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Abstract

In the Proterozoic Schist Belt of Nigeria, lenticular bodies of metabasites and meta-ultramafics are frequently intercalated within staurolite bearing metapelitic schists. Such a metamorphosed mafic-ultramafic complex is particularly well exposed in the Mokuro riverbed between the towns of Ife and Ilesha. These outcrops display contact relationships with the surrounding metasediments, as well as between the individual mafic and ultramafic rock types. The most common mafic rocks are indistinctly layered amphibolites, accompanied by apatite rich amphibolites and massive amphibolites, in part rich in ilmenite and pyrrhotite. Among the generally massive ultramafic rocks, nearly monomineralic amphibole rocks predominate, while chlorite-amphibole, talc-chlorite-amphibole and talc bearing olivine-chlorite-amphibole rocks occur in subordinate amounts.

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... Mafic-ultramafic rocks make up the large massif of Mokuro and also occur as boudins, varying from one decimetre to several metres across, scattered within the schists around Ife and southwest of Ilesha (Ige et al., 1998). The Mokuro body comprises mainly meta-pyroxenites, gabbro-norites and melanocratic amphibolites, alternating at the scale of some metres to tens of metres (Ige et al., 1998), and obliquely cut by the amphibolitic metamorphic foliation. ...
... Mafic-ultramafic rocks make up the large massif of Mokuro and also occur as boudins, varying from one decimetre to several metres across, scattered within the schists around Ife and southwest of Ilesha (Ige et al., 1998). The Mokuro body comprises mainly meta-pyroxenites, gabbro-norites and melanocratic amphibolites, alternating at the scale of some metres to tens of metres (Ige et al., 1998), and obliquely cut by the amphibolitic metamorphic foliation. Layered microperthite-pyroxene-amphibole-biotite metaquartzsyenites enriched in apatite are interbanded with metapyroxenites . ...
... Layers with up to 2% vol. apatite and others enriched in ilmenite (up to 5% TiO 2 ) disseminated sulphides (pyrrhotite) have been identified (Ige et al., 1998), as well as Au occurrences. Garnet-rich (up to 50% vol.) amphibolites and garnetiferous meta-anorthositic layers are also a minor component, as well as grunerite-diopside and siliceous epidotite-rich cherty rocks (Olade and Elueze, 1979). ...
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This paper presents new petrostructural data and a detailed geological map of the Ife-Ilesha area, a classical schist belt in southwest Nigeria. The overall syn-metamorphic flat-lying structures, recumbent folding and associated strong boudinage of the mafic and ultramafic rocks are the result of Pan-African syn-metamorphic stacking of thrust units above the Archæan basement. Supracrustal units are considered to be part of a former late Palæoproterozoic pericratonic cover, whereas mafic/ultramafic rocks may derive from a dismembered Proterozoic sill complex. It is proposed that this area is part of a Pan-African nappe system observed at mid-crustal levels. The northeastward tectonic vergence of units reconstructed from kinematic criteria is at variance with the southwestward motion of the nappes near the Pan-African suture 400 km to the west. Such stacking of units resulting from the same northeast-southwest compression at the scale of that part of Gondwana may correspond to the closure of a former basin floored by thinned Archæan crust with abundant magmatism of late Neoproterozoic age. Post-nappe upward doming of the slightly anatectic Archæan basement and upright folding of the already formed recumbent structures were linked to the initiation of the dextral strike-slip motion along the steep north-northeast-trending Ifewara mylonitic zone that compares well with similar shear zones of the Trans-Saharan Belt in the Tuareg Shield.
... The Mokuro body (Fig. 5) comprises mainly meta-pyroxenites, gabbro-norites and melanocratic amphibolites, alternating at the scale of some metres to tens of metres (Ige et al. 1998). Plagioclase-rich meta-gabbros are also a major constituent of the northern part of the body. ...
... Some thin strips of quartzite, biotite -staurolite and garnet -quartz schist occur in the central part of the body. Ige et al. (1998) have clearly pointed out the cumulate character of most rocks of the complex, using discriminant diagrams on trace elements and REE patterns, as well as identifying preserved petrographic and mineralogical features of typical igneous origin. They interpret the massif as a large basaltic sill with MORB-like characteristics, affected by crystal settling that produced the ultramafic cumulates. ...
... The close association of syenites with mafic rocks, however, suggests that part of the magmas was extracted from an enriched mantle, thus suggesting the same extensional setting that produced the 1.85 Ga sub-alkaline granitoids. The occurrence of boudins of mafic amphibolite and garnet-bearing meta-anorthosite within the micaschists implies that the body is a strongly boudinaged sill originally emplaced within the schists and quartzites prior to regional metamorphism, in agreement with the conclusions of Ige et al. (1998). However, detailed mapping and structural observations show that all contacts are tectonic and marked by sheared rocks (Fig. 5). ...
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The Borborema and Benin-Nigeria provinces of NE Brazil and NW Africa, respectively, are key areas in the amalgamation of West Gondwana by continental collision during the Brasiliano/Pan-African orogenies. Both are underlain by complex basement: Nigeria has c. 3.05 Ga Archaean crust but no known Palaeoproterozoic rocks >2.0 Ga; in NE Brazil, 2.6-3.5 Ga Archaean rocks form small cores within Palaeoproterozoic gneiss terrains affected by plutonism at c. 2.17 Ga. Both regions exhibit Late Palaeoproterozoic (c. 1.8 Ga) rift-related magmatism and metasedimentary sequences overlying the basement. The Seridó Group of NE Brazil (<0.65 Ga) is similar to the Igarra Sequence in SW Nigeria. The Ceará Group, which may date back to c. 0.85 Ga, is a passive margin deposit on crust thinned during initiation of an oceanic domain. In both provinces, basement and sedimentary cover were involved in tangential tectonics that resulted in crust-thickening by nappe-stacking associated with closure of this ocean. Frontal collision between c. 0.66 and 0.60 Ga later evolved to an oblique collision, generating north-south continental strike-slip shear zones at c. 0.59 Ga. In NE Brazil, the main Pan-African suture is probably buried beneath the Parnaiba Basin. The Transbrasiliano Lineament, interpreted as the prolongation of the Kandi-4°50 Lineament in Hoggar, may represent a cryptic suture.
... The samples were collected from the westernmost part of the Ilesha Schist belt (Fig. 1), which is dominated by metapelitic biotite schists and gneisses prekinematically intruded by the Oshu granite and numerous mafic and ultramafic sills and dykes. Postkinematic pegmatite bodies are common [for more details see Rahaman (1988) and Ige et al. (1998)]. Rb±Sr whole-rock dating on two pegmatites yielded ages of 562 AE 12 and 550 AE 15 Ma (Matheis & Caen-Vachette, 1988). ...
... In the metapelites, early recumbent isoclinal D 1 folds are transposed during a second folding event (D 2 ), which led to the formation of upright to west-vergent antiforms and synforms with northward plunging fold axes (e.g. Boose & Ocan, 1991;Rahaman, 1992;Ige et al., 1998). The dominant mineralogical banding is parallel to the D 2 fold axis and strikes NNE±SSW with a dip of 70±90 ESE. ...
