The Anna's Rust Sheet and related gabbroic intrusions in the Vredefort Dome-Kibaran magmatic event on the Kaapvaal Craton and beyond?

The Anna's Rust Sheet (ARS) and a suite of mineralogically and chemically related intrusions in the core and collar of the Vredefort Dome (in particular, the Vredefort Mafic Complex: VMC) represent a newly recognised type of high Ti gabbro in this central part of the Kaapvaal Craton. This lithology, referred to as the Vredefort Type IV mafic intrusion, is distinguished from chemically similar Type V intrusions (the Karoo dolerites) by the presence of glomeroporphyritic plagioclase and higher Th content and from Type III intrusions (≈ 1600 Ma gabbro) by the lack of cross-cutting pseudotachylitic breccia veinlets. Petrographic features and both major and trace element compositions of all Type IV intrusions are very similar. Based on its Rb-Sr isotope age and character, a gabbroic intrusion from Majuba Colliery (Mpumalanga Province) is also thought to belong to the ARS (Type IV) suite of tholeiitic intrusions. Rb-Sr isotopic analysis resulted in a preferred age of 1052±11 Ma (2ω) for biotite and plagioclase data for ARS, VMC and Majuba samples. The Rb-Sr age for the ARS is further supported by ⁴⁰Ar-³⁹Ar stepheating ages for plagioclase and pyroxene separates from two ARS and VMC samples, which favour formation of this gabbroic intrusion at ca 1000 Ma ago. These results suggest that an ≈ 120 m thick sheet intrusion may be present throughout a major part of the Vredefort Dome. While Kibaran-age (ca 1–1.2 Ga) alkaline, both mafic and felsic, magmatism, as well as tectonic and hydrothermal activity at that time, have been known in the central Kaapvaal Craton, a widespread tholeiitic magmatic component has now been added to this record. There is a strong likelihood that this magmatic event occurred throughout the southern African subcontinent and perhaps into Antarctica.     

Stratigraphic constraints on suture models for eastern Indonesia

Although collision in eastern Indonesia is now accreting the Australian continent to Southeast Asia, the small North and South Banda oceanic basins within the suture zone are interpreted as Late Cenozoic extensional features. Stratigraphic columns from the surrounding islands conform to one of three generalised patterns, two of which can be related to the margins of SE Asia (Sundaland) and the Australian continent, respectively. The third system, which is dominant in the outer Banda Arc and eastern Sulawesi, is associated with a microcontinent that was rifted from Australia in the Jurassic, drifted northwards ahead of Australia in the Cretaceous and collided with the Sundaland Margin in the Paleogene. Subsequent collapse of the resulting collision orogen led to rapid extension and the formation of the Banda Sea behind the Outer Banda Arc thrust belt. Eastern Indonesia thus duplicates a pattern familiar in the Mediterranean. The Tertiary compressional structures of the region cannot be explained solely in terms of the most recent collision, which began only in the Pliocene.     

Holocene Foraminifera as Indicators of Relative Estuarine–Lagoonal and Oceanic Influences in Estuarine Sediments of the River Murray, South Australia: Volume 53, Number 3 (2000), pages 378–391

In southeastern South Australia, the River Murray debouches through a coastal barrier separating euryhaline estuarine-lagoonal waters from the Southern Ocean. Depending upon the relative freshwater outflow of the river and ingress of the ocean, water salinity varies greatly within the lower estuary. Ammonia beccarii and Elphidium articulatum are euryhaline species of foraminifera that characterize the estuary and back-barrier Coorong Lagoon. The inner-shelf marine environment hosts an assemblage in which Discorbis dimidiatus, E. crispum, E. macelliforme, and various cibicidid species predominate. In cored sediments recovered from the shallow lower estuary, the relative abundance of A. beccarii + E. articulatum was compared with that of D. dimidiatus + E. crispum + E. macelliforme + other species. These data, and AMS radiocarbon ages determined for foraminifera and ostracods, provide evidence of a change from maximum oceanic influence (5255 ± 60 yr B.P.) to maximum estuarine influence (3605 ± 70 yr B.P.). Over this same time interval, sea level fell relatively by about 2 m. However, the event was also contemporaneous with falling water levels in several Victorian lakes, and it is thus attributed to onset of climatic aridity. Reduced precipitation in the River Murray catchment and reduced freshwater outflow enhanced development of the flood-tide delta and constriction of the mouth.     

