Search

A suite of mantle-derived xenoliths and xenocrysts in Cretaceous volcanic rocks from Mount Carmel, North Israel has been investigated to establish the regional paleo-geotherm and constrain the evolution of the lower crust and lithospheric mantle. Garnet pyroxenite xenoliths can be subdivided into low-MgO (MgO 15%) types. The low-MgO garnet websterites, with high equilibration pressures and temperatures (P = 20-23 kb, T = 1143-1192 â) and convex-upward REE patterns, are inferred to be cumulates from partial melts derived from a sub-lithospheric, fossilized plume head that impinged on the pre-existing lithospheric base during the early Mesozoic. In contrast, high-MgO pyroxenites, with lower T estimates, P defined by extrapolation to the inferred geotherm (1012-1046 â and ~ 14-15 kb) and mantle-like geochemical compositions, are deduced to represent cumulates from mafic magmas extracted from asthenospheric peridotitic mantle during late Proterozoic continental crustal growth in the region. Granulite xenoliths are mainly plagioclase-rich and equilibrated at depths of ~ 18-34 km. They have lower MgO contents but trace-element patterns similar to those of the low-MgO garnet pyroxenites, and probably originated as cogenetic crystallized melts and cumulates intruded around the Moho. U-Pb dating of apatite indicates ages around ~ 250 Ma, consistent with major magmatism dated by zircon xenocrysts. The lithospheric mantle has a two-layered structure, comprising a shallow (and relatively ancient) layer dominated by spinel peridotite and high-MgO garnet pyroxenites, and a deeper layer with fertile garnet-bearing peridotites intercalated with the low-MgO garnet pyroxenites., Author(s): Jiang-Gu Lu [sup.1] [sup.2], William L. Griffin [sup.2], Jin-Xiang Huang [sup.2], Hong-Kun Dai [sup.2] [sup.3], Montgarri Castillo-Oliver [sup.2], Suzanne Y. O'Reilly [sup.2] Author Affiliations: (1) grid.473484.8, 0000 0004 1760 [...]

Primary carbonates in kimberlites are the main CO.sub.2 carriers in kimberlites and thus can be used to constrain the original carbon and oxygen-isotope composition of kimberlite melts and their deep mantle sources. However, the contribution of syn- and post-emplacement processes to the modification of the C-O-isotope composition of kimberlites is yet to be fully constrained. This study aims to shed new light on this topic through a detailed textural, compositional (major and trace elements), and in situ C-O-Sr isotopic characterisation of carbonates in the Benfontein kimberlite sills (Kimberley, South Africa). Our multi-technique approach not only reveals the petrographic and geochemical complexity of carbonates in kimberlites in unprecedented detail, but also allows identification of the processes that led to their formation, including: (1) magmatic crystallisation of Sr-rich calcite laths and groundmass; (2) crystallisation of late groundmass calcite from hydrothermal fluids; and (3) variable degrees of crustal contamination in carbonate-rich diapirs and secondary veins. These diapirs most likely resulted from a residual C-O-H fluid or carbonate melt with contributions from methane-rich fluids from the Dwyka shale wall rock, leading to higher .sup.87Sr/.sup.86Sr and [delta].sup.18O, but lower [delta].sup.13C values than in pristine magmatic calcite. Before coalescing into the diapiric segregations, these fluids/melts also variably entrained early formed calcite laths and groundmass phases. Comparison between in situ and bulk-carbonate analyses confirms that O isotopic analyses of bulk carbonates from kimberlite rocks are not representative of the original isotopic signature of the kimberlite magma, whereas bulk C-isotope compositions are similar to those of the pristine magmatic carbonates. Calcite laths and most groundmass grains at Benfontein preserve isotopic values ([delta].sup.18O = 6-8â° and [delta].sup.13C = - 4 to - 6â°), similar to those of unaltered carbonatites worldwide, which, therefore, probably correspond to those of their parental melts. This narrow range suggests kimberlite derivation from a mantle source with little contribution from recycled crustal material unless the recycled material had isotopic composition indistinguishable from typical mantle values., Author(s): Montgarri Castillo-Oliver [sup.1] [sup.4], Andrea Giuliani [sup.1] [sup.2] [sup.3], William L. Griffin [sup.1], Suzanne Y. O'Reilly [sup.1], Russell N. Drysdale [sup.4] [sup.5], Adam Abersteiner [sup.6] [sup.7], Emilie Thomassot [sup.3] [...]

The Loma Baya complex in south-western Mexico is a volume of chromitite-bearing oceanic mantle that records a complex metamorphic history, defined by a first stage of hydrous metamorphism overprinted by a short-lived thermal event associated with an Eocene granite intrusion. During the hydrous metamorphism, the primary magmatic chromite-olivine assemblage was replaced by a secondary, porous intergrowth of Fe.sup.2+-rich chromite and chlorite. The heat supplied by an Eocene-age granite intrusion reversed the hydration reaction, producing chromite rims with perfectly developed crystal faces. This third-generation chromite is in equilibrium with highly magnesian (neoformed) olivine and defines a chemical trend analogous to the original magmatic one. The preservation of both reactions in the Loma Baya chromitite provides compelling evidence that the hydration of chromite can be reversed by either prograde metamorphism or any heating event, confirming previous thermodynamic predictions. Understanding these complex features is of particular interest due to the fact that changes in temperature and variable degrees of fluid/rock interaction during metamorphism and intrusion have also significantly affected the chromite-hosted IPGE carrier phases. Here, we propose that the metamorphic fluids involved in the hydrous metamorphism have caused the desulphurization of laurite RuS.sub.2, releasing minute particles of Ru-Os-Ir alloys Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1169-9) contains supplementary material, which is available to authorized users., Author(s): Jose M. Gonzalez-Jimenez[sup.1] [sup.2 ] , Martin Reich[sup.1] , Antoni Camprubi[sup.3] , Fernando Gervilla[sup.4] , William L. Griffin[sup.2] , Vanessa Colas[sup.5] , Suzanne Y. O'Reilly[sup.2] , Joaquin A. Proenza[sup.6] [...]

