Spinel Lherzolite Research Articles

Petrogenesis and Geological Implications of the Tianheyong Cenozoic Basalts, Inner Mongolia China

The Tianheyong basalts are basanites, which at least can be subdivided into three types of mineral assemblages. In general, the Tianheyong basalts are characteristics of low silica ( w(SiO 2)=43.97%–45.68%), enriched alkali ( w(K 2O+Na 2O) =5.91%–7.65%), enriched potassium ( w(K 2O)=2.04%–2.89%), high contents of titanium ( w(TiO 2)=2.18%–2.37%), high magnesium value (Mg #=68–76), and high contents of REE (ΣREE=(246.62–329.82)×10 −6). The chondrite normalized patterns of REE show right inclined smooth lines, extremely enriched in LREE, significantly fractionated from LREE and HREE, without obvious anomaly of Eu (δEu=0.90–1.02) and Ce (δCe=0.96–1.00), and the high enrichment of incompatible elements, especially, the peak value in high field strength elements (HFSE) of Nb, Ta, and Th indicate the OIB-like distribution patterns of multi-elements. The Tianheyong basalts are also high in compatible elements: Co ((39.1–48.9)×10 −6), Ni ((130–257)×10 −6), and Cr ((138–320)×10 −6). All of the above characteristics as well as the low degree of crystallization, the abundance in peridotite inclusions, and minor amount of xenocrysts, and the variation trend of elements indicate that the basalts were solidified from primitive basaltic magma. Trace element ratio of Ba/Rb (12–35) and variation of alkali metals might indicate the metasomatism of fluids in a source region of enriched lherzolites. Petrogenesis simulation indicates that the Tianheyong primitive basaltic magma might be the product of polybaric melting column with non-modal partial melting processes of enriched lherzolite, which straddle the boundary between garnet- and spinel-facies lherzolites. Magma might originate from low partial melting of source rocks(∼<5%), which have∼1% partial melting of garnet lherzolite and 2%–5% partial melting of spinel lherzolite. The integrative analysis indicates that the triggering mechanism of source rock partial melting might be the edge-driven convection of mantle, which results in that the Tianheyong basalts have the deepest origin depth as compared with those of eastern Jining and Hannuoba basalts.

Determination of platinum-group elements and rhenium in standard geological samples by isotope dilution with mass-spectrometric ending

A method is proposed for determining platinum-group elements, Ru, Pd, Os, Ir, Pt, and Re in geological samples by isotope dilution with mass spectrometric ending after sample decomposition with a mixture (1: 3) of conc. HCl + HNO3 in a microwave system and the chromatographic separation of the analytes from the matrix on an AG 50W × 8 cation exchanger. The concentrations and isotope ratios are determined on high-resolution mass spectrometers ELEMENT and ELEMENT2. The attained detection limits range from 0.005 ng/g (Ir) to 0.2 ng/g (Pd) in solid samples. The use of isotope dilution improves the accuracy of determination and takes into account the matrix effect and changes in plasma parameters on the analytical signal. The method is tested in the analysis of standard samples of spinel lherzolite (GP-13) and serpentinite (UB-N). The relative standard deviation of the results of analysis is 5–22%, depending on the element and its concentration level (Ru, Pd, Re, Pt). In determining Os and Ir, a certain decrease in the accuracy and repeatability of the results is noticed.

Detailed mineralogical-geochemical study of a large spinel Lherzolite xenolith in alkali basalt of Shavaryn Tsaram paleovolcano, Mongolia

Mineralogical-geochemical data obtained on a large xenolith of spinel lherzolite from basanites of Shavaryn Tsaram volcano (Middle Pleistocene) provide evidence that the xenolith was affected by heat and fluid that came from the basalt melt and induced geochemical heterogeneity of the xenolith. Olivine crystals from the central portion of the xenolith contain much more CaO than this mineral from the peripheral zones of the xenolith as well as in xenocrystals of the host basalts. The large xenolith was unevenly annealed by the host basalt melt, and this predetermined the heterogeneous diffusion-controlled “purification” of olivine crystals from their CaO admixture. The xenolith is heterogeneously enriched in LREE and some trace elements (Zr, Hf, Nb, Th, and U), and their distribution demonstrates unclearly pronounced concentric zoning: these elements enriched the outer portions of the xenolith. The enrichment of the xenolith in LREE and other mobile trace elements is explained by their occurrence not only as structurally bound (isomorphic) components in minerals but also in the form of nonstructural admixtures (contaminant), as minute grains of compounds hosted in the interstitial space and tiny cracks cutting mineral grains. The enrichment of the xenolith in these admixtures proceeded via their infiltration-controlled introduction with fluid from the basalt along minute cracks.

