Melting Of Lithosphere Research Articles

Late Triassic high‐Mg diorites and associated mafic dikes from the southern Zhangguangcai Range (NE China): Melting of juvenile arc crust and subarc mantle lithosphere

The Lesser Xing'an–Zhangguangcai Range is located in the eastern part of the Central Asian Orogenic Belt (CAOB). Early Mesozoic igneous rocks in this area can place constraints on the crustal growth mechanism in the Northeastern China. This paper reports a suite of Late Triassic high‐Mg diorites and mafic dikes from the southern Lesser Xing'an–Zhangguangcai Range. The mafic dikes have younger zircon U–Pb ages of 195 ± 2 Ma. They display intermediate SiO2 (50.58% to 58.20%) and high Na2O/K2O ratios of 1.90 to 3.20, low A/CNK ratios of 0.75 to 0.84, low (87Sr/86Sr)i ratios of 0.7045 to 0.7046, and high εNd(t) values of +3.1 to +3.5. Most zircons in the mafic dikes exhibit positive εHf(t) values from +8.6 to +16.0, with single‐stage Hf model ages ranging from 0.2 to 0.5 Ga in combination with their enrichment in LREE and LILEs and depletion in HFSEs. They are considered to be formed by melting of metasomatized subarc lithosphere mantle. The diorite and quartz diorite have older zircon U–Pb ages of 200 ± 3 and 202 ± 2 Ma, respectively. They have SiO2 = 54.45% to 61.18%, Al2O3 = 17.16% to 19.10%, TiO2 = 0.74% to 1.13%, and Mg# = 46.6 to 49.5, and they also display moderately depleted Sr–Nd (εNd(t) = +2.3 to +4.3) and zircon Lu–Hf (εHf(t) = +8.6 to +16.0) isotopic compositions. These features suggest that they were formed by melting of juvenile arc crust in active continental margin. Considering the occurrence of the contemporaneous and latter igneous rocks in the Zhangguangcai Range, we argue that the diorite, quartz diorite, and mafic dikes from the Lengshan area were also related to the westward subduction of Palaeo‐Pacific Plate (Izanagi Plate) beneath the Eurasian Plate. The strike‐like fault along the eastern margin of the Songneng–Zhuangguangcai Block may play an important role in the genesis of diorite melt.

Magnetotelluric Evidence for Asymmetric Simple Shear Extension and Lithospheric Thinning in South China

AbstractExtension and rifting of the lithosphere is fundamental to the evolution of the continents, but the mechanism by which the lithosphere thins under extension remains enigmatic. Using a new dense array of magnetotelluric data from the rifted margin of southeast China, we resolve the three‐dimensional electrical resistivity structure of the lithosphere to constrain the process of rifting and thinning. Our results reveal a brittle‐ductile transition zone in the Cathaysia Block, featuring low electrical resistivity and low seismic velocity at 15‐ to 20‐km depth. A southeast directed dip is resolved for the Jiangshan‐Shaoxing Fault—a suture zone originally formed by the Neoproterozoic collision of Yangtze and Cathaysia Blocks. This boundary fault had been reactivated to different extents by the Early Paleozoic and Early Mesozoic intracontinental orogenies of south China. Its present structural style, however, was interpreted as a lithospheric‐scale detachment fault that acted during the Late Mesozoic extension and rifting. Given the asymmetries of topography, electrical resistivity, Bouguer gravity anomaly, and Late Mesozoic volcanism across the Gan‐Hang Rift, we propose an asymmetric simple shear extension model for the south China rift system. Water content of up to 0.1 wt% and melt fraction of up to 1% are estimated at 70‐km depth beneath the central Wuyi Mountains, suggesting hydration and partial melting of the mantle lithosphere. The hydration weakening of the mantle lithosphere promoted both the gravitational instability and convective removal of the lowermost lithosphere in south China.

