Rare earth geochemistry of fused ophiolitic and alpine lherzolites—I. Othris,Lanzo and Troodos

Lherzolites from two Mediterranean peridotite masses have major and trace element data compatible with an origin as a fragment of relatively undepleted mantle. Field observations indicate a close association with in situ basaltic melt (gabbroic dikes and segregations) and a barren refractory residue (harzburgite) produced by the removal of the melt fraction.Two lherzolites Othris (ophiolite) and Lanzo (alpine¹ periodotite) have approximately chondritic rare earth abundances with a slight depletion in light rare earths. The refractory material is moderately to heavily depleted in light REE dependent on the efficiency of removal of basaltic melt. Lherzolite xenoliths from the Massif Central probably contain an interstitial light REE enriched fraction as the recalculated lherzolite is depleted and not light REE enriched like the actual whole rock. These basaltic xenoliths are similar in major, trace and REE profile to the Lanzo and Othris mantle lherzolites, giving some indication of source homogeneity in the Mediterranean area. Partial fusion calculations on the Othris and Lanzo peridotites reveal that tholeiitic liquids could be generated by 10–30% partial melting. Such tholeiitic liquids separated from the Othris mantle section and probably formed early sea floor in a small ocean basin. Alkalic basalts are also associated with the Othris ophiolite as an early rifting sequence, and such liquids could have been generated from the source Iherzolite but difficulties would occur in removing such a liquid from the refractory residue.     

Rare earth elements in alpine peridotites

The samples from alpine peridotite massifs (Beni Bouchera, Lherz and the Alps) have been analyzed for rare earth elements. The peridotites as a whole are characterized by various degrees of light REE depletion (Ce varies from 1.2 to 0.02 times chondrite) and a small variation in heavy REE (Yb varies about a factor of 2, from 1.3 to 2.2 times chondrite). They show a restricted and regular distribution in a Ce-Yb diagram, giving two types of linear trends for individual massifs (trend A for the Alps and Lizard; trend B for Beni Bouchera and Lherz, branching from trend A). The model calculations of partial melting based on the partition relations of REE among silicate minerals and melts suggest that trend A could represent a series of residua left after partial melting of garnet peridotite despite the fact that there is no garnet observed in the peridotites studied here. It is suggested that trend A would represent a melting event which predated the emplacement of the massifs and occurred at higher pressure (in the presence of garnet) than expected from the present mineralogy. The calculations also suggest that trend B could represent a partial melting event at lower pressures than trend A after the massifs uplifted into spinel peridotite field. It is also suggested that the REE concentrations of the mantle could be estimated as 2–2.5 times chondrite.     

Plagioclase lherzolite-residual mantle relationships within two eastern mediterranean ophiolites

The ophiolites of Othris, northern Greece, and Troodos, Cyprus, are of mixed lherzolite-harzburgite and harzburgite sub-type respectively. Within both ophiolites an entire spectrum of harzburgite, plagioclase harzburgite, lherzolite and plagioclase lherzolite interpreted respectively as residual and highly, moderately and slightly modified upper mantle has been recognised. Plagioclase lherzolite frequently contains gabbroic segregations generated by partial melting and incomplete extraction. Othris shows the full range of mantle peridotites while Troodos is dominated by harzburgite with very minor occurrences of plagioclase harzburgite and lherzolite. Chemically, the plagioclase lherzolites have low contents of the basaltophillic minor elements, and a composition slightly more residual than postulated upper mantle compositions, suggestive of a preceding phase of minor depletion. The Othris and Troodos ophiolites seem to have formed under fundamentally different environments — Othris as a marginal ophiolite at the inception on rifting of continental crust, and Troodos later in such an event when spreading was well established.     

SR-isotopic composition of clinopyroxenes from some mediterranean alpine lherzolites

Clinopyroxenes in the metamorphic alpine peridotites from Ronda, Béni Bouchera, Lanzo and Othris have ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios in the range of 0.70228 – 0.70370, similar to ocean ridge tholeiitic rock. Insofar as these lherzolites represent mantle sources, the present data allows simple evolutionary models relating basalt genesis and alpine peridotite. The highly radiogenic Sr reported in many whole rock alpine peridotites may be due to contamination in olivine and thus, earlier models that deny a simple relationship between alpine peridotite and the oceanic gabbros and basalts need a re-evaluation.     