The 3.1 Ga Nuggihalli greenstone belt in the Western Dharwar craton is comprised of chromitite-bearing sill-like ultramafic-mafic rocks that are surrounded by metavolcanic schists (compositionally komatiitic to komatiitic basalts) and a suite of tonalite-trondhjemite-granodiorite gneissic rocks. The sill-like plutonic unit consists of a succession of serpentinite (after dunite)-peridotite-pyroxenite and gabbro with bands of titaniferous magnetite ore. The chromitite ore-bodies (length [almost equal to]30-500 m; width [almost equal to]2-15 m) are hosted by the serpentinite-peridotite unit. Unaltered chromites from massive chromitites (>80 % modal chromite) of the Byrapur and Bhaktarhalli chromite mines in the greenstone belt are characterized by high Cr# (100Cr/(Cr + Al)) of 78-86 and moderate Mg# (100 Mg/(Mg + Fe.sup.2+)) of 45-55. In situ trace-element analysis (LA-ICPMS) of unaltered chromites indicates that the parental magma of the chromitite ore-bodies was a komatiite lacking nickel-sulfide mineralization. In the Ga/Fe.sup.3+# versus Ti/Fe.sup.3+# diagram, the Byrapur chromites plot in the field of suprasubduction zone (SSZ) chromites while those from Bhaktarhalli lie in the MOR field. The above results corroborate our previous results based on major-element characteristics of the chromites, where the calculated parental melt of the Byrapur chromites was komatiitic to komatiitic basalt, and the Bhaktarhalli chromite was derived from Archean high-Mg basalt. The major-element chromite data hinted at the possibility of a SSZ environment existing in the Archean. Altered and compositionally zoned chromite grains in our study show a decrease in Ga, V, Co, Zn, Mn and enrichments of Ni and Ti in the ferritchromit rims. Trace-element heterogeneity in the altered chromites is attributed to serpentinization. The trace-element patterns of magnetite from the massive magnetite bands in the greenstone belt are similar to those from magmatic Fe-Ti-V-rich magnetite bands in layered intrusions, and magnetites from andesitic melts, suggesting that magnetite crystallized from an evolved gabbroic melt. Enrichments of Ni, Co, Te, As and Bi in disseminated millerite and niccolite occurring within chromitites, and in disseminated bravoite within magnetites, reflect element mobility during serpentinization. Monosulfide solid solution inclusions within pyroxenes (altered to actinolite) in pyroxenite, and interstitial pyrites and chalcopyrites in magnetite, retain primary characteristics except for Fe-enrichment in chalcopyrite, probably due to sub-solidus re-equilibration with magnetite. Disseminated sulfides are depleted in platinum-group elements (PGE) due to late sulfide saturation and the PGE-depleted nature of the mantle source of the sill-like ultramafic-mafic plutonic rocks in the Nuggihalli greenstone belt. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1148-1) contains supplementary material, which is available to authorized users., Author(s): Ria Mukherjee[sup.1] [sup.2] , Sisir K. Mondal[sup.1] [sup.4] , Jose M. Gonzalez-Jimenez[sup.3] [sup.5] , William L. Griffin[sup.3] , Norman J. Pearson[sup.3] , Suzanne Y. O'Reilly[sup.3] Author Affiliations: (1) PGE [...]

Episodic refertilization and metasomatism of depleted subcontinental lithospheric mantle (SCLM) is rarely revealed within a single orogenic peridotite. We have investigated the Shenglikou peridotite massif in the North Qaidam Orogen (NE Tibet, China) to understand its origin and secular re-enrichment history and possible links with global tectonism, using multiple chemical and isotopic techniques. The massif contains two main groups of rocks, one rich in olivine (Ol) and the other rich in pyroxene (Pyro). In situ Re-Os isotope analyses on sulfides and previous reported whole-rock Os data show the Shenglikou peridotite massif has an Archean origin. The Ol-rich group has Mg# up to 0.923, Al.sub.2O.sub.3 contents down to 0.55 wt%, low FeO contents, whole-rock HREE ~0.01-0.1 times chondritic values and high spinel Cr# (up to 0.71). Modeling suggests that the Ol-rich peridotites are residues after ~30-40 % melt extraction at ~1600-1500 °C and [less than or equal to]5 GPa. Mineral compositions, oxygen-isotope compositions ([delta].sup.18O.sub.V-SMOW = 5.7 ± 0.1 â°) and attainment of Hf-isotope internal equilibration at ~1.5-1.4 Ga with initial ratios similar to that of depleted convective mantle indicate that the Ol-rich group was refertilized by silicate melts derived from the depleted asthenosphere at ~1.5-1.4 Ga, corresponding to one phase in the breakup of the Columbia supercontinent. On the other hand, chemical compositions and Hf-O isotopic signatures of the pyroxene-rich rocks suggest a second refertilization superimposed on the Archean lithospheric mantle at ~700 Ma, coeval with the global extension that affected Rodinia. Both groups show metasomatism by fluids, sourced from subducting slabs during the early Paleozoic convergence between the Qaidam and Qilian blocks. This study shows that the episodic refertilization and metasomatism of depleted Archean SCLM, temporally linked to supercontinental cycles, can be revealed in a single orogenic peridotite, and suggests that continental extension and convergence play an important role in the destruction of cratonic keels. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1126-7) contains supplementary material, which is available to authorized users., Author(s): Qing Xiong[sup.1] [sup.2] , William L. Griffin[sup.2] , Jian-Ping Zheng[sup.1] , Suzanne Y. O'Reilly[sup.2] , Norman J. Pearson[sup.2] Author Affiliations: (1) State Key Laboratory of Geological Processes and Mineral [...]

The Caledonian Nappe Complex of Arctic Norway provides rare insights into the interaction between mafic-ultramafic magmas and the deep continental crust. The Kalak Nappe Complex contains >25,000 [km.sup.3] of mafic igneous rocks, mostly layered gabbros, making up the 570-560 Ma Seiland Igneous Complex. The complex has been intruded by a series of ultramafic magmatic rocks, including the Nordre Bumandsfjord pluton. Field relationships in this pluton show that extremely fluid, dry, relatively Fe-rich ([Fo.sub.81]) dunite magmas intruded a pile of cumulate gabbros, with block stopping and intrusive brecciation. Diking on scales from mm to metres and extensive melting and assimilation of the gabbros attest to high temperatures, consistent with a 2-km-wide granulite-facies contact aureole. Major- and trace-element trends show that the dunites were progressively contaminated by a cpx-rich partial melt of the gabbros, producing a range of lithologies from dunite through lherzolites to wehrlite. Experimental studies of natural samples at 0.8-1 GPa define the dunite solidus at 1,650-1,700°C. In the average peridotite, contamination has produced a crystallisation interval of ca 400°C (1,600-1,200°C); this would provide large amounts of heat for melting and metamorphism and would maintain the fluidity of the magmas to relatively low T, consistent with field relationships. Thermochemical and dynamic modelling demonstrates that the dunitic primary magmas may represent the last melting of a rapidly ascending diapir of previously depleted subducted oceanic lithosphere. The mafic rocks of the Seiland Complex may already have been extracted from this diapir, and the late dikes of the province may reflect melting of the asthenosphere as the diapir spread out beneath the lithosphere. Ultramafic magmas, abundant in the Archean, may still be more common than usually assumed. However, they would only penetrate to the shallow crust under unusually extensional conditions, where ascent could outpace assimilation. Keywords Dunite magmas * Ultra-high-temperature magmas * Magmatic assimilation * Norwegian Caledonides * Deep crustal processes, Introduction The buoyant continental crust commonly acts as an effective filter for magmas rising from the asthenosphere; relatively dense mantle-derived magmas such as basalts appear at the surface only in [...]

