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Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ∼90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880–970 oC), and low-Mg# (70–80) plagioclase-bearing websterites with low equilibration temperatures (550–835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr–Nd (εNd = +11·41, 87Sr/86Sr = ∼0·7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ∼0·703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g. FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr–Nd isotopic ratios (εNd = -14·20– -16·74, 87Sr/86Sr = 0·7070–0·7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr = 0·706–0·711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian Ocean and the Late Mesozoic lithospheric extension of eastern Asia. [ABSTRACT FROM AUTHOR]

Igneous activity in the rear-arc of the Paleogene Urumieh–Dokhtar Magmatic Belt of Iran has to date been poorly studied. An example of such activity, Late Eocene potassic mafic to intermediate magmatic rocks in the Lahrud area of NW Iran, is the focus of this work. These lavas include olivine-bearing clinopyroxene-phyric basalts, analcime-bearing leucite–clinopyroxene-phyric basalts, andesites, and trachytes, and Paleocene–Early Eocene pyroclastic rocks. Monzo-syenite plugs (dated here at ∼37 Ma), clinopyroxene-phyric basaltic dikes, and leucite-bearing clinopyroxene-phyric basaltic dikes intrude older lavas and pyroclastic rocks. Olivine-bearing clinopyroxene-phyric basalts and analcime-bearing leucite–clinopyroxene-phyric basalts are characterized by large phenocrysts of olivine, clinopyroxene, leucite, and analcime. Clinopyroxene-rich enclaves and partially resorbed mantle xenoliths also occur. Olivine phenocrysts are zoned from high-Mg# cores (Mg# = 90) to Fe-rich rims (Mg# = 66). Clinopyroxene phenocrysts from the olivine-bearing clinopyroxene-phyric basalts, analcime-bearing leucite–clinopyroxene-phyric basalts and clinopyroxene crystals in the enclaves show complex oscillatory zoning, sieve textures, and resorption textures, but with systematic core–rim compositional trends. Their 87Sr/86Sr isotopic compositions measured in situ range from 0·7037 to 0·7068 (mean = 0·7052 ± 0·0004), suggesting negligible crustal assimilation during fractional crystallization. The Lahrud lavas are potassic and are enriched in light rare earth elements and large ion lithophile elements such as Th, Rb, K and U. High field strength elements (HFSE), such as Nb, are depleted in the olivine-bearing clinopyroxene-phyric basalts and analcime-bearing leucite–clinopyroxene-phyric basalts, but enriched in the trachytes and trachytic ignimbrites. The isotopic compositions vary: 87Sr/86Srt from 0·7045 to 0·7052, εNd(t) from +2·8 to +3·3, and εHf(t) from +7·2 to +7·7. The rocks have radiogenic lead 206Pb/204Pb from 18·66 to 18·76, 207Pb/204Pb from 15·58 to 15·62, and 208Pb/204Pb from 38·73 to 38·81. Modeling of major and trace elements using the MELTS algorithm indicates that the geochemical variations in the basaltic to andesitic rocks are reasonably explained by shallow fractional crystallization with some complications owing to source heterogeneity, crustal assimilation, and magma mixing. The isotopic data imply that partial melting of old sub-continental lithospheric mantle was not responsible for the Lahrud potassic magmas; Hf isotopes and Zr/Nb ratios suggest derivation from an enriched mantle wedge, whereas ratios of incompatible trace elements (e.g. La/Yb, Ba/La, Ce/Pb, Th/Yb) and high 87Sr/86Sr suggest mantle metasomatized by slab-derived fluids or melts dominated by a sediment component. Geochemical modeling using the Arc Basalt Simulator version 5 reveals that the HFSE-depleted, olivine-bearing, clinopyroxene-phyric basalts originated from a high-temperature mantle wedge (2·2 GPa, 1310°C) fluxed intensively (5%) by melts from a deep hot slab (6 GPa, 1000°C). The moderately HFSE-depleted, olivine-bearing, clinopyroxene-phyric basalts reflect melting of a lower-temperature mantle wedge (2·2 GPa, 1300°C) with a lesser amount of slab melt flux (4%) from a lower temperature and shallower slab (3 GPa, 866°C). In contrast, the leucite–clinopyroxene-phyric basalts and andesites are from a similar source to the moderately HFSE-depleted, olivine-bearing, clinopyroxene-phyric basalts (3 GPa, 866°C) but with a contribution from a lower temperature mantle wedge (2·2 GPa, 1270°C). During Late Eocene times, slab retreat and upper-plate extension occurred in the rear-arc region of the Urumieh–Dokhtar Magmatic Belt. The Lahrud potassic magmas were generated from a high-temperature mantle wedge, which resulted in melting of the slab; this slab melt flux further promoted melting of the mantle wedge. [ABSTRACT FROM AUTHOR]

