Garnet Lherzolite Research Articles

Sources and evolution of the Cenozoic suprasubduction magmatism of the Olyutorsky tectonic block, southern Koryak highland

This paper reports the results of an investigation of the geochemical and isotopic compositions of rocks formed during the Eocene suprasubduction magmatism in the Olyutorsky tectonic block. The contribution of various suprasubduction components to the formation of magmatic melts was estimated; the characteristics of the Eocene and Miocene-Quaternary suprasubduction magmatism of the Olyutorsky tectonic block were compared; and relations of the Cenozoic magmatism to the tectonic development of the block were evaluated. The Eocene-early Oligocene suprasubduction magmas were derived from geochemically and isotopically heterogeneous garnet lherzolites in a mantle wedge. The initially depleted lherzolites of the mantle wedge were probably locally and variably enriched by OIB-type mantle melts before the generation of island-arc magmas and then again depleted below the MORB level by the extraction of magmatic materials from them. In the Eocene, a considerable amount of quartz-feldspar sediments enriched in radiogenic Nd was consumed in the subduction zone, which resulted in a strong contamination of magmas derived from the garnet lherzolites of the mantle wedge. The later stages of subduction were accompanied by active generation of adakite magmas with depleted Nd isotope signatures and HFSE-rich melts showing no evidence for their contamination by sialic sediments. It was supposed that the Late Cenozoic subduction zone plunged northward beneath the Olyutorsky tectonic block. It was shown that the established characteristics of the suprasubduction magmatism of the Olyutorsky tectonic block could be related to Cenozoic spreading processes in the proto-Komandorsky basin of the Bering Sea.

Geochemical constraints on the origin of the late Jurassic proto-Caribbean oceanic crust in Hispaniola

The nature of the oceanic crust produced through rifting and oceanic spreading between North and South America during the Late Jurassic is a key element for the Caribbean plate tectonic model reconstruction. Located in the Cordillera Central of Hispaniola, the Loma La Monja volcano-plutonic assemblage (LMA) is com- posed of gabbros, dolerites, basalts, and oceanic sediments, as well as metamorphic equivalents, which represent a dismembered fragment of this proto-Caribbean oceanic crust. Petrologic and geochemical data show that the LMA have a relatively broad diversity in composition, which represent the crystallization products of a typical low-pressure tholeiitic fractionation of mid-ocean ridge basalts (MORB)-type parental magmas, ranging from N- to E-MORB. Three geochemical groups have been dis- tinguished in the volcanic sequence: LREE-flat to slightly LREE-enriched basalts of groups II and III occur inter- layered in the lower stratigraphic levels; and LREE- depleted basalts of group I in the upper levels. Mantle melt modeling suggests that group III magmas are con- sistent by mixing within a mantle melt column of low- degree (\1%) melts of a deep garnet lherzolite source and high-degree ((15%) melts of a shallow spinel source, and groups II and I magmas are explained with moderate to high (14-18%) and very high ((20%) fractional melting degrees of a shallower spinel mantle source, respectively. Thus, upward in the volcanic sequence of the LMA, the magmas represent progressively more extensive melting of shallower sources, in a plume-influenced spreading ridge of the proto-Caribbean oceanic crust. Nb/Y versus Zr/Y systematics combined with recent plate tectonic model reconstructions reveal that Caribbean Colombian oceanic plateau fragments in Hispaniola formed through melting of heterogeneous mantle source regions related with distinct plumes during at least from Aptian-Albian ((96 Ma) to Late Campanian.

P–T history of kimberlite-hosted garnet lherzolites from South-West Greenland

Exploration for diamonds in West Greenland has experienced a major boost within the last decade following the establishment of world-class diamond mines within the nearby Slave Province of the Canadian Arctic. Numerous companies have active programmes of diamond exploration and increasingly larger diamonds have been discovered, notably a 2.392 carat dodecahedral stone recovered by the Canadian exploration company Hudson Resources Inc. in January 2007. The Geological Survey of Denmark and Greenland (GEUS) is currently carrying out several studies aimed at understanding the petrogenesis of diamondiferous kimberlites in Greenland and the physical and chemical properties of their associated mantle source regions (e.g. Hutchison 2005; Nielsen & Jensen 2005).

