Proterozoic Mantle Research Articles

Lithospheric density structure beneath the dyke swarm region of the southern Bastar craton reveals a possible Proterozoic mantle plume: Inference from gravity and geoid study

Lithospheric density structure beneath the dyke swarm region of the southern Bastar craton reveals a possible Proterozoic mantle plume: Inference from gravity and geoid study

Proterozoic mantle melting recorded by the Re-Os isotopic systematics of ophiolites from the Qilian Orogenic Belt, northwestern China

• The Minhe serpentinites are mantle residues of multi-episode partial melting. • Fluid/melt metasomatism exerted a limited impact on the Re-Os systematics. • Re-Os ages provide temporal constraints on Proterozoic melt extraction. Re-Os isotopes of the mantle-derived ultramafic rocks can provide temporal constraints on the melt extraction of ancient mantle. Here, new geochemistry and Re-Os isotopes of serpentinites from the Minhe ophiolite in the Qilian Orogenic Belt (QOB) are presented. Their geochemical compositions (e.g., abundances of heavy rare earth elements, Al 2 O 3 , Yb, Re, and Th/Yb ratio) suggest that protoliths of the serpentinites originated as mantle residues. They underwent multiple episodes of melt depletion, leading to a highly refractory nature and high degrees of partial melting (> 18%). Moreover, subchondritic 187 Re/ 188 Os (0.0023 – 0.0399) and 187 Os/ 188 Os (0.1135 – 0.1210) ratios of the serpentinites, and decoupling of the Re-Os systematics and metasomatic proxies (e.g., LOI, Sr, La, and Th/Yb), demonstrate that fluid/melt metasomatism exerted a limited impact on their Re-Os isotopic systematics. Their Re-depletion ages (T RD ), varying from 1.22 – 2.21 Ga, are analogues to their Re-Os model ages (T MA , 1.29 – 2.31 Ga). Therefore, Re-Os isotopic ages recorded by ophiolites in QOB indicate that at least two major melt-extraction episodes occurred in the lithospheric mantle during Proterozoic (1.2 – 1.6 Ga and 2.0 – 2.2 Ga). Notably, the 1.2 – 1.6 Ga mantle melt-depletion event may have a genetic affinity with the 1.1 – 1.7 Ga crustal growth. Mantle rocks from an ophiolite have a great potential in tracking ancient geological events, such as Precambrian mantle melting.

Petrogenesis and Geodynamic Implications of a Newly Discovered Basanite Dike in Zaolin, Jingdezhen City, South China

Abstract A recently discovered basanite dike in the Zaolin area of Jingdezhen, South China, contains mantle xenocrysts such as kink-banded olivines, olivines + orthopyroxenes assemblage, and chromites. In addition, polymorphic carbonates of the MgCO3–FeCO3 series occur as augens, either independently or interspersed with diopside and spinel in the matrix. The rock is characterized by high Cr and Ni contents, high whole-rock Mg# values (0.66–0.72), and high Ca/Al (0.72–1.03) and TFeO/MgO (1.1–1.3) ratios and is alkali-rich with Na2O > K2O. The trace-element partition patterns are similar to those of other basanites in eastern China as well as ocean island basalts. Whole-rock geochemical analyses show depleted Sr and Nd isotopic compositions (86Sr/87Sr=0.70358–0.703853, εNd=2.52–6.73). These data indicate that the rock has experienced negligible crustal contamination, should be derived from asthenospheric mantle, or mixed by the MORB with EMI/EMII mantle and have been carbonated. The calculated T–P conditions of the melt in equilibrium with xeno-olivine are 1160–1320°C at the mantle depth. The high Cr# values of the spinel xenocrysts indicate that the lithospheric mantle under the Jingdezhen area was probably relict Proterozoic mantle. The Ar–Ar plateau age and the isochron and inverse isochron ages for the matrix of the basanite are all 44 Ma. The basanite, as well as other alkaline basalt or lamprophyre dikes in southeastern China, formed in a rifting regime during the Eocene.