The Cadomian orogen of NW Iran includes a series of metamorphic rocks with zircon U-Pb ages between ca 562 and 505 Ma (Ediacaran to middle Cambrian). The Ediacaran-Cambrian basement is intruded by a series of Late Eocene-Late Oligocene I-type granitic rocks. U-Pb geochronology, integrated with geochemical and isotopic data for the basement rocks in NW Iran, provides further evidence of a Cadomian (562-505 Ma) arc-related magmatic event lasting ∼ 60 Myr. Cadomian magmatism in Iran was a part of a ∼ 100 Myr long episode of subduction-related arc magmatism at the northern margin of Gondwana. Zircon Hf-isotope compositions show that during Cadomian magmatic arc activity, juvenile arc magmas interacted with reworked Archean crust to generate the Ediacaran-Cambrian igneous rocks. Our results document both inheritance of old zircons and the presence of zircons with juvenile signatures in NW Iran, suggesting that the geotectonic setting for the Cadomian rocks was an Ediacaran continental magmatic arc and probably a neighboring back-arc basin. The occurrence of Ediacaran ophiolitic slices in NW Iran may provide evidence of back-arc basin opening at that time. Cenozoic plutonism in NW Iran is part of an Eocene-Oligocene magmatic 'flare-up' along the Urumieh-Dokhtar Magmatic Belt in central Iran, which lasted for ca 30 Myr. The melts responsible for the formation of these rocks had an essentially juvenile signature with minor contamination by Archean to Cadomian middle-lower continental crust. Continuous convergence between Arabia and Iran was accompanied by the transition of SW Eurasia from a compressional to an extensional convergent plate margin in Eocene-Oligocene times, leading to orogenic collapse, core-complex formation, exhumation of Cadomian crust and a major increase in arc magmatism. [ABSTRACT FROM AUTHOR]

Despite a relatively ‘uniform’ fertile composition (Al2O3 = 2·19–4·47 wt %; Fo% = 89·2 ± 0·3%; Cr#Spl = 8·9 ± 1·5%), the Montferrier peridotite xenoliths show a wide range of S contents (22–590 ppm). Most sulphides are interstitial and show peculiar pyrrhotite–pentlandite intergrowths and low abundances of Cu-rich phases. Sulphide-rich samples are characterized by strong enrichment in the light rare earth elements and large ion lithophile elements without concomitant enrichment of the high field strength elements. Such trace-element fractionation is commonly ascribed to metasomatism by volatile-rich melts and/or carbonatitic melts. S and Se (11–67 ppb), as well as S/Se (up to ≈12 000), are correlated with La/Sm. Cu, however, remains broadly constant (30 ± 5 ppm). These features strongly suggest that the percolation–reaction of such volatile-rich fluids has led to sulphide enrichment with an atypical signature marked by strong fractionation of the chalcophile elements (i.e. S vs Se and Cu). S-rich xenoliths are also characterized by high (Pd/Ir)N (1·2–1·9; where subscript N indicates normalized to chondrite), (Pd/Pt)N between 1·5 and 2·2, and (Os/Ir)N up to 1·85. Despite the relative uniform fertile composition of the xenoliths, Re/Os ranges between 0·02 and 0·18. 