, and subsequently retrograded to amphibole-bearing garnet lherzolite ( M 6) and chlorite-amphibole-bearing spinel lherzolite ( M 7). Detailed analyses of reaction textures and mineral compositions revealed several stages of metasomatism related to oceanic and continental subduction and exhumation.

Anomalous deepening of a belt of intraslab earthquakes in the Pacific slab crust under Kanto, central Japan: Possible anomalous thermal shielding, dehydration reactions, and seismicity caused by shallower cold slab material

A belt of intraslab seismicity in the Pacific slab crust parallel to iso‐depth contours of the plate interface has been found beneath Hokkaido and Tohoku. Hypocenter relocations have shown that this seismic belt does not run parallel to but obliquely to the iso‐depth contours beneath Kanto, deepening toward the north from ∼100 km to ∼140 km depth. The depth limit of the contact zone with the overlying Philippine Sea slab is located close to and parallel to this obliquely oriented seismic belt, suggesting that the deepening of the seismic belt there is caused by the contact with the overlying slab. The contact with this cold slab hinders the heating of the Pacific slab crust by hot mantle wedge, which would cause delay of eclogite‐forming phase transformations and hence deepening of the seismic belt there. The depth limit of the subducting low‐velocity crust also deepens toward the north, supporting this idea.

Na depletion in modern adakites via melt / rock reaction within the sub-arc mantle

Interaction between slab-derived melt and mantle peridotite and the role of slab melt as a metasomatizing agent in the sub-arc mantle is being increasingly recognized. Adakite, the slab melt erupted on the surface, usually exhibits anomalously high MgO, CaO, Cr and Ni contents that indicate interaction with mantle peridotitite. Here we note that Cenozoic adakites have Na 2O contents below 5.8 wt.% with ∼95% samples lower than 5.0 wt.%, and are generally depleted in this component relative to experimental basalt partial melts (mostly beyond 5.0 wt.% and up to 9.0 wt.% Na 2O) produced under 1.5–3.0 GPa conditions that are most relevant to adakite production. We interpret the adakite Na depletion to be also a consequence of the melt / rock reaction that takes place within the hot mantle wedge. During ascent and reaction with mantle peridotite, primary adakite melts gain mantle components MgO, CaO, Cr and Ni but lose Na 2O, SiO 2 and perhaps K 2O to the mantle, leading to Na-rich mantle metasomatism. Selective assimilation of predominately mantle clinopyroxene, some spinel and minor olivine at high T/P has been considered to be an important process in producing high-Mg adakites from primary low-Mg slab melts [Killian, R., Stern, C. R., 2002. Constraints on the interaction between slab melts and the mantle wedge from adakitic glass in peridotite xenoliths. Eur. J. Mineral. 14, 25–36]. In such a process, Na depletion in the assimilated melt is the result of dilution due to the increase in melt mass. Phase relationships in the reaction system siliceous melt + peridotite and quantitative calculation suggest that assimilation of mantle clinopyroxene, olivine and spinel and fractional crystallization of sodic amphibole and orthopyroxene, under conditions of moderate T/P and increasing melt mass, is also an important process that modifies the composition of adakites and causes the Na depletion.

Suboceanic earthquake location and seismic structure in the Kanto district, central Japan

Abstract We present a combined method, using sP depth-phase data and double-difference arrival times, to determine the precise hypocenter locations of earthquakes that occur under the Pacific Ocean outside of the area covered by the land-based seismic network. We assess the effectiveness of the combined method using a data set of P- and S-wave arrival times and sP depth phase from suboceanic earthquakes recorded by both land-based seismic stations and offshore seismic stations (OFS). The hypocenters of the offshore earthquakes relocated using the combined method are consistent with those determined using the standard location method and OFS data. The differences in the hypocenters relocated by the two methods are less than 4 km. We applied the method to the subduction region that underlies the Kanto district, central Japan, and located a large number of earthquakes that occurred beneath the Pacific Ocean. We then determined the detailed 3D seismic velocity structure by inverting a large number of arrival times of P- and S-waves and sP depth phase from the relocated earthquakes in the study region. High-velocity anomalies related to the cold subducting Pacific slab and low-velocity anomalies related to the hot mantle wedge are clearly imaged. Beneath active volcanoes, low-velocity zones are visible from the surface to a depth of 100 km, reflecting fluids released by dehydration of the subducting Pacific slab. Strong lateral heterogeneities are revealed on the upper boundary of the Pacific slab beneath the forearc region. The low-velocity areas under the offshore region are associated with low seismicity and weak interplate coupling. A low-velocity layer is imaged along the upper boundary of the Philippine Sea slab in the northern part of Kanto district, which may reflect dehydration of the slab. Our tomographic images indicate that the overlaying Philippine Sea plate has effects on the spatial distribution of active volcanoes related to the subducting Pacific slab in the study region.

