Flat Rare Earth Element Patterns Research Articles

Caractérisation, genèse et évolution géodynamique du terrain de Guerrero (Mexique occidental)

The Guerrero terrane (western Mexico) is composed of Late Jurassic – Early Cretaceous plutono-volcanic and volcano-sedimentary sequences of the Alisitos–Teloloapan arc that accreted to the North American craton at the end of the Early Cretaceous. The geodynamic evolution of the Guerrero terrane is that of the Alisitos–Teloloapan intraoceanic arc, partly built on continental crust and partly on oceanic crust. The growth of the arc was likely linked to the subduction of the Arperos and Olvidada basins fringing the North American borderland. The subduction was dipping west-south-west.The continent-based segment of the arc, which is presently exposed mainly in northwestern Mexico, is composed of aerial and submarine K-rich calc-alkaline basalts, andesites, and rhyolites and of siliceous pyroclastic rocks interbedded with Aptian–Albian bioclastic carbonates or red beds bearing dinosaurus foot prints. The calc-alkaline basalts and andesites show light rare earth elements enriched patterns and high concentrations in large ion lithophile elements. The siliceous andesites and rhyodacites display low contents in Y and heavy rare earth elements, uncommon for such calc-alkaline SiO2-saturated rocks. This depletion is likely linked to amphibole fractionation and to the presence of sphene and zircon, minerals known to concentrate the heavy rare earth elements.In contrast, the magmatic arc sequences built on oceanic crust, that crops out in central-southern areas of the Guerrero terrane, show an evolution with time. The activity of the arc began with depleted tholeiitic igneous rocks, followed first by mature tholeiitic basalts, then by calc-alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of Early Cretaceous age (Neocomian). At the end of the arc development, in Late Aptian–Albian, calc-alkaline pillow basalts and andesites erupted at the volcanic front whereas shoshonitic basalts emitted backwards. In the late Early Cretaceous, the arc drifted towards the north and collided with the craton. Arc tholeiites are characterized by flat rare earth element patterns or slightly depleted in light rare earth elements and by high εNd ratios. The calc-alkaline plutonic and volcanic rocks show light rare earth elements enriched patterns and their εNd ratios decrease with time. This decrease of the εNd ratios suggests that either the mantle source of the calc-alkaline rocks was contaminated by subducted terrigenous sediments derived from an old continental crust (North American craton) or that these calc-alkaline rocks derive from the partial melt of an oceanic island basalt source present in the mantle wedge. The shoshonitic features of the basalts are marked by the presence of sanidine in the groundmass and the high levels of K2O, Ba, and Sr of the unaltered rocks.

Early basic magmatism in the evolution of the northern marginal zone of the archean limpopo belt

Basaltic rocks enclosed in quartzo-feldspathic gneisses and metamorphosed to granulite grade at 2.9 Ga from the North Marginal Zone of the Limpopo Belt are subdivided into three types. Group A basalts are tholeiitic and have flat rare earth element (REE) patterns. Field and chemical evidence suggest that these metabasic rocks may have originated as basaltic lavas prior to the 2.9 Ga metamorphism. Group B basalts are enriched in trace elements relative to Group A, particularly in the LREE. This group merges into a smaller group of basalts (Group C), which are highly enriched in trace elements. Group B and possibly Group C basalts are tentatively identified as dykes which cut the quartzo-feldspathic gneisses but predate the metamorphism. The cause of the trace-element enrichment in Groups B and C cannot be uniquely identified for whilst some evidence suggests crustal contamination it is impossible to unambiguously choose between crustal contamination, a small percentage of melting and source region heterogeneity. Trace-element patterns for the Group B basalts show a striking similarity to those for Phanerozoic basalts from this region and which are thought to have been derived from an enriched mantle source.

Distinctive REE patterns for tholiitic and calc-alkaline magma series co-occurring at Adatara volcano, Northeast Japan.

