Slab Window Research Articles

Chronological and geochemical constraints on the pre-variscan tectonic history of the Erzgebirge, Saxothuringian Zone

A combined U–Pb zircon geochronological and whole-rock isotopic and geochemical study has been carried out on high-grade orthogneiss, meta-basite, and meta-sediments from the Erzgebirge. The results indicate multiple pulses of Ediacaran–Ordovician magmatism in a transitional volcanic-arc to rift-basin setting. Orthogneiss from high-pressure nappes exhibit a step-like pattern of inherited zircon ages and emplacement ages of 500–475 Ma. In contrast, granite gneiss from the medium-pressure core of the Erzgebirge is characterised by three pulses of magmatism in the Early Cambrian, Late Cambrian, and Early Ordovician. A trend of decreasing Th/U ratios in zircon is observed to c.500 Ma, after which significant increases in the trend and variability of the data is inferred to mark the transition from arc-related to rift-related magmatism. Sediments deposited in the Early Cambrian have continental island arc affinity. Major detrital peaks in the Ediacaran and subordinate Tonian, Palaeoproterozoic, and Neoarchaean data are consistent with an Avalonian-Cadomian Arc and West African Craton derivation. The Early Cambrian sediments were locally reworked by a thermal event in the Ordovician resulting in leucocratic banding and recorded in Ordovician zircon rims characterised by systematically lower Th/U ratios. Ptygmatically folded leucocratic bands containing Ordovician zircon rims, associated with low Th/U ratios, are further observed in the granite gneiss core of the Erzgebirge. Variscan ages are rare, except in a fine-grained high-pressure micaschist, which contains exclusively small, structure-less, zircon with a weighted mean age of 350 ± 2 Ma. These data, along with a re-evaluation of previously published data, have been interpreted as the product of flattening subduction during the Early Cambrian; followed by the opening of slab windows in the Late Cambrian; and finally delamination in the Early Ordovician. Delamination of the orphaned slab led to asthenospheric upwellings triggering extension, bimodal magmatic pulses, recycling of fertile crust, high-temperature metamorphism, and cratonisation of relatively young crust.

Coeval high Ba-Sr arc-related and OIB Neoproterozoic rocks linking pre-collisional magmatism of the Ribeira and Araçuaí orogenic belts, SE-Brazil

Two arc systems have been described within the Neoproterozoic Ribeira Belt: the inner continental magmatic arc and the outer intra-ocean magmatic arc. New geochemical and U-Pb and Lu-Hf (LA-ICPMS) data from zircons of the Marceleza-Leopoldina stocks, belonging to the inner magmatic arc system, in the boundary region between the Ribeira and Araçuaí orogenic belts are presented. The intermediate to felsic magmatic rocks have high Ba, Sr, K/Rb, LILE and LREE contents. Low Rb, Th, U, Nb, and very low Y and HREE contents point to their generation in a supra-subduction tectonic scenario, from melting of the asthenospheric wedge above a subducting oceanic plate. Zircon U-Pb (LA-ICPMS) data indicates crystallization ages between ca. 650 and 600 Ma, with high-grade metamorphic overprint at ca. 582 Ma. Tonian inherited zircons were found in the older rocks, suggesting that the development of the continental magmatic arc might have begun earlier. Associated basic (gabbros and monzogabbros) to intermediate (diorites and monzodiorites) rocks are geochemically heterogeneous. Some represent less evolved calc-alkaline basaltic magmas, while relatively more enriched varieties are intraplate basalts with OIB components. Together with lithogeochemical data, Lu-Hf signatures showing TDM model ages between ca. 1.7 to 1.3 Ga, with initial ɛHf values ranging from slightly negative values (−0.9 to −3.75) to strongly negative (up to −10.4) indicate a magmatic arc developed on continental margin environment (i.e., continental-type arc). The studied rocks represent the Rio Doce arc in the studied region and are correlated with other arc segments located from the north, in the Araçuaí Orogen, to the southernmost Ribeira Orogen, defining a c. 1000 km long continental-type magmatic arc. This inner arc system first collided against the São Francisco craton during the Neoproterozoic Brasiliano orogeny around 580 Ma. The geochemical diversity of coeval intermediate to felsic rocks and the variability of associated basic rocks are suggestive of a complex evolution of the subduction zone system. Possible mechanisms for the enrichment in incompatible elements of magmatic products may have involved slab roll-back and subduction of fore-arc rocks, and the influx of ascending asthenosphere associated with slab windows (slab break off, ridge subduction) prior to collision.