Byline: Claudio Marchesi (1,2), William L. Griffin (2), Carlos J. Garrido (3), Jean-Louis Bodinier (1), Suzanne Y. O'Reilly (2), Norman J. Pearson (2) Keywords: Subcontinental lithospheric mantle; Partial melting; Proterozoic; Re--Os isotopes; Sulfide; Ronda massif Abstract: The western part of the Ronda peridotite massif (Southern Spain) consists mainly of highly foliated spinel-peridotite tectonites and undeformed granular peridotites that are separated by a recrystallization front. The spinel tectonites are interpreted as volumes of ancient subcontinental lithospheric mantle and the granular peridotites as a portion of subcontinental lithospheric mantle that underwent partial melting and pervasive percolation of basaltic melts induced by Cenozoic asthenospheric upwelling. The Re--Os isotopic signature of sulfides from the granular domain and the recrystallization front mostly coincides with that of grains in the spinel tectonites. This indicates that the Re--Os radiometric system in sulfides was highly resistant to partial melting and percolation of melts induced by Cenozoic lithospheric thermal erosion. The Re--Os isotopic systematics of sulfides in the Ronda peridotites thus mostly conserve the geochemical memory of ancient magmatic events in the subcontinental lithospheric mantle. Os model ages record two Proterozoic melting episodes at ~1.6 to 1.8 and 1.2--1.4 Ga, respectively. The emplacement of the massif into the subcontinental lithospheric mantle probably coincided with one of these depletion events. A later metasomatic episode caused the precipitation of a new generation of sulfides at ~0.7 to 0.9 Ga. These Proterozoic Os model ages are consistent with results obtained for several mantle suites in Central/Western Europe and Northern Africa as well as with the Nd model ages of the continental crust of these regions. This suggests that the events recorded in mantle sulfides of the Ronda peridotites reflect different stages of generation of the continental crust in the ancient Gondwana supercontinent. Author Affiliation: (1) Geosciences Montpellier, UMR 5243, CC 60, CNRS-Universite Montpellier II, Place E. Bataillon, 34095, Montpellier, France (2) GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia (3) Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC-Universidad de Granada, Avenida Fuentenueva s/n, 18002, Granada, Spain Article History: Registration Date: 13/07/2009 Received Date: 02/02/2009 Accepted Date: 13/07/2009 Online Date: 14/08/2009 Article note: Communicated by J. Hoefs.

Byline: Giovanna T. Sapienza (1), William L. Griffin (2), Suzanne Y. O'Reilly (2), Lauro Morten (1) Keywords: Gabbro xenoliths; In situ Sr isotopes; Sr--Nd--Hf isotope data; MARID; Hyblean Plateau Abstract: In situ trace-element and isotopic [(.sup.87]Sr/ [.sup.86]Sr) data and whole-rock Sr--Nd--Hf data on 12 gabbro xenoliths from the Hyblean Plateau (south-eastern Sicily) illustrate the complex petrogenetic evolution of this lithospheric segment. The gabbros formed by precipitation of plagioclase + clinopyroxene from a HIMU-type alkaline melt, then were cryptically metasomatized by a low-Rb, high- 87.sup.Sr/ [.sup.86]Sr fluid, and finally infiltrated by an exotic, late Fe--Ti-rich melt with 87.sup.Sr/ [.sup.86]Sr ~ 0.7055, carrying high concentrations of Sr, Rb and HFSE. The geochemical and isotopic features of both the metasomatizing fluid and the Fe--Ti-rich melt are compatible with their common derivation by the progressive melting of an amphibole--phlogopite--ilmenite metasomatic domain (MARID-type?) that probably resided within the subcontinental lithospheric mantle. Therefore, both the astenosphere and the lithosphere underneath the Hyblean Plateau contributed to the petrogenesis of the gabbros. Sm--Nd dating yields an age of 253 +- 60 Ma for the cumulitic pile, roughly coinciding with a hydrothermal event recorded by crustal zircons in the area. We suggest that the Hyblean Plateau suffered a thermal event--probably related to lithospheric thinning and upwelling and melting of the asthenosphere--in Permo-Triassic time (the opening of the Ionian Basin?). The induced perturbation in the lithosphere caused consequent melting of some previously metasomatised portions. Author Affiliation: (1) Dipartimento di Scienze della Terra e Geologico-Ambientali, Universita di Bologna, Piazza Porta San Donato 1, 40126, Bologna, Italy (2) ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, Macquarie, NSW, 2109, Australia Article History: Registration Date: 28/05/2008 Received Date: 20/02/2008 Accepted Date: 28/05/2008 Online Date: 17/06/2008 Article note: Lauro Morten deceased, November 18, 2006. Communicated by J. Hoefs.

Group II (143-117 Ma) and Group I kimberlites (108-74 Ma) intrude across the southwest boundary of the Kaapvaal craton, sampling the same volume of the subcontinental lithospheric mantle (SCLM) in two time slices. Major and trace element analyses of 3699 peridotitic garnet xenocrysts were used to construct paleogeotherms for 17 kimberlite localities, and to place each garnet at its depth of origin. The Ti contents of each garnet and the calculated [X.sub.Mg] of its coexisting olivine were projected onto a southwest-northeast section across the craton boundary, and splines were used to interpolate between the virtual boreholes in each age group. The sections show that the cratonic SCLM extends at least 75 km southwest of the mapped craton boundary, suggesting a dipping contact. Marked differences between the time-slice sections show that between 117 and 108 Ma the SCLM on both sides of the craton boundary was heated and chemically refertilized by infiltrating asthenosphere-derived melts, thinning the depleted layer by ~40 km. The thermal and geochemical changes record a significant tectonothermal event that may be related to changes in the stress field associated with opening of the South Atlantic, or with mantle upwelling. Keywords: South Africa, lithosphere evolution, lithosphere structure, mantle xenocrysts, mantle garnets.

