The competing models for the origin and internal evolution of granitic pegmatites in the light of melt and fluid inclusion research

In this paper we discuss the main petrogenetic models for granitic pegmatites and how these models have evolved over time. We suggest that the present state of knowledge requires that some aspects of these models to be modified, or absorbed into newer ones. Pegmatite formation and internal evolution have long supposed the need for highly water- and flux-enriched magmas to explain the differences between pegmatites and other intrusives of similar major element composition. Compositions and textural characteristics of fluid and melt inclusions in pegmatite minerals provide strong evidence for such magmas. Furthermore, we show that melt inclusion research has increased the number of potential flux components, which may include H₂O, OH^?, CO₂, HCO ₃ ^? , CO ₃ ^2? , SO ₄ ^2? , PO ₄ ^3? , H₃BO₃, F , and Cl, as well as the elements Li, Na, K, Rb, Cs, and Be, herein described as melt structure modifiers. In this paper we emphasize that the combined effect which these components have on the properties of pegmatite melts is difficult to deduce from experimental studies using only a limited number of these components. The combination and the amount of the different magmatic species, together with differences in the source region, and variations in pressure and temperature cause the great diversity of the pegmatites observed. Some volatile species, such as CO ₃ ^2? and alkalis, have the capacity to increase the solubility of H₂O in silicate melt to an extraordinary degree, to the extent that melt-melt-fluid immiscibility becomes inevitable. It is our view that the formation of pegmatites is connected with the complex interplay of many factors.     

Nb–Ta–(Ti–Sn) oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the Borborema Province,Northeastern Brazil

The Borborema Pegmatitic Province (BPP), northeastern Brazil, is historically important for tantalum mining and also famous for top-quality specimens of exotic Nb–Ta oxides and, more recently, for the production of gem quality, turquoise blue, ‘Paraíba Elbaite.’ With more than 750 registered mineralized rare-element granitic pegmatites, the BPP extends over an area of about 75 by 150 km in the eastern part of the Neoproterozoic Seridó Belt. The Late Cambrian pegmatites are mostly hosted by a sequence of Neoproterozoic cordierite–sillimanite biotite schists of the Seridó Formation and quartzites and metaconglomerates of the Equador Formation. The trace-element ratios in feldspar and micas allow to classify most pegmatites as belonging to the beryl–columbite phosphate subtype. Electron microprobe analyses (EMPA) of columbite, tapiolite, niobian–tantalian rutile, ixiolite and wodginite group minerals from 28 pegmatites in the BPP are used to evaluate the effectiveness of Nb–Ta oxide chemistry as a possible exploration tool, to trace the degree of pegmatite fractionation and to classify the pegmatites. The columbite group mineral composition allows to establish a compositional trend from manganoan ferrocolumbite to manganocolumbite and on to manganotantalite. This trend is typical of complex spodumene- and/or lepidolite-subtype pegmatites. It clearly contrasts with another trend, from ferrocolumbite through ferrotantalite to ferrowodginite and ferrotapiolite compositions, typical of pegmatites of the beryl–columbite phosphate subtype. Large scatter and anomalous trends in zoned crystals partially overlap and conceal the two main evolution patterns. This indicates that a large representative data set of heavy mineral concentrate samples, collected systematically along cross-sections, would be necessary to predict the metallogenetic potential of individual pegmatites. Other mineral species, e.g. garnets and/or tourmaline, with a more regular distribution than Nb–Ta oxides, would be more appropriate and less expensive for routine exploration purposes. The currently available Nb–Ta oxide chemistry data suggest the potential for highly fractionated Ta–Li–Cs pegmatites in the BPP, so far undiscovered, and encourages further, more detailed research.     

Tantalite-(Mn) from the Borborema Pegmatite Province,northeastern Brazil: conditions of formation and melt- and fluid-inclusion constraints on experimental studies

Carbon dioxide-rich fluid and carbonate-rich aluminosilicate melt inclusions in tantalite-(Mn) from the Alto do Giz pegmatite in the Borborema Pegmatite Province, northeastern Brazil were investigated to constrain the formation of the host crystals. The results demonstrate that in the Alto do Giz pegmatite, water- and alkaline carbonate-rich fluids and melts are responsible for the transport and deposition of tantalite-(Mn) at temperatures around 600°C and about 4 kbar. Moreover, evidence is presented to show that during crystallization of the tantalite-(Mn), three different components coexisted, which are now trapped as separate inclusions: two immiscible silicate melts (types A and B melt inclusions) and a CO₂-rich aqueous fluid. We hypothesize that immiscible fluid separation may have been a critical factor in producing the water- and alkaline carbonate-rich fluids and melts necessary for Ta and Nb transport. Since the tantalite-(Mn) crystallized during pegmatite formation, this mechanism must also have implications for pegmatite genesis in general.     