Electrically Anisotropic Crust From Three‐Dimensional Magnetotelluric Modeling in the Western Junggar, NW China

AbstractAn island arc environment related to intraoceanic subduction or ridge subduction from the Late Carboniferous to the Early Permian has been proposed for the formation of Western Junggar, NW China, situated in the southwest of Central Asian Orogenic Belt. However, the details and consequences of the subduction remain controversial. To further identify the relics of the Late Carboniferous subduction at depth, three magnetotelluric profiles were deployed. Phase tensors, real induction arrows, and three‐dimensional (3‐D) isotropic resistivity models indicate the presence of a 3‐D anisotropic resistivity structure in the crust. Consequently, a 3‐D anisotropic resistivity model was constructed by forward modeling. The 3‐D anisotropic resistivity model contains two azimuthally anisotropic middle‐upper crust anomalies with minimum resistivity striking N90°E at depths of 5 to 20 km, and an azimuthally anisotropic lower crust anomaly with minimum resistivity striking N20°E at depths of 20 to 46 km. The electrically anisotropic anomalies are closely related to relatively high shear‐wave velocities with significant negative radial anisotropy. The anisotropies in the middle‐to‐upper crust and the lower crust are related to the oceanic ridge (N90°E) subduction with a slab window in the Late Carboniferous and the remnant oceanic slab, respectively. The magnetotelluric observations support a NW70° subduction with an intraoceanic ridge‐transform system in the Late Carboniferous and a remnant oceanic slab trapped in Western Junggar.

Receiver function mapping of mantle transition zone discontinuities beneath Alaska using scaled 3-D velocity corrections

SUMMARYThe mantle transition zone is the region between the globally observed major seismic velocity discontinuities around depths of 410 and 660 km and is important for determining the style of convection and mixing between the upper and the lower mantle. In this study, P-to-S converted waves, or receiver functions, are used to study these discontinuities beneath the Alaskan subduction zone, where the Pacific Plate subducts underneath the North American Plate. Previous tomographic models do not agree on the depth extent of the subducting slab, therefore improved imaging of the Earth structure underneath Alaska is required. We use 27 800 high quality radial receiver functions to make common conversion point stacks. Upper mantle velocity anomalies are accounted for by two recently published regional tomographic S-wave velocity models. Using these two tomographic models, we show that the discontinuity depths within our CCP stacks are highly dependent on the choice of velocity model, between which velocity anomaly magnitudes vary greatly. We design a quantitative test to show whether the anomalies in the velocity models are too strong or too weak, leading to over- or undercorrected discontinuity depths. We also show how this test can be used to rescale the 3-D velocity corrections in order to improve the discontinuity topography maps.After applying the appropriate corrections, we find a localized thicker mantle transition zone and an uplifted 410 discontinuity, which show that the slab has clearly penetrated into the mantle transition zone. Little topography is seen on the 660 discontinuity, indicating that the slab has probably not reached the lower mantle. In the southwest, P410s arrivals have very small amplitudes or no significant arrival at all. This could be caused by water or basalt in the subducting slab, reducing the strength at the 410, or by topography on the 410 discontinuity, preventing coherent stacking. In the southeast of Alaska, a thinner mantle transition zone is observed. This area corresponds to the location of a slab window, and thinning of the mantle transition zone may be caused by hot mantle upwellings.

Syncollisional gabbro in the Irtysh shear zone, Eastern Kazakhstan: Compositions, geochronology, and geodynamic implications

New data of petrography, mineralogy, and geochemistry from the Irtysh shear zone in Eastern Kazakhstan reveal three types of gabbro: (1) dolerite and gabbrodolerite, (2) biotite-bearing gabbronorite and gabbrodiorite, and (3) layered peridotite-gabbro intrusions. Rocks of types 1 and 2 occur mainly as small bodies and share similarity in major- and trace-element compositions. Layered gabbro (type 3) forms the largest intrusion in the area and has a different composition: higher CaO and Al2O3 but lower K2O, TiO2, P2O5, Zr, Nb, REE. Correspondingly, the gabbro of two first types may result from differentiation of the same tholeiite magma, whereas the layered gabbro rather derived from a magma intermediate between tholeiite and calc-alkaline series. According to geochemical and isotopic signatures, both types of primary magma originated from the same sublithospheric depleted mantle by low- and high-degree partial melting of spinel lherzolite. The obtained 317–313 Ma U-Pb and Ar-Ar ages of the gabbro fall within the time of collisional events in the area. The suggested model explains the origin of syncollisional gabbro as intrusion of ascending mafic melts from a mantle wedge at the transform boundary of colliding plates along the Irtysh shear zone where strike-slip faulting produced slab windows open to inputs of hot asthenospheric material.