Rare earth element (REE) concentrations were precisely determined by inductively coupled argon plasma/atomic emission spectrometry (ICP) for 9 tholeiitic and 13 associated calc-alkaline rocks from Adatara volcano, located at the volcanic front of Northeast Japan arc. The tholeiitic samples commonly display nearly flat REE patterns, whereas the calc-alkaline samples characteristically show concave light REE-enriched patterns. In both suites, the REEs, with the exception of Eu, are positively correlated to varying extents with K2O and other incompatible elements. Eu concentrations in the calc-alkaline suite do not vary significantly, resulting in significant negative Eu anomalies. In the tholeiitic suite, chondrite-normalized (Ce/Yb)CN ratios are low and show little variation (1.54–1.80) over a significant range in SiO2 content (52–62 wt%). In contrast, (Ce/Yb)CN in the calc-alkaline suite show much greater variation (2.17 to 2.94) over a smaller range in SiO2 content (56 to 62 wt%) and increase systematically with SiO2. REE data for the tholeiitic suite are consistent with a crystal fractionation model that successfully reproduces the observed major and trace-element variations. A similar fractionation model is compatible with major-element and some trace-element variations for the calc-alkaline suite, but results in excessively high heavy REE contents, and fails to reproduce the elevated (Ce/Yb)CN ratios for the most acidic samples. The preferential enrichment of light REEs may reflect additional processes such as: (1) assimilation, (2) incorporation of a differentiated liquid (or evolved magma) that has undergone fractionation of heavy REE-rich minerals, and (3) mixing of primary magma derived by a small degree of partial melting from a common (amphibolitic or hornblende gabbroic) source. Although the tholeiitic and calc-alkaline suites possess distinct 87Sr/86Sr, Rb/Ba and Zr/Nb ratios, different magma sources are not required to produce the distinct REE patterns of the coexisting suites. An equilibrium batch partial melting model can reproduce both the flat (tholeiitic) and concave (calc-alkaline) REE patterns from a common peridotitic source by larger (10–15%) and smaller (5%) degrees of partial melting, respectively. Therefore, both tholeiitic and calc-alkaline suites could have been derived from a common lherzolitic magma source, provided that the calc-alkaline suite underwent incorporation (assimilation) of isotopically distinct (lower crust-derived) materials prior to evolution.

Petrology of the Rainy Lake area, Minnesota, USA-implications for petrotectonic setting of the archean southern Wabigoon subprovince of the Canadian Shield

The Rainy Lake area in northern Minnesota and southwestern, Ontario is a Late Archean (2.7 Ga) granite-greenstone belt within the Wabigoon subprovince of the Canadian Shield. In Minnesota the rocks include mafic and felsic volcanic rocks, volcaniclastic, chemical sedimentary rocks, and graywacke that are intrucded by coeval gabbro, tonalite, and granodiorite. New data presented here focus on the geochemistry and petrology of the Minnesota part of the Rainy Lake area. Igneous rocks in the area are bimodal. The mafic rocks are made up of three distinct suites: (1) low-TiO2 tholeiite and gabbro that have slightly evolved Mg-numbers (63–49) and relatively flat rare-earth element (REE) patterns that range from 20–8 x chondrites (Ce/YbN=0.8–1.5); (2) high-TiO2 tholeiite with evolved Mg-numbers (46–29) and high total REE abundances that range from 70–40 x chondrites (Ce/YbN=1.8–3.3), and (3) calc-alkaline basaltic andesite and geochemically similar monzodiorite and lamprophyre with primitive Mg-numbers (79–63), enriched light rare-earth elements (LREE) and depleted heavy rare-earth elements (HREE). These three suites are not related by partial melting of a similar source or by fractional crystallization of a common parental magma; they resulted from melting of heterogeneous Archean mantle. The felsic rocks are made up of two distinct suites: (1)low-Al2O3 tholeiitic rhyolite, and (2) high-Al2O3 calc-alkaline dacite and rhyolite and consanguineous tonalite. The tholeiitic felsic rocks are high in Y, Zr, Nb, and total REE that are unfractionated and have pronounced negative Eu anomalies. The calcalkaline felsic rocks are depleted in Y, Zr, and Nb, and the REE that are highly fractionated with high LREE and depleted HREE, and display moderate negative Eu anomalies. Both suites of felsic rocks were generated by partial melting of crustal material. The most reasonable modern analog for the paleotectonic setting is an immature island arc. The bimodal volcanic rocks are intercalated with sedimentary rocks and have been intruded by pre- and syntectonic granitoid rocks. However, the geochemistry of the mafic rocks does not correlate fully with that of mafic rocks in modern are evvironments. The low-TiO2 tholeiite is similar to both N-type mid-ocean-ridge basalt (MORB) and low-K tholeiite from immature marginal basins. The calc-alkaline basaltic andesite is like that of low-K calc-alkaline mafic volcanic rocks from oceanic volcanic arcs; however, the high-TiO2 tholeiite is most similar to modern E-type MORB, which occurs in oceanic rifts. The conundrum may be explained by: (1) rifting of a pre-existing immature arc system to produce the bimodal volcanic rocks and high-TiO2 tholeiite; (2) variable enrichment of a previously depleted Archean mantle, to produce both the low- and high-TiO2 tholeiite and the calc-alkaline basaltic andesite, and/or (3) enrichment of the parental rocks of the high-TiO2 tholeiite by crustal contamination.

