Time of formation and peak of Variscan HP-HT metamorphism of quartz-feldspar rocks in the central Erzgebirge,Saxony, Germany

The Variscan Erzgebirge represents an antiform with a core of gneisses and mica schists, surrounded by a phyllitic mantle. The Gneiss-Eclogite Unit (GEU), in the central part, is a composite tectonometamorphic assemblage characterized by a HP-HT imprint and comprises migmatitic para- and orthogneisses, HT mylonites, HP granulites, eclogites and garnet peridotites. It is tectonically sandwiched between two major units with distinctly lower PT histories. The GEU experienced a characteristic “kinked” retrograde PT path after HP-HT equilibration with: (1) strong near-isothermal decompression at high temperatures; (2) extensive re-equilibration at medium pressures, followed (3) by rapid cooling during continued uplift. We dated zircons (Pb-Pb evaporation) from granitoid orthogneisses and metapelites of the GEU. The orthogneisses contain euhedral, long-prismatic zircons of igneous origin that provided protolith ages between 470 and 524 Ma. Metapelites retain well-preserved granulite-facies mineral assemblages and contain spherical, multifaceted metamorphic zircons that grew near the peak of HP/HT metamorphism. Inclusions of prograde HP phengite (∼15 kbar) and rutile are included in one such zircon. Metamorphic zircons of three samples from different localities yielded identical ²⁰⁷Pb/²⁰⁶Pb ages of 340.5 ± 0.7 Ma, 341.2 ± 0.5 Ma and 341.6 ± 0.5 Ma respectively. Consideration of these zircon ages with published ³⁹Ar/⁴⁰Ar white mica ages suggests fast cooling and uplift rates in excess of 50 °C/Ma and 4 km/Ma. This is typical for large-scale extensional tectonic unroofing of the ultra-deep part of a fossil, thickened Variscan continental crust (>60 km) during continuing continental collision and orogenic collapse. Received: 5 June 1997 / Accepted: 7 January 1998     

Spare Capacity (2003) and Peak Production in World Oil

Reliable estimates of minimum spare capacity for world oil production can be obtained by comparing production statistics before and following the collapse of the Iraqi oil industry in March 2003. Spare production was at least 3.2 M b/d (million barrels/day), and was concentrated in the Middle East, mostly in Saudi Arabia (1.7 M b/d), but also in Kuwait and the UAE. This indicates that, assuming Iraqi production returns to its prewar level, a peak in world oil production resulting from resource constraints alone (ignoring political factors) before 2008 can be rejected. This also implies that with an invigorated Iraqi industry, and with further increases in production in the Former Soviet Union and other non-OPEC areas, there would be significant short-term downward pressure on oil prices and strains within OPEC. These transient issues do not alter the projection for a resource-constrained peak or plateau in non-OPEC production between 2010 and 2018.     

Rb-Sr geochronology in favour of polymetamorphism in the Pan African Damara belt of Namibia (South West Africa)

New Rb-Sr whole rock age data are reported from two metasedimentary sequences of the Damara Supergroup in the central Pan African Damara belt of Namibia (South West Africa).Calc-granofels rocks of the Karibib Formation (Swakop Group) near Usakos are dated at 665±34 Ma (⁸⁷Rb=1.39×10^–11 a^–1) which is interpreted as reflecting a high-grade metamorphic event predating widespread granite intrusion.Pyroxene-bearing feldspathic gneisses of the Khan Formation (Nosib Group) from the Khan-Swakop River area east of Swakopmund show incomplete homogenization at 474±16 Ma. In view of similar ages obtained on the nearby Rössing alaskite granite and on biotites from a variety of rock assemblages this age is interpreted as reflecting a second Damaran metamorphic event rather than a specific stage in a long cooling history as previously thought.It is probable that the new ages characterize two distinct Pan-African tectono-metamorphic events previously named Katangan and Damaran episodes respectively (Clifford, 1967). The younger of these has affected large areas of south western Africa both within orogenic zones and on the neighbouring Kalahari Craton and may reflect crustal processes of sub-continental proportion during the closing stages of the Pan-African tectogenesis.

