Asthenospheric Upwelling Research Articles

Crustal and upper mantle structure beneath SE China from joint analysis of receiver functions and Rayleigh-wave dispersion

SUMMARY Various geodynamic models, such as a mantle plume, lithospheric delamination and the subduction of palaeo-Pacific Plate, have been proposed to account for the Mesozoic magmatic activity and related mineralization in SE China. However, previous seismic velocity models are insufficient in resolution to constrain the accompanying crust–mantle interactions, which limits our understanding of the deep geodynamic processes responsible for the Mesozoic magmatism in SE China. In this study, we construct a new 3-D shear wave velocity model of the crust and upper mantle beneath SE China using joint inversion of receiver functions and Rayleigh-wave dispersion. We also determine the crustal thicknesses and Vp/Vs ratios by H–k stacking of receiver functions. Compared to the western Cathaysia Block, the coastal volcanic-intrusive belt (CVIB) southeast of the Zhenghe-Dapu Fault shows relatively high-velocity anomalies in the lower crust and high Vp/Vs ratios, indicating significant underplating of mafic material beneath the CVIB. Furthermore, it is noticeable that the asthenospheric low-velocity anomalies are stronger beneath the CVIB than beneath the interior of SE China, suggesting a southeastward increase in the intensity of asthenospheric upwelling and partial melting, which was likely responsible for the strong volcanism and the inferred mafic underplating beneath the CVIB. Our observations provide new evidence and more details for the northwestward subduction and subsequent rollback of the palaeo-Pacific Plate causing the Late Mesozoic magmatism in SE China.

Origin of carbonate melts in orogenic belts by anatexis of downthrust carbonate sediments

Carbonate melts produced by partial melting of the carbonated mantle or by liquid immiscibility from mantle-derived silicate melts provide insights into the deep carbon cycle. Sedimentary geochemical signatures in some of the carbonate melts have been attributed to subducted material within their mantle sources. However, subducted carbonates at slab tops may not melt but be transported to the deep mantle, where sedimentary signatures would be diluted by mantle rocks. Here, we report the intruded Cenozoic carbonate melt dykes from the Yanshan orogen – an intracontinental orogenic belt far from the subduction zone – which show strong limestone-like geochemical features. Xenocrysts from the deep crustal basement, together with Sr-Nd-C-O isotope variations, indicate assimilation of the deep continental crust accompanied by CO2 degassing. We suggested they were formed by melting of downthrust limestone strata in the Yanshan orogen, triggered by underplating with basaltic melts during the Cenozoic upwelling of asthenosphere below the area. A compilation of global Phanerozoic carbonate magmas shows that this model can apply to many carbonate melts in orogenic belts which show much more evolved Sr-Nd isotopes and heavier C-O isotopes than those in intraplate settings. Trace elements of these carbonate melts indicate shallow derivation with a negligible contribution of mantle rocks. Spatial-temporal distribution of the orogenic carbonate melts supports them being related to continental crustal collision rather than oceanic subduction. Our findings highlight the carbonate sediments deposited in the continental crust as an important source that may deeply downthrust in the orogenic belt and melt during the post-collision stage.

A top-down control on upper crustal inheritance on the south-eastern Tibetan Plateau

Mantle tectonics, such as asthenospheric upwelling and lithospheric decoupling, usually controls subsequent fluid formation and migration from deep to shallow levels in the lower lithosphere and promotes rock failure and deformation. Based on the concept whereby the fluid migration location defines the structural inheritance, we aim to relate crustal processes to mantle tectonics using fluids revealed by a new magnetotelluric (MT) array across the Ailao Shan-Red River belt. As imaged via a 3D resistivity model, two lower-lithospheric conductive anomalies are determined as consistent with the Dian-Qiong (DQ) suture and Song Da (SD) belt and interpreted to contain interconnected melt. Because potassic magmatism was enabled by partial melting in the lower lithosphere, we could infer that the DQ and SD have reworked as the major fluid migration channels. Fluid migration is considered to drive lithospheric decoupling at a low-viscosity conductive layer, which is inferred feeding by aqueous and melt fluids originating from the two channels and diffusing at depths from 15 to 20 km. Constrained by the geochronology results, this weak layer could have provided convenience to induce the entirely upper crustal translation-rotation within the time interval between potassic magmatism and strike-slip of the Red River fault. This translation-rotation process is inherited from the underlying mantle processes and may further be remotely affected by the upper crustal movement of the Tibetan Plateau, conforming with the crustal rotation observed to the north of our study region. This work provides a compelling example of the tectonic control of the mantle on inherited responses in the crust.Plain Language Summary Crustal deformation is usually controlled by mantle tectonic processes, especially the southeastern Tibetan Plateau. Some models have proposed that lithospheric shearing and lower-crustal flow have controlled the crustal deformation in the Cenozoic. However, crustal rotation models show less possibility of a channel flow and relate the crustal deformation to the remote effect of India-Asia collision. Our study investigates above hypotheses by imaging the electrical resistivity of rocks beneath the major Red River Fault area in southeastern Tibet, using magnetotelluric data with high quality in a dense array. The lithosphere of the study area is interpreted to have been divided into two horizontal systems by a fluid diffusion layer at the bottom of upper crust. Resistivity low in the lower lithosphere is inferred due to partial melting of deep lithosphere and reworking of the paleo sutures in the Late Cenozoic, which have spread at the bottom of upper crust. Rather than the channel flow, this fluid migration process sensitively reflected in our model relates the mantle tectonics to crustal rotation by providing rheological conditions. We, hence, propose an inherited structure model featured by lithospheric decoupling and upper-crustal translation-rotation that may occur before the major strike-slip event.