Palaeoproterozoic Mafic magmatism in the Indian Shield: Petrologic, Geochemical and Thermal constraints on Proterozoic Mantle

Palaeoproterozoic Mafic magmatism in the Indian Shield: Petrologic, Geochemical and Thermal constraints on Proterozoic Mantle

The geodynamic setting of Dulan eclogite-type rutile deposits in the North Qaidam orogen, western China

Abundant eclogite-type rutile deposits have recently been discovered along the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in China. The identified titanium reserves exceed 3 million tons and range in grade from 1.0% to 4.5%. The rutile deposits of economic interest are found within two types of eclogite. However, the origin and original geodynamic setting of the two types of rutile-bearing eclogites remain controversial. This study therefore investigates ore and mineralization bodies belonging to the two types by way of detailed field work and geochronological, geochemical and Sr–Nd–Hf isotopic compositional analysis, taking the Dulan eclogite-type rutile deposit as its case study. Type-1 rutile-bearing eclogites enclosed in granitic gneisses are found to have Neoproterozoic protolith ages of 845 ± 4 Ma and an eclogite-facies metamorphic age of ca. 433 Ma. Their high TiO2, TFe2O3, and P2O5 contents cannot have been produced by rutile-formation during UHP metamorphism but could have arisen through the accumulation of ilmenite and apatite in a high-Ti protolith. They have ocean-island basalt (OIB)-like trace element patterns, with enrichment in light rare-earth elements (LREEs) and high field strength elements (HFSEs) (Nb, Ta, Zr, and Hf). The Hf isotopic compositions of the residual magmatic zircon cores in Type-1 rutile-bearing eclogites (εHf (845) = +1.4 to +6.0, TDM = 1.2–1.5 Ga) indicate that their parental magmas were derived from the Proterozoic mantle. However, they show initial 87Sr/86Sr values of 0.71059–0.71326 and εNd (845) values of −1.5 to −2.5, suggesting significant crustal contamination. Thus, the Type-1 rutile-bearing eclogites may have originated through partial melting of an OIB-type mantle plume source during the Neoproterozoic rifting of the Rodinia supercontinent. In contrast, the Type-2 rutile-bearing eclogites, which are hosted in a mafic–ultramafic ophiolitic complex, have protolith ages of 449 ± 3 Ma and eclogite-facies metamorphic ages of ca. 435–433 Ma. Correlations between the whole-rock MgO and the major and trace element contents of Type-2 rutile-bearing eclogites suggest that they are derived from cumulates with alternating olivine and pyroxene-rich and ilmenite-rich layers. They have mid-ocean-ridge basalt (MORB) and OIB-like trace element patterns, indicating extensive fractional crystallization and the possibility that there were multiple injections of magma during the formation of the protolith. Meanwhile, the +0.6 to +4.3 εNd (449) values and −1.8 to +11.8 εHf (449) values of the Type-2 rutile-bearing eclogites indicate that the protoliths may have been generated by partial melting of a mantle wedge source metasomatized by slab-derived components. The short interval (ca. 14 my) between magma generation and eclogite-facies metamorphism, in combination with the type of rock assemblage within which they are found, indicates that the Type-2 rutile-bearing eclogites originated in a supra-subduction zone (SSZ) ophiolite. Type-1 and Type-2 rutile-bearing eclogites have similar eclogite-facies metamorphic ages of ca. 435–433 Ma, indicating that the Type-1 and SSZ-related Type-2 protoliths were incorporated into a subduction mélange that was dragged down the subduction channel and underwent eclogite-facies metamorphism via subduction of the Qaidam Block, eventually forming eclogite-type rutile deposits.

A mantle plume origin for the Palaeoproterozoic Circum-Superior Large Igneous Province

The Circum-Superior Large Igneous Province (LIP) consists predominantly of ultramafic-mafic lavas and sills with minor felsic components, distributed as various segments along the margins of the Superior Province craton. Ultramafic-mafic dykes and carbonatite complexes of the LIP also intrude the more central parts of the craton. Most of this magmatism occurred ∼1880Ma. Previously a wide range of models have been proposed for the different segments of the CSLIP with the upper mantle as the source of magmatism.New major and trace element and Nd-Hf isotopic data reveal that the segments of the CSLIP can be treated as a single entity formed in a single tectonomagmatic environment. In contrast to most previous studies that have proposed a variety of geodynamic settings, the CSLIP is interpreted to have formed from a single mantle plume. Such an origin is consistent with the high MgO and Ni contents of the magmatic rocks, trace element signatures that similar to oceanic-plateaus and ocean island basalts and εNd-εHf isotopic signatures which are each more negative than those of the estimated depleted upper mantle at ∼1880Ma. Further support for a mantle plume origin comes from calculated high degrees of partial melting, mantle potential temperatures significantly greater than estimated ambient Proterozoic mantle and the presence of a radiating dyke swarm. The location of most of the magmatic rocks along the Superior Province margins probably represents the deflection of plume material by the thick cratonic keel towards regions of thinner lithosphere at the craton margins. The primary magmas, generated by melting of the heterogeneous plume head, fractionated in magma chambers within the crust, and assimilated varying amounts of crustal material in the process.