Reinhard Maack

Ohne Zusammenfassung

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Reinhard Maack

     

Die Problematik paläogeographischer Rekonstruktion

Zusammenfassung Mit der heute wohl begründeten Annahme von Lageveränderungen der geotektonischen Großeinheiten während der Erdgeschichte ist die paläogeographische Rekonstruktion problematisch geworden, da sie nicht auf der Basis der heutigen geographischen Lagebeziehungen aufbauen kann. Auf der anderen Seite ist gerade die paläogeographische Rekonstruktion, die unvoreingenommen möglichst alle Einzeldaten der verschiedenen Spezialdisziplinen berücksichtigt, die Voraussetzung für eine kritische Analyse der wechselnden Lagebeziehungen. Gerade seine Problematik macht den paläogeographischen Rekonstruktionsversuch zum unentbehrlichen methodischen Hilfsmittel geologischer und geotektonischer Forschung — unter der Voraussetzung, daß man sich dabei nicht vom heutigen Kartenbild leiten läßt. An den Beispielen des Amazonas-Beckens, des Atlantischen Ozeans und der Paläogeographie Mittelamerikas—Westindiens werden diese Zusammenhänge dargelegt.

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By the well-founded hypothesis of changes in the position of geotectonical major units during earth history, a paleogeographical reconstruction becomes problematical, since it cannot be based on the present geographical situation. On the other hand, it is above all the paleogeographical reconstruction which considers the greatest possible number of single data of the various branches of earth sciences, thus being the presupposition for a critical analysis of the changing position of the geotectonical units. By its complexity, the attempt of paleogeographical reconstruction is an indispensable methodical aid for geological and geotectonical research — under the condition that it is done independently of the actual geographical map. These correlations are demonstrated on the examples of the Amazon basin, the Atlantic Ocean and the paleogeography of Middle America and the West Indies.

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Résumé L'hypothèse bien fondée des changements de position des blocs continentaux pendant l'histoire de la terre, rend problématique la réconstruction paléogéographique, puisque celle-ci ne peut pas se baser sur la situation géographique actuelle. De l'autre coté, c'est surtout la réconstruction paléogéographique qui tient compte des dates individuelles des branches géoscientifiques particulières, étant ainsi le fondement pour une analyse critique des changements de position des éléments tectoniques. Par son caractère problématique, l'essai d'une réconstruction paléogéographique est une aide indispensable pour la recherche géologique et géotectonique — sous la condition qu'on ne regard pas la figure des cartes modernes. L'auteur démontre ces relations par l'exemple du bassin des Amazones, de l'Océan atlantique et de la paléogéographie de l'Amérique centrale et des Indes occidentales.

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Resumo A hipótese é bem fundamentada que, durante a história da terra aconteceram alterações da posiçào dos blocos continentais; em consequência disso a reconstrução palaogeográfica é um pouco problemática; pois não pode basear-se na configuração geográfica atual. Por outro lado, é exatamente a reconstrução paleogeográfica, baseada nas observações e experiências das diversas disciplinas geológicas, que constitui o único fundamento certo de uma análise das alterações consecutivas da posição das unidades geotectônicas. A tentativa da reconstrução paleogeográfica torna-se, dêste modo, um método indispensável para a pesquisa geológica e geotectônica, caso que a reconstrução paleogeográfica é executada independentemente da configuração geográfica atual. Estas relações são demonstradas pelos exemplos do desenvolvimento geológico da bacia do Amazonas, do Ozeano Atlântico e da América Central e das ilhas centralamericanas.

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Die Zukunft des Menschen aus paläontologischer Sicht

Zusammenfassung Ausgehend von den regelhaften Ablaufsformen in der stammesgeschichtlichen Entfaltung werden der stammesgeschichtliche Werdegang der Hominiden und die ihn bestimmenden Umbildungstendenzen diskutiert, die vor allem durch die Cerebralisation beherrscht werden. Die Frage, ob die orthogenetisch verlaufende Fortbildung der Hominiden eine aus der Entwicklungsrichtung abgeleitete, voraussagende Projektion in die Zukunft möglich macht, muß verneint werden; denn die excessive Cerebralisation führt den Menschen aus dem biologischen Zwangsablauf heraus in eine durch Entscheidungsfreiheit gekennzeichnete Sonderstellung.

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Proceeding from the regular forms of evolution of the Humanoids, the phylogenic processes involved and the controlling remodeling tendencies, dominated principally by cerebralisation, are discussed.The question whether the orthogenetic course of further evolution of the Humanoids would make it possible to predict a future image — derived from the evolutionary trends — must be negated. This follows from the fact that excessive cerebralisation leads the Human-beings out of the biological inevitable course into a special situation characterised by free judgement.

