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The Canadian Mineralogist Vol. 46, pp. 19-40 (2008) DOI : 10.3749/canmin.46.1.19
MAJOR- AND TRACE-ELEMENT COMPOSITION OF OLIVINE, PEROVSKITE, CLINOPYROXENE, Cr–Fe–Ti OXIDES, PHLOGOPITE AND HOST KAMAFUGITES AND KIMBERLITES, ALTO PARANAÍBA, BRAZIL Leone MELLUSO§ Dipartimento di Scienze della Terra, Università degli Studi di Napoli Federico II, Via Mezzocannone 8, I–80134 Napoli, Italy
Michele LUSTRINO Dipartimento di Scienze della Terra, Università degli Studi di Roma “La Sapienza”, P.le Aldo Moro 5, Roma, Italy and CNR Istituto di Geologia Ambientale e Geoingegneria (I.G.A.G.) c/o Università di Roma “La Sapienza”, I–00185 Roma, Italy
Excelso RUBERTI Instituto de Geociências, Universidade de São Paulo, Rua do Lago, São Paulo, SP, Brazil
Pietro BROTZU Dipartimento di Scienze della Terra, Università degli Studi di Napoli Federico II, Via Mezzocannone 8, I–80134 Napoli, Italy
Celso de BARROS GOMES Instituto de Geociências, Universidade de São Paulo, Rua do Lago, São Paulo, SP, Brazil
Lucio MORBIDELLI Dipartimento di Scienze della Terra, Università degli Studi di Roma “La Sapienza”, P.le Aldo Moro 5, Roma, Italy and CNR Istituto di Geologia Ambientale e Geoingegneria (I.G.A.G.) c/o Università di Roma “La Sapienza”, I–00185 Roma, Italy
Vincenzo MORRA Dipartimento di Scienze della Terra, Università degli Studi di Napoli Federico II, Via Mezzocannone 8, I–80134 Napoli, Italy
Darcy P. SVISERO Instituto de Geociências, Universidade de São Paulo, Rua do Lago, São Paulo, SP, Brazil
Fosco d’AMELIO Dipartimento di Scienze della Terra, Università degli Studi di Napoli Federico II, Via Mezzocannone 8, I–80134 Napoli, Italy
Abstract An integrated whole-rock petrographic and geochemical study has been carried out on kamafugites and kimberlites of the Late Cretaceous Alto Paranaíba igneous province, in Brazil, and their main minerals, olivine, clinopyroxene, perovskite, phlogopite, spinels and ilmenite. Perovskite is by far the dominant repository for light lanthanides, Nb, Ta, Th and U, and occasionally other elements, reaching concentrations up to 3.4 3 104 chondrite values for light lanthanides and 105 chondrite for Th. A very strong fractionation between light and heavy lanthanides (chondrite-normalized La/Yb from ~175 to ~2000) is also observed. This is §
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likely the first comprehensive dataset on natural perovskite. Clinopyroxene has variable trace-element contents, likely due to the different position of this phase in the crystallization sequence; Sc reaches values as high as 200 ppm, whereas the lanthanides show very variable enrichment in light over heavy REE, and commonly show a negative Eu anomaly. The olivine, phlogopite (and tetra-ferriphlogopite), Cr–Ti oxide and ilmenite are substantially barren minerals for lanthanides and most other trace elements, with the exception of Ba, Cs and Rb in mica, and V, Nb and Ta in ilmenite. Estimated mineral / whole-rock partition coefficients for lanthanides in perovskite are similar to previous determinations, though much higher than those calculated in experiments with synthetic compositions, testifying once more to the complex behavior of these elements in a natural environment. The enormous potential for exploitation of lanthanides, Th, U and high-field-strength elements in the Brazilian kamafugites, kimberlites and related rocks is clearly shown. Keywords: trace elements in minerals, perovskite, clinopyroxene, phlogopite, kamafugites, kimberlites, Alto Paranaíba, Brazil.