Roles of Melting and Metasomatism in the Formation of the Lithospheric Mantle beneath the Leizhou Peninsula, South China

This study characterizes the nature of fluid interaction andmelting processes in the lithospheric mantle beneath the Yingfenglingand Tianyang volcanoes, Leizhou Peninsula, South China, usingin situ trace-element analyses of clinopyroxene, amphibole andgarnet from a suite of mantle-derived xenoliths. Clinopyroxenesfrom discrete spinel lherzolites exhibit large compositionalvariations ranging from extremely light rare earth element (LREE)-depletedto LREE-enriched. Trace-element modelling for depleted samplesindicates that the Leizhou lherzolites are the residues of amantle peridotite source after extraction of 1–11% meltgenerated by incremental melting in the spinel lherzolite fieldwith the degree of melting increasing upwards from about 60km to 30 km. This process is consistent with gradational meltingat different depths in an upwelling asthenospheric column thatsubsequently cooled to form the current lithospheric mantlein this region. The calculated melt production rate of thiscolumn could generate mafic crust 5–6 km thick, whichwould account for most of the present-day lower crust. The formationof the lithospheric column is inferred to be related to Mesozoiclithosphere thinning. Al-augite pyroxenites occur in compositexenoliths; these are geochemically similar to HIMU-type oceanisland basalt. These pyroxenites postdate the lithospheric columnformation and belong to two episodes of magmatism. Early magmatism(forming metapyroxenites) is inferred to have occurred duringthe opening of the South China Sea Basin (32–15 Ma), whereasthe most recent magmatic episode (producing pyroxenites withigneous microstructures) occurred shortly before the eruptionof the host magmas (6–0·3 Ma). Trace-element traversesfrom the contacts of the Al-augite pyroxenite with the spinelperidotite wall-rock in composite xenoliths record gradientsin the strength and nature of metasomatic effects away fromthe contact, showing that equilibrium was not attained. Significantenrichment in highly incompatible elements close to the contacts,with only slight enrichment in Sr, LREE and Nb away from thecontact, is inferred to reflect the different diffusion ratesof specific trace elements. The observed geochemical gradientsin metasomatic zones show that Sr, La, Ce and Nb have the highestdiffusion rates, other REE are intermediate, and Zr, Hf andTi have the lowest diffusion rates. Lower diffusion rates observedfor Nb, Zr, Hf and Ti compared with REE may cause high fieldstrength element (HFSE) negative anomalies in metasomatizedperidotites. Therefore, metasomatized lherzolites with HFSEnegative anomalies do not necessarily require a carbonatiticmetasomatizing agent. KEY WORDS: China; lithosphere; mantle xenoliths; clinopyroxene trace elements; mantle partial melting; mantle metasomatism; trace-element diffusion rates     

The Othris Ophiolite, Greece: A snapshot of subduction initiation at a mid-ocean ridge

The mantle section of the Tethyan-type Othris Ophiolite, Greece, records tectono-magmatic processes characteristic of both mid-ocean ridges and supra-subduction zones. The Othris Ophiolite is a remnant of the Jurassic Neotethys Ocean, which existed between Eurasia and Gondwanaland. Othris peridotites range from fertile plagioclase lherzolites to depleted harzburgites. Abundances of Al₂O₃ and CaO show well-defined inverse linear correlations with MgO, suggesting that the Othris peridotites formed as residua from variable degrees of partial melting.

Peridotites from the Fournos Kaïtsa and western Katáchloron sub-massifs are similar to abyssal peridotites and can be explained by a multistage model with some melting in the garnet stability field followed by moderate degrees of anhydrous near-fractional melting in the spinel stability field. In contrast, the peridotites from the Metalleio, Eretria, and eastern Katáchloron sub-massifs, and the Vourinos ophiolite are highly depleted and have extremely low concentrations of Al₂O₃ and heavy rare earth elements. These peridotites have enriched light REE contents compared to the middle REE. These residua are best modelled by hydrous melting due to a flux of slab-derived fluid to the mantle wedge during melting.

The occurrence of both styles of melting regimes within close spatial and temporal association in the same ophiolite is explained by intra-oceanic thrusting and forced subduction initiation at (or near) a mid-ocean ridge. Thus, the Othris Ophiolite, and probably Tethyan-type ophiolites in general, represent a transient phase of plate tectonic reorganisation rather than quasi-steady state plate tectonics.     