Paleocene A-type igneous suites in the Sikhote-Alin (the East Asian continental margin): Petrological, geochronological, isotopic, and geodynamic constraints

Global geodynamic reconstruction models of the Pacific margin of Asia refer to materials collected throughout Japan, Korea and northeastern China, but they lack data on southern Sikhote-Alin. Therewith, the Sikhote-Alin orogenic belt (NE Asia) constituted a single eastern margin of the paleo-Asian continent with the abovementioned territories in the Paleocene. New isotopic, geochemical, and geochronological data show that Paleocene igneous activity (∼61–55 Ma) is widely developed in southern Sikhote-Alin. Bulk rock compositions of the igneous rocks of this period yield ferroan, peraluminous, calc-alkaline to alkaline affinities, highly abundant LILE and HFSE (with pronounced Ba, Sr, Eu, and Ti negative anomalies) and depleted HREE. The initial melts, displaying Zr + Y + Ce + Nb > 350 ppm and 10,000 × Ga/Al > 2.6, derived from an OIB-like mantle source crystallised under fairly reducing conditions (below FMQ buffer), and yield high temperature of zircon saturation (>850 °C), indicating typical A-type granite and related volcanic rock features. It is logical to associate variations in A-type rock geochemical composition with an enrichment of the upper part of the magma chamber with fluid-mobile components involving a redistribution of major and trace elements through fluid-magmatic differentiation. Strong depletion in Ca, Mg, Ba, Sr, Eu in the A-type rocks is caused by an outflow of these elements by an oxidizing, initially reduced, acidic fluid beyond the zone of magma generation. Whole-rock Sr-Nd isotope data argue for the generation of the A-type rocks by melting of dominantly a juvenile mantle component with a subordinate component of the ancient crust. Isotope variations of igneous rocks of the reference area: 87Sr/86Sr(t) (0.7024–0.7118), εNd(t) (−0.9 to −5.1) and TDM2 (934–1277 Ma), result from the mixing of the OIB-like mantle source with selective melts or from the metapelite contamination of the Samarka terrane accretionary prism and of the Zhuravlevka-Amur turbidite basin, later followed by fluid-magmatic differentiation that led to the formation of anatectic or hybrid melts. We further suggest that the origin of the A-type granites and related volcanic rocks is the result of the oblique interaction of oceanic and continental plates. This interaction accounts for the simultaneous formation of tears in the slab, enabling sub-slab asthenospheric upwelling, and strike-slip fault-related extensional structures in the overriding continental plate.

Progressively elevated geothermal gradients for Miocene metamorphism in the Himalayan orogen

The property of regional metamorphism at convergent plate margins is substantially related to the change of geothermal gradients, resulting in different types of crustal reworking through metamorphic dehydration and partial melting. The relationship between crustal metamorphism and thermal evolution of collisional orogens can thus be deciphered by investigating the change of metamorphic thermobaric ratios in both space and time. This is illustrated by a combined study of field observation, petrographic observation, whole-rock and mineral major and trace elements, laser ablation−inductively coupled plasma−mass spectrometry zircon and monazite U-Th-Pb isotopes and trace elements, and phase equilibrium modeling of granitic gneisses from the Cona area, southern Tibet, in the Himalayan orogen. The results reveal variations in spatial level, protolith age, petrological composition, and metamorphic pressure−temperature−time path along an N-S transect across the Higher Himalayas. On this basis, three types of granitic gneisses were identified for the Miocene metamorphism. Type I gneiss was formed at 16.2−14.7 Ma under upper amphibolite facies conditions of 720−735 °C and 0.77−0.82 GPa, Type II gneisses were formed at 18.3−14.3 Ma under granulite facies conditions of 765−795 °C and 0.99−1.06 GPa, and Type III gneisses were formed at 21.8−13.0 Ma under high-pressure granulite facies conditions of 850−875 °C and 1.40−1.45 GPa. These rocks underwent anatectic metamorphism at lower to upper crustal depths from 22 to 13 Ma, and their metamorphic thermobaric ratios increase from 586−803 °C/GPa at 22−18 Ma to 878−967 °C/GPa and finally to 1071 °C/GPa at 15−13 Ma. This increase corresponds to a progressive increase in metamorphic geothermal gradients from 16.1−22.0 °C/km to 24.1−26.5 °C/km and finally to 29.4 °C/km. Therefore, the crustal rocks in the Himalayan orogen experienced the three types of regional metamorphism at the three different geothermal gradients in the Miocene. The progressively elevated geothermal gradients are attributed to continental rifting due to asthenospheric upwelling consequential to thinning of the lithospheric mantle in the collisional orogen. This has important implications for the tectonic evolution of collisional orogens in the post-collisional stage.

