Spinel Lherzolite Research Articles

Geochronology and geochemistry of Eocene-aged volcanic rocks around the Bafra (Samsun, N Turkey) area: Constraints for the interaction of lithospheric mantle and crustal melts

40Ar–39Ar age, whole-rock chemical, and Sr–Nd isotope data are presented for the post-collisional, Eocene (51.3–44.1Ma)-aged volcanic rocks from the Bafra (Samsun) area in the western part of the Eastern Pontides (N Turkey) aiming to unravel their sources and evolutionary history. The studied Eocene volcanic rocks can be divided into two groups: analcime-bearing (tephritic lava flows and dykes) and analcime-free (basaltic to trachytic lava flows and basaltic dykes). The analcime-bearing volcanic rocks have a fine-grained porphyritic texture with clinopyroxene phenocrysts, whereas analcime-free volcanic rocks show a variety of textures including hyalo-microlitic microgranular porphyritic, intersertal, trachytic, fluidal, and glomeroporphyritic. The volcanic rocks also show evidence of mineral-melt disequilibrium textures such as sieved, rounded, and corroded plagioclases, partially melted and dissolved clinopyroxenes and poikilitic texture. Petrochemically, the parental magmas of the volcanic rocks evolved from alkaline to calc-alkaline lava suites and include high-K and shoshonitic compositions. They display enrichments in light rare earth and large ion lithophile elements such as Sr, K, and Rb, as well as depletions in high field strength elements such as Nb, Ta, Zr, and Ti, resembling subduction-related magmas. The analcime-bearing and -free volcanic rocks share similar incompatible element ratios and chondrite-normalised rare rearth element patterns, indicating that they originated from similar sources. They also have relatively low to moderate initial 87Sr/86Sr (0.7042–0.7051), high positive εNd(t) values (+0.20 to +3.32), and depleted mantle Nd model ages (TDM1=0.63–0.93Ga, TDM2=0.58–0.84Ga). The bulk-rock chemical and Sr–Nd isotope features as well as the high Rb/Y and Th/Zr, but low Nb/Zr and Nb/Y ratios, indicate that the volcanic rocks were derived from a lithospheric mantle source that had been metasomatised by slab-derived fluids. Trace element modelling suggests that the parental magma(s) of the volcanic rocks represent mixtures of melts derived by low-degree (~5–10%) partial melting of spinel–lherzolite (40–85%) and garnet–lherzolite (15–60%) mantle sources. Sr–Nd isotopic modelling also suggests that a 25–35% lower crustal component was added in the parental magmas; AFC modelling additionally indicates minor upper crustal contamination during the evolution of the volcanic rocks. In conclusion, integration of the geochemical, petrologic, and isotopic data with regional geology suggests that the analcime-bearing and -free volcanic rocks evolved from parental magma(s) derived from melts of a subcontinental lithospheric mantle and lower crustal sources.

The basaltic volcanism of the Dumisseau Formation in the Sierra de Bahoruco, SW Dominican Republic: A record of the mantle plume-related magmatism of the Caribbean Large Igneous Province

The basaltic volcanism of the Dumisseau Fm in the Sierra de Bahoruco, SW Dominican Republic, offers the opportunity to study, on land, the volcanism of the Caribbean Large Igneous Province (CLIP). It consists of an at least 1.5km-thick sequence of submarine basaltic flows and pyroclastic deposits, intruded by doleritic dykes and sills. Three geochemical groups have been identified: low-Ti tholeiites (group I); high-Ti transitional basalts (group II); and high-Ti and LREE-enriched alkaline basalts (group III). These geochemical signatures indicate a plume source for all groups of basalts, which are compositionally similar to the volcanic rocks that make up various CLIP fragments in the northern region of the Caribbean Plate. Trace element modelling indicates that group I magmas are products of 8–20% melting of spinel lherzolite, group II magmas result 4–10% melting of a mixture of spinel and garnet lherzolite, and group III basalts are derived by low degrees (0.05–4%) of melting of garnet lherzolite. Dynamic melting models suggest that basalts represent aggregate melts produced by progressive decompression melting in a mantle plume. There is no compositional evidence for the involvement of a Caribbean supra-subduction zone mantle or crust in the generation of the basalts. Two 40Ar/39Ar whole-rock ages reflect the crystallisation of group II magmas at least in the late Campanian (~74Ma) and the lower Eocene (~53Ma). All data suggest that the Dumisseau Fm is an emerged fragment of the CLIP, which continues southward through the Beata Ridge

The behavior of Fe3+/∑Fe during partial melting of spinel lherzolite

This study presents an internally consistent model for the behavior of Fe3+/∑Fe during partial melting of spinel lherzolite. The Fe3+/∑Fe ratio for olivine is calculated on the basis of point defect thermodynamics, and the oxidation states of iron in the other solid phases are calculated using Fe3+/Fe2+ distribution between olivine and orthopyroxene, clinopyroxene, or spinel. Conservation of mass is used to relate the Fe3+/Fe2+ ratio of partial melt to the concentrations of Fe3+ and Fe2+ in the initial and residual solids as a function of pressure, temperature, and oxygen fugacity. Results from isobaric batch melting calculations demonstrate that the Fe3+/∑Fe ratio of the partial melt decreases with increasing melt fraction. Conversely, the Fe3+/∑Fe ratio of the partial melt increases with increasing melt fraction during decompression batch melting. The relative oxygen fugacity of the upper mantle depends on both the oxidation state of iron and mantle potential temperature. Results from incremental decompression melting calculations in which 1% melt is produced for each 100MPa of decompression and then removed from the residual solid indicate that relative oxygen fugacity calculated from the oxidation state of iron in basaltic glass does not represent a unique value for the oceanic upper mantle but, rather, reflects conditions in the lower portion of the melting regime. A 100°C change in mantle potential temperature produces a change in relative oxygen fugacity of ∼0.8logunits, similar to the global range inferred from mid-ocean ridge basalt glasses. It is necessary, therefore, to compare relative oxygen fugacity calculated from basaltic glass with proxies for potential temperature before drawing conclusions on heterogeneity of the oxidation state of iron in the oceanic upper mantle. Results from model calculations also suggest that the sub-arc mantle is intrinsically more oxidizing than the oceanic mantle because it is cooler. The global correlation between Fe3+/∑Fe and H2O in basaltic glasses may, in part, reflect the solidus-lowering influence of the latter. Secular variations in relative oxygen fugacity of the upper mantle due to cooling of the Earth over geologic time do not appear to be large enough to have influenced the nature of volcanic outgassing significantly enough to have contributed to the Great Oxidation Event.