Geochemical make-up of oceanic peridotites from NW Turkey and the multi-stage melting history of the Tethyan upper mantle

We present the whole-rock and the mineral chemical data for upper mantle peridotites from the Harmanc?k region in NW Turkey and discuss their petrogenetic–tectonic origin. These peridotites are part of a Tethyan ophiolite belt occurring along the ?zmir-Ankara-Ercincan suture zone in northern Turkey, and include depleted lherzolites and refractory harzburgites. The Al₂O₃ contents in orthopyroxene and clinopyroxene from the depleted lherzolite are high, and the Cr-number in the coexisting spinel is low falling within the abyssal field. However, the orthopyroxene and clinopyroxene in the harzburgites have lower Al₂O₃ contents for a given Cr-number of spinel, and plot within the lower end of the abyssal field. The whole-rock geochemical and the mineral chemistry data imply that the Harmanc?k peridotites formed by different degrees of partial melting (~%10–27) of the mantle. The depleted lherzolite samples have higher MREE and HREE abundances than the harzburgitic peridotites, showing convex-downward patterns. These peridotites represent up to ~16 % melting residue that formed during the initial seafloor spreading stage of the Northern Neotethys. On the other hand, the more refractory harzburgites represent residues after ~4–11 % hydrous partial melting of the previously depleted MOR mantle, which was metasomatized by slab-derived fluids during the early stages of subduction. The Harmanc?k peridotites, hence, represent the fragments of upper mantle rocks that formed during different stages of the tectonic evolution of the Tethyan oceanic lithosphere in Northern Neotethys. We infer that the multi-stage melting history of the Harmanc?k peridotites reflect the geochemically heterogeneous character of the Tethyan oceanic lithosphere currently exposed along the ?zmir-Ankara-Erzincan suture zone.     

西藏班公湖—怒江缝合带中段蓬湖蛇绿岩中的洋脊型二辉橄榄岩

蓬湖蛇绿岩产于西藏藏北湖区的蓬湖西侧,属班公湖-怒江缝合带中段白拉拉弄-依拉山亚带。该蛇绿岩主要由地幔橄榄岩、堆晶岩和辉绿岩等组成。其中地幔橄榄岩由方辉橄榄岩和二辉橄榄岩组成。蓬湖二辉橄榄岩的橄榄石Fo值介于88.85～90.33之间、斜方辉石的Al2O3含量范围在4.26%～6.60%。与原始地幔相比,蓬湖二辉橄榄岩岩石有较高的MgO含量和较低的Al2O3、CaO和TiO2等易熔组分含量;稀土元素总量介于1.11×10-6～1.53×10-6之间,明显低于原始地幔值,配分模式为轻稀土轻微亏损。在原始地幔微量元素蛛网图中,蓬湖二辉橄榄岩显示Rb、Zr亏损,U、Ta、Sr强烈富集特征。蓬湖二辉橄榄岩的铂族元素总量介于22.9×10-9～27×10-9之间,PGEs球粒陨石标准化图解显示其为接近原始地幔的"平坦型"。以上特征与深海橄榄岩相似,指示它们可能形成于大洋中脊环境。定量模拟估算表明,蓬湖二辉橄榄岩可能来源于地幔中尖晶石相二辉橄榄岩源区,系经历了约5%～10%的部分熔融残余。蓬湖堆晶岩矿物结晶顺序为橄榄石-单斜辉石-斜长石,其中异剥橄榄岩中的单斜辉石Mg#值介于86.92～89.93之间、橄榄石Fo平均值为84.45,明显不同于MOR型蛇绿岩堆晶岩。蓬湖堆晶岩的矿物组成、岩浆结晶顺序和矿物成分均与俯冲带上SSZ型蛇绿岩形成的堆晶岩类似。以上结果表明,蓬湖二辉橄榄岩形成于大洋脊环境,为尖晶石二辉橄榄岩源区经历了不超过10%部分熔融的残余,后期由于洋内俯冲作用经历了岩石-熔体反应,形成了SSZ型堆晶岩和含较高Cr#值尖晶石的方辉橄榄岩。     

Integrated Models of Basalt Petrogenesis: A Study of Quartz Tholeiites to Olivine Melilitites from South Eastern Australia Utilizing Geochemical and Experimental Petrological Data

