Slab Window Research Articles

The tectonomagmatic evolution of structure-lithologic complexes in the Tannu-Ola zone, Tuva, in the Late Vendian–Early Cambrian ( from geochemical, Nd isotope, and geochronological data)

In the Tannu-Ola zone, there are island-arc (Late Vendian) and subduction-accretionary (first half of the Early Cambrian) complexes whose mantle sources were generated from melts produced in suprasubduction conditions as a result of the melting of mantle wedge peridotites. The island arc, whose fragment is present in the Tannu-Ola zone, formed tentatively at 570 Ma and ceased as an individual structure at the Vendian–Cambrian boundary, though the subduction continued till the middle Early Cambrian. From the Vendian–Cambrian boundary to middle Early Cambrian, the accretion of island-arc, back-arc, oceanic, and metamorphic structure-lithologic complexes took place at this segment of the Central Asian Fold Belt. From middle to late Early Cambrian, the Tannu-Ola zone was an active continental margin, where basalts with island-arc characteristics, quartz microdiorites of adakite composition, basalts, dolerites, and quartz microdiorites of intraplate type, and diorite-tonalite-plagiogranite association of island-arc type formed. To explain the spatial and temporal conjugation of different magmatism manifestations on the active margin, we applied the slab–window model.

The origin of high-Mg magmas in Mt Shasta and Medicine Lake volcanoes, Cascade Arc (California): higher and lower than mantle oxygen isotope signatures attributed to current and past subduction

We report the oxygen isotope composition of olivine and orthopyroxene phenocrysts in lavas from the main magma types at Mt Shasta and Medicine Lake Vol- canoes: primitive high-alumina olivine tholeiite (HAOT), basaltic andesites (BA), primitive magnesian andesites (PMA), and dacites. The most primitive HAOT (MgO ( 9 wt%) from Mt. Shasta has olivine d 18 O( d 18 OOl) values of 5.9-6.1%, which are about 1% higher than those observed in olivine from normal mantle-derived magmas. In con- trast, HAOT lavas from Medicine Lake have d 18 OOl values ranging from 4.7 to 5.5%, which are similar to or lower than values for olivine in equilibrium with mantle-derived magmas. Other magma types from both volcanoes show intermediate d 18 OOl values. The oxygen isotope composi- tion of the most magnesian lavas cannot be explained by crustal contamination and the trace element composition of olivine phenocrysts precludes a pyroxenitic mantle source. Therefore, the high and variable d 18 OOl signature of the most magnesian samples studied (HAOT and BA) comes from the peridotitic mantle wedge itself. As HAOT magma is generated by anhydrous adiabatic partial melting of the shallow mantle, its 1.4% range in d 18 OOl reflects a heter- ogeneous composition of the shallow mantle source that has been influenced by subduction fluids and/or melts sometime in the past. Magmas generated in the mantle wedge by flux melting due to modern subduction fluids, as exemplified by BA and probably PMA, display more homogeneous composition with only 0.5% variation. The high-d 18 O values observed in magnesian lavas, and prin- cipally in the HAOT, are difficult to explain by a single- stage flux-melting process in the mantle wedge above the modern subduction zone and require a mantle source enriched in 18 O. It is here explained by flow of older, pre- enriched portions of the mantle through the slab window beneath the South Cascades.