Geochemistry and tectonomagmatic significance of Lower Cretaceous island arc lavas from the Devils Racecourse Formation, eastern Jamaica

Abstract The Benbow Inlier in Jamaica contains the Devils Racecourse Formation, which is composed of a Hauterivian to Aptian island arc succession. The lavas can be split into a lower succession of basaltic andesites and dacites/rhyolites, which have an island arc tholeiite (IAT) composition and an upper basaltic and basaltic andesite sequence with a calc-alkaline (CA) chemistry. Trace element and Nd–Hf isotopic evidence reveals that the IAT and CA lavas are derived from two chemically similar mantle wedge source regions predominantly composed of normal mid-ocean ridge-type spinel lherzolite. In addition, Th-light rare earth element/high field strength element–heavy rare earth element ratios, Nd–Hf isotope systematics, (Ce/Ce*) n-mn and Th/La ratios indicate that the IAT and CA mantle wedge source regions were enriched by chemically distinct slab fluxes, which were derived from both the altered basaltic portion of the slab and its accompanying pelagic and terrigenous sedimentary veneer respectively. The presence of IAT and CA island arc lavas before and after the Aptian–Albian demonstrates that the compositional change in the Great Arc of the Caribbean was the result of the subduction of chemically differing sedimentary material. There is therefore no evidence from the geochemistry of this lava succession to support arc-wide subduction polarity reversal in the Aptian–Albian.

Deccan plume, lithosphere rifting, and volcanism in Kutch, India

Kutch (northwest India) experienced lithospheric thinning due to rifting and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event. Alkalic lavas, containing mantle xenoliths, form plug-like bodies that are aligned along broadly east–west rift faults. The mantle xenoliths are dominantly spinel wehrlite with fewer spinel lherzolite. Wehrlites are inferred to have formed by reaction between transient carbonatite melts and lherzolite forming the lithosphere. The alkalic lavas are primitive (Mg# = 64–72) relative to the tholeiites (Mg# = 38–54), and are enriched in incompatible trace elements. Isotope and trace element compositions of the tholeiites are similar to what are believed to be the crustally contaminated Deccan tholeiites from elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all except two alkalic basalts plot in a tight cluster that largely overlap the Indian Ridge basalts and only slightly overlap the field of Reunion lavas. This suggests that the alkalic magmas came largely from the asthenosphere mixed with Reunion-like source that welled up beneath the rifted lithosphere. The two alkalic outliers have an affinity toward Group I kimberlites and may have come from an old enriched (metasomatized) asthenosphere. We present a new model for the metasomatism and rifting of the Kutch lithosphere, and magma generation from a CO 2-rich lherzolite mantle. In this model the earliest melts are carbonatite, which locally metasomatized the lithosphere. Further partial melting of CO 2-rich lherzolite at about 2–2.5 GPa from a mixed source of asthenosphere and Reunion-like plume material produced the alkalic melts. Such melts ascended along deep lithospheric rift faults, while devolatilizing and exploding their way up through the lithosphere. Tholeiites may have been generated from the main plume head further south of Kutch.

Petrology of the Yugu peridotites in the Gyeonggi Massif, South Korea: Implications for its origin and hydration process

AbstractPeridotites exposed in the Yugu area in the Gyeonggi Massif, South Korea, near the boundary with the Okcheon Belt, exhibit mylonitic to strongly porphyroclastic textures, and are mostly spinel lherzolites. Subordinate dunites, harzburgites, and websterites are associated with the lherzolites. Amphiboles, often zoned from hornblende in the core to tremolite in the rim, are found only as neoblasts. Porphyroclasts have recorded equilibrium temperatures of about 1000°C, whereas neoblasts denote lower temperatures, about 800°C. Olivines are Fo90–91 in lherzolites and Fo91 in a dunite and a harzburgite. The Cr# (= Cr/(Cr + Al) atomic ratio) of spinels varies together with the Fo of olivines, being from 0.1 to 0.3 in lherzolites and around 0.5 in the dunite and harzburgite. The Na2O content of clinopyroxene porphyroclasts is relatively low, around 0.3 to 0.5 wt% in the most fertile lherzolite. The Yugu peridotites are similar in porphyroclast mineral chemistry not to continental spinel peridotites but to sub‐arc or abyssal peridotites. Textural and mineralogical characteristics indicate the successive cooling with hydration from the upper mantle to crustal conditions for the Yugu peridotites. Almost all clinopyroxenes and amphiboles show the same U‐shaped rare earth element (REE) patterns although the level is up to ten times higher for the latter. The hydration was associated with enrichment in light REE, resulting from either a slab‐derived fluid or a fluid circulating in the crust. The mantle‐wedge or abyssal peridotites were emplaced into the continental crust as the Yugu peridotite body during collision of continents to form a high‐pressure metamorphic belt in the Gyeonggi Massif. The peridotites from the Gyeonggi Massif exhibit lower‐pressure equilibration than peridotites, with or without garnets, from the Dabie–Sulu Collision Belt, China, which is possibly a westward extension of the Gyeonggi Massif.

Platinum‐group elements and rhenium in mantle xenoliths from the East Sayan volcanic field (Siberia, Russia): evaluation of melt extraction and refertilization processes in lithospheric mantle of the Tuva‐Mongolian massif

AbstractWe determined the concentrations of platinum‐group elements (PGE) and rhenium in granular spinel lherzolites entrained as xenoliths in the Late Cenozoic volcanic rocks of the East Sayan within the Sarkhoi palaeo‐arc block of the Tuva‐Mongolian massif. Major element, PGE and rhenium variations in the East Sayan xenoliths can be explained by impregnation of up to 15% of arc‐type melt into initially depleted mantle harzburgite. Such a refertilization process probably took place in the Middle Neoproterozoic, when the Tuva‐Mongolian massif was in a subduction environment. East Sayan xenoliths show close similarities to Vitim xenoliths, whose host basalts erupted within an off‐cratonic crustal block. Both East Sayan and Vitim xenoliths are different from cratonic and circum‐cratonic peridotite xenoliths of worldwide localities.

