Late Cenozoic Basalts Research Articles

Mantle melting in regions of thick continental lithosphere: Examples from Late Cretaceous and younger volcanic rocks, Southern Rocky Mountains, Colorado (USA)

Major- and trace-element data together with Nd and Sr isotopic compositions and 40Ar/39Ar age determinations were obtained for Late Cretaceous and younger volcanic rocks from north-central Colorado, USA, in the Southern Rocky Mountains to assess the sources of mantle-derived melts in a region underlain by thick (≥150 km) continental lithosphere. Trachybasalt to trachyandesite lava flows and volcanic cobbles of the Upper Cretaceous Windy Gap Volcanic Member of the Middle Park Formation have low εNd(t) values from −3.4 to −13, 87Sr/86Sr(t) from ~0.705 to ~0.707, high large ion lithophile element/high field strength element ratios, and low Ta/Th (≤0.2) values. These characteristics are consistent with the production of mafic melts during the Late Cretaceous to early Cenozoic Laramide orogeny through flux melting of asthenosphere above shallowly subducting and dehydrating oceanic lithosphere of the Farallon plate, followed by the interaction of these melts with preexisting, low εNd(t), continental lithospheric mantle during ascent. This scenario requires that asthenospheric melting occurred beneath continental lithosphere as thick as 200 km, in accordance with mantle xenoliths entrained in localized Devonian-age kimberlites. Such depths are consistent with the abundances of heavy rare earth elements (Yb, Sc) in the Laramide volcanic rocks, which require parental melts derived from garnet-bearing mantle source rocks. New 40Ar/39Ar ages from the Rabbit Ears and Elkhead Mountains volcanic fields confirm that mafic magmatism was reestablished in this region ca. 28 Ma after a hiatus of over 30 m.y. and that the locus of volcanism migrated to the west through time. These rocks have εNd(t) and 87Sr/86Sr(t) values equivalent to their older counterparts (−3.5 to −13 and 0.7038–0.7060, respectively), but they have higher average chondrite-normalized La/Yb values (~22 vs. ~10), and, for the Rabbit Ears volcanic field, higher and more variable Ta/Th values (0.29–0.43). The latter are general characteristics of all other post– 40 Ma volcanic rocks in north-central Colorado for which literature data are available. Transitions from low to intermediate Ta/Th mafic volcanism occurred diachronously across southwest North America and are interpreted to have been a consequence of melting of continental lithospheric mantle previously metasomatized by aqueous fluids derived from the underthrusted Farallon plate. Melting occurred as remnants of the Farallon plate were removed and the continental lithospheric mantle was conductively heated by upwelling asthenosphere. A similar model can be applied to post–40 Ma magmatism in north-central Colorado, with periodic, east to west, removal of stranded remnants of the Farallon plate from the base of the continental lithospheric mantle accounting for the production, and western migration, of volcanism. The estimated depth of the lithosphere-asthenosphere boundary in north-central Colorado (~150 km) indicates that the lithosphere remains too thick to allow widespread melting of upwelling asthenosphere even after lithospheric thinning in the Cenozoic. The preservation of thick continental lithospheric mantle may account for the absence of oceanic-island basalt–like basaltic volcanism (high Ta/Th values of ~1 and εNd[t] > 0), in contrast to areas of southwest North America that experienced larger-magnitude extension and lithosphere thinning, where oceanic-island basalt–like late Cenozoic basalts are common.

Hf isotope compositions of basaltic rocks from Jeju Island, South Korea: Implications for the origin of the dichotomous isotopic distribution of late Cenozoic magmatism in East Asia

