Flat Heavy Rare Earth Element Patterns Research Articles

Petrogenesis of A-type granitoids from the Wallagga area, western Ethiopia: constraints from mineralogy, bulk-rock chemistry, Nd and Sr isotopic compositions

Abstract A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplaced at the contact between low- and high-grade terranes, constitute a significant proportion of the granitoid rocks in the Precambrian of western Ethiopia. These granitoids are characterized by Fe-rich biotite, ferro-hornblende, alkali-amphiboles, and alkali pyroxenes. High total alkalis, high FeO T /MgO (particularly in the peralkaline varieties and syenite), enriched rare earth element (REE), Y, Nb, Ta and low CaO, MgO, and Sr abundance characterize these granitoids. Chondrite-normalized REE patterns show enriched light REE, moderate to strong negative Eu anomalies, and more or less flat heavy REE patterns. Wide ranges of major and trace element compositions among these different A-type granitoids are attributed to variations of source compositions, crystal fractionation, and assimilation. Aluminum-in-hornblende barometric and coexisting amphibole-plagioclase thermometric estimates show that the Ganjii monzogranite crystallized at 600–630 °C and 5–6 kbar. Compared to A-type magmas elsewhere, this temperature estimate is low, suggesting that it might represent conditions of crystallization close to the solidus. Major and trace element modelling of the Ganjii monzogranite suggests derivation by crystal fractionation dominated by plagioclase, hornblende, and biotite from monzodioritic magma. A binary mixing model involving monzodiorite marginal facies and microgranitic dyke end members could not explain the chemical variation in the Ganjii monzogranite, suggesting that the effect of magma mixing on the petrogenetic evolution of the granite was insignificant. The peralkaline to mildly peraluminous Homa gneissic granite and the Tuppii granite, characterized by variable mineralogy, major, and trace element compositions are formed as a result of combined effects of fractionation and assimilation. The initial 87 Sr / 86 Sr of 0.70281 and positive e Nd (625 Ma) values (>+4) for the Ganjii monzogranite are consistent with generation of parental monzodioritic magma from the mantle. The depleted mantle Nd model age ( T DM ) of 0.86 Ga for this granite is coeval with early magmatic stages in the Arabian Nubian shield elsewhere. Despite the occurrence of Mesoproterozoic xenocrystic zircons in the Ganjii monzogranite, the Sr and Nd isotopic compositions indicate insignificant contribution of older crustal materials. The Tullu Kapii quartz syenite yielded e Nd (625 Ma) of +2.4 and +3, consistent with the results obtained from the Ganjii monzogranite, suggesting generation from a juvenile source. However, the older T DM (1.1 Ga) estimated for the Tullu Kapii quartz syenite indicates derivation from a relatively older source or contribution of pre-Pan-African crustal material, whose existence is indirectly substantiated by Mesoproterozic zircon inheritance and T DM ≅1.5 Ga obtained from a sample of calc-alkaline Ujjukka granitoids. Overall, A-type magmas in the area may have derived as a result of orogenic collapse that caused decompression melting of subcontinental lithospheric mantle during late Neoproterozoic. The abundance of comparable mantle-derived granitoids in other parts of the Arabian Nubian shield indicates considerable new crustal addition in Neoproterozoic time.

Geochemistry of xenolithic eclogites from West Africa, part 2: origins of the high MgO eclogites

Oxygen isotope, mineral trace element, and measured and reconstructed whole-rock compositions are reported for the high MgO eclogite xenolith suite (16 to 20 wt% MgO in the whole rock) from the Koidu Kimberlite complex, Sierra Leone. In contrast to the previously published data for low MgO eclogites (6 to 13 wt% MgO) from this area, high MgO eclogites equilibrated at higher temperatures (1080 to 1130°C vs. 890 to 930°C) have only mantlelike δ 18O and show variable degrees of light rare earth element (REE) enrichment. Analyses of multiple mineral generations suggest that the heterogeneous REE patterns of the high MgO eclogites reflect variable degrees of metasomatic overprinting. High MgO and Al 2O 3 contents of the eclogites suggest a cumulate origin, either as high-pressure (2 to 3 GPa) garnet–pyroxene cumulates or low-pressure (<1 GPa) plagioclase–pyroxene–olivine cumulates. Trace element modeling suggests a low-P origin for eclogites with flat heavy REE patterns and a high-P origin for eclogites with fractionated heavy REE. Flat heavy REE patterns, the presence of Sr anomalies, and low to moderate transition element contents in the low-P group are consistent with a low-pressure origin as metamorphosed olivine gabbros and troctolites. These metagabbroic high MgO eclogites either could represent the basal section of subducted oceanic crust or foundered mafic lower continental crust. In the former case, the metagabbroic high MgO eclogites may be genetically related to the Koidu low MgO suite. Crystal fractionation trends suggest that the metapyroxenitic high MgO eclogites formed at lower pressures than their current estimated equilibrium pressures (>4 GPa).