New petrological and geochemical data on mid-Paleozoic island-arc volcanics of northern Sierra Nevada, California: evidence for a continent-based island arc

The mid-Paleozoic volcanics of northern Sierra Nevada consist of the Sierra Buttes rhyolites, the Taylor basalts and andesites, and the Keddie Ridge basalt–latite–rhyolite suite. The Sierra Buttes calc-alkaline rhyolites display strong light rare-earth element enrichment and negative εNd values. The Taylor basalts and andesites in the northern Hough and Genesee blocks exhibit calc-alkaline affinities (REE rare-earth element patterns highly enriched in LREE), whereas in the southern Hough block they are tholeiitic (flat rare-earth element patterns). The abundance of silicic lavas, the low εNd values of both the Sierra Buttes and Taylor volcanics and the δ18O values of the Sierra Buttes rhyolite and Bowman Lake trondjhemite provide evidence that the northern Sierra Nevada island arc was continent based. The Keddie Ridge differentiated volcanics, characterized by high Zr, Y, Nb, K, and light rare-earth elements, are geochemically similar to a shoshonite suite. Their eruption at the end of the mid-Paleozoic volcanic episode suggests a reversal of subduction, uplift, and block faulting in the island arc.The mid-Paleozoic volcanics of the northern Sierra Nevada are thought to represent the remnant of a mature island arc because calc-alkaline rocks predominate over tholeiitic ones, the lavas display a K enrichment with time, and the volcanics are evolved in their isotopes, compared with rocks erupted in young or primitive island arcs.

Pillow basalts of the Angayucham terrane: Oceanic plateau and island crust accreted to the Brooks Range

The Angayucham Mountains (north margin of the Yukon‐Koyukuk province) are made up of an imbricate stack of four to eight east‐west trending, steeply dipping, fault slabs composed of Paleozoic (Devonian to Mississippean), Middle to Late Triassic, and Early Jurassic oceanic upper crustal rocks (pillow basalt, subordinate diabase, basaltic tuff, and radiolarian chert). Field relations and geochemical characteristics of the basaltic rocks suggest that the fault slabs were derived from an oceanic plateau or island setting and were emplaced onto the Brooks Range continental margin. The basalts are variably metamorphosed to prehnite‐pumpellyite and low‐greenschist facies. Major element analyses suggest that many are hypersthene‐normative olivine tholeiites. Classification based on immobile trace elements confirms the tholeiitic character of most of the basalts but suggests that some had primary compositions transitional to alkali basalt. Although field and petrographic features of the basalts are similar, trace element characteristics allow definition of geographically distinct suites. A central outcrop belt along the crest of the mountains is made up of basalt with relatively flat rare earth element (REE) patterns. This belt is flanked to the north and south by LREE (light rare earth element)‐enriched basalts. Radiolarian and conodont ages from interpillow and interlayered chert and limestone indicate that the central belt of basalts is Triassic in age, the southern belt is Jurassic in age, and the northern belt contains a mixture of Paleozoic and Mesozoic ages. Data for most of the basalts cluster in the “within‐plate basalt” fields of trace element discriminant diagrams; none have trace‐element characteristics of island arc basalt. The Triassic and Jurassic basalts are geochemically most akin to modern oceanic plateau and island basalts. Field evidence also favors an oceanic plateau or island setting. The great composite thickness of pillow basalt probably resulted from obduction faulting, but the lack of fault slabs of gabbro or peridotite suggests that obduction faults did not penetrate below oceanic layer 2, a likely occurrence if layer 2 were anomalously thick, as in the vicinity of an oceanic island. The presence of basaltic tuff interbeds indicates proximity to an explosive basaltic eruptive center. The juxtaposition of submarine basalts of differing chemical affinity and age, adjacent to higher‐grade Paleozoic metamorphic rocks of the Brooks Range to the north, may be explained by obduction of internally complex (thickened) oceanic crust formed in an ocean plateau setting. Emplacement and rotation of thrust plates to steep attitudes occurred during accretion of the Brooks Range passive margin, probably beginning in the Late to Middle Jurassic.

