Cenozoic Alkaline Basalts Research Articles

Influences of the Stagnant Pacific Slab Beyond Its Westernmost Edge: Insights From the Cenozoic Alkaline Basalts in the Dariganga Volcanic Field, SE Mongolia

AbstractIt remains uncertain whether a stagnant slab in the mantle transition zone can affect the asthenospheric mantle beyond its leading edge. To address this question, we investigated Cenozoic alkaline basalts from the Dariganga volcanic field (DVF) in southeastern Mongolia. The DVF is located west of North–South Gravity Lineament (NSGL) in Eastern China, which is spatially coincident with the seismically detected stagnant Pacific slab front. Basalts from the DVF consist of nephelinite, basanite and alkali olivine basalt. These rocks have relatively high Nb/U (average = 58) and Nb/La (>1) ratios and radiogenic Nd–Hf isotopic compositions. They also have high Ca/Al (0.60–1.13), Zn/FeT (13.5–16.5), and FeO/MnO (77–112) ratios as well as low δ26Mg (−0.42‰ to −0.26‰) values, reflecting an asthenospheric mantle source modified by carbonated eclogite‐derived melts. Pb–Nd–Hf isotope characteristics indicate that the carbonated eclogite‐derived melts likely originated from the stagnant Pacific slab. Although Cenozoic basalts from both the east of the NSGL (ENSGL) and DVF domains exhibit light δ26Mg values, basalts from the ENSGL nevertheless have lower δ26Mg values than those in the DVF domain. This suggests a gradual westward decline in the amount of carbonated melts/fluids derived from the stagnant Pacific slab. This variation trend, combined with a more fertile and oxidized asthenospheric mantle toward the ENSGL, indicates that the stagnant slab has affected the mantle and created a compositional aureole beyond its leading edge, which substantially contributed to the formation of the alkaline basalts in the DVF.

Deep mantle cycle of chalcophile metals and sulfur in subducted oceanic crust

Understanding the subduction-driven recycling of economically important chalcophile metals (e.g., Cu, Au, Ag, and platinum-group elements, PGEs) and volatiles (e.g., sulfur) is critical for assessing models of ore formation. However, whether the sulfide remains stable in recycled altered oceanic crust (AOC) in the mantle is unclear, and the influence of AOC-derived metasomatic agents on the extraction efficiency of chalcophile elements from the deep mantle remains poorly understood. Here we report whole-rock Cu-Ag-Au-Re-PGEs of Cenozoic alkaline basalts at Hannuoba, eastern China, which were derived from the interaction of carbonated eclogite (recycled AOC)-derived melts with lithospheric peridotites. Volatile-rich primitive melts from carbonated eclogite are poor in strongly chalcophile metals (e.g., ∑PGE < 1 ng/g, Au < 1 ng/g, Cu < 30 μg/g). Remarkably, primitive melts show significant fractionation of elements with variable chalcophile affinities, such as lower Cu/Ag ratios than those in primitive mantle and MORBs. These features reflect the sequestration of these metals in residual mono-sulfide solid solution (MSS) during low-degree melting of the AOC in the deep mantle. Given the retention of sulfides in the AOC, partial melts derived from it are rich in sulfur but poor in strongly chalcophile metals. The Hannuoba alkaline melts scavenged some strongly chalcophile metals from peridotites (especially PGEs) during their ascent. However, this did not necessarily lead to the formation of chalcophile metal-enriched melts (e.g., Au < 1 ng/g and Cu < 60 μg/g). These processes appear to be common in eastern China, as evidenced by the low abundances of Cu and Au found in many primitive intraplate alkaline basalts. We propose that strongly chalcophile elements in subducted AOC are efficiently transported into, and stored in, the deep mantle, with a small proportion returning to the exosphere through volcanism. Moreover, the chalcophile metal-poor and volatile-rich melts of subducted AOC could also explain the addition of abundant sulfur (secondary sulfides), but limited chalcophile metals to the metasomatized mantle.