187Os/188Os is extremely variable even within a single sample and can be as high as 0·1756 for the most S-rich samples. Sulphides show highly fractionated and variable abundances of the highly siderophile elements (HSE) [0·03 &leq; (Pd/Ir)N < 1283] and Re–Os isotopic composition (0·115 < 187Os/188Os < 0·172). Such variation can be observed at the thin-section scale. Whole-rock and in situ sulphide data demonstrate that chalcophile and HSE systematics and the Os isotopic composition of the upper mantle could be significantly modified through metasomatism, even with volatile-rich fluids. These features highlight the complex behaviour of the HSE in fluid–rock percolation–reaction models and suggest a complex interplay between sulphide addition (crystallization or sulphidation) and partial equilibration of pre-existing sulphide. The specific fractionations observed in chemical proxies such as S and Se, Os and Ir, and Pd and Pt, as well as the low abundance of Cu-rich sulphides, suggest that sulphide addition may not have occurred via sulphide melts. Rather, we infer that S was present as a dissolved species in a (supercritical) oxidizing, volatile-rich fluid (C–O–H–S ± Cl) along with other chalcophile and siderophile elements such as Os, Pd, Re and Au. The highly radiogenic Os composition of this fluid ( 187Os/ 188Os > 0·17) would imply that such fluids are derived from an uncommon type of mantle source possibly related to carbonatite melts. [ABSTRACT FROM AUTHOR]

Apatites crystallized from different types of igneous rocks show significant variations in the abundances of some minor and trace elements. In this study, electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry were used to determine the concentrations of 25 minor and trace elements in apatite separated from three principal rock types of the Transhimalayan igneous plutonic suite: S-type granites, the I-type Gangdese batholith and post-collisional adakites. F, Mn, Sr and rare earth elements (REE) in apatite vary systematically with the composition of the host magma and thus have high potential as petrogenetic tracers. More specifically, the F and Mn contents of apatite can be used as an indicator of magma aluminosity or differentiation index. Combined with Sr and REE data, which show significant variations in apatite from different rock types, these elements are useful for constructing ‘discrimination diagrams’. This study also reveals that apatite has the capacity to retain geochemical information about the host magma through the course of magmatic evolution. Systematic variations of Sr and REE in apatite with bulk-rock aluminosity are the results of partition competition with pre-existing and coexisting major and accessory minerals in silicate melts, and thus are useful for more detailed investigations of petrogenetic processes such as fractional crystallization and magma mixing, which is signaled by inconsistent Eu anomalies, Sr abundances and REE patterns relative to bulk-rock compositions. [ABSTRACT FROM PUBLISHER]

Major- and trace-element data on the constituent minerals of garnet peridotite xenoliths hosted in early Paleozoic (457–500 Ma) kimberlites and Neogene (16–18 Ma) volcanic rocks within the North China Craton are compared with those from the pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSD-PP1) in the tectonically exhumed Triassic (∼220 Ma) Sulu ultrahigh-pressure (UHP) terrane along its southern margin. P–T estimates for the Paleozoic and Neogene peridotite xenoliths reflect different model geotherms corresponding to surface heat flows of ∼40 mW/m2 (Paleozoic) and ∼80 mW/m2 (Neogene). Garnet peridotite xenoliths or xenocrysts from the Paleozoic kimberlites are strongly depleted, similar to peridotites from other areas of cratonic mantle, with magnesium olivine (mean Fo92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet (and spinel) peridotite xenoliths hosted in Neogene basalts are derived from fertile mantle; they have high Al2O3 and TiO2 contents, low-Mg-number olivine (mean Fo89.5), low-Cr garnet and diopside with flat rare earth element (REE) patterns. The differences between the Paleozoic and Neogene xenoliths suggest that a buoyant refractory lithospheric keel present beneath the eastern North China Craton in Paleozoic times was at least partly replaced by younger, hotter and more fertile lithospheric mantle during Mesozoic–Cenozoic times. Garnet peridotites from the Sulu UHP terrane have less magnesian olivine (Fo91.5), and lower-Cr garnet than the Paleozoic xenoliths. The diopsides have low heavy REE (HREE) contents and sinusoidal to light REE (LREE)-enriched REE patterns. These features, and their high Mg/Si and low CaO and Al2O3 contents, indicate that the CCSD-PP1 peridotites represent a moderately refractory mantle protolith. Details of mineral