... At present little is known about the metamorphic evolution in the Ilesha Schist belt. P±T data are available only from amphibolites and meta-ultrabasites infolded with the Ilesha metaclastics, which yielded peak P±T conditions of about 550±620 C at 1Á5±3 kbar (Ige et al., 1998). ...
Article
Garnet-bearing assemblages of K-rich and K-poor metapelites from the Ilesha Schist belt, SW Nigeria, are investigated. K-rich samples contain the assemblages (A) garnet–staurolite–muscovite–chlorite–magnetite, (B) andalusite–garnet–staurolite–muscovite–chlorite–magnetite and (C) sillimanite–andalusite–garnet–muscovite–chlorite–magnetite. K-poor samples contain the assemblages (D) garnet–staurolite–cordierite–chlorite and (E) garnet–cordierite–chlorite ± staurolite. All assemblages contain quartz, plagioclase, biotite and ilmenite. P – T pseudosections calculated in the system CaO–Na 2 O–K 2 O–TiO 2 –MnO–FeO–MgO–Al 2 O 3 &puncsp;–SiO 2 &puncsp;–H 2 O ± O 2 suggest peak metamorphism at 590 ± 20°C at 5 ± 0·5 kbar, followed by retrogression to 550°C at 3·0 kbar, in agreement with field evidence, domain assemblages, mineral compositions, modes and geothermobarometry. The absence of compositional zonation shows that garnet in all investigated rocks nucleated and grew at constant P–T–X in equilibrium with associated minerals on the thin-section scale. However, the garnet-in reaction did not begin until the establishment of a significant temperature overstep of ∼80°C, with consequent rapid dehydration of chlorite- and muscovite-bearing assemblages resulting in interface-controlled garnet growth under locally hydrostatic pressure conditions. Garnet in rock types A and B formed with a characteristic pattern of biotite and quartz inclusions—a ‘house of cards’ texture. Additionally, clusters and aggregates of unzoned garnet grains formed in rock types A and D. Garnet resorption in rock types A–C resulted from prograde staurolite- and retrograde andalusite-forming reactions.
... However, Zeh and Holness (2003) specifically constrained the PeT conditions of 590 ± 20 C at 5 ± 0.5 kbar for the Ilesha Schist Belt. Ige et al. (1998) constrained a temperature range of 570e625 C from the metamorphosed mafic-ultramafic complex of Mokuro and llesha Schist Belt in the southwestern Nigeria. ...
Article
We report the first detailed petrological and pressure-temperature data from the pelitic schists and am-phibolites of the Keffi area, north-central Nigeria, which is located on the eastern flank of the western Nigerian based, Great Nigerian Schist Belt. In one fresh exposure of the schist, staurolite-bearing and staurolite-absent, garnet-rich assemblages occur. All pelitic samples contain garnet, quartz, biotite, plagioclase, chlorite and ilmenite, but the staurolite-bearing assemblage contains euhedral to subhedral staurolites and very subordinate retrograde chlorites in addition. Mineral compositions applied to calculate metamorphic PeT conditions using different approaches reveal a temperature range of 570e630 C for the garnet-biotite geothermometry. PeT pseudosection analyses calculated using THERMOCALC software for the suitable rock types, constrain garnet/staurolite equilibration within the range of 6.4e7.7 kbar and 610 e630 C. Empirical calculations and pseudo-section approaches indicate a clockwise P-T path for the rocks of the study area. The result of geothermobarometry (peak conditions) from this study is consistent with previous PeT estimations for the Pan-African episode on several areas within the Trans-Saharan Belt. All evidences point towards a magmatic arc tectonic setting for this area of study.
... In some places however extensive and minable deposits are found. There have been studies on muscovite as an accessory (minute crystals associated with minerals) in rocks (Kayode, 1972;Ige et al., 1998;Ige and Holness, 2002), but there has been no attempt at analysing the muscovite books for industrial uses. ...
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Thermoluminescence of gamma-irradiated muscovite samples from two deposits in Southwestern Nigeria have been studied. Their glow curves and TL sensitivity are shown to be deposit dependent. TL peaks are observed around 238 and 330∘C in one, and only at 238∘C (as measured with a heating rate of 8∘Cs-1) in the other. The dose responses of the glow peaks have a super-linear behaviour at low doses followed by linearity and then become sub-linear at high doses. The TL sensitivity of the annealed samples was found to be lower than that of unannealed ones and a possible explanation is given. Fading studies show that over a period of one month, the TL signal from the irradiated sample remained relatively stable. Comparison of the results from the present work with those found in the literature showed that, though the TL properties may be deposit dependent, muscovite might possess some properties that are similar irrespective of deposit. These results further indicate the potential of muscovite as a phosphor in retrospective dosimetry and TL dating.
Article
The Ilesha schist belt comprises predominantly of metasediments and mafic ultramafic units, within the basement gneiss-migmatite complex. In the western part, metapelitic schists form a major part of the metasedimentary complex and are essentially composed of several varieties of quartz-mica schist. Samples consist of varying proportions of quartz, chlorite, andalusite, biotite, cordierite, plagioclase, sillimanite and almandine. Qualitative and quantitative pressure and temperature (P-T) estimates suggest variation along the metamorphic field-gradient from 3.5 kbar and 470°C to 6 kbar and 600°C corresponding to the middle amphibolite facies. A monaxite date of ca 580Ma has been obtained from the metapelites with the peak metamorphic assemblage of garnet-biotite-staurolite.
Article
Two laterite profiles on ultramafic rocks in Mokuro-Ife area of southwestern Nigeria, were investigated for the formation of secondary minerals and trace element behavior during humid tropical weathering. Microscopic and X-ray data indicate that the rocks alter to talc, tremolite and chromiferous chlorite. Chlorite becomes thre major stable component and is concentrated in the weathering profile. Geochemical data further indicated that with increasing depth, Fe2O3 and Al2O3 contents decrease, whereas SiO2 and MgO increase. Trace elements such as Ni, Co, Cr, Mn and Cu are all concentrated in the lattice of goethite. Comparison with data on soils overlying associated metapelitic schist shows that geochemical investigation on residual soil may be used for mapping purposes in area with thick weathering overprint.
Article
A group of soapstone statues found near the village of Esie in southwestern Nigeria is one of the largest collections of prehistoric sculpture found in sub-Saharan Africa. The statues were found at Esie by people of the Igbomina sub-grouping of the Yoruba ethnic group when they settled in the Esie area some 300 years ago. The origin of the statues remains unknown. Several areas of soapstone in the Nigerian Schist Belt are considered as potential source areas for the Esie statues. Mineral assemblages, textures, and compositions of talc, cummingtonite, and chlorite from some of the statues suggest a source for the statue soapstone from the Agbonda area, the closest proposed source area to Esie. However, mineralogic data from one of the statues are unlike any of the proposed source areas. The result of our study, together with earlier studies of Stevens, Hambolu and Onabajo provides strong evidence that the soapstone statues were carved from local ultramafic rocks from Esie area.