The Paleoproterozoic Otish Gabbro suite and coeval dyke swarms of the Superior Province: Probing the ca. 2.17 Ga mantle

The uraniferous, Proterozoic Otish Basin of the southeastern Superior Province hosts shallow-dipping sills and northeast to northwest-trending dykes of the Otish Gabbro suite and the north-trending Matoush dyke. Otish Gabbro sills comprise two distinct geochemical groups that reflect different degrees of crystal accumulation in a common olivine-tholeiitic parental magma. Group 1 Otish gabbros approach liquid compositions, whereas Group 2 gabbros are crystal cumulates of olivine+plagioclase+clinopyroxene. The high Nb/Yb, La/Yb, Th/Yb and low Zr/Nb ratios of the Otish Gabbro sills reflect trace element-enriched, mantle-derived parental liquids, modified by the addition of a crustal component. A north-trending Matoush dyke records a second magmatic event in the Otish Basin, genetically unrelated to the emplacement of the Otish Gabbro sills. In contrast to the gabbros, the strongly altered Matoush dyke has more fractionated trace element patterns and strong enrichments in light rare earth elements (LREE) that are indicative of a lamprophyric affinity.The Otish Gabbro sills are part of a large ca. 2.17Ga magmatic event that also generated the extensive Biscotasing and Payne River dyke swarms in the south-central and northeastern Superior Province, respectively. The contemporaneous mafic dyke swarms experienced relatively minor crustal contamination and their high Nb/Zr and Nb/Yb ratios suggest derivation from fertile mantle sources. Furthermore, the low Ti/Yb ratios of the ca. 2.17Ga magmas are consistent with melting at relatively shallow depths (<100km), implying removal of the depleted Archean lithosphere from a large portion of the Superior Province and replacement by a more fertile Proterozoic mantle. The variable geochemical composition of the ca. 2.17Ga mafic magmas indicates melting of a compositionally heterogeneous mantle and appears inconsistent with a common distal mantle plume source. A more suitable petrogenetic model for the ca. 2.17Ga dyke swarms of the Superior Province is vertical rise of mantle-derived magmas in a rifting environment.

Metasomatic enrichment of Proterozoic mantle south of the Kaapvaal Craton, South Africa: origin of sinusoidal REE patterns in clinopyroxene and garnet