Continental flood basalts, derived from mantle plumes that rise from the convecting mantle and possibly as deep as the core mantle boundary, are major hosts for world-class Ni-Cu-PGE ore deposits. Each plume may have a complex history and heterogeneous composition. Therefore, some plumes may be predisposed to be favourable for large-scale Ni-PGE mineralisation ('fertile'). Geochemical data from 10 large igneous provinces (LIPs) have been collected from the literature to search for chemical signatures favourable for Ni-PGE mineralisation. The provinces include Deccan, Kerguelen, Ontong Java, Paranfi, Ferrar, Karoo, Emeishan, Siberia, Midcontinent and Bushveld. Among these LIPs, Bushveld, Siberia, Midcontinent, Emei Mt and Karoo are 'fertile', hosting magmatic ore deposits or mineralisation of various type, size and grade. They most commonly intruded through, or on the edges of, Archaean-Paleoproterozoic cratonic blocks. In contrast, the 'barren' LIPs have erupted through both continental and oceanic crustal terranes of various ages. Radiogenic isotopic signatures indicate that almost all parental LIP magmas are generated from deep-seated mantle plumes, and not from the more widespread depleted asthenospheric mantle source: this confirms generally accepted plume models. However, several important geochemical signatures of LIPs have been identified in this study that can discriminate between those that are 'fertile' or 'barren' in terms of their Ni PGE potential. The fertile LIPs generally contain a relatively high proportion of primitive melts that are high in MgO and Ni, low in [Al.sub.2][O.sub.3] and [Na.sub.2]O, and are highly enriched in most of the strongly incompatible elements such as K, P, Ba, Sr, Pb, Th, Nb, and LREE. They have relatively high Os contents ([greater than or equal to] 0.03 to 10 ppb) and low Re/Os ( Barren LIPs contain fewer high-MgO lavas. The barren LIP lavas in general have low Os contents (mostly [less than or equal to] 0.02 ppb) with high Re/Os (10-[greater than or equal to] 200). They show isotopic variations between plume and EM2 geochemical signatures and have high Rb/Ba ratios. These signatures may indicate involvement of deep recycled material in the mantle sources or crustal contamination for barren LIPs, but low degrees of interaction with old lithospheric-type roots. Keywords: continental flood basalts; nickel and PGE mineralisation; mantle plumes; lithospheric mantle; cratonic lithosphere; lithospheric sulfides

Byline: Sonja Aulbach (1,2), William L. Griffin (1,3), Norman J. Pearson (1), Suzanne Y. O'Reilly (1), Buddy J. Doyle (4) Keywords: Mantle peridotite origins; Mantle melting; Plume mantle; Subcontinental lithosphere formation; Lac de Gras lithospheric mantle Abstract: Major-element compositions of minerals in peridotite xenoliths from the Lac de Gras kimberlites provide constraints on the mode of lithosphere formation beneath the central Slave Craton, Canada. Magnesia contents of reconstructed whole rocks correlate positively with NiO and negatively with CaO contents, consistent with variable partial melt extraction. Alumina and [Cr.sub.2]O.sub.3 contents are broadly positively correlated, suggestive of melt depletion in the absence of a Cr--Al phase. Garnet modes are high at a given [Al.sub.2]O.sub.3 content (a proxy for melt depletion), falling about a 7 GPa melt depletion model. These observations, combined with high olivine Mg and major-element relationships of FeO-poor peridotites (3 GPa (residual FeO content being a sensitive indicator of melt extraction pressure), and similar high pressures of last equilibration ([proportional to]4.2 to 5.8 GPa), provide multiple lines of evidence that the mantle beneath the central Slave Craton has originated as a residue from high-pressure melting, possibly during plume subcretion. Apparent low melt depletion pressures for high-FeO peridotites (>7.5 wt%) could suggest formation in an oceanic setting, followed by subduction to their depth of entrainment. However, these rocks, which are characterised by low SiO.sub.2 contents ( Author Affiliation: (1) GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW, 2109, North Ryde, Australia (2) Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada, T6G 2E3 (3) CSIRO Exploration and Mining, N. Ryde, NSW, 2113, Australia (4) Lithosphere Services, 4009 Edinburgh Street, Burnaby, BC, Canada, V5C 1R4 Article History: Registration Date: 03/04/2007 Received Date: 27/09/2006 Accepted Date: 02/04/2007 Online Date: 08/05/2007 Article note: Communicated by T.L. Grove.

The first in situ Hf and U-Pb isotope analyses of zircon separates from Mesozoic granites in southern Tibet identify a significant, previously unknown stage of magmatism. Igneous zircons (n = 34) from a granite within the Gangdese batholith show a weighted mean [sup.206]Pb/[sup.238]U age of 188.1 [+ or -] 1.4 Ma and [[epsilon].sub.Hf](T) (the parts in [10.sup.4] deviation of initial Hf isotope ratios between the zircon sample and the chondritic reservoir) values between +10.4 and +16.8, suggesting predominantly Early Jurassic intrusive activity with a juvenile mantle contribution. Of 40 inherited zircons from two Cretaceous S-type granites in the northern magmatic belt, 23 delineate a slightly older [sup.206]Pb/[sup.238]U age cluster between 188 and 210 Ma. These zircons have [[epsilon].sub.Hf](T) values from -3.9 to -13.7, yielding crustal Hf model ages from ca. 1.4 to 2.1 Ga, suggesting a major episode of crustal growth in Proterozoic time and remelting of this crust in Early Jurassic time. Combining these with literature data, we interpret the Jurassic Gangdese magmatism as an early product of the Neo-Tethyan subduction that played a long-lasting role in the tectonic evolution of southern Tibet prior to the India-Asia collision. Keywords: zircon, Hf isotope, U-Pb dating, Tibet, Neo-Tethyan subduction.

Zircons have been extracted from garnet peridotite and its wall rock (gneiss), which was intersected by the pre-pilot hole of the Chinese Continental Scientific Drilling project (CCSD-PP1) in the Sulu ultrahigh-pressure (UHP) metamorphic belt. The peridotitic zircons record early Mesozoic UHP metamorphism ([sup.206]Pb/[sup.238]U age of 223.5 [+ or -] 7.5 Ma), but their Hf isotope compositions indicate that the protolith of the peridotite is at least Mesoproterozoic in age (minimum depleted-mantle ages [[T.sub.DM]] of 1.4 Ga). Zircons from the gneiss also reflect the Mesozoic metamorphism, with a cluster of [sup.206]Pb/[sup.238]U ages at 224.5 [+ or -] 11.5 Ma; a trail of discordant grains indicates a protolith age older than 800 Ma, consistent with [T.sub.DM] model ages of younger than 1.2 Ga. The peridotitic zircons have trace-element patterns similar to kimberlitic and/or carbonatitic zircons, while those from the gneiss have affinities with zircons from syenites/monzonites. The differences suggest that the metasomatic agents that affected the peridotitic zircons were derived from the asthenospheric mantle rather than from subducted continental crust. The strong depletion of the CCSD-PP1 peridotite in basaltic components, and the relatively unradiogenic Hf isotopic compositions (e.g., -16.3 to ~-13.8 [[epsilon].sub.Hf]) of the peridotitic zircons, indicate that the peridotitic body is a fragment of refractory Archean mantle that experienced Mesoproterozoic metasomatism and represents a tectonic intrusion into younger crust. Keywords: peridotitic zircon, Hf isotope, trace element, U-Pb dating, Sulu UHP terrane, China.