Insights on the Interplay of Rifting, Transcrustal Magmatism and Formation of Geothermal Resources in the Central Segment of the Ethiopian Rift Revealed by 3‐D Magnetotelluric Imaging

AbstractThe Main Ethiopian Rift is accompanied by extensive volcanism and the formation of geothermal systems, both having a direct impact on the lives of millions of inhabitants. Although previous studies in the region found evidence that asthenospheric upwelling and associated decompression melting provide melt to magmatic systems that feed the tectono‐magmatic segments in the rift valley, there is a lack of geophysical models imaging these regional and local scale transcrustal structures. To address this challenge, we use the magnetotelluric method and image subsurface electrical conductivity to examine the magmatic roots of Aluto volcano, quantify and interpret the melt distribution in the crust considering established concepts of continental rifting processes and constrain the formed geothermal system. Specifically, we combined regional (maximum 30 × 120 km2) and local (15 × 15 km2) magnetotelluric data sets and obtained the first multi‐scale 3‐D electrical conductivity model of a segment of the central Main Ethiopian Rift. The model unravels a magma ponding zone with up to 7 vol. % melt at the base of the crust (30–35 km b.s.l.) in the western part of the rift and its connection to Aluto volcano via a fault‐aligned transcrustal magma system. Melt accumulates at shallow crustal depths (≥4 km b.s.l.), thereby providing heat for Aluto's geothermal system. Our model suggests that different volcano‐tectonic lineaments in the rift valley share a common melt source. The presented model provides new constraints on the melt distribution below a segment of the rift which is important for future geothermal developments and volcanic hazard assessments in the region.

Fertile mantle domains with different metasomatic features beneath the eastern North China Craton: Transformation from variably auto-metasomatized asthenospheric upwelling

The eastern North China Craton is known for replacement of refractory lithospheric mantle by fertile one relevant to the (Paleo-) Pacific subduction. However, the formation mechanism of the fertile mantle domain remains enigmatic, thus hindering profound understanding of the associated lithosphere-asthenosphere interaction. To explore the deep dynamic processes, we present petrology, in-situ element and Sr isotopic data on spinel peridotite xenoliths entrained by the Cenozoic basalt in the Qingyuan county, Liaoning Provence.The Qingyuan mantle xenoliths can be categorized into two groups based on textural characteristics and mineral compositions. Group A peridotites consist of lherzolites containing patch of fine-grained aggregates and have low olivine Fo (100×Mg/(Mg + Fe) ≤ 89, atomic ratios) and mineral major-element composition trends deviating from residual mantle. The coarse-grained clinopyroxenes outside the patch and the patch-hosted fine-grained clinopyroxenes and amphiboles display similar light rare earth element (LREE)-enriched patterns and comparable 87Sr/86Sr ratios (0.7035–0.7039, 0.7040–0.7062 and 0.7029–0.7033, respectively). These characteristics suggest that Group A peridotites may represent fragments of lithospheric mantle strongly modified by melt impregnation. In contrast, Group B xenoliths, typically lherzolites, have protogranular texture without the patches of fine-grained aggregates. The olivines have moderate Fo (ca. 90–91) and mineral major-element compositions highly resembling melting residua. The melting is estimated to occur at about 1370–1590 °C and 2–4 GPa, corresponding to mantle potential temperatures of around 1450–1550 °C. The clinopyroxenes show variably enriched REE patterns and depleted 87Sr/86Sr ratios (0.7020–0.7044). These peridotites are thus interpreted as fragments of residual lithospheric mantle transformed from small-scale asthenospheric upwelling. Diffusion modeling of trace elements in clinopyroxenes suggests ubiquitous Mesozoic-Cenozoic metasomatism in the studied peridotites.Combining with other Cenozoic basalt-borne peridotite xenoliths from the eastern NCC and associated tectonic setting, we conclude that the fertile lithospheric mantle domains with different metasomatic features may be transformed from variably auto-metasomatized asthenospheric upwelling. That is, the subduction-related small-scale upwelling not only leaves behind residual domains with weak metasomatism but also produces mantle regions strongly infiltrated by the decompressional melts. This model links the mantle domains of different formation mechanisms to coherent geodynamic processes and emphasizes the complex mantle processes associated with the fluctuated lithosphere-asthenosphere boundary.