Rifting evolution of the Malvinas basin, offshore Argentina: New constrains from zircon U–Pb geochronology and seismic characterization

The Malvinas basin, emplaced offshore southern Patagonia, is an asymmetric extensional basin formed by NNW-striking faults. In this contribution we present a review and updated structural framework for the Malvinas basin, together with two new U–Pb zircon ages for the synrift series. By extrapolation from the neighboring Austral basin, the Middle Jurassic Serie Tobífera unit has typically been interpreted as the synrift infill of the Malvinas basin and correlated to the V2 event of the Chon Aike Magmatic Province. A tuff sample recovered from Serie Tobífera in a well in the Malvinas basin, confirmed this correlation and yielded a crystallization age of 169.6 ± 2 Ma (U–Pb in zircon). Furthermore, a volcanic breccia obtained from a deeper section within the synrift, yielded a Late Triassic age (215 Ma, U–Pb in zircon). This is the first Triassic age in the Malvinas basin and allowed correlation of these deposits with the El Tranquilo basin in the Deseado Massif (onshore Patagonia). The synextensional emplacement of the retroarc Chon Aike Magmatic Province is associated with trenchward migration of the coetaneous volcanic arc. To explain the rotational extension observed in the Chon Aike structural fabric at continental plate-scale, we follow the differential rollback model for the Jurassic evolution of southern Patagonia and introduce a vertical slab tear between Patagonia and Antarctica. The slab tear model provides a border condition for the differential rollback and explains the lateral displacement between Eastern and Western Gondwana during the Mozambique Channel opening. Moreover, the asthenospheric window opened by the slab tear would have produced a thermal anomaly that triggered the opening of the Weddell Sea in the Late Jurassic between Eastern and Western Gondwana.

Cretaceous to Miocene magmatism, sedimentation, and exhumation within the Alaska Range suture zone: A polyphase reactivated terrane boundary

AbstractThe Alaska Range suture zone exposes Cretaceous to Quaternary marine and nonmarine sedimentary and volcanic rocks sandwiched between oceanic rocks of the accreted Wrangellia composite terrane to the south and older continental terranes to the north. New U-Pb zircon ages, 40Ar/39Ar, ZHe, and AFT cooling ages, geochemical compositions, and geological field observations from these rocks provide improved constraints on the timing of Cretaceous to Miocene magmatism, sedimentation, and deformation within the collisional suture zone. Our results bear on the unclear displacement history of the seismically active Denali fault, which bisects the suture zone. Newly identified tuffs north of the Denali fault in sedimentary strata of the Cantwell Formation yield ca. 72 to ca. 68 Ma U-Pb zircon ages. Lavas sampled south of the Denali fault yield ca. 69 Ma 40Ar/39Ar ages and geochemical compositions typical of arc assemblages, ranging from basalt-andesite-trachyte, relatively high-K, and high concentrations of incompatible elements attributed to slab contribution (e.g., high Cs, Ba, and Th). The Late Cretaceous lavas and bentonites, together with regionally extensive coeval calc-alkaline plutons, record arc magmatism during contractional deformation and metamorphism within the suture zone. Latest Cretaceous volcanic and sedimentary strata are locally overlain by Eocene Teklanika Formation volcanic rocks with geochemical compositions transitional between arc and intraplate affinity. New detrital-zircon data from the modern Teklanika River indicate peak Teklanika volcanism at ca. 57 Ma, which is also reflected in zircon Pb loss in Cantwell Formation bentonites. Teklanika Formation volcanism may reflect hypothesized slab break-off and a Paleocene–Eocene period of a transform margin configuration. Mafic dike swarms were emplaced along the Denali fault from ca. 38 to ca. 25 Ma based on new 40Ar/39Ar ages. Diking along the Denali fault may have been localized by strike-slip extension following a change in direction of the subducting oceanic plate beneath southern Alaska from N-NE to NW at ca. 46–40 Ma. Diking represents the last recorded episode of significant magmatism in the central and eastern Alaska Range, including along the Denali fault. Two tectonic models may explain emplacement of more primitive and less extensive Eocene–Oligocene magmas: delamination of the Late Cretaceous–Paleocene arc root and/or thickened suture zone lithosphere, or a slab window created during possible Paleocene slab break-off. Fluvial strata exposed just south of the Denali fault in the central Alaska Range record synorogenic sedimentation coeval with diking and inferred strike-slip displacement. Deposition occurred ca. 29 Ma based on palynomorphs and the youngest detrital zircons. U-Pb detrital-zircon geochronology and clast compositional data indicate the fluvial strata were derived from sedimentary and igneous bedrock presently exposed within the Alaska Range, including Cretaceous sources presently exposed on the opposite (north) side of the fault. The provenance data may indicate ∼150 km or more of dextral offset of the ca. 29 Ma strata from inferred sediment sources, but different amounts of slip are feasible.Together, the dike swarms and fluvial strata are interpreted to record Oligocene strike-slip movement along the Denali fault system, coeval with strike-slip basin development along other segments of the fault. Diking and sedimentation occurred just prior to the onset of rapid and persistent exhumation ca. 25 Ma across the Alaska Range. This phase of reactivation of the suture zone is interpreted to reflect the translation along and convergence of southern Alaska across the Denali fault driven by highly coupled flat-slab subduction of the Yakutat microplate, which continues to accrete to the southern margin of Alaska. Furthermore, a change in Pacific plate direction and velocity at ca. 25 Ma created a more convergent regime along the apex of the Denali fault curve, likely contributing to the shutting off of near-fault extension-facilitated arc magmatism along this section of the fault system and increased exhumation rates.