Sommaire Nous présentons les résultats d’une étude pétrographique et géochimique intégrée des kamafugites et kimberlites de la ceinture tardi-crétacée de la province ignée de Alto Paranaíba, au Brésil, et les minéraux importants que ces roches renferment: olivine, clinopyroxène, pérovskite, phlogopite, spinelles et ilménite. La pérovskite constitue le réservoir dominant des terres rares légères, Nb, Ta, Th et U, et dans certains cas, d’autres éléments, atteignant des concentrations allant jusqu’à 3.4 3 104 fois les valeurs chondritiques pour les terres rares légères et jusqu’à 105 fois la valeur chondritique pour le Th. Un très fort fractionnement a séparé les terres rares légères et lourdes (rapport La/Yb normalisé à une chondrite entre 175 et 2000, environ). Nos données seraient probablement les premières compréhensives sur la pérovskite naturelle. Le clinopyroxène possède des teneurs assez variables en éléments traces, probablement à cause de différentes positions de ce minéral dans la séquence de cristallisation. Le Sc y atteint des teneurs de 200 ppm, tandis que les terres rares légères accusent un enrichissement très variable sur les terres rares lourdes; une anomalie négative en Eu est développée assez couramment. L’olivine, la phlogopite (ainsi que la tétra-ferriphlogopite), les oxydes à Cr–Ti et l’ilménite sont relativement stériles en terres rares et la plupart des autres éléments, sauf pour Ba, Cs et Rb dans le mica, et V, Nb et Ta dans l’ilménite. Les coefficients de partage estimés entre minéral et roche globale pour les terres rares dans la pérovskite sont semblables aux déterminations antérieures, quoique beaucoup plus élevées que celles que l’on peut simuler par expériences sur des analogues synthétiques. Ce décalage indique une fois de plus le comportement complexe de ces éléments dans les milieux naturels. Nos données soulignent clairement le potentiel énorme qu’ont les kamafugites, kimberlites et roches associées du Brésil pour exploitations de ressources en terres rares, Th, U et éléments à champ électrostatique élevé. (Traduit par la Rédaction)
Mots-clés: éléments traces dans les minéraux, pérovskite, clinopyroxène, phlogopite, kamafugites, kimberlites, Alto Paranaíba, Brésil.
Introduction The Alto Paranaíba Igneous Province is one of the largest potassic–ultrapotassic provinces in the world (~3000 km2, >15000 km3; Gibson et al. 1995, Brod et al. 2000, Araújo et al. 2001, Gomes & Comin-Chiaramonti 2005). The rocks are represented by intrusions, diatremes, lava flows, highly altered pyroclastic rocks (also known as Mata da Corda Formation), and several intrusive complexes (Fig. 1). Kamafugites, kimberlites, and intrusive and cumulate lithologies have been found throughout the province (Gomes & Comin-Chiaramonti 2005, and references therein). The association of cumulate lithologies with carbonatites is well known at Araxá (the largest Nb mine in the world), Tapira, Salitre and Catalão [Gomes & Comin-Chiaramonti (2005), and references therein]. In this paper, we have determined the major- and trace-element contents of whole rocks and the main rock-forming mineral phases in kimberlites and kamafugites. A subset of samples has been chosen to calculate semiquantitative partition-coefficients of elements between mineral phases and kamafugite–kimberlite
whole rocks, reliable values of which are still very scarce in the literature.
Geological Setting These rocks were emplaced during the Late Cretaceous (~90–75 Ma; Gibson et al. 1995, Sgarbi et al. 2004, Gomes & Comin-Chiaramonti 2005, Carlson et al. 2007) in and on top of Proterozoic metamorphosed crustal sequences of the Brasilia mobile belt (Almeida et al. 2000). This belt represents a strongly folded terrane (with ages of peak metamorphism around 790 and 630–610 Ma; Pimentel et al. 2000) forming the western boundary of the São Francisco craton. Some of the Alto Paranaíba kimberlites are known to carry diamond and xenoliths of garnet lherzolite (e.g., Três Ranchos). Petrochemical and Sr, Nd, Os and Pb isotopic studies carried out on the Alto Paranaíba rocks suggest several constraints about chemical and isotopic features of mantle sources of magmatism: a) high contents of incompatible elements in mafic, mantle-derived rocks are likely to be the result of partial melting of subcontinental lithospheric mantle sources undergoing intense
kamafugites and kimberlites, alto paranaíba, brazil
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Fig. 1. Geological setting of the Alto Paranaíba volcanic rocks, with sampling sites (filled circles).
metasomatism prior to melting (e.g., Gibson et al. 1995, Carlson et al., 1996, 2007); b) the range of lithologies and the chemical and isotopic differences between main petrographic types (kimberlites and kamafugites) require partial melting of variously metasomatized mantle sources or melting at different depths or crystal
fractionation, or a combination thereof (Carlson et al. 1996, 2007, Bizzi et al. 1995, Gibson et al. 1995, Thompson et al. 1998, Araújo et al. 2001). The rocks forming the Alto Paranaíba Province are interpreted to result from upwelling of asthenospheric melts through the relatively thin lithospheric lid located above the
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Trindade plume during the Late Cretaceous (the Proterozoic Brasilia Belt), even though the chemical influence of this plume has been seriously questioned (Brotzu et al. 2005, Carlson et al. 2007). The Alto Paranaíba mafic ultrapotassic rocks are considered to be the result of melting lithospheric mantle metasomatized in the Proterozoic (Carlson et al. 2007), and a similar origin has been hypothesized for the coeval or slightly younger potassic and ultrapotassic dikes and alkaline intrusions of the Serra do Mar Province, southeast of the Alto Paranaíba (Brotzu et al. 2005, Thompson et al. 1998). It is interesting to highlight similarities and differences between these volcanic rocks and the roughly coeval kimberlites of formerly neighboring regions like southern Africa, which are subject of intensive studies (e.g., Becker & le Roex 2006, and references therein) or the Italian kamafugites, which are emplaced in a completely different tectonic setting (Conticelli & Peccerillo 1992, Conticelli et al. 2002, 2004). Kamafugites and kimberlites of the Alto Paranaíba have been used as case studies because of their peculiar enrichment in lanthanides and other “usually” incompatible elements (e.g., Araújo et al. 2001).