Isotropic and radially anisotropic S-velocity structure beneath the Arabian plate inferred from surface wave tomography

We estimated Rayleigh- and Love-wave group-velocity dispersion curves using dense seismic networks in Saudi Arabia and nearby countries. We merged our data with group- and phase-velocity dispersion curves from previous studies to produce a new isotropic and radially anisotropic S-velocity model for the Arabian plate and nearby regions. Strong negative radial anisotropy (VSH < VSV) is observed in the crustal depth beneath the southern Red Sea, but weak positive radial anisotropy (VSH > VSV) and relatively thick lithosphere are found beneath the northern Red Sea, implying different tectonics between the two regions. We found low-velocity anomalies beneath Cenozoic volcanism in the Arabian Peninsula, but corresponding radial anisotropic perturbations are observed differently in spatiotemporal regimes depending on source origin. From the strong positive radial anisotropy at asthenospheric depth, which correlates with a circular pattern of azimuthal anisotropy from shear-wave splitting, we infer toroidal mantle flow around regions of thick lithosphere including an eastern part of the Arabian platform, the Mesopotamian Plain, and the Zagros belt. On the contrary, weak radial anisotropy is found beneath the thin lithosphere of the Arabian shield and in the thick lithosphere of the Mesopotamian, which may imply vertical flow due to asthenospheric upwelling and cratonic growth, respectively.

Andean evolution, orogenic deformation and uplift of the Western Cordillera and Altiplano of southern Peru, northern Bolivia and Chile: Eocene-Oligocene lithospheric delamination

Reappraisal of Andean evolution in southern Peru, northern Bolivia and northern Chile provides support for models of orogenic cyclicity in which Andean crustal shortening in the Western Cordillera led to Eocene-Oligocene lithospheric delamination which triggered a renewed pulse of Andean uplift. The sedimentary fill of the Arequipa "rift" back-arc basin (Rhaetian-Neocomian) was controlled by two major graben-bounding fault systems: the Condoroma-Caylloma and Cincha-Lluta-Incapuquio; as well as the Cusco-Lagunillas fault system separating the back-arc region from the Cusco-Puno high. These thinned cortical inherited structures, were the most favourable places to undergo a positive tectonic inversion at the beginning of Andean compression (Albian), and then during Late Cretaceous-Paleocene shortening, they continued to develop as east-vergent crustal decollements which allowed crustal doubling and Western Cordillera thickening. The crustal thickening of >45 km took place along the Condoroma-Caylloma inverted graben where the lower crust, which was probably enriched in basalts, was subjected to high-pressure metamorphism leading to eclogitization, and became denser than the underlying mantle lithosphere and therefore delamination occurred. The subsequent asthenospheric upwelling led to lithospheric thinning, asthenospheric mantle magmatism of the first stage of Andahuaylas-Yauri batholith emplacement (∼48-43 Ma), which crystallized under temperatures of 1200–700 °C and pressures of 1.0–0.5 GPa (>40-30 km deep). This process was followed by the rapid surface uplift of the Western Cordillera that reached 3.1 km and fed the Kayra and Lower Moquegua basins, located on the Altiplano and in the forearc respectively. The delamination process was still active during 43–30 Ma, with second-stage magmatism of the Andahuaylas-Yauri batholith, emplaced continuously from deep mafic intrusions (40-30 km) to superficial porphyries (3 km). This indicates an important uplift of the Western Cordillera to 3.3 km, supported by mantle upwelling and the continuous flat-slab compression that prevented normal faults from developing, and rather produced the shortening of the delaminated crust. The Anta-Soncco foreland basins (43-30 Ma) of the Altiplano and Moquegua basin of the forearc, received the products of Cordilleran denudation at a rate of up to 700 m/My. The alkaline potassic magmatism (29–27 Ma) could represent episodes of post-delamination magmatism and the late stage of flat subduction.