Petrology and mineralogy of the La Peña igneous complex, Mendoza, Argentina: An alkaline occurrence in the Miocene magmatism of the Southern Central Andes

The La Peña alkaline igneous complex (LPC) is located in the Precordillera (32°41′34″ S - 68°59′48″ W) of Mendoza province, Argentina, above the southern boundary of the present-day flat-slab segment. It is a 19 km2 and 5 km diameter subcircular massif emplaced during the Miocene (19 Ma) in the Silurian-Devonian Villavicencio Fm. The LPC is composed of several plutonic and subvolcanic intrusions represented by: a cumulate of clinopyroxenite intruded by mafic dikes and pegmatitic gabbroic dikes, isolated bodies of malignite, a central intrusive syenite that develops a wide magmatic breccia in the contact with clinopyroxenite, syenitic and trachytic porphyries, a system of radial and ring dikes of different compositions (trachyte, syenite, phonolite, alkaline lamprophyre, tephrite), and late mafic breccias. The main minerals that form the LPC, ordered according to their abundance, are: pyroxene (diopside, hedenbergite), calcium amphibole (pargasite, ferro-pargasite, potassic-ferro-pargasite, potassic-hastingsite, magnesio-hastingsite, hastingsite, potassic-ferro-ferri-sadanagaite), trioctahedral micas (annite-phlogopite series), plagioclase (bytownite to oligoclase), K-feldspar (sanidine and orthoclase), nepheline, sodalite, apatite group minerals (fluorapatite, hydroxylapatite), andradite, titanite, magnetite, spinel, ilmenite, and several Cu–Fe sulfides. Late hydrothermal minerals are represented by zeolites (scolecite, thomsonite-Ca), epidote, calcite and chlorite.The trace element patterns, coupled with published data on Sr-Nd-Pb isotopes, suggest that the primary magma of the LPC was generated in an initially depleted but later enriched lithospheric mantle formed mainly by a metasomatized spinel lherzolite, and that this magmatism has a subduction-related signature. The trace elements pattern of these alkaline rocks is similar to other Miocene calc-alkaline occurrences from the magmatic arc of the Southern Central Andes.Mineral and whole-rock chemical compositions support the interpretation that a first batch of a tephritic magma produced a cumulate of clinopyroxenite (clinopyroxene + magnetite + apatite) and a residual melt that crystallized as malignite at a shallow emplacement level (<5 km). Fractional crystallization in a deep chamber, coupled with rock assimilation, produced successive magma pulses that gave the composite central syenite, the syenitic and trachitic porphyries, and trachytic dikes. The latest rocks (phonolites) reflect an extreme fractionation with Ca, Al and K removal by feldspars. Mingling relationships between tephrites-basanites and trachytes-phonolites, plus compositional variations linked to the reabsorption surface observed in the K-feldspar from the phonolitic dikes, suggest recharge events with local mixing in the late-stages of the LPC evolution.The emplacement of the LPC melts at a shallow crustal level was favored by the aperture of extensional NNW-SSE fractures genetically linked to a local brittle shear zone, active at 18–19 Ma during a strong compressional event, that initiated the regime of flat-slab subduction in this part of the Andes.

Asthenosphere–lithosphere interactions in Western Saudi Arabia: Inferences from 3He/4He in xenoliths and lava flows from Harrat Hutaymah

Extensive volcanic fields on the western Arabian Plate have erupted intermittently over the last 30Ma following emplacement of the Afar flood basalts in Ethiopia. In an effort to better understand the origin of this volcanism in western Saudi Arabia, we analyzed 3He/4He, and He, CO2 and trace element concentrations in minerals separated from xenoliths and lava flows from Harrat Hutaymah, supplemented with reconnaissance He isotope data from several other volcanic fields (Harrat Al Birk, Harrat Al Kishb and Harrat Ithnayn). Harrat Hutaymah is young (<850ka) and the northeasternmost of the volcanic fields. There is a remarkable homogeneity of 3He/4He trapped within most xenoliths, with a weighted mean of 7.54±0.03 RA (2σ, n=20). This homogeneity occurs over at least eight different xenolith types (including spinel lherzolite, amphibole clinopyroxenite, olivine websterite, clinopyroxenite and garnet websterite), and encompasses ten different volcanic centers within an area of ~2500km2. The homogeneity is caused by volatile equilibration between the xenoliths and fluids derived from their host magma, as fluid inclusions are annealed during the infiltration of vapor-saturated magmas along crystalline grain boundaries. The notable exceptions are the anhydrous spinel lherzolites, which have a lower weighted mean 3He/4He of 6.8±0.3 RA (2σ, n=2), contain lower concentrations of trapped He, and have a distinctly depleted light rare earth element signature. 3He/4He values of ~6.8 RA are also commonly found in spinel lherzolites from harrats Ithnayn, Al Birk, and from Zabargad Island in the Red Sea. Olivine from non-xenolith-bearing lava flows at Hutaymah spans the He isotope range of the xenoliths. The lower 3He/4He in the anhydrous spinel lherzolites appears to be tied to remnant Proterozoic lithosphere prior to metasomatic fluid overprinting.Elevated 3He/4He in the western harrats has been observed only at Rahat (up to 11.8 RA; Murcia et al., 2013), a volcanic field situated above thinned lithosphere beneath the Makkah-Medinah-Nafud volcanic lineament. Previous work established that spinel lherzolites at Hutaymah are sourced near the lithosphere-asthenosphere boundary (LAB), while other xenolith types there are derived from shallower depths within the lithosphere itself (Thornber, 1992). Helium isotopes are consistent with melts originating near the LAB beneath many of the Arabian harrats, and any magma derived from the Afar mantle plume currently appears to be of minor importance.