The Thickness of the Mantle Lithosphere and Collision-Related Volcanism in the Lesser Caucasus

The Lesser Caucasus mountains sit on a transition within the Arabia–Eurasia collision zone between very thin lithosphere (<100 km) to the west, under Eastern Anatolia, and a very thick lithospheric root (up to 200 km) in the east, under western Iran. A transect of volcanic highlands running from NW to SE in the Lesser Caucasus allows us to look at the effects of lithosphere thickness variations on the geochemistry of volcanic rocks in this continental collision zone. Volcanic rocks from across the region show a wide compositional range from basanites to rhyolites, and have arc-like geochemical characteristics, typified by ubiquitous negative Nb–Ta anomalies. Magmatic rocks from the SE, where the lithosphere is thought to be thicker, are more enriched in incompatible trace elements, especially the light rare earth elements, Sr and P. They also have more radiogenic ⁸⁷Sr/⁸⁶Sr, and less radiogenic ¹⁴³Nd/¹⁴⁴Nd. Across the region, there is no correlation between SiO₂ content and Sr–Nd isotope ratios, revealing a lack of crustal contamination. Instead, ‘spiky’ mid-ocean ridge basalt normalized trace element patterns are the result of derivation from a subduction-modified mantle source, which probably inherited its subduction component from subduction of the Tethys Ocean prior to the onset of continent–continent collision in the late Miocene. In addition to the more isotopically enriched mantle source, modelling of non-modal batch melting suggests lower degrees of melting and the involvement of garnet as a residual phase in the SE. Melt thermobarometry calculations based on bulk-rock major elements confirm that melting in the SE must occur at greater depths in the mantle. Temperatures of melting below 1200°C, along with the subduction-modified source, suggest that melting occurred within the lithosphere. It is proposed that in the northern Lesser Caucasus this melting occurs close to the base of the very thin lithosphere (at a depth of ∼45 km) as a result of small-scale delamination. A striking similarity between the conditions of melting in NW Iran and the southern Lesser Caucasus (two regions between which the difference in lithosphere thickness is ∼100 km) suggests a common mechanism of melt generation in the mid-lithosphere (∼75 km). The southern Lesser Caucasus magmas result from mixing between partial melts of deep lithosphere (∼120 km in the south) and mid-lithosphere sources to give a composition intermediate between magmas from the northern Lesser Caucasus and NW Iran. The mid-lithosphere magma source has a distinct composition compared with the base of the lithosphere, which is argued to be the result of the increased retention of metasomatic components in phases such as apatite and amphibole, which are stabilized by lower temperatures prior to magma generation.

Hydrous melting of metasomatized mantle wedge and crustal growth in the post-collisional stage: Evidence from Late Triassic monzodiorite and its mafic enclaves in the south Qinling (central China)

Triassic collision between the Yangtze and North China blocks is a key aspect of the evolution of the Paleo-Tethys in East Asia. This paper reports age and geochemistry for Late Triassic monzodiorite and its mafic enclaves from the south Qinling (central China). The host monzodiorite and mafic enclaves have identical zircon U-Pb ages of 227 ± 3 Ma and 221 ± 3 Ma, respectively. The host monzodiorite displays moderate SiO2 (62.44–63.85 wt%) and MgO (3.44–4.47 wt%) contents and high Cr (157–244 ppm) and Ni (78–108 ppm) contents. It has evolved whole-rock Sr-Nd isotopic compositions (εNd(t) = –6.2 to –5.0) and slightly positive zircon εHf(t) values (up to +6.4). Given these characteristics in combination with high Th/Nb and Nb/Yb ratios, the host monzodiorite is considered to have been derived from hydrous melting of metasomatized mantle lithosphere. Its moderate Sr/Y and low Yb/Lu ratios indicate the fractional crystallization of hornblende. The mafic enclaves have lower SiO2 (52.85–58.53 wt%) and higher MgO (8.26–9.45 wt%) contents. Most zircons in the mafic enclaves display positive εHf(t) values of +0.9 to +16.5. These features indicate that the pristine mafic melt was derived from depleted mantle lithosphere. Minor grains in the mafic enclaves display lower εHf(t) values (−4 to 0) than the zircons in the host monzodiorite, suggesting that the mafic melt had incorporated some evolved crustal component before it intruded into the host monzodiorite chamber. In summary, in the circumstance of slab break-off and asthenosphere upwelling, hydrous melting of mantle lithosphere has contributed greatly to crustal-derived granites in the Qinling orogenic belts. These results have the following implications for the Triassic granites in the Qinling orogenic belts: (1) hydrous melting of metasomatized mantle wedge greatly contributes to crustal growth in orogenic processes; and (2) mantle-derived hydrous mafic melts induced the extensive melting of crust and led to the voluminous Triassic granites in the Qinling orogen.