Chemical geodynamics of Western Anatolia

We report major and trace element concentrations and Nd–Sr–Pb isotopic data of 10 post-collisional volcanic domains in Western Anatolia, a seismically active part of the Alpine–Himalayan belt in the Aegean extensional province. Our objective is to provide geochemical constraints for tectono-magmatic processes shaping the late Cenozoic geodynamic evolution of Western Anatolia. Calc-alkaline volcanic rocks occurring to the north of the Izmir–Ankara–Erzincan suture zone show arc-like trace elements and isotopes and were formed by the melting of the metasomatized Neotethyan mantle-wedge; this process was facilitated by asthenospheric upwelling resulting from slab delamination. Calc-alkaline and alkaline volcanic rocks from within the Izmir–Ankara–Erzincan suture zone also show the imprint of subduction fluids in their major and trace elements, but their isotopic compositions indicate derivation from a metasomatized lithospheric mantle followed by assimilation of ancient crust. Volcanics along the N–S-oriented Kirka–Afyon–Isparta trend were derived from the lithospheric mantle that was metasomatized by fluids from the older subduction of the African plate. Golcuk–Isparta volcanic rocks show an asthenospheric imprint; the latter was a consequence of upwelling following a tear in the subducting African lithosphere. Shoshonitic Kula volcanic rocks show very high trace element concentrations, OIB mantle-like trace elements, and Nd–Sr–Pb isotopic signatures, and were formed by partial melting of the upwelling asthenospheric mantle; this event was synchronous with the Aegean extension and possibly also with slab window formation due to ruptures in the African plate. Inherent in the above chemical geodynamic models are the high ϵNd(0) values (+6.4) of the end-member volcanic rocks, implying the presence of an asthenospheric source beneath Western Anatolia that is responsible for the currently observed high heat flow, low Pn wave velocities, high seismicity, and tectonic activity.

Mantle flow through the Northern Cordilleran slab window revealed by volcanic geochemistry

The Northern Cordilleran slab window formed beneath western Canada concurrently with the opening of the Californian slab window beneath the southwestern United States, beginning in Late Oligocene–Miocene time. A database of 3530 analyses from Miocene– Holocene volcanoes along a 3500-km-long transect, from the northern Cascade Arc to the Aleutian Arc, was used to investigate mantle conditions in the Northern Cordilleran slab window. Using geochemical ratios sensitive to tectonic affi nity, such as Nb/Zr, we show that typical volcanic arc compositions in the Cascade and Aleutian systems (derived from subduction-hydrated mantle) are separated by an extensive volcanic fi eld with intraplate compositions (derived from relatively anhydrous mantle). This chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the Northern Cordilleran slab window. We suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge, which had developed during extensive early Cenozoic arc and backarc volcanism in western Canada. High heat fl ow throughout the western Canadian Cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window-induced mantle upwelling.

The North Patagonian batholith at Paso Puyehue (Argentina-Chile). SHRIMP ages and compositional features

New U–Pb SHRIMP zircon ages together with a geochemical and isotopic (Sr and Nd) study have been carried out on a hornblende gabbro , a biotite granodiorite , and host andesitic rocks of the North Patagonian batholith at Paso Puyehue, close to the Argentinean-Chilean border. The results yield ages of 18 ± 1 Ma for the gabbros and 12.4 ± 0.3 Ma for the granodiorites. An age of 125 ± 2 Ma was obtained for the host volcanic rock, and an age of 22 ± 2 Ma for the basaltic trachy-andesite. The intrusive suite is composed of dominant hornblende-biotite granodiorites and subordinate hornblende gabbros and diorites . These form a typical metaluminous calc-alkaline series. The time-isotopic magmatic evolution of plutonic and volcanic rocks shows an early Eocene beginning of disparate isotopic ratios as a consequence of coeval primitive and more crustal compositions with no spatial constraints. This disparate isotopic assemblage is also coeval with the presence of the Aluk-Farallon-SAM triple junction at this latitude and the shift of the calc-alkaline magmatism from the batholith axis to the back-arc. Considerations are made for the possibility of a Paleogene slab window by slab detachment, and the development of a new subduction front since the beginning of Nazca Plate development at the 23 Ma major ocean plate rearrangement. ► First Tertiary U-Pb SHRIMP zircon ages of the North Patagonian batholith. ► Disparate isotopic asemblage of coeval primitive and more crustal compositions. ► A new subduction front at the 23 Ma major ocean plate rearrangement.