Xenoliths of mantle peridotite have been sampled from four kimberlite intrusions, Melton Wold, Hebron, Uintjiesberg and Markt, emplaced through the Mesoproterozoic Namaqua-Natal Belt, along the southern border of the Kaapvaal Craton. Although many of the xenoliths are heavily altered, constituent clinopyroxene, garnet and phlogopite are fresh and have been analysed by electron microprobe for major elements and by laser ablation ICP-MS for trace elements. Primitive mantle-normalised REE abundances in clinopyroxene are all strongly LREE enriched and show a range of patterns including uniformly MREE–HREE sloped (referred to here as ‘normal’), sinusoidal and humped sinusoidal patterns. HREE abundances are extremely low (Yb = 0.3–0.06 × PM). REEN patterns in coexisting garnets show a similar range of patterns. When normalised to primitive mantle values, trace element patterns in some clinopyroxenes show strong relative depletion in Rb–Ba, Ta–Nb and Ti, with some samples also being relatively depleted in Zr–Hf. These trace element characteristics are indistinguishable from those found in clinopyroxene and garnet from peridotites from the adjacent cratonic mantle. Numerical modelling of reactive porous flow of an enriched metasomatic melt through a geochemically depleted peridotite matrix can account for the full range in observed REEN patterns. The relative depletion in Rb–Ba, Ta–Nb and Ti can be accounted for by an early crystallisation of phlogopite from the percolating melt. The relative depletion in Zr–Hf in some clinopyroxenes requires either zircon to crystallise in the proximal metasomatic assemblage, or metasomatism by a carbonatitic melt. Modelling results, together with the absence of clinopyroxene with depleted or even partially enriched REEN patterns, suggest that all clinopyroxene has been modally introduced through metasomatism into an initially highly depleted harzburgitic protolith. The range in Sr and Pb isotopic composition of the clinopyroxenes indicates regional metasomatism by melts of various compositions. The strong HREEN depletion is interpreted to reflect the effect of initial melt depletion in the early Proterozoic, with melting extending into the spinel stability field requiring an oceanic realm, and again later in the Mesoproterozoic (Namaqua Orogeny). The superimposed incompatible element enrichment indicates subsequent multiple enrichment events by rising alkaline melts similar in composition to kimberlite or ultramafic alkaline lamprophyre, possibly related to Mesozoic plume upwelling beneath the region, that reintroduced clinopyroxene into the depleted Proterozoic harzburgite protolith.

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.

The Early Proterozoic Matachewan Large Igneous Province: Geochemistry, Petrogenesis, and Implications for Earth Evolution

The Matachewan Large Igneous Province (LIP) is interpreted to have formed during the early stages of mantle plume-induced continental break-up in the early Proterozoic. When the Matachewan LIP is reconstructed to its original configuration with units from the Superior Craton and other formerly adjacent blocks (Karelia, Kola, Wyoming and Hearne), the dyke swarms, layered intrusions and flood basalts, emplaced over the lifetime of the province, form one of the most extensive magmatic provinces recognized in the geological record. New geochemical data allow, for the first time, the Matachewan LIP to be considered as a single, coherent entity and show that Matachewan LIP rocks share a common tholeiitic composition and trace element geochemistry, characterized by enrichment in the most incompatible elements and depletion in the less incompatible elements. This signature, ubiquitous in early Proterozoic continental magmatic rocks, may indicate that the Matachewan LIP formed through contamination of the primary magmas with lithospheric material or that the early Proterozoic mantle had a fundamentally different composition from the modern mantle. In addition to the radiating geometry of the dyke swarms, a plume origin for the Matachewan LIP is consistent with the geochemistry of some of the suites; these suites are used to constrain a source mantle potential temperature of c. 1500–1550°C. Comparison of these mantle potential temperatures with estimated temperatures for the early Proterozoic upper mantle indicates that they are consistent with a hot mantle plume source for the magmatism. Geochemical data from coeval intrusions suggest that the plume head was compositionally heterogeneous and sampled material from both depleted and enriched mantle. As has been documented with less ancient but similarly vast LIPs, the emplacement of the Matachewan LIP probably had a significant impact on the early Proterozoic global environment. Compilation of the best age estimates for various suites shows that the emplacement of the Matachewan LIP occurred synchronously with the Great Oxidation Event. We explore the potential for the eruption of this LIP and the emission of its associated volcanic gases to have been a driver of the irreversible oxygenation of the Earth.

The geochemistry and petrogenesis of the Paleoproterozoic du Chef dyke swarm, Québec, Canada

The du Chef dyke swarm in southern Québec, Canada is composed of numerous northeast trending, greenschist-amphibolite facies, gabbronoritic dykes that crop out either side of the Grenville Front. The age of the du Chef swarm (2408±3Ga) has led previous authors to suggest a genetic link between the du Chef dykes and coeval swarms (including the Ringvassøy, Scourie, Widgemooltha and Sebanga) preserved on other Archean cratons. These now disparate dyke swarms are proposed to have formed in response to mantle plume-induced continental breakup during the early Proterozoic. This work represents the first geochemical study of the du Chef dykes and shows that the swarm evolved through fractional crystallisation of a tholeiitic parent magma that remained largely uncontaminated during its residence in, and ascent through, the crust. We also show that the primary magma for the du Chef swarm was derived through partial melting of an enriched region of the mantle, with a similar trace element composition to the modern-day HIMU reservoir and that the magma produced was significantly hotter than the ambient mantle at the time. We contend that the du Chef dykes are the product of early Proterozoic mantle plume magmatism and may help pinpoint an ancient hotspot centre that initiated continental break up along the margin of the Superior Craton at ∼2.4Ga. Other dyke swarms proposed to be genetically linked with the du Chef dykes record a distinctly different petrogenetic history to that of the du Chef dykes, as evidenced by their more volcanic arc-like geochemical signature. These contrasting geochemical signatures in supposedly cogenetic continental tholeiitic rocks may be evidence of early Proterozoic mantle heterogeneity sampled by the rising du Chef mantle plume.