Seismic imaging of the entire arc of Tohoku and Hokkaido in Japan using P-wave, S-wave and sP depth-phase data

In order to better understand seismic structure and seismotectonics of the entire arc of Tohoku and Hokkaido in Japan, we combined arrival time data from earthquakes beneath Tohoku and Hokkaido land areas, and beneath the Pacific Ocean to determine the three-dimensional (3D) velocity structures (Vp and Vs) under the entire Northeast (NE) Japan–Kuril arc. We adopted 176,431 P-wave and 110,953 S-wave arrival times, from 5123 local earthquakes, and 2843 sP depth-phase data from 385 events that occurred beneath the Pacific Ocean. The 385 suboceanic events were accurately relocated by using P-wave, S-wave and sP depth-phase arrival time data jointly. The obtained results confirmed the major features delineated by previous studies and revealed some new features of the structural heterogeneity beneath NE Japan and the Kuril forearcs. High-velocity anomalies of the cold subducting Pacific slab and low-velocity anomalies in the hot mantle wedge were imaged clearly. Strong lateral heterogeneities were revealed on the upper boundary of the Pacific slab under the forearc region, which showed a good correlation with the spatial distribution of large interplate earthquakes. These results indicated that strong coupling sections (or asperities) and weak-coupled or decoupled patches might exist along the upper boundary of the Pacific slab. Widespread low-velocity anomalies were visible in the forearc mantle above the subducting Pacific slab, which might reflect serpentinization of the forearc mantle associated with the dehydration process of the subducting slab. Our results also showed a general tendency for seismic coupling in the asperities to be located around low-velocity areas on the slab boundary under the suboceanic region.

Pressure–Temperature Evolution of a Late Palaeozoic Paired Metamorphic Belt in North–Central Chile (34°–35°30′S)

In the Chilean Coastal Cordillera, two units, the Western and Eastern Series, constitute coeval parts of a Late Palaeozoic paired metamorphic belt dominated by siliciclastic metasediments. The Western Series also contains rocks from the upper oceanic crust and represents an accretionary prism. Omnipresent high-pressure conditions are reflected by Na–Ca-amphibole and phengite in greenschists. Peak PT conditions of 7 0–9 3 kbar, 380–420 C point to a metamorphic gradient of 11–16 C/km. Three unique occurrences of blueschist yield deviating conditions of 9 5–10 7 kbar, 350– 385 C and are interpreted as relics from the lowermost part of the basal accretion zone preserving the original gradient of 9–11 C/km along the subducting slab. Pervasive ductile deformation related to basal accretion occurred near peak PT conditions. Deformation and PT evolution of the metapsammopelitic rocks is similar to that of the metabasites. However, a rare garnet mica-schist yields peak PT conditions of 9 6–14 7 kbar, 390–440 C reflecting a retrograde stage after cooling from a high-temperature garnet-forming stage. It is considered to be an exhumed relic from the earliest siliciclastic rocks subducted below a still hot mantle wedge. A retrograde overprint of all rock types occurred at 300–380 C. Continuous reactions caused crystal growth and recrystallization with abundant free water mostly under strain-free conditions. They record a pressure release of 3–4 kbar without erasing peak metamorphic mineral compositions. The Eastern Series lacks metabasite intercalations and represents a less deformed retro-wedge area. In the study area it was entirely overprinted at a uniform depth at 3 0 5 kbar with temperatures progressively rising from 400 C to 720 C towards the coeval Late Palaeozoic magmatic arc batholith. The interrelated pattern of PT data permits a conceptual reconstruction of the fossil convergent margin suggesting a flat subduction angle of 25 with continuous basal accretion at a depth of 25–40 km and a short main intrusion pulse in the magmatic arc. The latter was accompanied by the formation of a thermal dome in the retro-wedge area, which remained stable relative to the vertically growing accretionary prism characterized by cyclic mass flow.