chemistry indicate that this protolith experienced complex metasomatism by asthenosphere-derived melts or fluids in Mesoproterozoic, and subsolidus re-equilibration involving fluids/melts derived from the subducted Yangtze continental crust during UHP metamorphism in the early Mesozoic. Tectonic extension of the subcontinental lithospheric mantle of the North China Craton and exhumation of the Sulu UHP rocks in the early Mesozoic induced upwelling of the asthenosphere. Peridotites sampled by the Neogene basalts represent newly formed lithosphere derived by cooling of the upwelling asthenospheric mantle in Jurassic–Cretaceous and Paleogene time. [ABSTRACT FROM PUBLISHER]

This study characterizes the nature of fluid interaction and melting processes in the lithospheric mantle beneath the Yingfengling and Tianyang volcanoes, Leizhou Peninsula, South China, using in situ trace-element analyses of clinopyroxene, amphibole and garnet from a suite of mantle-derived xenoliths. Clinopyroxenes from discrete spinel lherzolites exhibit large compositional variations ranging from extremely light rare earth element (LREE)-depleted to LREE-enriched. Trace-element modelling for depleted samples indicates that the Leizhou lherzolites are the residues of a mantle peridotite source after extraction of ∼1–11% melt generated by incremental melting in the spinel lherzolite field with the degree of melting increasing upwards from about 60 km to 30 km. This process is consistent with gradational melting at different depths in an upwelling asthenospheric column that subsequently cooled to form the current lithospheric mantle in this region. The calculated melt production rate of this column could generate mafic crust 5–6 km thick, which would account for most of the present-day lower crust. The formation of the lithospheric column is inferred to be related to Mesozoic lithosphere thinning. Al-augite pyroxenites occur in composite xenoliths; these are geochemically similar to HIMU-type ocean island basalt. These pyroxenites postdate the lithospheric column formation and belong to two episodes of magmatism. Early magmatism (forming metapyroxenites) is inferred to have occurred during the opening of the South China Sea Basin (32–15 Ma), whereas the most recent magmatic episode (producing pyroxenites with igneous microstructures) occurred shortly before the eruption of the host magmas (6–0·3 Ma). Trace-element traverses from the contacts of the Al-augite pyroxenite with the spinel peridotite wall-rock in composite xenoliths record gradients in the strength and nature of metasomatic effects away from the contact, showing that equilibrium was not attained. Significant enrichment in highly incompatible elements close to the contacts, with only slight enrichment in Sr, LREE and Nb away from the contact, is inferred to reflect the different diffusion rates of specific trace elements. The observed geochemical gradients in metasomatic zones show that Sr, La, Ce and Nb have the highest diffusion rates, other REE are intermediate, and Zr, Hf and Ti have the lowest diffusion rates. Lower diffusion rates observed for Nb, Zr, Hf and Ti compared with REE may cause high field strength element (HFSE) negative anomalies in metasomatized peridotites. Therefore, metasomatized lherzolites with HFSE negative anomalies do not necessarily require a carbonatitic metasomatizing agent. [ABSTRACT FROM AUTHOR]

In situ laser ablation inductively coupled plasma mass spectrometry analysis of trace elements, U–Pb ages and Hf isotopic compositions of magmatic zircon from I- and S-type granitoids from the Lachlan Fold Belt (Berridale adamellite and Kosciusko tonalite) and New England Fold Belt (Dundee rhyodacite ignimbrite), Eastern Australia, is combined with detailed studies of crystal morphology to model petrogenetic processes. The presented examples demonstrate that changes in zircon morphology, within single grains and between populations, generally correlate with changes in trace element and Hf-isotope signatures, reflecting the mixing of magmas and changes in the composition of the magma through mingling processes and progressive crystallization. The zircon data show that the I-type Kosciusko tonalite was derived from a