Subsolidus phase relations in the system Zr-Fe-Ti-O in equilibrium with metallic iron. Implications for lunar petrology

An experimental study of the system Zr-Fe-Ti-O in equilibrium with metallic iron has been conducted at atmospheric pressure and in the temperature range 950–1300° C, with the purpose of modelling sub-solidus relations between Zr-bearing Fe-Ti oxide phases in lunar rocks. The phase relations are governed by the coexistence of the Fe-Ti oxides with ZrO₂ at temperatures below 1247° C, but with the new cubic ZFT phase at higher temperatures. ZFT, approximately Zr Fe Ti O₅, is probably related to the so-called stabilized cubic zirconia and could represent a synthetic equivalent of some lunar Zr-Fe-Ti rich oxide minerals.The zirconium contents in all Fe-, Ti and Fe-Ti-oxide phases increase with increasing temperature. At each temperature, Zr is incorporated preferentially in the Ti-oxide (up to 11 wt% ZrO₂), followed by ferropseudobrookite (up to 6 wt% ZrO₂), ilmenite (max. 4 wt%) and wüstite (max. 2.5 wt%). Ulvöspinel coexisting with ilmenite (+Fe⁰+ ZrO₂ and/or ZFT) always contains less than 1.2 wt% ZrO₂, whereas the same mineral in assemblage with wüstite-ZrO₂-Fe⁰ displays even higher ZrO₂ contents than ilmenite (above 4 wt%). Considering that the values determined here in the synthetic parageneses represent saturation concentrations at high temperatures in a simple model system, the ZrO₂ concentrations in the corresponding lunar minerals should generally be significantly lower. This is realized, except for the so-called Cr-Zr-Ca armalcolites which display ZrO₂ contents in the range 3.5–7 wt% and are thus probably related rather to Zr-rich oxide minerals than to armalcolite itself.     

Tsunami Hazard in the Eastern Mediterranean: Strong Earthquakes and Tsunamis in the Corinth Gulf,Central Greece

The exhaustive review of a long number of historical documents, books, reports,scientific and press reports, instrumental recordings, previous catalogues andpersonal field observations, concluded with the production of a completely newtsunami catalogue for the Corinth Gulf, Central Greece, which is arranged in theformat adopted by the GITEC group for the new European Tsunami Catalogue.The catalogue is presented in three sections: the Quick-Look Table, the Quick-LookAccounts File and the References File. An Appendix explains why some particularsea disturbances were not included in the new catalogue although they were consideredas tsunami events by previous researchers. Past history clearly shows that most tsunamis in the Corinth Gulf are produced by strong (Ms 5.5) offshore and near shore earthquakes. However, seismic or aseismic sliding of coastal and submarine sediments is a significant factor in tsunamigenesis. Calculations based on the random model indicate that the probability for at least one tsunami occurrence of intensity TI 2 TI 3 and TI 4 within 50 years equals 0.851, 0.747 and 0.606, respectively. From the intensity–frequency relationship the mean return period of tsunami intensity TI 2, TI 3 and TI 4 equals to 16, 40 and 103 years. The tsunami geographicaldistribution, however, is non-random with a clear trend for the tsunamigenesis todecrease drastically from west to east within the Corinth Gulf. In fact, the probabilityfor a strong earthquake to cause a tsunami of TI 3 in the Corinth Gulf consideredas an entity is 0.35, while in the western part of the Gulf it goes up to 0.55. Therefore, the rapid and accurate determination of the earthquake focal parameters is of great importance in an algorithm of a real-time tsunami warning system in the Corinth Gulf.     

Sm−Nd dating of multiple garnet growth events in an arc-continent collision zone,northwestern U.S. Cordillera

Integrated petrologic and Sm–Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite,P–T studies indicate a single stage of metamorphism with final equilibration at 600°C and 8–9 kbar. The Sm–Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128±3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A⁴⁰Ar/³⁹Ar age for this hornblende indicates cooling through 525°C at 119±2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm–Nd garnet — whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent ages of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at 128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America.     

Formation of Ni–Cu–Platinum Group Element sulfide mineralization in the Sudbury Impact Melt Sheet

Summary The Ni–Cu–Platinum Group Element (PGE) sulfide deposits of the Sudbury Structure have provided a major portion of the worlds total nickel production and their host rocks have been the subject of numerous research studies, yet a number of perplexing problems remain to be solved. On the one hand, studies seeking to explain the formation of the Sudbury Structure have now converged on a genetic model which proposes that the Main Mass and Offset Dykes of the Sudbury Igneous Complex (SIC) were produced by crystallization of an impact-generated melt sheet. On the other hand, these models have yet to be fully reconciled with the production of the very large volume of magmatic Ni, Cu, Co, and PGE-rich sulfide mineralization and the associated mafic rock types. This paper explores this problem using new precious metal data from the Main Mass and Offset Dykes. These data are used to understand the relationships between these rocks, and to provide constraints on how the Ni–Cu–PGE sulfide ore deposits fit into the geological evolution of the Sudbury Structure.In the two drill cores selected for study in this project, the Mafic Norite has 1–5 modal percent pyrrhotite plus chalcopyrite, and elevated Ni (40–1000ppm), Cu (40–1140ppm), and PGE (1.9–7.8ppb Pd, 1.8–7.3ppb Pt); this is overlain by Felsic Norite that contains pyrrhotite, and has a wide range in concentration of Ni (13–257ppm), Cu (7–328ppm), and PGE (<0.01–6.4ppb Pd, <0.01–5ppb Pt). For a similar range of MgO, the upper portion of the Felsic Norite unit has 5–10 times lower Ni and Cu abundances than within-plate basalts and local crustal rocks, and PGE abundance levels are mostly below analytical determination limits. Stratigraphic studies of other compositional profiles around the SIC demonstrate that this depletion signature of Ni, Cu, and PGE is widespread and developed not only above mineralized embayments and offsets, but also above barren sections of the lower contact of the SIC.The depletion of the upper part of the Felsic Norite in Ni, Cu and PGE is presumably due to equilibration of the magma with magmatic sulfide, and accumulation of this dense sulfide liquid. Results of modeling indicate that the parental magma giving rise to the Mafic and Felsic Norites had initial Ni and Cu contents of 210 and 110ppm, respectively. In addition, Ni, Cu and PGE tenors calculated in 100% sulfide from the Copper Cliff Offset average 13% Cu, 6% Ni, 18ppm Pd, and 19ppm Pt indicating that these sulfides had formed by fractionation from magmas that contained 310ppm Ni, 310ppm Cu, 18ppb Pd and 19ppb Pt. These values are factors of 3 to 5 higher than the Ni, Cu, Pd, and Pt contents of the Onaping Formation with average values of 55ppm Ni, 48ppm Cu, and 4.9ppb Pd as well as the marginal sulfide-poor phase of the Worthington Offset quartz diorite, which has average values of 61ppm Ni, 59ppm Cu, 2.8ppb Pd and 4.0ppb Pt. Both the Onaping Formation and the marginal quartz diorite are believed to represent the initial composition of a large component of the melt sheet. There is therefore a fundamental problem in reconciling the initial metal contents of the SIC magma as indicated by the marginal phases of the Offset dykes and that of the Onaping Formation with the composition of the SIC magma at the times of formation of the sulfides as indicated by their Ni, Cu and PGE tenors.It is proposed that because the SIC melt sheet was initially superheated with a temperature of 1700°C, it was able to dissolve 5 times as much S as it could at its liquidus temperature of 1200°C. It was also initially composed of an emulsion of mafic and felsic melts (Marsh and Zieg, 1999), which may have formed discrete magma cells. As the temperature of the melt sheet decreased, some of these magma cells became S-saturated and the resultant Ni–Cu–PGE sulfides settled downwards and on reaching magma cells lower in the melt sheet were re-dissolved thereby raising the Ni, Cu and PGE contents of the lower magma cells. It was from these enriched magma cells that precipitation of the ore-forming Ni–Cu–PGE sulfide melts eventually took place.The mineral potential of Offset and embayment structures appears to be empirically linked to the thickness of the overlying noritic rocks; for example, the most heavily mineralized embayments and Offset Dykes are located in areas where the Felsic Norite is thickest. It appears unlikely that the entire 1–3km-thick melt sheet was convectively mixing throughout its lateral extent, and so the heterogeneity in sulfide distribution was retained after crystallization and cooling.     