The mantle sources of late Cenozoic intraplate basalts in East Asia can be divided into two distinct domains on the basis of Sr–Nd–Pb isotope systematics: DMM–EM1 in the north and DMM–EM2 in the south. However, the origin of this dichotomous isotopic distribution remains unclear. Here, we present new Hf isotope data for basaltic rocks from Jeju Island, South Korea, and compare the data with published data for coeval basalts from East Asia to understand the nature of the enriched components. The Jeju basalts have 176Hf/177Hf ratios of 0.282861–0.282965 and εHf values of +3.2 to +6.8. In an HfNd isotope plot, the basalts plot along the terrestrial array, without marked decoupling between Hf and Nd isotopes. Late Cenozoic basalts in East Asia fall mainly in the field of global ocean-island basalts. However, basalts in NE Asia tend to be much less radiogenic than those in SE Asia. In addition, some samples deviate from the terrestrial array, reflecting the involvement of various lithologies in the source. Major- and trace-element contents of the Asian basalts indicate the presence of subducted oceanic crust (eclogite/pyroxenite) in the mantle source. However, such crust is not a plausible source for the isotopically enriched components, given the less radiogenic Pb isotope compositions and higher 207Pb/206Pb ratios (0.816–0.917) of the late Cenozoic basalts compared with the HIMU end-member. Instead, subducted sediments could have been the enriched source. The representative EM1-type Wudalianchi basalts in NE Asia have high (Zr/Zr*)N ratios that show a negative correlation with εHf values, indicating the presence of ancient zircon-bearing sediments in the source. In contrast, SE Asian basalts have high Th/La ratios, suggesting the involvement of pelagic clay-rich sediments. A lack of meaningful correlations between Th/La ratios and εHf values indicates that the sediments are relatively young and were recycled with only a short residence time in the mantle source. PbNd isotope modeling shows that ancient (~2.2 Ga) sediments were involved in the mantle source of the NE Asian basalts, compared with relatively young sediments in the mantle source of the SE Asian basalts.

Bifurcation of mantle plumes by interaction with stagnant slabs in the mantle transition zone: Evidence from late Cenozoic basalts within Southeast Asia

Mantle plumes can encounter stagnant slabs in the mantle transition zone (MTZ) during their ascent from the lower mantle to Earth’s surface. However, the interactions between stagnant slabs and mantle plumes remain poorly understood. This study presents new data on the spatial thermal−lithological−chemical heterogeneity in the mantle over a vast area of Southeast Asia that reflects the variable influence of a late Cenozoic mantle plume. Site U1433 of International Ocean Discovery Program (IODP) Expedition 349 in the southwest sub-basin of the South China Sea (SCS) is located near the center of this seismically imaged mantle upwelling. This area also contains mid-ocean ridge basalt (MORB) derived from a peridotite-dominated and normal temperature mantle source with a depleted Nd-Hf and enriched 207Pb/206Pb isotopic composition. In contrast, MORB from Site U1431 of IODP Expedition 349 within the eastern sub-basin of the SCS and other intraplate basalts from the Hainan-Leizhou area, the eastern Indochina block, and elsewhere in the SCS are dominantly sourced from higher temperature pyroxenite-enriched mantle material that records mantle plume activity. These observations are in contrast with previous models involving a single plume rising from the lower mantle beneath the SCS that then tilted toward Hainan Island, China. Instead, it is likely that the observations from this region are indicative of plume-stagnant slab interaction. The shallow upper mantle beneath Site U1433 has not been influenced by a mantle plume as the presence of stagnant slabs within the MTZ prevented further upwelling of the plume. In contrast, a mantle plume passed through the weaker MTZ within the eastern sub-basin and circum-SCS areas, allowing plume-related magmatism to occur. The bifurcation of a deep-rooted mantle plume by stagnant slabs in the MTZ produced a cluster of small mantle upwellings that ascended to shallow depths, consistent with new multiscale global tomographic data for Southeast Asia.

Carbon Enrichment in the Lithospheric Mantle: Evidence from the Melt Inclusions in Mantle Xenoliths from the Hainan Basalts

AbstractIt is generally believed that the lithospheric mantle and the mantle transition zone are important carbon reservoirs. However, the location of carbon storage in Earth's interior and the reasons for carbon enrichment remain unclear. In this study, we report CO2‐rich olivine‐hosted melt inclusions in the mantle xenoliths of late Cenozoic basalts from the Penglai area, Hainan Province, which may shed some light on the carbon enrichment process in the lithospheric mantle. We also present a detailed petrological and geochemical investigation of the late Cenozoic basalts and mantle xenoliths from northern Hainan Island. The collected samples of late Cenozoic Hainan Island basalts belong to both alkaline and subalkaline series, showing fractionated REE patterns with high (La/Yb)Nvalues of 3.52–11.77, which are typical for OIB. Based on Al‐in‐olivine thermometry, the temperatures estimated for the mantle xenoliths can be divided into two groups. One group has temperatures of less than 1050°C, and the other group has temperature ranging from 1050°C to 1282°C. Clinopyroxene (La/Yb)N–Ti/Eu and clinopyroxene Ca/Al–Mg#diagrams indicate that the mantle peridotite experienced metasomatism from both silicate and carbonate melts. Melt inclusions in the olivine of mantle xenoliths include (1) CO2bubble–rich melt inclusions; (2) multiphase melt inclusions (glass + CO2bubble + daughter minerals); (3) pure glass melt inclusions. Magnesite is a daughter mineral in the olivine‐hosted melt inclusions, which could be interpreted as a secondary mineral formed by the interactions of CO2‐rich fluids with an olivine host, due to post‐entrapment effects. The glasses in olivine‐hosted melt inclusions have high SiO2contents (60.21–77.72 wt%). Our results suggest that a considerable amount of CO2‐rich melt inclusions are captured in the lithospheric mantle during metasomatism. The lithospheric mantle can therefore act as is a ‘carbon trap’, with much CO2being absorbed by the lithospheric mantle in this way.

