Petrology of Whiteschists and Associated Rocks at Mautia Hill (Tanzania): Fluid Infiltration during High-Grade Metamorphism?

Talc–kyanite schists (whiteschists), magnesiohornblende–kyanite–talc–quartzschists and enstatite–sapphirine–chlorite schistsoccur at Mautia Hill in the East African Orogen of Tanzania.They are associated with metapelites and garnet–clinopyroxene–quartzmetabasites. Geobarometry (GASP/GADS equilibria) applied tothe latter two rock types indicates a peak pressure of P = 10–11kbar. These results are confirmed by the high fO₂ assemblagehollandite–kyanite–quartz and late-stage manganianandalusite that contains up to 19·5 mol. % Mn₂SiO₅. Maximumtemperatures of T = 720°C are inferred from late-stage yoderite+ quartz. A clockwise P–T evolution is constrained byprograde kyanite inclusions in metapelitic garnet and late-stagereaction rims of cordierite between green yoderite and talcthat reflect conditions at least 3–4 kbar below the peakpressure. Oxidizing conditions are recorded throughout the metamorphichistory of the whiteschists and chlorite schists, as indicatedby the presence of haematite coexisting with pseudobrookiteand/or rutile. Increasing water activity near peak pressuresis thought to have led to the breakdown of the high-pressureassemblages (Tlc–Ky–Hem and Mg-Hbl–Ky–Hem)and the subsequent formation of certain uncommon minerals, e.g.yellow sapphirine, Mn–andalusite, green and purple yoderite,piemontite and boron-free kornerupine. The proposed increasein water activity is attributed to fluid infiltration resultingfrom the devolatilization of underlying sediments during metamorphism. KEY WORDS: fluid infiltration; high-pressure amphibolite facies; East African Orogen; Pan-African; whiteschist     

Diatoms from the Middle and Late Weichselian and the Early Flandrian period on Andøya, north Norway

BOREAS Foged, N. 1978 03 01: Diatoms from the Middle and Late Weichselian and the Early Flandrian period on Andøya, north Norway. Boreas, Vol. 7, pp. 41–47. Oslo. ISSN 0300–9483.
From cores from a depth of 9.9 m up to 5.1 m below the present surface of a mire situated approx. 36 m above sea level on Andøya, north Norway, 47 samples were analysed for diatoms. Some 240 taxa were recorded, chiefly in Late Weichselian and Early Flandrian material. They were subdivided according to their halobion and pH relation. On the whole, the pH reaction of the environment was neutral, but it changed from faintly acid to faintly alkaline during the sedimentation of the Late Weichselian material.     

A symposium on Nordic Quaternary research in 1986

Different topics of Quaternary geology were dealt with at a symposium arranged in 1986 in Århus, Denmark: (1) reviews on recent Quaternary research in the Nordic countries, (2) studies of stratigraphy and geological processes in the Fennoscandian Border Zone, which demonstrate that the Quaternary deposition pattern is highly influenced by tectonic movements, and (3) Quaternary research as it can be applied to geotechnical investigations in the search for raw materials, in connection with pollution, and with conservation.     

Oxygen and carbon isotope compositions of Upper Cretaceous chalk from the Danish sub-basin and the North Sea Central Graben

The oxygen and carbon isotopic compositions of Upper Cretaceous chalk have been studied from a large number of outcrops and boreholes in the Danish sub-basin and the North Sea Central Graben. The carbon isotopic compositions vary from +0.50%_o to +3.00%_oδ¹³C which correspond to the expected carbon isotopic composition of carbonate precipitated from normal sea water. The oxygen isotopic compositions of samples from outcrops and near-surface drilled sections fall in the range from —0.50%_o to — 2.00%_oδ¹⁸O, which is close to the expected values for carbonate produced by coccoliths in the Late Cretaceous sea. A geographic distribution exists in the Danish sub-basin showing relatively heavy oxygen isotope values in the centre of the basin and slightly more negative values in the eastern part of the basin. Primarily the phenomenon is thought to reflect minor variations in the oceanographic parameters in the Late Cretaceous sea. The deep subsurface sections represent a depth interval of 0–3100 m. Mechanical compaction in the uppermost part of the sequences apparently has no influence on the isotopic composition. Chemical compaction dominates at greater depth, leading to temperature-induced isotopic re-equilibration in the water-rock system. In these sequences the oxygen isotope values range from ? 1.50%_o to — 7.50%_oδ¹⁸O and are significantly correlated with depth of burial, cementation and porosity. The data indicate that pressure-dissolution, recrystallization, reprecipitation and ion-exchange between solid carbonate phase and pore water, are all actively involved in the oxygen isotope re-equilibration process. This process is believed to take place in a diagenetically closed system and is tentatively divided into two phases: 1) an early diagenetic phase which takes place at porosities down to approximately 20% in which the chalk possesses a pore-water controlled isotopic re-equilibration system and 2) a late diagenetic phase at porosities below 20% in which the re-equilibration process increasingly becomes influenced by the rock-introduced isotopic change in the composition of the formation water.     

Coastal morpho-stratigraphy and Holocene relative sea level changes at Tuapaat, southeastern Disko Island, central West Greenland

The geomorphology of a coarse clastic coastal landscape at Tuapaat (69°24'N 52°36'W), southeastern Disko Island, central West Greenland, is described, and a coastal morpho-stratigraphy of the landscape is constructed. ¹⁴C ages on marine shells, whale bones, peat and gyttja are used to construct relative sea level changes throughout the Holocene. The emergence of SE Disko Island occurred in the early part of the Holocene. The Holocene marine limit is situated ca. 80m a.s.l. Between 4.7 and 1.0ka BP, the relative sea level approached the present sea level and it has probably been below present sea level between 4.7 ka BP and the present. The morpho-stratigraphy in the lowest part of the coastal landscape at Tuapaat suggests a complex late Holocene relative sea level history which includes at least 3-4 transgressions during the last ca. 2.5 ka.