Article
Tese (doutorado)—Universidade de Brasília, Instituto de Geociências, 2007. A Província Borborema (Nordeste do Brasil) exibe o registro de uma evolução geológica precambriana policíclica complexa, iniciada no Arqueano e encerrada, ao final do Neoproterozóico, com a orogenia Brasiliana/Pan-Africana. É a continuação, na América do Sul, da Província Benin/Nigéria (África do Oeste) que apresenta com ela notáveis semelhanças geológicas. A orogenia Brasiliana/Pan-Africana foi responsável, entre 660 e 570 Ma, pela amalgamação final de Gondwana Ocidental. Na África do Oeste, a identificação de zonas de suturas materializadas por rochas metamórficas de alta pressão e seqüências ofiolíticas e de arcos magmáticos oceânicos deixa claro o caráter colisional dessa orogenia que envolveu os cratons São Luis-Oeste Africa e São Francisco-Congo. No nordeste do Brasil, o caráter colisional ou não dessa orogenia ainda é polemico. Por entender que a evolução das coberturas metassedimentares da Província Borborema, do início da sua deposição ao seu envolvimento na orogenia brasiliana, é um elemento chave na compreensão da formação de Gondwana, resolvemos estudar detalhadamente uma porção do Grupo Ceará, extensa seqüência metassedimentar aflorando no Domínio Ceará Central (sub-província Borborema Setentrional). Os resultados apresentados nesta tese representam uma síntese das informações colhidas durante o mapeamento, na escala de 1:100.000, de uma área de cerca de 5.200 km2 onde as relações entre Grupo Ceará e embasamento arqueano/paleoproterozóico são particularmente bem expostos. Esses dados foram complementados por um estudo geocronológico combinando dados isotópicos Sm-Nd (idades modelo) e datações de grãos de zircão e monazita pelos métodos U-Pb convencional e U-Pb SHRIMP (grãos de zircão detrítico). Na região estudada, considerações ligadas a diferenças nas condições metamórficas, a existência de contatos tectônicos internos e a incongruências nas direções de transporte tectônico, materializadas por lineações minerais, resultaram na subdivisão do Grupo Ceará em cinco subunidades. Essa subdivisão tem valor local. Uma das subunidadea apresenta, intercalados nos metapelitos dominantes, frequentes níveis anfibolíticos. Esses anfibolitos, geralmente ricos em granadas, derivam de sills ou derrames basálticos com idade de ca. 750 Ma e εNd(750) levemente positivos indicando que são derivados de magmas com leve contaminação crustal. São correlacionáveis com metariolitos do mesmo Grupo, datados por outros autores em ca. 780 Ma. São representantes de um vulcanismo bimodal associado ao rifteamento do continente arqueano/paleoproterozóico. O registro desse episódio magmático foi identificado também nos grãos de zircão detrítico de uma amostra de metapelito que apresentou uma população com idade compreendida entre 800 e 750 Ma. Com o afinamento crustal decorrente do rifteamento, iniciou-se a sedimentação do Grupo Ceará. Os dados isotópicos Sm-Nd sugerem uma forte componente do embasamento na proveniência do material detrítico, com grande parte das idades modelo variando no intervalo 2.000/2.500 Ma e εNd(750) francamente negativo. Entretanto, alguns paragnaisses apresentam idades modelo mesoproterozóicas e εNd(750) levemente negativos ou positivos, mostrando que alguns horizontes foram alimentados essencialmente por material juvenil associado ao magmatismo do rift. A evolução progressiva do metamorfismo do Grupo Ceará durante a orogenia brasiliana, desde condições de fácies eclogítico até condições de fácies anfibolito de alta temperatura/baixa pressão, numa trajetória de sentido horário, mostra que esta seqüência foi envolvida numa subducção seguida de exumação, implicando, a exemplo do que aconteceu no Oeste Africano, num modelo colisional para a orogenia brasiliana. A datação de monazitas de mobilizados mostra que a migmatização aconteceu em torno de 610 Ma. A exumação do pacote de metassedimentos foi acompanhada da formação de um empilhamento de nappes sobre o embasamento arqueano/paleoproterozóico. A colocação final das nappes aconteceu em condições de fácies anfibolito baixo. A estruturação das nappes, com direção de transporte tectônico NNW-SSE e mergulhos baixos para W, difere totalmente da estruturação do embasamento que apresenta mergulhos baixos para SE e direções de transporte tectônico NNE-SSW. Esse contraste, na ausência de dados conclusivos, pode ser interpretado de duas maneiras: ou as nappes chegaram sobre um embasamento frio e estruturado durante o paleoproterozóico, deixando pouco registro dúctil brasiliano no autóctone, ou o embasamento conserva o registro de um episódio anterior da orogenia brasiliana, com cinemática de transporte diferente. _____________________________________________________________________________ ABSTRACT The Borborema Province (NE Brazil) represents the South American continuation of the Benin/Nigeria Province (W Africa), with which displays many geological similarities. Its geological framework results from a complex polycyclic evolution, which begins in Archaean times and ends with the Brasilian/Pan African Orogeny, at the end of the Neoproterozoic. The Brasilian/Pan African orogeny was responsible for the West Gondwana amalgamation, between 660 and 540 Ma. In W Africa, well-defined suture zones showing both high-pressure metamorphic rocks and ophiolitic sequences, as well as oceanic magmatic arcs support the collisional character of this orogeny, which includes the São Luis-West Africa and the São Francisco-Congo Cratons. In NE Brazil, the collisional character of the brasilian orogeny is still controversial. The evolution of the monocyclic metasedimentary covers of the Borborema Province - from sediment deposition to deformation and metamorphism during the Brasilian Orogeny - is a key element to understand the formation of West Gondwana. The Ceará Group is one of these metasedimentary sequences. It crops out in the Central Ceará Domain (Northern Borborema Province) and has been chosen to carry out a detailed geological study that includes a 1:150.000- scale mapping of a 5.220 km2 area, in which the relationship between the monocyclic cover and the polycyclic basement are particularly well-exposed. In the studied area the Ceará Group is composed by five nappes separated by concordant, late-metamorphic low-angle ductile thrusts. These nappes are made up of essentially metapelitic rocks that show both distinctive metamorphic and structural behaviors. One of these units (Guia Nappe) exhibits bodies of garnet amphibolites that have been interpreted as former basaltic sills and/or flows metamorphosed under eclogitic conditions. These rocks show ca. 750 Ma, with slightly positive εNd resulting from continental crustal contamination during the ascent of the magma, being possibly correlated with metarhyolites dated by other authors (ca. 780 Ma, U/Pb). The magmatic set are being interpreted as rift-related, Tonian-age bimodal volcanism, also registered in detrital zircon grains found in a paragneiss sample from the same unit (800 to 750 Ma). Deposition of the sediments from the Ceará Group is coeval with rifting and thinning of the Archaean/Palaeoproterozoic continental crust. Sm-Nd model ages in the metasediments suggest a strong contribution of Palaeoproterozoic sources for detrital material, being most of the TDM ages ranging between 2.0 and 2.5 Ga with strongly negative εNd. However, Mesoproterozoic ages and either slightly negative or positive εNd found in some samples imply in essentially juvenile material as sources for some of the sedimentary horizons. The progressive metamorphic evolution of the Ceará Group during the Brasilian Orogeny followed a clockwise path from eclogitic- to HT/LP amphibolite conditions. As in the W Africa, this fact imply in an evolution that involved partial subduction followed by exhumation as a consequence of continental collision. Both conventional and SHRIMP U/Pb estimates in monazite grains from migmatitic mobilizates indicate that the HT/LP metamorphism occurred at ca. 610 Ma. The exhumation of the metasediments from the Ceará Group was done as nappe stacking over the Archaean/Palaeproterozoic basement. The final emplacement of these nappes occurred under retrograde conditions (lower amphibolite to greenschist facies). The nappe pile shows W-dipping, low-angle foliations associated to roughly NNW-SSEoriented stretching lineations. This structural organization is highly contrasting with that found in the basement, where E-dipping, low angle foliations are related to NNE-SSW-oriented stretching lineations. Two hypotheses may explain this contrast: i) the Brasilian nappes were thrusted over cold basement that had been already structured during the Palaeoproterozoic, leading to few- or none Brasilian imprint; or ii) the basement framework was acquired during a former Brasilian event that shows distinct tectonic transport direction.
