Proterozoic Lithospheric Mantle Research Articles

Marie Byrd Land lithospheric mantle: a review of the xenolith record

Abstract The Marie Byrd Land (MBL) lithospheric mantle xenolith record comprises over 100 samples from a range of localities spanning both major crustal terranes that comprise MBL: Ross and Amundsen provinces. Coarse granular to porphyroclastic in texture, the xenoliths are predominantly Type I spinel-bearing lherzolites to harburgites, but include rare dunite and pyroxenite examples. Garnet is absent and no hydrous phases, such as amphibole or mica, have been reported to date, although traces of apatite may be present. Characterisation of the lithospheric mantle composition and its evolution however, is hampered by patchy and uneven geochemical analyses across the xenolith suite. Nonetheless, a picture emerges of a heterogeneous lithosphere beneath both Ross and Amundsen Provinces. Previously published and new data reported here are consistent with samples ranging from variably cryptically metasomatized residua from variable (10–25%) degrees of partial melt extraction to refertilized compositions. Limited isotopic data point to a complex history, providing evidence for both ancient Proterozoic lithospheric mantle and preservation of Ordovician events. The Sr–Nd–Pb composition of the sampled lithospheric mantle overlaps the common low-µ isotopic endmember identified in Cenozoic magmatism from MBL and the wider West Antarctic Rift System.

Petrogenesis of Eocene shoshonitic rocks from South-Central British Columbia: Melting of Proterozoic lithospheric mantle

The Terrace Mountain complex is a part of a discontinuous north-west trending belt of Eocene volcanic centres, which stretches for about 1000 km in southern and central interior British Columbia and probably extends into the northwestern United States. The 55–45 Ma chain is related to east-vergent subduction of oceanic plates under the North American continent. The volcanic rocks of the belt are mainly of calc-alkaline intermediate to felsic composition, typical of distal continental arcs. However, close to the Canada-United States border, in the Terrace Mountain volcanic complex, there are alkaline (shoshonitic) mafic and intermediate rocks with an unusual isotopic composition compared to most other rocks of the belt. These rocks, which are hosted in an extensional basin/half-graben associated with regional strike-slip faulting in a continental arc setting, have highly negative ɛNd(t) values, Proterozoic neodymium depleted mantle model ages and elevated initial 87Sr/86Sr ratios. The two rock units, which are >700 m thick were emplaced within a short (<1–2 Ma) magmatic episode at ~52 Ma. The older volcanic pulse was composed of basaltic trachyandesites while the younger pulse included trachyandesites. The rocks are enriched in strongly incompatible trace elements and underwent fractional crystallization. Both suites are related to a parent magma, which was sourced during extension of the Cordilleran orogen by partial melting of heterogeneous subduction-modified sub-continental lithospheric mantle composed of amphibole-phlogopite-bearing peridotite. Nd isotopic characteristics require an old LREE-enriched slab in the mantle suggesting that the source was separated from convecting mantle since the Proterozoic.

Resistivity structures of western Victoria, Australia from 2D and 3D modelling of magnetotelluric data

SUMMARY A long period magnetotelluric (MT) survey, comprising 39 sites over an area of 270 by 150 km, has identified partial melt within the thinned lithosphere of Quaternary Newer Volcanics Province (NVP) in southeast Australia. MT inversion models reveal several important tectonic features and unravel critical information about the tectonics of the area. The models have imaged a conductive anomaly beneath the NVP at ~40-80 km depth, which is consistent with the presence of 1.5-4% partialmelt in the lithosphere. The conductive zone is located within thin juvenile oceanic lithospheric mantle, which was accreted onto thicker Proterozoic continental lithospheric mantle, suggesting that the NVP origin is due to decompression melting within the asthenosphere, promoted by lithospheric thickness variations in conjunction with rapid shear. In addition, inversion modelling shows that there is a conductivity contrast across the Moyston Fault that suggests the transition from Proterozoic continental lithospheric mantle under the Delamerian Orogen to the Phanerozoic lithospheric mantle under the Lachlan Orogen.