Хорго вулканы үүсэл, хэлбэрийн онцлог

The book, Cenozoic alkaline basalts of Mongolia and their deep inclusion, volcano existing during Cenozoic was firstly written by V.V.Kepejenskas in 1979 was the first comprehensive monograph including integration of results in many years. The aim of my paper is to define volcanic origin of the Khorgo as the one representative and its morphology or landscape based on such monograph. In Mongolia, there is nothing about study on morphological features by volcanic activity. There are much morphology formed due to effusive effect of lava with basic content in which it’s very crucial to find out which type of morphology had been distributed and relation between origin preferring Khorgo volcanic issues. Thickness of crust of Tariat volcano was 45 km during its activity (Stoch, Kopylova 1995; Ionov 2002). Harris (2009) Due to the ground hot spot, continent became thinner resulted as Khangai magmatic activity.Many intermissive breakdowns through earth crust were formed here. Therefore, due to such formation of intermissive breakdown or one of the main breakdown extending from north-west to south east became stream for basic lava resulted in formation of Khorgo volcano. Lava stream consisting of silicon oxide less than 45% and predominant kali content with fluidity formed as barrier blocking the Suman river valley which led to the formation of Terkhiin Tsagaan lake. Research showed that it was lava elevation among morphologies formed by lava with basic content. The steepness of crater of Khorgo volcano is around 45 degrees while its depth and diameters are 100 m and 500 m respectively. Scientists have predicted that the age. Khorgo volcano as approximately 1.8-2.0 million years in quaternary. Therefore, it’s needed to precise the age of its rocks.

Depletion and refertilisation of the lithospheric mantle below the Kapsiki plateau (Northern Cameroon Volcanic Line) deduced from trace element and H2O systematics in mantle xenoliths

Clinopyroxene bearing spinel harzburgites recovered from Cenozoic alkaline basalts from the Kapsiki plateau were investigated with the aim of understanding depletion and enrichment processes in the subcontinental mantle below a major tectonic line. Concentrations of major and trace elements (including H) were determined in-situ in olivine, orthopyroxene, clinopyroxene and spinel by EPMA, LA-ICP-MS and FTIR. The xenoliths are refractory with high olivine (0.91–0.92) and orthopyroxene (0.92–0.93) Mg# coupled with relatively high modal proportions of clinopyroxene (5–8%) and a remarkably wide range in clinopyroxene Mg# (0.91–0.94), interpreted to reflect high degrees of partial melting followed by modal and chemical metasomatic refertilization. A fractional partial melting model of orthopyroxene trace elements in a depleted mantle indicates 20–30% of melting. Clinopyroxenes show two groups based on their CaO contents (18.7–18.9 and 20.6–22.6 wt %) suggesting a secondary origin and modal metasomatism. The low Ca clinopyroxenes were the latest to be formed, and the wide range of clinopyroxene Mg# shows a lack of equilibrium with the coexisting minerals. The high Ca clinopyroxenes record ancient metasomatic events that are also documented in orthopyroxene. Cryptic metasomatism is shown by strong progressive enrichments in LREEs and MREEs of orthopyroxene and clinopyroxene. Th vs La and Sr vs Nd of clinopyroxene are used to identify different generations of metasomatism. Ti/Eu vs (La/Yb)N and Zr/Hf systematics indicate that the metasomatising agent has both a carbonatite and a silicate component. The structural hydroxyl contents of the different mineral phases are very low (<1 ppm for olivine, 40–48 ppm for orthopyroxene and 164–277 ppm for clinopyroxene). The very low water contents compared to typical sub-continental peridotites could be attributed to the depleted nature of the xenoliths or reflect lower water activities of carbonatitic melts relative to alkaline melts. Estimated fO2 values expressed as ΔlogFMQ, (where FMQ corresponds to the fayalite magnetite-quartz oxygen buffer) range from -0.42 to -0.26 and are consistent with minor OH stretching regions of 3520 cm-1 in orthopyroxene, related to a ferric iron defect.The combination of textural investigations, major, and trace element and H2O data suggests that a refractory subcontinental mantle was affected by several episodes of intraplate metasomatism dominated by silicate-carbonatite melts above the North-Eastern end of the Cameroon Volcanic Line.

Zircon Xenocrysts from Cenozoic Alkaline Basalts of the Ratanakiri Volcanic Province (Cambodia), Southeast Asia—Trace Element Geochemistry, O-Hf Isotopic Composition, U-Pb and (U-Th)/He Geochronology—Revelations into the Underlying Lithospheric Mantle