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ABSTRACT Volcanic arc basalts are all characterized by a selective enrichment in incompatible elements of low ionic potential, a feature thought to be due to the input of aqueous fluids from subducted oceanic crust into their mantle source regions. Island are basalts are additionally characterized by low abundances [for a given degree of fractional crystallization) of incompatible elements of high ionic potential, as feature for which high degrees ot'melting, stability of rninor residual oxide phases, and remelting of depleted mantle are all possible explanations. Calc-alkaline basalts and shoshonites are additionally characterised by enrichment of Th, P and the light REE in addition to elements of low ionic potential, a feature for which one popular explanation is th contamination of their mantle source regions by a melt derived from subducted sediments. By careful selection of variables, discrimination diagrams can be drawn which highlight these various characteristics and therefore enable volcanic arc basalts to he recognized in cases where geological evidence is ambiguous. Plots of Y against Cr, K[Yb, Ce/Yb, or Th/Yb against Ta/Yb, and Ce/Sr against Cr are all particularly successful and can be modelled in terms of vectors representing different petrogenctic processes. An additional plot of Ti/Y against Nb/Y is useful for identifying 'anomalous' volcanic arc settings such as Grenada and parts of the Aleutian arc. Intermediate and acid rocks from volcanic are settings can also be recognized using a simple plot of Ti against Zr. The lavas from the Oman ophiolite complex provide a good test of the application of these techniques. The results indicate that the complex was made up of back-arc oceanic crust intruded by the products of volcanic arc magmatism.
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The pressure-temperature conditions of the reactions governing the high-pressure stability of talc were investigated in experiments on the bulk composition Mg3Si4O10(OH)2 + H2O at 2.9-6.8 GPa, 650-820°C, using piston-cylinder and multianvil apparatus. The reaction talc = enstatite + coesite + vapor was bracketed between 800 and 820°C at 2.90-2.95 GPa and between 770 and 780°C at 3.77-4.02 GPa. The lower-pressure bracket, which is just above the quartz-coesite phase transition, is consistent with some of the previous brackets on the reaction talc = enstatite + quartz + vapor and with the position of the talc dehydration reaction calculated using THERMOCALC v2.4. This revised version of THERMOCALC incorporates new compressibility and thermal expansivity data for talc. -from Authors
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The International Mineralogical Association's approved amphibole nomenclature has been revised to simplify it, make it more consistent with divisions generally at 50%, define prefixes and modifiers more precisely, and include new amphibole species discovered and named since 1978, when the previous scheme was approved. The same reference axes form the basis of the new scheme and most names are little changed, but compound species names like tremolitic hornblende (now magnesiohornblende) are abolished, as are crossite (now glaucophane or ferroglaucophane or magnesioriebeckite or riebeckite), tirodite (now manganocummingtonite), and dannemorite (now manganogrunerite). The 50% rule has been broken only to retain tremolite and actinolite as in the 1978 scheme; the sodic-calcic amphibole range has therefore been expanded. Alkali amphiboles are now sodic amphiboles. The use of hyphens is defined. New amphibole names approved since 1978 include nyböite, leakeite, kornite, ungarettiite, sadanagaite, and cannilloite. All abandoned names are listed. The formulae and source of the amphibole end-member names are listed and procedures outlined to calculate Fe3+ and Fe2+ where not determined by analysis.
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Variations in the F, Cl and OH contents of apatite are not constrained by crystal-chemical factors (in contrast to micas and amphiboles), and thus changes in the abundance of these components provide an indicator of halogen fugacity variations and insights into the degassing history of igneous rocks. Microprobe analysis of intercumulus apatites from the Stillwater Complex reveal that Cl-rich apatites, typically containing 6.0 wt % Cl, occur throughout the lower 1/3 of the complex excluding the Basal series. A change from Cl-rich to more F-rich apatite occurs within olivine-bearing zone I (OB I) of the Banded series, the host zone of the platiniferous J-M Reef. Although apatite compositions are somewhat variable above the J-M Reef, more F-rich apatites predominante and typically contain >1.2 wt % F and 2O3 + La2O3. The trend of first increasing, then decreasing Cl/F ratios with stratigraphic height is modeled by a vapor-driven zone refining process occurring within the cumulate pile causing Cl-enrichment in the interstitial melt accompanied by degassing at the top of the magma chamber causing overall loss of Cl from the magma as crystallization proceeded. The abrupt change from Cl-rich to more F-rich apatites within OB I is interpreted as the result of a breakdown of the Cl-rich zone refining front and mixing with Cl-poor supernatant melt. Any high temperature fluids that exsolved and circulated through the lower 1/3 of the complex must have been enriched in Cl and could have transported REE and trace metals.
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&#112Lenses of ultramafic rocks intercalated within a metagabbro–amphibolite sequence were encountered in the KTB pilot hole. A pervasive metamorphic overprint formed the dominating assemblage calcic amphibole–orthoamphibole–chlorite–talc. In this study the phase relations of ultramafic rocks are investigated in order to (a) constrain the stability field of this assemblage in general, and (b) define the equilibrium pressure–temperature (P–T) conditions of this assemblage in the ultramafic rocks from the KTB borehole. For that purpose, phase equilibria were calculated in the model systems CaO–MgO–Al2O3–SiO2-H2O (CMASH) and CaO–MgO–FeO–Al2O3–SiO2-H2O (CMFASH). Thereby, the continuous compositional change of solid solutions with pressure and temperature was modeled, including the Tschermak’s substitution and the MgFe-1 exchange. Based on these results, petrogenetic grids were constructed, revealing that calcic amphibole–orthoamphibole–chlorite–talc assemblages cover a stability field of <650–770&#117C/1M 14&#114kbar (CMASH) and <550–650&#117C/1&#7714&#114kbar (CMFASH), respectively. This explains the widespread occurrence of the considered assemblage. Based on the bulk rock composition of the KTB samples, a special P–T diagram was constructed, limiting the stability field of the calcic amphibole–orthoamphibole–chlorite–talc assemblage. At 580&#117C the stability field extends from 6 to 14&#114kbar pressure, and shrinks to 10–11&#114kbar at 630&#117C. Conventional estimates using the mineral compositions of the KTB samples yield a temperature around 630&#117C, at which the calculated stability field of calcic amphibole–orthoamphibole–chlorite–talc extends from 10 to 11&#114kbar.