Geology, mineralogy and magma evolution of Gunung Slamet Volcano, Java, Indonesia

Gunung Slamet, Central Java, is a large stratovolcano within the Sunda magmatic arc of Indonesia. The volcanic edifice includes products of two large overlapping Quaternary stratocones. Basaltic andesites and andesites, with rare basalts, dominate the western region of the complex, known as Slamet Tua (old); and basalts and basaltic andesites compose the eastern cone, called Slamet Muda (young). On the basis of stratigraphy, trace-element content, Zr Nb , Zr K and 87 Sr 86 Sr ratios, Slamet lavas can be broadly categorized as relating to high abundance magma (HAM) and low abundance magma (LAM) types. The Tua and Lebaksiu sequences generally comprise the LAM group, and are older, more salic and have higher 87 Sr 86 Sr ratios than those of HAM. LAM andesites contain some amphibole, but HAM andesites do not. Models involving randomized magma replenishment, tapping and fractionation (RTF) were developed to explain the geochemical features of both LAM and HAM rock groups. The salic lavas of the LAM suite can be generated if fractionation was dominant relative to replenishment and tapping in LAM magma chambers. Conversely, magma chambers with a high proportion of replenishment and tapping relative to fractionation can produce dominantly mafic lavas, such as those of the HAM suite. Concave-upward heavy-rare-earth element (HREE) patterns for LAM andesites are probably due to significant amphibole fractionation; HAM andesites display flat HREE patterns and do not require amphibole fractionation from parental basalts. The high TiO 2 contents of HAM basalts and basaltic andesites (relative to those of “average” arc rocks) are due to either suppressed crystallization—or minor accumulation—of Ti-magnetite, in conjunction with RTF processes.

Sm-Nd Isotopic and Geochemical Study of the Archean Tonalite-Amphibolite association from the eastern Indian Craton

We report the results of a Sm-Nd isotopic, major element and rare earth element (REE) study of the Older Metamorphic Group (OMG) tonalite-amphibolite association of the eastern Indian Craton. The Older Metamorphic Tonalite Gneisses (OMTG) have been previously dated to be 3.8 Ga using Sm-Nd isotope systematies, and 3.2–3.4 Ga by Rb-Sr and Pb-Pb dating. The results of this study indicate that the protoliths of the OMG amphibolites are 3.3 Ga isochron age=3.30±0.06 Ga, ɛNd= +0.9 ± 0.7), and therefore, the OMTG, which intrude into the associated amphibolites, cannot be any older than 3.3 Ga. The amphibolites display light REE enrichment ((Ce/Yb)N=2.2–6.7; La=30–100 x chondrite) and nearly flat heavy REE patterns ((Tb/Lu)N=1.2–1.9); the basaltic parents of the amphibolites were probably generated by the partial melting of a spinel lherzolite mantle. Strong linear relationships between the amphibolites and tonalites in 147Sm/144Nd-143Nd/144Nd space (isochron age =3.29±0.04 Ga, ɛNd= +0.8 ± 0.8) imply that they are genetically related. The tonalites display fractionated REE patterns (La=100–300 x chondrite) with moderate heavy REE depletions ((Tb/Lu)N=1.9–3.4). The isotopic, major element and REE data are consistent with the derivation of the OMTG from partial melting of OMG amphibolites or equivalent rocks at amphibolegarnet stabilization depths. An initial ɛNd(t) value of +0.9±0.7 for the amphibolites indicates the presence of a slightly depleted mantle source at 3.3 Ga with 147Sm/144Nd. between 0.20 and 0.22. It is suggested that the growth of continental crust in the eastern Indian craton occurred in response to magmatic underplating in a plume setting.

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.

Geology and geochemistry of Huronian rhyolites and low-Ti continental tholeiites from the Thessalon region, central Ontario

Bimodal volcanism associated with early phases of Huronian rifting in central Ontario, dated about 2450 Ma, produced low-Ti tholeiitic basalts and two varieties of crustally derived calc-alkaline rhyolite. Early tholeiites are characteristically highly evolved, have Mg* values from 30 to 50, and display pronounced enrichment in large-ion lithophile elements (LILE) and light rare-earth element (LREE) in comparison with modern oceanic basalts, fractionated heavy rare-earth element (HREE) patterns, and low Ti, Zr, P, Nb, Ba, and K abundances. Ti/Zr ratios rise progressively in early basalts and associated basaltic andesite fractionates from about 35 in early flows to 55 in central units. Late basalts also carry enriched LILE and LREE, but, in contrast to early types, have average Mg* values greater than 50 and lower rare-earth element (REE) abundances with flat HREE patterns. They also display negative Ba, Nb, and P anomalies on chondrite-normalized distribution diagrams, but lack low K, Zr, and Ti contents. Their Ti/Zr ratios of about 80 approach chondritic levels. Melting models suggest the differences are explained by lower degrees of fusion (as low as 10%) in a hydrated, LILE- and LREE-enriched peridotite during generation of the early basalts, leaving a residue containing appreciable garnet, amphibole, Ti oxides, zircon, and apatite.Erupted simultaneously with the basalts were two distinctive rhyolite types: (1) a low-LILE, high-LREE group (25% of analysed specimens), derived by −20% melting of granulitic siliceous tonalitic gneiss, presumably at deep crustal levels, and (2) a high-LILE, low-LREE group (75%), derived, probably at shallower levels, by ≤ 30% melting in granitic rocks with pegmatitic or leucogranitic compositions. Mutual magma mixing of basalts and rhyolites during early stages of volcanism produced abundant hybrid andesites, but the frequency of contamination is much lower in later units.Hypothetical subcontinental source compositions, calculated from the Raleigh equation, suggest that the Huronian mantle had already undergone a complex history. Low Ba, Nb, P, Ti, and depleted HREE abundances compared with abundances for modern oceanic basalts suggest that a basaltic melt had already been withdrawn from this source during Archean time. Subsequently, an episode of hydrous metasomatism enriched the source in LILE and LREE. The latter event resulted from (1) subcontinental mantle metasomatism by previous Archean subduction, (2) mantle metasomatism during the terminal Archean Kenoran Orogeny, or (3) a wave of hydrous metasomatism accompanying Huronian mantle convection immediately preceding volcanism.

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.