In-situ Sr isotope disequilibrium in plagioclases from Late Cenozoic basalts in Leiqiong area: Evidence for the role of the Hainan plume and mantle metasomatism due to a paleo-subduction event

The origin of the EM2 (enriched mantle 2)-type reservoir beneath the Southeast Asia is controversial. In this study, we present new in-situ major and trace element (olivine, clinopyroxene, and plagioclase), and Sr isotopic compositions (plagioclase) for Leiqiong basaltic rocks to explain the potential source of Leiqiong EM2-like reservoir. Leiqiong olivines show the normal zoning textures with in-situ trace element heterogeneity, Leiqiong plagioclases exhibit the complex zoning textures with in-situ87Sr/86Sr isotopic disequilibria. It likely suggests the contribution to an individual crystal by the incorporation of heterogeneous components. The olivine and plagioclase phenocrysts, and the calculated Leiqiong clinopyroxene-equilibrium melts show the features of OIB-type enriched compositions associated with the plume, likely indicating the presence of Hainan mantle plume. The clear disequilibrium of 87Sr/86Sr isotopic ratios among bulk-rock, plagioclase phenocryst, and groundmass likely indicates the isotopically heterogeneous mantle source and the influence of the recycled subduction-related H2O-enriched oceanic fluid/melts carried by the deep Hainan plume. Leiqiong olivines are crystallized by the incorporation of pyroxenitic mantle source, but different from those from olivine-rich peridotitic mantle lithology, likely due to the reaction between eclogite-derived silicate melts (recycled oceanic crust) and peridotite.

Pb−изотопная оценка возраста модельных источников позднекайнозойских базальтов вулканического поля Чанбайшань (Пектусан) и её значение в общей Pb−изотопной систематике Центральной Азии

From the 207Pb/204Pb – 206Pb/204Pb diagram, sources of Late Cenozoic basalts from the Changbaishan field, located on the Northeast China – North Korea border, are characterized by two age intervals: Late Archean (3.0–2.8 Ga) and Early Proterozoic (2.2–1.9 Ga). The latter interval coincides with the age estimates of the component of subducted sediments involved in their genesis and its upper boundary coincides with the Pb isotope age of the source of Pleistocene volcanic rocks in Northeast China.

Contribution of recycled sediments to the mantle reservoir beneath Hainan Island: Evidence from Sr, Nd, Pb, Hf, and Mg isotopic analyses of Late Cenozoic basalts

Magnesium isotopes are a useful tool for constraining the origin of basalts with EM-like isotopic signatures in relation to ancient subducted slabs and recycled materials incorporated in mantle plumes. In this study, we present new SrNdPbHf and Mg isotopic data that were used to determine the origin of the basalt on Hainan Island and investigate the EM mantle reservoir beneath the island. Cenozoic basalts from northern Hainan Island are mainly tholeiitic, with a small amount of alkaline basalts. The Hainan basalts exhibited depleted SrNd isotopic compositions and EM2-like Pb isotopic signatures. The δ26Mg values of the Hainan basalts ranged from −0.40‰ to −0.28‰. The origin of the low δ26Mg signature can be attributed to carbonate sediments from recycled oceanic slab. Hainan basalts show a negative concave curve relationship between 87Sr/86Sr and εNd values, a positive relationship between 206Pb/204Pb and 207Pb/204Pb values and exhibit an evolution trend from depleted mantle towards marine sediments. This indicates that Hainan Island basalts can be explained by the mixing between depleted mantle and marine sediments. Most Hainan basalts have higher K/U × 10−3 and Ba/Th ratios than primitive mantle (K/U × 10−3 ≈ 11.8, Ba/Th ≈ 83), moreover, display highly correlated K/U × 10−3 and Ba/Th compositions with low-pressure (6–8 GPa) carbonated melt released from initial sediments. Therefore, we speculate that the primitive mantle peridotite, coupled with the low-pressure carbonated melt, ultimately became the mantle source of Hainan Island basalts.