A Devonian Shoshonitic Appinite–Granite Suite in the North Qinling Orogenic Belt: Implications for Partial Melting of a Water-Fluxed Lithospheric Mantle in an Extensional Setting

AbstractAppinite–granite suites commonly occur in the final stage of collisional orogenic processes, providing a unique opportunity to reveal the properties of continental lithospheric mantle and crust–mantle geodynamics. In this paper, we present a systematic study of the petrology, mineral chemistry, whole-rock geochemistry and geochronology of the Xiong’erling pluton and adjacent appinite dikes in the northern margin of the North Qinling orogenic belt. The pluton is mainly composed of diorites, quartz monzonites and minor granites. The diorites and appinites have LA-ICP-MS zircon U–Pb ages of c. 389 Ma with variable εHf(t) values of −5.58 to +3.36 and TDM1 model ages peaking at c. 1133 Ma. These rocks belong to the shoshonitic series with high Ba–Sr content and were emplaced in an intraplate extensional environment. The quartz monzonites and granites are oxidized A-type granites and have zircon U–Pb ages of c. 396 and c. 378 Ma with negative εHf(t) values of −11.99 to −1.18 and TDM2 model age peaks at 1434, 1720, and 1888 Ma. The in situ87Sr/86Sr ratios of plagioclase from the mafic and felsic rocks are 0.70565-0.70752 and 0.70695-0.71008, respectively. The Xiong’erling pluton and the coeval appinite dikes represent a Devonian appinite–granite suite in the North Qinling orogenic belt. Asthenospheric upwelling during the intraplate extension triggered partial melting of a phlogopite- and amphibole-bearing garnet lherzolite mantle source that had been previously metasomatized by the subducted oceanic slab, leading to the formation of a primitive hydrous mafic magma. The rapid ascent of the water-rich magma along deep-seated active faults with fast crystallization of amphibole resulted in emplacement of the c. 389 Ma appinite dikes. Replacement of anhydrous minerals by amphibole and biotite via hydrous reaction during magma cooling resulted in the formation of the coeval diorites. The mafic magma underplating triggered episodic remelting of a late Paleoproterozoic to early Mesoproterozoic crustal source, leading to the generation of the c. 396–378 Ma quartz monzonites and granites. Combining our results with existing data, we identify a sequence of (1) northward subduction of the Shangdan oceanic crust beneath the Qinling block at c. 524–438 Ma resulting in island-arc calc-alkaline magmatism, (2) closure of the Shangdan Ocean indicated by collision between the Qinling block and the South Qinling terrane and slab failure magmatism at c. 438–410 Ma, and (3) post-collisional to intraplate extension with alkaline magmatism at c. 410–370 Ma. The Devonian extensive intraplate magmatism marks the end of the Paleozoic orogenesis in the North Qinling belt.

Crustal anisotropy of the South China block and tectonic implications

Exploiting the teleseismic waveform data from permanent seismic stations distributed in the South China block, we obtained the crustal anisotropy parameters at 51 stations by fitting the azimuthal variations of arrival times of the PMs converted wave of radial receiver functions. The results show the dominant direction of fast wave polarity in the South China block is NW–SE, which is basically consistent with the absolute plate motion direction. Combining the shear wave splitting and GPS results, we speculated that the tectonic extension of the Paleo-Pacific plate subduction dominates the crust–mantle anisotropy, which reflects the large-scale tectonic stress setting of the South China block. The fast wave directions of NE–SW and E–W calculated by a few stations might represent a local complex tectonic environment, and the crustal anisotropy of NE–SW is well related to the tectonic deformation in Paleozoic. The delay time of slow waves caused by crustal anisotropy ranges from 0.14 s to 0.71 s, with an average of 0.33 s. The delay time of Jiangnan orogenic belt, which located in the southeast of Yangtze block, is particularly large, which reflects the complex and intense intra-continental tectonic movements since Neoproterozoic. The crustal anisotropy along the southeast coast can be contributed to the local lithosphere underplating caused by the upwelling of asthenosphere in Cretaceous.

Petrogenesis of the Qiongduojiang Gabbro in Tethys Himalaya and Its Metallogenetic Implication

With the northward subduction and final closure of the Neo-Tethyan oceanic crust, the Indian and Eurasian plates finally collided together and underwent a strong collision orogenic event, resulting in large-scale crust–mantle magmatic interactions. In order to clarify the controversies about tectono-magmatic activities after the Indian–Eurasian continental collision, we report the newly dated Eocene Qiongduojiang gabbro explored in the Tethyan–Himalaya belt, southern Tibet. LA-ICP-MS zircon U-Pb dating shows that the crystallization age of the Qiongduojiang gabbro is 46.1 ± 1.7 Ma. The whole-rock major and trace elements, as well as Rb-Sr, Sm-Nd, and Pb isotopic data results, show that the Qiongduojiang gabbro is apparently depleted in Nd isotopes, is enriched in Pb isotopes, and has maintained a consistent 87Sr/86Sr(t) value. This paper argues that the E-MORB-like Qiongduojiang gabbro originated from asthenosphere upwelling caused by slab breakoff of the Neo-Tethyan oceanic plate. This event caused large-scale magmatic activities, a magmatic mixing process between ancient crust and deep mantle, and wild distribution of Eocene Gangdese plutons along the Yarlung–Tsangpo Suture Zone, and it rendered the subduction-modified Tibetan lithosphere fertile from the Gangdese porphyry Cu deposits.