Regional compositional variations of Late Tertiary bimodal rhyolite lavas across the Basin and Range/Colorado Plateau Boundary in western Arizona

Bimodal basalt‐rhyolite fields that erupted since 15 Ma form a NE trending line that crosses the boundary between the Basin and Range and the Colorado Plateau in western Arizona. The bimodal volcanism traverses physiographic province boundaries marked by changes in crustal thickness. Silicic volcanism in these fields postdates detachment faulting in the Basin and Range and generally decreases in age from south to north. We distinguish two rhyolite types that occur in these fields and summarize their geology, petrography, and geochemistry. Both high‐silica (HSR) and low‐silica rhyolites (LSR) occur in these fields. HSR lavas contain >75 wt % SiO2 (normalized to 100% volatile free), have high concentrations of incompatible elements (Rb, U, Th, Cs, Ta), low concentrations of compatible elements (Ba, Sr, Eu, Mg, Cr), and relatively flat rare earth element (REE) patterns (La/LuN <7). In contrast, LSR lavas have 68–74.5 wt % SiO2 (normalized to 100% volatile free), low concentrations of incompatible elements, high concentrations of compatible elements, and negatively sloping REE patterns (La/LuN >8). Further distinction of HSR and LSR lavas is made based on phenocryst assemblages. Trace element and isotopic studies suggest that both the HSR and LSR lavas contain a major crustal component and that the HSR lavas may have formed by extensive crystal fractionation of LSR parent magmas. HSR and LSR lavas display systematic compositional variations along the transect; however, their compositions diverge from south to north. HSR lavas erupted on the Colorado Plateau are more differentiated than those erupted in the Basin and Range, perhaps reflecting more extensive crystal fractionation while ascending the thicker crust of the Colorado Plateau province. In contrast, LSR lavas on the Colorado Plateau are more primitive than those in the Basin and Range. This seeming paradox may be related to lateral changes in source composition, possibly caused by a northerly increase in either metamorphic grade or bulk composition. Alternative models for the LSR compositional variations, such as magma mixing and mantle input into lower crustal sources, cannot be tested with our present data.

Geochemistry of Late Proterozoic amphibolites and ultramafic rocks, southeastern Canadian Cordillera

Late Proterozoic amphibolites and ultramafic rocks from the southeastern Canadian Cordillera have been analysed for major and trace elements in order to determine the nature and origin of the protoliths. Geologic relations indicate that these rocks were produced during an episode of continental rifting in the Precambrian. Based on rare-earth-element (REE) patterns, immobile-incompatible-element ratios, and characteristic elemental abundances, amphibolites are subdivided into alkaline and tholeiitic metabasalts. Alkaline basalts are recognized by their steep REE patterns, high Zr/Y, high TiO2 and P2O5 abundances, and low Y/Nb and Ti/Zr. Tholeiitic basalts are subdivided into three groups: (I) high-Mg#, high-field-strength-element (HFSE)-depleted, light-REE (LREE)-enriched tholeiites with flat heavy REE (HREE) patterns; (II) LREE-enriched tholeiites depleted in HREE; and (III) low-Mg# tholeiites with flat REE patterns. Ultramafic rocks occur as boudins of partially recrystallized Cr-spinel-bearing harzburgite or therzolite, enriched in LREE (Ce/Sm = 1.7–1.9), HFSE, CaO, Al2O3, and TiO2 relative to depleted mantle.Geochemical data suggest that the basalts were derived from a heterogeneous mantle source that underwent different degrees of partial melting with variable amounts of subsequent crystal fractionation of the melts. High Mg#, high Cr and Ni abundances, low HFSE abundances, and high olivine saturation temperatures suggest that group I tholeiites are primary mantle melts produced by a relatively high degree of partial melting of a LREE-enriched, HFSE-depleted source. Group II and III basalts have undergone moderate olivine and pryoxene and limited plagioclase fractionation. Mass-balance calculations suggest that the ultramafic rocks represent a crustally contaminated primary-mantle-derived melt.Les éléments majeurs et traces des amphibolites et des ultramafites, d'âge protérozoïque tardif, du sud-est de la Cordillère canadienne ont été analysés dans le but de déterminer la nature et l'origine des protolithes. Les relations géologiques indiquent que ces roches se sont formées durant un épisode de rifting continental dans le Précambrien. Les diagrammes des terres rares, les rapports des éléments immobiles et incompatibles et les compositions chimiques caractéristiques ont permis de subdiviser les amphibolites en métabasaltes tholéiitiques et alcalins. Les basaltes alcalins sont reconnus par les courbes abruptes dans les diagrammes des terres rares, les rapports Zr/Y élevés et les fortes teneurs en TiO2 et P2O5 et les rapports Y/Nb et Ti/Zr faibles. Les basaltes tholéiitiques sont subdivisés en trois groupes : (I) avec Mg# élevé, appauvrissement en éléments de force de champ élevée, tholéiites enrichies en terres rares légères avec courbe horizontale des terres rares lourdes; (II) tholéiites enrichies en terres rares légères et appauvries en terres rares lourdes; et (III) tholéiites avec Mg# faible et avec courbe horizontale des variations des terres rares. Les ultramafites se présentent en boudins formés d'harzburgite incluant un spinelle chromifère partiellement recristallisé ou de therzolite qui sont enrichies en terres rares légères (Ce/Sm = 1,7–1,9), en éléments à force de champ élevée, en CaO, Al2O3 et TiO2, comparativement à un manteau appauvri.

