The origin of carbonate mud in isolated carbonate platforms of Belize,Central America

Carbonate mud from three small isolated carbonate platforms of Belize (Central America) is largely a product of the breakdown of skeletal grains. The composition of the 63-20- and 20-4-µm fractions of 37 samples was determined by point counting under SEM; composition of the <4-µm fraction was assessed qualitatively under SEM. The 63-4-µm fractions are dominated by fragments of mollusks, the codiacean alga Halimeda, and other skeletal types. About one-third of the particles in the 63-4-µm fractions remained unidentified, probably due to obliteration of diagnostic features by early recrystallization processes such as micritization. Nanograins (<1 µm) and short (3-5 µm) aragonite needles are most common in the <4-µm fraction. These grains are interpreted to be largely fragments of codiacean algae (Halimeda, Penicillus). The 20-4- and <4-µm sediment fractions are composed of 80 and 75% aragonite on average, respectively. Trace element composition of strontium averages 7,900 ppm in the 20-4-µm fraction and in codiacean algae samples, and 5,600 ppm in the <4-µm fraction. Geochemical (trace element) data also argue against inorganic aragonite precipitation within the water column, and favor a skeletal origin. The lower strontium contents in the <4-µm fraction as compared with the 20-4-µm fraction may also be a consequence of early recrystallization processes.     

Der Basaltvulkanismus des Djebel Haroudj Ostfezzan/Libyen

A geological map of Djebel Haroudj is presented which shows the distribution of volcanoes and six major basalt flows over an area of approximately 45,000 square kilometres. Differences between the flows are based on age rather than petrographic composition. The oldest flows most probably erupted in the late Pliocene, though it is possible that locally flows of Miocene or even Oligocene age are present. The middle flows most probably occurred during the Pleistocene. The absence of erosional forms and drainage indicates that the youngest flows are of post-pluvial, Holocene age. The zones of volcanic activity at Djebel Haroudj are related to three major tectonic elements of very different ages (see alsoKlitzsch 1966 a): the early Paleozoic southern Haroudj uplift, the western edge of the Mesozoic Tibesti-Sirte uplift, and the south-eastern prolongation of the Hon graben fault system of late Cretaceous to Tertiary age.     

Shatter cones produced experimentally by impacts in limestone targets

Experimentally produced impact craters in limestone targets displayed millimeter-sized shatter cones within crater spallation zones. The craters have been produced by accelerating spherical metal projectiles by means of a light-gas gun. Variation of the impact velocity showed that at about 3 km/s shatter cone formation starts and is reproducible at any higher impact velocities. In most cases the cone apices were pointing in the direction of the impact center. The occurrence of shatter cones directly at the target surface (spallation zones of craters) does not support a theoretical model on shatter cone formation published by Gash (1971).     

Sur la paléoflore post-carbonifère de la bordure est du Bassin de Mourzouk (Libye)

Until recently the age of the continental sediments overlaying marine strata of Carboniferous age at the Murzuk Basin was unknown and long distance correlation was not possible because of lack of correlation criteria. The flora now described allows, for the first time, to identify the age of parts of this strata. Together with new sedimentological criteria of subdivision worked out by R.Glaeser, H.Kallenbach and I.Zöller which will be published later, the paleobotanical investigations contribute towards a final subdivision of these continental sediments. The terms “Nubian Series”, and “Posttassilien” will be abandoned, because they are useless within the area concerned. The creation of correlatable stratigraphical data is an important prerequisite for groundwater and mineral exploration within the large areas covered by these strata. Along the eastern Murzuk Basin these sediments containFilicophytae, Pteridophyllae,Pteridospermaphytae, Cycadophytae andConiferophytae. At Jebel Ben Ghnema the upper part of the lower third of the continental sediments contains plants which are known from triassic strata in Europe, India, North America, South America, South Africa, and Asia. At Jebel Ben Ghnema the top of the strata contains Jurassic plants known from Europe, North America, and Asia. Almost 300 kilometres further South at Jebel Ati near the top of these continental sediments plants of Jurassic age were found, too. It seems that the development of this flora has taken place under a generally warm and dry climate with wet seasons. From the paleophytographical point of view the flora described is of great interest, because it contains coexisting species of the Northern and Southern hemisphere. The Jurassic flora is comparable to that known from adjacent basins in Algeria and Egypt, and to flora known from Virginia.     