Geochemistry and Thermo-barometry of Mafic Igneous Rocks of the Almabulagh Complex (NW Iran)

The Almabulagh complex is located in the Sanandaj-Sirjan structural zone, NW of Hamadan. This complex consists of low-grade metamorphic rocks and two intrusions of felsic and mafic rocks. In this research, we studied the mafic intrusion. The intrusion has ophitic, subophitic and inter-granular textures and is mainly composed of plagioclase, clinopyroxene, amphibole and minor amounts of K-feldspar, quartz and opaque minerals. Secondary minerals include epidote, chlorite and amphiboles formed in the rim of clinopyroxene. Clinopyroxene is of augite type; amphibole is tschermakite; plagioclase is in the range of andesine-labradorite; K-feldspar is anorthoclase; and the opaque mineral is ilmenite. According to the bulk rock geochemistry and mineral chemistry of clinopyroxene, the mafic intrusion of the Almabulagh complex has been crystallized from a calc-alkaline magma and is related to subduction zone. In spider diagrams Nb displays a distinctive negative anomalies compared to the primitive mantle, which suggest a possible subduction-related magmatism. Emplacement temperature and pressure of mafic rocks measured by pyroxene-amphibole thermometer and amphibole barometer indicate temperature and pressure ranges of 750 to 900 °C and 6 to 7 kbar, respectively. Based on Sm/Yb and La/Sm ratios, the parental melt of the mafic rocks of the Almabulagh complex derived from partial melting of garnet–spinel lherzolite. According to the bulk rock geochemistry and mineral chemistry of the mafic rocks, they formed in a subduction area during Late Jurassic-Early Cretaceous before the Neo-Tethys closure in Sanandaj-Sirjan zone.

Mantle xenolith-xenocryst-bearing monogenetic alkali basaltic lava field from Kutch Basin, Gujarat, Western India: Estimation of magma ascent rate

Kutch rift basin of northwestern India is characterized by a topography that is controlled by a number of fault controlled uplifted blocks. Kutch Mainland Uplift, the largest uplifted block in the central part of the basin, contains alkali basalt plugs and tholeiitic basalt flows of the Deccan age. Alkali plugs often contain small, discoidal mantle xenoliths of spinel lherzolite and spinel wehrlite composition. Olivine occurs as xenocrysts (coarse, fractured, broken olivine grains with embayed margin; Fo>90), phenocrysts (euhedral, smaller, and less forsteritic ~Fo80), and as groundmass grains (small, anhedral, Fo75) in these alkali basalts. In a few cases, the alkali plugs are connected with feeder dykes. Based on the width of feeder dykes, on the sizes of the xenocrysts and xenoliths, thickness of alteration rim around olivine xenocryst, we estimate that the alkali magmas erupted at a minimum speed of 0.37km per hour. The speed was likely greater because of the fact that the xenoliths broke up into smaller fragments as their host magma ascended through the lithosphere.

Lithospheric mantle xenoliths sampled by melts from upwelling asthenosphere: The Quaternary Tasse alkaline basalts of southeastern British Columbia, Canada