Article
New experimental data on the upper thermal stability of chlorite in the system H2O-MgO-Al2O3-SiO2 are reported here and are combined with other experimental data, molar volumes of the solid phases, heat capacities, H2O fugacities, and activity expressions for cordierite, chlorite and orthopyroxene to obtain an array of univariant curves about an invariant point at 720 + or - 10oC and 2.75 + or - 0.3 kbar where chlorite, cordierite, forsterite, orthopyroxene, spinel and water are in equilibrium.-J.A.Z.
Article
The International Mineralogical Association's approved amphibole nomenclature has been revised in order to simplify it, make it more consistent with divisions generally at 50%, define prefixes and modifiers more precisely and include new amphibole species discovered and named since 1978, when the previous scheme was approved. The same reference axes form the basis of the new scheme and most names are little changed but compound species names like tremolitic hornblende (now magnesiohornblende) are abolished and also crossite (now glaucophane or ferroglaucophane or magnesioriebeckite or riebeckite), tirodite (now manganocummingtonite) and dannemorite (now manganogrunerite). The 50% rule has been broken only to retain tremolite and actinolite as in the 1978 scheme so the sodic calcic amphibole range has therefore been expanded. Alkali amphiboles are now sodic amphiboles. The use of hyphens is defined. New amphibole names approved since 1978 include nyböite, leakeite, kornite, ungarettiite, sadanagaite and cannilloite. All abandoned names are listed. The formulae and source of the amphibole end member names are listed and procedures outlined to calculate Fe ³⁺ and Fe ²⁺ when not determined by analysis.
Article
Phase relations in the ternary system Ni-As-S were determined in rigid silica-glass tubes between 700 degrees and 450 degrees C., with some additional data at lower temperatures. The only ternary phase in the system, corresponding to the mineral gersdorffite for which the accepted formula is NiAsS, has a large variation in its As/S ratio. It is homogeneous from Ni-As (sub 1.77) S (sub 0.23) to NiAs (sub 0.77) S (sub 1.23) at 700 degrees C. Gersdorffite in equilibrium with NiAs 2 (rammelsbergite or pararammelsbergite) has the composition NiAs (sub 1.80) S (sub 0.20) and NiAs (sub 1.72) S (sub 0.28) at 660 degrees and 450 degrees C., respectively. This is a difference of only 2 wt % As. The unit cell edge of gersdorffite between stoichiometric NiAsS and NiAs (sub 1.80) S (sub 0.20' is given by the equation: a = 5.6939 + 0.00047OX, where a is the unit cell edge of gersdorffite (in Aa) containing X weight percent NiAs 2 . Gersdorffite analyses from the literature are reviewed and several new specimens examined to demonstrate that natural gersdorffite also has a large variation in its As/S ratio. Complete solid solution exists between niccolite (Ni (sub 1+ or -x) As) and alpha Ni (sub 1-x) S (the high-temperature modification of millerite) above 595 degrees + or - 5 degrees C. The solvus was determined from 595 degrees to 400 degrees C. along the join from NiAs to NiS. Approximately 1 wt % S substituting for As lowers the polymorphic inversion of rammelsbergite to pararammelsbergite from 590 degrees + or - 10 degrees C. to 475 degrees + or - 25 degrees C. Stable assemblages in the synthetic system correspond closely to the associations found in nature.
Article
The pure magnesium orthorhombic amphibole, anthophyllite, has been synthesized and its upper and lower stability limits have been established by reversible hydrothermal experiments. The synthetic mineral has refractive indices n x=1·587±0·001, n y=1·602±0·005, n z=1·613±0·001, and unit-cell dimensions of a 0=18·61±0·02 Å, b 0=18·01±0·06 Å, c 0=5·24±0·01 Å. The mineral is stable over a narrow temperature range in the presence of the phase H 2O. At a P H2O of 1,000 bars the upper stability limit is 745°±10° C and the lower stability limit is 667°±8° C.Rate studies indicate that anthophyllite can nucleate at temperatures above its upper stability limit by disintegration of talc sheets into strips of double chains. The activation energy for this process is 150±30 kcal mol -1.Application of the data to rocks of the Balmat area, New York, suggests that the equilibrium pressure of water during the metamorphism was significantly less than the total pressure. The data indicate that monomineralic zones of anthophyllite in ultramafic rocks are due to the presence of a steep gradient in the activity of H 2O, or a steep gradient in temperature, or both, across the zones.
Article
Twenty-seven rock and seven mineral samples of SnNbTa-bearing and barren pegmatites from four localities in southwestern and one locality in central Nigeria have been investigated.Most of the pegmatites and corresponding main phase granodioritic units (Older Granites) were emplaced between 562-545 Ma; cooling to the muscovite blocking temperature was about 490 Ma. The initial Sr ratios range from 0.7155 to 0.7233, indicating an older crustal history of the sample material; there are no differences for barren and mineralized pegmatites, albitization does not affect the initial Sr ratios.The Ta-enriched pegmatites around Egbe, however, which are conformably emplaced into a basic to ultrabasic amphibolite sequence, are rather products of partial melting and leaching processes of basement units than the truly pegmatitic phase of the proximal Older Granite main phase. The time gap between their respective emplacement is about 100 Ma. The initial Sr ratio of 0.7678 for the pegmatites against 0.7229 for the Older Granite does not support a co-genetic origin.Biotite ages of 185/183 Ma from southwestern Nigeria correspond to similar ages formerly reported only around the Jos Plateau. The new evidence suggests a regionally more extensive thermal event in association with the emplacement of the anorogenic Younger Granite ring complexes of the Jos Plateau.
Article
The Shiant Isles Main Sill, of Tertiary age, is a classic example of a composite, differentiated alkaline basic sill. The first unit to be intruded was a 2 m thick olivine teschenite which was emplaced with phenocrysts of olivine ( mg > 83) [ mg ≡ Mg#] and, perhaps, plagioclase. This was intruded by a 24 m thick picrite sill consisting of a mush of melt and suspended olivine phenocrysts ( mg > 83) with a D-shaped modal profile. The 140 m thick picrodolerite-crinanite unit was formed by a magma carrying ∼ 10% olivine ( mg > 83) as the main phenocryst phase, together with some calcic plagioclase phenocrysts, being emplaced into the top of the picrite unit before the host rock was completely solidified. The olivine phenocrysts settled towards the bottom to form the picrodolerites. In-situ differentiation processes occurred under conditions of almost perfect fractional crystallization, during which very strongly zoned ophitic crystals of olivine (fayalitic rims) and clinopyroxene (hedenbergitic rims), and zoned laths of plagioclase (anorthoclase rims), formed. The last unit consists of ∼ 2 m of granular olivine picrodolerite which was intruded into the upper crinanites, again before the host rock was fully solid. The mineral zoning patterns are interpreted using published cation diffusion coefficient data, and used to show that the picrite unit might have cooled to the blocking temperatures for Mg and Fe diffusion in < 5 years, and that even the relatively thick crinanite unit cooled very fast, so preserving the zoned Fe-Mg olivine and pyroxene compositions. The compositions of coexisting ilmenites and spinels define a redox trend which initially lies close to fayalite-magnetite-quartz buffer conditions, but later becomes more reducing and approaches magnetite-wustite buffer conditions. The final stages of development occurred during sub-solidus deuteric processes and involved formation of analcime and zeolites, as well as localized sulphide mineralization.
