Petrology and geochronology of low-pressure mafic granulites in the Marydale Group, South Africa

Granulite- and amphibolite-facies metabasites occur within the Archaean Marydale Group (3.0 Ga) along the western edge of the mid-Proterozoic Kheis Tectonic Subprovince (1.8–1.3 Ga) of South Africa. At the northern end of the exposed Marydale Group, the metabasites are infolded with overlying quartzites from which they are separated by a low-angle fault contact. They contain two pyroxenes, hornblende and bytownite, but show widespread retrogression to coronas of almandine and hornblende. Geothermometric data for these assemblages indicate peak equilibration of the two-pyroxene assemblage at 690–760°C, and retrograde equilibration of garnet-hornblende pairs at 600–650°C. Barometric data are more uncertain though an estimate of 3–5 kbar is made from a consideration of hornblende chemistry. Using previously published data, a near-isobaric retrograde P-T path is inferred.

Rb---Sr ages of whole-rock hypersthene tonalites and mylonitized granites yield ages of 1353 ± 33 and 1355 ± 20 Ma, respectively, interpreted as the age of isotopic resetting during granulite-facies metamorphism. K---Ar hornblende ages of 1228 ± 61 and 1070 ± 48 Ma are recorded from fresh and sheared granulite-facies metabasites, respectively. These ages data the P-T path and show that the granulite-facies metamorphism predates the adjacent Namaqua orogeny that reset Rb---Sr systematics at ±1210 Ma.     

Seismic and sedimentologic features of Oxfordian-Kimmeridgian syn-rift sediments on the eastern margin of the Lusitanian Basin

Following deposition of widespread middle Oxfordian lacustrine carbonates and evaporites, the Lusitanian Basin was differentiated into a number of sub-basins. The Arruda sub-basin is a half graben basin situated some 30 km north of Lisbon. It accumulated over 2.5 km of Kimmeridgian siliciclastic sediments, and is bounded to the east by the Vila Franca de Xira fault zone. Carbonate deposition persisted over horsts along the fault zone from the Oxfordian to the early Kimmeridgian, and in places to the late Kimmeridgian, and shows a pronounced west-east facies zonation, with higher energy framestones and grainstones accumulating along the exposed western margins. Seismic data indicate a major gap between the horst blocks that acted as a conduit through which basement derived siliciclastics were fed westwards into the sub-basin to form a submarine fan system. The presence of large blocks of framestone carbonates encased in siliciclastics indicates that carbonate sedimentation occurred in abandoned parts of the fan system. The rapid changes of sediment thicknesses and facies types along the eastern margin of the Arruda sub-basin are indicative of contemporaneous strike-slip movements.

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Zusammenfassung Nach der weitverbreiteten Ablagerung mitteloxfordischer Seenkarbonate und Evaporite differenzierte sich das Lusitanische Becken in verschiedene Sub-Becken. Das Arruda Sub-Becken befindet sich ca. 30 km nördlich Lissabon und entspricht einem Halbgraben, in dem über 2.5 km mächtige Siliziklastika des Kimmeridgium zur Ablagerung kamen. Das Sub-Becken wird durch die Störungszone von Vila Franca de Xira nach Osten begrenzt. Auf Horststrukturen entlang der Störungszone wurden im Oxfordium und Unterkimmeridgium, z. T. auch bis ins Oberkimmeridgium Karbonate sedimentiert. Diese Karbonatschelfe weisen eine ausgeprägte Fazieszonierung von West nach Ost auf. Die westlichen Schelfränder sind durch höherenergetische Riffkalke (framestones) und Karbonatsande (grainstones) charakterisiert. Seismische Profile lassen eine große Lücke zwischen den Horstblöcken erkennen, durch welche Siliziklastika aus dem Hinterland ins Sub-Becken gelangten, wo sie einen submarinen Fächer aufbauten. Große Riffkalkblöcke innerhalb der Siliziklastika weisen auf Karbonatsedimentation in verlassenen Fächerbereichen hin. Die schnellen Mächtigkeits- und Fazieswechsel entlang des Ostrandes des Arruda Sub-Beckens sind durch synsedimentäre tektonische Bewegungen zu erklären, welche oftmals eine Dominanz der Lateralkomponente aufweisen.