Experimental Melting of Carbonated Peridotite at 6-10 GPa

Partial melting of magnesite-bearing peridotites was studied at 6–10 GPa and 1300–1700°C. Experiments were performed in a multianvil apparatus using natural mineral mixes as starting material placed into olivine containers and sealed in Pt capsules. Partial melts originated within the peridotite layer, migrated outside the olivine container and formed pools of quenched melts along the wall of the Pt capsule. This allowed the analysis of even small melt fractions. Iron loss was not a problem, because the platinum near the olivine container became saturated in Fe as a result of the reaction Fe2SiO4Ol = FeFe–Pt alloy + FeSiO3Opx + O2. This reaction led to a gradual increase in oxygen fugacity within the capsules as expressed, for example, in high Fe3+ in garnet. Carbonatitic to kimberlite-like melts were obtained that coexist with olivine + orthopyroxene + garnet ± clinopyroxene ± magnesite depending on P–T conditions. Kinetic experiments and a comparison of the chemistry of phases occasionally grown within the melt pools with those in the residual peridotite allowed us to conclude that the melts had approached equilibrium with peridotite. Melts in equilibrium with a magnesite-bearing garnet lherzolite are rich in CaO (20–25 wt %) at all pressures and show rather low MgO and SiO2 contents (20 and 10 wt %, respectively). Melts in equilibrium with a magnesite-bearing garnet harzburgite are richer in SiO2 and MgO. The contents of these oxides increase with temperature, whereas the CaO content becomes lower. Melts from magnesite-free experiments are richer in SiO2, but remain silicocarbonatitic. Partitioning of trace elements between melt and garnet was studied in several experiments at 6 and 10 GPa. The melts are very rich in incompatible elements, including large ion lithophile elements (LILE), Nb, Ta and light rare earth elements. Relative to the residual peridotite, the melts show no significant depletion in high field strength elements over LILE. We conclude from the major and trace element characteristics of our experimental melts that primitive kimberlites cannot be a direct product of single-stage melting of an asthenospheric mantle. They rather must be derived from a previously depleted and re-enriched mantle peridotite.

Diamondiferous peridotitic microxenoliths from the Diavik Diamond Mine, NT

Twenty-five diamonds recovered from 21 diamondiferous peridotitic micro-xenoliths from the A154 South and North kimberlite pipes at Diavik (Slave Craton) match the general peridotitic diamond production at this mine with respect to colour, carbon isotopic composition, and nitrogen concentrations and aggregation states. Based on garnet compositions, the majority of the diamondiferous microxenoliths is lherzolitic (G9) in paragenesis, in stark contrast to a predominantly harzburgitic (G10) inclusion paragenesis for the general diamond production. For garnet inclusions in diamonds from A154 South, the lherzolitic paragenesis, compared to the harzburgitic paragenesis, is distinctly lower in Cr content. For microxenolith garnets, however, Cr contents for garnets of both the parageneses are similar and match those of the harzburgitic inclusion garnets. Assuming that the microxenolith diamonds reflect a sample of the general diamond population, the abundant Cr-rich lherzolitic garnets formed via metasomatic overprinting of original harzburgitic diamond sources subsequent to diamond formation, conversion of original harzburgitic diamond sources occurred in the course of metasomatic overprint re-fertilization. Metasomatic overprinting after diamond formation is supported by the finding of a highly magnesian olivine inclusion (Fo95) in a microxenolith diamond that clearly formed in a much more depleted environment than indicated by the composition of its microxenolith host. Chondrite normalized REE patterns of microxenolith garnets are predominantly sinusoidal, similar to observations for inclusion garnets. Sinusoidal REEN patterns are interpreted to indicate a relatively mild metasomatic overprint through a highly fractionated (very high LREE/HREE) fluid. The predominance of such patterns may explain why the proposed metasomatic conversion of harzburgite to lherzolite appears to have had no destructive effect on diamond content.

Evolution of subcontinental lithospheric mantle beneath eastern China: Re–Os isotopic evidence from mantle xenoliths in Paleozoic kimberlites and Mesozoic basalts