Article
The rocks of the Iwo region of southwestern Nigeria comprise a migmatite-gneiss-granite complex and a metasupracrustal sequence. Though dominantly of amphibolite facies grade, pyroxene granulite and charnockite remnants are found in the migmatite-gneiss-granite complex. Old supracrustal remnants, best identified in resistant quartzite horizons, are also represented, and a variety of granitic plutonic rocks form well-marked circumscribed associations. The main metasupracrustal belt is associated with a major dislocation zone and has two principal lithologies - dominant amphibolite and pelite west of the fault zone and quartzite and quartzofeldspathic gneiss to the east. Oligoclase-granite gneiss and pegmatite are associated with the schist. All the rocks of the area have been affected to varying extents by late reworking, metasomatism, and granitic activity. On the basis of the indicated field relations and the limited radiometric age data available, a development sequence for the region is proposed. Archean crustal rocks are considered to have been involved in an Eburnian (1,950 ± 250 m.y.) reactivation of Proterozoic mobile belt type. In the Kibaran period (1,200 ± 200 m.y.), activity of greenstone belt-granite type brought about the formation of the Ife supracrustal-granite gneiss association, with extensive basement reactivation limited to the proximity of the downwarping supracrustal sequence. Late, widespread reworking and granitic plutonism is related to the Pan-African (Older Granite) event (600 ± 150 m.y.). The possible relevance of the proposed scheme to the interpretation of the development of the eastern parts of the West African platform is discussed.
Article
Schist belts in Nigeria occur in a 400 km wide zone which trends NNE, parallel to the boundary between the Pan-African Province and the West African Craton. A polycyclic migmatite—gneiss—granite terrain, ranging in age from Archaean to Lower Palaeozoic, flanks the schist belt zone to east and west and separates the individual belts within the zone. The schist belts fall into two age groups. The earlier group contains assemblages of mafic igneous rocks, pelitic schists and phyllites, banded iron formation and, locally, coarse grained clastics and carbonate rocks. These belts show complex structural styles and are extensively invaded by granitic plutons belonging to the widely distributed Pan-African magmatic suite. Schists and gneisses within and adjacent to some of these belts have been dated at ∼ 1100 Ma, and this group is thought to belong to the Kibaran Orogeny. Similar Kibaran rock associations can be recognised elsewhere in the Pan-African Province — in the Hoggar and Cameroun. The Kibaran Orogeny in this region is interpreted as a largely ensialic event (although with possible minor ocean basin formation) that led into the more intense and widespread Pan-African Orogeny. The later group of schist belts is characterised by coarse to fine grained clastics, insignificant mafic igneous rocks, a simple, straight structural style and an absence of internal granite plutons. These schist belts are believed to belong to the Pan-African Orogeny. Their sediments may have been deposited in a back-arc basin related to an E-dipping subduction zone at the western margin of the Pan-African Province.
Article
Major and trace element characteristics of Proterozoic amphibolites and talc-tremolite schists from the Ilesha area in the Pan-African orogenic domain of SW Nigeria suggest that they are metamorphosed tholeiitic basalts and associated peridotite flows or sills. The rather ‘primitive’ nature of these metabasalts as indicated by extremely low K, Rb and Rb/Sr values and high K/Rb ratios are interpreted as reflecting magma generation from a deep-seated subcrustal region due to rifting of a thinned sialic crust. The tholeiitic rather than alkaline affinity is attributed to partial melting at shallow depths in the mantle with only minor crustal contamination.The Nigerian Schist Belt is therefore envisaged as a Pan-African ensialic mobile belt derived from an aulacogen connected with an ocean lying to the west in the Dahomeyan-Pharusian belt during Late Precambrian times.
Article
Some high magnesian rocks (MgO 23–32%) having chemical similarity with rocks from komatititic suites were subjected to a detailed geochemical study which lead to a discussion of their petrogenesis. All rocks were subjected to regional metamorphism of upper greenschist to amphibolite facies. Consequently, the discussions on petrogenesis have relied heavily on the abundance of the less mobile elements such as rare earth elements and some transition elements (e.g., Ni and Cr).In general, the meta-ultramafites show almost flat heavy rare earth element and extensive enrichment in light rare earth elements [(La:Sm)N = 1.3−6.3 and (Ce:Yb)N = 0.8−28]. Ni (1700–4400 ppm) and Cr (1400–5200 ppm) are enhanced, which is consistent with their ultramafic parentage.The komatiitic meta-ultramafic rocks of southwestern Nigeria are highly enriched in most trace elements relative to komatiites from well-known areas of the world. This extensive enrichment is considered to be mostly a characteristic of the source region of these rocks.
Article
Phase relations in the system Ni-As were determined in rigid silica-glass tubes, in collapsible Au tubes, and by differential thermal analyses. The system includes the well established minerals maucherite (Ni 11 As 8 ), niccolite (Ni 1 + or -xAs), and NiAs 2 polymorphs rammelsbergite and pararammelsbergite. A phase with the composition of Ni 3 As (dienerite) could not be synthesized, and if this phase exists it must be stable only below 200 degrees C. Ni 5 -xAs 2 is stable to approximately 993 degrees C. and has a large variation in its Ni/As ratio. Maucherite, which is essentially restricted to Ni 11 As 8 composition, melts incongruently at 830 degrees + or -5 degrees C. to niccolite plus a liquid. The existence of a metastable form of Ni 11 As 8 appears to be likely. Niccolite, which is stable to 962 degrees + or -3 degrees C., also has a large variation in its Ni/As ratio. The niccolite solvus between NiAs and NiAs 2 is not useful as a geothermometer, however, since it is nearly vertical in the temperature range of geologic interest. The pararammelsbergite-rammelsbergite inversion was found to occur at 590 degrees C. under the vapor pressure of the assemblage when pure NiAs 2 is in equilibrium with niccolite. The inversion temperature is raised 22 degrees C./1000 bars, giving a H of 0.57 Kcal/mole at 590 degrees C. When pure NiAs 2 is in equilibrium with metallic As instead of niccolite, the inversion temperature is approximately 8 degrees C. higher. Investigation of the inversion temperatures of natural specimens of rammelsbergite and pararammelsbergite shows that solid solution of elements such as Fe, Co, and S may lower the inversion by more than 100 degrees C.
Article
Apatite is a ubiquitous intercumulus mineral in the Archean Munni Munni, Radio Hill, and Mount Sholl layered igneous complexes of the west Pilbara Block in Western Australia. Apatite compositions from the three intrusions are relatively rich in the fluorapatite endmember and become more so in cumulates derived from compositionally evolved magmas. The most extensive stratigraphic coverage is from the Munni Munni complex, where the mole fraction of fluorapatite increases from approximately 60–70% in cumulates of the ultramafic zone to over 90% near the top of the gabbroic zone. The chlorapatite component decreases from a high of 20–30 mol% in the ultramafic zone to less than 10% in the uppermost part of the gabbroic zone. The REE content of apatite parallels stratigraphic changes in whole rock incompatible element (Zr, Rb) concentrations and is consistent with equilibration with the bulk assemblage.