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Resumo Depois da sedimentação dos calcários lacustres e depósitos evaporíticos da idade Oxfordiano médio, a Bacia Lusitánica diferenciou-se em várias sub-bacias. A sub-bacia de Arruda está situada ca. de 30 km ao norte de Lisboa e corresponde a uma estrutura «half-graben» em que mais do que 2.5 km de sedimentos foram acumulados. Para leste, a sub-bacia é limitada pela zona das falhas de Vila Franca de Xira. Entre o Oxfordiano e o Kimeridgiano, calcários desenvolveram-se em cima dos blocos elevados (horsts) ao longo da zona de falhas. Estes calcários de tipo plataforma exibem uma distincta zonação de facies de oeste a leste. As margens occidentals das plataformas pequenas são caracterisadas por sedimentos recifais e areníticos. Cortes sísmicos indicam uma abertura grande entre os blocos elevados, pelo quai sedimentos siliciclásticos passaram de hinterland à sub-bacia formando um fan submarino. Grandes blocos recifais situados dentro dos depósitos siliciclásticos são evidências para sedimentação carbonática em várias áreas abandonadas dentro do fan. Mudanças rápidas das espessuras e das fácies dos sedimentos ao longo da margem oriental da sub-bacia de Arruda podiam ser explicadas por uma tectónica sinsedimentária dominada por movimentos horizontais.

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The influence of marine inflows on the chemical composition of groundwater in small islands: the example of the Cyclades (Greece)

 Marine contamination of groundwater may be caused by seawater intrusion and by salt spray. The role of both processes was studied in the Cyclades archipelago on four small islands (45–195 km²) whose aquifers consist essentially of fractured, weathered metamorphic rocks. Annual rainfall ranges from 400 to 650 mm and precipitation has high total dissolved solids contents of 45–223 mg l^–1. The chemical characteristics of the groundwater, whose salinity is from 0.4 to 22 g l^–1, are strongly influenced by seawater intrusion. However, the effect of atmospheric input is shown in certain water sampling locations on high ground elevation where the dissolved chloride contents may attain 200 mg l^–1. Received: 14 November 1995 · Accepted: 9 September 1996     

Geochemistry of Tertiary tholeiites and picrites from Qeqertarssuaq (Disko Island) and Nuussuaq, West Greenland with implications for the mineral potential of comagmatic intrusions

Tertiary continental flood basalts on Qeqertarssuaq and Nuussuaq in West Greenland contain ～3?km of picrites and variably contaminated tholeiites. The picrites are in the Naujánguit member of the Vaïgat Formation and they have 7–29?wt% MgO, La/Sm?=?0.9–2.1, and ¹⁴³Nd/¹⁴⁴Nd?=?0.51263–0.51307. They appear to have crystallised from high-Mg parental magmas (14.4–16.4?wt% MgO) with isotope and trace element ratios similar to recent Icelandic picrites. Discrete horizons of tholeiites, including the Asûk and?Kûgánguaq, have elevated SiO₂ (50–58 wt%), La/Sm?=?3–7, ⁸⁷Sr/⁸⁶Sr?=?0.70550–0.71224, and low ¹⁴³Nd/¹⁴⁴Nd?=?0.51234–0.51174. These lavas have low Cu and Ni abundances (typically 10–50?ppm Ni or Cu), and in the case of the Asûk on Qeqertarssuaq, they contain droplets of native iron. The low Cu and Ni contents are attributed to scavenging by magmatic sulphides formed in response to crustal contamination of picritic magmas. Two contamination trends are recognised, one to a sediment end-member with high Th/Nb and Archaean model Nd ages, and the other to a meta-igneous component with high La/Sm, low Th/Nb and Rb/Nb, and Proterozoic source ages. Overall, ²⁰⁶Pb/²⁰⁴Pb varies from 16.47–21.68. Both contamination trends are associated with low Cu and Ni, and high SiO₂, and it is argued that the magmatic sulphides were triggered by the increases in silica, rather than simply by the introduction of additional crustal-derived sulphur. Geochemically, the Asûk and Kûgánguaq rocks resemble the most contaminated Nadezhdinsky lavas of the Siberian Trap, which are widely regarded as the source of the Ni and Cu mineralisation in the giant Noril'sk deposits. Mass balance considerations indicate that the parental liquids to the contaminated magmas contained sufficient Ni, Cu, S and platinum group elements to form substantial magmatic sulphide deposits. However, unlike the lavas at Noril'sk, the contaminated (low Cu and Ni) West Greenland basalts are in isolated units with no evidence for a gradual recovery in Ni and Cu abundances with height in the lava column. Comparison with Noril'sk suggests that although significant quantities of metals were scavenged by sulphides in West Greenland, the metal contents of the sulphides may not have been upgraded by continued interaction with subsequent magma batches.     