Zircon xenocrysts from alkali basalts in Ratanakiri Province, Cambodia represent a unique low-Hf zircon within a 12,000 km long Indo-Pacific megacryst zone. Colorless, yellow, brown, and red crystals ({100}, {101}, subordinate {211}, {1103}), with hopper growth and corrosion features range up to 20 cm in size. Zircon chemistry indicates juvenile, Zr-saturated, mantle-derived alkaline melt (Hf 0.6–0.7 wt %, Y &lt;0.2 wt %, U + Th + REE (Rare-Earth Elements) &lt; 600 ppm, Zr/Hf 66–92, Eu/Eu*N ~1, positive Ce/Ce*N, HREE (Heavy REE) enrichment). Incompatible element depletion with increasing Yb/SmN from core to rim at ~ constant Hf suggests single stage growth. Ti-in-zircon temperatures (~570–740 °C) are lower than predicted by crystal morphology (800–900 °C) and decrease from core to rim (ΔT = 10–50 °C). The δ18O values (4.88 to 5.01‰ VSMOW (Vienna Standard Mean Ocean Water)) are relatively low for xenocrysts from the zircon Indo-Pacific zone (ZIP). The 176Hf/177Hf values (+ εHf 4.5–10.2) give TDepleted Mantle model source ages of 260–462 Ma and TCrustal ages of 391–754 Ma. The source magmas reflect variably depleted lithospheric mantle with little supracrustal input. Zircon U-Pb (0.88–1.56 Ma) and (U-Th)/He (0.86–1.02 Ma) ages are older than host basalt ages (~0.7 Ma), which suggests limited residence before transport. Zircon genesis suggests Zr-saturated, Al-undersaturated, carbonatitic-influenced, low-degree partial melting (&lt;1%) of peridotitic mantle at ~60 km beneath the Indochina terrane.

Mesozoic decratonization of the North China Craton by lithospheric delamination: Evidence from Sr-Nd-Hf-Os isotopes of mantle xenoliths of Cenozoic alkaline basalts in Yangyuan, Hebei Province, China

Mineral compositions, clinopyroxene trace-element and Sr-Nd-Hf isotopic compositions, whole-rock major element and platinum group element (PGE) contents, and Re-Os isotopes are reported for peridotite xenoliths in the Cenozoic alkali basalts from Yangyuan, Hebei Province (China), in order to constrain the nature of the subcontinental lithospheric mantle beneath the central part of the North China Craton, and further to investigate the mechanism of decratonization. The xenoliths mainly consist of spinel lherzolites and minor spinel pyroxenites and spinel harzburgites with equilibration temperatures of 890–1060°C. The olivine in these xenoliths have Mg number (%Fo) ranging from 88.2 to 91.6. According to the chemical characteristics, two groups of peridotites can be identified, i.e., group 1 are spinel lherzolites and spinel harzburgite that are depleted in Pt, Pd and Re (i.e., P-PGEs) relative to Os, Ir and Ru (i.e., I-PGEs) and their clinopyroxenes are depleted in light rare earth elements (LREEs), whereas, Group II are spinel lherzolites that are enriched in P-PGEs relative to I-PGEs and their clinopyroxenes are enriched in LREEs relative to HREEs. The spinel lherzolites have radiogenic 187Os/188Os ratios ranging from 0.1182 to 0.1262, whereas, the spinel harzburgites have relatively low 187Os/188Os ratios with Proterozoic model ages. Clinopyroxenes in the peridotite xenoliths have variable but relatively unradiogenic Sr (87Sr/86Sr=0.7026–0.7046) and radiogenic Nd (εNd=+2.0 to +14.7) and Hf (εHf=+8.8 to +37.9) isotopic compositions. All these geochemical features indicate that the subcontinental lithospheric mantle beneath Yangyuan had experienced variable degrees of melt extraction and mantle metasomatism by the silicate melts represented by spinel pyroxenites. The spinel pyroxenites are enriched in LREEs and P-PGEs relative to HREEs and I-PGEs. Clinopyroxenes in pyroxenite xenoliths have high 87Sr/86Sr ratios of 0.7054–0.7063, negative εNd (−3.4 to −15.8) and εHf (−2.0 to −22.0) values, and high 187Os/188Os ratios of 0.2196–0.3343, indicating that the silicate melts were derived from ancient lower continental crustal materials. Our data suggest that the lithospheric mantle beneath Yangyuan had been metasomatized by silicate melts derived from recycled ancient lower crust, indicating that decratonization of the North China Craton was possibly induced by delamination of the lower crust and lithospheric mantle.

Melting of dehydrated oceanic crust from the stagnant slab and of the hydrated mantle transition zone: Constraints from Cenozoic alkaline basalts in eastern China

The feasibility of oceanic igneous crust melting in stagnant slabs has previously been proposed from experimental petrological research. However, geochemical evidence for such melting has not yet been found from igneous rocks. We present evidence to suggest that melts from the igneous layer in the stagnant Pacific slab contributed to the source composition of basalts erupted in eastern China. Fe-rich (>13wt.%), Si-poor (<43wt.%) basalts occur only above the leading edge of the stagnant Pacific slab in eastern China, ~2000km west of the Pacific Plate trench. The source of these basalts has Nd–Hf and Sr–Nd–Pb isotopic compositions akin to those of the igneous layer in the Pacific slab, and their extremely low Rb and Pb contents suggest that their source material was modified by subduction processes. Together with forward modeling calculation on trace elements and isotope compositions, these geochemical characteristics imply that they received a contribution from fluid released from hydrated transition zone and dehydrated carbonate-bearing oceanic crust in the stagnant slab, without a long time-integrated ingrowth of Sr–Nd–Hf–Pb isotope systems, almost at the leading edge of the stagnant Pacific slab.