The ages of subcontinental lithospheric mantle beneath the North China and South China cratons are less well-constrained than the overlying crust. We report Re–Os isotope systematics of mantle xenoliths entrained in Paleozoic kimberlites and Mesozoic basalts from eastern China. Peridotite xenoliths from the Fuxian and Mengyin Paleozoic diamondiferous kimberlites in the North China Craton give Archean Re depletion ages of 2.6–3.2 Ga and melt depletion ages of 2.9–3.4 Ga. No obvious differences in Re and Os abundances, Os isotopic ratios and model ages are observed between spinel-facies and garnet-facies peridotites from both kimberlite localities. The Re–Os isotopic data, together with the PGE concentrations, demonstrate that beneath the Archean continental crust of the eastern North China Craton, Archean lithospheric mantle of spinel- to diamond-facies existed without apparent compositional stratification during the Paleozoic. The Mesozoic and Cenozoic basalt-borne peridotite and pyroxenite xenoliths, on the other hand, show geochemical features indicating metasomatic enrichment, along with a large range of the Re–Os isotopic model ages from Proterozoic to Phanerozoic. These features indicate that lithospheric transformation or refertilization through melt-peridotite interaction could be the primary mechanism for compositional changes during the Phanerozoic, rather than delamination or thermal-mechanical erosion, despite the potential of these latter processes to play an important role for the loss of garnet-facies mantle. A fresh garnet lherzolite xenolith from the Yangtze Block has a Re depletion age of ∼1.04 Ga, much younger than overlying Archean crustal rocks but the same Re depletion ages as spinel lherzolite xenoliths from adjacent Mesozoic basalts, indicating Neoproterozoic resetting of the Re–Os system in the South China Craton.

Li-Sr-Nd isotope signatures of the plume and cratonic lithospheric mantle beneath the margin of the rifted Tanzanian craton (Labait)

Lithium concentrations and isotopic compositions of olivine and 87Sr/86Sr and 143Nd/144Nd of coexisting clinopyroxene from peridotite xenoliths from the Quaternary Labait volcano, Tanzania, document the influence of rift-related metasomatism on the ancient cratonic mantle. Olivines show negative correlations between Fo content and both δ7Li and Li concentrations. Olivines in iron-rich peridotites (Fo85–87) have high Li concentrations (3.2–4.8 ppm) and heavy δ7Li (+5.2 to +6.6). In contrast, olivines in ancient, refractory peridotites have lower Li concentrations (∼2 ppm) and relatively light δ7Li (+2.6 to +3.5). This reflects mixing between ancient, refractory cratonic lithosphere and asthenosphere-derived rift magmas. A uniquely fertile, deformed, high-temperature garnet lherzolite, interpreted to be from the base of the lithosphere, has a 87Sr/86Sr of 0.7029 and 143Nd/144Nd of 0.51286, similar to HIMU oceanic basalts. It provides the best estimate of the Sr–Nd isotope composition of the upwelling mantle (i.e., plume, sensu lato) underlying this portion of the East African Rift, and is slightly less radiogenic compared to previous estimates of the plume that were based on rift basalts. Although elevated δ7Li are not exclusive to HIMU source regions, the data collectively indicate that the plume beneath Labait has HIMU characteristics in Sr, Nd and Li isotope composition.

Incorporation of Fe3+ in phase-X, A2–xM2Si2O7Hx, a potential high-pressure K-rich hydrous silicate in the mantle

AbstractPhase-X, a potential sink for K in the mantle, is a synthetic hydrous K-rich silicate formed by the breakdown of K-amphibole at high pressure. It has the general formula A2–xM2Si2O7Hx where A = K, Na, Ca, □ (vacancy), and M = Mg, Al, or Cr. No other isomorphic substitutions, either for the A or the M site, were reported for such a synthetic compound. Here we report the crystal structure and chemical composition of a crystal of phase-X containinglarg e amounts of trivalent Fe. This crystal was synthesized in the model system garnet lherzolite–K2CO3 at lower pressure (P = 7 GPa) and higher temperature (T = 1450–1650°C) with respect to the stability range reported in the literature (i.e. P = 9–17 GPa and T = 1150–1400°C). Quantitative analysis led to the following formula: . The lattice parameters (hexagonal setting) are: a = 5.005(1), c = 13.148(2) Å, V = 285.23(9) Å3, and Z = 2. The structure was refined in space group P63cm to R = 5.06% using199 independent reflections and consists of MO6 octahedra layers stacked alongthe c axis and linked together by Si2O7 groups. The Si2O7 groups form pillars in the layer that contains A atoms in the cavities between the pillars. The Raman spectrum in the OH-stretching region indicates the existence of two different OH environments. However, the position of H could not be determined. The substitution of Fe3+ for Mg shortens octahedral bond distances. In addition, the entry of Fe3+ in M induces geometrical changes to the adjacent A site. The crystal-chemical characteristics are compared with published data on synthetic Fe-free phase-X.