The age distribution of the crust is a fundamental parameter in modeling continental evolution and the rate of crustal accretion through Earth's history, but this is usually estimated from surface exposures. The exposed Yangtze craton in eastern China consists mainly of Proterozoic rocks with rare Archean outcrops. However, the U-Pb ages and Hf isotope systematics of xenocrystic zircons brought to the surface in lamproite diatremes from three Proterozoic outcrop areas of the craton suggest the widespread presence of unexposed Archean basement, with zircon age populations of 2900-2800 Ma and 2600-2500 Ma and Hf model ages of 2.6 to ca. 3.5 Ga or older. The zircons also record thermal events reworked on the craton ca. 2020 Ma (remelting of older crust) and 1000-850 Ma (addition of juvenile mantle material). The observation of deep crust significantly older than the upper crust will require revision of models for the rates of crustal generation through time. Keywords: crustal evolution, Hf isotopes, zircon geochronology, deep-crustal xenocryst, Archean basement, Yangtze craton, east China.

A suite of spinel peridotite xenoliths in Mesozoic basalts of the Tuoyun basin in the Tianshan area of northwest China has a high proportion of amphibole/ mica-bearing lherzolites, with high Cpx/Opx ratios (mean 0.74). Many aspects of mineral chemistry in the Tuoyun peridotites are intermediate between those of refractory Archean cratonic mantle and fertile Phanerozoic mantle. These include Ni/Cr and the contents of transition metals and Y in olivine and orthopyroxene and the abundances of elements such as Na, Al, Ti, Y, Sr and LREE in clinopyroxene. The data suggest that the mantle in Tuoyun is moderately depleted in basaltic components relative to both the refractory Archean mantle and the fertile Phanerozoic mantle. The wide variations in the CaO/[Al.sub.2][O.sub.3] (0.9-3.5) of whole rocks and LREE/HREE (0.8-14.2) and Ti/Eu (971-5,765) of clinopyroxenes in the Tuoyun peridotites are interpreted as the metasomatism of hydrous carbonatitic and potassic melt or the cumulative effects of mantle metasomatism by different agents (carbonatite and small-volume silicate melts) through time. The Tuoyun mantle shows closer affinity to the type of mantle found beneath the Proterozoic Cathaysia block, and especially to that beneath the East Central Asia Orogenic Belt (ECAOB), than to the mantle beneath the Archean North China Craton. This implies that the Tianshan subcontinental lithospheric mantle may have been generated during the accretion of the ECAOB. The high proportion of fine-grained microstructures, high Cpx/Opx ratio, obvious Ca enrichment and lower overall depletion in the Tuoyun mantle relative to that in other parts of the ECAOB reflect stronger mechanical and chemical modification of the Tuoyun mantle, near the translithospheric Talas-Ferghana strike-slip fault, which played a major role in controlling the strength of the mantle lithosphere and has channeled the upwelling mantle.

The Yangkou garnet-peridotite complex in the Sulu UHP terrane, eastern China, consists of several blocks, tens of meters across, enclosed within gneiss. The blocks consist mainly of garnet lherzolite and garnet clinopyroxenite with small amounts of eclogite. The complex displays a finely layered stRucture from outcrop to thin-section scales. Lherzolitic garnet is generally homogenous in composition ([Prp.sub.64-67][Alm.sub.23-24][Grs.sub.10-12][Sps.sub.1]). Garnet (PrpsI-63) from clinopyroxenite contains more pyrope component than eclogitic garnets ([Prp.sub.29-35]). Porphyroblastic garnet from clinopyroxenite and eclogite contains abundant exsolved rods of futile and apatite ([+ or -]Cpx). Lherzolitic garnet exhibits an unusual sinusoidal REE pattern characterized by a decrease in abundance from La to Pr and an increase from Nd to the HREE, and shows Ba and Pb enrichment and negative Nb and Ni anomalies in primitive mantle-normalized trace-element plots. In contrast, diopside shows a normal REE pattern with decreasing abundances from La to Lu, a peak at Ba and Pb, and depletion in Co and Ni. Garnet from eclogite has a REE pattern with increasing abundances from La to Lu, or a maximum at Eu. Eclogitic clinopyroxene shows humped REE patterns with a peak at Nd. Except for slightly lower [[delta].sup.18][O.sub.VSMOW] of +4.40% for olivine, O-isotopic compositions (+5.03 to +5.53%) of analyzed garnets and clinopyroxenes from all three rock types lie in the range of mantle [[delta].sup.18]O values of +5.1 to +6.6%0. Small O-isotope fractionations ([DELTA]Grt-Cpx: -0.16 to -0.35) in eclogite suggest isotopic equilibrium. On the basis of petrochemical and isotopic data, this garnet-peridotite complex was initially a mantle peridotite cut by mafic veins. These rocks experienced metasomatism and subduction-zone UHP metamorphism at 750 [+ or -] 50 [degrees]C and 40 to 45 kbar with a low thermal gradient. The fine-scale layering probably formed as a result of strong shear deformation during exhumation.

The Hf isotope composition of rutile in mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths from the Kimberley area of South Africa has been analyzed in situ by laser ablation microprobe--multicollector--inductively coupled plasma mass spectrometry to investigate metasomatic processes in the lithospheric mantle. The [sup.176]Hf/[sup.177]Hf ratio shows a wide range, from 0.2811 to 0.2858 ([[epsilon].sub.Hf] = -55 to +110); much of this range is found within single samples and even within single grains. We suggest that the MARID rutiles initially resulted from the interaction of an asthenospheric melt with ancient depleted harzburgitic mantle with low [sup.176]Hf/[sup.177]Hf ratios ( Keywords: MARID xenoliths, mantle rutile, hafnium isotopes, mantle xenoliths, kimberlitic metasomatism, South Africa lithosphere.