The Quaternary Tasse basalts are exposed near the north shore of Quesnel Lake in southeastern British Columbia. They host a variety of mantle xenoliths consisting predominantly of spinel lherzolite with minor dunite and pyroxenite. Mineralogically, the xenoliths are composed of olivine, orthopyroxene, clinopyroxene and spinel characterized by forsterite (Fo87–93), enstatite (En90–92), diopside (En45–50–Wo40–45–Fs5), and Cr-spinel (6−11wt.% Cr), respectively. All of the mantle xenoliths are coarse-grained and show granoblastic textures. Clinopyroxene and spinel display textural evidence for chemical reactions with percolating melts.The mantle xenoliths are characterized by restricted Mg-numbers (89−92) and low abundances of incompatible elements (Ba=2−11ppm; Sr=3−31ppm) and Yttrium (1−3ppm). On the basis of REE patterns, the xenoliths are divided into three groups reflecting the various degrees of mantle metasomatism: (1) Group 1 consists of concave-up LREE patterns (La/Smcn=0.48−1.16; Gd/Ybcn=0.71−0.92); (2) Group 2 possesses flat to moderately LREE-enriched patterns (La/Smcn=1.14−1.92; Gd/Ybcn=0.87−1.09); and (3) Group 3 is characterized by strongly LREE-enriched patterns (La/Smcn=1.53−2.45; Gd/Ybcn=1.00−1.32). On MORB-normalized trace element diagrams, the majority of the xenolith samples share the enrichment of LILE (Rb, Ba, K), U, Th, Pb, Sr and the depletion of HFSE (Nb, Ta, Ti, Y) relative to REE. These geochemical characteristics are consistent with a compositionally heterogeneous subcontinental lithospheric mantle source that originated as subarc mantle wedge peridotite at a convergent plate margin.The Tasse basalts have alkaline compositions characterized by low SiO2 (44−46wt.%) and high alkali (Na2O+K2O=5.1−6.6wt.%) contents. They are strongly enriched in incompatible elements (TiO2=2.4−3.1wt.%; Ba=580−797ppm; Sr=872−993ppm) and, display OIB-like trace element patterns (La/Smn=3.15−3.85; Gd/Ybn=3.42−4.61). They have positive εNd (+3.8 to +5.5) values, with 338−426Ma depleted mantle model ages, and display uniform OIB-like Sr (87Sr/86Sr=0.703346−0.703591) and Pb (206Pb/204Pb=19.40−19.58; 207Pb/204Pb=15.57−15.60; 208Pb/204Pb=38.99−39.14) isotopic compositions. The basalts erupted discontinuously along a >1000km long SE-NW-trending linear belt with minimal compositional variation indicative of a homogenous mantle source. The Sr−Nd−Pb isotope and trace element systematics of the alkaline basalts suggests that they originated from partial melting of an upwelling asthenospheric mantle source. Melting of the asthenospheric mantle might have stemmed from extension of the overlying lithosphere in response to the early stages of back-arc basin opening in the Omineca and Intermontane belts. Ridge subduction beneath the Canadian Cordillera might have played an important role in the weakening of the lithospheric mantle prior to its extension. Alternatively, melting of the upwelling asthenosphere in response to the delamination of the lithospheric mantle beneath the Rocky Mountain Trench might have generated the alkaline lavas.

Petrological constraints on evolution of continental lithospheric mantle beneath the northwestern Ethiopian plateau: Insight from mantle xenoliths from the Gundeweyn area, East Gojam, Ethiopia

Detailed petrographical observations and in-situ major- and trace-element data for minerals from ten spinel peridotite xenoliths from a new locality in Gundeweyn area, East Gojam, have been examined in order to understand the composition, equilibrium temperature and pressure conditions as well as depletion and enrichment processes of continental lithospheric mantle beneath the Ethiopian plateau. The peridotite samples are very fresh and, with the exception of one spinel harzburgite, are all spinel lherzolites. Texturally, the xenoliths can be divided into two groups as primary and secondary textures. Primary textures are protogranular and porphyroclastic while secondary ones include reaction, spongy and lamellae textures. The Fo content of olivine and Cr# of spinel ranges from 86.5 to 90.5 and 7.7 to 14.1 in the lherzolites, respectively and are 89.8 and 49.8, respectively, in the harzburgite. All of the lherzolites fall into the lower Cr# and Fo region in the olivine–spinel mantle array than the harzburgite, which indicates that they are fertile peridotites that experienced low degrees of partial melting and melt extraction. Orthopyroxene and clinopyroxene show variable Cr2O3 and Al2O3 contents regardless of their lithology. The Mg# of orthopyroxene and clinopyroxene are 87.3 to 90.1 and 85.8 to 90.5 for lherzolite and 90.4 and 91.2 for harzburgite, respectively. The peridotites have been equilibrated at a temperature and pressure ranging from 850 to 1100°C and 10.2 to 30kbar, respectively, with the highest pressure record from the harzburgite. They record high mantle heat flow between 60 and 150mW/m2, which is not typical for continental environments (40mW/m2). Such a high geotherm in continental area shows the presence of active mantle upwelling beneath the Ethiopian plateau, which is consistent with the tectonic setting of nearby area of the Afar plume. Clinopyroxene of five lherzolites and one harzburgite samples have a LREE enriched pattern and the rest exhibit LREE depletion relative to HREE. These suggest that the lithospheric mantle of the Ethiopian plateau has experienced at least two major processes, specifically, partial melting and metasomatism that produce LREE-depleted and -enriched signature of continental lithospheric mantle, respectively. There is also no clear relationship between degree of LREE enrichment and petrography of the studied peridotite.Based on our data, we conclude that the lithospheric mantle beneath Gundeweyn has experienced melt extraction during and/or before pan-African orogeny and then interacted with various degrees of asthenospheric melt. The interaction is probably related to mantle upwelling, which is mainly focused beneath East Africa rift system (EARS).

Mineral associations and major element compositions of base metal sulphides from the subcontinental lithospheric mantle of NE Spain

This study deals with textural types and major element compositions of Cu-Ni-Fe sulphides from spinel lherzolite, harzburgite and olivine websterite xenoliths found in alkali basaltic rocks of the Neogene-Quaternary volcanic zone of Catalonia (NE Spain). Sulphides in harzburgites and websterites are scarce. Four textural types have been distinguished: inclusions in silicates and spinel, trails of small droplets often radiating from inclusions, interstitial grains, and grains related to pyrometamorphic textures. The mineral associations are dominated by one or two low-temperature monosulphide solid solutions: mss1, mss2, occasionally accompanied by pyrrhotite, pentlandite and Cu-rich sulphides. Compositions of mss1 are more Fe-enriched in inclusions and interstitial grains than in grains related to pyrometamorphism. Compositions of mss2 are Ni-rich very close to pentlandite. Sulphide bulk compositions correspond to high-temperature monosulphide solid solution equilibrated with a relatively Cu-Ni enriched sulphide melt at 1100–1000 °C. The breakdown products of these earlier compositions could have been either equilibrated below 600, 300 °C or being at disequilibrium. A restitic origin is consistent with the main sulphide mineral associations, the estimated melt extraction for peridotites (<30 %) and with the fact that lherzolites are less affected by cryptic metasomatism than harzburgites. However, Ni exchange coefficients between olivine and the high-temperature monosulphide solid solution underestimate equilibrium values. This suggests that some lherzolites could derive from pervasive refertilization. The scarcity of sulphides in websterites is explained by S incompatible behaviour during the formation of earlier cumulates from the mafic alkaline magmas which caused the cryptic metasomatism.