Geochemistry and petrology of pyroxenite xenoliths from Cenozoic alkaline basalts, Bohemian Massif

Pyroxenites occur as rare, but important, mantle xenoliths within Cenozoic volcanic rocks of the Central European Volcanic Province (CEVP). We report the petrography, geothermobarometry, mineral chemistry as well as clinopyroxene trace-element and Sr-Nd isotopic compositions of six pyroxenite xenoliths hosted by Tertiary-Quaternary volcanic rocks from Kozakov (NE Bohemia), Dobkovicky and Kuzov (Ohre/Eger Rift), and Lutynia (SW Poland). Three Kozakov xenoliths record a complex nature and evolution. As suggested by contrasting estimated temperatures (596-663 degrees C and 1008-1067 degrees C) and textures, olivine clinopyroxenite and one websterite were likely derived from different depths and may represent crystallization products of transient melts. Another websterite contains symplectite pseudomorphs after garnet which originated by exsolution from highly aluminous clinopyroxene, and the garnet breakdown was associated with melt introduction. This websterite P-T conditions (17.3 to 21.4 kbar and 1080-1200 degrees C) correspond to derivation from depths of 55-69 km. The composition of websterite from Dobkovicky suggests its origin as a cumulate from melt derived from a highly depleted mantle source with a composition similar to Depleted MORB Mantle (DMM). The position and petrological significance of websterite from Kuzov remains unclear because of a lack of data from associated peridotites. At Lutynia, cryptic metasomatism of peridotites by a CO2-bearing alkaline melt was described previously, and the composition of melt calculated to be in equilibrium with Lutynia websterite clinopyroxene is very similar to that in equilibrium with metasomatized amphibole-bearing peridotite. Therefore, the Lutynia websterite may represent a cumulate from percolating melt that metasomatized the lithospheric mantle in this area.

Lithospheric petrology of the eastern Arabian Plate: Constraints from Al-Ashkhara (Oman) xenoliths

Mafic granulite and spinel lherzolite xenoliths from Cenozoic alkaline basalts near Al-Ashkhara, eastern Oman, have been selected for a systematic mineralogical, geochemical and Sr–Nd–Pb isotopic study. This is the only place in E Arabia where samples of both lower crust and upper mantle can be examined. Lower crustal xenoliths consist of two mineralogically and chemically distinct groups: gabbronorite (subequal abundances of ortho- and clino-pyroxene and plagioclase) and plagioclase pyroxenite (dominant pyroxene and subordinate plagioclase). Temperature estimates for lower crustal xenoliths using the two pyroxene geothermometer (T-Wells) yield 810–865 °C. The mineral assemblage (spinel–pyroxene–plagioclase) and Al content in pyroxene indicate that plagioclase-bearing xenoliths equilibrated at 5–8 kbar (13 and 30 km depth) in the lower crust. εNd and 87Sr/ 86Sr calculated at 700 Ma for Al-Ashkhara lower crustal xenoliths (+ 6.4 to + 6.6; 87Sr/ 86Sr = 0.7028 to 0.7039) are consistent with the interpretation that juvenile, mafic melts were added to the lower crust during Neoproterozoic time and that there was no discernible contribution from pre-Neoproterozoic crust. Upper mantle xenoliths consist of both dry and hydrous (phlogopite-bearing) lherzolites. These peridotites are more Fe-rich than expected for primitive mantle or melt residues and probably formed by pervasive circulation of melts that have refertilized pre-existing mantle peridotites. Mineral equilibration temperatures range from 990 to 1070 °C. Isotopic compositions calculated at 700 Ma are εNd = + 6.8 to + 7.8 and 87Sr/ 86Sr = 0.7016 to 0.7025, indicating depleted upper mantle. Pb isotopic compositions indicate that the metasomatism was relatively recent, perhaps related to Paleogene tectonics and basanite igneous activity. Nd model ages for the spinel peridotite xenoliths range between 0.59 and 0.65 Ga. The xenolith data suggest that eastern Arabian lower crust is of hotspot origin, in contrast to western Arabian lower crust, which mostly formed at a convergent plate margin. Geochemical and isotopic differences between lower crust and upper mantle indicate that these are unrelated, possibly because delamination replaced the E Arabian mantle root in Neoproterozoic time.