A K-Ar and 40Ar/39Ar study on white micas from the Brixen Quartzphyllite,Southern Alps

The Brixen Quartzphyllite, basement of the Southern Alps (Italy), consists of metasediments which had suffered progressive deformation and low grade metamorphism (p _max4 kbar, T _max375±25° C) during the Palaeozoic. It has been excavated by pre-Permian erosion, buried again beneath a pile of Permo-mesozoic to Cainozoic sediments (estimated T _max150° C), and is now exposed anew due to late Alpine uplift and erosion. The behavior of the K-Ar system of white micas is investigated, taking advantage of the narrow constraints on their thermal history imposed by the geological/stratigraphic reference systems.The six structurally and petrographically differing samples come from a single outcrop, whose position is roughly two kilometers beneath the Permian land-surface. White mica concentrates from five grain size fractions (<2 , 2–6 , 6–20 , 20–60 , 60–75 ) of each sample have been analyzed by the conventional K-Ar method, four selected concentrates additionally by the ⁴⁰Ar/³⁹Ar stepwise heating technique; furthermore, Ar content and isotopic composition of vein quartz were determined.The conventional ages of the natural grain size fractions (20–60 , 60–75) are in the range 316±8 Ma, which corresponds to the ⁴⁰Ar/³⁹Ar plateau age of 319.0±5.5 Ma within the error limits. The finer grain size fractions yield significantly lower ages, down to 233 Ma for fractions <2 . Likewise low apparent ages (down to 83 Ma) are obtained for the low temperature ⁴⁰Ar/³⁹Ar degassing steps.There is no correlation between microstructural generation of white mica prevailing in the sample and apparent age. This favours an interpretation of the 316±8 Ma values as cooling age; progressive deformation and metamorphism must be respectively older and their timing cannot be resolved by these methods. The data preclude any significant influence of a detrital mica component as well as of excess argon.The lower ages found for the fine grain-size fractions (respectively the low-T degassing steps) correspond to a near-surface period (p-T-minimum); the values are geologically meaningless. The effect is interpreted to result from partial Ar loss due to reheating during Mesozoic-Cainozoic reburial. A model based on diffusion parameters derived from the outgassing experiments and Dodson's (1979) equation yields a closure temperature of 284±40 °C for a cooling rate of 18° C/Ma. Furthermore, this model suggests that the observed argon loss of up to 5% may in fact have been induced by reheating to 150 °C for 50 Ma.     

Changes in stable isotope ratios of metapelites and marbles during regional metamorphism,Mount Lofty Ranges,South Australia: implications for crustal scale fluid flow

The Mount Lofty Ranges comprises interlayered marbles, metapsammites, and metapelites that underwent regional metamorphism during the Delamarian Orogeny at 470–515 Ma. Peak metamorphic conditions increased from lowermost biotite grade (350–400°C) to migmatite grade (700°C) over 50–55 km parallel to the lithological strike of the rocks. With increasing metamorphic grade, ¹⁸O values of normal metapelites decrease from 14–16 to as low as 9.0, while ¹⁸O values of calcite in normal marbles decrease from 22–24 to as low as 13.2 These isotopic changes are far greater than can be accounted for by devolatilisation, implying widespread fluid-rock interaction. Contact metamorphism appears not to have affected the terrain, suggesting that fluid flow occurred during regional metamorphism. Down-temperature fluid flow from synmetamorphic granite plutons (¹⁸O=8.4–8.6) that occur at the highest metamorphic grades is unlikely to explain the resetting of oxygen isotopes because: (a) there is a paucity of skarns at granite-metasediment contacts; (b) the marbles generally do not contain low-XCO₂ mineral assemblages; (c) there is insufficient granite to provide the required volumes of water; (d) the marbles and metapelites retain a several permil difference in ¹⁸O values, even at high metamorphic grades. The oxygen isotope resetting may be accounted for by along-strike up-temperature fluid flow during regional metamorphism with time-integrated fluid fluxes of up to 5x10⁹ moles/m² (10⁵ m³/m²). If fluid flow occurred over 10⁵–10⁶ years, estimated intrinsic permeabilities are 10^-20 to 10^-16m². Variations in ¹⁸O at individual outcrops suggest that time-integrated fluid fluxes and intrinsic permeabilities may locally have varied by at least an order of magnitude. A general increase in XCO₂ values of marble assemblages with metamorphic grade is also consistent with the up-temperature fluid-flow model. Fluids in the metapelites may have been derived from these rocks by devolatilisation at low metamorphic grades; however, fluids in the marbles were probably derived in part from the surrounding siliceous rocks. The marble-metapelite boundaries preserve steep gradients in both ¹⁸O and XCO₂ values, suggesting that across-strike fluid fluxes were much lower than those parallel to strike. Up-temperature fluid flow may also have formed orthoamphibole rocks and caused melting of the metapelites at high grades.This paper is a contribution to IGCP Project 304 Lower Crustal Processes     