single source of crustal origin, whereas the S-type Berridale adamellite had two distinct sources including a significant I-type magma contribution. Complex morphology and Hf isotope variations in zircon grains indicate a moderate contribution from a crustal component in the genesis of the I-type Dundee rhyodacite. The integration of data on morphology, trace elements and Hf isotope variations in zircon populations provides a tool for the detailed analysis of the evolution of individual igneous rocks; it offers new insights into the contributions of different source rocks and the importance of magma mixing in granite petrogenesis. Such information is rarely obtainable from the analysis of bulk rocks. [ABSTRACT FROM AUTHOR]

Taiwan is an active mountain belt created by the oblique collision of the northern Luzon arc with Asia. Late Pliocene extensional collapse of the northern Taiwan mountain belt (NTMB) was accompanied by magmatism that formed the Northern Taiwan Volcanic Zone (NTVZ; 2·8–0·2 Ma). The geochemical characteristics of the NTVZ magmas can thus provide constraints both for the mantle source composition and the geodynamic processes operating in the late orogenic stage of the region. The NTVZ volcanic rocks consist dominantly of calc-alkaline andesites and basalts, along with subordinate but heterogeneous lavas including low-K, shoshonitic and ultrapotassic magmas. From the NE to the SW in the NTVZ, the magmas show systematic compositional variations from low-K to calc-alkaline and then shoshonitic. This spatial geochemical variation, characterized by southwesterly increase in potassium and incompatible trace elements, appears to be subparallel to the southwestern part of the modern Ryukyu subduction system. Sr–Nd isotope ratios of the NTVZ volcanic rocks (87Sr/86Sr ≈ 0·70376–0·70551; 143Nd/144Nd ≈ 0·51259–0·51301) suggest that two mantle source components are involved in the magma generation, the asthenosphere and metasomatized subcontinental lithospheric mantle. These two components are represented by the 2·6 Ma Mienhuayu high-Mg basaltic andesites and the 0·2 Ma Tsaolingshan high-Mg potassic lavas, respectively. The latter are interpreted to be the products of small-degree melting of a phlogopite-bearing, harzburgite lithospheric mantle source metasomatized recently by the nearby Ryukyu subduction zone processes. The Sr–Nd–Pb isotope systematics and specific trace-element ratios of the NTVZ volcanic rocks suggest that melts derived from subducted sediments and fluids released from slab dehydration reactions were both involved in metasomatizing this mantle source. The unique spatial geochemical variation of the NTVZ volcanic rocks can be successfully modelled using variable degrees of partial melting of the mantle source regions, coupled with mixing of different melt components from depleted asthenospheric and metasomatized lithospheric mantle components beneath individual volcanic fields. It is inferred that mixing of melts from specific mantle components and the degree of partial melting are spatially and temporally related to the tectonic evolution of the northern Taiwan region, and not simply due directly to subduction zone processes. The overall NTVZ geochemical characteristics can be explained by various degrees of melting within an ascending region of the asthenospheric mantle, triggered by extensional collapse of the NTMB, and interaction of these melts with overlying fluid- and sediment-modified lithospheric mantle. [ABSTRACT FROM AUTHOR]

Radiogenic isotopic (Sr–Nd–Pb) and trace-element compositions of late Cenozoic basalts from two discrete geographical regions in North Queensland, Australia, can be used to identify contributions from geochemically distinctive mantle source components. The North Queensland basalts have positive Δ7/4Pb and Δ8/4Pb values (relative to the Northern Hemisphere Reference Line), and high 206Pb/204Pb and 87Sr/86Sr at given εNd relative to Tertiary basalts in New South Wales, eastern Australia. The northernmost Cooktown nephelinites in North Queensland are isotopically depleted with low 87Sr/86Sr and 206Pb/204Pb and high 143Nd/144Nd compared with the more southern enriched Atherton–Nulla basalts. Sr–Nd–Pb isotopic data fit