Cobalt and precious metals in sulphides of peridotite xenoliths and inferences concerning their distribution according to geodynamic environment: A case study from the Scottish lithospheric mantle

Abundances of precious metals and cobalt in the lithospheric mantle are typically obtained by bulk geochemical analyses of mantle xenoliths. These elements are strongly chalcophile and the mineralogy, texture and trace element composition of sulphide phases in such samples must be considered. In this study we assess the mineralogy, textures and trace element compositions of sulphides in spinel lherzolites from four Scottish lithospheric terranes, which provide an ideal testing ground to examine the variability of sulphides and their precious metal endowments according to terrane age and geodynamic environment. Specifically we test differences in sulphide composition from Archaean-Palaeoproterozoic cratonic sub-continental lithospheric mantle (SCLM) in northern terranes vs. Palaeozoic lithospheric mantle in southern terranes, as divided by the Great Glen Fault (GGF).Cobalt is consistently elevated in sulphides from Palaeozoic terranes (south of the GGF) with Co concentrations>2.9wt.% and Co/Ni ratios>0.048 (chondrite). In contrast, sulphides from Archaean cratonic terranes (north of the GGF) have low abundances of Co (<3600ppm) and low Co/Ni ratios (<0.030). The causes for Co enrichment remain unclear, but we highlight that globally significant Co mineralisation is associated with ophiolites (e.g., Bou Azzer, Morocco and Outokumpu, Finland) or in oceanic peridotite-floored settings at slow-spreading ridges. Thus we suggest an oceanic affinity for the Co enrichment in the southern terranes of Scotland, likely directly related to the subduction of Co-enriched oceanic crust during the Caledonian Orogeny. Further, we identify a distinction between Pt/Pd ratio across the GGF, such that sulphides in the cratonic SCLM have Pt/Pd≥chondrite whilst Palaeozoic sulphides have Pt/Pd<chondrite. We observe that Pt-rich sulphides with discrete Pt-minerals (e.g., PtS) are associated with carbonate and phosphates in two xenolith suites north of the GGF. This three-way immiscibility (carbonate-sulphide-phosphate) indicates carbonatitic metasomatism is responsible for Pt-enrichment in this (marginal) cratonic setting. These Co and Pt-enrichments may fundamentally reflect the geodynamic setting of cratonic vs. non-cratonic lithospheric terranes and offer potential tools to facilitate geochemical mapping of the lithospheric mantle.

Thermal and redox equilibrium conditions of the upper-mantle xenoliths from the Quaternary volcanoes of NW Spitsbergen, Svalbard Archipelago

Upper-mantle xenoliths in Cenozoic basalts of northwestern Spitsbergen are rocks of peridotite (spinel lherzolites) and pyroxenite (amphibole-containing garnet and garnet-free clinopyroxenites, garnet clinopyroxenites, and garnet and garnet-free websterites) series. The upper-mantle section in the depth range 50–100 km is composed of spinel peridotites; at depths of 80–100 km pyroxenites (probably, dikes or sills) appear. The equilibrium conditions of parageneses are as follows: in the peridotites—730–1180 °C, 13–27 kbar, and oxygen fugacity of −1.5 to +0.3 log. un.; in the pyroxenites—1100–1310 °C, 22–33 kbar. The pyroxenite minerals have been found to contain exsolved structures, such as orthopyroxene lamellae in clinopyroxene and, vice versa, clinopyroxene lamella in orthopyroxene. The formation temperatures of unexsolved phases in orthopyroxene and clinopyroxene are nearly 100–150 °C higher than the temperatures of the lamellae–matrix equilibrium and the equilibrium of minerals in the rock. The normal distribution of cations in the spinel structure and the equilibrium distribution of Fe2+ between the M1 and M2 sublattices in the orthopyroxenes point to the high rate of xenolith ascent from the rock crystallization zone to the surface. All studied Spitsbergen rock-forming minerals from mantle xenoliths contain volatiles in their structure: OH−, crystal hydrate water H2Ocryst, and molecules with characteristic CH and CO groups. The first two components are predominant, and the total content of water (OH– + H2Ocryst) increases in the series olivine → garnet → orthopyroxene → clinopyroxene. The presence of these volatiles in the nominally anhydrous minerals (NAM) crystallized at high temperatures and pressures in the peridotites and pyroxenites testifies to the high strength of the volatile–mineral bond. The possibility of preservation of volatiles is confirmed by the results of comprehensive thermal and mass-spectral analyses of olivines and clinopyroxene, whose structures retain these components up to 1300 °C. The composition of hypothetic C–O–H fluid in equilibrium (in the presence of free carbon) with the underlying mantle rocks varies from aqueous (>80% H2O) to aqueous–carbonic (~60% H2O). The fluid becomes essentially aqueous when the oxygen activity in the system decreases. However, there is no strict dependence of the redox conditions on the depth of formation of xenoliths.