Article
A system is presented whereby volcanic rocks may be classified chemically as follows:I. Subalkaline Rocks:A. Tholeiitic basalt series:Tholeiitic picrite-basalt; tholeiite; tholeiitic andesite.B. Calc-alkali series:High-alumina basalt; andesite; dacite; rhyolite.II. Alkaline Rocks:A. Alkali olivine basalt series:(1) Alkalic picrite–basalt; ankaramite; alkali basalt; hawaiite; mugearite; benmorite; trachyte.(2) Alkalic picrite–basalt; ankaramite; alkali basalt; trachybasalt; tristanite; trachyte.B. Nephelinic, leucitic, and analcitic rocks.III. Peralkaline Rocks:pantellerite, commendite, etc.A system is presented whereby volcanic rocks may be classified chemically as follows:I. Subalkaline Rocks:A. Tholeiitic basalt series:Tholeiitic picrite-basalt; tholeiite; tholeiitic andesite.B. Calc-alkali series:High-alumina basalt; andesite; dacite; rhyolite.II. Alkaline Rocks:A. Alkali olivine basalt series:(1) Alkalic picrite–basalt; ankaramite; alkali basalt; hawaiite; mugearite; benmorite; trachyte.(2) Alkalic picrite–basalt; ankaramite; alkali basalt; trachybasalt; tristanite; trachyte.B. Nephelinic, leucitic, and analcitic rocks.III. Peralkaline Rocks:pantellerite, commendite, etc.
Article
Apafite in most igneous intrusions has a high C1/F ratio. However, chlor-apatite has been reported in the lower portions of the Bushveld and Stillwater Complexes. This has been used as evidence supporting the early separation of a Cl-rich discrete hydrous fluid in these intrusions. Mineralogical evidence is presented here to demonstrate that the Bushveld Complex, at least, formed from a nearly anlaydrous magma, and did not release a hydrous fluid before apatite began to crystallize. It is suggested that apatite in the earliest cumulates equilibrated with trapped interstitial liquid, which converted it from the typical F-rich composition of cumulus apatite to a Cl-rieh composition. This is an analogous process to that in which cumulus marie minerals may become more Fe-rieh on cooling and reaction with interstitial liquid.
Article
The compositions of apatite crystals from seven Skaergaard rocks were obtained by electron-probe microanalysis. These span the range of occurrence from the base to the top of the exposed zones of the layered series. Apatite occurs as an intercumulus phase in the lower zones, but becomes a cumulus phase at a structural height of 1850 m ( c. 98 % crystallized), defining the base of Upper Zone b. All apatite analyses show a high F:Cl ratio. There is a slight but significant increase in F and decrease in Cl when the apatite becomes a cumulus phase, the F/Cl values changing from < 10 to > 30. Variations in F:Cl:OH are attributed to differential volatile migration from trapped intercumulus liquid sites. The apatite data provide new support for ferrodiorite-granophyre liquid immiscibility.
Article
The Basement complex of Nigeria is polycyclic and retains memories of events dating back to about 3000 m. y. As in other parts of Africa, it suffered its most pronounced deformation and remobilisation during the Pan African Orogeny (about 650-450 m. y. ago). A review of the geology of this terrain is presented following recent field and geochronological data. It is also shown that the basement may have participated in the 1300-900 m. y. Kibaran Orogeny. In addition, an attempt is made at discussing the evolution of the Precambrian rocks in Nigeria and the conclusion is made that most of the rocks evolved by ensialic rather than plate tectonic processes.
Article
Microprobe analysis of interstitial apatite from a 100-m section through the Merensky reef of the Bushveld Complex reveals that this stratigraphic section marks a transition from Cl-rich to more F-rich compositions in the Critical and Main zones, respectively. -from Authors
Article
This metavolcano-sedimentary sequence has previously been classified as a Proterozoic mobile belt. Field observations indicate, however, that this sequence has a typical (for the Archaean) granite-greenstone rock succession. Geochemical data could support this opinion. In spite of the lack of age determinations, it is possible to place these successions in the early Archaean. This new classification of the metavolcano-sedimentary sequence of the Ife-Ilesha area is of far-reaching economic importance. The known mineralizations of Au, cassiterite, columbite, tantalite and W in this area require further prospecting, not only in SW Nigeria but also in the other mobile belt region of Nigeria. Geochemical prospecting methods are a valuable tool for stratigraphic classification and the elucidation of the origin of the rocks in areas with strong lateritic weathering or metamorphic overprint.- C.N.
Article
Brown hornblende occurs in minor amounts in the Artfjllet gabbro and dolerites, except in quartz-dolerites where a pale green hornblende occurs. In the gabbro, brown hornblende is mostly Ti-bearing pargasite or kaersutite. It occurs along veins of orthopyroxene, as rims around and blebs in pyroxenes, with orthopyroxene in coronas between olivine and plagioclase and in coronas between ilmenite and plagioclase. In the olivine-dolerites and orthopyroxene-dolerites brown hornblende is ferroan titanian pargasite or ferroan kaersutite. The pale green hornblende in the quartz-dolerites is a magnesio-hornblende. The hornblendes in the dolerites are interstitial or granular, in some dolerites occurring as coarse oikocrysts. It is proposed that under certain conditions the Ti content of hornblende can be used as a thermometer, derived from experimental data of Helz (1973). Microstructures, compositions and formation temperatures (< 1,040="" c)="" show="" that="" the="" brown="" hornblende="" in="" the="" gabbro="" is="" not="" magmatic,="" but="" of="" subsolidus="" origin.="" probably="" it="" formed="" as="" a="" result="" of="" the="" introduction="" of="" water="" into="" the="" gabbro="" during="" a="" deformation="" event="" that="" occurred="" early="" in="" the="" cooling="" history="" of="" the="" gabbro.="" least-squares="" modelling="" of="" hornblende="" formation="" indicates="" that="" all="" magmatic="" minerals="" must="" have="" participated="" in="" the="" reaction="" and="" that="" the="" reaction="" probably="" was="" not="" isochemical.="" microstructures,="" compositions="" and="" formation="" temperatures="" (1,030-965="" c)="" of="" brown="" hornblende="" in="" the="" dolerites="" are="" consistent="" with="" late-stage="" crystallization="" from="" the="" magma.="" for="" the="" pale="" green="" hornblende="" in="" the="" quartz-dolerites="" a="" magmatic="" origin="" is="" likely,="" but="" cannot="" be="">
Article
Complex reaction textures in coronitic metagabbros and retrograded eclogites of the KTB pilot and an adjacent drilling provide evidence for a multistage metamorphic history in the Variscan basement of the NW Bohemian Massif. The eclogites show complete metamorphic recrystallization leaving no textural or mineral relics of their igneous precursors. In contrast, textural relics of the igneous protolith are still preserved in the metagabbros where the metamorphic overprint under high pressure conditions achieved only partial replacement of the initial assemblage plagioclase + augite + amphibole (+olivine or orthopyroxene?) + ilmenite to form the eclogite facies assemblage garnet + omphacite + kyanite + zoisite + quartz+rutile. The garnets in the metagabbros occur in the typical necklace fashion at the borders between the original plagioclase and mafic phase domains. In the same rocks, omphacite formed by a topotactic reaction mechanism replacing igneous augite as well as in smaller grains at the margins of the texturally igneous clinopyroxene where it occurs without fixed orientation with respect to the relict phase. Both eclogites and metagabbros show a partial breakdown under high pressure granulite (transitional to high pressure amphibolite) facies conditions during which omphacite broke down to vermicular symplectites of diopside + plagioclase. A later pervasive medium pressure metamorphism under amphibolite facies conditions led to the development of assemblages dominated by hornblende + plagioclase+titanite: phases prevailing in the overwhelming majority of the surrounding metabasites. Subsequent vein-associated retrogression produced minerals typical of the greenschist to zeolite facies. All metamorphic stages may be represented in a single thin section but although the overall reaction sequence is apparent, the obvious disequilibrium in the rocks makes the use of conventional geothermobarometry difficult. However, calculations made by assuming an approach to domainal equilibrium show that both the eclogite facies and early breakdown occurred above 10 kb. As the metamorphic unit hosting these particular metabasites is generally characterized by pressures below 10 kb these results have important implications for understanding the tectonometamorphic evolution of the region. The relationship between the studied rocks and other units in the NW Bohemian Massif exhibiting a multistage metamorphic evolution is discussed and possible tectonic models evaluated.