with two-component mixing between an isotopically depleted Indian Ocean mid-ocean ridge basalt source component and an enriched mantle component with an EM2 signature. The geochemical characteristics of the isotopically enriched Atherton–Nulla basalts are consistent with contributions from a subcontinental lithospheric mantle modified by subduction-related metasomatism. The isotopically depleted Cooktown nephelinites show HIMU-like incompatible element signatures that can be attributed to contributions from amphibole- and apatite-bearing assemblages in the lithospheric mantle. The low 206Pb/204Pb and high εNd of these depleted basalts are not correlated with their respective high U/Pb and low Sm/Nd parent/daughter element ratios. This decoupling implies that the formation of the metasomatic amphibole and apatite assemblages must be a close precursor of the magmatism, possibly connected with the eastward migration of the Indian Ocean asthenosphere and/or subduction at the northeastern margin of the Australian plate during early Tertiary time. [ABSTRACT FROM PUBLISHER]

Geological and geophysical evidence indicates that at least 100 km of Archaean to Proterozoic lithospheric mantle has been removed from beneath large areas of eastern and southeastern China during late Mesozoic to Cenozoic time. Mantle-derived xenoliths in Tertiary basalts from several localities across this region have been studied by X-ray fluorescence, electron microprobe and laser ablation microprobe–inductively coupled plasma-mass spectrometry to characterize this thinner lithosphere. Trace element patterns of clinopyroxenes in the peridotites from southeastern China can be divided into four groups: fertile garnet lherzolites, fertile spinel (± garnet) lherzolites, and depleted and enriched peridotites. The addition of Nb, Sr, light rare earth elements, but not of Ti and Zr, suggests a metasomatizing agent containing both H2O and CO2. This study also demonstrates that the negative Ti anomaly commonly observed in clinopyroxene from mantle peridotites cannot be balanced by the Ti in coexisting orthopyroxene, but can be explained by small degrees of partial melting, using appropriate distribution coefficients. Most of the peridotites from southeastern China, whether spinel or garnet facies, are highly fertile in terms of Al2O3 and CaO contents and mg-number; many resemble commonly used primitive mantle compositions. Modelling of trace element patterns in clinopyroxene indicates that most spinel and garnet peridotites from the Nushan, Mingxi and Niutoushan localities experienced less than 5%, and many less than 2%, partial melting. A few depleted spinel peridotites from Nushan, and all spinel peridotites from Mingxi, require 10–25% fractional partial melting; almost all spinel peridotites from the Qilin locality show evidence of higher degrees (6–25%) of fractional partial melting. At both Nushan and Mingxi, the more depleted compositions occur in the upper part of the lithospheric mantle, which now is ∼100 km thick. Garnet peridotites are essentially undepleted, and Y–Ga–Zr relationships of the garnets are typical of Phanerozoic mantle. The overall highly fertile nature of the existing lithosphere requires that the Archaean and Proterozoic mantle that existed beneath the region in Palaeozoic times has been largely or completely removed, and replaced by younger, hotter and more fertile material. This probably occurred by upwelling of asthenospheric material during late Mesozoic to Cenozoic time, underplating to form new lithosphere. The occurrence of rare depleted xenoliths may show that some older mantle material is residual and coexists with younger material beneath southeastern China. [ABSTRACT FROM PUBLISHER]