Neotectonics of the central parts of the Balkan Peninsula: Basic features and concepts

The neotectonic movements on the Balkan Peninsula occurred after the last intense thrusting (Early Miocene), and after the Early — Middle Miocene planation. They were controlled by extensional collapse of the Late Alpine orogen, and by extension behind the Aegean arc, and were influenced by the complicated vertical and horizontal movements in the Pannonian region. The Stara-planina and Dinarian-Hellenic linear neotectonic morphostructures inherited the Alpine orogenic zones (Balkanides and Dinarides-Hellenides) and bounded the Central-Balkan neotectonic region. The linear morphostructures were tilted towards the Pannonian and Euxinian basins and the North-Aegean trough.The Central-Balkan neotectonic region has a complicated block structure (horst-and-graben pattern) dominated by the NNW-SSE Struma and Vardar lineaments, the WNW-ESE Sava and Marica lineaments, and the Middle-Mesta and North Anatolian fault zones. The dominating Serbo-Macedonian neotectonic swell was rifted, and subsided along the Struma and Vardar lineaments. The range of the vertical neotectonic displacements reached a maximum of 3–4 km, and even up to 6 km at the edges of the Pannonian and Aegean basins. The general doming of the region was controlled by the isostatic uplift of a thickened crustal lens (Rhodope Massif) in the southern margin of the Eurasian plate. The collapse of the complicated domal structure began along the main (Struma, Vardar and Marica) lineaments in the central parts of the dome, and continued in the Pliocene and Quaternary along a more external contour bounded by the Stara-planina and Dinarian-Hellenic linear morphostructures.

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Zusammenfassung Die neotektonischen Bewegungen der Balkan-Halbinsel begannen nach den letzten intensiven Überschiebungen (frühes Miozän) und nach der frühbis mittelmiozänen Verebnung. Gesteuert wurden die Bewegungen durch den Dehnungskollaps des spätalpinen Orogens, der Dehnung hinter dem Ägäischen Bogen und den komplizierten vertikalen und horizontalen Bewegungen in der pannonischen Region. Die neotektonische Region des Zentralbalkans liegt zwischen den linearen, neotektonischen Morphostrukturen der Strara-planina und der Dinariden-Helleniden. Sie übernahmen die alpidischen Orogenzonen der Balkaniden und Dinariden-Helleniden und wurden zum Pannonischen-, dem Präkarpatischen- und dem Nordägäischen Trog geneigt.Die Region zeigt einen komplizierten Blockaufbau (Horst- und Grabenstrukturen), der von den NNW-SSE streichenden Struma- und Vardar-Lineamenten, von den WNW-ESE verlaufenden Sava- und Marica-Lineamenten und der Mittelmesta- und der Nordanatolischen Bruchzone dominiert war. Die Serbo-mazedonische neotektonische Schwelle war von Bruchspaltenbildung und Absenkung parallel der Struma- und Vardar-Lineamente betroffen. Die Höhe der vertikalen Versatzbeträge erreichte ein Maximum von 3–4 km; an den Rändern des Pannonischen und Ägäischen Beckens sogar mit bis zu 6 km. Die allgemeine Aufwölbung der Region wurde durch isostatische Hebung der verdickten Krustenteile (Rhodopisches Massiv) am Südrand der Eurasischen Platte bedingt. Der Kollaps der komplizierten Domstruktur begann in dessen Zentralteil entlang der Hauptlineamente (Struma-, Vardar- und Marica-Lineament) und setzte sich, während des Pliozäns und Quartärs, in den peripheren Bereichen, parallel zu den äußeren Begrenzungen (Balkaniden, Dinariden-Helleniden) der linearen Morphostrukturen, fort.

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Résumé Les mouvements néotectoniques dans la péninsule balkanique ont eu lieu après les derniers charriages d'âge miocène inférieur et la pénéplanation du Miocène inférieur et moyen. Ils ont été régis par l'affaissement extensionnel de l'orogène alpin tardif, par l'extension derrière l'arc égéen et par les mouvements verticaux et horizontaux complexes dans la région panonnienne. La région néotectonique centrebalkanique est située entre les morphostructures néotectoniques linéaires de Stara-Planina et des Dinarides-Hellénides. Celles-ci sont héritées des zones orogéniques alpines des Balkanides et des Dinarides-Hellénides et ont été inclinées vers les bassins panonnien, euxinien et nord-égéen.La région possède une structure en blocs (horsts et grabens) compliquée, dominée par les linéaments NNW-SSE de Struma et du Vardar, les linéaments WNW-ESE de Sava et de Marica et les zones faillées de Moyenne Mesta et d'Anatolie du nord. La ride néotectonique serbo-macédonienne a subi rifting et subsidence au long des linéaments de Struma et du Vardar. Les déplacements néotectoniques verticaux ont atteint 3 à 4 km au maximum, et même 6 km dans les bordures des bassins panonniens et égéen. Le soulèvement en dôme de la région a été provoqué par la montée isostatique d'une portion épaissie de l'écorce (massif du Rhodope) dans la marge méridionale de la plaque eurasiatique. L'affaissement de cette structure en dôme complexe a commencé le long des linéaments principaux (de Struma, Vardar et Marica) dans les parties centrales du dôme et a continué pendant le Pliocène et le Quaternaire le long d'un contour plus externe limité par les morphostructures néotectoniques linéaires de Stara-Planina et dinarohellénique.