Iron isotope evidence for multistage melt–peridotite interactions in the lithospheric mantle of eastern China

Iron isotopic data of the Cenozoic alkaline basalts and peridotite xenoliths entrained in the Cenozoic and Mesozoic basalts, eastern China, are reported to further constrain the Fe isotopic composition of the mantle and investigate the behavior of Fe isotopes during mantle processes. Nine Cenozoic alkaline basalts show limited variation in δ 57Fe that are within the ranges of oceanic basalts, which indicate homogeneous Fe isotope composition of the basalts. The lherzolites from the South China Block also display a limited Fe isotopic variation δ 57Fe of 0.041 to 0.077‰ and an average of 0.060‰ ± 0.029‰ (2 SD). This Fe isotopic composition is similar to that of the bulk silicate Earth, indicating that these samples represent mantle residues that underwent just limited degrees of partial melting. In contrast, both the mantle peridotites and their mineral separates from the North China Craton exhibit an extremely large Fe isotopic variation with δ 57Fe ranging from − 1.002‰ to 0.232‰. This may reflect heterogeneous Fe isotopic compositions of the lithospheric mantle beneath the North China Craton. An average δ 57Fe (− 0.066‰) calculated from all the mantle xenoliths in this study is obviously lower than the average value (0.198‰) of δ 57Fe for the Cenozoic alkaline basalts, consistent with the previous observation that basalts generally have heavier Fe isotopes than mantle peridotites. In individual minerals, δ 57Fe of the olivines (− 0.997 to 0.154‰) are systematically lighter than those of the coexisting pyroxenes (− 0.718 to 0.169‰ for orthopyroxene and − 0.642 to 0.304‰ for clinopyroxene, respectively). Phlogopite of apparently metasomatic origin has the heaviest iron isotopic composition amongst the mineral phases with δ 57Fe of 0.302 to 0.376‰. Extreme Fe isotope variation in these peridotites, in particular for the wehrlites, was probably produced by multistage melt–peridotite interactions, consistent with the petrological observation that some of the xenoliths analyzed such as wehrlites were the products of extensive mantle metasomatism. Positive correlations between δ 57Fe and other major and trace element indicators of metasomatism such as CaO and Rb further support the above suggestion. These characteristics suggest that mantle metasomatism or melt–rock interaction can significantly modify Fe isotopes and play an important role in producing Fe isotopic heterogeneity of the lithospheric mantle. Thus, Fe isotopes can provide important information concerning melt–rock interaction and evolution of the lithospheric mantle.

Thermal Evolution of the Lithosphere in a Rift Environment as Inferred from the Geochemistry of Mantle Cumulates, Northern Victoria Land, Antarctica

Among the abundant mantle xenoliths carried by the Cenozoic alkaline basalts of northern Victoria Land, Antarctica, we have studied a suite of clinopyroxene-rich cumulates collected at Browning Pass (Mt. Melbourne Volcanic Province), ranging in composition from wehrlites to clinopyroxenites. Clinopyroxenes belonging to both the Cr-diopside (wehrlites) and Al-augite series (ol-clinopyroxenites and clinopyroxenites) all show convex-upward REE patterns. Modal and cryptic metasomatism has variably affected the xenoliths, accounting for amphibole replacement of clinopyroxene and/or selective enrichment in incompatible elements. Chemical features, along with O–Sr–Nd isotopic data, indicate that both the parental magmas and the metasomatizing melts are related to the Cenozoic magmatic activity and imply the role of at least two mantle components with distinct isotopic fingerprints. The positive covariation between δ18Oolivine and the amount of modal olivine, and between δ18Oolivine and olivine Fo content, suggest that during the fractionation of olivine and pyroxene, the parent magma experienced a change in O-isotope composition; a low-δ18O melt component was not only added to the minerals during the metasomatic event but was also involved in the genesis of the parental melts. The Browning Pass cumulates are used to constrain the origin of the Antarctic Cenozoic magmatism from a heterogeneous mantle source whose depleted end-member is inferred to be the local lithospheric mantle, whereas the enriched end-member is represented by early metasomatic veins or domains emplaced into the depleted mantle during an amagmatic phase of rifting at the beginning of Ross Sea opening. Thermobarometric analysis of the process shows that the respective contribution to magma generation of the two end-members is related to the change of local thermal regime induced by an ‘edge effect’ in the mantle circulation at the lithospheric step between the thick East Antarctic craton and the thinned Ross Sea crust.