Mid-Paleozoic ridge subduction in the Central Beishan of the southern Altaids: evidence from geochemical, Sr–Nd and zircon U–Pb–Hf–O isotopic data of Gongpoquan volcanic rocks

The investigation of Mid-Paleozoic tectonics could constrain the relations of early and late Paleozoic subduction events in the southern Altaids and improve our understanding of the architecture of accretionary orogens. The Gongpoquan volcanic rocks are widespread in the Central Beishan and contain important information on the tectonics of the southern Altaids. Geochronological data constrain the eruptive time of the Gongpoquan volcanic rocks as early Silurian–early Devonian (434–411 Ma). The Gongpoquan andesites show similar geochemical characteristics to those of typical sanukitoids, including high MgO and compatible element concentrations, enrichments in large ion lithophile elements and light rare earth elements, and depleted isotopic compositions. The geochemical data show that the Gongpoquan andesites were generated through the partial melting of subducting sediments and subsequent melt–mantle peridotite interactions. The early Silurian basalts are Na-rich and exhibit high Nb/U and Nb/La ratios, an enrichment in high field strength elements and enriched isotopic compositions, making them akin to Nb-enriched basalts. Such Nb-enriched basalts should be derived from the melting of a mantle component similar to ocean island basalt in the mantle wedge. The early Devonian basalts are calc-alkaline, enriched in large ion lithophile elements and depleted in high field strength elements. They yield high Ba/Yb ratios and depleted isotopic compositions, indicating an origin via the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids. Our data prove that the Paleo-Asian Ocean may still have been subducted during the Mid-Paleozoic. During this process, a spreading ridge was subducted beneath the Central Beishan, resulting in the opening of a slab window, which generated the observed 431–411 Ma sanukitoids and Nb-enriched basalts. Supplementary Material: Analytical methods, U–Pb geochronological data, zircon Lu–Hf-O isotopic compositions, whole-rock major and trace element data, detailed descriptions of the zircon U–Pb geochronology and Hf–O isotopic compositions for analysed samples, representative field photographs and photomicrographs of samples, and representative cathodoluminescence images of analysed zircon grains are available at: https://doi.org/10.6084/m9.figshare.c.4461062

Geodynamic history and mantle evolution recorded in Cenozoic lavas of Sardinia

Two Cenozoic magmatic cycles occurred in Sardinia separated by a 5 myr gap: Oligo-Miocene subduction-related calc-alkaline magmatism associated with Sardinia migrating from the European continental margin to its current location, and Plio-Pleistocene intraplate alkaline magmatism associated with the opening of the southern Tyrrhenian basin. Based on an integration of new and published geochemical data on the Cenozoic volcanic rocks of Sardinia, and regional tectonics, we propose that a northward propagating slab tear formed south of Sardinia at the end of the Oligo-Miocene cycle. This was responsible for the transition from subduction-controlled calc-alkaline to extension-controlled mildly alkaline to peralkaline magmatism in the Sulcis area (SW Sardinia), and for the end of the Oligo-Miocene magmatism. The inflow of hotter material from the European Asthenospheric Reservoir (EAR) through the slab window could have produced the minor EAR-type alkaline lavas that erupted at the end of the Oligo-Miocene cycle and at the beginning of the Plio-Pleistocene cycle. The dominant younger Plio-Pleistocene lavas with EMI-type compositions probably formed from decompression melting of a shallower lithospheric mantle with little influence from the EAR-forming event. Supplementary material: A summary of Sulcis rocks petrography, representative whole-rock analysis of Sulcis rocks, Sr, Nd and Pb isotope analytical methods and results, a list of sources of whole-rock geochemical data from the literature and complementary diagrams are available at https://doi.org/10.6084/m9.figshare.c.4447802

Late Triassic ridge subduction of Paleotethys: Insights from high-Mg granitoids in the Songpan-Ganzi area of northern Tibet