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Zum Problem der Schichtlücken im Devon der Sierra Morena (Spanien)

Zusammenfassung Im Devon der nördlichen Sierra Morena und der Sierra de San Pedro treten Schichtlücken auf, die in den untersuchten Mulden ein unterschiedliches Ausmaß erreichen. Das Mitteldevon konnte nirgends nachgewiesen werden. Das Oberdevon folgt unmittelbar über unterdevonischen oder auch silurischen Schichten. Im südlichen Arbeitsgebiet kann sogar das Oberdevon fehlen, so daß unterkarbonische Sedimente über silurischen und älteren Schichten liegen.Für das Fehlen von Oberdevon kann Schwellenbildung und damit verbundene Abtragung vermutet werden. Die Schichtlücken im Unter- und Mitteldevon der übrigen Gebiete scheinen dagegen durch Unterbrechung der Sedimentzufuhr erklärbar zu sein.

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In the Devonian of northern Sierra Morena and Sierra de San Pedro there exist stratigraphical breaks of different amount in the investigated troughs. No Middle Devonian could be found. Upper Devonian rests directly upon Lower Devonian or Silurian rocks. Due to the occasional lack of Upper Devonian in the southern area under discussion even sediments of Lower Carboniferous age may superpose Silurian and older deposits.The absence of Upper Devonian is supposed to be caused by development of ridges and their erosion. However, stratigraphical breaks during Lower and Middle Devonian in the other areas seem to be explicable by non-deposition.

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Résumé Dans les synclinals étudiés du Dévonien de la Sierra Morena septentrionale et de la Sierra de San Pedro il y a des lacunes stratigraphiques atteignant de différentes ampleurs. Dévonien moyen ne fut nulle part observé. Dévonien supérieur suit immédiatement les couches du Dévonien inférieur ou bien du Silurien. Dans le secteur méridional il y a même des lacunes stratigraphiques du Dévonien supérieur, et les sédiments du Carbonifère inférieur reposent sur le Silurien ou sur des couches plus anciennes.Il est possible qu'un soulèvement d'un paléorelief accompagné de l'érosion soit responsable de la lacune du Dévonien supérieur. Quant aux lacunes du Dévonien inférieur et moyen des autres régions elles sont plutôt à expliquer par non-déposition.

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Rift-zone magmatism: Petrology of basaltic rocks transitional from CFB to MORB,southeastern Brazil margin

Compositions of basaltic samples from the southeastern Brazil passive margin (18°–24° S) depict the change from continental to oceanic lithosphere during the opening of the South Atlantic Ocean. Samples studied range from 138 to 105 m.y. old and are from 12 Petrobrás drill cores recovered from the coastline to about 150 km offshore in the Espirito Santo, Campos, and Santos basins. Compositions vary, ranging, for example, from 49–54 wt.% SiO₂, 0.5–3.0 wt.% TiO₂, 0.6–5.0 FeO^*/MgO, and 1-6 La/ Yb_(n), but can be grouped: (i) basalts enriched in incompatible elements, such as K (some K₂O>2.0 wt.%), Rb (>18 ppm), Zr (>120 ppm), and LREE (some FeO^* 16 wt.%; most with SiO₂ 51–54 wt.%), and resembling Serra Geral continental flood basalts (SG-CFB) of southern Brazil; (ii) basalts less enriched, or transitional, in incompatible elements, having K₂O <0.40 wt.% and flat REE patterns, and resemble N. Atlantic diabases and FAMOUS basalts; and (iii) one depleted sample, Ce/Yb_(n)=0.7, where Ce_(n)=4. Expressed in oceanic-basalt terminology and Zr-Nb-Y abundances, enriched samples are P- and T-type MORB (e.g., Zr/Nb 4–25), transitional samples are T-type (Zr/ Nb 8–27), and the depleted sample is N-type MORB (Zr/Nb>30).Trace-element ratios (e.g., Zr/Nb, Zr/Y) link the Brazil margin basalts to a heterogeneous mantle (attributed to metasomatic veining) of variably proportioned mixtures of depleted-mantle (N-MORB) and plume (P-MORB, e.g., Tristan hotspot) materials. The various compositions therefore reflect, in part, different zones of melting during the separation of Gondwanaland, where gradual decompression during rifting enabled concurrent melting of upper, more depleted (non- or sparsely-veined) mantle and enriched (densely-veined) mantle. Within the time represented, melting produced enriched, transitional, and depleted magmas that were emplaced subaerially, hypabyssally, and subaqueously; they mark the transition from CFB before rifting and separation (from deeper, enriched mantle) to N-MORB in the S. Atlantic afterwards (from non- or sparsely-veined upper mantle). While P-type mantle components account for the enriched compositions of some basalts (Zr/Nb<8), continental crust is largely responsible for that of others (e.g., Ti/Zr 40–57; La/Yb_(n) 5–6, and ¹⁸O+12.2 in one sample). Some may be contaminated expressions of otherwise T-type basalts free of crustal components. This study identifies CFB to be from sources similar to those for T- and P-type oceanic rocks, where individual CFB magmas may or may not have acquired crustal signatures.     

Scapolite-bearing and related calc-silicate layers from the alpujarride series. (Betic cordilleras of Southern Spain). A discussion on their origin and some comments

Generalized development of calc-silicate bands in several alpujarride units of the central and western segments of the Betic Zone, is interpreted as the result of metamorphic differentiation from original calcium-rich sedimentary beds in the sequence. Presence of Scapolite suggests that this phenomenon may have been made possible by the catalizing action of high anion concentrations in the pore fluid medium during metamorphism, probably caused by the existence of evaporic material in the original sediments. This hypothesis, which is further supported by anomalous high Ca, Na and K contents of the enclosing metapelites, leads to the correlation of the present calc-silicate bearing metamorphic formations with well-known permo-werfenian evaporitic series in other units of the Betic Zone. The use of this new criterion of stratigraphic correlation would indicate that highly metamorphosed permo-triassic sequences have often been misidentified as more ancient formations, thus causing an underestimation of the importance of alpine HT-LP metamorphism in the alpujarride tectonic domain.