Geochemical constraints on the origin of Archaean tonalitic-trondhjemitic rocks and implications for lower crustal composition

Summary The rare earth element (REE) concentrations of high-grade trondhjemitic gneisses from the Archean Kapuskasing Structural Zone, central Ontario, are similar to REE concentrations of Archaean tonalites and trondhjemites world-wide: REE patterns are steeply fractionated (La/Yb) N = 20–100), with variable positive Eu anomalies and fractionated HREE ((Gd/Yb) N = 2–4). Calculated models for the melting of a mafic source with a flat REE pattern and variable mineralogy show that amphibole alone is not capable of producing the large amount of REE fractionation observed in Archaean tonalites; garnet amphibolite (with >20% garnet), mafic garnet granulite or eclogite are the most likely sources. The presence of garnet constrains the mafic source to be within the lower crust (deeper than 25 km) or upper mantle. Such crustal or upper mantle melting depths are not geochemically distinguishable, yet have important implications for the composition of the Archaean lower crust.

The geochemistry of Lower Tertiary basic dykes in the Eastern Red Hills district, Isle of Skye, and their significance for the proposed magmatic evolution of the Skye Centre

AbstractMajor- and trace-element data are presented for basic dykes intruding the Eastern Red Hills district of the Tertiary igneous centre of Skye. In addition to a group of transitional basic intrusions, members of two other distinctive suites have been identified: (1) a series of basic alkaline intrusions (akin to the ‘Beinn Dearg Type’ of Harker, 1904), with characteristic high alkali contents, distinctive values of K2O/(K2O + Na2O) and rare-earth-element (REE) patterns similar to those of many of the Tertiary lavas of north Skye, and, (2) tholeiitic dykes, with low concentrations ofK2O, Ti/Zr values of c. 100, and flat REE patterns similar to those of the so-called Fairy Bridge magma type (Mattey et al., 1977). From these data and a consideration of the time relationships of the various components of the Skye Centre, agreement is found with Thompson et al.'s (1972, 1980) model of a mantle thermal anomaly which generates the magmatic sequence: transitional basic magmas, giving way to olivine-bearing tholeiitic magmas (at the culmination of the anomaly), and, finally, late-stage basic alkaline magmas at the end of the activity.

The basic lavas of the Eastern Red Hills district, Isle of Skye

Synopsis Major- and trace-element data from hydrothermally altered basic lavas from the Eastern Red Hills district of the Tertiary igneous centre of Skye indicate that members of two distinct assemblages are present—a transitional series, with characteristic light rare-earth-element (REE) enrichment ((Ce/Yb) N > 3.5), showing affinities to the voluminous plateau lavas of north Skye, and a distinctly tholeiitic assemblage typified by relatively flat REE patterns ((Ce/Yb) N < 3.5) and Ti/Zr 100–110. The REE data suggest that this tholeiitic group belongs to the previously defined Fairy Bridge magma type, which appears to have evolved in the upper crust by low-pressure crystal fractionation involving olivine and plagioclase. These tholeiites have REE patterns which are very similar to those of the subvolcanic granites of the district ((Ce/Yb) N ≈ 3.0–3.5), suggesting a probable cogenetic relationship between the tholeiites and the granites.