Source-to-sink relationships along the South-Central Chilean margin: evidence from detrital apatite fission-track analysis

Detrital fission-track studies on sedimentary basins surrounding eroding mountain belts provide a powerful tool to reconstruct exhumation histories of the source area. However, examples from active arc-trench systems are sparse. In this study, we report detrital apatite fission-track (AFT) data from Holocene and Pleistocene turbiditic trench and modern river sediments at the Chilean margin (36°S-47°S). Sediment petrography and detrital AFT data point to different major sediment sources, underlining the need for multidisciplinary studies: whereas sediment petrography indicates the erosion of large volumes of volcanic detritus, no such volcanic signal is seen in the detrital age pattern. Areally subordinate plutonic units are identified as the main, often unique sources. This result has important implications for studies of fossil systems, where the feeder areas are eroded, and where the youngest age population is often interpreted to indicate active volcanism. For the southernmost part of the study area in the Patagonian Andes, where the source area is mainly composed of granitoids, the sediment is derived from only small portions along the main divide, pointing to focused glacial erosion there. Our detrital AFT data show no exhumational signal that could be related to the subduction of the actively spreading Chile Ridge at c. 47°S and to the opening of a slab window beneath the South American Plate.

Supercontinent cycles, extreme metamorphic processes, and changing fluid regimes

The solid Earth supercontinent cycle is intimately related to a fluid cycle, and the fluid cycle has undergone secular changes throughout Earth history. We discuss the relationships between the supercontinental cycle with the timing and processes of ultrahigh-pressure (UHP) + ultrahigh-temperature (UHT) metamorphism and the release of fluids within the cycle. Two broad regions of contrasting fluid characteristics – water dominated and CO2 dominated – are identified, and their relation to the assembly and dispersal of supercontinents is evaluated. Subduction zones at convergent margins are present at all stages of the supercontinent cycle, but their relative length and locations change throughout the cycle. One of the main effects of subduction is to bring oceanic lithosphere, overlying sediments that are not scraped off and accreted, and hydrous minerals deep into the mantle; these protoliths represent the source material for many of the fluids deep in the mantle. We propose an integrated model of ridge subduction to explain the dynamics of UHP and UHT orogens. During ridge subduction, a slab window opens and is filled by upwelling mantle material. At moderate levels, near the base of the crust and in the lower domain, the slab window places hot asthenosphere against the base of the overlying plate in a region that would normally be cooled by the slab, leading to a belt of UHT metamorphic rocks at deep levels of the arc and forearc where hydrous fluids are absent and CO2 dominates. Below this part, the geometry of the slab window is such that hot asthenosphere from the slab window is placed against hot asthenosphere of the overriding plate, so the largest effects are probably related to the loss of cooling by the descending slab, and the deprivation (temporarily) of slab-derived sediments and fluids. Ridge subduction is also associated with considerable disruption of the accretionary wedge, higher sedimentation rates in the trench, and changes in the style of subduction that lead to both greater growth of the wedge and subduction of sediments to greater depths. Many of these sediments ultimately are subducted, and form thin orogenic wedges deep in the mantle, hydrating the mantle and bringing sialic material to depth. Deeply subducted material undergoes high and UHP metamorphism; their extrusion and return to the surface became more common in the Phanerozoic Earth due to the lubricated channels provided by water within the subduction zone.

Relief inversion triggered by subduction of an active spreading ridge: evidence from glacial morphology in Central Patagonia

Terra Nova, 23, 63–69, 2011AbstractThis study examines the response of the Central Patagonian Cordillera to the subduction of the active South Chile Ridge and the development of the asthenospheric slab window beneath Patagonia since the Pliocene. We investigated the eastern part of the Cordillera where the glacio‐volcanic piedmont is topographically inverted. Glacial morphology and associated deposits capping the top of these piedmonts reveal that during the Pliocene (after 3 Ma), glaciers were flowing to the east over a regional surface connecting the western Cordillera and the palaeo‐piedmont. N160‐trending normal faulting along the western edge of the inverted plateau disrupted the geomorphic surface after 3 Ma. At a regional scale, extensional tectonics is responsible for a relief inversion with a minimum value of 800 m. The timing of extensional tectonics is contemporaneous with the subduction of the spreading axis and the development of the slab window beneath Central Patagonia.