Erosion patterns and mantle sources of topographic change across the southern African Plateau derived from the shallow and deep records of kimberlites

AbstractFlow in the sublithospheric mantle is increasingly invoked as a mechanism to explain both modern and past surface topography, but the importance of this phenomenon and its influence at different localities are debated. Southern Africa is an elevated continental shield proposed to represent dynamically supported topography. However, this region is also characterized by a complex lithospheric architecture variably affected by Cretaceous heating, thinning, and metasomatic alteration. We used apatite (U‐Th)/He thermochronometry on 15 Cretaceous kimberlites from an ∼600 km long transect across the Kaapvaal Craton, combined with information from xenoliths in these pipes, to determine the plateau interior erosion history. The goal was to determine the relationships with lithospheric modification patterns and thereby better isolate the sublithospheric contribution to elevation. The results document a wave of erosion from west to east across the craton from ∼120 to &lt;60 Ma, initially focused along paleorivers and then retreating as a scarp across the landscape. This spatially variable erosion event was associated with limited modification of the Archean cratonic lithospheric mantle as recorded by mantle xenoliths and xenocrysts, implying that dynamic buoyancy sources may be required to explain the elevations. In contrast, off‐craton to the southwest, a more pronounced regional erosion phase at ∼110–90 Ma was coincident with significant modification of the Proterozoic lithospheric mantle. This relationship suggests that lithospheric processes were more important contributors to erosion and topographic change off‐craton than on‐craton. Together, these results suggest that lithospheric architecture can have an important control on the surface expression of mantle dynamics.

Trace element and Sr-Nd-Pb isotope geochemistry of Rungwe Volcanic Province, Tanzania: implications for a Superplume source for East Africa Rift magmatism

The recently discovered high, plume-like 3He/4He ratios at Rungwe Volcanic Province (RVP) in southern Tanzania, similar to those at the Main Ethiopian Rift in Ethiopia, strongly suggest that magmatism associated with continental rifting along the entire East African Rift System (EARS) has a deep mantle contribution (Hilton et al., 2011). New trace element and Sr-Nd-Pb isotopic data for high 3He/4He lavas and tephras from RVP can be explained by binary mixing relationships involving Early Proterozoic (+/- Archaean) lithospheric mantle, present beneath the southern EARS, and a volatile-rich carbonatitic plume with a limited range of compositions and best represented by recent Nyiragongo lavas from the Virunga Volcanic Province also in the Western Rift. Other lavas from the Western Rift and from the southern Kenya Rift can also be explained through mixing between the same endmember components. In contrast, lavas from the northern Kenya and Main Ethiopian rifts can be explained through variable mixing between the same mantle plume material and the Middle to Late Proterozoic lithospheric mantle, present beneath the northern EARS. Thus, we propose that the bulk of EARS magmatism is sourced from mixing among three endmember sources: Early Proterozoic (+/- Archaean) lithospheric mantle, Middle to Late Proterozoic lithospheric mantle and a volatile-rich carbonatitic plume with a limited range of compositions. We propose further that the African Superplume, a large, seismically anomalous feature originating in the lower mantle beneath southern Africa, influences magmatism throughout eastern Africa with magmatism at RVP and Main Ethiopian Rift representing two different heads of a single mantle plume source. This is consistent with a single mantle plume origin of the coupled He-Ne isotopic signatures of mantle-derived xenoliths and/or lavas from all segments of the EARS (Halldorsson et al., 2014).

SQUAW PEAK, ARIZONA: PALEOPROTEROZOIC PRECURSOR TO THE LARAMIDE PORPHYRY COPPER PROVINCE

Re-Os dating of two molybdenite samples from the Squaw Peak porphyry copper-molybdenum prospect in central Arizona returned essentially identical ages of 1,729 ± 7 and 1,738 ± 7 Ma. Therefore the prospect is not a component of the Laramide (Late Cretaceous-early Tertiary) porphyry copper province of southwestern North America as previously presumed. These Paleoproterozoic ages are similar to that of 1,740 ± 15 Ma for the I-type, magnetite-series Cherry batholith, within which the Squaw Peak porphyry stock and associated mineralization are located. Squaw Peak cannot be more than a few million years younger than volcanogenic massive sulfide (VMS) copper deposits in the nearby Jerome district, which are part of the Yavapai Supergroup, host to the Cherry batholith. These volcanic and intrusive rocks and their associated copper mineralization were formed in a juvenile island-arc setting and are now part of the Yavapai province, which was assembled and accreted to the Archean nucleus of North America by ~1.68 Ga. The Paleoproterozoic age for Squaw Peak in conjunction with the existence of the slightly older VMS deposits shows that the Laramide province of southwestern North America first developed its copper metallogenic signature >1,700 m.y. ago. The presence of this Paleoproterozoic copper mineralization may be taken as further support for recently proposed metasomatism of the mantle lithosphere during Paleoproterozoic subduction as a precursor to formation of at least part of the Laramide porphyry copper province. Similar spatial associations between inferred metal sources in Proterozoic mantle lithosphere and lowermost crust, relatively minor copper and/or molybdenum mineralization in Proterozoic magmatic arcs, and important post-Paleozoic porphyry copper and/or molybdenum provinces have recently been documented elsewhere, particularly in eastern China, and could prove to be of more general exploration significance.