Heavy-mineral concentrates (garnets, chromites) and xenoliths from 21 Cretaceous-Tertiary kimberlite intrusions have been used to map the lithospheric mantle beneath the Lac de Gras area in the central part of the Slave Province. Analyses of Nickel Temperature (TNi) and Zinc Temperature (TZn) have been used to place garnet and chromite xenocrysts, respectively, in depth context. Paleogeotherms derived from both xenoliths and concentrates lie near a 35 mW/m2 conductive model at T <= 900°C, and near a 38 mW/m2 model at higher T, implying a marked change in conductivity and/or a thermal transient. Plots of garnet composition vs TNi also show a sharp discontinuity in mantle composition at 900°C. Garnets from <145 km depth are ultradepleted in Y, Zr, Ti and Ga, whereas those from greater depths (to >= 200 km) are similar to garnets from Archean mantle world-wide. Relative abundances of garnet types indicate that the shallow layer consists of ~60% (clinopyroxene-free) harzburgite and 40% lherzolite, whereas the deeper layer contains 15-20% harzburgite and 80-85% lherzolite. T estimates on eclogite xenoliths show that all were derived from the deeper layer. Xenolith data and garnet compositions indicate that the shallow layer is more magnesian (Fo92-94) than the deeper layer (Fo91-92), and both layers are more olivine rich than South African or Siberian Archean peridotite xenoliths. The composition and sharply defined structure of the Lac de Gras lithosphere are unique within our current knowledge of Archean mantle sections. The shallow layer of this lithosphere section is similar to peridotites from some highly depleted ophiolites from convergent-margin settings, and may have formed in a similar situation during the accretion of the Hackett and Contwoyto terranes (magmatic arc and accretionary prism, respectively) to the ancient continental Anton terrane at 2·6-2·7 Ga. The deeper layer is interpreted as a plume head, which rose from the lower mantle and underplated the existing lithosphere at 2·6 Ga; evidence includes a high proportion of the superdeep inclusion assemblage (ferropericlase-perovskite) in the diamond population. This event could have provided heat for generation of the widespread 2·6 Ga post-tectonic granites. Proterozoic subduction from east and west may have modified the cratonic root, mainly by introduction of eclogites near its base. [ABSTRACT FROM PUBLISHER]

Primary sulphides occur both as micro-inclusions in major silicate and oxide phases and as individual grains in spinel lherzolite and pyroxenite xenoliths from Qilin, southeast China. Most of the lherzolite-hosted sulphide inclusions, typically 20-50 µm across, occur as isolated spheres or spheroids; host grains are olivine, clinopyroxene and orthopyroxene, but not spinel. In contrast, sulphide inclusions in pyroxenite are mostly 20-80 µm across and are almost exclusively hosted by clinopyroxene and spinel. These sulphide inclusions are typically multifaceted polygons, with their shapes epitaxially controlled by the host minerals. Isolated sulphide grains occur only in pyroxenite; they are up to 500 µm across, show evidence of deformation and are spatially associated with spinel. Lherzolite-hosted sulphide grains are polyphase assemblages that consist of pentlandite ± chalcopyrite ± Ni-poor monosulphide solid solution (mss1) ± Ni-rich monosulphide solid solution (mss2) ± cubanite ± heazlewoodite ± millerite ± bornite. Pyroxenite-hosted sulphide grains are pyrrhotite with minor chalcopyrite. All assemblages are likely to be the low-T ( <= 300°C) re-equilibrated products of high-T monosulphide solid solutions (MSS). The bulk compositions of these sulphide grains, estimated using proton microprobe analysis, show no consistent differences between the inclusion suites and intergranular sulphide grains, either in spinel lherzolite or pyroxenite samples. Average values of 111 proton microprobe analyses reveal that the lherzolite-hosted sulphides are rich in Ni (21%), Cu (<9%), Se (110 ppm) and platinum group elements (PGE) ( <= 30 ppm) but poor in Fe (37%) compared with the pyroxenite-hosted sulphides (Ni 1·4%, Cu <4%, Se 35 ppm, PGE absent, Fe 61·5%). Other trace elements (Co, Zn, As, Mo, Ag, Sn, Sb, Te and Pb) show no significant difference between the two suites. Lherzolite-hosted sulphides are inferred to be derived from immiscible sulphide melts trapped in residual mantle during partial melting. The sulphide melts had themselves undergone MSS fractionation before their incorporation into the depleted mantle rocks. In contrast, pyroxenite-hosted sulphides were produced by sulphur saturation during the crystallization of mafic magmas intruded into lherzolitic mantle. [ABSTRACT FROM PUBLISHER]