Petrologic, tectonic, and metallogenic evolution of the Ancestral Cascades magmatic arc, Washington, Oregon, and northern California

Present-day High Cascades arc magmatism was preceded by ∼40 m.y. of nearly cospatial magmatism represented by the ancestral Cascades arc in Washington, Oregon, and northernmost California (United States). Time-space-composition relations for the ancestral Cascades arc have been synthesized from a recent compilation of more than 4000 geochemical analyses and associated age data. Neither the composition nor distribution of ancestral Cascades magmatism was uniform along the length of the ancestral arc through time. Initial (>40 to 36 Ma) ancestral Cascades magmatism (mostly basalt and basaltic andesite) was focused at the north end of the arc between the present-day locations of Mount Rainier and the Columbia River. From 35 to 18 Ma, initial basaltic andesite and andesite magmatism evolved to include dacite and rhyolite; magmatic activity became more voluminous and extended along most of the arc. Between 17 and 8 Ma, magmatism was focused along the part of the arc coincident with the northern two-thirds of Oregon and returned to more mafic compositions. Subsequent ancestral Cascades magmatism was dominated by basaltic andesite to basalt prior to the post–4 Ma onset of High Cascades magmatism. Transitional tholeiitic to calc-alkaline compositions dominated early (before 40 to ca. 25 Ma) ancestral Cascades eruptive products, whereas the majority of the younger arc rocks have a calc-alkaline affinity. Tholeiitic compositions characteristic of the oldest ancestral arc magmas suggest development associated with thin, immature crust and slab window processes, whereas the younger, calc-alkaline magmas suggest interaction with thicker, more evolved crust and more conventional subduction-related magmatic processes. Presumed changes in subducted slab dip through time also correlate with fundamental magma composition variation. The predominance of mafic compositions during latest ancestral arc magmatism and throughout the history of modern High Cascades magmatism probably reflects extensional tectonics that dominated during these periods of arc magmatism. Mineral deposits associated with ancestral Cascades arc rocks are uncommon; most are small and low grade relative to those found in other continental magmatic arcs. The small size, low grade, and dearth of deposits, especially in the southern two-thirds of the ancestral arc, probably reflect many factors, the most important of which may be the prevalence of extensional tectonics within this arc domain during this magmatic episode. Progressive clockwise rotation of the forearc block west of the evolving Oregon part of the ancestral Cascades magmatism produced an extensional regime that did not foster signifi cant mineral deposit formation. In contrast, the Washington arc domain developed in a transpressional to mildly compressive regime that was more conducive to magmatic processes and hydrothermal fluid channeling critical to deposit formation. Small, low-grade porphyry copper deposits in the northern third of the ancestral Cascades arc segment also may be a consequence of more mature continental crust, including a Mesozoic component, beneath Washington north of Mount St. Helens.

Continental collision and slab break-off: A comparison of 3-D numerical models with observations