Osmium isotope evidence for Early to Middle Proterozoic mantle lithosphere stabilization and concomitant production of juvenile crust in Dish Hill, CA peridotite xenoliths

The 187Os/188Os compositions in peridotite samples from the sub-continental lithospheric mantle (SCLM) can be used to constrain the timing of melt extraction and potentially test the link between large-scale mantle melting and juvenile crust production. The SCLM has often experienced a complex history such that some lithophile elements such as REEs (rare earth elements) in these rocks typically record overprinting during metasomatism. New 187Os/188Os, major and trace element compositional data were obtained on sixteen Dish Hill peridotite xenoliths (California, USA) and are used to examine these issues. The samples show strong correlations between 187Os/188Os and indicators of melt depletion such as Lu abundance in clinopyroxene, modal abundance of clinopyroxene, bulk rock Al2O3 and the Cr# (Cr/(Cr+Al)) in spinel. These relationships indicate that metasomatism did not compromise the 187Os/188Os systematics. The data appear to form two melt depletion trends consistent with Re depletion model ages (TRD) obtained from the two Al2O3 versus 187Os/188Os trends are 2.1±0.5Ga and 1.3±0.3Ga (±95% conf.).It has been suggested that the SCLM under Dish Hill may be fragments of oceanic lithosphere emplaced as the result of Farallon plate subduction during the Late Cretaceous (Luffi et al., 2009). However, the strong melt depletion trends, major element compositions and Re-depletion ages are not consistent with the interpretation of this suite of xenoliths having an oceanic lithospheric origin. Rather, the 2.1Ga age coincides with Nd model ages of 2–2.3Ga (Bennett and DePaolo, 1987; Rämö and Calzia, 1998) for the overlying Mojavia crustal province. The 1.3Ga age is consistent with large-scale A-type magmatism in the nearby region at this time that is purported to be the result of mantle plume melting processes. Therefore, data from this study point to the SCLM under Dish Hill being formed by two ancient mantle-melting events, which could be the result of interleaving SCLM at depth. These interpretations indicate that the primary mechanism of SCLM formation under Dish Hill was through stabilization following partial melting in the convecting mantle that also results in contemporaneous juvenile crust production.

Across-arc geochemical variations in the Southern Volcanic Zone, Chile (34.5–38.0°S): Constraints on mantle wedge and slab input compositions

Crustal assimilation (e.g. Hildreth and Moorbath, 1988) and/or subduction erosion (e.g. Stern, 1991; Kay et al., 2005) are believed to control the geochemical variations along the northern portion of the Chilean Southern Volcanic Zone. In order to evaluate these hypotheses, we present a comprehensive geochemical data set (major and trace elements and O–Sr–Nd–Hf–Pb isotopes) from Holocene primarily olivine-bearing volcanic rocks across the arc between 34.5°S and 38.0°S, including volcanic front centers from Tinguiririca to Callaqui, the rear arc centers of Infernillo Volcanic Field, Laguna del Maule and Copahue, and extending 300km into the backarc. We also present an equivalent data set for Chile trench sediments outboard of this profile. The volcanic arc (including volcanic front and rear arc) samples primarily range from basalt to andesite/trachyandesite, whereas the backarc rocks are low-silica alkali basalts and trachybasalts. All samples show some characteristic subduction zone trace element enrichments and depletions, but the backarc samples show the least. Backarc basalts have higher Ce/Pb, Nb/U, Nb/Zr, and Ta/Hf, and lower Ba/Nb and Ba/La, consistent with less of a slab-derived component in the backarc and, consequently, lower degrees of mantle melting. The mantle-like δ18O in olivine and plagioclase phenocrysts (volcanic arc=4.9–5.6‰ and backarc=5.0–5.4‰) and lack of correlation between δ18O and indices of differentiation and other isotope ratios, argue against significant crustal assimilation. Volcanic arc and backarc samples almost completely overlap in Sr and Nd isotopic composition. High precision (double-spike) Pb isotope ratios are tightly correlated, precluding significant assimilation of older sialic crust but indicating mixing between a South Atlantic Mid Ocean-Ridge Basalt (MORB) source and a slab component derived from subducted sediments and altered oceanic crust. Hf–Nd isotope ratios define separate linear arrays for the volcanic arc and backarc, neither of which trend toward subducting sediment, possibly reflecting a primarily asthenospheric mantle array for the volcanic arc and involvement of enriched Proterozoic lithospheric mantle in the backarc. We propose a quantitative mixing model between a mixed-source, slab-derived melt and a heterogeneous mantle beneath the volcanic arc. The model is consistent with local geodynamic parameters, assuming water-saturated conditions within the slab.