The Tertiary to Recent basalts of Victoria and Tasmania havemineralogical and major element characteristics of magmas encompassingthe range from quartz tholeiites to olivine melilitites. Abundancesof trace elements such as incompatible elements, including therare earth elements (REE), and the compatible elements Ni, Coand Sc, vary systematically through this compositional spectrum.On the basis of included mantle xenoliths, appropriate 100 Mg/Mg+ Fe₊₂ (68–72) and high Ni contents many of these basaltsrepresent primary magmas (i.e., unmodified partial melts ofmantle peridotite). For fractionated basalts we have derivedmodel primary magma compositions by estimating the compositionalchanges caused by fractional crystallization of olivine andpyroxene at low or moderate pressure. A pyrolite model mantlecomposition has been used to establish and evaluate partialmelting models for these primary magmas. By definition and experimentaltesting the specific pyrolite composition yields parental olivinetholeiite magma similar to that of KilaeauIki, Hawaii (1959–60)and residual harzburgite by 33 per cent melting. It is shownthat a source pyrolite composition differing only in having0.3–0.4 per cent TiO₂ rather than 0.7 per cent TiO₂, isable to yield the spectrum of primary basalts for the Victorian-Tasmanianprovince by 4 per cent to 25 per cent partial melting. The mineralogiesof residual peridotites are consistent with known liquidus phaserelationships of the primary magmas at high pressures and thechemical compositions of residual peridotite are similar tonatural depleted or refractory lherzolites and harzburgites.For low degrees of melting the nature of the liquid and of theresidual peridotite are sensitively dependent on the contentof H₂O, CO₂ and the CO₂/H₂O in the source pyrolite. The melting models have been tested for their ability to accountfor the minor and trace element, particularly the distinctivelyfractionated REE, contents of the primary magmas. A single sourcepyrolite composition can yield the observed minor and traceelement abundances (within at most a factor of 2 and commonlymuch closer) for olivine melilitite (4–6 per cent melt),olivine nephelinite, basanite (5–7 per cent melt), alkaliolivine basalt (11–15 per cent melt), olivine basalt andolivine tholeiite (20–25 per cent melt) provided thatthe source pyrolite was already enriched in strongly incompatibleelements (Ba, Sr, Th, U, LREE) at 6–9 x chondritic abundancesand less enriched (2.5–3 x chondrites) in moderately incompatible(Ti, Zr, Hf, Y, HREE) prior to the partial melting event. Thesources regions for S.E. Australian basalts are similar to thosefor oceanic island basalts (Hawaii, Comores, Iceland, Azores)or for continental and rift-valley basaltic provinces and verydifferent in trace element abundances from the model sourceregions for most mid-ocean ridge basalts. We infer that thismantle heterogeneity has resulted from migration within theupper mantle (LVZ or below the LVZ) of a melt or fluid (H₂O,CO₂-enriched) with incompatible element concentrations similarto those of olivine melilitite, kimberlite or carbonatite. Asa result of this migration, some mantle regions are enrichedin incompatible elements and other areas are depleted. Although it is possible, within the general framework of a lherzolitesource composition, to derive the basanites, olivine nephelinitesand olivine melilitites from a source rock with chondritic relativeREE abundances at 2–5 x chondritic levels, these modelsrequire extremely small degrees of melting (0.4 per cent forolivine melilitite to 1 per cent for basanite). Furthermore,it is not possible to derive the olivine tholeiite magmas fromsource regions with chondritic relative REE abundances withoutconflicting with major element and experimental petrology argumentsrequiring high degrees (15 per cent) of melting and the absenceof residual garnet. If these arguments are disregarded, andpartial melting models are constrained to source regions withchondritic relative REE abundances, then magmas from olivinemelilitites to olivine tholeiites can be modelled if degreesof melting are sufficiently small, e.g., 7 per cent meltingfor olivine tholeiite. However, the source regions must be heterogenousfrom 1 to 5 x chondritic in absolute REE abundances and heterogerieousin other trace elements as well. This model is rejected in favorof the model requiring variation in degree of melting from 4per cent to 25 per cent and mantle source regions ranging fromLREE-enriched to LREE-depleted relative to chondritic REE abundances.     

Xenolith evidence for lithospheric melting above anomalously hot mantle under the northern Canadian Cordillera