Conditions and dynamics of subduction–collision and subsequent 3-D slab break-off and slab tear propagation are quantified, for the first time, using fully dynamic numerical models. Model results indicate that collision after the subduction of old, strong subducting oceanic slab leads to slab break-off at 20–25Myr after the onset of continental collision, and subsequently a slab tear migrates more or less horizontally through the slab with a propagation speed of 100–150mm/yr. In contrast, young, weak oceanic slabs show the first break-off already 10Myr after continental collision, and can experience tear migration rates up to 800mm/yr. Slab strength plays a more important role in the timing of slab break-off and the speed of a propagating slab tear than (negative) slab buoyancy does. Slab break-off is viable even for slabs that are supported by the viscosity jump and phase change between the upper and lower mantle.The density of the oceanic slab and the subducting continental block is important for the amount of continental subduction and the depth of slab break-off. A 40-km thick continental crust can be buried to depths greater than 200km, although this maximum depth is significantly less for younger or very weak slabs, or thicker continental crust. Slab break-off typically starts at a depth of 300km, mostly independent of mantle rheology, but, like continental crustal burial, can be shallower for young or buoyant plates. Our 3-D models illustrate how, due to the difference in necking in 2-D and 3-D, break-off has an intrinsic small preference to start as a slab window within the slab's interior, rather than as a slab tear at the slab edge. However, any significant asymmetry in the collision setting, e.g. earlier collision at one end of the subduction zone, would override this, and leads to slab tearing starting near one edge of the slab.These results put important new constraints on the dynamics of the collision and subsequent slab break-off for modern collision belts. For a proposed timing of the initial Arabia–Eurasia collision at 35Ma, break-off of the ~200-Myr-old Neo-Tethys slab is unlikely to have occurred before 15–10Ma. Furthermore, our results illustrate that shallow, early break-off of weak slabs provides a viable explanation for the absence of blueschists and ultra-high pressure metamorphism in the Precambrian geological record.

Paleomagnetic evidence for a post-1.2 Ma disruption of the Calabria terrane: Consequences of slab breakoff on orogenic wedge tectonics

In the past few years, a wealth of paleomagnetic data gathered from Neogene sediments consistently showed that since ca. 10 Ma the Calabria terrane coherently drifted ~500 km ESE-ward from the Sardinian margin, and rotated 15°–20° clockwise (CW) as a rigid microplate between 2 and 1 Ma. Here we report on a high-resolution paleomagnetic investigation of the Crotone forearc basin of northern Calabria. The integrated calcareous plankton biostratigraphy indicates early Pliocene (Zanclean) to late early Pleistocene (Calabrian) ages for 29 successful paleomagnetic sites and/or sections. Unexpectedly, four domains undergoing distinct rotations are documented. Two blocks have undergone a CW rotation statistically undistinguishable, for both timing and magnitude, from the rigid Calabria rotation documented in the past. Two additional ~10-km-wide blocks yielded a 30.8° ± 22.5° and 32.0° ± 9.2° post–1.2 Ma counterclockwise rotation, likely due to left-lateral shear along two NW-SE fault zones. We infer that since advanced early Pleistocene times, after the end of the uniform CW rotation, left-lateral strike-slip tectonics disrupted the Calabria terrane, overwhelming a widespread extensional regime accompanying the Calabria drift since late Miocene times. Seismological evidence reveals that only the southern part of the Ionian slab subducting below Calabria is continuous, while beneath northern Calabria a slab window between 100 and 200 km depth is apparent. We suggest that the partial breakoff of the Ionian slab after 1 Ma induced the fragmentation of the Calabria wedge, and that strike-slip faults from the Crotone basin decoupled “inactive” northern Calabria from southern Calabria, still drifting towards the trench.

Mapping Tertiary mid-ocean ridge subduction and slab window formation beneath Sundaland using seismic tomography

Mapping Tertiary mid-ocean ridge subduction and slab window formation beneath Sundaland using seismic tomography

Crustal and upper mantle structure of southeast Tibet from Rayleigh wave tomography

We have applied the two‐plane wave inversion technique to fundamental mode Rayleigh waves in southeast Tibet. Phase velocity variations are obtained at 15 periods from 20 to 143 s and are used to construct three‐dimensional shear wave velocity structure. A low‐velocity zone is imaged in the middle crust of southeast Tibet, in which velocity perturbation varies from −4% to 3%. The slow area might be associated with high temperature and partial melt and can flow ductilely, while the high‐velocity region could be relatively cold and mechanically strong enough to pass crustal strain, implying that coherent lithospheric deformation is possible with the presence of complicated crustal flow. Strong negative anomalies in the shallow crust are imaged at 92°E, correlating with the location of a north‐south rift zone. A fast mantle lid to a depth of ∼120 km is present beneath most of the study area. The lithosphere is significantly slower in the central part of the study area than in the west and east. To the east of 96°E, a subvertical high‐velocity zone is imaged at 120–240 km depths under the Bangong‐Nujiang Suture (BNS), which is more like the subduction of the Asian lithosphere. We interpret the high‐velocity anomalies to the south of the BNS as the subducted Indian lithosphere and those to the north as the Asian lithosphere. A vertical low‐velocity column is observed in the central part of the region from the lower crust to the midupper mantle, which is probably in part responsible for the rift at 92°E. The slow anomaly is likely associated with asthenosphere upwelling following the opening of a slab window that could have developed when the slab was torn apart vertically because of the significant change of subducting direction at the eastern syntaxis.