Article
The metagabbro-amphibolite sequences in the KTB pilot hole contain intercalations of talc-chlorite-amphibole felses (or ''hosbachites''), which show transitional contacts to the adjacent metagabbros. The hosbachites are characterized by relics of a primary igneous texture and still contain igneous minerals like clinopyroxene, biotite and pseudomorphs after olivine, while brown Ca-amphibole was presumably formed in a late-magmatic stage. The geological, textural, mineralogical and geochemical evidence indicates that the hosbachites were derived from ultramafic cumulates, differentiated from a basaltic magma, either in the inner parts of dolerite sills or in small gabbro intrusions. A pervasive metamorphic overprint under medium-pressure, amphibolite-facies conditions which was accompanied by penetrative deformation led to assemblages with green Ca-amphibole +/- anthophyllite +/- cummingtonite +/- tremolite/actinolite + clinochlore + talc + olivine + ilmenite +/- Cr-bearing spinel + sulfides. Phase relationships are consistent with a prograde P-T path leading to the formation of anthophyllite from olivine + talc at peak metamorphic temperatures of 640-700-degrees-C, at assumed pressures of 8-10 kbar, similar to those derived from mineral assemblages in the adjacent metabasites and metasediments. High-pressure relics locally present in coronitic metagabbros and retrograded eclogites of the KTB pilot hole were not recognized in the hosbachites. A retrograde overprint under greenschist-facies conditions led to the total replacement of igneous or metamorphic olivine by aggregates of antigorite + magnetite, chloritization of biotite and the formation of late tremolite/actinolite.
Article
The hydration of peridotites modelled by the system H2O-CaO-MgO-Al2O3-SiO2 has been treated theoretically after the method of Schreinemakers, and has been investigated experimentally in the temperature range 700°–900° C and in the pressure range of 8–14 kbar. In the presence of excess forsterite and water, the garnet- to spinel-peridotite transition boundary intersects the chlorite dehydration boundary at an invariant point situated at 865±5° C and 15.2±0.3 kbar. At lower pressures, a model spinel lherzolite hydrates to both chlorite- and amphibole-bearing assemblages at an invariant point located at 825±10° C and 9.3±0.5 kbar. At even lower pressures the spinel-to plagioclase-peridotite transition boundary intersects the dehydration curve for amphibole+forsterite at an invariant point estimated to lie at 855±10° C and 6.5±0.5 kbar. Both chlorite and amphibole were characterized along their respective dehydration curves. Chlorite was found to shift continuously from clinochlore, with increasing temperature, to more aluminous compositions. Amphibole was found to be tremolitic with a maximmum of 6 wt.% Al2O3. The experimentally determined curves in this study were combined with the determined or estimated stability curves for hydrous melting, plagioclase, talc, anthophyllite, and antigorite to obtain a petrogenetic grid applicable to peridotites, modelled by the system H2O-CaO-MgO-Al2O3-SiO2, that covers a wide range of geological conditions. Direct applications of this grid, although quite limited, can be made for ultramafic assemblages that have been extensively re-equilibrated at greenschist to amphibolite facies metamorphism and for some highgrade ultramafic assemblages that display clear signs of retrogressive metamorphism.
Article
Major-, trace- and rare-earth element chemistry studies of metabasite intercalations within different tectonic units of the East Bavarian crystalline basement leads to the following geochemical classification: The flaser amphibolites of the Erbendorf-Vohenstrauss Zone (ZEV) exhibit an enriched, E-MORB- or intraplate-like tholeiitic character, whereas the schistose and striped amphibolites of the ZEV and of the Tirschenreuth-Mahring Zone show N-MORB compositions. The metagabbros of the ZEV are transitional between these two types. The metabasites of the Erbendorf Greenschist Zone are similar to modern island-arc basalts (tholeiitic to calc-alkaline). The Fichtelgebirge crystalline complex contains amphibolites of enriched tholeiitic to alkaline i.e., intraplate character.In most of the investigated metabasites, a post-basaltic/post-gabbroic mobilization of the trace elements cannot be recognized. An exception is Ba which is generally enriched. This may be due to pre-metamorphic hydrothermal alteration processes and/or to a synmetamorphic chemical exchange with adjacent metasediments. The contact-metamorphic overprint of some flaser amphibolites from the ZEV by the intrusion of the Variscan Falkenberg granite led to enrichment in Li, Rb, K and W, a simultaneous depletion in Ca, Sr, Cr and Ni, and a decrease in the K/Rb ratio. Nb, Ce, P, Zr, Ti and V scatter in a much wider range than in the unaffected flaser amphibolites, although with no clear tendency for enrichment or depletion. A mobilization of P and the LREE's in some schistose and striped amphibolites of the ZEV and in the contact-metamorphosed flaser amphibolites is presumably a result of post-granitic hydrothermal alteration which is indicated by enrichment of As as a pathfinder element.
Article
Fifteen chondrites, including eight carbonaceous chondrites, were analyzed for rare earth element abundances by isotope dilution. Examination of REE for a large number of individual chondrites shows that only a small proportion of the analyses have flat unfractionated REE patterns within experimental error. While some of the remaining analyses are consistent with magmatic fractionation, many patterns, in particular those with positive Ce anomalies, can not be explained by known magmatic processes. Elemental abundance anomalies are found in all major chondrite classes. The persistence of anomalies in chondritic materials relatively removed from direct condensational processes implies that anomalous components are resistant to equilibrium or were introduced at a late stage of chondrite formation. Large-scale segregation of gas and condensate is implied, and bulk variations in REE abundances between planetary bodies is possible.
Metamorphism of Alpine peridotite and 0. A. IGE et al
  • B W Evans
Evans, B. W. 1977. Metamorphism of Alpine peridotite and 0. A. IGE et al.
Petrographic studies of metabasic and metaultramafics in relation to mineralization in Nigerian Schist belts
  • Elueze
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