Evolution of the Continental Margin of South to Central Vietnam and Its Relationship to Opening of the South China Sea (East Vietnam Sea)

AbstractThe continental margin of south to central Vietnam is notable for its high elevation plateaus many of which are covered by late Cenozoic basalt flows. It forms the westernmost margin of a wide continental rift of the South China Sea (East Vietnam Sea), and uplift has been considered a result of either rifting or younger intraplate basalt magmatism. To investigate margin development apatite thermochronometry was applied to a dense array of samples collected from across and along the margin of south to central Vietnam. Results, including thermal history models, identified a distinct regional episode of fast cooling between c. 37 and 30 Ma after which cooling rates remained low. The fast cooling coincides with a period of fast extension across the South China Sea (East Sea) region that preceded continental break‐up recorded by Oligocene grabens onshore. A thermal model is used test different processes that might influence the inferred cooling including a distinct pulse of exhumation; a decrease in exhumation followed by an associated transient decrease in geothermal gradients and, underplating coincident with rifting. Thermal relaxation following Mesozoic arc magmatism is ruled out as geotherms returned to background rates within 20 Myrs of emplacement, well before the onset of fast cooling. Models support fast cooling attributed to accelerated erosion during early stages of rifting. Some additional heating from either underplating and/or hot mantle upwellings is also possible. No evidence was found to support regional uplift associated with the intraplate magmatism, enhanced monsoon‐driven erosion, or seafloor spreading dynamics.

Fingerprinting subducted oceanic crust and Hainan Plume in the melt sources of Cenozoic Basalts from the South China Sea Region

AbstractGeochemical data compilation of Cenozoic basalts recovered from the South China Sea tectonic domain shows westward weakening of the influence of a focal zone‐like component in Nd–Hf, Nd–Pb and Sr–Pb, but not in Pb–Pb isotope spaces because the Pb isotopes are dominantly controlled by the high U/Pb component derived from the subducted Pacific oceanic slab. Low Th/U melt generated by recycling of marine carbonates, rather than the subduction‐related enriched mantle (EM2), signals the emplacement of the Hainan Plume at ~25 Ma. Radiogenic Hf in the pre‐existing ancient sub‐continental lithospheric mantle beneath the Cathaysia Block was greatly depleted by early‐stage magmatism influenced by the high U/Pb component. Hence, late Cenozoic basalts associated with the carbonatitic melts display contrasting Nd–Hf isotope covariations, with the Red River–Zhongnan Fault System as a dividing line, implying that the East and Southwest sub‐basins have been developed on the Cathaysia and Indochina Blocks respectively.

Metallic Iron in Basalts of the Malyi Yenisei Lava River: Results of Thermomagnetic Study

Thermomagnetic analysis of samples from two sections of lava layers of the Late Cenozoic basalt lava river in the Malyi Yenisei valley is carried out. The main magnetization carrier in the studied basalts is titanomagnetite with the Curie points of 100–120°C which is frequently substantially oxidized both single-phase and heterophase up to magnetite. It is likely that some part of metallic iron in the studied samples has also been oxidized and even disintegrated, which resulted in the significant scatter of iron concentration across the flow against which, however, the increasing trend of iron concentration in the lava flow from the top downwards is observed. The relative magnitude of this increase (iron concentration gradient along the vertical of the lava flow) is almost constant for all flows of the lava sequence probably indicating the decisive role of gravity in iron particle precipitation in the lava. Based on the synthesis of iron particle data from different objects and different regions of the world, this correlation with gravity also follows from the very similar shapes of particle size histograms of iron. This is most clearly seen from the same particle size modes (10–20 μm). Another important finding is that this constant mode of iron particle size does not depend on age and origin of rock as well as on the type of particle source (terrestrial or extraterrestrial).