Origin of Late Palaeoproterozoic Great Vindhyan basin of North Indian shield: Geochemical evidence from mafic volcanic rocks

On the western and southern margins of the sickle shaped Vindhyan basin of north Indian shield, there are basal Vindhyan mafic volcanic rocks referred to as Khairmalia volcanics and Jungel volcanics respectively. These volcanics vary in composition from low-Ti tholeiite to high-Ti alkali basalt showing close affinity with continental flood basalts (CFB) and ocean island basalts (OIB) respectively. The parental magmas of Khairmalia and Jungel alkali basalts were formed by different degrees of partial melting of a garnet lherzolite. The magma of Khairmalia tholeiites was generated by a relatively higher degrees of partial melting of a garnet + spinel lherzolite. The geochemical data coupled with available geological and geophysical data favour a rift type origin of this basin which evolved as a peripheral basin showing many similarities with Paleogene Himalayan foreland basin. The existing radiometric age data suggest that the origin of Vindhyan basin is linked with Aravalli–Satpura orogeny. At about 1800–1600 Ma collision occurred along the Aravalli-Delhi fold belt (ADFB) and Central Indian Tectonic Zone (CITZ) with west and south subduction respectively. During this process the subducting lithosphere suffered extensional deformation on its convex side and some pre-existing large faults in the already thin leading edge of subducted plate also reactivated and tapped magma generated by decompressional melting of the subcontinental mantle. The simultaneous processes such as flexural subsidence, reactivation of pre-existing faults, heating, thermal cooling and contraction during volcanism, resulted in the formation of curvilinear warp parallel to the emerging mountain front. The Lower Vindhyan volcano–sedimentary succession was deformed and exposed to erosion before the deposition of Upper Vindhyan rocks. The orogenic forces were active intermittently throughout the Vindhyan sedimentation.

Secular evolution of the Neoproterozoic lithospheric mantle underneath the northern margin of the Yangtze Block, South China

The 820-Ma Wangjiangshan, 814-Ma Beiba, 780-Ma Bijigou and 746-Ma Luojiaba mafic–ultramafic intrusions in the Hannan region of the northern margin of the Yangtze Block, South China, are used to constrain the secular evolution of the lithospheric mantle. These intrusions are mainly composed of gabbros, but the Beiba and Wangjiangshan intrusions also contain diorite. Rocks from these four intrusions have arc-like geochemical characteristics with high Ba/Nb (13–733), Ba/La (2–150), Sr/Nb (28–2320), Pb/Nb (0.2–13) and Ba/Th (107–7150) ratios relative to the primitive mantle. The Bijigou and Wangjiangshan intrusions have relatively constant initial 87Sr/ 86Sr ratios (0.703267 to 0.703729) and εNd( t) values (+ 2.95 to + 3.87). The Luojiaba intrusion has initial 87Sr/ 86Sr ratios (0.703690 to 0.703914) and εNd values (+ 0.93 to + 1.34) similar to those of the Beiba intrusion ( 87Sr/ 86Sr = 0.703768 to 0.704586; εNd = + 0.23 to + 1.98). The average zircon εHf values of the Bijigou and Wangjiangshan intrusions (+ 7.38 to + 9.58) are higher than those of the Luojiaba (+ 5.73) and Beiba intrusions (+ 4.76). The Wangjiangshan, Bijigou and Luojiaba intrusions do not show crustal contamination, whereas the Beiba intrusion shows evidence of crustal contamination. Modeling calculations reveal that the Wangjiangshan, Luojiaba and Beiba intrusions were produced by partial melting of garnet–spinel lherzolite to spinel lherzolite mantle, whereas the Bijigou intrusion was derived from a garnet lherzolite to garnet–spinel lherzolite mantle source. The arc-related geochemistry of these intrusions suggests that the lithospheric mantle sources were modified by slab fluids. The Luojiaba intrusion with high Th and high-field-strength-elements (HFSE) was derived from a mantle source enriched by sediment melts. Combined with 950 Ma arc volcanic rocks in the region, these intrusions support a model in which the lithospheric mantle beneath the northern margin of the Yangtze Block was enriched by slab-derived fluids since 950 Ma, and then depleted by melt extraction at ∼ 780 Ma, eventually further enriched by slab-derived melts at ∼ 746 Ma.