A comparison of mantle xenolith suites along the northern Canadian Cordillera reveals that the xenoliths from three suites exhibit bimodal populations whereas the xenoliths from the other four suites display unimodal populations. The bimodal suites contain both fertile lherzolite and refractory harzburgite, while the unimodal suites are dominated by fertile lherzolite xenoliths. The location of the three bimodal xenolith suites correlates with a newly discovered P-wave slowness anomaly in the upper mantle that is 200 km in width and extends to depths of 400–500 km (Frederiksen AW, Bostock MG, Van Decar JC, Cassidy J, submitted to Tectonophysics). This correlation suggests that the bimodal xenolith suites may either contain fragments of the anomalously hot asthenospheric mantle or that the lithospheric upper mantle has been affected by the anomalously hot mantle. The lherzolite xenoliths in the bimodal suites display similar major element compositions and trace element patterns to the lherzolite xenoliths in the unimodal suites, suggesting that the lherzolites represent the regional lithospheric upper mantle. In contrast, the harzburgite xenoliths are highly depleted in terms of major element composition, but their clinopyroxenes [Cpx] have much higher incompatible trace element contents than those in the lherzolite xenoliths. The major element and mildly incompatible trace element systematics of the harzburgite and lherzolite xenoliths indicate that they could be related by a partial melting process. The lack of textural and geochemical evidence for the former existence of garnet argues against the harzburgite xenoliths representing actual fragments of the deeper anomalous asthenospheric mantle. Furthermore, the calculated P-wave velocity difference between harzburgite and lherzolite end-members is only 0.8%, with the harzburgites having higher P-wave velocities. Therefore the 3% P-wave velocity difference detected teleseismically cannot be produced by the compositional difference between the lherzolite and harzburgite xenoliths. If temperature is responsible for the observed 3% P-wave velocity perturbation, the anomalous mantle is likely to be at least 200 °C higher than the surrounding mantle. Taken together these data indicate that the refractory harzburgite xenoliths represent the residue of 20–25% partial melting of a lherzolite lithospheric mantle. The incompatible trace element enrichment of the harzburgites suggests that this melting was accompanied by the ingress of fluids. The association of the bimodal xenolith suites with the mantle anomaly detected teleseismically suggests that anomalously hot asthenospheric mantle provided both the heat and volatiles responsible for the localized melting and enrichment of the lithospheric mantle. Received: 16 May 1997 / Accepted: 25 October 1997     

Origin of some magmas in oceanic and circum-oceanic regions

Partial melting experiments on spinel-lherzolite, a rock which probably occurs in relatively shallow parts of the oceanic upper mantle, demonstrate that alkali basaltic melt is formed at depths of at least 20 kbar whereas tholeiitic melt is formed at lower pressures (< 15 kbar) under anhydrous conditions. The specimen studied was a relatively iron-rich natural spinel-lherzolite (Fe/Mg+Fe=0.15) and the melts produced have ratios comparable to those obtained in basalts. Slight increase of degree of partial melting produces picritic melt over a wide pressure range. Under hydrous (water-excess) conditions, andesitic melt is produced by partial melting of the same natural spinel-lherzolite and a synthetic lherzolite. The melting experiments on two different abyssal tholeiites from the Mid-Atlantic Ridge suggest that the derivation of olivine tholeiite from a more mafic magma or a mantle peridotite (lherzolite) is possible, but is limited to depths shallower than 25 km under essentially anhydrous conditions, whereas plagioclase tholeiite may have been formed by fractional crystallization at depths of about 20 km in the presence of a small amount (～ 2 wt.%) of water.It is suggested that under mid-ocean ridges, partial melting of spinel-lherzolite at depths shallower than 60 km would produce olivine-tholeiitic magma, which differentiates at shallower levels (20–25 km) under either essentially anhydrous or hydrous (but vapor-absent) conditions to produce abyssal tholeiites of olivine-tholeiite type or plagioclase-tholeiite type. It may be also possible that the former olivine-tholeiite is generated by direct partial melting of plagioclase-lherzolite. Alkali basalts in the oceanic region may be generated at depths greater than 50 km by relatively small degree of partial melting. Along island arcs and continental margins, where the subduction zones probably exist, partial melting of lherzolite would take place in the presence of water that may be supplied by breakdown of hydrous minerals in the subducted oceanic crust, thereby producing andesitic magmas. High-alumina basalt magma could be produced by partial melting of the dehydrated oceanic crust in the subduction zone at depths between 40 and 60 km, where garnet is unstable above the solidus.     

Gabbroic rocks in ophiolitic occurrences from East Othris, Greece: petrogenetic processes and geotectonic environment implications

East Othris area consists of scattered ophiolitic units, as well as ophiolitic mélange occurrences, which encompass gabbroic rocks. These rocks have been affected by low-grade ocean floor metamorphism (T?<?350°C and P?<?8?kbar). Based on their petrography, mineral chemistry and geochemistry gabbroic rocks have been distinguished into gabbros and diorites, with the latter being divided into two groups. Gabbros seem to have been formed from moderate to high partial melting degrees (~8–25%) of a highly depleted mantle source, while group (1) diorites have been differentiated after variable fractionation processes (up to 30%). Group (2) diorites seem to have been derived from low partial melting degrees (~3%) of a fertile or moderately depleted mantle source and with extensive fractionation processes (~50%). Geochemical results suggest that partial melting processes occurred at relatively shallow depths, in the plagioclase-spinel stability field, while amphibole chemistry data indicate shallow level crystallization. Chondrite and PM-normalized patterns, Th/Yb, and Nb/Th ratios as well as mineral chemistry analyses show that gabbros and group (1) diorites (with relatively low PM-normalized Nb and Ta values and negative Ti anomalies) suggest subduction processes, while group (2) diorites are MORB or BAB related. Some gabbros have been characterised as high-Mg, being compositionally similar to picrites or boninites. Variability in extent of partial melting of the mantle source and the different geotectonic environment affinities are consistent with a supra-subduction zone (SSZ) origin of the east Othris ophiolites. The fact that IAT related rocks are more abundant in east rather than in west Othris may possibly be explained by a slab rollback model retreating to the east within the Pindos oceanic basin.     