Lithosphere tearing and foundering during continental subduction: Insights from Oligocene−Miocene magmatism in southern Tibet

The distribution of Oligo−Miocene magmatic rocks from southern Tibet in space and time yields critical information on the geometry and deformation of the subducted Indian lithosphere which impacts on plateau growth following the India and Eurasia collision. A growing body of geophysical evidence has shown that the subducted Indian lithosphere beneath the Tibetan Plateau has been torn apart. However, the spatiotemporal distribution and cause of the tearing remain enigmatic. Timing of the post-collisional magmatic rocks in southern Tibet exhibits four patterns of decreasing ages; magmatism began earlier in the west and east Himalayan syntaxis and evolved to two age undulations in the central southern Tibet. Seismic images show that regions of slab window (both 90°E and 84°E) and flattened subducted lithosphere (both 86°E and 81°E) are present at depth of 135 km. Correspondingly, increasing mineral crystallization temperatures (absolute value of 50 °C) were recorded in the Oligo−Miocene ultrapotassic-potassic rocks at 90°E and 84°E, while opposing trends were shown by coeval ultrapotassic-potassic rocks at 86°E and 81°E. Besides, the melting depth of the Oligo−Miocene ultrapotassic-potassic primitive melts decreases from nearly 100 km to 70 km between 81°E and 90°E, probably indicating progressive rising of the lithosphere-asthenosphere boundary. Such variations were possibly the results of the focused flow and upwelling of asthenosphere, which advanced rapidly but diachronously through weakened and torn sectors within the overlying Indian slab. The upwellings probably induced diachronously upward bending of the residual Indian slab and its flattening, which accelerated the tearing of the Indian lithosphere during continental subduction.

Petrogenesis of Late Campanian Alkaline Igneous Rocks in Eastern Anatolia: Magmatism related to a subduction transform edge propagator (STEP) fault?

ABSTRACT In eastern Anatolia, the Divriği-Hekimhan Magmatic Province (DHMP) includes ~77–69 Ma alkaline rock units which are located to the NW of the Baskil Arc of ~85–74 Ma. The magmatic rocks are composed of nepheline (Ne)–to quartz (Q)–normative alkaline basaltic to trachytic/syenitic units. Among them, the basaltic rocks are composed of plagioclase (Or2–11Ab32–51An39–64) + clinopyroxene (Wo47–51En35–42Fs9–16) + Fe-Ti oxide ± alkali feldspar (Or57–98Ab2–42An1) ± biotite ± olivine. Their 87Sr/86Sr(I) ratios and ɛNd(I) values vary in the ranges of 0.70591–0.70871 and−3.2–1.6, respectively. The subvolcanic trachytic rocks are composed of perthitic alkali feldspar phenocryst in a matrix of feldspar (Or45–61Ab38–54An0–2), biotite, and Fe-Ti oxides. The trachytic volcanic rocks are made up of feldspar (Or38–63Ab34–59An1–4) in a fine-grained matrix. Their 87Sr/86Sr(I) ratios and ɛNd(I) values vary in the ranges of 0.70532–0.70952 and−3.2–0.7, respectively. The syenitic rocks in the region contain both quartz- and nepheline-sodalite-bearing syenites. Geochemical features reveal that the Ne-normative basaltic magmas have undergone mafic mineral fractionation coupled with crustal contamination to produce the Q-normative derivatives. Enhanced differentiation of the Ne- and Q-normative fractionated magmas via feldspar-dominated fractionation created the silica-undersaturated and -oversaturated trachytic magmas, respectively. During the feldspar-dominated differentiation, the re-melting of accumulated alkali feldspars in the magma chamber likely gave rise to the formation of trachytic rocks with alkali feldspar-like whole rock compositions. The final products of the Ne-normative magmas are represented by the phonolites and foid-syenites with silica-undersaturated eutectic compositions. A geochemical evaluation of the basaltic rocks revealed that the alkaline magmatism mainly originated from a shallow asthenospheric mantle source which had previously been metasomatized by oceanic to continental subduction. We suggest that the DHMP was formed in response to STEP fault-controlled rolling back of the northward subducting slab of the Baskil Arc, which created a localized gap for asthenospheric upwelling.