Diamonds and their mineral inclusions from the A154 South pipe, Diavik Diamond Mine, Northwest territories, Canada

Mineral inclusions recovered from 100 diamonds from the A154 South kimberlite (Diavik Diamond Mines, Central Slave Craton, Canada) indicate largely peridotitic diamond sources (83%), with a minor (12%) eclogitic component. Inclusions of ferropericlase (4%) and diamond in diamond (1%) represent “undetermined” parageneses. Compared to inclusions in diamonds from the Kaapvaal Craton, overall higher CaO contents (2.6 to 6.0 wt.%) of harzburgitic garnets and lower Mg-numbers (90.6 to 93.6) of olivines indicate diamond formation in a chemically less depleted environment. Peridotitic diamonds at A154 South formed in an exceptionally Zn-rich environment, with olivine inclusions containing more than twice the value (of ∼ 52 ppm) established for normal mantle olivine. Harzburgitic garnet inclusions generally have sinusoidal rare earth element (REE N) patterns, enriched in LREE and depleted in HREE. A single analyzed lherzolitic garnet is re-enriched in middle to heavy REE resulting in a “normal” REE N pattern. Two of the harzburgitic garnets have “transitional” REE N patterns, broadly similar to that of the lherzolitic garnet. Eclogitic garnet inclusions have normal REE N patterns similar to eclogitic garnets worldwide but at lower REE concentrations. Carbon isotopic values ( δ 13C) range from − 10.5‰ to + 0.7‰, with 94% of diamonds falling between − 6.3‰ and − 4.0‰. Nitrogen concentrations range from below detection (< 10 ppm) to 3800 ppm and aggregation states cover the entire spectrum from poorly aggregated (Type IaA) to fully aggregated (Type IaB). Diamonds without evidence of previous plastic deformation (which may have accelerated nitrogen aggregation) typically have < 25% of their nitrogen in the fully aggregated B-centres. Assuming diamond formation beneath the Central Slave to have occurred in the Archean [Westerlund, K.J., Shirey, S.B., Richardson, S.H., Gurney, J.J., Harris, J.W., 2003b. Re–Os systematics of diamond inclusion sulfides from the Panda kimberlite, Slave craton. VIIIth International Kimberlite Conference, Victoria, Canada, Extended Abstracts, 5p.], such low aggregation states indicate mantle residence at fairly low temperatures (< 1100 °C). Geothermometry based on non-touching inclusion pairs, however, indicates diamond formation at temperatures around 1200 °C. To reconcile inclusion and nitrogen based temperature estimates, cooling by about 100–200 °C shortly after diamond formation is required.

Water partitioning between mantle minerals from peridotite xenoliths

The speciation and amount of water dissolved in nominally anhydrous silicates comprising eight different mantle xenoliths has been quantified using synchrotron micro-FTIR spectroscopy. Samples studied are from six geographic localities and represent a cross-section of the major upper mantle lithologies from a variety of tectonic settings. Clinopyroxene contains between 342 and 413 ppm H2O. Orthopyroxene, olivine and garnet contain 169–201, 3–54 and 0 to <3 ppm H2O, respectively. Pyroxenes water contents and the distribution of water between ortho- and clinopyroxene is identical regardless of sample mineralogy (D water cpx/opx = 2.1 ± 0.1). The total water contents of each xenolith are remarkably similar (113 ± 14 ppm H2O). High-resolution spectroscopic traverses show that the concentration and speciation of hydrous defects dissolved in each phase are spatially homogeneous within individual crystals and identical in different crystals interspersed throughout the xenolith. These results suggest that the amount of water dissolved in the silicate phases is in partial equilibrium with the transporting melt. Other features indicate that xenoliths have also preserved OH signatures of equilibrium with the mantle source region: Hydroxyl stretching modes in clinopyroxene show that garnet lherzolites re-equilibrated under more reducing conditions than spinel lherzolites. The distribution of water between pyroxenes and olivine differs according to xenolith mineralogy. The distribution of water between clinopyroxene and olivine from garnet peridotites (D water cpx/oliv (gnt) = 22.2 ± 24.1) is a factor of four greater than mineral pairs from spinel-bearing xenoliths (D water cpx/oliv (sp) = 88.1 ± 47.8). Such an increase in olivine water contents at the spinel to garnet transition is likely a global phenomenon and this discontinuity could lead to a reduction of the upper mantle viscosity by 0.2–0.7 log units and a reduction of its electrical resistivity by a factor of 0.5–0.8 log units.