The time of final closure of the Paleotethys Ocean is still hotly debated in particular regarding the Jinsha Ocean in northern Tibet. This study reports new geochronologic, geochemical and zircon Hf isotopic data of six granodiorites-diorites, which intrude the Mid-Late Triassic Songpan-Ganzi complex (SGC). LA-ICP-MS zircon U-Pb analyses of three quartz diorites (214.9 ± 2.2, 211.0 ± 1.3, 205.8 ± 1.4 Ma) and three granodiorites (210.8 ± 2.7, 206.7 ± 3.3, 205.9 ± 3.5 Ma) gave a Late Triassic age range (214.9–205.8 Ma). These granodiorite-diorite rocks have relatively high MgO (up to 9.1%) and Mg# (up to 70.2), Al2O3, Cr, Ni, but low TiO2, and high magnesium andesite/diorite (HMA) chemical signatures with low Sr and Y content, and slight-moderate negative Eu anomaly. Also, the dioritic enclaves and Late Triassic detrital zircons (as young as 208 Ma) were captured by studied rocks. Hf isotope data show overall slight to moderate negative zircon εHf(t) values (−1.97 to −5.76, −4.12 to −6.72) and heterogeneous TDMC ages (1.38−1.61Ga, 1.51−1.68Ga) for quartz diorites and granodiorites respectively, together with other published Hf isotopic data in SGC, are in highly concert with those around ~240 and ~440Ma magmas (as potential sources for SGC turbidites) in east Kunlun orogenic belt. Taken together, the HMA magmas were probably derived from a reaction between subducted marine sedimentary siliceous melts and peridotites from a mantle wedge.The linear distribution of Late Triassic HMAs and their eastward-younging trend (from northern Qiangtang to southern SGC), in combination with the contrasting distribution of volcanic rocks in the northern Qiangtang terrane, lead us to suggest that a slab window opened in the downgoing oceanic crust with two diverse slab angles during oblique subduction. Accordingly, the final closure of the Paleotethys Ocean was later than 205 Ma, and most likely in the end of the Late Triassic to Early Jurassic.

Nd-O-Hf isotopic decoupling in S-type granites: Implications for ridge subduction

The generation of S-type granites in the continental crust generally requires partial melting of metasedimentary rocks at relatively high temperatures, and is typically associated with syn- or post-collisional tectonics. These melts are generally expected to inherit the geochemical characteristics of their metasedimentary protoliths, and their Nd, Hf, and O isotopic compositions have been widely used to trace the origin of the magmas. Here we show that these isotopic systems are decoupled in S-type granites from the Chinese Altai, displaying a wide range of zircon ɛHf(t) values (−0.7 to +6.2), and a limited range of whole rock ɛNd(t) values (−6.1 to −2.2) and zircon δ18O values (+8.49‰ to +12.91‰). Zircon xenocrysts are common in these plutons, and show markedly negative ɛHf(t) values (from −30.7 to −18) and highly positive δ18O values (from +9.61‰ to +13.03‰). In addition, these granites exhibit high Lu/Hf (0.15 to 1.38) ratios, and substantial depletions in Zr and Hf. We develop a new model to explain the observed relationship between residual zircon content and Nd–Hf–O isotope decoupling. We propose that these S-type granites formed from partial melting of metasedimentary crustal rocks, associated with a ridge subduction-related heat source. Limited crustal heating, due to mantle upwelling through the slab window on a small scale, may have resulted in significant disequilibrium melting of the metasedimentary crust. Our findings provide new insights into melting processes in the deep continental crust.

Petrogenesis and geological significance of charnockite in the Yinshan Block of North China Craton

ABSTRACTA comparative study was performed on the petrography, geochemistry and geochronology of charnockite and granulite in the Xiwulanbulang (XWLBL) area, northern margin of the North China Craton, NW China. Inclusions within garnet in the charnockite are used to identify the mineral assemblage in the granulite during peak metamorphism. The formation of charnockite is attributed to the anatectic of the protolith, as a result of granulite-facies metamorphism during the same tectono-thermal event. Anatexis occurred mainly during the post-peak isothermal depressurization stage (granulite and charnockite yield peak P–T conditions of 800–850°C and 1.0–1.2 GPa, and 750°C and 0.9–1.0 GPa, respectively), as inferred from the metamorphic evolution and P–T conditions derived from analyses of metamorphic minerals by electron microprobe (EMP). The garnets in the charnockite were a residual or peritectic mineral facies during anatexis, and the charnockite was the product of crystallization from melt with abundant residual minerals. Charnockite has similar geochemical characteristics to felsic granulite, although it differs in having 1) an uneven distribution of major and trace elements; 2) strong depletions in the large-ion lithophile element Cs, the heat-producing elements U and Th, and the high-field-strength elements Nb, Ta, P and Ti; and 3) both Eu-enriched and Eu-depleted patterns that are characteristic of granite formed by largely in situ anatectic. The geochemical data indicate that the XWLBL charnockite formed in a subduction-related volcanic arc setting. On the basis of our results, combined with geological data on the Neoarchean structural evolution of the Yinshan Block and εHf(t) values of 1.60–7.81, we propose that the anatectic of charnockite was related to slab break-off during mid-ocean-ridge subduction, which resulted in the ascent of mantle magma through the slab window.