Volcanic Markers of the Post-Subduction Evolution of Baja California and Sonora, Mexico: Slab Tearing Versus Lithospheric Rupture of the Gulf of California

The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: “regular” calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the “no-slab” regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.

Late Devonian–Early Permian A-type granites in the southern Altay Range, Northwest China: Petrogenesis and implications for tectonic setting of “A 2-type” granites

The Altay Range is an important part of the Central Asian Orogenic Belt (CAOB) that contains A-type granites that have been attributed to a post-collisional setting during the Late Carboniferous–Permian. However, our new LA-ICP-MS zircon U–Pb age data demonstrates that there were two episodes of A-type granite magmatism in the southern Altay Range. A Late Carboniferous to Early Permian (308–291 Ma) suite does occur in the vicinity of Qiakuerte but a distinct Late Devonian (382–367 Ma) suite is present in the Kouan area. The early Kouan A-type granites are mainly composed of alkali-feldspar granites, which are approximately synchronous with adakites, boninites, high-Ti basalts, picrites, ophiolitic rocks, and high temperature-low pressure metamorphic rocks, whereas the late Qiakuerte A-type granites mainly consist of arfvedsonite and aegirine-bearing granites, which are approximately coeval with some mafic–ultramafic rocks in the southern Altay Range. A-type granites are commonly classified as A 1 and A 2 sub-types, which are considered to be generated in anorogenic (rift or plume-related) or post-collisional settings, respectively. Both suites of the southern Altay Range are geochemically similar to typical A 2-type granites, e.g., high K 2O + Na 2O, FeO/MgO, and Ga/Al, and Y/Nb values, and low CaO, Ba, Sr, and Eu contents. The Kouan A-type granites have relatively low (La/Yb) N (2.7–5.9), high ε Nd( t) (+6.7 to +7.7) and Nb/La (0.59–1.67), and variable ε Hf( t) (+5.0 to +14.1) values, whereas the Qiakuerte A-type granites have comparatively high (La/Yb) N (4.2–9.2), low ε Nd( t) (+5.3 to +6.0) and Nb/La (0.33–0.74), and variable ε Hf( t) (+10.0 to +18.7) values. The early Kouan A-type granites may have been generated by partial melting of a mafic source containing more depleted mantle-derived components than the late Qiakuerte A-type granites. We suggest that the Late Carboniferous to Early Permian Qiakuerte granites were formed in a post-collisional extensional setting, as is typical of A 2-type granites. However, the Late Devonian Kouan A-type granites were more plausibly generated in an extensional setting as a result of slab window caused by a ridge subduction. In this case, the upwelling of asthenosphere through the slab window provided the source of parental magmas, or the heat for the generation of Devonian magmas. Therefore, our results suggest that A 2-type granites can also form in a ridge subduction-related extensional setting.