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.

The Hudson Bay Lithospheric Experiment (HuBLE): insights into Precambrian plate tectonics and the development of mantle keels

Abstract Hudson Bay Lithospheric Experiment (HuBLE) was designed to understand the processes that formed Laurentia and the Hudson Bay basin within it. Receiver function analysis shows that Archaean terranes display structurally simple, uniform thickness, felsic crust. Beneath the Palaeoproterozoic Trans-Hudson Orogen (THO), thicker, more complex crust is interpreted as evidence for a secular evolution in crustal formation from non-plate-tectonic in the Palaeoarchaean to fully developed plate tectonics by the Palaeoproterozoic. Corroborating this hypothesis, anisotropy studies reveal 1.8 Ga plate-scale THO-age fabrics. Seismic tomography shows that the Proterozoic mantle has lower wavespeeds than surrounding Archaean blocks; the Laurentian keel thus formed partly in post-Archaean times. A mantle transition zone study indicates ‘normal’ temperatures beneath the Laurentian keel, so any cold mantle down-welling associated with the regional free-air gravity anomaly is probably confined to the upper mantle. Focal mechanisms from earthquakes indicate that present-day crustal stresses are influenced by glacial rebound and pre-existing faults. Ambient-noise tomography reveals a low-velocity anomaly, coincident with a previously inferred zone of crustal stretching, eliminating eclogitization of lower crustal rocks as a basin formation mechanism. Hudson Bay is an ephemeral feature, caused principally by incomplete glacial rebound. Plate stretching is the primary mechanism responsible for the formation of the basin itself.

The Aguilar pluton (23°12′ S–65°40′ W; NW Argentina): Petrological implications on the origin of the Late Jurassic intraplate magmatism in the Central Andes

The Late Jurassic Aguilar pluton is located in NW Argentina, about 300–400 Km east of the Tarapacá basin, representing the backarc basin linked to the Jurassic volcanic arc. This small-size and compositionally heterogeneous pluton intruded the metasedimentary rocks of the Ordovician Santa Victoria Group, along the Cobres-Salinas Grandes lineament. A revision of published geochemical data in the light of new field and petrological results, allows us to propose a model concerning the petrogenesis and emplacement mechanisms of Aguilar pluton and to discuss its geodynamic setting. The pluton is mainly composed of metaluminous and nearly peraluminous granitoids, showing the geochemical characteristics of ferroan granites. The volumetrically subordinate mafic rocks are both ne- and hy-normative, and their primary magmas were generated by partial melting of a pristine Proterozoic mantle. Aguilar rocks display a rather limited range in (87Sr/86Sr)i, compared to the entire rift-related plutonic suite, i.e., 0.703198–0.704601, and εNdt from −1.06 to 3.82, calculated at 149 Ma. Fractional crystallization of mantle-derived magmas and crustal contamination processes explain the evolution to produce strongly silica-oversaturated magmas, which emplaced in the continental crust. The petrological data indicate that magma emplacement and cooling occurred at rather shallow depth. Multiple injections of magma batches into the magma chamber caused mingling and mixing processes early in the crystallization history. The Aguilar pluton is one of the several igneous complexes whose formation was associated with the extensional tectonics active during Late Jurassic-Early Cretaceous in NW Argentina. Based on the geological position and the igneous rocks affinity, we exclude that the Late Jurassic magmatism was generated in an orogenic setting and envisage that it was linked to the early extensional phase that preceded the Cretaceous continental rifting, related to the break–up of the South America-Africa continents.