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Zusammenfassung Das häufige Auftreten von Kalksilikatbändern (manchmal mit Skapolith) in einigen alpujarriden Einheiten, besonders im zentralen und westlichen Abschnitt der betischen Kordillere, wird erklärt als Ergebnis einer metamorphen Differentiation von ursprünglich Ca-reichen Schichten. Die Anwesenheit von Skapolith legt eine hohe Anionenkonzentration der intergranularen fluiden Phasen während der Metamorphose nahe, bedingt durch evaporitisches Material der ursprünglichen Sedimente. Diese Hypothese, zusätzlich gestützt durch den extrem hohen Gehalt der umgebenden Pelite an löslichen Kathionen (Ca, Na, K), erlaubt die Korrelation der Kalksilikatbänder-haltigen Formationen mit aus anderen betischen Einheiten bekannten evaporitischen Serien des Permowerfenïan. Auf die stratigraphische Korrelation angewendet, legt dies nahe, daß öfters hochmetamorphe, permotriadische Serien fälschlicherweise als älter betrachtet wurden, was zu einer Unterschätzung der alpinen Metamorphose (Niederdruck-Hochtemperatur) im alpujarridem Bereich führte.

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Resumen El desarrollo bastante generalizado de bandas de silicatos calcicos, entre los cuales se encuentra a veces una escapolita, en varias unidades alpujárrides de los segmentos central y occidental de la Zona Bética, es interpretado como el resultado de un proceso de diferenciación metamórfica a partir de lechos sedimentarios originalmente ricos en calcio. La presencia de escapolita sugiere que este hecho ha sido probablemente posible gracias a la existencia de una alta concentración de aniones en el fluido metamórfico intergranular, condicionada por la existencia de material evaporitico en los sedimentos originales. Esta última hipótesis que es ulteriormente apoyada por un contenido anómalamente alto de cationes solubles (Ca, Na, K) en las metapelitas encajantes, permite correlacionar las formaciones que contienen los lechos de silicatos calcicos con conocidas series evaporiticas de edad permowerfenense en otras unidades de la Zona Bética. La aplicación de este nuevo criterio de correlación estratigráfica sugiere que series permotriásicas altamente metamorfizadas han sido a menudo erróneamente interpretadas como más antiguas, hecho que ha conllevado a una substimación de la importancia del metamorfismo (BP-AT) alpino en el dominio tectónico alpujárride.

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Résumé Le developpement très fréquent de roches litées à silicates calciques dans quelques unités alpujarrides des regions centrale et occidentale de la Zone Bétique, est interpreté comme la conséquence d'un processus de différenciation métamorphique à partir de lits riches en calcium dans les sequences sédimentaires originelles. La présence de scapolite suggére que ce phénomène a été rendu possible par l'intervention d'un cataliseur naturel, tel que des hautes concentrations en anions dans le milieu fluide intergranulaire pendant le métamorphisme, occasionées probablement par l'existence de materiel évaporitique dans les sédiments originels.Cette hypothese, qui se trouve additionellement supportée par des teneurs élevées en cations solubles des métapelites encaissantes, permet d'assimiler les formations qui contiennent les lits à silicates calciques avec des séries évaporitiques permowerféniennes, bien connues dans d'autres unités des zones internes des Cordilléres Bétiques. L'application de ce nouveau critère de corrélation stratigraphique indiquerait que des séquences permotriasiques fortement affectées par le métamorphisme ont été souvent à tort interprétées comme plus anciennes, amenant ainsi a une sous-estimation de l'importance du métamorphisme alpin (BP-HT) dans le domaine tectonique alpujarride.

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Der Spessart als Teilgebiet der Mitteldeutschen Schwelle

Zusammenfassung Ein Teil der Mitteldeutschen Schwelle (R.Brinkmann 1948), der Spessart, wird sowohl bezüglich seines Innenbaues (Tektonik, Stratigraphie, Petrogenese des Grundgebirges), als auch bezüglich seiner Funktion als Lieferant für die jungpaläozoisch-mesozoischen Sedimente des Wetterau-Beckens betrachtet.Über diese Betrachtungen hinaus lassen sich Vergleiche zu anderen Gebieten der Mitteldeutschen Schwelle durchführen.Ganz offenbar ist die Grenzzone zwischen Saxothuringischer und Rhenoherzynischer Zone des Varistischen Gebirges in Mitteleuropa keine zufällige Ortslage für die Mitteldeutsche Schwelle und das Saar-Nahe-(Wetterau-)Becken. - Diese Kristallinzone hat tatsächlich zu verschiedenen Zeiten in der Erdgeschichte umgebungsdifferente Bewegungsimpulse erfahren. Sie ist darüber hinaus ohne Zweifel ein krustenversteifendes Element, was sich bereits mindestens aus ihrer modifizierenden Wirkung auf die großen Bruchsysteme der Mittelmeer-Mjösen-Zone im Bereich des nördlichen Oberrheingrabens und der südlichen Hessischen Senke ablesen läßt.

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The Spessart, as part of the Central German geanticline (Mitteldeutsche Schwelle, R.Brinkmann 1948) is discussed with respect to both its internal structure (tectonics, stratigraphy, petrogenesis of the basement) and its importance as a source region of Upper-Paleozoic-Mesozoic sediments which were deposited in the basin of the Wetterau. This leads to comparisons with other regions of the Central German geanticline.Obviously the regional situation of the Central German geanticline as well as the basins along Saar and Nahe(-Wetterau) between the hercynian zones of Saxothuringia and Rhenohercynia in Central Europe is not chance.The basement belt actually has been affected to vertical movements several times during its geologic history. Apart from this this belt is a consolidating element for the crust. This can clearly be seen by watching the great fault systems which can be traced from the Mediterranean to the Norwegian Lake Mjösen (Mittelmeer-Mjösen-Zone). They are modified by the Central German geanticline in the north of the Upper Rhine Valley and in the southern low-lands of Hesse.

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Résumé Une partie du bombement de l'Allemagne centrale (R.Brinkmann 1948), le Spessart, est étudiée tant sous l'aspect de sa structure interne (tectonique, stratigraphie, pétrogenèse du socle cristallin) que sous celui de son rôle comme domaine nourricier en sédiments du bassin Wetterau, du Paléozoïque supérieur au Mésozoïque.Partant de ces considérations, on peut faire des comparaisons avec d'autres régions du bombement de l'Allemagne centrale. Il est tout-à-fait évident que la zone limitrophe comprise entre les zones saxothuringienne et rhénohercynienne de la chaîne varisque de l'Europe Centrale n'est pas quelconque par rapport au bombement de l'Allemagne centrale et au bassin de la Sarre et de la Nahe (-basin de Wetterau). Cette zone cristalline a effectivement subi à différentes époques de l'histoire terrestre des mouvements distincts de ceux de son entourage. En outre elle constitue sans doute un élément qui a raidi la croûte terrestre, ce qui apparaît par l'influence modificatrice qu'elle a exercé dans la région septentrionale du Fossé du Rhin supérieur et de la dépression méridionale de la Hesse sur le grand système de fractures tectoniques de la zone Mer Méditerranée-Mjösen.