Petrochemistry, oxygen isotopes and U‐Pb SHRIMP geochronology of mafic–ultramafic bodies from the Sulu UHP terrane, China

AbstractTwo Rongcheng eclogite‐bearing peridotite bodies (Chijiadian and Macaokuang) occur as lenses within the country rock gneiss of the northern Sulu terrane. The Chijiadian ultramafic body consists of garnet lherzolite, whereas the Macaokuang body is mainly meta‐dunite. Both ultramafics are characterized by high MgO contents, low fertile element concentrations and total REE contents, which suggests that they were derived from depleted, residual mantle. High FeO contents, an LREE‐enriched pattern and trace‐element contents indicate that the bulk‐rock compositions of these ultramafic rocks were modified by metasomatism. Oxygen‐isotope compositions of analysed garnet, olivine, clinopyroxene and orthopyroxene from these two ultramafic bodies are between +5.2‰ and +6.2‰ (δ18O), in the range of typical mantle values (+5.1 to +6.6‰). The eclogite enclosed within the Chijiadian lherzolite shows an LREE‐enriched pattern and was formed by melts derived from variable degrees (0.005–0.05) of partial melting of peridotite. It has higher δ18O values (+7.6‰ for garnet and +7.7‰ for omphacite) than those of lherzolite. Small O‐isotope fractionations (ΔCpx‐Ol: 0.4‰, ΔCpx‐Grt: 0.1‰, ΔGrt‐Ol: 0.3–0.4‰) in both eclogite and ultramafic rocks suggest isotopic equilibrium at high temperature. The P–T estimates suggest that these rocks experienced subduction‐zone ultrahigh‐pressure (UHP) metamorphism at ∼700–800 °C, 5 GPa, with a low geothermal gradient. Zircon from the Macaokuang eclogite contains inclusions of garnet and diopside. The 225 ± 2 Ma U/Pb age obtained from these zircon may date either the prograde conditions just before peak metamorphism or the UHP metamorphic event, and therefore constrains the timing of subduction‐related UHP metamorphism for the Rongcheng mafic–ultramafic bodies.

Lithospheric control on late Cenozoic magmatism at the boundary of the Tuva-Mongolian Massif, Khubsugul area, northern Mongolia

Late Cenozoic lavas from the western wall of the Khubsugul rift trough were erupted within the Tuva-Mongolian Massif with a pre-Vendian basement, and the lavas in the eastern wall of the trough were erupted within Early Caledonian terranes. The composition of the lavas was determined to vary across the strike of the boundary of the Tuva-Mongolian Massif. The western wall of the trough is dominated by hawaiites and contains subordinate volumes of basanites and much lower amounts of olivine tholeiites and basaltic tra- chyandesites. The eastern wall contains, in addition to hawaiites, widespread olivine tholeiites and basaltic andesites with subordinate amounts of basaltic trachyandesites. The boundary zone contains practically all rock types (except basaltic andesites) in roughly equal proportions. The trace-element simulations of the partial melt- ing processes demonstrates that the basaltic magmas were produced mainly by 0.5-5% partial melting of garnet lherzolite, with the probable mixing with partial melts derived from spinel lherzolite. The main factor control- ling the compositional variations of the lavas was likely the variable depths of their derivation due to variations in the lithosphere thickness at the boundary of the Tuva-Mongolian Massif. Based on the assumption that the source of the magmas was relatively homogeneous and on the results of simulations with the use of experimen- tal data on peridotite melting, we concluded that the asthenospheric sources of the basaltic magmas occurred at depths of 75 ± 10 km ( 24.6 ± 3.2 kbar) beneath the Tuva-Mongolian Massif and at 60 ± 12 km (20.1 ± 3.8 kbar) beneath the Early Caledonian terranes.

Multiple metasomatism in Sulu ultrahigh‐P garnet peridotite constrained by petrological and geochemical investigations