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)

U-Pb zircon and titanite ages and Sr-Nd-Hf isotope constraints on the timing and evolution of the Petrohan-Mezdreya pluton (Western Balkan Mts, Bulgaria)

A combination of methods is applied in the present study to define the exact age of the Petrohan and Mezdreya plutons and trace their magma evolution. Field, petrological, and geochemical studies of the Petrohan pluton revealed its complex evolution and emphasized the role of magma mingling and mixing, complementary to the normal assimilation and fractional crystallization (AFC) processes. Using high-precision conventional U-Pb (CA)-ID-TIMS zircon and titanite dating in combination with CA-LA-ICP-MS zircon dating and tracing, we suggest an incremental growth of a common Petrohan-Mezdreya pluton. It was assembled over minimum 4.5 Ma from 311.14±0.48 Ma to 307.54±0.54 Ma. The younger age of the gabbro (308.12±0.33 Ma), compared with the age of granodiorites (311.14±0.48 Ma), provides numerical proofs for magma replenishment during the assembling of the Petrohan pluton. Whole-rock strontium-neodymium (initial 87Sr/86Sr ratios of 0.70521–0.70527 to 0.70462 and 143Nd/144Nd of 0.51221 to 0.51210) and Hf-zircon isotope data (ε-Hf from –5.8 to +3.6) argue for interaction of mantle derived magma with crustal melts but also mixing and mingling and transfer of zircon grains between the gabbroic and granitic melts. Possible petrogenetic scenario includes melting of subcontinental mantle lithosphere and crust and evolution trough AFC, FC and mingling/mixing processes. Considering the Petrohan-Mezdreya pluton as part of the Variscan orogeny in SE Europe, our new data support the accretion/collision of both the Balkan and Sredna Gora/Getic units with Moesia in the Early Carboniferous followed by syn- and post-collisional Carboniferous and Permian magmatism.

Petrology, geochemistry and geodynamic setting of Eocene-Oligocene alkaline intrusions from the Alborz-Azerbaijan magmatic belt, NW Iran

The Kaleybar, Razgah and Bozqush (KRB) intrusions were studied to better understand subduction-related Eocene-Oligocene alkaline magmatism in NW Iran. The bulk of intrusions mainly consist of Si-undersaturated rocks including foid-bearing monzonite and syenite (nepheline syenite, pseudoleucite syenite) with some foid-bearing diorite and gabbro. In addition, they are spatially associated with Si-saturated rocks ranging in composition from monzo-diorite to syeno-granite. The main mafic rock-forming minerals of the studied rocks are olivine (Fo44Fa56), clinopyroxene (diopside to augite), biotite (Mg-biotite through Fe-biotite), amphibole (ferro-pargasite and magnesio-hastingsite with Mg#<0.55), and garnet (Ti-andradites). Based on whole rock geochemistry, the foid-syenites and associated rocks show mildly alkaline (shoshonitic) affinity. The content of SiO2, K2O + Na2O, and K2O/Na2O ratio ranges from 47.8 to 60.7 wt.%, 5.31 to 16.33 wt.%, and 0.6 to 3.2, respectively. The intrusions are commonly metaluminous, with an aluminum-saturation index (ASI) ranging from 0.66 to 1.01. Almost all the rocks display similar arc-related geochemical features characterized by the enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) together with the depletion in high field strength elements (HFSE). The chondrite-normalized REE patterns show no to marked negative Eu anomaly (Eu/Eu* = 0.55 to 1.12), (La/Yb)N = 8.16 to 31, (La/Sm)N = 2.80 to 10.59, and (Tb/Yb)N = 0.84 to 2.40. The evaluation of the REE patterns for the KRB magmas and the comparison of the trace element ratios with experimental studies indicate a chemically enriched lithospheric mantle source composed of garnet-spinel-lherzolite that have underwent a low degree of partial melting <5% to generate the KRB intrusions. Based on the present data, we infer that the mantle source was contaminated by a subduction component and the melting of the mantle lithosphere occurred by local extension in an overall convergent regime in NW Iran. The extension regime during the Eocene is proposed to be the result of the Neo-Tethys slab roll-back and the Sevan-Akera-Qaradagh (SAQ) slab break-off.