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Herrn Prof. Dr. Dr. h. c.Erich Bederke zum 70. Geburtstag gewidmet.     

Petrology of Serra Geral (Paraná) continental flood basalts,southern Brazil: crustal contamination,source material,and South Atlantic magmatism

The Serra Geral (Paraná) continental flood basalt (SG-CFB) province in Brazil is associated with the Jurassic-Cretaceous breakup of Gondwanaland and the transition of continental to oceanic magmatism during the opening of the South Atlantic Ocean. A suite of 24 samples representative of the SG-CFB in Rio Grande do Sul, southern Brazil, shows a compositional continuum from basalt (50–53 wt% SiO₂, Mg# 60-45), to basaltic andesite, to andesite, rhyodacite, and rhyolite (73 wt% SiO₂). Certain compositional aspects of the mafic rocks (e.g., TiO₂, K₂O, CaO, Zr/Nb, Zr/Y, Ti/Zr) resemble those of basaltic dikes and flows associated with the opening of the North Atlantic Ocean.Fractionation trends are apparent in MgO variation diagrams and calculations show that basalt-basaltic andesite continuums are largely due to removal of plagioclase and clinopyroxene. These mafic rocks can be categorized as (i) having higher or lower incompatible-element contents (e.g. K₂O 0.6–1.5 wt%; Rb 12–43 ppm; Ba 125–240 ppm) due to incorporation of Brazilian Archean crust or rhyolitic magma by basalt from a particular source material or to partial-melting differences of that source, and (ii) as having higher or lower TiO₂, Sr, Ba, and P contents due to source heterogeneities. Crustal components are obvious in certain basaltic samples, as where K₂O > 1 wt%, SiO₂ > 51%, and TiO₂ 1%, but are insignificant in others (e.g., compositions close to those of South Atlantic basin basalts). Calculations indicate origins for intermediate and silicic rocks by removal of pl, cpx, and Ti-magnetite from basaltic andesite, but crust and magma-mixing affecting basaltic-andesite fractionates were likely also involved. Where contamination is insignificant, Zr, Nb, and Y abundances indicate T-type MORE source material like that for certain S. Atlantic Ocean basalts. Source material was essentially a 91 hybrid of N-type and P-type MORB components. N-type MORB lithosphere followed SG-CFB because decompression due to rifting crust enabled partial melting of uppermost (depleted) mantle.     

Sulfur isotope ratios of Icelandic rocks

Sulfur isotope ratios have been determined in 27 selected volcanic rocks from Iceland together with their whole rock chemistry. The ³⁴S of analyzed basalts ranges from –2.0 to +0.4 with an average value of –0.8 Tholeiitic and alkaline rocks exhibit little difference in ³⁴S values but the intermediate and acid rocks analyzed have higher ³⁴S values up to +4.2 It is suggested that the overall variation in sulfur isotope composition of the basalts is caused by degassing. The small range of the ³⁴S values and its similarity to other oceanic and continental basalts, suggest that the depleted mantle is homogeneous in its sulfur isotope composition. The ³⁴S of the depleted mantle is estimated to be within the range for undegassed oceanic basalts, –0.5 to +1.0     

Sulphur and sulphate-oxygen isotopes and the origin of the silvermines deposits,Ireland

New sulphur and sulphate-oxygen isotope measurements for the main discordant and stratiform lead-zinc-barite orebodies at Silvermines Co. Tipperary, allow reappraisal of previously offered differing interpretations (Graham, 1970; Greig et al., 1971) of the bearing of sulphur isotopes on the genesis of this important Irish deposit. The following aspects of the data are confirmed: barite ³⁴ S-values range from 17–21, similar to lower Carboniferous seawater sulphate: stratiform sulphide lens pyrites have ³⁴ S-values ranging from –13 to –36; vein sulphide ³⁴ S-values range from –8 to 4; sulphide ³⁴ S-values increase upwards and outwards respectively in the related discordant and stratiform G orebodies; galena-sphalerite isotope palaeotemperatures are not too consistent, ranging from 40 to 430°C (using the calibration of Czamanske and Rye (1974). New facts are as follows: barite ¹⁸O-values range from –13 to –17, stratiform barites ranging from 13 to 14.5; sulphides separated from a single stratiform ore lens hand specimen usually have ³⁴ S_sl > ³⁴ S_ga > ³⁴ S_py; the outward decrease in ³⁴ S-values in the stratiform G orebody is confined to the first few hundred feet only; pyrite ³⁴ S-values progressively increase downwards through one stratiform sulphide orebody; yet variations of 13 occur within a single colloform pyrite structure from another stratiform orebody. It is concluded that there were at least two sources of sulphur, seawater sulphate and deep-seated sulphur. The former was the dominant source of all sulphate and, via biogenic reduction, of the sulphur in the bulk of the stratiform sulphide. The latter was the source of the sulphur in the vein sulphides. There was minimal isotopic interaction between the cool seawater sulphate and the warm unwelling ore fluid sulphur species, even though the latter precipitated under near isotopic equilibrium conditions when the temperature dropped and/or the pH and Eh increased. The lack of isotopic equilibrium between pyrite and ore sulphides in the stratiform ore lenses may result from the latter having precipitated slightly later than the former because of solubility relationships. Overall the present isotopic evidence supports considerable geological evidence favoring a syngenetic origin for the stratiform Silvermines orebodies.     

A preliminary study of D/H ratios of chlorites

Chemistry dependence of D of chlorites is inferred from data for natural chlorites. D of water equilibrated with those chlorites is estimated to be –2–8.     