AbstractThe pre‐pilot hole (PP1) of the Chinese Continental Scientific Drilling Project (CCSD) recovered drill core samples from a 118 m‐thick section of peridotites located at Zhimafang in the southern Sulu UHP terrane, China. The peridotites consist of phlogopite‐bearing garnet lherzolite, harzburgite, wehrlite and dunite. Some peridotite layers contain magnesite and Ti‐clinohumite, and are characterized by LREE and LILE enrichment and HFSE depletion. Phlogopite (Phl) occurs in the peridotite matrix and is LILE‐enriched with low Zr/Hf ratios (0.19–0.60). Phlogopite shows a mantle signature in H and O isotopes (δ18O: +5.4‰ to +5.9‰, and δD: −76‰ to −91‰). Ti‐clinohumite (Ti‐Chu) is Nb and Ta‐enriched and has higher Ti and HREE concentrations than phlogopite. Magnesite (Mgs) occurs as megacrysts, as a matrix phase, and as veins (±Phl ± Ti‐Chu), and contains low REEtotal contents (&lt;0.3 ppm) with a flat REE pattern. The δ18O values (+5.5‰ to +8.0‰) of magnesite are in the range of primary carbonatite, but the δ13C values (−2.4‰ to −3.4‰) are slightly more positive than those of the mantle and of primary carbonatite. Petrochemical data indicate that the Zhimafang peridotite was subjected to three episodes of metasomatism, listed in succession from oldest to youngest: (1) crystallization of phlogopite in the mantle caused by infiltration of K‐rich hydrous fluid/melt; (2) formation of Mgs and Mgs ± Phl ± Ti‐Chu veins possibly caused by infiltration of mantle‐derived carbonatitic melt with a hydrous silicate component; and (3) replacement of magnesite, garnet and diopside by dolomite and secondary hydrous phases caused by a crust‐related, CO2‐bearing, aqueous fluid. Stable isotopic compositions of phlogopite and magnesite indicate metasomatic agents for events (1) and (2) are from an enriched mantle. Multiple metasomatism imposed on mantle peridotite of variable composition led to significant compositional heterogeneity at all scales within the Zhimafang peridotite.

Densities of metapelitic rocks at high to ultrahigh pressure conditions: What are the geodynamic consequences?

Current geodynamic models of continental collision involving (ultra)high pressure complexes imply that even deeply subducted continental crust is significantly lighter than the ultrabasic upper mantle. To test this implication, we have investigated density changes of major components of continental crust, in particular metagreywacke and metapelite, as a function of pressure and temperature using a Gibbs free energy minimization approach. Pseudosections were calculated for fixed chemical compositions and the P– T range of 10–40 kbar, 600–1000 °C. Selected compositions were those of natural psammopelitic rocks, average crustal components, various theoretical mixtures of quartz, plagioclase, illite, chlorite and Fe,Ti-oxides, and finally mid-ocean ridge basalt and lherzolite for comparison. Calculated densities were presented as density maps (isochors in P– T diagrams). In general, observed densities of psammopelitic rocks increase with rising pressure due to the formation of advancing amounts of garnet, Na-pyroxene, and kyanite. A common assemblage, for instance, at 25 kbar/800 °C consists of phengite, quartz, jadeite, garnet, kyanite, magnetite, and rutile. After overstepping the quartz–coesite transition the density of a mean psammopelitic rock (3.35 g/cm 3) is almost as high as that of garnet lherzolite. Calculations with other pelitic compositions demonstrate that the resulting densities (up to 3.5 g/cm 3) can even exceed that of a garnet lherzolite due to high contents of garnet. Our calculations suggest that (i) even non-basic crustal material can sink into the Earth's mantle to fertilize it and (ii) the proportion of low-density granitic rocks in deeply subducted continental crust must be relatively high to claim buoyancy forces for a return of this crust to the surface.

Late Miocene igneous rocks of Samos: the role of tectonism in petrogenesis in the southeastern Aegean

Abstract Late Miocene igneous rocks of Samos, in the southeastern Aegean Sea, comprise monzodiorite and minor granite of the Katavasis complex, trachyte and rhyolite of the Ambelos volcanic centre, and bimodal basalt–rhyolite at basin margins. Six new K–Ar ages, together with existing geochronology and biostratigraphy, show that the Katavasis complex and Ambelos centre date from 10–11 Ma and basalt–rhyolite from 8 Ma, correlating with cooling ages for the Katavasis complex and an unconformity in the basin fill. Monzodiorite, granite, trachyte and basalt all have similar radiogenic isotopes. Monzodiorite and basalt have similar trace element compositions and could result from 5–10% partial melting of enriched garnet lherzolite in the subcontinental lithosphere. Variations in trace elements suggest that trachyte and monzodiorite evolved by fractional crystallization from a parental magma similar to the younger basalt. The Katavasis and Ambelos rocks were synchronous with regional extension and listric faulting, which created opportunities for mid-crustal magma chambers and magma fractionation. Basalt extrusion was synchronous with the onset of north–south strike-slip faulting, which permitted more rapid transfer of magma to the surface. Late Miocene strike-slip faulting propagated from north to south in western Anatolia and the southeastern Aegean Sea, providing pathways for different types of mantle melts.