The mineral chemistry and origin of xenoliths from the lavas of Anjouan,Comores Archipelago,Western Indian Ocean

Mineralogical data for xenoliths occurring as inclusions in the fissure erupted alkali basalts and the basanitic tuffs of Anjouan reveal three xenolith suites: 1) the lherzolites, 2) the dunites and wehrlites, 3) the gabbros and syenites. The dunite-wehrlite suite and the gabbro suite are shown to represent high-level cumulate sequences resulting from ankaramitic fractionation of the hy-normative shield-building lavas and cotecictic fractionation of the alkali basalt lavas respectively, whilst the syenitic xenoliths represent evolved high-level intrusions. Mineralogical and rare earth element (REE) data indicate that the most likely origin for the spinel lherzolite xenoliths is by extraction of a basaltic phase from spinel peridotite, leaving a light REE-poor spinel lherzolite residuum. REE models, constructed using model peridotite assemblages, imply that the hy-normative basalt lavas may be derived by partial melting of spinel peridotite at pressures of <20–25 kb leaving a residual lherzolite, and that the alkali basalt and basanite melts are formed by small degrees of melting of a garnet-peridotite source at pressures of >20–25 kb. The spinel lherzolite source for the hy-normative basalts has been accidentally sampled during explosive eruption of the alkali basalt and basanite magmas.     

Trace-element geochemistry of archean greenstone belts

Progressive alteration, diagenesis, and low-grade metamorphism of Archean greenstone belts often leads to redistribution of alkali and related trace elements. Transition metals and rare earths are relatively resistant to these processes and hence are most useful in evaluating petrologic problems.Depleted Archean tholeiite (DAT) exhibits flat REE distributions and low LIL-element contents while enriched Archean tholeiite (EAT) exhibits slightly enriched REE patterns and moderate LIL-element contents. DAT is grossly similar to modern rise and are tholeiites and EAT to cale-alkaline and oceanic island tholeiites. Archean and esites fall into three categories: depleted Archean andesite (DAA) exhibits flat REE patterns, negative Eu anomalies and low LIL-element contents; low-alkali Archean andesite (LAA) shows minor light REE enrichment and low LIL-element contents; and high-alkali Archean andesite (HAA) shows light REE enrichment and high LIL-element contents. LAA is grossly similar to modern cale-alkaline andesites, but DAA and HAA do not have modern analogues. Archean depleted siliceous volcanics (DSV) exhibit depletion in heavy REE and Y compared to modern siliceous volcanics whereas undepleted varieties (USV) are similar to modern ones. Almost all Archean volcanic rocks, regardless of composition, are enriched in transition metals compared to modern varieties. Archean graywackes are similar in composition to Phanerozoic graywackes. Rock associations in Archean greenstones suggest the existence of two tectonic settings.Magma model studies indicate that partial melting has left the strongest imprint on trace-element distributions in greenstone volcanics. Three magma source rocks are necessary (listed in order of decreasing importance): ultramafic rock, eclogite, and siliceous granulite. Trace-element studies of Archean graywackes indicate a mixed volcanic—granitic provenance with minor ultramafic contributions.Alkali and related trace-element contents of Archean volcanics have been interpreted in terms of both undepleted and depleted upper mantle sources. Preferential enrichment of transition metals in Archean volcanics may have resulted from upward movement of immiscible liquid sulfide droplets with Archean magmas, depleting the source area in these elements. Initial Sr isotope distributions in Archean volcanics indicate the upper mantle during the Archean was heterogeneous in terms of its Rb/Sr ratio.     