Mantle peridotites from the Dinaridic ophiolite belt and the Vardar zone western belt, central Balkan: A petrological comparison

The nearly parallel Dinaridic ophiolite belt and the Vardar zone western belt are assumed to be the remnants of two distinct oceanic basins, constituting different parts of the Tethys Ocean that separated the Gondwana and Eurasia continents during Mesozoic time. These belts comprise numerous large peridotite massifs and small bodies whose petrology was poorly known. This paper presents a large set of internally consistent analytical data for peridotites, including primary mineralogy, major-element chemistry and clinopyroxene geochemistry for massifs of both the ophiolitic belts. We propose, discuss and apply a set of mineralogical, geochemical and petrologic criteria that allow a recognition of the probable geodynamic setting of formation of the ultramafic massifs. Ultramafic massifs of the Vardar zone western belt gradually change in composition northwards from depleted spinel lherzolites (Banjska massif) to depleted harzburgites (Maljen massif); these bodies originated in the same geodynamic setting, probably a back-arc spreading center. By contrast, the Dinaridic belt ultramafic massifs include several different types that formed in different geodynamic environments. Orogenic lherzolites, interpreted as subcontinental peridotites (Kozara, Čavka, Borja, Sjenički Ozren and Bistrica massifs) are dominated by fertile spinel and plagioclase lherzolites with subordinate amounts of depleted spinel lherzolite, spinel harzburgite, rare dunite and very rare vein garnet clinopyroxenite. The inferred subcontinental peridotites of these massifs are not co-magmatic with neighboring basalts and cannot be considered as members of a single ophiolitic assemblage. Massifs of two other types are less common in the Dinaridic ophiolite belt. These are composed of spinel lherzolite–harzburgite (Zlatibor and possibly Bosanski Ozren massifs) and depleted harzburgite (Tuzinje and Brezovica massifs); both probably originated in a suprasubduction environment. The available data suggest that the studied ultramafic rocks formed in two different oceanic basins, probably marginal seas. We also suggest that the orogenic lherzolites that dominate the central-northern part of the Dinaridic ophiolite belt formed as a result of continental lithosphere extension after closure of the Dinaride oceanic basin and were later tectonically incorporated into the ophiolitic mélange.

The Lesvos mafic–ultramafic complex, Greece: Ophiolite or incipient rift?

On the island of Lesvos in the north-eastern Aegean Sea, thrust sheets of mantle peridotite override a tectonic mélange containing metasediments, metabasalts and a few metagabbros. Previous workers have collectively called the above rock types the “Lesvos ophiolite” and suggested it represents Neotethyan (Jurassic) oceanic crust of N-MORB and IAT affinities. However, the complete absence of a sheeted dyke complex and of a layered cumulate sequence, in turn overlain by isotropic gabbros does not support the ophiolite description. The basalts of the mélange define two distinct groups on the basis of Nb/Y values: the Vatera (low Nb/Y) and the Mélange (high Nb/Y) groups. The Vatera basalts show multi-element profiles and REE patterns lying well within the field of N-MORB with the exception of Y and HREE abundances that are lower than those of N-MORB and match those of E-MORB. They originated by ∼ 10% melting of depleted MORB mantle that carried a small amount (∼ 3%) of an enriched (OIB-like) component, corroborated by moderate initial εNd values (+ 4.6–+ 6.2). The Mélange basalts, in contrast, have multi-element profiles and REE patterns typical of within-plate compositions, transitional between E-MORB and OIB. They originated by < 5% melting of spinel (and occasionally garnet) lherzolite that carried a small amount (∼ 3%) of recycled crustal component, as supported by low initial εNd values (+ 3.8–+ 5.1). The Lesvos peridotites comprise both spinel lherzolites and harzburgites. Using mineral compositions and calculations of temperature and oxygen fugacity, the lherzolites plot in the fields of alpine-type and continental peridotites. Trace element modelling suggests they have experienced anhydrous melting in the stability field of spinel but also garnet lherzolite; calculated degrees of melting are generally small and never exceed 14%. There is no evidence for refertilisation or hydrous melting in a supra-subduction zone environment for the lherzolites. The harzburgites, despite being considerably more altered than the lherzolites, also display evidence of anhydrous melting in the stability field of spinel. However, they also display spoon- and dish-shaped REE patterns that can be produced by contamination of the harzburgites by a percolating OIB-like component with an inherent recycled sedimentary component, which is necessary to explain their isotopic shift towards EM2 compositions. SHRIMP dating of gabbro zircons revealed an Upper Permian (253 Ma) crystallisation age and a Middle Triassic (236 Ma) resetting of the zircon rims, compatible with the coeval and widespread rift-related intrusion of A-type granites in the eastern Mediterranean. Gabbro zircon ages of 777, 539 and 338 Ma strongly suggest inheritance from the basement into which the gabbros were intruded and correspond to ages of distinct terranes recently recognized in the Hellenides. We propose that the Lesvos mélange and ultramafic rocks best represent an incipient continental rift setting of Triassic age, similar to what has been described for the Italian Ligurides, that led to the subsequent formation of a branch of Neotethys, and that the term “Lesvos ophiolite” should be used with due caution or abandoned altogether.