Microcontinents among the accretionary complexes of the Central Asia Orogenic Belt: In situ Re–Os evidence

Major elements and Re–Os isotope ratios were analysed in situ on individual sulfide grains in spinel peridotite xenoliths hosted by Quaternary intraplate basalts from the Tariat volcanic field, Central Mongolia. The sulfides are dominantly high-temperature (>900°C) Fe-rich monosulfide solid solution (MSS). Some sulfides with low Ni contents may be residual MSS, whereas other sulfides defining a negative Ni–Cu correlation may be crystallization products of fractionated sulfide melts. The subchondritic 187Re/188Os and 187Os/188Os of some sulfides also indicate they are residual MSS. Os isotope compositions of sulfides reveal the presence of Archean to Proterozoic lithospheric mantle beneath the region. The sulfides have TRD model ages ranging from 3.0 to 0.2Ga, with peaks at 1.5–1.3, 1 and 0.7–0.5Ga. The peak ages are indicative of significant events in the lithospheric mantle at those times. The timing of these events is remarkably consistent with those of the major crust-building events within the Tarvagatay Terrane where the Tariat volcanic field is located. The similarity in the ranges of crustal U–Pb ages and Nd model ages, and our sulfide Os model ages, suggests that the sulfide ages may date metasomatic events in the underlying lithospheric mantle, which were related to tectonothermal events that affected the overlying crust. Radiometric ages from the Tarvagatay Terrane appear to correspond to the Archean model ages from its SCLM counterpart. The last two events (1.1 and 0.7–0.5Ga) recorded in the Tarvagatay Terrane suggest involvement of the “CAOB mantle” and development of significant juvenile crustal growth in the orogeny.

Garnet pyroxenite, eclogite and alkremite xenoliths from the off-craton Gibeon Kimberlite Field, Namibia: A window into the upper mantle of the Rehoboth Terrane

Different types of mantle xenoliths, including garnet peridotite, garnet pyroxenite, eclogite, and alkremite, occur in kimberlites of the Gibeon Kimberlite Field (GKF), Namibia. The off-craton GKF is situated within the Palaeoproterozoic Rehoboth Terrane, about 400–500 km west of the inferred margin of the Archaean Kaapvaal Craton. Petrological data are available only for garnet peridotite; the other three rock types are described here for the first time. Both types of eclogite, Type I and II, are present in the GKF. The criteria to distinguish between the two types, i.e. Na 2O in garnet and K 2O in clinopyroxene, based on cratonic samples are less suited for off-craton eclogite xenoliths. As demonstrated in this study, Na 2O contents and Cr 2O 3/K 2O ratios in clinopyroxene are useful proxies for classifying off-craton eclogites. All GKF eclogites are bi-mineralic garnet-clinopyroxene rocks with rare accessory spinel and phlogopite. Minor rutile, amphibole and K-feldspar occur only in Type I eclogites. Alkremite is dominated by primary garnet and corundum and contains minor spinel and clinopyroxene of probably secondary origin. Garnet pyroxenite is websteritic and contains clinopyroxene, orthopyroxene and garnet. The garnet pyroxenite protolith was a high-pressure clinopyroxene-rich cumulate, in which garnet exsolution from Al-rich clinopyroxene took place by cooling below 1350 °C. Genetically linked with the garnet pyroxenite is Type II eclogite, which also formed by crystallization from a melt in the mantle. In contrast, the nature of Type I eclogite remains ambiguous. It may be of similar origin as Type II eclogite, or it may represent fragments of subducted oceanic crust. The equilibrium temperature of garnet pyroxenite of ca. 770 °C is consistently lower than that of eclogite (965–1190 °C). The pressure estimate for garnet pyroxenite (21–23 kbar) implies that the rocks formed and re-equilibrated at some 65–75 km depth. The inferred eclogite depth range is 90–130 km. A similar difference in temperature, and by implication also in depth, between pyroxenite and eclogite is known from other xenolith suites in southern Africa. These localities are more than 1000 km apart and extend from the Namibian GKF in the west to the Rietfontein and Kaalvallei kimberlites in South Africa and beyond to the area around the Premier diamond mine close to Pretoria. This implies a common thermal and lithological depth pattern for large parts of southern Africa, with a layer rich in garnet pyroxenite being underlain by an eclogite-rich layer. This supports a recent proposition ( Bell et al., 2003; Janney et al., 2010) that the thermal state and thickness of Proterozoic off-craton and Archaean cratonic lithosphere were largely similar prior to Mesozoic Gondwana break-up, which caused thermal erosion of the off-craton lithosphere with respect to the Kaapvaal Craton. In many cases, Proterozoic lithospheric mantle may represent reworked Archaean rather than juvenile Proterozoic material (e.g. Begg et al., 2009). If this also applies to the GKF mantle lithosphere has yet to be shown. The correlation inferred from this study may also be explained in terms of similar processes that formed and modified both Archaean and Proterozoic mantle lithosphere.