Water and Oxygen Fugacity in the Lithospheric Mantle Wedge beneath the Northern Canadian Cordillera (Alligator Lake)

AbstractAssessing water contents of subduction zone mantle peridotites can gain insight into the compositions of slab‐derived fluids/melts and the active margin water cycle. Here eight mantle xenoliths from Alligator Lake (northern Canadian Cordillera) are examined to address these issues. The harzburgites have less water, on average, but are more oxidized (ΔFMQ ~ 0.1) than the lherzolites (ΔFMQ ~ −1.0). The lherzolites have major and trace element compositions close to primitive mantle, while the harzburgite major element and heavy rare earth element compositions are indicative of higher degrees of melt depletion but with light rare earth element‐enriched profiles. Correlations between lherzolite pyroxene water contents and bulk rock Ba/Nb and Ba/Yb ratios likely result from interaction with subduction related fluids. The trace element compositions of the harzburgite clinopyroxenes are successfully modeled by melting of a fertile mantle lithosphere and interaction with a carbonatite melt. Correlations between the harzburgite water contents and clinopyroxene Ca/Al ratios and Mg# are also consistent with the influence of carbonatite metasomatism. Metasomatism likely resulted from opening of a slab window beneath the region, detected as a low‐velocity seismic anomaly, which heated and mobilized a heterogeneous mantle lithosphere veined with carbonatite. This study confirms that subduction zone mantle lithosphere is not necessarily more water‐rich or more oxidized than oceanic lithosphere or other off‐cratonic settings. Moreover, local oxidation is not necessarily related to the ingress of subduction zone fluids but can also be related to melting of a heterogeneous lithosphere following heating above a slab window.

Imaging Lithospheric Discontinuities Beneath the Northern East African Rift Using S-to-P Receiver Functions.

Imaging the lithosphere is key to understand mechanisms of extension as rifting progresses. Continental rifting results in a combination of mechanical stretching and thinning of the lithosphere, decompression upwelling, heating, sometimes partial melting of the asthenosphere, and potentially partial melting of the mantle lithosphere. The northern East African Rift system is an ideal locale to study these processes as it exposes the transition from tectonically active continental rifting to incipient seafloor spreading. Here we use S‐to‐P receiver functions to image the lithospheric structure beneath the northernmost East African Rift system where it forms a triple junction between the Main Ethiopian rift, the Red Sea rift, and the Gulf of Aden rift. We image the Moho at 31 ± 6 km beneath the Ethiopian plateau. The crust is 28 ± 3 km thick beneath the Main Ethiopian rift and thins to 23 ± 2 km in northern Afar. We identify a negative phase, a velocity decrease with depth, at 67 ± 3 km depth beneath the Ethiopian plateau, likely associated with the lithosphere‐asthenosphere boundary (LAB), and a lack of a LAB phase beneath the rift. Using observations and waveform modeling, we show that the LAB phase beneath the plateau is likely defined by a small amount of partial melt. The lack of a LAB phase beneath the rift suggests melt percolation through the base of the lithosphere beneath the northernmost East African Rift system.

Early Silurian adakitic high‐Mg diorite from the Longshan area: Implication for melting of mantle lithosphere in the south‐eastern Qilian Orogenic Belt

The Longshan area is located in the junction zone between the Qinling and eastern Qilian orogenic belts. Early Palaeozoic igneous rocks in this area can provide important insights into the evolution of Shangdan Ocean. This paper reports new petrology, chronology, major and trace element geochemistry, Sr–Nd isotope, and zircon Lu–Hf isotopic compositions for the Early Silurian pyroxene diorite in the Longshan area. The pyroxene diorite has a zircon U–Pb age of 432 ± 1 Ma (MSWD = 0.25, n = 27). The pyroxene diorite intrusion displays low SiO2 (53.34 to 56.32 wt.%), high Na2O (3.11% to 3.79 wt.%) and Sr (1112 to 1234 ppm) contents, and moderate Mg# (52.70 to 55.00) values, high Sr/Y (67 to 84) ratios, depleted in Y and HREE; these features are identical with the low‐Si adakite or sanukitoids that derived from melting of metasomatized mantle wedge. The pyroxene diorite intrusion has evolved Sr–Nd isotopic compositions, (87Sr/86Sr) i = 0.705286 to 0.705310, εNd(t) = −3.4 to −3.9. Zircons from the pyroxene diorite display variable εHf(t) values of −3.6 to +0.1. These features suggest that the pyroxene diorite intrusion was derived from a hydrous melting metasomatized mantle wedge which has been modified by slab melts. The occurrence of pyroxene diorite indicates an Early Silurian active continental margin setting in the south‐eastern Qilian Orogenic Belt.