Petrology, geochemistry and Re[sbnd]Os isotopes of peridotite xenoliths from Maguan, Yunnan Province: Implications for the Cenozoic mantle replacement in southwestern China

Petrology, geochemistry and ReOs isotopes of peridotite xenoliths from Maguan (Yunnan Province) are reported in this paper with the aims of constraining the age and evolution of the lithospheric mantle beneath the western margin of the Cathyasia block. The Maguan mantle xenoliths contain predominantly fertile lherzolites with whole-rock Al2O3 contents of 2.42–4.99wt.%, and subordinate clinopyroxene-poor lherzolites with Al2O3 contents of 1.19–1.98wt.%. Their whole-rock CaO, Al2O3 and Na2O decrease along with the increase of MgO, following melt depletion trends. This suggests that the Maguan lherzolites represent mantle residues after variable degrees of partial melting. Clinopyroxenes in the fertile lherzolites display flat to depleted REE patterns, whereas those in the clinopyroxene-poor lherzolites are variably enriched in LREE. Modeling results of Y and Yb contents in clinopyroxenes suggest that the fertile lherzolites have experienced ~1–5% degrees of partial melting, in contrast with ~10–15% for the clinopyroxene-poor lherzolites. Both fertile and clinopyroxene-poor lherzolites have similarly high equilibrium temperatures, i.e., 911–1120°C versus 919–941°C, respectively. The whole-rock 187Os/188Os ratios of clinopyroxene-poor lherzolites vary from 0.11764 to 0.12506, which are slightly lower than most fertile lherzolites (0.12272-0.12854). Their 187Os/188Os ratios show no correlation with 187Re/188Os ratios or bulk-rock Al2O3 contents. The rhenium depletion ages (TRD) of the lherzolites range from 0.15 to 1.08Ga, whereas the clinopyroxene-poor lherzolites have TRD ages of 0.64–1.67Ga. This suggests the co-existence of Phanerozoic and Proterozoic mantle beneath the western Cathyasia block. Alternatively, the whole lithospheric mantle beneath Maguan was likely formed during the Phanerozoic, given the resemblance of their Os isotopic ratios with those of abyssal peridotites. The latter explanation is consistent with the fact that all the studied samples plot along the oceanic trend in a plot of olivine modes versus Fo contents. We suggest that the enriched mantle that was existed beneath the western Cathyasia block during the Late Cretaceous or Eocene–Oligocene has been replaced by juvenile and depleted mantle, which probably occurred during the Cenozoic.

Petrology, geochemistry and Re–Os isotopes of peridotite xenoliths from Yantai, Shandong Province: Evidence for Phanerozoic lithospheric mantle beneath eastern North China Craton

Petrology, geochemistry and Re–Os isotopes of peridotite xenoliths from Yantai (Shandong Province) are reported in this paper with aims of constraining the age and evolution of the lithospheric mantle beneath eastern North China Craton. The Yantai xenoliths contain predominant harzburgites and subordinate lherzolites. Although their highly incompatible element compositions have been modified by metasomatism, the heavy rare earth element (REE) and Y contents in the Yantai peridotites are primarily governed by partial melting, which started in garnet stability field then continued in spinel stability field after breakdown of garnet to two pyroxenes and spinel. Such a polybaric melting produced a residual mantle in which degree of depletion decreases with depth. Os isotopic ratios of the most refractory peridotites (Al2O3<1.2wt.%) range from 0.117 to 0.126, yielding TRD model ages between 0.5 and 1.7Ga. This suggests co-existence of Phanerozoic and Proterozoic mantle beneath Shandong Province. Alternatively, the whole lithospheric mantle beneath Yantai was likely formed during the Phanerozoic, given the resemblance of their Os isotopic ratios with those of abyssal peridotites. The latter interpretation is consistent with the fact that all the studied samples plot along the oceanic trend in a plot of forsterite content in olivine versus olivine mode. It also gains further support from the contrasting εNd between the late Mesozoic lithospheric mantle and Cenozoic mantle beneath the region. The data presented in this study therefore argue for a complete replacement of the cratonic mantle by upwelling asthenosphere.