Double difference relocation of earthquakes in Sunda-Banda transition zone, Indonesia

Accurate earthquake hypocenter location is very important for seismicity analysis. In this study we used 593 earthquakes information from International Seismological Center (ISC) catalogue from January 2009 to December 2016 for hypocenter relocation in Sunda-Banda transition zone. We applied double difference method and one dimensional AK135 for the velocity model. The result shows that position and orientation of earthquakes are changed significantly. Distribution of earthquakes are narrower in the subducting slab, which is associated to the geological setting in this region. A prominent feature found in this study is a lack of seismicity between 240 and 400 km depth that could be related to stress transition from extensional to compressional regime. Another feature is aseismic zone within 100 -120 km depth around 120°E in the subducting slab which suggest a presence of a slab window.

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

Based on tectonic reconstructions, Whittaker et al. (2007) proposed that a slab window formed beneath Sundaland between 70 and 43 Ma, due to subduction of the Wharton Basin spreading ridge between India and Australia. They suggest that extension in the Java Sea region at this time was exacerbated as a result of upwelling asthenosphere associated with the slab window. Active ridge subduction and subsequent slab window formation can severely affect basin formation, heatflow and petroleum systems development on the overriding margin. A slab window forms between diverging plates when a mid-ocean ridge is subducted, leading to anomalous thermal effects like increased mantle wedge temperatures and thermal gradients in the overlying crust. Whittaker et al.?s (2007) kinematic reconstructions rely on restoring now-subducted lithosphere based on preserved ocean crust, but the inherent uncertainties in this process call for an independent evaluation of this model. Mantle seismic tomography models provide qualitative boundary conditions for modelled tectonic histories. We compare seismic tomography models with the model of Whittaker et al. (2007) at a range of mantle depths to confirm the existence of a slab window, and obtain bounds for its maximum regional extent. We identify a break in the high-velocity, down-going Indian-Australian slab at depths between 950-1350 km and longitudes between 85° and 110°, supporting the presence of a slab window. However, we find that the window is located approximately 5° further north and 10°-15° further west than previously proposed, implying that the Wharton Ridge was subducted farther west than previously suggested.

Effects of slab-window, alkaline volcanism, and glaciation on thermochronometer cooling histories, Patagonian Andes

Southern Patagonia is a prime example of ongoing oceanic ridge collision and slab-window formation sustained over several million years. The impact of these phenomena on the thermal structure and exhumation of the crust have been mainly assessed with low-temperature thermochronology of bedrock samples. Here, we infer thermal histories from new and existing thermochronological data from the region of most recent ridge collision. In particular, we evaluate the potential far-reaching thermal effects of the evolving slab window, which have previously been considered responsible for patterns of late Miocene reheating associated with back-arc alkaline volcanism. Our model results define protracted cooling since ∼15 Ma and stepwise exhumation since the late Miocene. The pattern of stepwise exhumation closely matches the onset of Patagonian glaciation at 7 Ma and the successive pulse of glacial incision coeval with neotectonic activity since 3–4 Ma that are also documented by independent geological and geomorphological evidence in the region. Importantly, our findings challenge the recently suggested lack of glacial erosion and incision since 5 Ma in this region. Furthermore, in contrast to previous modelling studies, we find that the available data do not evidence a previously proposed northward-propagating heating event associated with alkaline volcanism. We hypothesize that the anomalous alkaline volcanism in the Patagonian back-arc might be related to trench-orthogonal tears aligned with transform faults in the subducting plate. The substantial differences from the previous modelling procedure on some of the same samples is demonstrated to result from an important lack of convergence in model runs.