Implications of subduction and subduction zone migration of the Paleo-Pacific Plate beneath eastern North China, based on distribution, geochronology, and geochemistry of Late Mesozoic volcanic rocks

Several major volcanic zones are distributed across the eastern North China Craton, from northwest to southeast: the Greater Xing’an Range, Jibei-Liaoxi, Xishan, and Songliao Basins, and the Yanji, Huanghua, and Ludong volcanic zones. The Huanghua depression within the Bohai Bay Basin was filled by middle Late Mesozoic volcanic rocks and abundant Cenozoic alkaline basalts. Zircon LA-ICP-MS and SHRIMP U–Pb dating show that basic–intermediate volcanic rocks were extruded in the Early Cretaceous of 118.8 ± 1.0 Ma (weighted mean 206Pb/238U age), before Late Cretaceous acid lavas at 71.5 ± 2.6 Ma. An inherited zircon from andesite has a Paleoprotoerozoic core crystallization age of 2,424 ± 22 Ma (206Pb/207Pb age) indicating that the basement of the Bohai Bay Basin is part of the North China Craton. Early Cretaceous basic and intermediate lavas are characterized by strong enrichments in LREE and LILE and depletions in HREE and HFSE, indicating a volcanic arc origin related to oceanic subduction. Depletion in Zr only occurs in basic and intermediate volcanic rocks, while depletions in Sr and Ti exist only in acid samples, indicating that the acid series is not genetically related to the basic–intermediate series. Formation ages and geochemical features indicate that the Late Cretaceous acid lavas are products of crustal remelting in an extensional regime. Combined information from all these volcanic zones shows that subduction-related volcanic rocks were generated in the Jibei-Liaoxi and Xishan volcanic zones during the Early Jurassic, about 60 Ma earlier than their analogues extruded in the Huanghua and Ludong volcanic zones during the Early Cretaceous. This younging trend also exists in the youngest extension-related volcanism in each of these zones: Early Cretaceous asthenosphere-derived alkaline basalts in the northwest and Late Cretaceous in the southeast. A tectonic model of northwestward subduction and continuous oceanward retreat of the Paleo-Pacific Plate is proposed to explain the migration pattern of both arc-related and post-subduction extension-related volcanic rocks. As the subduction zone continuously migrated, active continental margin and backarc regimes successively played their roles in different parts of North China during the Late Mesozoic (J1–K2).

Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ57Fe are in a range of −0.25 to 0.14‰ for olivine, −0.17 to 0.17‰ for orthopyroxene, −0.21 to 0.27‰ for clinopyroxene, and −0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.

Iron valency in minerals of xenoliths and redox state of the upper mantle (by Mössbauer spectroscopy data)

The subjects of the research were mantle xenoliths from Cenozoic alkaline basalts in the Baikalian–Mongolian region. The Mossbauer spectroscopy was used to study iron valence in mineral structures. The mineral compositions were analyzed by microprobe technique. The values of ΔlgfO2 for xenoliths is vary: DP: −0.9− (−1.7); TD: −0.9−(−1.8); VP: −0.1–(−0.8); BA: −0.1−(−0.8). p − T equilibrium conditions of mineral association in xenoliths correspond to graphite stability field, ΔlgfO2 values evidence of predominance of CO2 and H2O in the mantle fluid. The heterogeneity in redox state corresponds to thermal heterogeneity. The deduced equation allows estimating the value of oxygen fugacity only the data of chemistry and φSp.

Zircon U-Pb dating on the Mesozoic volcanic suite from the Qingshan Group stratotype section in eastern Shandong Province and its tectonic significance