Multistage metamorphism of orogenic garnet–lherzolite from Zhimafang, Sulu UHP terrane, E. China: Implications for mantle wedge convection during progressive oceanic and continental subduction

Seven stages of mineral assemblage were recognized in the Zhimafang orogenic garnet lherzolite from the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China, which suggest that it experienced progressive mantle wedge convection during subduction of previous oceanic and subsequent continental slabs. M 1 is recorded by inclusions of high-Mg olivine, high-Mg–Al–Cr–Ca orthopyroxene and high-Mg clinopyroxene in the high Ca–Cr cores of the garnet porphyroclasts, suggesting that the Zhimafang peridotite was probably a garnet lherzolite originated from a deep and hot mantle wedge above the previously subducted oceanic slab. M 2 is represented by inclusions of high-Mg–Al chromite, high-Mg olivine and high-Mg orthopyroxene in the low-Cr mantles of the garnet porphyroclasts and core of the matrix garnet, suggesting that the rock was convected to shallow and hot mantle wedge, and was transformed to refractory spinel harzburgite or spinel dunite due to high degree of decompressional partial melting. M 3 is manifested by the origin of metasomatic clinopyroxene and orthopyroxene porphyroblasts with inclusions of high-Fe chromite, tremolitic or edenitic amphibole, phlogopite, barite and pyrite, indicating that the rock was convected to shallow, cold and wet corner of the mantle wedge, and was transformed to a fertile amphibole-bearing spinel lherzolite during continental subduction. The lherzolites was subsequently transformed to high-pressure (HP) amphibole-bearing garnet lherzolite ( M 4) and UHP garnet lherzolite ( M 5), indicating its convection to deep mantle wedge. Finally, the UHP garnet–lherzolite exhumed to the Earth’s surface, together with the UHP terrane, and subsequently retrograded to amphibole-bearing garnet lherzolite ( M 6) and chlorite-amphibole-bearing spinel lherzolite ( M 7). Detailed analyses of reaction textures and mineral compositions revealed several stages of metasomatism related to oceanic and continental subduction and exhumation.

Multi-stage melt–rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

Spinel and plagioclase peridotites from the Mt.Maggiore (Corsica, France) ophiolitic massif record a composite asthenosphere–lithosphere history of partial melting and subsequent multi-stage melt–rock interaction. Cpx-poor spinel lherzolites are consistent with mantle residues after low-degree fractional melting (F = 5–10%). Opx + spinel symplectites at the rims of orthopyroxene porphyroclasts indicate post-melting lithospheric cooling (T = 970–1,100°C); this was followed by formation of olivine embayments within pyroxene porphyroclasts by melt–rock interaction. Enrichment in modal olivine (up to 85 wt%) at constant bulk Mg values, and variable absolute REE contents (at constant LREE/HREE) indicate olivine precipitation and pyroxene dissolution during reactive porous melt flow. This stage occurred at spinel-facies depths, after incorporation of the peridotites in the thermal lithosphere. Plagioclase-enriched peridotites show melt impregnation microtextures, like opx + plag intergrowths replacing exsolved cpx porphyroclasts and interstitial gabbronoritic veinlets. This second melt–rock interaction stage caused systematic chemical changes in clinopyroxene (e.g. Ti, REE, Zr, Y increase), related to the concomitant effects of local melt–rock interaction at decreasing melt mass, and crystallization of small (<3%) trapped melt fractions. LREE depletion in minerals of the gabbronoritic veinlets indicates that the impregnating melts were more depleted than normal MORB. Preserved microtextural evidence of previous melt–rock interaction in the impregnated peridotites suggests that they were progressively uplifted in response to lithosphere extension and thinning. Migrating melts were likely produced by mantle upwelling and melting related to extension; they were modified from olivine-saturated to opx-saturated compositions, and caused different styles of melt–rock interaction (reactive spinel harzburgites, vs. impregnated plagioclase peridotites) depending on the lithospheric depths at which interaction occurred.

Neoproterozoic–Early Devonian magmatism in the Antigonish Highlands, Avalon terrane, Nova Scotia: Tracking the evolution of the mantle and crustal sources during the evolution of the Rheic Ocean

The Antigonish Highlands lies in the Avalon terrane (Avalonia) in the Canadian Appalachians and records four episodes of continental rift-related bimodal magmatism ranging in age from Late Neoproterozoic to Middle Devonian. The oldest episode (ca. 615 Ma Clydesdale Formation) was emplaced in a local rift setting in an Andean-style arc when Avalonia resided along the northern Gondwanan margin. Early Cambrian magmatism (Arbuckle Brook Formation) occurred in a local transtensional setting after arc magmatism had given way to a San Andreas-type transform environment. Middle Ordovician (Dunn Point and Bears Brook formations) magmatism occurred in a local rift in an ensialic island arc setting, analogous to the modern Taupo Volcanic Zone in northern New Zealand, after Avalonia had separated from Gondwana. Middle Devonian (McArras Brook Formation and correlatives) occurred after Avalonia had collided with Laurentia. In each episode, mafic rocks are characterized by high FeO t, FeO t/MgO, TiO 2, Zr/Y and Ti/Y typical of differentiated within-plate mafic magmas. Multi-element spider-diagrams indicate that all four episodes were generated in an enriched sub-continental lithospheric mantle. LIL, HFS and REE patterns indicate that the Clydesdale, Dunn Point/Bears Brook and McArras Brook formations were sourced in the shallow (spinel lherzolite) mantle, and that the Arbuckle Brook Formation was generated in the deeper (garnet lherzolite) mantle. In each episode, coeval felsic rocks were generated by crustal anatexis. Taken together, the Sm–Nd isotopic data for the mafic rocks form an envelope that defines the evolution of the mantle source beneath the Antigonish Highlands that was enriched between 0.8 and 1.1 Ga and has an average Sm/Nd ratio of ca. 0.24 (a value that is typical of an enriched mantle source). Sm–Nd isotopic data for the felsic rocks are characterized by a recurrence of T DM ages between 0.95 and 1.0 Ga, interpreted to reflect repeated melting of the lower crust that was itself derived from a depleted-mantle source between 0.95 and 1.1 Ga. The remarkable geochemical and isotopic similarity of the Neoproterozoic, to Middle Devonian mafic and felsic magmas indicate that the crust and sub-continental lithospheric mantle beneath the Antigonish Highlands were coupled during the rift and drift of Avalonia from the Gondwanan margin during the formation of the Rheic Ocean in the Ordovician, its accretion to Baltica and to Laurentia in the Silurian, and post-accretionary strike-slip movement of Avalon along the Laurentian margin. Although the Sm–Nd isotopic data suggest that Avalonian basement and lithospheric mantle formed over the same time interval (0.8 to 1.1 Ga), the divergence of their respective envelopes with time reflects their differing ε Nd values and Sm/Nd ratios. We suggest that Avalonian crustal basement and its enriched mantle were both formed between 0.8 and 1.1 Ga in a Panthalassa-type ocean that surrounded the supercontinent Rodinia and were accreted to the northern Gondwanan margin at about 650 Ma, prior to the oldest magmatic event in the Antigonish Highlands.