Petrological and geochemical constraints on the origin of garnet peridotite in the North Qaidam ultrahigh-pressure metamorphic belt, northwestern China

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, NW China, a large garnet peridotite massif has been exhumed together with felsic gneisses and eclogite during Early Paleozoic continental collision between North China Craton and Qaidam–Qilian Block. Four rock types are recognized in this massif: dunite without garnet, dunite with garnet, garnet lherzolite and garnet pyroxenite. The bulk compositions of these rocks retain depleted major element characteristics, but possess elevated abundances of incompatible trace elements, especially the fluid-mobile large ion lithophile elements (LILE, e.g., Rb, Cs, Ba, Sr, U, and Pb). The unradiogenic Nd ( ε Nd[460 Ma] − 0.5 to − 6.8) isotopes of these rocks are consistent with their flat to light rare earth element (LREE)-enriched patterns. Petrological and geochemical data altogether suggest that the protoliths of the garnet peridotite massif represent rocks from a subarc system in the Early Paleozoic. These rocks include subarc lithospheric harzburgite and a subarc cumulate assemblage from the ascending mantle wedge melts. These rocks were then further metasomatized by the ascending melts before being transported to depths of 150–200 km or more by the subducting slab-induced mantle wedge corner flow. Their subsequent exhumation to the upper crust is likely to have been accomplished during continental collision with a recovery rate of about 7.5−10 mm/year.

Origin and geodynamic significance of Upper Cretaceous lamprophyres from the Villány Mts (S Hungary)

In the Villany Mts of southern Hungary, ocelli-bearing porphyritic lamprophyre dykes and sills of Upper Cretaceous age occur sporadically, intruding Mesozoic carbonate rocks. They at places contain metasomatised mantle xenoliths and quartz xenocrysts of crustal origin. They are moderately fractionated with significant LILE and LREE enrichments and a notable Nb–Ta negative anomaly. Trace elements indicate that they formed in an intraplate environment by very low degree partial melting of a metasomatised garnet lherzolite mantle source that was enriched by earlier subduction. Based on petrography, geochemistry and age constraints, they differ from other Mesozoic basic rocks of the Tisza block (Mecsek Mts and Slavonian basalts); however, they show a significant geochemical similarity to the Upper Cretaceous lamprophyre dyke swarm from NE Transdanubia (northwestern Hungary) situated on the Alcapa microplate. Thus we suggest that lamprophyres from the Villany Mts and NE Transdanubia could have originated from the same or similar enriched asthenospheric mantle sources.

High-field-strength elements in carbonatitic rocks: Geochemistry, crystal chemistry and significance for constraining the sources of carbonatites

Carbonatites and related rocks exhibit a significant variation in their content of high-field-strength elements (HFSE), i.e. Ti, Nb, Ta, Zr and Hf. The average abundances of these elements (calculated for 119 localities worldwide) decrease from phoscorites and silicocarbonatites to calciocarbonatites to magnesiocarbonatites. The analytical data currently available for ferrocarbonatites are insufficient to establish if this trend persists through the entire carbonatitic series. In comparison with the primitive mantle, the average carbonatite has significantly higher Nb/Ta and Zr/Hf ratios, and lower Zr/Nb and Zr/Ta ratios (35, 60, 1 and 29, respectively). Alkali-ultramafic rocks associated with carbonatites exhibit, on average, higher Zr/Nb ratios and no (or little) depletion in heavy HFSE (Ta and Hf). Implications of these data for constraining the source(s) of intracratonic carbonatites are discussed using the Kola Alkaline Province (NW Russia and Finland) as an example. The bulk of incompatible HFSE (> 85%) is estimated to reside in Ti, Nb and Zr oxide minerals (in particular, perovskite, pyrochlore, ilmenite, baddeleyite and zirconolite) and zircon, whereas a significant proportion of Ti is contained in magnetite and ferromagnesian silicates. The lowest recorded Nb/Ta and Zr/Hf ratios in the primary HFSE oxides and titanite (but not zircon) are significantly below the primitive-mantle values, which makes these phases an effective vehicle of heavy-HFSE fractionation at crustal levels. The HFSE geochemistry of carbonatites en masse is not consistent with their derivation from a hypothetical carbonate-rich alkali-ultramafic (e.g., nephelinitic) magma by either liquid immiscibility or crystal fractionation. The bulk of carbonatites are interpreted to originate by low-degree partial melting of a metasomatized HFSE 5+-enriched mantle source with elevated Nb/Ta and Zr/Hf ratios. The trace-element signature of carbonatites cannot be adequately explained by clinopyroxene-controlled decoupling of HFSE 5+ and HFSE 4+ during partial melting, and requires the presence of amphibole and, possibly, refractory phlogopite in their mantle source. The HFSE geochemistry, field relations and relative timing of Kola carbonatites and associated silicate rocks are consistent with their derivation by low-degree melting of a vertically heterogeneous amphibole-bearing mantle reservoir, followed by differentiation of the primitive magmas. The previously established volume, tectonic and geochemical constraints suggest that the metasomatic enrichment of the mantle may have resulted from reaction of garnet lherzolite with carbonate melts of asthenospheric provenance, which is in agreement with the published experimental and mantle-xenolith studies. There is no evidence for the presence of refractory rutile (i.e., subducted crust) in the postulated mantle source.