A geochronological study of zircon U-Pb on the volcanic rocks from the stratotype section of the Qingshan Group within the Jiaozhou Basin, eastern Shandong Province, is presented. The zircons were analyzed using the method of in situ ablation of a 193 nm excimer laser system coupled with an up to date ICP-MS system. Among the three formations of the Qingshan Group, zircons recovered from the lowest part of the Houkuang Fm. were dated at 106±2 Ma (95% confidence, the same below), whereas those from the lower and upper parts of the Shiqianzhuang Fm. were given ages of 105±4 Ma and 98±1 Ma, respectively. A spatially decreasing trend for the Mesozoic magmatic timing from west to east in the province is observed through comparing the data of this study with those by previous works on the Qingshan volcanic lavas occurring at western Shandong and within the Yishu fault zone. The Qingshan volcanic rocks are constituent of the ‘Shoshonite Province’ in East China. Exposed at most provinces of central East China along the Tan-Lu fault and the Yangtze fault zones, these volcanic suites are characterized by shoshonite and high-K calcalkalic rocks in lithology and thought to be correlated with the partial melting of continental mantle in genesis. It is also shown that the Qingshan potassic volcanic suite from eastern Shandong basins is distinctly younger than those from other areas of the shoshonite province. By contrary, ages of the Mesozoic to Cenozoic alkaline basalts, sourced by asthenospheric mantle, from both northern Huaiyan basin and northern Dabie belt along the Tan-Lu fault zone and from the Ningwu, Lishui and Luzong basins along the Yangtze fault zone are observably older than those occurring within eastern Shandong. The revealed temporal and spatial patterns in magmatism for the two types of volcanic suites make an important geochronological constraint on the Mesozoic to Cenozoic dynamic evolution model of the subcontinental lithosphere in East China.

Preliminary study on noble gas isotopes of mantle-derived peridotite xenoliths along the Tan-Lu Fault belt, Eastern China

In order to understand the nature of noble gas reservoirs beneath continents, elemental and isotopic compositions of noble gases were analyzed from spinel-lherzolites with anhydrous mineral assemblages (i.e., olivine+clinopyroxene+orthopyroxene +spinel±apatite±garnet); these are considered to be typical of the upper lithospheric mantle. Isotopic compositions of the noble gases were measured for peridotite xenoliths enclosed in Cenozoic alkaline basalts in eastern-central China to provide insight on mantle characteristics. The results show that the noble gas abundances and isotopic compositions in mantle-derived xenoliths from eastern-central China vary widely:4He abundances from 0.0024×10−6 to 25.0×10−6 (cm3stp/g);3He/4He ratios range between 0.33RA and 4.62 RA;40Ar/36Ar ratios from 250.56 to 1233.86;40Ar abundances from 0.97×10−6 to 34.18×10−6(cm3/g). Only three measured Ne isotopes show a degree of homogeneity:20Ne from 0.3 to 0.7 (cm3stp/g);20Ne/22Ne ratios from 10.5 to 11.0; and20Ne abundances are also rather uniform, from 35.8×10−13 to 47.8×10−13 (cm3stp/g), and129Xe/130Xe ratios from 6.50 to 6.70. It can be seen that3He/4He ratios vary largely, and40Ar/36Ar ratios are significantly lower than MORB and Proterozoic crust, but close to that of atmospheric values. On the basis of the noble gas abundances and isotope compositions, it can be concluded that Ne, Kr and Xe in mantle-derived xenoliths from NE China have atmospheric origin. The results also show the heterogeneity of noble gases in peridotite from the upper mantle in eastern China, which accords with the results of previous geochemical studies.

He and Ar isotopes in mantle megacryst minerals from Nushan and Yingfengling in Southeast China

He and Ar isotopic compositions of megacrystal minerals from mantle xenoliths were measured by the technique of vacuum crushing extraction. The used samples were clinopyroxene, garnet and ilmenite in Cenozoic alkaline basalts, which were from Nushan in Anhui Province and Yingfengling in Guangdong Province, respectively, and represented materials from the upper mantle in the continental margin of SE China. The results show3He/4He ratios of 7.99 Ra to 8.58 Ra, consistent with the characteristic ratios of the MORB-type mantle.40Ar/39Ar ratios vary from 313 to 909, suggesting a binary mixing between the MORB-type mantle and air argons. This may reflect the incorporation of the air argon absorbed in oceanic sediments into the mantle beneath the continental margin by subduction of oceanic plate. This study presents the first report that ilmenite megacrysts contain abundant fluid inclusions and noble gases in the mantle xenoliths.

Continental crust and lithospheric mantle interaction beneath North China: isotopic evidence from granulite xenoliths in Hannuoba, Sino-Korean craton

Mafic granulite and pyroxenite xenoliths from Cenozoic alkaline basalts at Hannuoba, Hebei Province, North China have been selected for a systematic geochemical and Sr–Nd–Pb isotopic study, which provides a unique opportunity to explore nature of the lower crust and the interaction between the continental crust and lithospheric mantle beneath an Archean craton. The major, compatible and incompatible elements and radiogenic isotopes of these xenoliths suggest great chemical heterogeneity of the lower crust beneath the Hannuoba region. Petrological and geochemical evidences indicate a clear cumulate origin, and most likely, they are related to basaltic underplating in different geological episodes. However, the Sr–Nd–Pb isotopic compositions of the xenoliths reveal a profound enriched source signature (EM I) with some influence of EM II, which implies that some portion of pre-existing, old metasomatized subcontinental lithospheric mantle could have played an important role in their genesis. It is suggested that the interaction between continental crust and subcontinental mantle as manifested by basaltic underplating would be closely related to regional tectonic episodes and geodynamic processes in the deep part of subcontinental lithospheric mantle.