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Résumé En partant des règles sur la dérivation des formes dans l'évolution phylogénétique, l'auteur discute l'évolution des Hominidés et les tendances déterminantes à la transformation, principalement dominées par la cérébralisation. A la question de savoir si la continuation orthogénétique en cours de l'évolution des Hominidés peut faire l'objet d'une prédiction en la projetant selon sa ligne directrice, on ne peut que répondre négativement: la cérébralisation excessive conduit l'homme hors des contraintes biologiques dans une position particulière caractérisée par la liberté de décision.

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Borborema Pegmatitic Province: geological and geochemical characteristics

The geological features of the pegmatites from the Borborerna Pegmatitic Province (BPP) are described, combining data from the literature with new field and laboratory observation. A geochemical study was performed against this geological background to test the crystallization model of mineralized pegmatites against barren pegmatites and to compare the BPP with other provinces, fields or individual pegmatites throughout the world. The field evidence (mode of emplacement, textural relationships, zonation and mineralogy) as well as the geochemical characteristics support the pegmatite crystallization model proposed by London (1990). The geochemical and mineralogical evidence places these pegmatites as “medium Ta-mineralized” compared to other pegmatite provinces.     

Neue Beobachtungen zur Paläogeographie des Devon auf dem brasilianischen Block

Zusammenfassung Neuere Beobachtungen im Jatobá-Becken (Pernambuco — Brasilien), dem nördlichsten Ausläufer der Reconcavo-Zone von Bahia haben überraschenderweise dort eine Vertretung von Unterdevon und unterstem Unterkarbon ergeben. Das Cariri-Konglomerat im Liegenden der kretacischen Araripe-Serie (West-Pernambuco) muß nach neuen Beobachtungen ebenfalls ins Unterdevon eingereiht werden. Gleiches gilt für den stratigraphisch bisher in der Luft hängenden Tombador-Sandstein im zentralen Bahia, dessen Profil weitgehend dem Unterdevon-Profil von Piaui entspricht. Die Devondecke auf dem nordöstlichen Teil des brasilianischen Blockes war ursprünglich viel ausgedehnter als bisher vermutet werden konnte. Eigenartig ist die auf die weiten Entfernungen gleichförmige grobklastische Fazies auch in offensichtlich küstenferneren Bereichen, die kaum eine andere Deutung als driftglaziale Einflüsse im Unterdevon-Meer zuläßt, was früher auch schonMaack für das Unterdevon in Paraná vermutet hatte. Die aus paläomagnetischen Messungen erschlossene Lage des brasilianischen Blockes während des Devon stimmt damit gut überein.

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Recent observations in the basin of Jatobá (Pernambuco — Brasil), the extreme northern end of the Reconcavo-Zone of Bahia, resulted in the surprising discovery of Subdevonian and lowest Carboniferous. Recent studies also found out that the Cariri-conglomerat in the underlying bed of the cretaceous Araripe-series (West-Pernambuco) must also be classified in the Subdevonian, as well as the Tombador sandstone in central Bahia, whose profil resembles that of the Subdevonian of Piaui. The Devonian nappe on the north-eastern part of the Brasilian block was originally much larger than it has been assumed. The clastic facies even in regions far from the coast, is very strange. It can only be interpreted as glacial deposits in the subdevonian ocean, a suggestion which has already been made byMaack for the Subdevonian in Paraná. This agrees with the situation of the Brasilian block during the Devonian, infered from paleomagnetic measurements.

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Resumen Conforme as observações mais recentes há uma representação do Devoniano inferior e do Carbonífere mais inferior na bacia de Jatobá (Pernambuco — Brasil), a extremidade setentrional da zone de Revoncavo. O conglomerado de Cariri na lapa da série Araripe, deve ser atribuido ao Devoniano inferior conforme observações recentes. O arenito Tombador da Bahia, cujo perfil corresponde muito bem ao do Devoniano inferior de Piaui, tern a mesma posição estratigráfica. No bloco brasileiro, a capa, devoniana era originalmente muito mais extensa do que se supôs até agora. A fácies clástica, muito monótona por todas estas amplas áreas, é um fenomeno muito estranho que pode ser interpretado só pela idéia que no mar devoniano houve gelo flutuante. No Devoniano do Paraná, R.Maack pronunciou a mesma idéia. As medições paleomagnéticas indicam uma posição do bloco brasileiro durante o Devoniano, que bem corresponde a esta interpretação.

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Das Gondwana-Inlandeis in Südbrasilien

Zusammenfassung Entgegen den früheren Annahmen wird gezeigt, daß das südbrasilianische Gondwana-Inlandeis vom Süden (Riograndensischer Schild) stammt. Die mächtige, weder durch Glazialstauchungen noch durch glaziale oder interglaziale Erosion gestörte und unterbrochene Ablagerungsfolge, die damit ein vom europäischen Glazialdiluvium völlig abweichendes Verhalten zeigt, ist dadurch bedingt, daß das Eis in ein weiträumiges, fortschreitende epirogene Absenkung zeigendes Becken hineinfloß.Mit Unterstützung des Conselho Nacional d. Pesquisas, Rio de Janeiro.