Enrichment of HFSE in chlorite‐harzburgite produced by high‐pressure dehydration of antigorite‐serpentinite: Implications for subduction magmatism

Depletion of high‐field‐strength trace elements (HFSE) relative to normal mid‐ocean basalts (N‐MORB) is the most distinctive geochemical fingerprint of subduction magmatism. Proposed hypotheses advocate that this “subduction” signature is acquired during melting and/or fluid transfer either in the mantle wedge or in the crust of the subducting oceanic plate. Here we provide field‐based and geochemical evidence showing that high‐pressure dehydration of antigorite‐serpentinite produces chlorite‐harzburgite relatively enriched in HFSE due to the stabilization of F‐OH‐Ti‐clinohumite intergrowths with prograde olivine. Available experimental data indicate that in hydrated, intermediate to warm subduction zones, clinohumite‐olivine intergrowths can be stable in prograde chlorite‐harzburgite olivine at subarc depths. In these settings, deserpentinization may act as a source of fluids leaching large‐ion lithophile elements (LILE), Pb, and Sr from the overlying crust and sediments on their way up to the mantle wedge. Stabilization of chlorite‐harzburgites with clinohumite‐olivine intergrowths in the mantle wedge, on the other hand, acts as a sink of HFSE by selectively fractionating them from other incompatible trace elements in fluids emanating from the slab. Resulting arc fluids in equilibrium with wedge chlorite‐harzburgite are strongly depleted in HFSE and transfer this depletion to the overlying hot mantle wedge, where subduction magmas are generated.

Dynamical causes for incipient magma chambers above slabs

Using a regional upper-mantle model with an unprecedented spatial resolution of ∼100 m, we have investigated at multiple resolutions the character of incipient magma chambers forming under oceanic arcs. The magma chambers are formed from wave-like structures propagating upward along descending slabs and consist of compositionally buoyant, hydrated, partially molten subducted crustal and mantle material. These wave structures are 300–500 °C colder than the mantle wedge and may have an upward velocity of >1 m/yr. Inverted temperature structures and transitory bimodal magmatism are plausible consequences of finger-like penetration of relatively cold, hydrated material of the incipient magma chambers into the hot mantle wedge. Apart from forming and periodic feeding of the magma chambers, “cold” waves may also transport upward thousands of cubic kilometers of subducted material and may cause the rapid exhumation of ultrahigh-pressure rocks along slabs.

A geodynamic model of the Southern Apennines accretionary prism

ABSTRACTThe Apennines comprise a Neogen—Quaternary accretionary prism that shows several anomalies with respect to classic alpine‐type mountain belts, namely (i) low elevation, (ii) a shallow new Moho below the core of the belt, (iii) high heat flow in the internal parts, (iv) mainly sedimentary cover involved in the prism, (v) a deep foredeep and (vi) a fully developed back‐arc basin. The suction exerted by a relatively eastward migrating mantle can determine the eastward retreat of the subduction zone and an asthenospheric wedging at the retreating subduction hinge. Heat flow, geochemical and seismological data support the presence of a hot mantle wedge underlying the western side of the Apenninic accretionary prism. A thermal model of the belt with foreland dipping isotherms fits with deepening of the seismicity toward the east. Mantle volatiles signatures are also widespread in springs along the Apennines.

Variscan exhumation of a subducted Paleozoic continental margin: The basal units of the Ordenes Complex, Galicia, NW Spain

A structural and metamorphic study was carried out in the basal units of the Ordenes Complex in Spain, thought to represent a subducted part of the Paleozoic margin of Gondwana. According to their metamorphic evolution, this part of the margin was subducted at the onset of the Variscan Orogeny, becoming part of an accretionary complex developed below a colliding element built previously. Variations in the PT conditions of the first high‐pressure metamorphic event along the units indicate a polarity of the subduction to the west. Subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units, whereas the convergence continued. Recumbent folds and thrusts developed along with successive normal detachments. Compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading. The unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units.