Chromite in Komatiites, II. Modification during Greenschist to Mid-Amphibolite Facies Metamorphism

Chromite compositions in komatiites are influenced by metamorphicprocesses, particularly above 500°C. Metamorphosed chromiteis substantially more iron rich than igneous precursors, asa result of Mg–Fe exchange with silicates and carbonates.Chromite metamorphosed to amphibolite facies is enriched inZn and Fe, and depleted in Ni, relative to lower metamorphicgrades. Relative proportions of the trivalent ions Cr³⁺, Al³⁺and Fe³⁺ are not greatly modified by metamorphism up to loweramphibolite facies, although minor Fe³⁺ depletion occurs duringtalc–carbonate alteration at low temperature. SignificantAl is lost from chromite cores above 550°C, as a resultof equilibration with fluids in equilibrium with chlorite. ElevatedZn content in chromite is restricted to rocks with low (metamorphic)Mg/Fe ratios, and is the result of introduction of Zn duringlow-temperature alteration, with further concentration and homogenizationduring prograde metamorphism. Cobalt and Mn also behave similarly,except where carbonate minerals are predominant in the metamorphicassemblage. Chromite at amphibolite facies is typically extensivelyreplaced by magnetite. This is the result of incomplete metamorphicreaction between chromite and chlorite-bearing silicate assemblages.Magnetite compositions at the inner chromite–magnetiteboundary are indicators of metamorphic grade. KEY WORDS: chromite; komatiite; spinel; metamorphism; Zn     

The Late Cretaceous Impact of the Trindade Mantle Plume: Evidence from Large-volume, Mafic, Potassic Magmatism in SE Brazil

When the subcontinental lithospheric mantle undergoes heatingand/or extension, some of the earliest mafic melts to be generatedare those rich in volatUes and potassium. In some cases, e.g.when a plume impinges on thick cratonic lithosphere or whenthe amount of extension is very small, K-rich mafic igneousrocks may be the only surface expression of mantle melting.The Alto Paranaiba Igneous Province, in SE Brazil, is one ofthe world's most voluminous mafic potassic provinces (>15000km³),which until recently was relatively unknown. The magmas wereemplaced into a narrow Proterozoic mobile belt close to thesurface margin of the Sao Francisco craton, and it is one ofseveral Cretaceous alkaline igneous provinces that are locatedaround the margin of the Parana sedimentary basin in Braziland Paraguay.Detailed geochemical analyses of samples from throughoutthe Alto Paranaiba Igneous Province show that it is composedof a relatively diverse suite of ultrapotassic-potassic, ultramaficmqfic,silica-undersaturated lavas and hypabyssal intrusions, i.e.kimberlites, madupitic olivine lamproites and kamafugitic rocks.These all have very high concentrations of incompatible traceelements and are all strongly enriched in light rare earth relativeto heavy rare earth elements (e.g. La/Yb=50-230). Wide variationsin major element ratios, which are unrelated to the effectsof crystal fractionation in these magmas (e.g. CaO/Al₂O₃), suggestthat the mafic potassic rocks were derived from a heterogeneousmantle source. They show relatively restricted ranges of initial⁸⁷Sr/⁸⁶Sr (070436-070588) and Nd₂₅ values of -4 to -8, intermediatebetween Group I and II South African kimberlites. T_DM Nd isotopemodel ages of 900 Ma suggest that the magmas were derived bythe remobilization of subcontinental lithospheric mantle thathad been enriched by small-volume K-rich melt fractions sincethe Late Proterozoic.New K/Ar ages for mica separates show thatthe kimberlites, madupitic olivine lamproites and kamafugiticrocks were emplaced together with large carbonatite-bearingplutonic complexes at 85 Ma. Reconstructions of plate motionsshow that, at this time, the location of the Alto ParanaibaIgneous Province coincided with the postulated position of thepresent-day Trindade(or Martin Vaz) plume. We propose that thewidespread Late Cretaceous alkaline magmatism in SE Brazil mayhave been caused by impingement of this plume on the base ofthe subcontinental lithosphere. Heat penetrating the lithosphere,both by conduction and advection by asthenospheric-source decompressionmelts, may have caused melting of the readily fusible partsof the lithospheric mantle and the genesis of mafic potassicand (after fractionation) carbonatite magmas. The Proterozoicmobile belt (the Brasilia Belt) appears to have acted as a Hhinspofrelative to the adjacent Sao Francisco craton, allowing greaterupwelling and melting of the asthenosphere. Subsequently, asthe craton passed over the plume, volcanism was switched off'until the Early Tertiary when the plume reemerged from beneaththe westward drifting South America continent and was the magmasource for oceanic-islands and seamounts of the Trindade-Vitriachain. Corresponding author     

Engineering-geological mapping of tropical soils for land-use planning and geotechnical purposes: A case study from Jamaica, West Indies