Diamonds from the Buffalo Head Hills, Alberta: Formation in a non-conventional setting

Kimberlite pipes K11, K91 and K252 in the Buffalo Head Hills, northern Alberta show an unusually large abundance (20%) of Type II (no detectable nitrogen) diamonds. Type I diamonds range in nitrogen content from 6 ppm to 3300 ppm and in aggregation states from low (IaA) to complete (IaB). The Type IaB diamonds extend to the lowest nitrogen concentrations yet observed at such high aggregation states, implying that mantle residence occurred at temperatures well above normal lithospheric conditions. Syngenetic mineral inclusions indicate lherzolitic, harzburgitic, wehrlitic and eclogitic sources. Pyropic garnet and forsteritic olivine characterize the peridotitic paragenesis from these pipes. One lherzolitic garnet inclusion has a moderately majoritic composition indicating a formation depth of ∼ 400 km. A wehrlitic paragenesis is documented by a Ca-rich, high-chromium garnet and very CaO-rich (0.11–0.14 wt.%) olivine. Omphacitic pyroxene and almandine-rich garnet are characteristic of the eclogitic paragenesis. A bimodal δ 13C distribution with peaks at − 5‰ and − 17‰ is observed for diamonds from all three kimberlite pipes. A large proportion (∼ 40%) of isotopically light diamonds ( δ 13C < −10‰) indicates a predominantly eclogitic paragenesis. The Buffalo Head Terrane is of Lower Proterozoic metamorphic age (2.3–2.0 Ga) and hence an unconventional setting for diamond exploration. Buffalo Hills diamonds formed during multiple events in an atypical mantle setting. The presence of majorite and abundance of Type II and Type IaB diamonds suggests formation under sublithospheric conditions, possibly in a subducting slab and resulting megalith. Type IaA to IaAB diamonds indicate formation and storage under lower temperature in normal lithospheric conditions.

Archean to Proterozoic depletion in Cape Verde lithospheric mantle

Two groups of protogranular spinel-lherzolite and harzburgite xenoliths occur in Late Tertiary necks on Sal Island, Cape Verde Archipelago. Large protogranular clinopyroxenes from lherzolites (cpx 8–17 vol%) have high Cr2O3 and low CaO contents, and upward-convex rare earth element (REE) patterns (LaN = 3.6– 5.3; SmN = 8.3–12.8; YbN = 2.4–4.8). Orthopyroxenes show very high Cr2O3 contents and flat HREE distributions, with YbN ranging from 0.7 to 0.9. Small secondary clinopyroxenes in harzburgites (cpx 1–3 vol%) are characterized by strong enrichment of LREE and MREE (LaN = 7.9–64.1; SmN = 36.6–95.4) with positively fractionated HREE patterns (YbN = 4.9–13.1). Orthopyroxenes in harzburgites reflect high degrees of partial melting, with low Al2O3 contents ( 25%) of a spinel-bearing protolith, whereas the lherzolites reflect low-degree partial melting of a garnet-bearing lherzolite that re-equilibrated in the spinel stability field. Both lithotypes have been metasomatised by kimberlite-like melts, leaving veins of K-rich glass + K-feldspar. In situ Re–Os analyses of intergranular sulfides from lherzolites yield values of 187Os/188Os varying from 0.1037 + 0.0020 to 0.1256 + 0.0022. TRD model ages define three groups: 3320 + 150 Ma, 2060 + 140 Ma and 996 + 73 Ma (1 s). The age patterns in the Re–Os data, the juxtaposition of strongly depleted harzburgites and (originally) garnet lherzolites, and evidence of kimberlite-like metasomatism suggest that part of the ancient African subcontinental lithospheric mantle was incorporated into the newly formed oceanic lithosphere during the opening of the Atlantic ocean.