Fluid inclusion study of the Ballinglen W-Sn-sulphide mineralization, SE Ireland

Scheelite-mineralized microtonalite sheets occur on the SE margin of the end-Caledonian Leinster Granite in SE Ireland. Scheelite, polymetallic sulphides and minor cassiterite occur in veins in the microtonalites, disseminated throughout the greisened microtonalite sheets and in the adjacent wallrocks. Two major mineralized vein types occur in the microtonalite sheets: (1) Scheelite ± arsenopyrite ± pyrrhotite occur in quartz-fluorite veins, generally without a muscovite selvage; (2) Sphalerite ± chalcopyrite ± pyrite ± galena ± cassiterite ± stannite occur in quartz + fluorite veins with a coarse muscovite selvage and are often intergrown with the muscovite. Quartz-hosted fluid inclusions were examined from representative samples of both vein types using petrographic, microthermometric and laser Raman spectroscopic techniques. Three distinct types of fluid inclusions have been recognized. Primary, vapour rich Type 1 inclusions in quartz from the scheelite-mineralized veins are of H₂O-CO₂-CH₄-N₂ ± H₂S ± NaCl composition and formed between 360–530 °C. Primary and secondary, liquid-rich Type 2 fluid inclusions in the base metal sulphide-mineralized veins are of H₂O-CH₄-N₂ ± H₂S-NaCl composition and formed between 340–480 °C. They also occur as pseudosecondary and secondary inclusions in scheelite-mineralized veins. Late dilute, low temperature H₂O-NaCl + KCl fluid inclusions may be related to late-Caledonian convection of meteoric waters around the cooling Leinster Granite batholith. Received: 4 September 1996 / Accepted: 23 May 1997     

The reaction MgCr2O4 + SiO2 = Cr2O3 + MgSiO3 and the free energy of formation of magnesiochromite (MgCr2O4)