Detrital zircon double-dating of forearc basin strata reveals magmatic, exhumational, and thermal history of sediment source areas

Research Article| January 30, 2019 Detrital zircon double-dating of forearc basin strata reveals magmatic, exhumational, and thermal history of sediment source areas Eva Enkelmann; Eva Enkelmann † 1University of Calgary, Department of Geoscience, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada †eva.enkelmann@ucalgary.ca Search for other works by this author on: GSW Google Scholar Sonia K. Sanchez Lohff; Sonia K. Sanchez Lohff 2University of Cincinnati, Department of Geology, 500 Geology Physics Building, Cincinnati, Ohio 45221-0013, USA Search for other works by this author on: GSW Google Scholar Emily S. Finzel Emily S. Finzel 3University of Iowa, Department of Earth and Environmental Science, 115 Trowbridge Hall, Iowa City, Iowa 52242, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Eva Enkelmann † 1University of Calgary, Department of Geoscience, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada Sonia K. Sanchez Lohff 2University of Cincinnati, Department of Geology, 500 Geology Physics Building, Cincinnati, Ohio 45221-0013, USA Emily S. Finzel 3University of Iowa, Department of Earth and Environmental Science, 115 Trowbridge Hall, Iowa City, Iowa 52242, USA †eva.enkelmann@ucalgary.ca Publisher: Geological Society of America Received: 22 May 2018 Revision Received: 04 Oct 2018 Accepted: 21 Nov 2018 First Online: 31 Jan 2019 Online Issn: 1943-2674 Print Issn: 0016-7606 © 2019 Geological Society of America GSA Bulletin (2019) 131 (7-8): 1364–1384. https://doi.org/10.1130/B35043.1 Article history Received: 22 May 2018 Revision Received: 04 Oct 2018 Accepted: 21 Nov 2018 First Online: 31 Jan 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Eva Enkelmann, Sonia K. Sanchez Lohff, Emily S. Finzel; Detrital zircon double-dating of forearc basin strata reveals magmatic, exhumational, and thermal history of sediment source areas. GSA Bulletin 2019;; 131 (7-8): 1364–1384. doi: https://doi.org/10.1130/B35043.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Subduction along the southern margin of Alaska, USA, has been ongoing since at least the Jurassic. However, the character of the subducting slab has changed through time and has included subduction of normal oceanic crust, a spreading ridge, and an oceanic plateau. The latter two resulted in a change of subduction mode by inducing a shallow subduction angle. Geologic processes and landscape evolution of the overriding plate were affected by these variations in subduction mode and are recorded in the forearc basin strata. We investigate the Cenozoic to modern sediment of the forearc Cook Inlet basin in south-central Alaska. Here we use a double-dating approach that combines fission track dating and U-Pb dating on individual detrital zircon grains. In total we analyzed more than 1700 zircons from Eocene to Pliocene strata and modern river sand that has eroded from the surrounding regions of the Cook Inlet basin. The double-dating approach combined with the existing knowledge of the regional geology allows us to discriminate between magmatic cooled grains of extrusive and shallow intrusive rocks, exhumational cooled grains, and thermal reset grains. We find that the erosion of both shallow and deep intrusive arc rocks dominate the detrital age signal, while syn-depositional extrusive grains are lacking. The erosion of rocks that have been thermally altered during the subduction of a spreading ridge dominates the fission track (FT) age signal. This pattern is particularly prominent in the accretionary prism where ages in the most inboard (older) portion have not been thermally reset, but thermal resetting is prevalent in the outboard (younger) portion located proximal to the Paleocene–Eocene near-trench intrusions. Thermal alteration is also evident in the region of the arc that was affected by the passage of the asthenospheric slab window. The erosional signal of the more inboard arc and backarc region (Alaska Range) is characterized by exhumational FT ages of deep-seated rocks, which currently provide material into the forearc basin. This age signal results from flat-slab subduction of the Yakutat microplate, which transfers stress far inboard and produces significant mountain building and deformation. This exhumational age signal, however, is not recorded in the late Cenozoic strata, suggesting that the modern landscape developed since <3 Ma. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Shoshonitic magmatism in the Paleoproterozoic of the south-western Siberian Craton: An analogue of the modern post-collision setting

The Siberian Craton was assembled in a Paleoproterozoic episode at about 1.88 Ga by the collision of older blocks, followed at about 1.86 Ga by post-collisional felsic magmatism. We have found a set of extremely fresh mica-bearing lamprophyre-looking rocks within the Sharyzhalgay metamorphic complex of the south-western Siberian Craton. Zircon from these rocks yields a UPb TIMS age of 1864.7 ± 1.8 Ma, which coincides perfectly with the peak of the post-collisional granite ages and postdates by ~15 Ma the peak of ages obtained for metamorphism. The same ages were reported earlier for a mafic dyke with ocean island basalt (OIB) geochemical signatures and a Pt-bearing mafic-ultramafic intrusion found in the same region. Mineralogy, major and trace element geochemistry and Sr-Nd-Pb isotopes show that the studied rocks (1) have shoshonitic affinity, (2) are hybrid rocks with mineral assemblages which could not be in equilibrium, (3) where derived by recycling of an Archean crustal source and (4) resemble post-collision Tibetan shoshonitic series. The genesis of these rocks is considered to be due to melting of crustal lithologies and metasomatized lithospheric mantle within a subducted slab. Some of the resulting melts ascended through the lithospheric column and fractionated to low-Mg absarokites, whereas other melts were contaminated by orthopyroxenitic mantle material and attained unusual high-Mg mafic compositions. According to our model, the post-collisional magmatism (shoshonite- and OIB-type) occurred due to upwelling of hot asthenosphere through a slab window, when the active collision ceased as a result of the slab break off and loss of the slab pull force. Overall, our study shows that in the Paleoproterozoic shoshonitic melts were emplaced within a similar tectonic setting as seen today in modern orogenic systems.