Geochemical investigation of Archaean Bimodal and Dwalile metamorphic suites, Ancient Gneiss Complex, Swaziland

The bimodal suite (BMS) comprises leucotonalitic and trondhjemitic gneisses interlayered with amphibolites. Based on geochemical parameters three main groups of siliceous gneiss are recognized: (i) SiO2 14%, and fractionated light rare-earth element (REE) and flat heavy REE patterns; (ii) SiO2 and Al2O3 contents similar to (i) but with strongly fractionated REE patterns with steep heavy REE slopes; (iii) SiO2 > 73%, Al2O3 < 14%, Zr ∼ 500 ppm and high contents of total REE having fractionated light REE and flat heavy REE patterns with large negative Eu anomalies. The interlayered amphibolites have major element abundances similar to those of basaltic komatiites, Mg-tholeiites and Fe-rich tholeiites. The former have gently sloping REE patterns, whereas the Mg-tholeiites have non-uniform REE patterns ranging from flat (∼ 10 times chondrite) to strongly light REE-enriched. The Fe-rich amphibolites have flat REE patterns at 20–30 times chondrite. The Dwalile metamorphic suite, which is preserved in the keels of synforms within the BMS, includes peridotitic komatiites that have depleted light REE patterns similar to those of compositionally similar volcanics in the Onverwacht Group, Barberton, basaltic komatiites and tholeiites. The basaltic komatiites have REE patterns parallel to those of the BMS basaltic komatiites but with lower total REE contents. The Dwalile tholeiites have flat REE patterns. The basic and ultrabasic liquids were derived by partial melting of a mantle source which may have been heterogeneous or the heterogeneity may have resulted from sequential melting of the mantle source. The Fe-rich amphibolites were derived either from liquids generated at shallow levels or from liquids generated at depth which subsequently underwent extensive fractionation.

Trace element composition of Tertiary volcanic rocks of northeast Japan.

Abundances of trace elements including rare earth elements (REE) were determined by instrumental neutron activation analysis for twenty three Tertiary volcanic rocks of northeast Japan. Positive correlations were confirmed in the following couples of elements: Hf-La, K-La, Th-La and Fe-Sc. Unlike Quaternary volcanic rocks, Tertiary volcanic rocks show no definite correlation between the inclination of REE patterns and the distance of sampling sites from the estimated volcanic front of the Tertiary. The presence of low alkali tholeiites with relatively flat REE patterns in the Japan Sea side suggests that Tertiary volcanism is qualitatively different from Quaternary one.

Origin of the rhyolitic rocks of the taupo volcanic zone, New Zealand

Abstract Major element, Rb, Sr, Ba, Cr and V analyses as well as 13 new rare earth element (REE) analyses are presented for the greywacke basement surrounding the Taupo Volcanic Zone (TVZ). On this basis the basement rocks are divided into a Western Basement of approximately andesitic composition (∼ 62% SiO2) and an Eastern Basement of approximately granodiorite composition (∼ 75% SiO2). These analyses, 5 new REE analyses for the rhyolites, and published data for the volcanic rocks of TVZ are used to investigate the petrogenesis of rhyolitic rocks in the area. Least-squares mixing calculations for major elements show that 88% fractional crystallisation of high-alumina basalt produces a liquid of rhyolitic bulk composition, but Rayleigh fractionation models show that the trace element concentrations of the rhyolites are inconsistent with basalt fractionation. 57% fractionation of the assemblage plagioclase (35.6%), orthopyroxene (9.7%), clinopyroxene (7.8%), ilmenite (0.6%) and magnetite (3.4%) from a plagioclase-pyroxene andesite can produce liquids of rhyolitic bulk composition. REE concentrations produced by this model are consistent with those observed in the rhyolites but predicted Ba and Rb values are lower and V concentrations are higher than those in the rhyolites. Andesite fractionation also produces an unrealistic fractionation of the Cr/V ratio. A non-modal melting model involving ∼35% melting of a granulitic assemblage (plagioclase + quartz + clinopyroxene + orthopyroxene + biotite + magnetite + cordierite) with a bulk composition equivalent to the Western Basement can reproduce the REE pattern of the rhyolites as well as the concentrations of Rb and Ba. Sr values remain anomalously high, but the Cr/V ratio does not indicate fractionation. Absolute values of Cr and V are within the uncertainties of published crystal—liquid partition coefficients. The rhyolites have relatively flat REE patterns (La/Yb ∼ 7.5), as do the greywackes (La/Yb ∼ 8.2), so it is therefore unlikely that the rhyolites equilibrated with a garnet or amphibole-bearing assemblage.