Low-temperature heat capacity measurements for MgCr₂O₄ have only been performed down to 52 K, and the commonly quoted third-law entropy at 298 K (106 J K⁻¹ mol⁻¹) was obtained by empirical extrapolation of these measurements to 0 K without considering the magnetic or electronic ordering contributions to the entropy. Subsequent magnetic measurements at low temperature reveal that the Néel temperature, at which magnetic ordering of the Cr³⁺ ions in MgCr₂O₄ occurs, is at ∼15 K. Hence a substantial contribution to the entropy of MgCr₂O₄ has been missed. We have determined the position of the near-univariant reaction MgCr₂O₄+SiO₂=MgSiO₃+Cr₂O₃. The reaction, which has a small positive slope in P-T space, has been bracketed at 100 K intervals between 1273 and 1773 K by reversal experiments. The reaction is extremely sluggish, and lengthy run times with a flux (H₂O, BaO-B₂O₃ or K₂O-B₂O₃) are needed to produce tight reversal brackets. The results, combined with assessed thermodynamic data for Cr₂O₃, MgSiO₃ and SiO₂, give the entropy and enthalpy of formation of MgCr₂O₄ spinel. As expected, our experimental results are not in good agreement with the presently available thermodynamic data. We obtain Δ _f H ^∘ ₂₉₈=−1759.2±1.5 kJ mol⁻¹ and S ^∘ ₂₉₈=122.1±1.0 J K⁻¹ mol⁻¹ for MgCr₂O₄. This entropy is some 16 J K⁻¹ mol⁻¹ more than the calorimetrically determined value, and implies a value for the magnetic entropy of MgCr₂O₄ consistent with an effective spin quantum number (S') for Cr³⁺ of 1/2 rather than the theoretical 3/2, indicating, as in other spinels, spin quenching. Received: 9 May 1997 / Accepted: 28 July 1997     

Environmental geology problems in the Tyrrhenian coastal area of Santa Marinella, province of Rome, central Italy

 The Tyrrhenian resort of S. Marinella (central Italy) is subjected to significant anthropogenic pressures during the summer vacation period, a common situation all along the Italian coast. Located 65 km NW of Rome on the southern slopes of the Tolfa Mountains, S. Marinella is built on a gently sloping, E–W trending belt which is cut by 14 N–S oriented ephemeral streams that discharge into the Tyrrhenian Sea. The low to medium permeability turbiditic sandstones which outcrop along this belt belong to the Late Cretaceous Pietraforte unit. Three environmental problems are addressed in this study. The first problem is related to the high water supply demand during the summer months which has forced local residents to dig a large number of wells. Extensive pumping from these wells has caused salt-water intrusion into the Pietraforte, thus compromising the domestic use of the groundwater. The second problem consists of the illegal dumping of urban solid waste, material that represents a hazard during significant rain events as well as a possible cause of groundwater contamination. The final issue addressed concerns the flooding potential of the 14 ephemeral streams that cross the inhabited area of S. Marinella, a risk which is highlighted by the disastrous flood which occurred on 2 October 1981 and during the period of the Roman Emperor Settimio Severo (205 A.D.). Some suggestions are proposed to mitigate and contain the effects of these problems. Received: 7 November 1995 / Accepted: 5 December 1996     

The economic mineral potential of the mid-Proterozoic Waterberg Group, northwestern Kaapvaal craton, South Africa

The 1900–1700 Ma Waterberg Group in the main Waterberg fault-bounded basin consists of dominantly coarse siliciclastic red beds with minor volcanic rocks. The sedimentary rocks were deposited mainly by alluvial fans, fluvial braidplains and transgressive shallow marine environments, with lesser lacustrine and aeolian settings. Uplifted, largely granitic source areas were located along the Thabazimbi-Murchison lineament (TML) fault system in the south, and along the Palala shear zone in the northeast. Palaeoplacer titanomagnetite-ilmenite-zircon heavy mineral deposits, best developed in the Cleremont Formation in the centre of the basin, reflect initial fluvial reworking and subsequent littoral marine concentration. Coarse alluvial cassiterite placer deposits are found in the Gatkop area in the southwest of the basin, and appear to have been derived from stanniferous Bushveld Complex lithologies south of the TML. Hydrothermal zinc and U-Cu mineralisation in the Alma lithologies in the same area appears to be related to the TML fault system. Small manganese deposits and anomalous tungsten values occur in the south of the basin, where they are again closely spatially associated with the TML. Copper-barium mineralisation is found associated with dolerite dykes, and in stratigraphically controlled, inferred syngenetic settings. The most interesting of these apparently syngenetic occurrences is found within green coloured reduced mudrocks and inferred volcanic rocks, at an unconformity developed within the overall red bed sequence of the Waterberg Group, adjacent to the TML in the southwest of the basin. The most important potential mineralisation in the main Waterberg basin thus encompasses shoreline placer Ti and the possibility of substantial sediment-hosted copper deposits. Received: 31 May 1996 / Accepted: 17 February 1997     