On Zones of Potential High Seismic Activity on Mars

The joint analysis of current data on the topography and nonequilibrium part of the Martian gravity field allowed us to obtain a detailed pattern of the distribution of nonhydrostatic stresses in the Martian interior. The criteria for selection of possible sources of marsquakes were large values of shear stress against the background of significant tensional stresses. The rheological models considered were the elastic model, the one with the lithosphere, and the model with the lithosphere and possible melting zones in it. Independently from the rheological model chosen, zones of high shear and tensional stresses in the Martian crust and mantle were revealed beneath the Hellas and Argyre impact basins, Mare Acidalium, Arcadia Planitia, and Valles Marineris.

Tectono-Magmatic Evolution of the South Atlantic Continental Margins with Respect to Opening of the Ocean

The history of the opening of the South Atlantic in Early Cretaceous time is considered. It is shown that the determining role for continental breakup preparation has been played by tectono-magmatic events within the limits of the distal margins that developed above the plume head. The formation of the Rio Grande Rise–Walvis Ridge volcanic system along the trace of the hot spot is considered. The magmatism in the South Atlantic margins, its sources, and changes in composition during the evolution are described. On the basis of petrogeochemical data, the peculiarities of rocks with a continental signature are shown. Based on Pb–Sr–Nd isotopic studies, it is found that the manifestations of magmatism in the proximal margins had features of enriched components related to the EM I and EM II sources, sometimes with certain participation of the HIMU source. Within the limits of the Walvis Ridge, as magmatism expanded to the newly formed oceanic crust, the participation of depleted asthenospheric mantle became larger in the composition of magmas. The role played by the Tristan plume in magma generation is discussed: it is the most considered as the heat source that determined the melting of the ancient enriched lithosphere. The specifics of the tectono-magmatic evolution of the South Atlantic is pointed out: the origination during spreading of a number of hot spots above the periphery of the African superplume. The diachronous character of the opening of the ocean is considered in the context of northward progradation of the breakup line and its connection with the northern branch of the Atlantic Ocean in the Mid-Cretaceous.

Intraplate Deformation of Oceanic Crust in the West Somali Basin: Insights From Long‐offset Reflection Seismic Data

AbstractInterpretation of new high‐resolution seismic reflection sections reveals intraplate deformation of oceanic crust in the West Somali Basin, between the East Africa margin and the Davie Ridge and its northern extension. The deformation of the oceanic crust takes the form of 1–2 km amplitude buckle folds with a 20–30 km wavelength associated with thrust faults. Reverse faulting is localized along deformation corridors, about 15–30 km wide that we interpret as inherited oceanic fracture zones of the extinct oceanic spreading center. Deformation of the oceanic crust in the westernmost segment started before Hauterivian time (~132 Ma), while oceanic accretion was ongoing to the east of the Davie Ridge, and stopped at Aptian time (~120 Ma), when spreading ceased in the whole Somali Basin. Prior to the deformation of the oceanic crust, large volcanic edifices have been emplaced on top of the oceanic crust along preexisting oceanic fracture zones. Well‐defined groups of reflectors, within the mantle (&lt;~20 km below top basement), are interpreted as large shear zones that tapped a connection to deep lithospheric melt channels leading to magmatic intrusions within the crust and emplacement of postseafloor‐spreading volcanic edifices within deformation corridors.

Mantle heterogeneity, plume-lithosphere interaction at rift controlled ocean-continent transition zone: Evidence from trace-PGE geochemistry of Vempalle flows, Cuddapah Basin, India