Refertilization of Jurassic oceanic peridotites from the Tethys Ocean — Implications for the Re–Os systematics of the upper mantle

The influence of melt migration and solid state mixing of pyroxenites on Re–Os isotopic and major element systematics of oceanic mantle rocks have been studied in serpentinized spinel lherzolites and associated spinel and spinel–garnet pyroxenite layers from the Totalp ultramafic massif near Davos (eastern Swiss Alps). The ultramafic body originally was emplaced and serpentinized on the Jurassic Liguria–Piedmont ocean floor and was little modified during the Alpine orogeny. Field and petrographic observations indicate that refertilization of the peridotites occurred by migration of melt between abundant websterite layers and the peridotites, as well as by mechanical stretching and thinning of websterite layers down to the subcentimeter scale, thus producing fertile lherzolite compositions.Osmium concentrations in the peridotites are in the range observed for normal upper mantle peridotites, whereas Re concentrations in some peridotites are substantially higher than typical mantle values (up to 0.76 ppb). Initial Os isotopic compositions and Re/Os ratios in the peridotites range from subchondritic to suprachondritic (γOs (160 Ma) between −5.5 and +7.2). Re/Os, γOs (160 Ma) and Re concentrations correlate with fertility indicators such as Al2O3 and Na2O, suggesting limited influence of serpentinization on Re–Os systematics of the peridotites. These correlations extend beyond estimates for primitive upper mantle (PUM) compositions suggesting that depletion by partial melting cannot have been the only process producing the variations.The pyroxenites generally have higher Re (up to 3.2 ppb Re), but lower Os concentrations than lherzolites, and suprachondritic initial γOs (+9.0 to +497). Websterites with suprachondritic γOs (160 Ma) alternate with peridotites with chondritic or even subchondritic 187Os/188Os, indicating substantial small-scale Os isotopic heterogeneity. The combined petrographic observations and contrasting compositions of peridotites and pyroxenites indicate that the correlations of γOs (160 Ma) with fertility indicators reflect mixing of peridotites with a long-term depletion history and Re-enriched melts. The pyroxenites may represent residues of eclogites that melted during asthenospheric upwelling or could be cumulates which precipitated from asthenospheric melts.These data provide new evidence that the lower section of the suboceanic lithospheric mantle contains fertile components that are enriched in Re and radiogenic Os. Melt migration evidently does not completely homogenize small-scale Os isotopic heterogeneities in the lower lithosphere, thus accounting for the presence of old depleted peridotites in young oceanic lithospheric mantle.

Asthenospheric signature in fertile spinel lherzolites from the Viliga Volcanic Field in NE Russia

Abstract Mantle xenolith bearing olivine melanephelinites from the Okhotsk sector of the Okhotsk–Chukotka Volcanic Belt (OCVB), northeastern Russia, occur as small isolated volcanoes emplaced within massive late Early to Late Cretaceous subduction-related calc-alkaline rocks. The xenoliths are typical medium- to fine‐grained anhydrous mainly spinel lherzolites that are strongly to weakly foliated with intensive to minor recrystallization to equigranular texture. The primitive mantle normalized whole-rock REE have flat patterns or patterns with slightly elevated light REE (LREE) ((La/Yb) N =0.48–1.38). The REE in clinopyroxenes have systematically decreasing normalized abundances from Sm to La, implying that the LREE enrichments in the whole‐rock REE patterns are attributed to circulation of minor intergranular fluids or melts. Equilibration temperatures and pressures calculated for the Viliga samples are in the range of 1050–1160 °C and 15–21 kbar, respectively. Ca diffusion rates in olivine reveal a rapid transport to the surface (2–6 days) of these peridotites. Model calculations have shown that the fertile lherzolites can be produced by 2–9% batch melting, whereas the depleted peridotites require 15% batch melting of a primitive source. The cessation of the interaction between the palaeo-Pacific plate and the NE Russian margin at c . 87 Ma apparently caused a ‘piecemeal’ collapse of the former followed by intrusion and ascent of olivine melanephelinitic magma, which entrained xenoliths from the asthenospheric mantle of the subducted plate during the Pliocene through the generated window(s). Moreover, clinopyroxenes that have low 87 Sr/ 86 Sr and high 143 Nd/ 144 Nd and plot in and above the mid-ocean ridge basalt (MORB) field are consistent with an upwelling asthenospheric mantle through the window(s) created by the ‘piecemeal’ collapse of the palaeo-Pacific plate.