Distribution of rare-earth elements (REE) in size fractions of recent sediments of the Indian Ocean

A sample of each of five sediment types from the Indian Ocean, all presenting a flat shale-normalized REE pattern has been divided into eight size fractions (> 35; 20–35; 5–20; 2–5; 1–2; 0.5–1; 0.2–0.5 and < 0.2 μm). Each fraction was examined for mineralogy and REE content. REE patterns of coarser fractions exhibit a negative cerium anomaly related to biogenic components, while fine fractions composed almost exclusively of smectite, show a positive Ce anomaly. Thus flat whole-sample REE patterns which are usually interpreted as indicating a ferrigenous origin can also be due to a mixture of Cedepleted and -enriched phases. The REE distribution was studied in fifteen other sediments. Thirteen fine-size fractions (< 0.5 μm) exhibit a positive Ce anomaly and two are Ce-depleted while the REE patterns of whole samples are flat, slightly Ce-depleted or -enriched. The Ce enrichment of most fine fractions could occur during submarine alteration of volcanic material forming smectite, but may also be produced or accentuated during reworking or subsequent to the deposition of sediments.

The rare earth element contents and the origin of the sparry magnesite mineralizations of Tux-Lanersbach, Entachen Alm, Spiessnaegel, and Hochfilzen, Austria, and the lacustrine magnesite deposits of Aiani-Kozani, Greece, and Bela Stena, Yugoslavia

The rare earth element distribution patterns of magnesite and dolomite samples from the magnesite mineralizations of Tux-Lanersbach (Innsbruck quartz phyllite), Entachen Alm, Spiessngel, and Hochfilzen (graywacke zone) have been determined by neutron activation spectrometry. The finely dispersed magnesite in the dolomitic country rock shows rather flat rare earth element patterns. The magnesite and dolomite from veins exhibit extremely low light rare earth element contents. This feature is attributed to long-range mobilization of magnesite.The tectonic structure of the graywacke zone and of the Innsbruck quartz phyllite suggests that a maior perturbation of the regional geothermal gradient was produced by overthrusting. Owing to the irregular form of the geothermal gradient, low temperature Mg (super +2) ion-rich solutions, generated by dehydration reactions and equilibrated with calcite and dolomite, penetrated dolomite-bearing sheets at higher temperatures during their ascent. The dolomite of the higher tectonic units was thereby metasomatically altered to magnesite. Such a regional metamorphism is displayed by the positive Eu anomalies in the rare earth element distribution patterns of the Spiessngel dolomites. This Eu anomaly indicates that the dolomite from this locality crystallized from solutions involved in the decomposition of feldspars. The rare earth element contents of the magnesites and huntites from lacustrine basins of Bela Stena and of Aiani-Kozani are lower than those of the sparry magnesites. This reflects the different sources of the Mg (super +2) -bearing solutions and depositional environments.

Rare earth geochemistry of Lewisian granulite-facies gneisses, northwest Scotland: Implications for the petrogenesis of the Archaean lower continental crust

Rare earth element (REE) data, together with data for major elements and 14 other trace elements, are presented for ultramafic, mafic, intermediate, tonalitic, trondhjemitic, anorthositic and microline gneisses, representative of the range of rock types making up the 2.9-b.y. Lewisian granulite complex of northwest Scotland. The data are used to constrain petrogenetic models for the Archaean lower crust. Ultramafic gneisses have flat REE patterns with 3–5 times chondrite abundance. The more Fe-rich mafic gneisses show slight light-REE enrichment, range up to 40 times chondritic and some have negative Eu anomalies. Intermediate gneisses have more fractionated REE distributions (Ce N/Yb N= 4−25) but with rather constant heavy REE. Tonalitic gneiss REE patterns are also strongly fractionated, show variable heavy-REE depletion and have positive Eu anomalies. REE patterns of trondhjemitic gneisses are very strongly fractionated (Ce N/Yb N up to ∼ 300), show strong heavy-REE depletion and most have marked positive anomalies. Anorthosites and microcline gneisses have similar REE distributions to the trondhjemites in spite of different major element compositions. Modelling of the REE and other trace element patterns of processes such as fractional crystallisation and partial melting suggests that whereas the mafic gneisses can be related by low-pressure fractional crystallisation, the more silicic gneisses can only be related by high-pressure partial melting of a mafic source. Most of the gneisses represent liquid compositions; few can be regarded as cumulates or the residues of partial melting. The computed average Lewisian granulite does have a positive Eu anomaly, but this is imparted by the more fractionated tonalites and trondhjemites and not by the more mafic components of the gneiss complex. Elements such as K, Rb, Cs, Th and U are removed from the lower crust by a fluid (CO 2-rich) not a melt phase during granulite-facies metamorphism. The Archean lower crust is more silicic than implied by the andesite model for continental growth. The most suitable tectonic environment to account for the generation and intermixing of the different low-pressure and high-pressure magmas or rock types (and the presence of high-grade metasediments) would be one similar to the Cordilleran belt of South America. Underplating by subduction zone or mantle derived liquids may be a significant process in Archaean continental growth.