Secular evolution of the lithospheric mantle beneath the eastern North China craton: evidence from peridotitic xenoliths from Late Cretaceous mafic rocks in the Jiaodong region, east-central China

The Mesozoic lithospheric mantle beneath the North China craton remains poorly constrained relative to its Palaeozoic and Cenozoic counterparts due to a lack of mantle xenoliths in volcanic rocks. Available data show that the Mesozoic lithospheric mantle was distinctive in terms of its major, trace element, and isotopic compositions. The recent discovery of mantle peridotitic xenoliths in Late Cretaceous mafic rocks in the Jiaodong region provides an opportunity to further quantify the nature and secular evolution of the Mesozoic lithospheric mantle beneath the region. These peridotitic xenoliths are all spinel-facies nodules and two groups, high-Mg# and low-Mg# types, can be distinguished based on textural and mineralogical features. High-Mg# peridotites have inequigranular textures, high Mg# (up to 92.2) in olivines, and high Cr# (up to 55) in spinels. Clinopyroxenes in the high-Mg# peridotites are generally LREE-enriched ((La/Yb)N>1) with variable REE concentrations, and have enriched Sr–Nd isotopic compositions (87Sr/86Sr = 0.7046–0.7087; 143Nd/144Nd = 0.5121–0.5126). We suggest that the high-Mg# peridotites are fragments of the Archaean and/or Proterozoic lithospheric mantle that underwent extensive interaction with both carbonatitic and silicate melts prior to or during Mesozoic time. The low-Mg# peridotites are equigranular, are typified by low Mg# ( < 90) in olivines, and by low Cr# ( < 12) in spinels. Clinopyroxenes from low-Mg# peridotites have low REE abundances (ΣREE = 12 ppm), LREE-depleted REE patterns ((La/Yb)N < 1), and depleted Sr–Nd isotopic features, in contrast to the high-Mg# peridotites. These geochemical characteristics suggest that the low-Mg# peridotites represent samples from the newly accreted lithospheric mantle. Combined with the data of mantle xenoliths from the Junan and Daxizhuang areas, a highly heterogeneous, secular evolution of the lithosphere is inferred for the region in Late Cretaceous time.

Hf–Nd isotopic decoupling in continental mantle lithosphere beneath Northeast China: Effects of pervasive mantle metasomatism

Nd–Hf isotopic decoupling has frequently been observed in the continental and oceanic mantle, but its origin remains controversial. Here we present combined elemental and Sr–Nd–Hf isotopic study on peridotite xenoliths entrained in Cenozoic basalts from Shuangliao and Jiaohe in Northeast China, which provides insight into this issue. The data reveal a heterogeneous lithospheric mantle beneath Northeastern China, consisting of fertile (type I) to strongly refractory (type II) peridotites. Type I peridotites are largely shielded from late metasomatism, thus preserving information of depletion events. Nd model age suggests a Proterozoic lithospheric mantle beneath NE China. Type II peridotites are mostly refractory harzburgites and show ubiquitous enrichment of incompatible elements. They are further divided into two sub-groups. Clinopyroxenes from type IIa samples have high and wide Lu/Hf (0.34–1.3) and very radiogenic Hf isotopic ratios ( ε Hf = 44.4–63.8). Hf concentration is generally low (0.12–0.43 ppm) and plots along or slightly above the modeled partial melting depletion trend. In contrast, Nd content in type IIa clinopyroxenes is significantly higher than the modeled concentrations in residues at a given degree of melt depletion. The difference in enrichment of Hf and Nd translates to decoupling of Lu/Hf–Sm/Nd ratios and of Nd–Hf isotopes ( ε Nd = −1.3 to 8.4). Clinopyroxenes from most of type IIb peridotites have relatively low Lu/Hf ratios (0.04–0.24) and coupled Nd–Hf isotopes. Both Hf and Nd plot significantly above the depletion trend; their concentrations are governed by the equilibrium partitioning between percolating melt and peridotites. The distinct geochemical characteristics of type IIa and type IIb clinopyroxenes may have resulted from chromatographic percolation of small volumes of silicate melts, in which percolation fronts of incompatible elements are dependent on their relative incompatibilities.