Zr contents of glaucophane-bearing meta-basalts of Western Crete,Greece

Chemical analyses of hornblendes from different regional metamorphic terrains and from rocks of different metamorphic grade have been compared. Hornblendes of lowpressure type are distinguished from hornblendes of high-pressure type by their Al^VI and Si contents. The Ti content of hornblende is shown to increase with metamorphic temperature from the greenschist-amphibolite transition facies to the hornblende-granulite facies.     

Relics of eclogite facies assemblages in the Ceará Central Domain, NW Borborema Province, NE Brazil: Implications for the assembly of West Gondwana

The Borborema Province, in the NE of Brazil, is a rather complex piece in the Brazil–Africa puzzle as it represents the junction of the Dahomeyide/Pharusian, Central African, Araçuai and Brasilia fold belts located between the West-African/São Luis, Congo/São Francisco and Amazonas craton. The correlation between the Dahomeyides from W-Africa (Ghana, Benin, Togo, and Mali) and the Borborema Province involves the Médio Coreaú and Central Ceará domains. The inferred continuation of the main oceanic suture zone exposed in the Dahomeyides of W Africa is buried beneath the Phanerozoic Parnaíba Basin in Brazil (northwest of the Médio Coreaú domain) where some high density gravity anomalies may represent hidden remnants of an oceanic suture. In addition to this major suture a narrow, nearly continuous strip composed of mainly mafic pods containing relics of eclogite-facies assemblages associated with partially migmatized granulite-facies metapelitic gneisses has been found further east in the NW Borborema Province. These high pressure mafic rocks, interpreted as retrograded eclogites, are located between the Transbrasiliano Lineament and the Santa Quitéria continental arc and comprise primitive to evolved arc-related rocks with either arc- or MORB-type imprints that can indicate either deep subduction of oceanic lithosphere or roots of continental and oceanic magmatic arcs. Average peak P–T conditions under eclogite-facies metamorphism (T = 770 °C and P = 17.3 kbar) were estimated using garnet–clinopyroxene thermometry and Jd content in clinopyroxene. Transition to granulite-facies conditions, as well as later widespread re-equilibration under amphibolite facies, were registered both in the basic and the metapelitic rocks and suggest a clockwise P–T path characterized by an increase in temperature followed by strong decompression. A phenomenon possibly related to the exhumation of a highly thickened crust associated with the suturing of the Médio Coreaú and Central Ceará domains, two distinct crustal blocks separated by the Transbrasiliano Lineament.     

Das System KFeSi3O8−KAlSi3O8−H2O mit Untersuchungen über die Mikroklin-Sanidin-Beziehungen

Zusammenfassung Im System KFeSi₃O₈–KAlSi₃O₈ wird eine Mischungslücke gefunden, welche den Bereich von 10–60 Mol. % K-Fe-Feldspat umfaßt. Die Mischkristalle links und rechts der Mischungslücke verhalten sich ähnlich wie ihre benachbarten Endglieder. Das Fehlen von intermediären Phasen auf der Eisenseite und die Mischungslücke machen es wahrscheinlich, daß das Verhalten des K-Fe-Feldspates nicht auf das Verhalten des K-Al-Feldspates extrapoliert werden darf.

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Summary In the system KFeSi₃O₈–KAlSi₃O₈ a miscibility-gap is found from 10 to 60 Mol.% K-Fe-felspar. The mixed crystals on the right and left side of the miscibility-gap show a behaviour similar to the corresponding end-members. The lack of intermediate phases on the iron-side and the miscibility-gap make probable that one cannot extrapolate the behaviour of the K-Fe-felspar to the behaviour of the K-Al-felspar.

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Mit 3 Textabbildungen

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Herrn Professor Dr.F. Machatschki zum 70. Geburtstag gewidmet.