An appraisal using magnetic data of the continent-to-ocean transition structure west of Iberia

SUMMARYAbout half of the rifted margins purportedly formed by extension with minor magmatism. The conceptual models of those magma-poor systems are greatly influenced by the continent-to-ocean transition structure of the archetypal magma-poor West Iberia Margin. In the past, interpretation of magnetic data of West Iberia has been used to constrain conceptual rifting models, including the structure of the transition from the exhumed mantle domain to the oceanic crust formed at a spreading centre. However, uncertainties on geophysical data were generally not considered leading to overdetailed interpretations. We use synthetic magnetic modelling to show that magnetic data acquired at sea level cannot resolve subhorizontal lithological layering in deep-water continental margins. We then present new magnetic modelling guided by a refined velocity model of the wide-angle seismic IAM-9 profile that shows that the magnetic J-anomaly correlates with oceanic crust that abuts exhumed mantle across a vertical boundary. This well-constrained observation supports that seafloor spreading initiated abruptly, terminating mantle exhumation. Conventional wisdom dictates that the sudden efficient melt extraction relates to a mechanical threshold during lithospheric thinning and concomitant asthenospheric upwelling under which melt can migrate toward the surface. However, our results support that mantle melting creating oceanic crust was probably not driven by gradual lithospheric thinning and asthenospheric upwelling, but by seafloor spreading centre propagation that cut across the lithosphere, creating the abrupt structure.

Mixing of magmas from multiple sources in the petrogenesis of multi‐stage Early Cretaceous syenites in the Wulingshan alkaline complex, northern North China Craton: Evidence from enclaves

There are several different models for the origin of syenites, but the role of magma mixing in the formation of syenites remains unclear. The Wulingshan alkaline complex in the northern North China Craton consists mainly of porphyritic syenite and syenite with abundant enclaves. These enclaves may provide new insights into the petrogenesis of syenites. We obtained zircon U–Pb age, mineral chemistry, whole‐rock major and trace elements, and Sr–Nd isotopic data for the enclaves and their host rocks to constrain the petrogenesis and identify the role of magma mixing during the formation of the different syenites in the Wulingshan alkaline complex. The results of zircon U–Pb dating indicate that the enclaves and host rocks crystallized contemporaneously at ca. 133 Ma. The enclaves contain abundant clinopyroxene, amphibole, and biotite, and their average Nb/Ta (18.46) and Th/Ce (0.04) ratios are similar to those of the mantle. The enclaves have relatively high Fe2O3T, MgO, and CaO contents, and their SiO2contents are equivalent to those of intermediate rocks, indicating that they formed by mixing the mafic and felsic magmas to some extent. The different mineral assemblages, major and trace elements and isotopic compositions of the enclaves in the porphyritic syenite and syenite indicate that they are two batches of parental magma with different properties derived from the mantle. The porphyritic syenite and syenite have high SiO2and low Fe2O3T, MgO, and CaO contents, as well as low V, Cr, Co, and Ni contents. These major and trace element characteristics of the host rocks require the involvement of crustal components. The Sr–Nd isotopic compositions of the enclaves and the host syenitoids plot on a mixing curve between enriched lithospheric mantle and lower crust, indicating that these rocks are the products of magma mixing and crust–mantle interaction. Considering the contact relationships, the geochemistry data, and the mixing model presented in the paper, we propose that the upwelling of the hot asthenosphere heated the overlying enriched lithospheric mantle and triggered low‐degree partial melting. The alkalic mafic magmas derived from enriched mantle sources were mixed with felsic magmas generated by partial melting of the lower (or upper) crust to form the different syenites.

Extremely low δ56Fe in arc tholeiites linked to ferrocarbonate recycling: Implications for Fe enrichment in the Awulale Arc, Central Asia

Recycling of Fe-rich materials through subduction may affect the element budgets and redox properties of the mantle, thus influencing the differentiation trends and mineralization types of mantle-derived magmas. However, the effects of different recycled Fe-rich materials on the mantle are dependent on their lithologies, which are still poorly constrained. Stable Fe isotopes can act as useful tracers for distinguishing among different recycled Fe-rich lithologies, and their imprints may be documented in mantle-derived magmas. This study focuses on the Fe isotopes of ferrobasalts and the associated dacites and magnetite ores in the Chagangnuoer Fe deposit of the Awulale Arc, Central Asia, to identify recycled Fe-rich materials and explore the Fe enrichment mechanism in continental arcs. Our results indicate that the ferrobasalts and dacites possess the lowest known δ56Fe, −0.40‰ ± 0.04‰ (2SE), among their counterparts worldwide. The low δ56Fe signatures are considered to originate from a hybridized mantle source, which may have been modified by recycled ferrocarbonates. The recycled ferrocarbonates may have melted during the decompressional heating stage of the slab subduction of the South Tianshan Ocean, coupled with asthenospheric upwelling under “wet” mantle conditions. The addition of ferrocarbonate melts to the mantle might have decreased the oxygen fugacity of the mantle wedge to below the fayalite−magnetite−quartz buffer, accounting for the Fe enrichment in arc tholeiites and large-scale Fe mineralization along the Awulale. Notably, our study reveals a novel carbonate recycling pathway in the cold subduction zones, where ferrocarbonates were subducted into the mantle and then recycled by the upwelling asthenosphere to mix with the fluid-metasomatized mantle through mantle convection.