Mafic xenoliths from the Paleozoic Fuxian kimberlites in the North China craton include garnet granulite, and minor pyroxene anaphibolite, metagabbro, anorthosite and pyroxenite. The formation conditions of the amphibolites are estimated at 745-820[degrees]C and 7.6-8.8 Kb (25 30 km); the granulites probably are derived from greater depths in the lower crust. LAM-ICPMS U-Pb dating of zircons from four granulites reveals multiple age populations, recording episodes of magmatic intrusion and metamorphic recrystallisation. Concordant ages and upper intercept ages, interpreted as minimum estimates for the time of magmatic crystallisation, range from 2,620 to 2,430 Ma in three granulites, two amphibolites and two metagabbros. Lower intercept ages, represented by near-concordant zircons, are interpreted as reflecting metamorphic recrystallisation, and range from 1,927 to 1,852 Ma. One granulite contains two metamorphic zircon populations, dated at 1,927[+ or -]55 Ma and 600 700 Ma. Separated minerals from one granulite and one amphibolite yield Sm-Nd isochron ages of 1,619 [+ or -] 48 Ma ([sup.143]Nd/[sup.144]Nd)I=0.51078), and 1,716 [+ or -] 120 Ma ([sup.143]Nd/ [sup.144]Nd)i=0.51006), respectively. These ages are interpreted as recording cooling following metamorphic resetting; model ages for both samples are in the range 2.40-2.66 Ga. LAM-MC-ICPMS analyses of zircon show a range in [sup.176]Hf/[sup.177]Hf from 0.28116 to 0.28214, corresponding to a range of [[epsilon].sub.Hf] from -34 to + 12. The relationships between [sup.207]pb/[sup.206]Pb age and [[epsilon].sub.Hf] show that: (1) the granulites, amphibolites and metagabbro were derived from a depleted mantle source at 2.6-2.75 Ga; (2) zircons in most samples underwent recrystallisation and Pb loss for 100-200 Ma after magmatic crystallisation, consistent with a residence in the lower crust; (3) metamorphic zircons in several samples represent new zircon growth, incorporating Hf liberated from breakdown of silicates with high Lu/Hf; (4) in other samples metamorphic and magmatic zircons have identical [sup.176]Hf/[sup.177]Hf, and the younger ages reflect complete resetting of U-Pb systems in older zircons. The Fuxian mafic xenoliths are interpreted as the products of basaltic underplating, derived from a depleted mantle source in Neoarchean time, an important period of continental growth in the North China craton. Paleoproterozoic metanaorphic ages indicate an important tectonic thermal event in the lower crust at 1.8-1.9 Ga, corresponding to the timing of collision between the Eastern and Western Blocks that led to the final assembly of the North China craton. The growth of metamorphic zircon at 600-700 Ma may record an asthenospheric upwelling in Neoproterozoic time, related to uplift and a regional disconformity in the North China craton.

Small volumes of garnet peridotite occur within large dunite bodies in the Western Gneiss Region of the Norwegian Caledonides. Previous Sm-Nd analyses of garnet peridotite yield Proterozoic ages close to the crystallization ages of the host gneisses. In situ (laser-ablation multicollector--inductively coupled plasma--mass spectrometry) Re-Os analysis of sulfides in the garnet peridotites gives a range of Proterozoic and Archean model ages. An Archean (2.7-3.1 Ga) protolith age is supported by whole-rock Re-Os data for sulfide-free dunites from several bodies. The Archean ages imply a melt-extraction event in the lithospheric mantle that predates the growth of the known Proterozoic upper crust in the region. Keywords: Norwegian Caledonides, mantle peridotites, sulfide ages, Re-Os ages, Archean lithospheric mantle.

U-Pb and Hf isotope analyses of zircons from felsic granulite xenoliths in Mesozoic volcanics reveal Early Archean ([greater than or equal to] 3.6 Ga) lower crust beneath the younger ( Keywords: North China craton, Hf isotopes, zircon U-Pb dating, lower crustal granulites, Archean crustal evolution.

The Os isotope compositions of sulfides in mantle xenoliths from the Penghu Islands, Taiwan Strait, reveal the presence of Proterozoic subcontinental lithospheric mantle beneath the highly extended southeast margin of the South China block. Both [T.sub.RD] (Re depleted) model ages for individual sulfides and model ages estimated from the initial [sup.187]Os/[sup.188]Os ratios of Re-Os mixing lines require that some volumes of the subcontinental lithospheric mantle formed prior to 2.3-1.9 Ga. Later events in the subcontinental lithospheric mantle may be recorded by [T.sub.RD] model ages of 1.5-1.2 Ga and ca. 0.9 Ga. The events recognized in the subcontinental lithospheric mantle are consistent with those known in the crust of the mainland South China block. The sulfide Os isotope data show that Proterozoic lithosphere beneath the South China block has survived the extensive Mesozoic Yanshanian magmatism on the continental margin and has not been delaminated even during the severe lithospheric extension that led to the subsidence of the Taiwan Strait. Keywords: lithospheric mantle, Re-Os, sulfides, Taiwan Strait, Proterozoic, South China block.

Quenched carbonate-silicate inclusions in lherzolitic clinopyroxene macrocrysts, derived from 200 km beneath the Slave craton in northern Canada, are interpreted as natural samples of mantle carbonatites. Oxygen, carbon, and strontium isotope data provide evidence for the involvement of subducted crustal material in the origin of these carbonatites, supporting suggestions that carbon recycling by subduction is an important prerequisite for carbonatite magmatism. The compositional range of the inclusions suggests that the parent melt was decreasing in silica content as it was trapped in the host crystal, a trend that is predicted experimentally. Isotopic disequilibrium between the carbonatitic inclusions and the host clinopyroxene indicates that they were trapped shortly before kimberlite eruption, suggesting a temporal link between the entrapment of the carbonatite in the host and the Paleocene eruption of the kimberlite. Keywords: carbonatites, isotopes, subduction, Slave province, mantle melts, xenoliths.

Trace element abundances in igneous zircons, as determined by electron microprobe and laser-ablation microprobe ICPMS analysis, are shown to be sensitive to source rock type and crystallisation environment. The concentrations of 26 trace elements have been determined for zircons from a wide range of different rock types and reveal distinctive elemental abundances and chondrite-normalised trace element patterns for specific rock types. There is a general trend of increasing trace element abundance in zircons from ultramafic through mafic to granitic rocks. The average content of REE is typically less than 50 ppm in kimberlitic zircons, up to 600-700 ppm in carbonatitic and lamproitic zircons and 2,000 ppm in zircons from mafic rocks, and can reach per cent levels in zircons from granitoids and pegmatites. Relatively flat chondrite-normalised REE patterns with chondrite-normalised Yb/Sm ratios from 3 to 30 characterise zircons from kimberlites and carbonatites, but Yb/Sm is commonly over 100 in zircons from pegmatites. Th/U ratios typically range from 0.1 to 1, but can be 100-1000 in zircons from some carbonatites and nepheline syenite pegmatites. The geochemical signatures characteristic of zircon from some rock types can be recognised in bivariate discriminant diagrams, but multivariate statistical analysis is essential for the discrimination of zircons from most rock types. Classification trees based on recursive partitioning techniques provide a rapid means of relating parent rock type to zircon trace element analysis; zircons from many rock types can be discriminated at confidence levels of 75% or more. These trees allow recognition of the provenance of detrital zircons from heavy mineral concentrates, and significantly enhance the usefulness of zircon in regional crustal studies and as an indicator mineral in mineral exploration.