Corundum,Cr-muscovite rocks at O'Briens,Zimbabwe: the conjunction of hydrothermal desilicification and LIL-element enrichment — geochemical and isotopic evidence

Monomineralic domains of chlorite, corundum and Cr muscovite coexist over a kilometer scale within ultramafic schists of the Harare greenstone belt (2.73 Ga). This exotic lithological association includes the conjunction of some of the most aluminous (Al₂O₃88 wt%) and potassic (K₂O10 wt%) rocks known. The paragenetic sequence developed from chloritecorundumcorundum+ diaspore: Cr muscovite variably overprinted both the corundum and chloritite domains. Terminal stages were marked by sporadic production of andalusite+quartz, and finally margarite.Chlorite (Cr₂O₃=0.31–2.65 wt%), corundum (0.79–2.66 wt%), and diaspore are all Cr-rich varieties. The chromian (Cr₂O₃3.86 wt%) paragonitic muscovite incorporates up to 17% of the paragonite molecule, and significant Mg and Fe substitutions.The suite of rocks are characterized by chondritic Ti/Zr ratios (–x=107), systematically enhanced Cr (up to 14000 ppm) and Ni (up to 1200 ppm) abundances, low levels of the alteration-insensitive incompatible elements Th, Ta, Nb. Chlorite, corundum and Cr muscovite represent progressive stages in the incremental metasomatic alteration of a komatiite precursor. Mass balance calculations, constrained by the isochemical behaviour of Ti, Zr and Hf reveal that the komatiite chloritite transformation involved volumetric contractions of 60% by hydrothermal leaching of Si, Fe, Mn, Ca and Na. Reaction of chloritite to corundum involved further volumetric reductions of 50% due to essentially quantitative loss of Si, Fe, Mn, Mg, K and Ca. Conversion of corundum to muscovite required additions of Si, K, Fe, Mn, Mg, Rb and Ba at 50–200% dilation. K, Rb, Ba, Li and Cs are enriched by up to 2×10³ over background abundances in ultramafic rocks, and the suite is also enriched in B, Se, Te, Bi, As, Sb and Au. REE were extensively leached during chloritite-corundum stages, whereas LREE additions accompany development of muscovite. Ti, Zr, Hf and Al were all concentrated by selective leaching of mobile components, but absolute additions of Al accompanied development of the corundum domains due to Al precipitation in response to depressurization.Corundum ( ¹⁸O=3.5–4.8), muscovite ( ¹⁸O=6.7–7.5) and chlorite (4.5–5.6) are isotopically uniform and formed at 380–520° C from a fluid where ¹⁸O=5.6–6.9. The corundum is ¹⁸O depleted relative to either igneous or anatectic counterparts (O_cor=7.6–8.2), or to gibbsitic laterites ( ¹⁸O=12–17).Previous genetic schemes involving metamorphism of exhalites or bauxite, or Si-undersaturation of magmas, can all be ruled out from the data. The chloritite, corundum, Cr-muscovite association represents sequential alteration products of ultramafic rocks by high temperature, low pH hydrothermal solutions carrying LIL-elements, and in which excursions of pH and/or degree of quartz undersaturation account for the mineralogical transitions. A deep level acid epithermal system, or fluid advection across steep inverted thermal gradients in a thrust regime could account for required hydrothermal conditions.     

Talksynthesen im Hinblick auf semisalinare Bedingungen

Zusammenfassung Vor einigen Jahren wurde Talk als Begleitmineral in Salzlagerstän erstmalig gefunden, und zwar in den amerikanischen Salzlagerstätten in Texas und Neu-Mexiko (Balley), in dem englischen Kalisalzvorkommen bei Eskdale in Ost-Vorkshire (Stewart), sowie im Hallstätter Salzberg des alpinen Haselgebirges (Mayrhofer undSChauberger). Auch in Deutschland wurde kürzlich bei der Erdgasbohrung Frenswegen 3 im Emsland in den oberen Lagen des Zechsteinanhydritprofils ein talkantiges Mineral identifiziert (Füchtbauer undGold-Schmidt). Die drei erstgenannten Talkvorkommen zeigen in der Art des Auftretens wesentliche gemeinsame Züge, aus denen man schließen kann, daß sie in der Lagerstätte selber entstanden sind, nicht aber dadurch, daß Talk als Verunreinigung in die Lagerstätte gelangte. Bei näherer Betrachtung bietet sich die Auffassung an, daß der Talk semisalinar (Leonhardt undBerdesinski) gebildet wurde, durch die Einwirkung Mg²⁺-haltiger Reaktionslösungen auf SiO₂-haltige Substanzen, die durch Zuflüsse usw. in die Lagerstätte gelangten. Von diesen Überlegungen ausgehend war zu prüfen, ob sich Talk wirklich bildet bei der Einwirkung von Mg-Laugen auf SiO₂, und zwar bei milden Metamorphosebedingungen, d. h. bei möglichst niedriger Temperatur und niedrigem Druck, entsprechend den wahrscheinlichen Bedingungen in den Salzlagerstätten. Bei den Versuchen wurden MgCl₂-haltige und Carnallit-Lösungen verwandt und stark gealtertes SiO₂-Gel, Quarz feinkristallin, Na₂Si₂O₅ · 2H₂O und Wasserglas als Bodenkörper gewählt. Die Reaktionsprodukte wurden vorwiegend röntgenographisch nach dem Debye-Verfahren untersucht und dann mit, abnehmendem Bildungsgrad in die 3 Gruppen: 1. Talk, 2. Talkprodukt, 3. Hinbildung zum Talk bzw. nur geringe Umbildung eingestuft.Um weitere, differenziertere Aussagen machen zu können, wurden typische Proben mit einer Talkvergleichssubstanz nach der Guiniermethode mit zweigeteilter Kamera untersucht; es wurden auch chemische Analysen und Anfärbtests durchgeführt und von bestimmten Synthesebildungen die mittleren Brechungsindizes bestimmt.Versuche mit SiO₂-Gel als Bodenkörper führten bei einer Reaktionstemperatur von 150° C noch zu guten Talkprodukten. Die 100° C-Versuche sind noch nicht abgeschlossen; ihre Ergebnisse bleiben einer späteren Veröffentlichung vorbehalten. Aber ein Versuch ergab bereits eine Hinbildung zum Talk. Die Synthesen mit Quarz als Bodenkörper erforderten wesentlich längere Reaktionszeiten, führten aber auch bis herab zu 150° C zur Hinbildung zum Talk. Bei Wasserglas (gallertig-dickflüssig) und Na₂Si₂O₅ · 2 H₂O waren diese Hinbildungen bis herab zu einer Reaktionstemperatur von 135° C zu verfolgen. So kann man feststellen, daß sich SiO₂-Substanzen semisalinar, bei milden Umwandlungsbedingungen zum Talk hin umsetzen lassen.     