This study reports major, trace, rare earth and platinum group element compositions of lava flows from the Vempalle Formation of Cuddapah Basin through an integrated petrological and geochemical approach to address mantle conditions, magma generation processes and tectonic regimes involved in their formation. Six flows have been identified on the basis of morphological features and systematic three-tier arrangement of vesicular-entablature-colonnade zones. Petrographically, the studied flows are porphyritic basalts with plagioclase and clinopyroxene representing dominant phenocrystal phases. Major and trace element characteristics reflect moderate magmatic differentiation and fractional crystallization of tholeiitic magmas. Chondrite-normalized REE patterns corroborate pronounced LREE/HREE fractionation with LREE enrichment over MREE and HREE. Primitive mantle normalized trace element abundances are marked by LILE-LREE enrichment with relative HFSE depletion collectively conforming to intraplate magmatism with contributions from sub-continental lithospheric mantle (SCLM) and extensive melt-crust interaction. PGE compositions of Vempalle lavas attest to early sulphur-saturated nature of magmas with pronounced sulphide fractionation, while PPGE enrichment over IPGE and higher Pd/Ir ratios accord to the role of a metasomatized lithospheric mantle in the genesis of the lava flows. HFSE-REE-PGE systematics invoke heterogeneous mantle sources comprising depleted asthenospheric MORB type components combined with plume type melts. HFSE-REE variations account for polybaric melting at variable depths ranging from garnet to spinel lherzolite compositional domains of mantle. Intraplate tectonic setting for the Vempalle flows with P-MORB affinity is further substantiated by (i) their origin from a rising mantle plume trapping depleted asthenospheric MORB mantle during ascent, (ii) interaction between plume-derived melts and SCLM, (iii) their rift-controlled intrabasinal emplacement through Archean–Proterozoic cratonic blocks in a subduction-unrelated ocean-continent transition zone (OCTZ). The present study is significant in light of the evolution of Cuddapah basin in the global tectonic framework in terms of its association with Antarctica, plume incubation, lithospheric melting and thinning, asthenospheric infiltration collectively affecting the rifted margin of eastern Dharwar Craton and serving as precursors to supercontinent disintegration.

Petrogenesis of the Late Cenozoic collision volcanism in the central part of the Lesser Caucasus (Azerbaijan)

The Late Cenozoic volcanics of the Lesser Caucasus have similar trace-element and REE patterns with negative anomalies of Nb, Ta, Hf, and Zr. They are highly enriched in Rb, Ba, Th, and La and depleted in Ti, Yb, and Y with respect to N-MORB, which indicates their formation from the subduction-metasomatized lithospheric mantle. Partial melting of the subcontinental mantle lithosphere and crustal assimilation and fractional crystallization controlled the magma evolution in the collisional magmatic belts.

Plume-stagnant slab-lithosphere interactions: Origin of the late Cenozoic intra-plate basalts on the East Eurasia margin

Intra-plate basalts of ~35–0Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature (Tp), while the Shandong basalts have the lowest Tp. We infer that a deep high-Tp plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low-Tp plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at ~35Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation.

Petrogenesis and origin of the Upper Jurassic-Lower Cretaceous magmatism in Central High Atlas (Morocco): Major, trace element and isotopic (Sr-Nd) constraints

During an uplift phase, which lasted ca. 40 Ma, from the Late Jurassic (165 Ma) to the Early Cretaceous (125 Ma), transitional to moderately alkaline magmatic series were emplaced in the Central High Atlas. The corresponding magmatic products include basaltic lava flows erupted within wide synclines and intrusive complexes composed of layered mafic intrusions and monzonitic to syenitic dykes emplaced along narrow anticlinal ridges. The igneous rock sequence within the intrusive complexes is composed of troctolites, olivine-gabbros, oxide-gabbros, monzonites and syenites. The chemical compositions of the various intrusive rocks can be accounted for by crystal accumulation, fractional crystallization and post-magmatic remobilization. The evolution from the troctolites to the syenites was mainly controlled by a fractional crystallization process marked by early fractionation of olivine, plagioclase and clinopyroxene, followed by separation of biotite, amphibole, apatite, and Ti-magnetite. Hydrothermal activity associated with emplacement of the intrusions within the Jurassic limestones modified the elemental and the Sr isotopic composition of the hydrothermally altered rocks In particular the monzonitic to syenitic dykes underwent an alkali metasomatism marked by depletion in K and Rb and enrichment in Na and Sr. As a result, their Sr isotopic composition was shifted towards higher initial Sr isotopic ratios (0.7067–0.7075) with respect to the associated gabbros (0.7036–0.7046). On the contrary, the Nd isotopic compositions were preserved from isotope exchange with the limestones and vary in a similar range to those of the gabbros (+1.6 < εNdi < +4.1). The isotopic and the trace element ratios of the uncontaminated samples were used to constrain the source characteristics of this magmatism. The Sr-Nd isotopic data and the incompatible element ratios (e.g. La/Nb, Zr/Nb, Th/U, Ce/Pb) are consistent with generation from an enriched upper mantle similar to an ocean island basalt source. Melting of the subcontinental metasomatized lithosphere is tentatively related to small-scale shallow mantle upwelling and asthenospheric uprise at the triple junction between the western High Atlas, the Middle Atlas and the eastern High Atlas domains during a period of relative tectonic quiescence.