Types of metasomatism in mantle xenoliths enclosed in Neogene-Quaternary alkaline mafic lavas from Catalonia (NE Spain)

Abstract Mantle xenoliths in within-plate alkaline mafic lavas from NE Spain are mainly anhydrous spinel lherzolites and harzburgites, grading into each other, and subordinate pyroxenites. Peridotites followed an earlier melt depletion caused by mantle decompression and subsequent metasomatism. Two main types of metasomatism are differentiated affecting mainly the harzburgites: a silicate-melt metasomatism of Fe–Ti type and a carbonatite metasomatism. Both types are recognized in the nearby Pyrenean peridotite massifs, but the presence of hydrous minerals is less frequent in the xenoliths. The two metasomatic styles could have been generated by the intrusion of Cretaceous alkaline magmas, if a chromatographic fractionation–reaction process at decreasing melt mass took place. This would account for the evolution of the original alkaline silicate percolating melt towards a carbonatite-rich melt, allowing the coexistence in both space and time of the two metasomatic styles. Metasomatism in lherzolites could be explained in the same way. The pyroxenite xenoliths are interpreted as cumulates from these alkaline basic magmas that crop out in the area as rare camptonite dykes. Interaction with host lavas is minor and could explain the partial melting, enrichment and disequilibrium observed in a deformed composite xenolith and sporadic veins filled with glass.

The role of HIMU metasomatic components in the North African lithospheric mantle: petrological evidence from the Gharyan lherzolite xenoliths, NW Libya

Abstract The Neogene–Quaternary alkali-basalt–hawaiite lavas of the Gharyan volcanic field (NW Libya) contain mantle xenoliths. These mostly consist of protogranular spinel lherzolites with superimposed metasomatic textures represented by reaction patches where primary orthopyroxene (opx), clinopyroxene (cpx) and spinel (sp) are the main reacting phases. The secondary parageneses include clinopyroxene (cpx2), olivine (ol2) and feldspar (feld) as reaction rims around opx, spongy-textured clinopyroxene with recrystallized portions (cpx2±feldspar), and brown spinel destabilized in a higher Cr/(Cr+Al) black vermicular aggregate (sp2) generally associated with feldspar microlites. Cpx2 are typically depleted in Na 2 O and Al 2 O 3 relative to cpx; feldspar includes both alkali-feldspar (Or 17–51) and plagioclase (An 23–64). Bulk rocks have flat heavy rare earth element (HREE) patterns (1.2–2.3 times chondrite) and are variably enriched in light REE (LREE; La N /Yb N up to 6.6). The constituent clinopyroxenes are characterized by flat HREE distributions (8–14.5 times chondrite) and variable LREE enrichment with La N /Yb N up to seven, which generally conform to the bulk-rock chemistry. Samples relatively unaffected by metasomatism have clinopyroxene Sr–Nd isotopic composition ( 87 Sr/ 86 Sr down to 0.7023, 143 Nd/ 144 Nd up to 0.5139) that approaches the depleted mantle (DM), suggesting that the lithospheric mantle beneath the area underwent a long-term depletion probably by pre-Palaeozoic extraction of basic melts. The remaining samples approach 87 Sr/ 86 Sr c . 0.7030, 143 Nd/ 144 Nd c . 0.5130, with 206 Pb/ 204 Pb up to 19.66. These data imply that the causative agents of metasomatism were Na-alkali silicate melts with a clear HIMU affinity, in accordance with the isotopic signature of the host lavas ( 87 Sr/ 86 Sr=0.7032, 143 Nd/ 144 Nd=0.5130, 206 Pb/ 204 Pb=19.60). This prevalent HIMU geochemical signature is comparable with that recorded in Cenozoic alkaline basic lavas and associated mantle xenoliths from other occurrences of the northern–central African lithosphere, suggesting a common regional sub-lithospheric component. The relatively low 3 He/ 4 He of the Gharyan xenoliths (5.3–6.5 R a ) indicates that this component originates within the upper mantle and is unrelated to the deep-seated mantle plume source of the Ethiopian–Yemen plateau basalts. Therefore, the Cenozoic volcanic districts of the Saharan belt could be related to smaller-scale shallow mantle upwellings (also referred to as ‘hot fingers’) triggered by intraplate reactivation of regional tectonic lineaments within the Pan-African cratonic basement, as a foreland reaction of the African–Europe collisional system.