The Xinchang peridotite xenoliths reveal mantle replacement and accretion in southeastern China

The Xinchang mantle xenoliths, including garnet lherzolites, spinel lherzolites and harzburgites, have been studied for the nature and age of the lithospheric mantle beneath southeastern (SE) China. The spinel harzburgites have refractory compositions, with 0.95–1.73wt.% Al2O3, 0.65–1.47wt.% CaO, 0.03–0.16wt.% TiO2 and 2134–2422ppm Ni. The lherzolites (both spinel- and garnet-) have more fertile compositions, containing 2.4–5.43wt.% Al2O3, 2.17–3.22wt.% CaO, 0.1–0.38wt.% TiO2 and 1733–2055ppm Ni. Olivines in the harzburgites have Fo contents of 88.7–91.4, which is 88.4–90.2 for the lherzolites. Both clinopyroxene and orthopyroxene have lower Al2O3 but higher Cr2O3 contents than those in the lherzolites. Modeling of the Y and Yb contents in clinopyroxenes indicates that the spinel harzburgites have been subjected to ca. 20% degrees of partial melting. Reaction textures, in particular the sieve-texture in clinopyroxene, is widely developed in the Xinchang mantle xenoliths, which resulted from reaction with the host lavas. The sieve-textured clinopyroxenes show compositional zonation; the sieve-textured rims have lower Na2O and Al2O3 but higher Cr2O3 and CaO than the inclusion-free cores. The inclusion-free cores of clinopyroxene in the harzburgites show clearly enriched characteristics in trace elements, reflecting they have been metasomatized before the entrainment in the host lavas. Clinopyroxenes have high Ti/Eu but low La/Ya ratios, suggesting that the Xinchang mantle xenoliths were metasomatized by silicate melts. The spinel harzburgites have 187Os/188Os of 0.11999–0.12258, giving TRD age of 0.99–1.35Ga relative to the primitive upper mantle (PUM). In comparison, both spinel and garnet lherzolites have more radiogenic 187Os/188Os ratios (0.12424–0.12801) and younger model ages (0.22–0.75Ga). We explain that the spinel harzburgites represent the ancient lithosphere relic beneath the Xinchang region, whereas both spinel and garnet lherzolites represent the newly accreted mantle. This reflects that the lower portion of the Proterozoic mantle has been removed and replaced by younger, hotter and juvenile mantle that was accreted from the asthenosphere. The harzburgites have high equilibrium temperatures of 1112–1174°C, which are indistinguishable from those of the garnet lherzolites (1108–1198°C). This suggests that the lithosphere relic has been heated by the newly accreted asthenosphere. The mantle replacement and accretion is a common process in SE China and could be related to the lithospheric extension since the Mesozoic.

Detrital pyrope garnets from the El Kseibat area, Algeria: A glimpse into the lithospheric mantle beneath the north-eastern edge of the West African Craton

Pyrope garnet grains found in Cretaceous and Quaternary alluvial sediments in the El Kseibat area (Algeria) suggest the presence of kimberlites along the NE margin of the West African Craton. The garnets have been studied using major- and trace-element chemistry to obtain information about the composition and thermal properties of the lithospheric mantle beneath this area. Most of the garnet grains are lherzolitic in composition (group G9), but range up to high Cr contents (>10% Cr 2O 3); two grains are harzburgitic (G9/G10). Three differently metasomatised groups of pyrope garnets were distinguished: (1) depleted grains that have low Sr, Ti, and Zr contents; (2) grains having low Sr and Ti and high Zr contents; and (3) grains with high Ti and Zr contents. Each group reflects a different stage and intensity of metasomatism. A range of chondrite-normalised rare earth element (REE n) patterns also reflects several styles of metasomatism. Based on geochemical features, most grains appear to be derived from Archean to Proterozoic mantle. The geotherm estimated from the garnet compositions is typically cratonic ( ca 40 mW/m 2), but somewhat higher than that estimated for the interior of the Craton using heat flow constraints (33 ± 8 mW/m 2). Depleted garnets give P– T estimates up to 950 °C, 40 kbar; those showing melt-related metasomatism (high Ti, Zr) go up to 1400 °C and >50 kbar. Nearly all garnet grains were derived from the graphite stability field at depths of 100–170 km, but some may be derived from the diamond stability field. The pyrope garnets of the El Kseibat area indicate that their lithospheric-mantle source has a lherzolitic composition as a result of several stages of metasomatism.

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.