Conditions of sulfide formation in the metasomatized mantle beneath East Antarctica

Garnet–spinel lherzolites from Antarctica and peridotites from Mongolia were fluid saturated, which is indicated by the presence of fluid inclusions in their minerals. Flows of reactive fluids caused extensive metasomatic alteration of mantle materials. The cryometric and Raman spectroscopic investigation of the Antarctic xenoliths showed that their fluid was a complex mixture of CO2, N2, H2S, and H2O with a density of up to 1.23 g/cm3. The entrapment of fluids was accompanied by the formation of clusters of numerous sulfide inclusions. The compositions of these inclusions correspond to a Ni-rich sulfide melt and a monosulfide solid solution. The partition coefficient of Ni between them (DNimss/melt) ranges from 0.99 to 3.23, which suggests that the two-phase sulfide assemblages in the partly decrepitated inclusions equilibrated at 920–1060°C. In order to refine the initial P-T conditions of the development of the Antarctic peridotites, the results of our investigation were evaluated in the light of experimental data on (1) the stability field of the two-phase assemblage mss + sulfide melt, (2) the solidus of peridotite + 0.9CO2 + 0.1 H2O, and (3) isochores of 0.8CO2 + 0.2N2 fluid. The obtained parameters are close to 1270–1280°C and 2.2 GPa and lie near the Sp–Gar boundary. The temperature of the existence of sulfide melt at a pressure of 2.2 GPa must be near 1300°C and corresponds to the boundary between the occurrence of carbon as CO2 fluid and carbonate (carbonate melt).

Petrology, geochemistry, and geochronology of mafic rocks from the Taoxinghu Devonian ophiolite, LongmuCo–Shuanghu–Lancang suture zone, northern Tibet: evidence for an intra-oceanic arc–basin system

A newly discovered Devonian ophiolite located in the Taoxinghu area of central Qiangtang on the Qinhai–Tibet Plateau is described. The ophiolite consists of gabbro and diabasic dikes, and invasive cumulate gabbros-leucogabbros. The ophiolite has undergone greenschist facies metamorphism and minor deformation. Dating of the metagabbro by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb techniques yielded a weighted mean age of 367.2 ± 3.3 Ma (Late Devonian). Whole-rock geochemical analyses show that the rocks belong to the tholeiite series, with weak depletion in light rare-earth elements (LREEs), almost no Eu anomalies, weak enrichment in large-ion lithophile elements (LILEs), depletion in Nb and Ta, and weak negative Hf and Ti anomalies. These characteristics are similar to those of back-arc basin basalts. Together, these characteristics suggest that the rocks of the Devonian ophiolite formed by ~30% partial melting of spinel lherzolite, which was enriched by interaction with aqueous fluids during the late-generation phases; there is no evidence of subduction-related melting. The Devonian ophiolite rocks in the Taoxinghu area were first discovered in the LongmuCo–Shuanghu–Lancang suture zone. Detailed geochemical analyses show that the rocks formed in a back-arc ocean basin environment, indicating that the LongmuCo–Shuanghu–Lancang suture zone in central Qiangtang represents a Late Devonian intra-oceanic subduction zone in the Palaeo–Tethys Ocean. The discovery of the central Qiangtang Devonian ophiolite provides essential data for understanding the evolutionary history of the Palaeo–Tethys Ocean, and for identifying and understanding the roles of the different tectonic units on the Qinghai–Tibet Plateau.

Generation of ca. 900–870 Ma bimodal rifting volcanism along the southwestern margin of the Tarim Craton and its implications for the Tarim–North China connection in the early Neoproterozoic

Large scale rift related volcanic suites are widely accepted as the evidence of the initial breakup of supercontinents and thus can be used to reconstruct the relative position of the cratons. Numerous early Neoproterozoic volcanic rocks have been identified in the Tiekelik Belt, southwestern part of the Tarim Craton. The Sailajiazitage volcanic rocks in the Tiekelik Belt are composed of a suite of bimodal volcanics and related pyroclastic rocks. The geochemical characteristics resemble those of continental flood basalts (CFBs) formed in the shallower mantle dominated by spinel–lherzolite with EM2 affinities. The rhyolites have been dated at 896±11Ma at the base, 881±14Ma at the middle and 872±8Ma at the top by U–Pb method on zircon. They erupted in an intraplate rift environment and are interpreted to have formed by crustal melting. Hf isotopes of individual dated zircon grains show a wide range of initial εHf values, from strongly negative (down to −28) to highly positive (up to +12.8), indicating a mixture of mantle- and crustal-derived materials. The bimodal volcanic suite is overlain by the Bochatetage Formation sediments, deposited sometime after 797±12Ma. An identical ca. 900–870Ma age for rift-related volcanic rocks indicate Neoproterozoic regional extensional processes in the southern margin of the Tarim–North China, which allow us to speculate on the correlation of the Tarim and North China cratons during the Neoproterozoic. This voluminous bimodal magmatism resembles the rift-related magmatism in the North China, West African, Congo and São Francisco cratons of age ca. 925–870Ma, suggesting that the Tarim Craton was located close to these cratons during early Neoproterozoic, or alternatively, that such early Neoproterozoic rift magmatism was widespread over large parts of Rodinia.