Mantle flow and dynamic topography associated with slab window opening: Insights from laboratory models

We present dynamically self-consistent mantle-scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics, mantle flow and associated dynamic topography over geological time scales. The adopted setup consists of a two-layer linearly viscous system simulating the subduction of a fixed plate of silicone (lithosphere) under negative buoyancy in a viscous layer of glucose syrup (mantle). Our experimental setting is also characterized by a constant-width rectangular window located at the center of a laterally confined slab, modeling the case of the interaction of a trench-parallel spreading ridge with a wide subduction zone. We found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab. On the contrary, slab-induced mantle circulation, quantified both in the vertical and horizontal sections using the Feature Tracking image analysis technique, is strongly modified. In particular, rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components, with the importance of each evolving according to kinematic stages. Mantle coming from the oceanic domain floods through the slab window, indenting the supra-slab mantle zone and producing its deformation without any mixing between mantle portions. The opening of the slab window and the upwelling of sub-slab mantle produce a regional-scale non-isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca. 1 and 5 km in nature. Assuming that our modeling results can be representative of the natural behavior of subduction zones, we compared them to the tectonics and volcanism of the Patagonian subduction zone. We found that the anomalous backarc volcanism that has been developing since the middle Miocene could result from the lateral flow of sub-slab mantle and that part of the Patagonian uplift could be dynamically supported.

Geochemical and zircon U–Pb study of the Huangmeijian A-type granite: implications for geological evolution of the Lower Yangtze River belt

The Early Cretaceous Huangmeijian Pluton is an A-type granite located on the northern bank of the Lower Yangtze River in Anhui Province, east-central China. It intruded the SE edge of the Early Cretaceous Luzong volcanic basin. The moderate- to coarse-grained granite is mainly composed of alkali feldspar, plagioclase, and quartz and has a typical A-type geochemical signature. LA-ICP-MS zircon dating yielded a weighted mean 206Pb/238U age of 127.1 ± 1.4 Ma, similar to other A-type granites in the Lower Yangtze River belt, indicating an Early Cretaceous extensional environment. Temperatures calculated using the Ti-in-zircon thermometer suggest that the magma formed under high-temperature conditions (720–880°C). The low calculated Ce(IV)/Ce(III) ratio based on zircon rare earth element patterns indicates low oxygen fugacity for this A-type magma. Previous studies suggested that eastern China was an active plate margin related to the Early Cretaceous subduction of the Pacific and Izanagi plates. The ridge between these two plates probably passed under the Lower Yangtze River belt, forming A-type granites and adakites. The Huangmeijian Pluton is roughly the same age within error but is marginally older than the Baijuhuajian A-type granite in the eastern part of the Lower Yangtze River belt. A-type granite genesis in the Lower Yangtze River belt only lasted for 2–3 million years and slightly predates the transition from regional extension to compression. All these can be plausibly interpreted by the ridge subduction model, that is, A-type granites formed because of mantle upwelling through the slab window during subduction of the ridge separating the Pacific and Izanagi plates.

Magmatic history and evolution of the Central American Land Bridge in Panama since Cretaceous times

Chemical compositions for 310 igneous rocks from the Cordillera de Panama and the Sona and Azuero peninsulas were supplemented by 40 Ar/ 39 Ar dating and Sr-, Nd-, Pb-, and O-isotope analysis to determine the magmatic evolution and oceanic plate interactions over the past 100 Ma in western Panama. An initial phase of intraplate magmatism, having geochemical characteristics of the Galapagos hotspot, formed the oceanic basement of the Caribbean large igneous province from 139 to 69 Ma. Younger accreted terranes with enriched trace element patterns (accreted ocean island basalt [OIB]) were amalgamated between 70 and 20 Ma. A second magmatic phase in the Azuero and Sona peninsulas has trace element patterns (Sona-Azuero arc) suggesting the initiation of subduction at 71–69 Ma. Arc magmatism continued in the Chagres basin region (Chagres-Bayano arc) from 68 to 40 Ma. A third phase formed discrete volcanic centers across the Cordillera de Panama (Cordilleran arc) from 19 to 5 Ma. The youngest phase consists of isolated volcanic centers of adakitic composition (Adakite suite) in the Cordillera de Panama that developed over the past 2 million years. Initiation of arc magmatism at 71 Ma coincides with the cessation of Galapagos plateau formation, suggesting a causal link. The transition from intraplate to arc magmatism occurred relatively quickly and introduced a new enriched mantle source. The arc magmatism involved progressive transition to more homogeneous intermediate mantle wedge compositions through mixing and homogenization of subarc magma sources through time and/or the replacement of the mantle wedge by a homogeneous, relatively undeleted asthenospheric mantle. Adakite volcanism started after a magmatic gap, enabled by the formation of a slab window.