Study of the distribution of Trace elements in soils in and around Mysore city,Karnataka

Soil samples collected from various places in and around Mysore were analyzed for the total trace elements such as Fe, Mn, Cu, Zn, Pb, and Cd. The results of the analysis indicate that the concentration of lead and cadmium in soils is below 2.5 g ml^–1 and 0.2 g ml^–1, respectively, which are the minimum detection levels, whereas the concentration of iron, manganese, copper, and zinc in most of the samples is within the global average ranges of 3%, 500–1000 g g^–1, 15–40 g g^–1, and 50–100 g g^–1, respectively. The investigated area has the presence of gneisses and schists, in which partly there are igneous intrusions and pegmatitic intrusions. There are amphibolite enclaves in gneisses that account for the higher concentration of trace elements. The lower concentration may be attributed to the presence of silicic type of rock.     

Deformation and metamorphism at the base of the Helgeland nappe complex,northwest of Grong (Northern Norway)

Pelites, carbonate-silicate rocks and mafic rocks occurring at the base of the Helgeland nappe complex northwest of Grong (N. Norway) were subjected together with the crystalline basement to medium grade metamorphism and threefold deformation during Caledonian times.During the first act of deformation temperature increased and reached a maximum of 600–650°C at 6–7 Kb after the cessation of the first deformation event. At the peak of metamorphism granitic, aplitic and pegmatitic rocks were formed.During the second and third acts of deformation the temperature decreased to 500–400°C. Mylonites, formed during the second act of deformation, indicate that there was nappe movement from NW to SE. Simple shearing with direction of the main elongation parallel to the fold axes was associated with the second deformation. During the third deformation simple shearing also occurred. However, elongation took place oblique to the fold axes and the sense of rotation was opposite to that of the second deformation.

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Zusammenfassung An der Basis des Helgeland-Komplexes nordwestlich von Grong (Nord-Norwegen) wurden pelitische, kalksilikatische und basische Gesteine zusammen mit dem Kristallinen Grundgebirge in kaledonischer Zeit von drei aufeinanderfolgenden Faltungsakten erfaßt. Dabei herrschten amphibolitfazielle Temperaturen.Während der ersten Deformation stieg die Temperatur an. Das Maximum von 600–650°C wurde, bei einem Druck von 6–7 Kb, nach der ersten und vor der zweiten Deformation erreicht. Gleichzeitig intrudierten Granite, Aplite und Pegmatite. Während der zweiten und dritten Deformation sank die Temperatur wieder auf 500–400°C. Im Zuge des zweiten Deformationsaktes entstanden stellenweise Mylonite, die auf Deckenbewegungen von NW nach SE hindeuten. Zweite und dritte Deformation sind beide rotational, während der zweiten liegt die Hauptdehnungsrichtung parallel zu den Faltenachsen, während der dritten aber schräg zu ihnen. Der Rotationssinn der dritten Deformation ist dem der zweiten entgegengesetzt.

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Résumé En Norvège septentrionale, au NW de Grong, la base du complexe d'Helgeland composée de metapélites, de roches calc-silicatiques et de roches mafiques, et le soubassement cristallin de ce complexe ont subi ensemble trois actes de déformation successifs à des températures du faciès amphibolites, pendant les temps calédoniens.Durant le premier acte la température a monté. Le maximum de 600–650° C, sous une pression de 6–7 kbar, a été atteint après la fin de cette déformation et avant la deuxième; des roches granitiques, aplitiques et pegmatitiques se sont formées au cours de ce stade. Durant le deuxième et le troisième acte de déformation, la température diminuait à 500–400° C. Des mylonites, indiquant un transport des nappes du NW vers le SE, ont été formées au cours de la deuxième phase. Celle-ci est caractérisée par un mouvement rotatoire par cisaillement simple, l'élongation étant parallèle aux axes des plis. La troisième phase était égalément une phase de mouvements rotatoires, mais en sens opposé et avec un élongation oblique aux axes.

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Monte Senga-Senga,a domal structure in Central Moçambique

A circular structure, first recognized on aerial photographs, occurs at the northern end of the Gairezi-Barué highlands in the province of Manica e Sofala. This feature, known as Mte. Senga-Senga, was visited briefly and was found to consist of a nearly perfectly circular core of granitic gneiss of Mozambique Orogenic Belt type overlain by outward-dipping quartzites and pelites of the late-Precambrian Umkondo System. Although the structure of this feature is problematical, it might be a mantled gneiss dome. Other domes occur elsewhere in the area, but Senga-Senga which is completely encircled by the mantle rocks, and may represent a high structural level, is particularly worthy of a more detailed study.

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Zusammenfassung Eine kreisförmige Struktur wurde auf Luftbildern am Nordende des Gairezi-Barué Hochlandes in der Provinz Manica e Sofala entdeckt. Eine Kurzbegehung dieser Struktur, die als Mte. Senga-Senga bekannt ist, ergab einen nahezu kreisrunden Kern aus granitischem Gneis des Mozambique-Gebirgsgürtel-Typus, welcher von nach außen geneigten Quarziten und Peliten des oberpräkambrischen Umkondo-Systems überlagert wird.Obwohl die Struktur dieses Gebildes problematisch ist, könnte es sich um einen verdeckten Gneisdom handeln. Es gibt andere Dome in der Region, jedoch ist Senga-Senga vollständig von Deckgebirge umgeben und kann ein hohes Strukturniveau repräsentieren. Weitergehende Studien wären sehr wertvoll.

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Résumé Une structure circulaire, d'abord reconnue sur photographies aériennes, existe dans la partie nord des Monts Gairezi-Barué dans la province de Manica et Sofala. Ce fait, connu sous le nom de Mte. Senga-Senga, a été visité brièvement et consiste en un noyau presque parfaitement circulaire de gneiss granitique appartenant à la zone orogénique du Mozambique; il est recouvert de quartzites et pélites inclinés vers l'extérieur et appartenant aux Systeme de Umkondo datées du Précambrien supérieur. Bien que cette structure soit problématique on peut penser à un dôme de gneis «coiffé». D'autres dômes se trouvent à d'autres endroits dans cette région, mais Senga-Senga qui est entièrement entouré par des roches de l'enveloppe et peut représenter un niveau structural élevé, mérite une étude plus détaillée.

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