Carbonatites from Eastern Paraguay and genetic relationships with potassic magmatism: C, O, Sr and Nd Isotopes

Summary Geochemical characteristics were systematically determined for Early Cretaceous samples of carbonatitic rocks from Eastern Paraguay (Rio Apa, Amambay and Central Provinces). The data show that all the occurrences have an enriched isotopic signature and that the carbonatites have negligible or absent crustal signature. A petrogenetic model (parent liquids, fractional crystallization, hydrothermal interactions and weathering) is proposed as a function of incompatible trace element, stable (O-C) and radiogenic (Sr-Nd) isotope variations with the aim to test the significance of carbonatitic complexes as a marker of the metasomatized subcontinental lithospheric mantle. The results indicate that the carbonatites and primary carbonates from eastern Paraguay, and those from the north eastern Paraná Basin (SE Brazil), were affected by metasomatic events distinct in time and composition.

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Karbonatite aus Ost-Paraguay und ihre genetische Beziehung zu Kalium-Magmatismus: C O, Sr und Nd isotope

Zusammenfassung Die geochemischen Charakteristika von frühkretazischen Karbonatitproben aus Ostparaguay (Rio Alpa, Amambay und Zentrale Provinzen) wurden untersucht. Die Daten belegen, daß alle Vorkommen eine isotopische Anreicherungssignatur zeigen und daß ihnen eine entsprechende Krustensignatur fehlt. Ein Petrologisches Modell (Ausgangsschmelze, fraktionierte Kristallisation, hydrothermale Interaktion und Verwitterung) wird auf Grund der Verteilung der inkompatiblen Spurenelemente, der stabilen (C-O) und radiogenen (Sr-Nd) Isotope vorgeschlagen. Es versucht die Bedeutung der Karbonatitkomplexe als Markerhorizonte des metasomatischen subkontinentalen Mantels zu überprüfen. Die Ergebnisse zeigen, daß die Karbonatite und die primären Karbonate in Ostparaguay, und jene aus dem Paraná Becken SüdostBrasiliens durch zeitlich und zusammensetzungsmäßig unterschiedliche metasomatische Prozesse erfaßt wurden.

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With 10 Figures     

High-resolution sequence stratigraphy of the Lower Triassic ‘Buntsandstein’ in the Netherlands and northwestern Germany

Detailed log correlations of the largely fluvio-lacustrine Lower Triassic Buntsandstein (Late Permian-Early Anisian), carried out on 80 wells in the Dutch onshore and offshore areas, can be linked to northwest-German high-resolution sequence stratigraphy. The correlations show that cyclic sedimentation occurred in large parts of the basin. Seven 1st-order sequences are recognised, namely the Main Claystone, Rogenstein, Volpriehausen, Detfurth, Hardegsen, Solling and Lower Röt Sequences. They are overlain by the lower part of the Upper Röt–Lower Muschelkalk Sequence. Distinct sequence boundaries have been identified at the bases of four sequences: Volpriehausen, Detfurth, Solling and Upper Röt. The higher-order sequences consist of fining-upwards cycles with a thickness of up to tens of metres. The sequences are laterally persistent and have a characteristic expression on gamma-ray and sonic logs. In the Lower Buntsandstein, they display a uniform character throughout most of the area, with only minor differences in thickness or lithology. NNE-oriented lows and swells were formed during deposition of the Volpriehausen, Detfurth and Hardegsen Sequences. Uplift prior to the deposition of the Solling Sequence caused deep erosion on the swells in the basin and minor erosion in the lows. The high-resolution sequences probably represent alternating, relatively wet and dry climatic periods, with a periodicity of about 100 000 years. An analysis of the sequences suggests that their reduced thickness on the swells is mainly the effect of erosion. This is supported by analyses of the accumulation patterns and rates.