Trace-element abundances in the shallow lithospheric mantle of the North Atlantic Craton margin: Implications for melting and metasomatism beneath Northern Scotland

AbstractBulk rock geochemistry and major- and trace-element compositions of clinopyroxene have been determined for three suites of peridotitic mantle xenoliths from the North Atlantic Craton (NAC) in northern Scotland, to establish the magmatic and metasomatic history of subcontinental lithospheric mantle (SCLM) below this region. Spinel lherzolites from the southernmost locality (Streap Com'laidh) have non-NAC mantle compositions, while the two northern xenolith suites (Loch Roag and Rinibar) are derived from the thinned NAC marginal keel. Clinopyroxene compositions have characteristic trace-element signatures which show both 'primary' and 'metasomatic' origins. We use Zr and Hf abundances to identify ancient cryptic refertilization in 'primary' clinopyroxenes. We suggest that Loch Roag and Rinibar peridotite xenoliths represent an ancient Archaean-Palaeoproterozoic SCLM with original depleted cratonic signatures which were overprinted by metasomatism around the time of intrusion of the Scourie Dyke Swarm (∼2.4 Ga). This SCLM keel was preserved during Caledonian orogenesis, although some addition of material and/or metasomatism probably also occurred, as recorded by Rinibar xenoliths. Rinibar and Streap xenoliths were entrained in Permo-Carboniferous magmas and thus were isolated from the SCLM ∼200 Ma before Loch Roag xenoliths (in an Eocene dyke). Crucially, despite their geographical location, lithospheric mantle peridotite samples from Loch Roag show no evidence of recent melting or refertilization during the Palaeogene opening of the Atlantic.

Geochemical Characteristics of Volcanic Rocks from ODP Site 794, Yamato Basin: Implications for Deep Mantle Processes of the Japan Sea

AbstractDeep mantle processes and the dynamic mechanism of magmatism in the Japan Sea Basin are important processes that have not been studied in detail. In this paper, systematic evaluation of basalt samples from the ocean drilling program Site 794 in the Japan Sea was performed, which included petrography, whole‐rock major‐ and trace‐element analysis, Sr‐Nd‐Pb isotopic composition, and electron microprobe analysis of plagioclase and clinopyroxene. These basalts belong to the tholeiitic series with porphyritic texture and massive Ca‐rich plagioclase, clinopyroxene, and minor olivine phenocrysts. The basalts are characterized as flat rare earth elements and high‐field‐strength elements with remarkably low ratios of (La/Yb)N (0.75–2.51), significant positive anomalies of Ba, Sr, and Rb and no Eu anomaly (δEu = 0.99–1.36). The samples showed relatively high 87Sr/86Sr (0.70425–0.70522), 207Pb/204Pb (15.511–15.610), and 208Pb/204Pb (38.064–38.557) values and a low 143Nd/144Nd ratio (0.51271–0.51295). The basalts from Site 794 can be divided into upper, middle, and lower volcanic rocks (UVR, MVR, and LVR) on the basis of their stratigraphic level. The MVR was geochemically derived from the depleted mantle, whereas the UVR and LVR originated from a nondepleted and relatively enriched mantle source with contributions from subducted Pacific plate fluid and sediments. Use of geothermobarometers indicates that the crystallization pressure for the UVR and LVR (6.25–11.19 kbar) was significantly higher than that of the MVR (3.48–5.84 kbar). The UVR and LVR may have been derived from the low‐degree (5%–10%) partial melting of spinel lherzolite, while the MVR originated from a shallower mantle source with a high degree (10%–20%) of partial melting. In addition, the geochemical characteristics of the samples are consistent with a younger age (13–17 Ma) and the depleted composition of the MVR and an older age (17–23 Ma) and slightly enriched composition of the UVR and LVR. Therefore, temporal changes in the mantle source from old and enriched to young and depleted and subsequently to old and nondepleted may have been associated with progressive lithospheric extension and thinning, as well as at least two episodes of diverse asthenospheric upwelling and pull‐apart tectonic motion in the Yamato Basin.

Geochemistry and petrogenesis of mafic–ultramafic suites of the Irindina Province, Northern Territory, Australia: Implications for the Neoproterozoic to Devonian evolution of central Australia

Petrological and geochemical data for magmatic mafic–ultramafic suites of the Irindina and Aileron provinces of the Eastern Arunta region, Northern Territory, Australia constrain the petrogenesis and tectonic setting of magmatic events covering ~500millionyears. Six geochemically distinct magmatic suites, here named A–F, have been identified and provide evidence of the tectonic history of this region and also are linked to two mineralisation-related magmatic events: the Lloyd Gabbro (Ni–Cu–PGE mineralisation) and the Riddoch Amphibolite (Cyprus-style Cu–Co volcanogenic massive sulphide mineralisation).The whole-rock geochemistry of Suites A and F is indicative of melts derived from a range of mantle depths (garnet to spinel lherzolite) and source enrichment. Suite D is likely related to the ~1070Ma Warakurna/Giles event of central Australia, including the Alcurra (Musgrave) and Stuart (Arunta) dyke swarms, and likely formed through either: a) melting of subduction modified, sub-continental lithospheric mantle (SCLM) by an upwelling mantle plume; or b) a combination of intra-plate tectonic processes involving a long-lived thermal anomaly, lithospheric-scale architecture that focussed magmatism, and large-scale tectonism. Suite F represents more alkaline magmas, derived from a deeper source, but most likely formed during the same Warakurna LIP event (possibly contemporaneously) as Suite D. Suite E (the Riddoch Amphibolite) was most likely emplaced in a back-arc basin (BAB) setting at ~600Ma, coincident with Delamerian subduction and BAB formation along the eastern Proterozoic margin of Australia from Queensland to the eastern Arunta and possibly further south. Subsequent destabilisation of the SCLM underneath the North Australian Craton generated the ~510Ma Kalkarindji LIP in the form of Suite B intrusions that assimilated some of the older Suite E (Riddoch) material. This event is locally known as the ~506Ma Stanovos Igneous Suite and represents the most voluminous magmatic event within the study area. Suite A samples are more alkaline and cannot be correlated with any known magmatic events; however, they probably represent an unidentified alkaline phase of the Kalkarindji LIP event derived from a deep mantle source region. This suite is similar to the more widespread Kalkarindji and Suite B/Stanovos magmas in that these magmas assimilated arc material (i.e., Suite D) prior to emplacement. The last magmatic event occurred during the exhumation and inversion of the Irindina basin during the 450–300Ma Alice Springs Orogeny, resulting in the formation of melts derived from a deep mantle source. These melts subsequently became contaminated en route to emplacement as the ~409Ma Suite C Lloyd Gabbro.