Steps in lithospheric thickness within eastern Australia, evidence from surface wave tomography

A series of steps in the lithospheric thickness of eastern Australia are revealed by the latest seismic surface wave tomographic model and calculations of the horizontal gradient of shear wave speed. The new images incorporate data from the recent Tasmal experiment, improving resolution in continental Australia. Through comparisons with surface geology and geochemical studies, it is possible to infer that the steps in lithospheric thickness are related to boundaries between blocks of different age. The westernmost boundary marks the edge of the Archaean to Early‐Proterozoic core of the continent. A second lithospheric boundary is observed in the central part of east Australia. To the west of this line, geochemical evidence suggests that there is Proterozoic lithospheric mantle, and this boundary may therefore represent the change from Proterozoic to Phanerozoic basement. The structure on the eastern margin of the continent is dominated by slow velocities, suggesting that in this area the continental lithosphere is very thin. There is a strong correlation between the slow wave speeds and the location of both the highest topography and recent volcanic activity. Inland of the continental margin, a zone of strong gradients in the seismic wave speed is observed, indicating a distinct step in lithospheric structure. If the step in lithospheric thickness was in place prior to volcanism, it may have acted as a boundary, with volcanism mainly occurring beneath the thinner lithosphere to the east.

Mantle structure beneath the western edge of the Colorado Plateau

Teleseismic traveltime data are inverted for mantle Vp and Vs variations beneath a 1400 km long line of broadband seismometers extending from eastern New Mexico to western Utah. The model spans 600 km beneath the moho with resolution of ∼50 km. Inversions show a sharp, large‐magnitude velocity contrast across the Colorado Plateau‐Great Basin transition extending ∼200 km below the crust. Also imaged is a fast anomaly 300 to 600 km beneath the NW portion of the array. Very slow velocities beneath the Great Basin imply partial melting and/or anomalously wet mantle. We propose that the sharp contrast in mantle velocities across the western edge of the Plateau corresponds to differential lithospheric modification, during and following Farallon subduction, across a boundary defining the western extent of unmodified Proterozoic mantle lithosphere. The deep fast anomaly corresponds to thickened Farallon plate or detached continental lithosphere at transition zone depths.

Mediterranean Tertiary lamproites derived from multiple source components in postcollisional geodynamics

In the Mediterranean area, lamproitic provinces in Spain, Italy, Serbia and Macedonia have uniform geological, geochemical and petrographic characteristics. Mediterranean lamproites are SiO 2-rich lamproites, characterized by relatively low CaO, Al 2O 3 and Na 2O, and high K 2O/Al 2O 3 and Mg-number. They are enriched in LILE relative to HFSE and in Pb, and show depletion in Ti, Nb and Ta. Mediterranean lamproites show huge regional variation of Sr, Nd and 207Pb/ 204Pb isotopic values, with 87Sr/ 86Sr range of 0.707–0.722, εNd range from −13 to −3, and 207Pb/ 204Pb range of 15.62–15.79. Lamproitic rocks are derived from melts with three components involved in their origin, characterized by contrasting geochemical features which appear in 206Pb/ 204Pb, 87Sr/ 86Sr and 143Nd/ 144Nd space: (i) a mantle source contaminated by crustal material, giving rise to crust-like trace element patterns and radiogenic isotope systematics, (ii) an extremely depleted mantle characterized by very low whole-rock CaO and Al 2O 3, high-Fo olivine and Cr-rich spinel, which isotopically resembles European peridotitic massifs and lithospheric mantle; (iii) a component originating from the convecting mantle, characterized by unradiogenic 87Sr/ 86Sr and radiogenic 143Nd/ 144Nd and 206Pb/ 204Pb. These components demand multistage preconditioning of the lamproite-mantle source, involving an episode of extreme depletion, followed by involvement of terrigenous sediments, and finally interaction with melts originating from the convecting mantle, some of which are probably carbonatitic. We use our data on Mediterranean lamproites to characterize the mantle composition under the whole Alpine-Himalaya belt. Lamproites are an integral part of postcollisional volcanism, and are the most extreme melting products from a mantle which is ubiquitously crustally metasomatized. Enriched isotope signatures in Himalayan volcanics can also be explained by the involvement of subducted sediments instead of by proterozoic mantle lithosphere.