An area of a Tertiary and Cretaceous deposits, 100–310 m above sea level and on predominant 20–45°, highly dissected, concave slopes was investigated to assess its geotechnical characteristics and land use planning implications. Lithologies include turbidites, fluvial conglomerates, breccias, sandstones, mudrocks, carbonates, granodiorite, acid dykes, chloritized and epidotized volcanics, andesite and Holocene fluvial deposits. Bedrock is highly fractured and sheared, hydrothermally altered and highly weathered. Soils are quite variable, with a strong geological control on soil properties. Soils are generally sandy to gravely, with local silts and clays in mudrocks, within Holocene alluvium or in hydrothermally altered and sheared granodiorites. The soils are composed of 0.14–80.00% gravel; 5.36–62.50% sand, 2.33–50.55% silt, 0.17–51.50% clay, and total fine content between 2.50 and 94.50%, with natural moisture between 4 and 44% during the dry season, plastic limit between 6 and 35%, liquid limit between 19 and 83% and plasticity index between 1 and 59%. Sandy soils have residual friction angles between 19 and 39°, PI values less than 35% with cohesion between 1.00 and 5.27 KN/m² for cohesive samples. ASTM classification of soils include soil groups GC, GM, GW, GP, SP, SM, SC, SM, SC, ML, CL and CH.

Based on the characteristics of the terrain, the geotechnical and land use planning problems include high landslide frequency and susceptibility, soil erosion, fluvial and reservoir sedimentation, high debris flow hazard, cut slope failures, potentially expansive clays and silts, seepage erosion and soil piping, differential settlement in interlayered competent and incompetent lithologies, high solution erosion and potential subsidence over limestones, waste water disposal problems and groundwater pollution and seismic induced settlement and ground failures. These phenomena suggest that systematic site investigations should be conducted prior to the utilization of these areas for construction and development in order to minimize the deleterious effects resulting from ground failure.     

An engineering geological appraisal of the sandstones of the Clarens Formation,Lesotho, in relation to tunnelling

Summary The Delivery Tunnel South forms part of a system of tunnels for conveying water from Lesotho to South Africa. The tunnel was excavated primarily by tunnel boring machine in the sandstones of the Clarens Formation. These sandstones are uniformly graded and fine grained with quartz being the most abundant mineral. They are mostly of aeolian origin. Their unconfined compressive strength varies appreciably, from moderately strong to extremely strong. The boreability of the sandstones was investigated by the Norwegian Institute of Technology tests and the total hardness. These tests showed that these sandstones would be relatively easy to bore, although moderately abrasive. The prediction proved correct. In fact, excavation of the tunnel was completed 20 months ahead of schedule, the rock conditions being better than expected. The long-term durability of these sandstones was investigated to establish criteria for concrete lining of the tunnel. A series of tests were undertaken to evaluate the durability. These included a number of soaking tests using different fluid media, wet and dry testing, erosion tests and brushing tests. This testing programme suggested that concrete lining of the intact sandstone was probably only required where the strength of the sandstone was less than 20 MPa and that therefore only a small percentage of the tunnel need be lined.     

Über Eckfluren und andere Probleme der Talbildung

Ohne Zusammenfassung     

Petrology and Age of the Pre-Otavi Basement Granite at Franzfontein, Northern South-West Africa

Chemical, spectrographic, and isotopic analyses are presentedfor the Franzfontein alkali granite and constituent minerals.This rock has the chemical character of granties produced byliquid crystal equilibrium. Dated at 1, 700?70m.y. by the constituentzircon, its crystallization formed part of a major period (theHuabian episode) of batholithic granite emplacement in northernSouth-West Africa. The occurrence of these crystalline rocksin the core of the Huab anticline defines the maximum possibleage of the overlying Otavi Facies sediments, precluding theircorrelation with the Transvaal System of South Africa. The imprint of the Damara metamorphism (Damaran episode) isreflected in the Sr/Rb age (560?30 m.y.) obtained for the biotite:the inversion of biotite to stilpnomelane and chlorite probablyrepresents the mineralogical effects of that metamorphis. Isotopicdata indicate that changes in the relative concentrations ofRb and Sr differed significantly in plagioclase and microcline;such data from feldspars in metamorphic rocks should, therefore,be interpreted with caution.     

Bemerkung zu dem „Serpentintalk” aus dem Zechsteinanhydritprofil der Erdgasbohrung Frenswegen 3 im Emsland

Zusammenfassung Im folgenden werden die Röntgendaten — mit Einschränkung auch die Brechungsindizes — des von H.Füchtbauer und H.Goldschmidt untersuchten Serpentintalks den Untersuchungsergebnissen eigener Talksyntheseprodukte gegenübergestellt. Es zeigt sich eine sehr gute Übereinstimmung. Die Talksynthesen wurden im Hinblick auf semisalinare Mineralbildungen in Salzlagerstätten bei milden Metamorphosebedingungen durchgeführt. Ein Vergleich mit den Untersuchungsergebnissen der thermischen Zersetzung und der Differentialthermoanalyse, wie sie von H.Füchtbauer und H.Goldschmidt durchgeführt wurden, war nicht möglich, da die Reaktionsprodukte nicht nach diesen Methoden untersucht werden konnten. Für eine nähere Bestimmung des Serpentintalks wäre eine Gegenüberstellung weiterer Untersuchungsergebnisse sehr wichtig.