Characterization of the sub-continental lithospheric mantle beneath the Cameroon volcanic line inferred from alkaline basalt hosted peridotite xenoliths from Barombi Mbo and Nyos Lakes

We carried out detailed petrographic, major and trace element geochemical, microstructural and FTIR analyses on eight characteristic ultramafic xenoliths from Nyos and Barombi Mbo Lakes in the continental sector of the Cameroon Volcanic Line (CVL). The studied xenoliths are spinel lherzolites showing lithologies similar to the other xenoliths reported previously along the CVL. They have protogranular and porphyroclastic textures. One of the Barombi xenolith contains amphibole, which had not been previously reported in this locality. Amphibole is common in the Nyos xenoliths suite. Peridotite xenoliths from both localities show some chemical heterogeneity, but Barombi xenoliths generally are less depleted in basaltic elements with respect to Nyos xenoliths. Trace element compositions of Nyos spinel lherzolites show a moderately depleted initial (premetasomatic) composition and variable enrichment in REE. Evidence for both modal and cryptic metasomatism is present in Nyos xenoliths. Rare earth element patterns of clinopyroxene suggest that interaction between mafic melts and the upper mantle occurred beneath the Nyos locality. Barombi Mbo xenoliths, on the other hand, record a small degree of partial melting. The Barombi Mbo xenoliths have weak, dominantly orthorhombic olivine crystal preferred orientations, whereas Nyos ones have strong axial-[010] patterns, which may have formed in response to transpression. Nominally anhydrous mantle minerals (NAMs) of the Barombi Mbo xenoliths show generally higher bulk concentrations of ‘water’ (70–127 ppm) than Nyos xenoliths (32–81 ppm). The Barombi Mbo xenoliths could originate from a juvenile segment of the lithospheric mantle, which had been originally part of the asthenosphere. It became a part of the lithosphere in response to thermal relaxation following the extension, forming a weakly deformed lower lithospheric mantle region along the CVL. The Nyos xenoliths, however, represent a shallow lithospheric mantle bearing imprints of several depletion and enrichment events probably prior or following the extension (at ∼30 Ma).

Evolution of the Hazelton arc near Terrace, British Columbia: stratigraphic, geochronological, and geochemical constraints on a Late Triassic – Early Jurassic arc and Cu–Au porphyry belt

Understanding the development of island arcs that accreted to the North American craton is critical to deciphering the complex geological history of the Canadian Cordillera. In the case of the Hazelton arc (part of the Stikine terrane, or Stikinia) in northwestern British Columbia, understanding arc evolution also bears on the formation of spatially associated porphyry Cu–Au, epithermal, and volcanogenic massive sulfide deposits. The Hazelton Group is a regionally extensive, long-lived, and exceptionally thick Upper Triassic to Middle Jurassic volcano-sedimentary succession considered to record a successor arc that was built upon the Paleozoic and Triassic Stikine and Stuhini arcs. In central Stikinia, near Terrace, British Columbia, the lower Hazelton Group (Telkwa Formation) comprises three volcanic-intrusive complexes (Mt. Henderson, Mt. O’Brien, and Kitselas) that, at their thickest, constitute almost 16 km of volcanic stratigraphy. Basal Telkwa Formation conglomerates and volcanic rocks were deposited unconformably on Triassic and Paleozoic arc-related basement. New U–Pb zircon ages indicate that volcanism initiated by ca. 204 Ma (latest Triassic). Detrital zircon populations from the basal conglomerate contain abundant 205–233 Ma zircons, derived from regional unroofing of older Triassic intrusions. Eleven kilometres higher in the section, ca. 194 Ma, rhyolites show that arc construction continued for &gt;10 million years. Strata of the Nilkitkwa Formation (upper Hazelton Group) with a U–Pb zircon age of 178.90 ± 0.28 Ma represent waning island-arc volcanism. Telkwa Formation volcanic rocks have bimodal silica concentrations ranging from 48.1 to 62.8 wt.% and 72.3 to 79.0 wt.% and display characteristics of subduction-related magmatism (i.e., calc-alkaline differentiation with low Nb and Ti and high Th concentrations). Mafic to intermediate rocks form a differentiated suite that ranges from high-Al basalt to medium- to high-K andesite. They were derived from hydrous melting of isotopically juvenile spinel lherzolite in the mantle wedge and from subsequent fractional crystallization. Compared to basalts and andesites (εNd = +5 to +5.5), rhyolites have higher positive εNd values (+5.9 to +6.0) and overlapping incompatible element concentrations, indicating that they are not part of the same differentiation suite. Rather, the rhyolites formed from anatexis of arc crust, probably caused by magmatic underplating of the crust. This study documents a temporal and spatial co-occurrence of Hazelton Group volcanic rocks with a belt of economic Cu–Au porphyry deposits (ca. 205–195 Ma) throughout northwestern Stikinia. The coeval relationship is attributed to crustal underplating and intra-arc extension associated with slab rollback during renewed or reconfigured subduction beneath Stikinia, following the demise of the Stuhini arc in the Late Norian.