Pb, Sr, and Nd isotope data for Cenozoic basalts in eastern Australia indicate that mantle isotope signatures of Pacific Ocean mid-oceanic ridge basalts (MORB) type characterize the lava-field basalts (55-14 Ma) in southeastern Australia, whereas mantle isotope signatures of Indian Ocean MORB type characterize younger basalts (6-0 Ma) from northeastern Australia. This discovery further constrains the secular distribution of major asthenospheric mantle reservoirs represented by the Pacific and Indian Oceans MORB sources during and following the breakup of eastern Gondwana and tracks, for the first time, the locus of the boundary of the two reservoirs beneath the Australian continent. These new data fill the gap between previous Indian Ocean MORB-Pacific Ocean MORB boundary locations determined from backarc basin basalts in the southwestern Pacific Ocean and ocean-floor basalts in the Southern Ocean. We propose that the Indian Ocean MORB source is a long-term asthenospheric reservoir beneath most of eastern Gondwana and that the westward migration of the Pacific Ocean MORB source may have been associated with the opening of the Tasman Sea (ca. 85-60 Ma) along a broad front southeast of the Australian continent.

A study on granulite and pyroxenite xenoliths from the Delegate basaltic pipes in the Paleozoic Lachlan fold belt in southeastern Australia is given. An analysis of the Sm-Nd isotopes of the minerals and rocks and U-Pb zircon dating is used to carry out the experiments. Findings of the study will help clarify the evolution of geological processes within the crust and upper mantle in the region. Among these are the mafic growth into the lower crust and upper mantle and the development of granitoid melts.

The carbonates present in the mantle peridotite xenoliths from Spitsbergen precipitated from carbonate melts that later moved to the surface to form the xenoliths. The melts reacted with the host lherzolites to form carbonate minerals with different compositions and fine-grained regions with wehrlitic assemblages. The carbonates have a low Na content and high-Mg numbers. They range from Mg-containing calcite to dolomite. The carbonates are present with (Al,Cr,Ti)-rich clinopyroxene and (Ca,Mg)-rich olivine.

Mineral inclusions in the corundum grains show a different range of mineral phases, such as Nb-Ta oxides, Ca feldspars, zircons, Fe,Cu-sulphide and Ca,Th-phosphates, crystallized with corundum megacrysts. The unique chemical features of mineral inclusions indicate that granite/syenite pegmatite and bonatite components are involved in the formation of corundum megacrysts. This inclusion model give a new 'hybrid rock hypothesis' for corundum megacrysts.

A study shows that in 11 lava-field provinces in New South Wales and Queensland, eastern Australia, oceanic island basalt (OIB)-type source is the dominant component. Subcontinental lithospheric mantle (SCLM) contributes other significant components. These facts are obtained from the analysis of isotopic signatures and trace element patterns. The geochemical characteristics of SCLM elements are similar to those of spinel lherzolite xenoliths.

A suite of eclogites from the Roberts Victor kimberlite has been extensively characterized in terms of petrology and geochemical compositions (Greau et al. in Geochim Cosmochim Acta 75(22):6927-6954, 2011; Huang et al. in Lithos 142-143:161-181, 2012a). In the present study, the water contents of eclogitic garnet and omphacite were analyzed by Fourier transform infrared spectrometry. Garnet does not contain measureable OH in any sample. The water content of omphacite in the studied eclogites ranges from 211 to 1,496 ppm. Mantle metasomatism has modified the water content of some of the eclogites, while others retain water contents characteristic of their original environment. The OH contents of the metasomatized eclogites may be mainly controlled by the [H.sub.2]O fugacity and mineral compositions. The OH contents of the non-metasomatized samples are interpreted to be more sensitive to their mantle equilibration temperature, pressure, and the local fugacities of [H.sub.2]O and [O.sub.2]. The calculated water content of the metasomatic medium is similar to that of carbonatitic-kimberlitic melts/fluids. Eclogites contain more water than peridotites recorded in the literature (341 ± 161 vs 122 ± 54 ppm) and represent an important water reservoir in the lithospheric mantle wherever they occur. This is an important parameter to be considered in the interpretation of mantle processes and geophysical data such as seismic wave speeds and electrical conductivity, and in geodynamic modeling. Keywords Eclogites * Xenoliths * Water content * Mantle metasomatism * Kaapvaal craton, Introduction Hydrogen (H), as one of the main volatile elements, plays an important role in Earth's evolution and dynamics (Dasgupta and Dixon 2009; and references therein). It is incorporated into [...]

Spinel lherzolite and wehrlite xenoliths from the Cenozoic Calatrava volcanic field carry the geochemical imprint of metasomatic agents that have affected the subcontinental lithospheric mantle beneath Central Iberia. Some xenoliths (mainly wehrlites) were enriched in REE, Sr, P, and C[O.sub.2] by silicic-carbonate-rich metasomatic melts/ fluids, while others record the effects of subduction-related hydrous silicate fluids that have precipitated amphibole and induced high Ti/Eu in primary clinopyroxene. The petrographic observations and geochemical data suggest that interstitial glass in the xenoliths represent the quenched products of Si-rich melts that infiltrated the mantle peridotite shortly before the entrainment of the xenoliths in the host magmas that erupted ca 2 million years ago. During their infiltration, the metasomatic melts reacted with peridotite, resulting in silica enrichment, while remobilizing grains of iron-rich monosulfide solid solution (Fe-rich Mss) initially enclosed in, or intergranular to, primary olivine and pyroxenes. In situ laser ablation inductively coupled plasma-mass spectrometry analysis of single sulfide grains reveals that the Fe-rich Mss in glass shows platinum-group element (PGE) patterns and [sup.187]Os/[sup.188]Os compositions identical to the Fe-rich Mss occurring as inclusions in, or at grain boundaries of primary silicates. Moreover, independent of its microstructural position, Fe-rich Mss exhibits PGE and [sup.187]Os/[sup.188]Os signatures typical of Mss either residual after partial melting or crystallized directly from sulfide melts. Our findings reveal that young metasomatic melt(s)/fluid(s) may carry remobilized sulfides with PGE and Os-isotopic signatures identical to those of texturally older sulfides in the peridotite xenolith. These sulfides thus still provide useful information about the timing and nature of older magmatic events in the subcontinental mantle. Keywords Mantle sulfides * Mantle xenoliths * Mantle metasomatism * Mantle melts * PGE and Re-Os signatures * Lithospheric mantle * Spain lithosphere, Introduction The in situ analysis of PGE and Re-Os isotopic compositions by laser sampling of individual sulfide grains in mantle xenoliths provides critical information on episodes of partial melting or [...]