Late Archean to Early Proterozoic lithospheric mantle beneath the western North China craton: Sr–Nd–Os isotopes of peridotite xenoliths from Yangyuan and Fansi

Sr–Nd–Os isotopic analyses are presented for peridotite xenoliths from Tertiary alkali basalts in Yangyuan and Fansi with the aim of identifying and characterizing the relics of ancient lithospheric mantle that survived lithospheric removal in the western North China Craton (NCC). The analyzed samples are residual lherzolites and harzburgites, ranging from fertile to highly depleted (0.36–4.0 wt% Al 2O 3) composition. Some LREE-enriched samples are characterized by moderate 87Sr/ 86Sr (0.7044 to 0.7047) and low ε Nd (− 6.9 to − 10.6), pointing to an EMI-type signature. This is distinct from the predominant depleted isotopic composition in mantle xenoliths from eastern China. Os isotopic ratios range from 0.1106 to 0.1325. The lower limit is the most unradiogenic value measured so far for Cenozoic basalt-borne xenoliths from eastern China. Two samples show radiogenic Os ratios higher than that of the primitive upper mantle, one sample has an anomalously high Os concentration (> 9 ppb). These samples also show high La/Yb, consistent with the addition of radiogenic components during the infiltration of asthenosphere-derived and/or subduction-related melts in the lithospheric mantle. The remaining samples define positive correlations between 187Os/ 188Os and indices of melt extraction, which yield a model age of ∼ 2.6 Ga. This age of melt extraction overlaps with the Nd model age of the overlying crust, indicating a coupled crust–mantle system in the western NCC. This contrasts with the decoupled nature in the eastern NCC, suggesting distinct mantle domains underneath the NCC. Such a heterogeneous age structure of the upper mantle is compatible with the view that the lithospheric removal was largely limited to the eastern NCC.

Shear wave splitting as a diagnostic of variable anisotropic structure of the upper mantle beneath central Fennoscandia

We analyze splitting of shear waves recorded during the SVEKALAPKO passive seismic experiment in south-central Finland to study fabrics of the mantle lithosphere of the Precambrian region and thus to bring information into a debate on existence of plate tectonics or its forms in the early stage of continent formation. Geographical variations of the splitting parameters and their distinct dependence on direction of wave propagation through the upper mantle allow us to identify six domains of the central Fennoscandian mantle lithosphere, including the Proterozoic–Archean transition, and to model their fabrics by joint inversion of body wave anisotropic parameters. Fabrics of the Archean mantle lithosphere can be approximated by a peridotite aggregate with lineation a dipping to the NE. On the other hand, anisotropy of the Proterozoic mantle lithosphere is weaker and we model its fabric by the (a, c) foliations dipping to the SE. We present a 3D self-consistent anisotropic model of the Proterozoic and Archean upper mantle along the SW-NE profile in the south-central Finland. Boundaries of inter-growing wedges of the Proterozoic and Archean mantle lithospheres explain the longitudinal and shear wave propagation and polarization, mantle xenolith ages, surface wave tomography and location of the upper mantle reflectors. We interpret the six anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric which was created before these micro-continents assembled.

Evolution of the Lithospheric Mantle beneath the Marsabit Volcanic Field (Northern Kenya): Constraints from Textural, P–T and Geochemical Studies on Xenoliths

Xenoliths hosted by Quaternary basanites and alkali basalts from Marsabit (northern Kenya) represent fragments of Proterozoic lithospheric mantle thinned and chemically modified during rifting in the Mesozoic (Anza Graben) and in the Tertiary–Quaternary (Kenya rift). Four types of peridotite xenoliths were investigated to constrain the thermal and chemical evolution of the lithospheric mantle. Group I, III and IV peridotites provide evidence of a cold, highly deformed and heterogeneous upper mantle. Textures, thermobarometry and trace element characteristics of minerals indicate that low temperatures in the spinel stability field (� 750– 800 � Ca t<1.5 GPa) were attained by decompression and cooling from initially high pressures and temperatures in the garnet stability field (970–1080 � Ca t 2.3–2.9 GPa). Cooling, decompression and penetrative deformation are consistent with lithospheric thinning, probably related to the development of the Mesozoic to Paleogene Anza Graben. Re-equilibrated and recrystallized peridotite xenoliths (Group II) record heating (from � 800 � Ct o � 1100 � C). Mineral trace element signatures indicate enrichment by mafic silicate melts, parental to the Quaternary host basanites and alkali basalts. Relationships between mineral textures, P–T conditions of equilibration, and geochemistry can be explained by metasomatism and heating of the lithosphere related to the formation of the Kenya rift, above a zone of hot upwelling mantle.