An Early Cretaceous slab window beneath central Tibet, SW China: Evidence from OIB‐like alkaline gabbros in the Duolong area

AbstractThe geodynamic mechanism of the late Early Cretaceous magmatic flare‐up in the collisional zone between the Lhasa and Qiangtang terranes in Tibet is controversial because of a scarcity of robust evidence. To address this problem, we report geochronological, geochemical and Hf isotopic data for the newly discovered Gufeng gabbros from the Duolong Cu–Au mineral district of the western Bangong–Nujiang Suture Zone (BNSZ). The gabbro samples, dated at 126.3 ± 1.8 Ma, show geochemical similarities to typical ocean island basalt (OIB) and have positive εHf(t) values of +3.3 to +6.9. The gabbros were generated by decompression melting of deep upwelling asthenosphere. This event is best explained by slab break‐off and the resultant development of a slab window beneath central Tibet.

Abyssal Peridotite as a Component of Forearc Mantle: Inference from a New Mantle Xenolith Suite of Bankawa in the Southwest Japan Arc

Lithology and petrologic nature of the forearc mantle have been left unclear due to the very limited sampling to date. Here, we present petrological data on a forearc peridotite suite obtained as xenoliths in an alkali basalt dike (7.5 Ma) from the Bankawa area in the Southwest Japan arc for our better understanding of the forearc mantle. The host alkali basalt is of asthenosphere origin, and passed through a slab window with slight chemical modification by the slab-derived component. The Bankawa peridotite suite is comprised of lherzolites, which contain various amounts of secondary phlogopite and were metasomatized to various degrees. The least metasomatized lherzolite exhibits Fo91 of olivine, Cr/(Cr + Al) = 0.3 of chromian spinel, and depletion of middle to light rare-earth elements in clinopyroxene, and is overall similar to an abyssal lherzolite. It had originally formed at the proto-Pacific Ocean and then was trapped at a eastern margin of Eurasian continent by initiation of subduction. The forearc mantle peridotite formed as a residue of proto-arc magma formation is depleted harzburgite as represented by the peridotites obtained from the forearc seafloor, but can be less depleted abyssal peridotite if being devoid of partial melting or reaction with magmas after entrapment.

Phosphorus and Potassium Metasomatic Enrichment in the Mantle Source of thec. 1450–1425 Ma Michael–Shabogamo Gabbro of Eastern Laurentia

A c. 1450 Ma, east-west-oriented suite of Proterozoic mafic intrusions in eastern Canada, located between Labrador City and coastal Labrador near Rigolet, has been divided into two groups: the Shabogamo Gabbro, which is exposed west of Churchill Falls, and the Michael Gabbro, which crops out east of Churchill Falls. Both suites of gabbro are within-plate tholeiites that range from sub-alkaline basalt to basaltic andesite in composition and are enriched in P2O5 and K2O. Collectively, the two suites are better interpreted as a single magmatic event, termed the Michael- Shabogamo Gabbro (MSG), with ages ranging from 1425 to 1450 Ma, based on new U-Pb isotope dilution thermal ionization mass spectrometry baddeleyite ages of 144963, 143764 and 1435610 Ma, along with previously published ages. Within the MSG, two geochemical groups are evident. Most notably, each is characterized by differing P2O5/Zr (P/Zr) ratios: (1) a high P/Zr group, c. 1425-1437 Ma; (2) a low P/Zr group, c. 1435-1450 Ma. The positive slope of P2O5 versus Zr data arrays for each group indicates that melting of apatite was pivotal in influencing the rare earth element concentrations of the MSG magmas and their secular changes in geochemistry. K2O is enriched in the MSG relative to enriched mid-ocean ridge basalts and is correlated with P2O5, Ba and Rb. Multiple origins for the enrichment of P2O5 and K2O in the MSG are considered and the available evidence is most consistent with melting of a metasomatized subcontinental lithospheric mantle source interpreted to originate by infiltration of partial melts of phosphate-rich sediments. A slab window is proposed to have facilitated lithospheric melting at c. 1425-1450 Ma. (Less)