The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

Jurassic age volcanic rocks of the Stonyford volcanic complex(SFVC) comprise three distinct petrological groups based ontheir whole-rock geochemistry: (1) oceanic tholeiites; (2) transitionalalkali basalts and glasses; (3) high-Al, low-Ti tholeiites.Major and trace element, and Sr–Nd–Pb isotopic dataindicate that the oceanic tholeiites formed as low-degree partialmelts of normal mid-ocean ridge basalt (N-MORB)-source asthenospheresimilar in isotope composition to the East Pacific Rise today;the alkalic lavas were derived from an enriched source similarto that of E-MORB. The high-Al, low-Ti lavas resemble second-stagemelts of a depleted MORB-source asthenosphere that formed bymelting spinel lherzolite at low pressures. Trace element systematicsof the high-Al, low-Ti basalts show the influence of an enrichedcomponent, which overprints generally depleted trace elementcharacteristics. Tectonic discrimination diagrams show thatthe oceanic tholeiite and alkali suites are similar to present-daybasalts generated at mid-oceanic ridges. The high-Al, low-Tisuite resembles primitive arc basalts with an enriched, alkalibasalt-like overprint. Isotopic data show the influence of recycledcomponents in all three suites. The SFVC was constructed ona substrate of normal Coast Range ophiolite in an extensionalforearc setting. The close juxtaposition of the MORB-like olivinetholeiites with alkali and high-Al, low-Ti basalts suggestsderivation from a hybrid mantle source region that includedMORB-source asthenosphere, enriched oceanic asthenosphere, andthe depleted supra-subduction zone mantle wedge. We proposethat the SFVC formed in response to collision of a mid-oceanridge spreading center with the Coast Range ophiolite subductionzone. Formation of a slab window beneath the forearc duringcollision allowed the influx of ridge-derived magmas or themantle source of these magmas. Continued melting of the previouslydepleted mantle wedge above the now defunct subduction zoneproduced strongly depleted high-Al, low-Ti basalts that werepartially fertilized with enriched, alkali basalt-type meltsand slab-derived fluids. KEY WORDS: CRO; oceanic basalts; California     

Geochemistry of the Archean Bulawayan Group,Midlands greenstone belt,Rhodesia

The Bulawayan Group in the Midlands greenstone belt can be divided into three formations. The Mafic Formation is composed principally of pillowed, low-K tholeiites and minor bedded chert. The Maliyami Formation and conformably overlying Felsic Formation are composed of calc-alkaline tholeiites, andesites, and dacites with andesites dominating in the Felsic Formation. Minor rhyolite quartz porphyries and ultramafic bodies also occur in the section. The Bulawayan Group near Que Que is perhaps the least altered and metamorphosed Archean greenstone succession known. The absence of andesite and related rocks, the association of bedded chert, and the consistently low K₂O, Rb, and Sr contents of Mafic Formation tholeiites suggest that they represent Archean oceanic rise tholeiites. The compositions of tholeiites and andesites of the Maliyami Formation, however, suggest that they represent an emerging arc system. The Felsic Formation is interpreted as a more advanced stage in the evolution of this arc system.Trace-element model calculations favor an origin for Mafic Formation tholeiites involving about 30% partial melting of a lherzolite source. Similar calculations are consistent with an origin for Maliyami Formation tholeiites, Maliyami and Felsic Formation andesites, and Midlands rhyolites involving, respectively, 50, 20–30, and 10% equilibrium melting of eclogite or garnet amphibolite (of Mafic Formation tholeiite composition). The low K₂O, Rb, and Sr contents of Mafic Formation tholeiites suggest that they were derived from an upper mantle source as depleted in these elements as the oceanic upper mantle is today.A plate tectonic model is proposed for the Bulawayan Group in which the Mafic Formation is derived from a depleted lherzolite source beneath a spreading center in a marginalsea basin and the Maliyami and Felsic Formations and associated rhyolites are produced by partial melting of eclogite in a descending slab located west of the basin.     

High-Cr and High-Al Podiform Chromitites,Western China: Relationship to Partial Melting and Melt/Rock Reaction in the Upper Mantle

Podiform chromitites include both high-Cr and high-Al varieties with distinctly different geochemical characteristics. A comparison of high-Cr and high-Al deposits in western China has demonstrated that both varieties are magmatic in origin and that chromite compositions reflect the degree of partial melting in the mantle source area. The chromitites of the Sartohay ophiolite of Xinjiang Province have chromites with low Cr numbers (<70) and are hosted in highly depleted harzburgites. In both deposits melt/wall rock interaction has produced highly depleted dunite envelopes around the chromitites. In Sartohay, high-Al magmas reacted with lherzolites to produce high-Al dunites and harzburgites, whereas in Luobusa the reaction between more refractory melts and depleted harzburgites yielded only highly depleted dunite envelopes. This study suggests that high-Al deposits can occur in weakly depleted mantle sequences (lherzolite ophiolite type or transitional type) that are locally depleted by melt/rock reaction in the immediate vicinity of the chromitite pods.     

Depletion and enrichment processes in the lithospheric mantle beneath the Kola Peninsula (Russia): Evidence from spinel lherzolite and wehrlite xenoliths

A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites.

Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO₂. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship.     

The Pleistocene-Recent Tonga-Kermadec Arc Lavas: Interpretation of New Isotopic and Rare Earth Data in Terms of a Depleted Mantle Source Model

New REE data, and new Nd, O, Sr, and Pb isotopic data are presentedand integrated with previous data for this low-K intra-oceanicarc suite. Geochemically, the arc tholeiites and basaltic andesitesrange from extremely HFS element depleted (northern Tonga) tonear N-MORB-like HFS element abundances in L'Esperance (southernKermadecs). LIL elements (Sr, Rb, K, Rb, Ba, Th) show the characteristicselective enrichment generally recognized in arc magmas, andthus indicate decoupling of the HFS and LIL elements. Modellingsuggests a compositionally variable source (mantle wedge) alongthe arc, ranging from restite after remelting an N-type MORBsource (northern end), to progressively less depleted, MORB-likesources southwards. Thus, the low HFS/LIL element ratios areinterpreted in terms of HFS depletion followed by LIL elementenrichment associated with subduction; broad correlations occurbetween Zr/Ba and Sr/Nd ratios (fractionation corrected) and87Sr/86Sr and ¹⁴³Nd/¹⁴⁴Nd ratios. Derivation of the arc magmas from depleted peridotote requiressuperimposed fractional crystallization, which has been modelledthermodynamically using SILMIN (Ghiorso, 1985), utilizing experimentallyproduced partial melts from depleted lherzolite (Jaques &Green, 1980). It is shown that the arc tholeiites and basalticandesites (and also high Mg-andesites) are potentially developedat low pressures 5 kb), from parental magmas also generatedat relatively low pressure ( 10 kb). These data further suggestthat a southward increasing depth of magma segregation (correlatingwith Benioff Zone geometry) could account for differences inchemistry between the Tonga and Kermadec arc segments. The mechanism of LILE enrichment is still highly problematic,but it is suggested that the model of Tatsumi et al. (1986)may account for much of the geochemical data; this involvesrelatively shallow release, via fluids, of LIL elements intooverlying peridotite beneath the fore-arc region. Induced convectionin the mantle wedge moves the metasomatized mantle into thezones of magma generation. The development of the inferred, variably depleted mantle wedgesource is here related to active back-arc spreading which isslightly older and more rapid behind the northern region ofthe arc. The lherzolite restite from this spreading processis interpreted to undergo further partial melting in the metasomatizedmantle wedge overlying the subduction zone, involving inducedconvection. The back-arc island of Niua fo'ou is geochemically quite distinctfrom the arc magmas, being similar to N-MORB in its trace elements,but to OIB in its isotope ratios.     

The evolution of the Arabian lower crust and lithospheric mantle — Geochemical constraints from southern Syrian mafic and ultramafic xenoliths

Geochemical compositions of lower crustal and lithospheric mantle xenoliths found in alkali basaltic lavas from the Harrat Ash Shamah volcanic field in southern Syria place constraints on the formation of the Arabian–Nubian Shield in northern Arabia. Compositions of lower crustal granulites are compatible with a cumulate formation from mafic melts and indicate that they are not genetically related to their host rocks. Instead, their depletion in Nb relative to other incompatible elements points to an origin in a Neoproterozoic subduction zone as recorded by an average depleted mantle Sm–Nd model age of 630 Ma.Lithospheric spinel peridotites typically represent relatively low degree (< 10%) partial melting residues of spinel lherzolite with primitive mantle compositions as indicated by major and trace element modelling of clinopyroxene and spinel. The primary compositions of the xenoliths were subsequently altered by metasomatic reactions with low degree silicate melts and possibly carbonatites. Because host lavas lack these signatures any recent reaction of the lherzolites with their host magma can be ruled out. Sm–Nd data of clinopyroxene from Arabian lithospheric mantle lherzolites yield an average age of 640 Ma suggesting that the lithosphere was not replaced since its formation and supporting a common origin of the Arabian lower crustal and lithospheric mantle sections.The new data along with published Arabian mantle xenolith compositions are consistent with a model in which the lithospheric precursor was depleted oceanic lithosphere that was overprinted by metasomatic processes related to subduction and arc accretion during the generation of the Arabian–Nubian Shield. The less refractory nature of the northern Arabian lithosphere as indicated by higher Al, Na and lower Si and Mg contents of clinopyroxenes compared to the more depleted nature of the south Arabian lithospheric mantle, and the comparable low extent of melt extraction suggest that the northern Arabian lithosphere formed in a continental arc system, whereas the lithosphere in the southern part of Arabia appears to be of oceanic arc origin.