Partial melting of stagnant oceanic lithosphere in the mantle transition zone and its geophysical implications

Widespread low velocity anomalies have been observed in the upper mantle around many oceanic subduction zones. Fluid or melt released from a stagnant slab may have contributed to the formation of these anomalies. Furthermore, slab partial melting or dehydration is also thought to be closely related to the origin of intraplate volcanoes (i.e., Changbaishan). However, experimental evidence on the process of slab partial melting is very limited. Here, our experimental results show that partial melting of stagnant oceanic lithosphere may occurs for temperatures above ~1300–1400°C, with residual phases composed of wadsleyite/ringwoodite+garnet+clinopyroxene/stishovite/akimotite. The density of melt was approximately 1.0–1.5g/cm3 less dense than the surrounding mantle, which provided a buoyancy force for ascent to the upper mantle across the 410-km seismic discontinuity. The ascending melt may react with mantle peridotite, leading to the formation of a variably metasomatized mantle, which may contribute to the formation of the observed low velocity anomalies above stagnant slab. Re-melting of the metasomatized mantle may have contributed to the origin of the intraplate volcanoes, e.g., Changbaishan volcanoes. We suggest that partial melting of stagnant oceanic lithosphere in the MTZ may have close relations with the origin of the big mantle wedge beneath eastern China.

Introduction to special section: Subsurface expression of igneous systems and their impacts on petroleum systems

Magma commonly forms during continental breakup in response to stretching and thinning of the lithosphere and decompression melting of the asthenospheric mantle ([White and McKenzie, 1989][1]). This magma may stall during its ascent and intrude the crust in the form of sills, dikes, or inclined

The 2014–15 eruption and the short-term geochemical evolution of the Fogo volcano (Cape Verde): Evidence for small-scale mantle heterogeneity

Recurrent eruptions at very active ocean island volcanoes provide the ideal means to gain insight on the scale of spatial variations at the mantle source and on temporal changes of magma genesis and evolution processes. In 2014, after 19years of quiescence, Fogo volcano (Cape Verde Archipelago) experienced a new eruption, with the vents located 200m from those of the 1995 eruption, and less than 2000m from those of the 1951 event. This offered a unique opportunity to investigate the existence of small-scale mantle heterogeneities and the short-term compositional evolution of magmas erupted by a very active oceanic volcano like Fogo. Here we present petrological and geochemical data from the early stages of the Fogo's most recent eruption – started on November 23, 2014 – and compare them with the signature of previous eruptions (particularly those of 1995 and 1951).The magmas erupted in 2014 are alkaline (up to 23.4% and 0.94% of normative ne and lc, respectively) with somewhat evolved compositions (Mg #<56), ranging from tephrites to phonotephrites. The eruption of phonotephritic lavas preceded the effusion of tephritic ones. Lavas carried to the surface clinopyroxene and kaersutite phenocrysts and cognate megacrysts, which indicate that the main stages of magma evolution occurred in magma chambers most probably located at mantle depths (25.6±5.5km below sea level). This was followed by a shallower (<1.5km below sea level) and shorter (≈50days) magma stagnation before the eruption. 2014 magmas have more unradiogenic Sr and more radiogenic Nd compositions than those of the previous 1951 and 1995 eruptions, which generally have less radiogenic Pb ratios. These isotopic differences – coming from quasi-coeval materials erupted almost in the same place – are remarkable and reflect the small-scale heterogeneity of the underlying mantle source. Moreover, they reflect the limited isotopic averaging of the source composition during partial melting events as well as the inefficient homogenization within the plumbing system when on route to the surface. The lid effect of an old and thick lithosphere is considered of utmost importance to the preservation of a significant part of source heterogeneity by erupted magmas. The decrease in the contribution of an enriched component to the Fogo magmas in the 2014 eruption marks a change on the volcano short-term evolution that was characterized by a progressive increase of the importance of such a component. Nb/U ratios of the 2014 lavas are similar, within 2σ, to the mean value of OIB, but significantly lower than those reported for the 1995 and 1951 eruptions. This is considered to reflect the lack of significant mixing of the 2014 magmas with lithospheric melts, as opposed to what is here hypothesised for the two previous eruptions.