Groundmass perovskite has been dated by LA-ICPMS in 135 kimberlites and related rocks from 110 localities across southern Africa. Sr and/or Nd isotopes have been analysed by LA-MC-ICPMS in a subset of these and integrated with published data. The age distribution shows peaks at 1,600-1,800, 1,000-1,200, 500-800 and 50-130 Ma. The major 'bloom' of Group I kimberlites at ca 90 ± 10 Ma was preceded by a slow build-up in magmatic activity from ca 180 Ma. The main pulse of Group II kimberlites at 120-130 Ma was a distinct episode within this build-up. Comparison of the isotopic data with seismic tomography images suggests that metasomatized subcontinental lithospheric mantle (SCLM) with very low [ζ.sub.Nd] and high [sup.87]Sr/[sup.86]Sr, (the isotopic signature of Group II kimberlites) was focused in low-Vs zones along translithospheric structures. Such metasomatized zones existed as early as 1,800 Ma, but were only sporadically tapped until the magmatic build-up began at ca 180 Ma, and contributed little to the kimberlitic magmas after ca 110 Ma. We suggest that these metasomatized volumes resided in the deep SCLM and that their low-melting point components were 'burned off' by rising temperatures, presumably during an asthenospheric upwelling that led to SCLM thinning and a rise in the ambient geotherm between 120 and 90 Ma. The younger Group I kimberlites therefore rarely interacted with such SCLM, but had improved access to shallower volumes of differently metasomatized, ancient SCLM with low [sup.87]Sr/[sup.86]Sr and intermediate [ζ.sub.Nd] (0-5). The kimberlite compositions therefore reflect the evolution of the SCLM of southern Africa, with metasomatic-enrichment events from as early as 1.8 Ga, through a major thermal and compositional change at ca 110 Ma, and the major kimberlite 'bloom' around 90 Ma. Keywords Perovskite * Kimberlite * U-Pb age * Sr-Nd isotopes * Lithospheric evolution * Mantle metasomatism, Introduction The problems of dating kimberlites, and understanding their sources, continue to be a research focus worldwide, because these rocks represent our best source of information on the composition and [...]

New seismic data suggest a possible coincidence under southeastern Australia of the Mohorovicic discontinuity (Moho) and the crust-mantle boundary. The data taken from deep seismic refraction tests conducted in western Victoria provide important details about the crust-mantle boundary and its stratigraphy and architecture. Seismic data show that the spinel herzolites in mantle wall-rock emanate from a depth similar to the Moho, suggesting a coincidence with a seismic Moho. This interpretation, however, is not conclusive and may be disputed.

Robins and Bennett (2013) express 'surprise' at the 'resurrection' of our interest in the remarkable field relations so well illustrated in the Nordre Bumandsfjord (as spelled in the relevant dialect) [...]

Abstract: U–Pb dating and Hf-isotope data from detrital zircons have been used to analyse the crustal evolution of the eastern and western parts of the Cathaysia Block in SE China. Zircons from the Oujiang River in eastern Cathaysia indicate that the basement is dominantly Paleoproterozoic (1850–1870Ma, 2100–2400Ma) in age with minor Archean components; it was extensively reworked in Jurassic–Cretaceous time (100–155Ma) to produce the widespread Yanshanian magmatic suite. Both the 1850–1870Ma event and the Yanshanian magmatism show wide ranges in Hf-isotope composition, consistent with mixing between crustal and juvenile magmas. Marked downstream changes in the relative proportions of zircon age populations emphasize the care required in using detrital zircon data to estimate continental growth rates. Zircons from the North River indicate that the crust of western Cathaysia was generated mainly during Neoproterozoic time, although it contains some Archean (2500–3500Ma) to Mesoproterozoic components. This crust was strongly reworked during Caledonian (ca. 450Ma), Indosinian (ca. 240Ma) and Early Yanshanian (ca. 160Ma) thermal events; there is little evidence for juvenile crustal growth in any of these events. Although the Yanshanian granitoids of the eastern and western parts of the Cathaysia give similar mean Hf model ages (consistent with published Nd model ages), this is a coincidence; they have different crustal sources. The distinct patterns of crustal evolution suggest that eastern and western Cathaysia may represent separate microcontinents, accreted to the older Yangtze craton and transposed by extensive strike-slip faults. [Copyright &y& Elsevier]

Understanding the formation, preservation and destruction of continental lithosphere in the paradigm of plate tectonics remains one of the "holy grails" of solid Earth Science. It is well recognized that the eastern North China Craton (NCC) experienced Mesozoic replacement of thick, ancient and refractory lithospheric mantle by a thin and fertile one, in response to the (Paleo-) Pacific subduction. However, the deep lithospheric responses to the long lifespan of the Paleo-Asian Ocean on the north of the craton are still poorly constrained. Based on a comprehensive review of the evolution of the NCC and the Paleo-Asian Ocean, we reappraise the available data on deep lithospheric (uppermost mantle and lower crust) xenoliths from northwestern NCC in conjunction with new U Pb ages and trace elements of zircon xenocrysts, and bulk-rock Pb-isotope data for pyroxenite xenoliths and mafic dykes. The northwestern NCC, far from the tectonic regimes on the south and east, has great potential to preserve the cratonic responses to the evolution of the Paleo-Asian Ocean. The fertile lithospheric mantle underlying the ancient northwestern NCC is characterized by low concentrations of CaO in olivine (<0.1 wt%), TiO 2 in spinel (typically <0.2 wt%), Wo component in orthopyroxene (<0.3), Ca/Al (<6), and high Na in clinopyroxene. These characteristics are distinct from those of refertilised cratonic mantle and oceanic mantle and resemble those of cooled asthenosphere. Lithospheric mantle domains with radiogenic Pb isotope compositions (e.g., 207Pb/204Pb > 16.7, 206Pb/204Pb = ~19) are recorded by Cr-pyroxenite and Al-pyroxenite xenoliths from the westernmost NCC. Such a radiogenic domain could be produced by lithospheric isolation from convecting mantle containing ancient recycled materials at ~1.4 Ga and was probably related to the opening of the Paleo-Asian Ocean. Zircon xenocrysts and crustal xenoliths from across the northwestern NCC record vigorous Mesoproterozoic (e.g., ~1.35 Ga) and Paleozoic (470–230 Ma) magmatic and thermotectonic events. The parental melts of the zircon xenocrysts are inferred to have been basaltic andesitic to andesitic in composition; the crystallization temperatures peak at ~900 °C, comparable to the equilibration temperatures of the lower crustal xenoliths. They are probably the deep crustal record of the opening and secular subduction of the Paleo-Asian Ocean. Late Cretaceous mafic rocks from the westernmost NCC, with elemental and Sr Nd isotopic compositions analogous to those of Cenozoic alkali basalts across eastern Asia, have relatively radiogenic Pb isotope compositions (206Pb/204Pb = 18.5–19). Similar radiogenic components are recognized in the Cenozoic basalts from the eastern NCC and the southeastern coast of China, and these signatures suggest the ubiquitous presence of recycled Paleo-Asian oceanic slab materials in the convecting mantle during the Late Mesozoic to Cenozoic. Collectively, the deep lithospheric modification of the northern NCC provides an archive of the fate of the Paleo-Asian Ocean, including the generation of fertile lithospheric mantle from cooled asthenosphere during its formation and melt infiltration during subduction and closure. This study shows the great potential of deep lithosphere at continental margins to reveal the secular interaction between the subducting oceanic slab, convecting mantle wedge and overlying lithosphere. [ABSTRACT FROM AUTHOR]