Mantle-plume model for the origin of Archaean greenstone belts based on trace element distributions

TRACE element studies of Archaean greenstone volcanics indicate the existence of two major types of tholeiite1–3: depleted Archaean tholeiite (DAT) showing flat rare earth element (REE) patterns similar to modern rise tholeiites and enriched Archaean tholeiite (EAT) exhibiting slight light REE enrichment and generally greater concentrations of LIL (large-ion lithophile) elements than DAT. Included in the DAT category are komatiitic tholeiites. DAT and associated ultramafic flows and sills seem to make up from 50 to 90% of Archaean greenstone successions. EAT becomes relatively more abundant at stratigraphically higher levels and in the upper parts of minor volcanic cycles within greenstone successions. Although overall EAT is generally less abundant than DAT, it accounts for up to 50% of some greenstone sections2,4. DAT seems to be absent in the upper 10–20% of most greenstone successions.

285. Influence of alloying elements on the mechnical properties and recrystallization point of Ta-33 per cent Nb alloy: A M Levin et al, Izv VUZ Tsvetnaya Met, No 3, 1967, 122–127

The Saga and Sangsang ophiolites are located about 600 and 450 km west of Lhasa and represent a western extention of the central portion of the Yarlung Zangbo Suture Zone (YZSZ) ophiolite belt. The Saga massif comprises fresh mantle lherzolite and cpx-harzburgite, an ophiolite mélange (± amphibolite), metamorphosed mafic crustal rocks (meta-gabbro, meta-basalts and amphibolites) and a sequence of uppermost crustal rocks (chert, basaltic lavas, diabase sills and dikes). The Sangsang ophiolite consists of an ophiolite mélange (harzburgite) and upper mantle harzburgite with minor lavas and gabbro.Peridotites from both massifs show variable degrees of serpentinization. Their Mg# varies between 0.89 and 0.91. All peridotites show distinct flat REE (rare earth elements) patterns with La/YbN ratios close to 1, probably indicative of a refertilized mantle. The Olivine-Spinel equilibrium and the spinel chemistry for the Saga (Cr# ~ 0.10–0.22) and Sangsang (Cr# ~ 0.30–0.55) peridotites suggest that the Saga peridotites have a deeper mantle provenance (> 20 kbar) and have undergone lower degrees of partial melting (5–12%) than the Sangsang peridotites (< 15 kbar; 17–30%). The composition of the Saga peridotites is similar to the composition of pre-oceanic peridotites while peridotites from the Sangsang massif resemble abyssal and subduction-related peridotites. Mafic rocks from both ophiolites have basalt and basaltic andesite compositions. They are slightly depleted in light REE with respect to the heavy REE, with La/YbN between 0.5 and 0.8 and with a small negative Ta-Nb anomaly suggesting the presence of a subduction component. The abundances of incompatible elements in these mafic rocks are similar to N-MORB (mid-ocean ridge basalt) or back-arc-basin basalts (BABB).Our data suggest that the Saga and Sangsang ophiolites belong to an intraoceanic suprasubduction zone segment as postulated for other ophiolitic massifs in the eastern portion of the YZSZ. However, the geochemistry of mantle rocks from these two massifs is different compared to each other (fertile vs. refractory) and compared to other YZSZ ophiolites suggesting different petrogenetic histories. Field relationships and geochemical data furthermore suggest that the Saga and Sangsang ophiolites were formed in a complex arc–back-arc setting where at least two subducting slabs must have been active. This study places one more piece in the YZSZ puzzle and lead to a better understanding of the morphology of the convergence zone before the final stage of collision which led to the present configuration of the suture zone.