Analytical Techniques Major- and trace-element concentrations were obtained with an X-ray fluorescence unit (Philips PW1400) at Centro Interdipartimentale di Strumentazioni per Analisi Geomineralogiche (CISAG), Università di Napoli. Details on the analytical techniques are identical to those reported in Brotzu et al. (2005, 2007). The concentration of additional trace-elements, including the lanthanides, have been obtained with inductively coupled plasma – mass spectrometry at Actlabs (http://www.actlabs.com). Electron-microprobe analyses have been obtained with a JEOL superprobe (15 kV, 20 nA, beam diameter ranging from 5 to 20 mm) at the Instituto de Geociências of the University of São Paulo, using natural and synthetic oxides and elements as standards. The concentration of trace elements in minerals was obtained with laser-ablation microprobe linked with an inductively coupled plasma – mass spectrometer (LAM–ICP–MS) at the CNR Istituto di Geoscienze e Georisorse, Pavia (Italy), utilizing a double focusing sector field analyzer (Finnigan Mat Element), coupled with a Q-switched Nd:YAG laser source (Quantel Brilliant), whose fundamental emission in the near-IR region (1064 nm) is converted to 266 nm by two harmonic generators (Tiepolo et al. 2003). Helium was used as carrier gas, mixed with Ar downstream of the ablation cell. The spot diameter varied in the range 30–100 mm. Reference sample BCR2-g was used for calibration, with 44Ca as internal standard for clinopyroxene and amphibole, and 29Si for orthopyroxene. Precision and accuracy, both
better than 10% for concentrations at the ppm level, were assessed from repeated analyses of SRN NIST612 reference standard. Detection limits were typically in the range 100–500 ppb for Sc, 10–100 ppb for Sr, Zr, Ba, Gd and Pb, 1–10 ppb for Y, Nb, La, Ce, Nd, Sm, Eu, Dy, Er, Yb, Hf and Ta, and, usually, 1 ppb for Pr, Th and U. The complete mineral chemical dataset (ca. 500 electron microprobe analyses and 100 laser-ablation microprobe data) can be requested to the authors.
Petrography and Composition of the Alto Paranaíba Kamafugites and Kimberlites Results of whole-rock major and trace element analyses of the samples are reported in Table 1. Using the classification of Foley et al. (1987) and Le Maitre et al. (2002), the samples from the Alto Paranaíba Igneous Province fall in the kamafugite field. The nomenclature of the volcanic rocks of the Alto Paranaíba region is a difficult task, owing to peculiar parageneses and to the variable mineralogical and chemical composition of the same lithologies in the hand specimen and thin section. The kamafugites considered here (Presidente Olegario, Veridiana, Santa Rosa, Canas, Malaquias; Fig. 1) are generally porphyritic, with variable size and amount of phenocrysts of variably serpentinized olivine and zoned clinopyroxene. Perovskite (particularly abundant as zoned idiomorphic microphenocrysts at Malaquias), poikilitic phlogopite, Fe–Ti oxides, leucite and analcime pseudomorphs after leucite occur as microphenocrysts, or are confined to the groundmass (Fig. 2). The kamafugites are relatively rich in Ca (9.9–14.7 wt.% CaO), Mg (8.2–21.4 wt.% MgO) and Ti (3.6–5.5 wt.% TiO2) and poor in Si (35.9–42.2 wt.% SiO2). The K content (not higher than 2.9 wt.% K2O) is probably lower than the original content, because of Nametasomatism that caused the formation of analcime. The kimberlites (Indaiá, Limeira I, Pântano, Três Ranchos; Fig. 1) are porphyritic and are made up of rounded and altered macrocrysts and microcrysts of olivine, monticellite (Indaiá), perovskite (particularly abundant at Pântano; Fig. 2), phlogopite and apatite in an altered glassy groundmass (Fig. 2). The origin of the olivine macrocrysts is controversial, because it is difficult to distinguish crystals of mantle origin (xenocrysts) from phenocrysts. The kimberlites are rich in Ca (8.3–15.4 wt.% CaO) and Mg (23.4–29.3 wt.% MgO), whereas Ti (0.9–2.3 wt.% TiO2), Al (