Geochemistry of Late Cenozoic basalts from the Crary Mountains: characterization of mantle sources in Marie Byrd Land, Antarctica

Late Cenozoic (9.34 to 0.04 Ma) alkaline basalts from the Crary Mountains, eastern Marie Byrd Land, Antarctica have low 87 Sr/ 86 Sr (0.70267–0.70283), moderately high 143 Nd/ 144 Nd (0.51286–0.51291) and high 206 Pb/ 204 Pb (19.9–20.9) ratios. The Crary Mountains basalts together with Hobbs Coast basalts from western Marie Byrd Land have the strongest HIMU signature ( 206 Pb/ 204 Pb>20.6 ) in the West Antarctic Rift System (WARS). There is no evidence of crustal contamination and their trace-element patterns are similar to HIMU basalts from oceanic islands. Higher concentrations of some large-ion lithophile (LIL) elements, particularly K, relative to oceanic HIMU, suggest melting of an enriched mantle. Models for small degrees of partial melting between 2% and 3% of a mantle peridotite containing 2.5% amphibole are consistent with trace-element variations in the basalts. The Crary data confirm the presence of a moderately depleted, lower-μ ( μ= 238 U/ 204 Pb ) mantle component with a 206 Pb/ 204 Pb signature between 19.3 and 19.8 in the source regions of West Antarctic basalts. The lower-μ component, defined by the focus of West Antarctic data arrays on 2-D and 3-D isotope plots, is prevalent as a mixing end-member throughout the WARS, whereas the HIMU component, defined by 206 Pb/ 204 Pb ratios greater than 20.5, is present at only a few localities. The positive correlation of many highly incompatible trace-elements with 206 Pb/ 204 Pb ratios indicate that smaller-degree melts preferentially sample the HIMU component, whereas larger degree melts sample the lower-μ component. We also suggest, based on variations in elements moderately incompatible to highly compatible in the mantle phases phlogopite and amphibole, that the lower-μ component is more hydrous than the HIMU source. The lower-μ and HIMU components are also found in alkaline basalts from Tasmania and New Zealand which were adjacent to the Antarctica coast prior to the mid-Cretaceous fragmentation of the proto-Pacific margin of Gondwanaland. We propose that before continental breakup, a HIMU-type mantle plume was trapped and stored beneath a much more extensive pre-existing metasomatised layer within the Gondwanaland lithosphere. Source regions for all known extreme HIMU basalts in the continental borderlands of the southwest Pacific would be encompassed by a modest plume head, 600–800 km in diameter, emplaced around 100 Ma. We suggest that an earlier metasomatic event, possibly related to the Jurassic Bouvet-plume, enriched the originally depleted upper mantle in highly incompatible elements. A fertile lithosphere formed by plume-driven melts and fluids during the Jurassic would allow for the widespread development of the present-day isotopic signature of the lower-μ source.

Mantle-derived xenoliths with hotspot type helium in Cenozoic alkali, basalt, northwestern Kyushu, Japan

The 3He/4He ratio of mantle-derived material is not uniform and the distinct 3He/4He signatures are found in association with different tectonic environments. Helium isotopic signature of hot-spot regions such as Hawaii, Iceland, and Rbunion range from 9 to 30 Ra, where Ra is 3He/4He of the atmosphere = 1.4 x 10 -6, which indicates that the hot-spot volcanisms tap a deeper region of the mantle enriched in 3He than MORB reservoir. Cenozoic alkaline basalts from back-arc region of the Japanese island-arc system are less depleted in Ta, Nb and Ti and less enriched in K, Sr, Ba and Rb compared with samples from northeastem Japan. These geochemical features lead to Nakamura et al. (1985) to suggest that the alkali volcanisms of this area were not directly related with subduction processes but were triggered by plumes originated from greater depth. Furthermore, it was suggested that the alkali basalt magmatism may represent the final stage of a mantle diapirism which was most active in the Miocene and caused the opening of the Sea of Japan (Iwamori, 1991). We measured noble gas isotopic compositions of xenoliths in Cenozoic alkali basalt, Takashima, northwestern Kyushu to investigate possible contribution of deep mantle plume.