Petrogenesis of late Cenozoic basalts from Dalinor, Inner Mongolia: Implications for lateral mantle heterogeneity in eastern China

We present major element, trace element and Sr-Nd-Pb-Hf isotopic compositions of the late Cenozoic basaltic rocks from Dalinor, Inner Mongolia, on the west of the Greater Hinggan Mountain Range in Northeast Asia, and discuss the origin of mantle source heterogeneity beneath Northeast Asia. The early-stage volcanoes (EV) from Dalinor produced predominantly quartz tholeiitic and olivine tholeiitic basalts whereas the late-stage volcanoes (LV) produced alkali basalts. Variations in the degree of partial melting and in fractionation crystallization could explain most of the changes in major and trace element compositions within individual groups. Trace element ratios and Sr-Nd-Pb-Hf isotope ratios between the EV and LV groups indicate a heterogeneous mantle source. The EV group is characterized by an ocean island basalt (OIB)-like spider diagram and positive anomalies of Sr, Ba and K, moderately LREE-enriched pattern ((La/Yb)N = 5.7–8.2), high Ba/Nb (10.2–16.5), and negative correlation between Dy/Yb and SiO2. Two thirds of the EV samples have low Nb/U (<37). The source rock for the EV lavas was amphibole-bearing metasomatized lithospheric mantle. The LV lavas have high TiO2 content (2.57–3.65 wt%), high Nb/U (42.4–58.8) and Indian-MORB-like SrNd isotopic compositions, indicating their asthenospheric origin. The Sr-Nd-Pb isotopic compositions of the Dalinor lava overlap those of the Southeast China basalts, and are consistent with source mixing between a depleted mantle component (DM) and an enriched mantle EM2 component. The western part of the trans-Greater Hinggan Mountain Range mantle source profile has a DM + EM2 signature, whereas the eastern part of the profile has a DM + EM1 signature. This lateral mantle source heterogeneity cannot be explained by the stagnant Pacific slab in the mantle transition zone. Instead, we propose that the EM2 component originated from the descending slab during a slab avalanche in the lower mantle near the westernmost leading edge of stagnant slab, before finally residing in the lithospheric mantle.

Mantle sources and magma genesis of Late Cenozoic basalts in Weizhou Island, Guangxi, China

Mantle sources and magma genesis of Late Cenozoic basalts in Weizhou Island, Guangxi, China

Geochemical constraints on the origin of late Cenozoic basalts in the Mt. Changbai volcanic field, NE China: evidence for crustal recycling

ABSTRACT We conducted a systematic geochemical study of the Mt. Changbai basalts (MCBs) in NE China to better constrain their mantle sources and petrogenesis. The MCBs display ocean island basalt-like trace element patterns (e.g., positive Nb–Ta anomalies) and variable MgO contents (3.2–10.0 wt.%), trace element ratios (e.g., Ba/Nb and Rb/Nb), and Sr–Nd–Pb–Hf isotopic ratios (87Sr/86Sr(i) = 0.704497–0.705629; εNd(i) = – 2.8 to +3.3; 206Pb/204Pb(i) = 16.9–18.1; εHf = −1.2 to +6.2), which indicate derivation from a heterogeneous mantle source. The least evolved samples (MgO > 7 wt.%), with moderate Fe/Mn and FC3MS (defined as FeOT/CaO – 3MgO/SiO2) values, require a hybrid pyroxenite–peridotite mantle source. The isotopic compositions indicate the mantle source is a mixture between depleted mid-ocean ridge basalt mantle and two enriched mantle components. Our geochemical and published geophysical data indicate that these two enriched components are recycled ancient sediments and Pacific oceanic crust, which were derived from the mantle transition zone. An upwelling wet plume from the mantle transition zone transported these two enriched components into the asthenospheric mantle. These enriched components were then transformed into pyroxenite and/or eclogite through melt–rock reaction or mechanical mixing with asthenospheric mantle peridotite. We further propose that partial melting of this heterogeneous asthenospheric mantle containing fusible pyroxenite and/or eclogite (i.e., the enriched components) and refractory peridotite produced the MCBs.