Petrogenesis of slab-derived trondhjemite–tonalite–dacite/adakite magmas

ABSTRACT:The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic process. Experimental partial melting data for basalt over a wide range of pressures (1–32 kbar) and temperatures (700–1150°C) have shown that melt compositions are primarily trondhjemite–tonalite–dacite (TTD). High-Al (> 15% Al2O3at the 70% SiO2level) TTD melts are produced by high-pressure (≥ 5 kbar) partial melting of basalt, leaving a restite assemblage of garnet + clinopyroxene ± hornblende. A specific Cenozoic high-Al TTD (adakite) contains lower Y, Yb and Sc and higher Sr, Sr/Y, La/Yb and.Zr/Sm relative to other TTD types and is interpreted to represent a slab melt under garnet amphibolite to eclogite conditions. High-Al TTD with an adakite-like geochemical character is prevalent in the Archean as the result of a higher geotherm that facilitated slab melting. Cenozoic adakite localities are commonly associated with the subduction of young (<25 Ma), hot oceanic crust, which may provide a slab geotherm (≍9–10°C km−1) conducive for slab dehydration melting. Viable alternative or supporting tectonic effects that may enhance slab melting include highly oblique convergence and resultant high shear stresses and incipient subduction into a pristine hot mantle wedge. The minimumP–Tconditions for slab melting are interpreted to be 22–26 kbar (75–85 km depth) and 750–800°C. ThisP–Tregime is framed by the hornblende dehydration, 10°C/km, and wet basalt melting curves and coincides with numerous potential slab dehydration reactions, such as tremolite, biotite + quartz, serpentine, talc, Mg-chloritoid, paragonite, clinohumite and talc + phengite. Involvement of overthickened (>50 km) lower continental crust either via direct partial melting or as a contaminant in typical mantle wedge-derived arc magmas has been presented as an alternative to slab melting. However, the intermediate to felsic volcanic and plutonic rocks that involve the lower crust are more highly potassic, enriched in large ion lithophile elements and elevated in Sr isotopic values relative to Cenozoic adakites. Slab-derived adakites, on the other hand, ascend into and react with the mantle wedge and become progressively enriched in MgO, Cr and Ni while retaining their slab melt geochemical signature. Our studies in northern Kamchatka, Russia provide an excellent case example for adakite-mantle interaction and a rare glimpse of trapped slab melt veinlets in Na-metasomatised mantle xenoliths.

The system tonalite-peridotite-H2O at 30 kbar, with applications to hybridization in subduction zone magmatism

We present data on the phase relationships of mixtures between natural tonalite and peridotite compositions with excess H2O at 30 kbar, and on the composition of the piercing point where the peridotite-tonalite mixing line intersects the L(Ga,Opx) reaction boundary. These data, in conjunction with earlier analogous data along peridotite-granite and basalt-granite mixing lines, permit construction of a pseudoternary liquidus projection that is relevant to interaction of peridotite with slab-derived magmas. Knowledge of the liquidus phase and temperature for a range of compositions within this projection enables us to map primary crystallization fields for quartz, garnet, orthopyroxene, clinopyroxene, and olivine, and to estimate the distribution of isotherms across the projection. Using this projection, we explore the consequences of peridotite assimilation by mafic to intermediate (basalt to dacite) hydrous slab-derived melts. Progressive assimilation under isothermal conditions results in garnet precipitation as the melt composition traverses the garnet liquidus surface and then garnet+orthopyroxene crystallization once the melt reaches the L(Ga,Opx) field boundary. The melt is constrained to remain on this field boundary and further assimilation of peridotite simply results in continued precipitation of garnet+orthopyroxene until the melt is consumed. The product is a hybrid solid assemblage consisting of Ga+ Opx. It is noteworthy that this process drives the melt composition in a direction nearly perpendicular to the mixing line between peridotite and the initial melt. If assimilation occurs with increasing temperature (as might occur if a slab-derived magma rises into the hotter mantle wedge), intermediate magmas (e.g. andesites) will again precipitate garnet until they reach the L(Ga,Opx) reaction boundary at which point Ga re-dissolves and orthopyroxene precipitates as the melt composition moves up-temperature along this boundary. The product of this process is a hybrid solid assemblage with garnet subordinate to orthopyroxene. For more mafic initial compositions (e.g. basalts) originally plotting in the Cpx field, it appears possible to avoid field boundaries involving garnet and shift in composition more directly toward peridotite, if assimilation is accompanied by a sharp increase in temperature. Considering published REE evidence (arguing against garnet playing a significant role in the genesis of many subduction-related magmas) in light of our results, it appears unlikely that peridotite assimilation by intermediate magmas under conditions of constant or increasing temperature is an important process in subduction zones. However, if assimilation is accompanied by an increase in temperature, our data do permit the derivation of high-Mg basalts from less refractory precursors (e.g. high-Al basalts) by peridotite assimilation.