Geochemistry of Paleoproterozoic metavolcanic rocks from the southern Ashanti volcanic belt, Ghana: Petrogenetic and tectonic setting implications

Geochemical data are presented for Paleoproterozoic metavolcanic rocks from the southern Ashanti volcanic belt with the aim of inferring their petrogenesis and tectonic setting in which they were formed. The metavolcanic rocks, which are predominantly basalts/basaltic andesites and andesites, have high Cr and Ni contents, indicating that they have not undergone significant fractional crystallization from mantle-derived melts. Two types of basalts/basaltic andesites, Type I and Type II, were identified. The Type I basalts show flat to slightly LREE-depleted patterns with (La/Sm) N = 0.69–1.03, (La/Yb) N = 0.57–0.71 and minor negative and positive Eu anomalies (Eu/Eu* = 0.92–1.24). The Type II basalts/basaltic andesites show fractionated REE patterns with (La/Sm) N = 1.34–2.31, (La/Yb) N = 2.08–4.25 and minor positive Eu anomalies (Eu/Eu* = 1.09–1.13). The andesites also show fractionated REE patterns, (La/Sm) N = 1.97–2.78 and (La/Yb) N = 4.11–8.48 with minor positive to non-existent Eu anomalies (Eu/Eu* = 0.99–1.15). N-MORB-normalized, trace element patterns show that the Type II basalts/basaltic andesites and the andesites have geochemical patterns characterized by enrichment in LILE relative to HFSE and in LREE relative to HREE. The andesites and the Type II basalts/basaltic andesites exhibit characteristics of subduction zone-related magmas; i.e., the former displays strong Nb and Ti anomalies and relatively high Th/Nb ratios (0.69–95) whereas the latter displays strong negative Nb anomalies but relatively smaller Ti anomalies and lower Th/Nb ratios (0.35–0.37). The Type I basalts/basaltic andesites are generally moderately enriched in LILEs and depleted in HFSEs and HREEs relative to N-MORB. Like the andesites and the Type II basalts/basaltic andesites, the Type I basalts/basaltic andesites display subduction-related trace element characteristics of positive Ba and Sr anomalies together with slightly negative Nb–Ta anomalies. The low Th/Nb ratios of 0.06–0.11 coupled with their MORB-like REE patterns indicate no or negligible amount of sedimentary component in the Type I basalts/basaltic andesites. Crustal contamination may be ruled out as the cause of the negative Th and HFSE anomalies observed in the basaltic and andesitic rocks. Rather the anomalies are interpreted, on the basis of Th–Nb–La–Ce, to reflect a recycled slab-derived lithosphere component. The LREE-depleted tholeiitic Type I basalts/basaltic andesites exhibit back-arc basin geochemical signatures. The calc-alkaline andesites and the Type II basalts/basaltic andesites show intra-oceanic island arc signatures. The high Mg basaltic/andesitic rocks may, however, have evolved in a forearc setting. We, therefore, infer an intra-oceanic island arc-forearc-backarc setting for the Paleoproterozoic Birimian metavolcanic rocks from the southern Ashanti volcanic belt. REE modeling indicates that the magma from which the Type I basalts were formed was generated by about 17–30% partial melting of a depleted mantle (DMM) composed of spinel lherzolite in an extensional marginal basin. The parental magmas of the high Mg calc-alkaline basaltic/andesitic rocks were produced in the forearc by about 20–30% partial melting of the metasomatized mantle, and the Type II basalts/basaltic andesites and the andesites are related by fractional crystallization.

Platinum-group element abundances and Os isotope composition of mantle peridotites from the Mamonia complex, Cyprus

Platinum-group element (Os, Ir, Ru, Pt and Pd) abundances and Re–Os isotopic composition of fifteen peridotites (eleven spinel lherzolites and four spinel harzburgites) from the Mamonia complex, Cyprus, were determined as well as major and trace element compositions of bulk-rocks and minerals. Spinel lherzolites show excellent correlation between parameters indicating the degree of melting — e.g. Fo content in olivine, Cr# (Cr/(Cr + Al)) of Cr-spinel, Al and Yb concentrations in clinopyroxene. The degree of partial melting, calculated using Cr-spinel compositions, range from 1% to 9%. The PGE contents in spinel lherzolites show correlation with each other and with the spinel compositions, and thus can be related to the partial melting of the mantle source. The PGE abundances of the mantle source are estimated using the least depleted spinel lherzolite samples: Os 3.6 ± 0.5, Ir 3.4 ± 0.5, Ru 6.5 ± 0.9, Pt 6.1 ± 0.2, Pd 3.9 ± 0.2 (in ppb). Spinel harzburgites, despite a good correlation between the whole rock major element abundances and mineral compositions (e.g., Yb in clinopyroxene and Cr# of spinel), indicate no relationship between the Fo content of olivine and the Cr# of spinel. Hence, the harzburgites cannot be the residuum of simple partial melting, but have a more complex origin (e.g. melt percolation). Pt/Ir ratios increase in the harzburgites as Pt increases. Similar behavior of Pt and Pd is observed in abyssal and SCLM harzburgites, and explained by sulfide precipitation during melt percolation. Rhenium concentrations in most Mamonia peridotites are significantly higher than in the primitive mantle and does not show correlations with PGE. Indeed Re concentrations tend to increase with the Cr# of spinel. Thus its distribution is not governed by partial melting and we suggest that Re addition to the peridotites of Mamonia occurred during serpentinization. The Re–Os model ages of peridotites form three age clusters at 250 Ma, 600–800 Ma and > 1000 Ma. The youngest age is consistent with the age of magmatic rocks in the Mamonia Complex, whereas the “oldest” peridotites may belong to remnants of the subcontinental mantle.