Subduction‐Induced Upwelling of a Hydrous Transition Zone: Implications for the Cenozoic Magmatism in Northeast China

AbstractThe widespread Cenozoic basalts in northeast China are commonly thought to be related to the stagnation of the Pacific slab in the transition zone and its deep dehydration. By incorporating experimentally constrained phase diagrams of hydrous mantle and melting conditions at high pressures into two‐dimensional petrological‐thermomechanical models, here we model the interaction of a subducting slab with a hydrous transition zone (TZ) and examine its potential role in generating intracontinental magmatism. The model results show that descending of the oceanic slab first forces up the material in the TZ. Depending on the water content in the TZ, the upwelling hydrous material may undergo dehydration melting above the TZ. As a large slab stagnates within the TZ owing to the lower mantle resistance, the deep melting migrates progressively toward the overriding continent's interior, generating plutonic/volcanic rocks in the continental crust far away from the trench. The amount of deep melting and surface magmatism is obviously related to the amount of water stored in the TZ. In contrast, the water stored in the cold core of the subducting slab and released in the transition zone does not generate melting as it is entirely absorbed by mantle transition zone nominally anhydrous minerals. Comparing the model results with the distribution of the late Cenozoic basalts in northeast China, we suggest that the intracontinental magmatism there was likely caused by deep dehydration melting induced by the slab‐transition zone interaction.

Geochemical comparison of late Mesozoic and early Cenozoic volcanic rocks in South Mongolia

The Mesozoic-Cenozoic volcanic rocks are widely distributed in the interior of the East Asia and document the tectonic transition of East Asia. We present new geochronology and geochemistry data of late Cretaceous-early Cenozoic basalts in Bayantsagaan and Han-Uul volcanic provinces in South Mongolia, in order to explore their petrogenesis and geodynamic settings. The volcanic rocks in the Bayantsagaan and Han-Uul field yielded K-Ar ages of 90.55±1.93 Ma and 55.49±1.49 Ma, respectively. The volcanic rocks in South Mongolia can be subdivided into to alkaline basalts and tholeiitic series, and are characterized by ocean island basalts (OIB) trace elements features, such as enrichment of light REE relative to heavy REE and enrichment in large ion lithophile elements (LILE) with positive K anomaly. Compared with the late Cretaceous, the early Cenozoic basalts show a decrease in the contents of HREE and an increase of Nb and Ta. Crustal contamination and fractional crystallization are insignificant in the genesis of late Cretaceous-early Cenozoic basalts South Mongolia. The available Sr-Nd isotope results indicate that a mixing depleted (DM) and enriched mantle (EM) signature characterize in late Cretaceous volcanic rocks, which derived from magmas from the asthenosphere with some contributions of metasomatized subcontinent lithospheric mantle, whereas the early Cenozoic basalts are ascribed to contributions from the asthenospheric mantle. We propose that the generation of the late Cretaceous-early Cenozoic volcanism (90-40 Ma) in Mongolia is probably related to the shallow mantle upwelling (asthenosphere) induced by the edge convection along the northern margin of the North China Craton (NCC), triggered by a far-field effect of Indo-Asian collision.

In Situ LA-ICP-MS Analysis of Minerals Hosted by Late Cenozoic Basaltic Rocks from Thailand

Shortly after the cessation of seafloor spreading, intraplate magmatism affected large areas in the South China Sea (SCS) region. The origin and geodynamic setting of the post-spreading volcanism is still in debate, for many previous studies have focused on petrogenesis and mantle source of the late Cenozoic basalts from the SCS region. In this study, we obtained in situ major element compositions (by using Electron microprobe analysis—EMPA) and trace element compositions (by using laser ablation inductively coupled plasma mass spectrometry— LA-ICP-MS) for minerals (clinopyroxenes (Cpx), plagioclases (Pl), and olivines (Ol)) hosted by late Cenozoic basaltic rocks from Thailand. The results showed that the olivines had forsterite contents between 60.12% and 84.74%. Clinopyroxene were diopside and augite, and they were enriched in light rare earth elements (LREEs) (LaN/YbN = 1.93–4.27) and depleted in large-ion lithophile elements (LILEs). Mineral compositions (mainly based on clinopyroxene) confirmed that these late Cenozoic basaltic rocks were of an intraplate affinity and were similar to contemporaneous basaltic fields in the SCS region (Southern Vietnam, Northern Hainan, and SCS seamounts). Plagioclases were predominantly labradorite, with a few andesine and bytownite, and they were enriched in LREEs and Ba, Sr, and Pb, and most of them exhibited strong positive Eu anomalies. The source lithology of Thailand basaltic rocks could be garnet pyroxenite. The mantle potential temperature beneath Thailand is in the range of 1448–1467 °C, which can be comparable to those beneath Southern Vietnam and Northern Hainan, indicating the Thailand basaltic rocks could be produced by the Hainan mantle plume. In addition, the crystallization temperature of clinopyroxenes (1145–1214 °C) and plagioclase (1067–1133 °C) and their composition characteristics indicate that the magmatic processes have a conspicuous characteristic of fast rate of magma upwelling. Thus, we proposed that the deep geodynamic setting of Thailand late Cenozoic basaltic rocks is similar to those of the whole SCS region, and Hainan mantle plume plays a significant role in the petrogenesis of these basaltic rocks.