Sluggish Slab Rollback at the Early Stage of Flux Melting During Subduction Initiation: Li Isotopic Evidence From the Coto High‐Al Chromite Deposit, Zambales Ophiolite, Philippines

AbstractThe compositions of chromitites and dunites from Moho transition zone (MTZ) of the Coto block of the Zambales ophiolite, Philippines, are used to investigate the geodynamic transition from anhydrous to hydrous magmatism during subduction initiation (SI). Chromite grains in the chromitites have Cr# values [100 × Cr/(Cr + Al)] and TiO2 contents ∼35–50 and 0.05–0.30 wt.%, respectively, intermediate between those of chromite in typical MORB‐like lavas (Cr#, ∼20–60; TiO2, ∼0.6–1.7 wt.%) and boninites (Cr#, ∼70–85; TiO2, &lt;0.4 wt.%). Olivine grains in the dunites have δ7Li values varying from ∼−2‰ to +21‰ with most between +10‰ and +15‰, beyond that of normal mantle (+4 ± 2‰) but comparable to those of some arc lavas (up to +12‰). The data set indicates that parental magmas of the high‐Al chromitites originated from hydrated harzburgitic mantle sources and formed temporally between MORB‐like and boninitic magmatism during SI, resulting from the early stage of flux melting in the Zambales proto‐forearc mantle. Modeling of Li diffusion reveals that the MTZ cooled down at a minimum rate of 0.1°C/yr in order to preserve the large δ7Li variation of olivine in the dunites, comparable to the thermal conditions below ultra‐slow to slow spreading ridges. Such a stage of transitional magmatism, although displaying notable slab contributions, took place at a sluggish period of slab rollback and asthenospheric upwelling, leading to a trough level of heat flow and magma production during the entire course of SI.

High‐Resolution Broadband Lg Attenuation Structure of the Anatolian Crust and Its Implications for Mantle Upwelling and Plateau Uplift

AbstractThe Anatolian Plateau, currently experiencing rapid uplift and westward escape, records both the termination of oceanic subduction and the conversion to continental collision. The crustal response to the transition of the subduction environment from eastern to western Anatolia can be inferred by the seismic velocity and attenuation structures. With this study, we construct a broadband Lg‐wave attenuation model for the Anatolian Plateau and use it to constrain lateral crust heterogeneities linked to this transition. Crustal Lg attenuation links late Cenozoic magmatism with asthenospheric upwelling by characterizing the lithospheric thermal structure. The widely distributed strong attenuation observed in eastern Anatolia may be related to the crustal partial melting due to mantle upwelling after the delamination and subsequent break‐off of the Bitlis slab. Lithospheric dripping in central Anatolia likely facilitates the mantle flows through the window between the Cyprus and Aegean slabs, which results in the piecemeal low anomaly in central Anatolia.

Age and petrogenesis of mafic granulites from central Madurai block, south India: implications on regional tectonics

AbstractThe Precambrian Southern Granulite Terrane (SGT) of south India is well-known for the preservation of high- to ultrahigh-temperature (HT-UHT) granulites, prominently exposed in its central part forming a linear belt referred to as the Kambam UHT belt. This belt also hosts widespread occurrences of mafic granulites that are observed in close spatial association with the HT-UHT granulites. This study presents detailed petrology, geochemistry and geochronology of representative mafic granulites from the area to understand their petrogenesis and tectonic setting. The results demonstrate that mafic granulites are low- to medium-K tholeiites, with continental arc affinity, formed by the partial melting of a subduction-modified enriched mantle source. The composition of the parent mantle source is modelled with a spinel/garnet lherzolite contribution ratio between 100/0 and 70/30, suggesting the mixing of spinel and garnet bearing melts during asthenosphere upwelling. Zircon U–Pb geochronology of mafic granulites constrains their emplacement between 612 Ma and 625 Ma, that subsequently underwent metamorphism between 581 Ma and 531 Ma. This overlaps with the timing of HT-UHT metamorphism in the Kambam UHT belt bracketed between 593 and 532 Ma. Zircon Hf isotopic studies reveal parent magma generation from reworked melting sources involving Archean and Proterozoic components. These results propose an alternative heat source for the formation of HT-UHT granulites in the Kambam UHT belt which can be designated as a major terrane boundary within the SGT.