Nature and evolution of Late Cretaceous lithospheric mantle beneath the eastern North China Craton: Constraints from petrology and geochemistry of peridotitic xenoliths from Jünan, Shandong Province, China

Mantle xenoliths entrained in a newly discovered Late Cretaceous basaltic breccia at Jünan, Shandong Province, China provide important constraints on the nature and evolution of the Late Mesozoic lithospheric mantle beneath the eastern North China Craton. The basaltic breccia erupted at 67 Ma contains abundant mantle and lower crustal xenoliths, including peridotites, pyroxenites, and granulites. The peridotitic xenoliths are all spinel-facies lherzolites and two types of lherzolites, high-Mg # and low-Mg #, occur. The low-Mg # lherzolites are dominant peridotitic xenoliths and have coarse-grained texture. These lherzolites are characterized by the occurrence of low Mg # (Fo < 90) olivine, orthopyroxene, and clinopyroxene and the LREE-depleted REE patterns ((La/Yb) N = 0.3–0.8) in cpxs. These mineral features and estimated equilibration temperatures (950–1040 °C) are very similar to those of lherzolite xenoliths entrained in the widespread Cenozoic basalts, eastern China. This similarity suggests that the low-Mg # spinel lherzolites represent the newly accreted lithospheric mantle. The high-Mg # lherzolites are very few and typified with high Mg # olivine (Fo = 92), high Cr # spinel, and convex-upward cpx REE pattern ((La/Yb) N = 1.5 and (La/Nd) N = 0.2). Its refractory geochemical features are completely different from those of low-Mg # lherzolites, but similar to those xenoliths from Archean and Proterozoic lithospheric mantle on the craton, and thus represent the relic of old refractory lithospheric mantle. The convex-upward cpx REE pattern implies an involvement of melt/rock interaction in its origin. No obvious differences in the equilibrium temperature of low-and high-Mg # peridotites suggest that the newly accreted mantle was close contact with the overlying old lithospheric mantle at the time of basaltic eruption.

Geochemistry of South African On- and Off-craton, Group I and Group II Kimberlites: Petrogenesis and Source Region Evolution

Bulk-rock geochemical compositions of hypabyssal kimberlites, emplaced through the Archaean Kaapvaal craton and Proterozoic Namaqua–Natal belt, are used to estimate close-to-primary magma compositions of Group I kimberlites (Mg-number 1⁄4 0 82–0 87; 22–28 wt % MgO; 21–30 wt % SiO2; 10–17 wt % CaO; 0 2–1 7 wt % K2O) and Group II kimberlites (Mgnumber 1⁄4 0 86–0 89; 23–29 wt % MgO; 28–36 wt % SiO2; 8–13 wt % CaO; 1 6–4 6 wt % K2O). Group I kimberlites are distinguished from Group II by their lower Ba/Nb ( 22) ratios. The distinct rare earth element patterns of the two types of kimberlites indicate a more highly metasomatized source for Group II kimberlites, with more residual clinopyroxene and less residual garnet. The similarity of Sr and Nd isotope ratios and diagnostic trace element ratios (Ce/Pb, Nb/U, La/Nb, Ba/Nb, Th/Nb) of Group I kimberlites to ocean island basalts (OIB), but more refractory Mg-numbers and Ni contents, are consistent with derivation of Group I kimberlites from subcontinental lithospheric mantle (SCLM) that has been enriched by OIB-like melts or fluids. Source enrichment ages and plate reconstructions support a direct association of these melts or fluids with Mesozoic upwelling beneath southern Africa of a mantle plume(s), at present located beneath the southern South Atlantic Ocean. In contrast, the geochemical characteristics of both onand off-craton Group II kimberlites show strong similarity to calc-alkaline magmas, particularly in their Nb and Ta depletion and Pb enrichment. It is suggested that Group II kimberlites are derived from both Archaean and Proterozoic lithospheric mantle source regions metasomatized by melts or fluids associated with ancient subduction events, unrelated to mantle plume upwelling. The upwelling of mantle plumes beneath southern Africa during the Mesozoic, at the time of Gondwana break-up, may have acted as a heat source for partial melting of the SCLM and the generation of both Group I and Group II kimberlite magmas.