Interaction of a Spreading Mantle Plume Head and the Ancient Lithosphere: Studying Mantle Xenoliths in Basalts and Lamprophyre Diatremes of Western Syria

Abstract —Our study of mantle xenoliths in the Cretaceous lamprophyre diatremes and late Cenozoic plateau basalts of western Syria has shown that the ancient lower crust that existed in the Cretaceous and was composed of garnet granulites and eclogite-like rocks was replaced by mantle peridotites in the late Cenozoic. We conclude that the heads of the local (secondary) plumes of the present-day Afro-Arabian thermochemical mantle plume responsible for the regional basaltic magmatism reached the basement of the ancient upper sialic crust, where they spread, leading to a displacement of the mafic lower crust.

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

Geochemical constraints on the spatial distribution of recycled oceanic crust in the mantle source of late Cenozoic basalts, Vietnam

This study presents a comprehensive analysis of the major and trace element, mineral, and Sr, Nd, Pb and Mg isotopic compositions of late Cenozoic intraplate basaltic rocks from central and southern Vietnam. The Sr, Nd, and Pb isotopic compositions of these basalts define a tight linear array between Indian mid-ocean-ridge basalt (MORB)-like mantle and enriched mantle type 2 (EM2) components. These basaltic rocks contain low concentrations of CaO (6.4–9.7wt%) and have high Fe/Mn ratios (>60) and FeO/CaO–3MgO/SiO2 values (>0.54), similar to partial melts derived from pyroxenite/eclogite sources. This similarity is also supported by the composition of olivine within these samples, which contains low concentration of Ca and high concentrations of Ni, and shows high Fe/Mn ratios. The basaltic rocks have elevated Dy/Yb ratios that fall within the range of melts derived from garnet lherzolite material, although their Yb contents are much higher than those of modeled melts derived from only garnet lherzolite material and instead plot near the modeled composition of eclogite-derived melts. The Vietnamese basaltic rocks have lighter δ26Mg values (−0.38±0.06‰) than is expected for the normal mantle (−0.25±0.07‰), and these values decrease with decreasing Hf/Hf* and Ti/Ti* ratios, indicating that these basalts were derived from a source containing carbonate material. On primitive mantle-normalized multi-element variation diagrams, the central Vietnamese basalts are characterized by positive Sr, Eu, and Ba anomalies. These basalts also plot within the pelagic sediment field in PbPb isotopic space. This suggests that the mantle source of the basalts contained both garnet peridotite and recycled oceanic crust. A systematic analysis of variations in geochemical composition in basalts from southern to central Vietnam indicates that the recycled oceanic crust (possibly the paleo-Pacific slab) source material contains varying proportions of gabbro, basalt, and sediment. The basalts from south-central Vietnam (12°N–14°N) may be dominated by the lowest portion of the residual slab that contains rutile-bearing plagioclase-rich gabbroic eclogite, whereas the uppermost portion of the recycled slab, including sediment and basaltic material with small amounts of gabbro, may be a major constituent of the source for the basalts within the central region of Vietnam (14°N–16°N). Finally, the southern region (10°N–12°N) contains basalts sourced mainly from recycled upper oceanic crust that is basalt-rich and contains little or no sediment.

The smectite to corrensite transition: X-ray diffraction results from the MH-2B core, western Snake River Plain, Idaho, USA

AbstractThe MH-2B borehole, a part of Project HOTSPOT, was drilled to a depth of 1821 m in late Cenozoic basalts, hyaloclastites and interbedded lake sediments, on the Mountain Home Air Force Base in southern Idaho, USA. Drillers encountered hot water (145°C) under artesian pressure at 1745 m in a narrow zone of highly fractured rock associated with a major sub-surface fault. X-ray diffraction (XRD) analysis identified corrensite (with and without smectite) between 1700 and 1800 m, but only smectite above 1700 m and below 1800 m. This corrensite horizon contains a relatively narrow zone of fracturing and hot artesian water near its centre but for the most part occurs in relatively massive basalt flows. No evidence was found for randomly interstratified chlorite-smectite.