Palinspastic restoration of NAVDat and implications for the origin of magmatism in southwestern North America

Simultaneous palinspastic restoration of deformation and volcanism illuminates relationships between magmatism and tectonics in western North America. Using ArcGIS, we retrodeformed the NAVDat (North American Volcanic Database, navdat.geongrid.org) using the western North America reconstruction of McQuarrie and Wernicke (2005). From these data sets we quantitatively compare rates of magmatism and deformation and evaluate the age, composition, and migration of Cenozoic volcanism from 36 Ma to present. These relationships are shown in a series of palinspastic maps as well as animations that highlight migrating extension and volcanism with time. Western North America is grouped into eight different regions with distinct relationships between strain and volcanism to evaluate competing hypotheses regarding the relationship of extension to continental magmatism. A first‐order observation from this study is that magmatism throughout the Basin and Range appears to be primarily driven by plate boundary effects, notably subducting and foundering slabs as well as slab windows. Exceptions include the Yellowstone hotspot system along the northern border of our study area and late‐stage (<8 Ma) passive, extension‐related asthenospheric upwelling along the eastern and western margins of the Basin and Range. The palinspastic reconstructions presented here highlight that the classic, high‐angle, Basin and Range faulting that comprises most of the physiographic Basin and Range Province commenced during a magmatic lull. More broadly, with the exception of the Rio Grande rift we find that pulses of magmatism lag the onset of extension. These observations largely contradict the active rifting model where magmatism triggers Basin and Range extension.

Secular variations in magmatism during a continental arc to post-arc transition: Plio-Pleistocene volcanism in the Lake Tahoe/Truckee area, Northern Sierra Nevada, California

In the Lake Tahoe–Truckee River area of northeastern California, a suite of latest Pliocene to Pleistocene volcanic rocks, termed the Tahoe–Truckee volcanic field (TTVF), overlies Miocene to Pliocene volcanic rocks of the Ancestral Cascades arc. The lavas post-date the passage of the south edge of the subducting Juan de Fuca plate, and represent a secular transition from continental arc to post-arc volcanism associated with a slab window. Compared to the older arc volcanic rocks, TTVF lavas are small in volume, non-porphyritic, and mildly alkaline. TTVF lavas have a subduction signature in primitive mantle-normalized plots but, compared to the older arc rocks, are enriched in the light rare earth elements, Nb, and Ta, and depleted in the large ion lithophile elements. Isotopically, TTVF rocks overlap with Ancestral Cascade arc lavas, although most TTVF rocks fall at the high 87Sr/ 86Sr–low 143Nd/ 144Nd end of the range of Ancestral Cascade compositions. Pb and oxygen isotope ratios overlap completely. Although the incompatible element systematics are consistent with increased importance of a within-plate mantle source component in the TTVF, the more radiogenic Sr and less radiogenic Nd isotopic compositions are not and require a source that is old. We propose that, like the Ancestral Cascades arc in this region, the post-arc lavas are primarily tapping a lithospheric mantle source, but for the TTVF, melting is triggered by asthenospheric upwelling around the south edge of the Juan de Fuca slab. TTVF lavas include a much lower proportion of melts from the (now, ex-) mantle wedge, since slab fluids are no longer supplied to the mantle beneath this region. The lithospheric mantle source in the TTVF may be similar chemically and mineralogically to that of the Big Pine volcanic field and Long Valley caldera in the Western Great Basin. This example of slab window volcanism is unusual in that the lavas are not melts of a mantle source that is more depleted in incompatible elements than the arc source.