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Zusammenfassung Neue Rb-Sr-Gesamtgesteins-Isochronenalter von zwei sedimentären Abfolgen der spätpräkambrischen Damara-Supergruppe in der Zentralzone des Pan-Afrikanischen Damara-Orogens von Namibia (Südwest-Afrika) deuten auf eine mehrphasige metamorphe Geschichte hin.Kalk-Granofels-Gesteine der Karibib-Formation (Swakop-Gruppe) südöstlich von Usakos ergaben ein Alter von 665±34 M. J. (⁸⁷Rb=1,39×10^–11 a^–1) und wir interpretieren dieses Alter als das Resultat isotoper Homogenisierung während einer intensiven Regionalmetamorphose, die vor dem Eindringen weitverbreiteter Granite stattfand.Pyroxenführende und feldspathaltige Gneise der Khan-Formation (Nosib-Gruppe) aus dem Gebiet der Khan- und Swakop-Flüsse östlich von Swakopmund zeigen unvollständige Homogenisierung ihrer Sr-Isotope um 474±16 M. J. In Anbetracht ähnlicher Alter für den benachbarten Rössing-Alaskitgranit und für Biotite verschiedener Gesteinstypen aus dem gesamten Zentralbereich des Orogens interpretieren wir obiges Alter eher als den Höhepunkt einer zweiten Regionalmetamorphose als ein bestimmtes Stadium in der post-orogenen Abkühlungsgeschichte.Es ist wahrscheinlich, daß unsere Altersbestimmungen zwei deutlich voneinander getrennte pan-afrikanische tektono-metamorphe Phasen charakterisieren, vor allem wenn man sie in Zusammenhang mit publizierten Daten betrachtet, und wir nehmen an, daß diese den vonClifford (1967) postulierten Katanga- und Damara-Episoden entsprechen. Die jüngere dieser Episoden hat nicht nur die pan-afrikanischen orogenen Bereiche im südwestlichen Afrika, sondern auch weite Teile des Kalahari-Kratons erfaßt und mag auf tektogenetische Prozesse von nahezu kontinentalen Ausmaßen in und unter der Kruste im frühen Paläozoikum zurückzuführen sein.

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Résumé Deux séquences métasédimentaires du supergroupe du Damara ont été datées par la méthode Rb/Sr.Des gneiss calciques («granofels») de la formation de Karibib (groupe de Swakop), récoltés près d'Usakos ont été datés à 665±34 Ma (avec⁸⁷Rb=1.39× 10^–11 a^–1). Cet âge est interprété comme le résultat d'un événement métamorphique de degré é levé antérieur à la mise en place des grandes masses de granite.Des gneiss feldspathiques à pryoxène de la Formation de Khan (Groupe de Nosib), échantillonnés dans la région de la Khan River — Swakop à l'Est de Swakopmund, montrent une homogénéisation isotopique incomplète à 474±16 Ma. Des âges voisins ayant été obtenus pour l'Alaskite de Rössing qui affleure dans la même zone et pour les biotites de différentes roches, cet âge est interprété comme le résultat d'un second métamorphisme damarien plutôt que comme un stade de la longue histoire de refroidissement comme cela avait été imaginé auparavant.Ces nouveaux âges caractériseraient les deux épisodes tectonométamorphiques distincts de l'orogénie Pan-Africaine, qui avaient été appelés respectivement épisode katangien et épisode demarien parClifford (1967). Le plus récent de ces événements a affecté une grande partie de l'Afrique du S.W. aussi bien dans les zones orogéniques que dans le craton voisin du Kalahari; il peut refléter une évolution crustale d'échelle subcontinentale se produisant pendant les derniers stades du tectogène Pan-Africain.

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Rb/Sr - , - ; , .- , , 665±34 (⁸⁷Rb=1,39 10^–1a^–1); , . , , , , 474 ± 16 . - ; , , . , - - , , . , 1967 , . - - , ; , , .

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Contribution No. 27 of the South African Geodynamics Project, presented at the 9th Colloquium on African Geology, Göttingen, April 1977.     

Pre-Imbrian craters and basins: ages,compositions and excavation depths of Apollo 16 breccias

Breccia fragments have been analyzed from the 2–4 mm sieve fraction of three Apollo 16 soils collected in the vicinity of North Ray Crater (63503,17 at Station 13; 67603,1 and 67703,14 at Station 11). Ar³⁹-Ar⁴⁰ ages, Ar³⁷-Ar³⁸ exposure ages, abundances of major and certain trace elements, and petrographie data relevant to thermal history have been obtained for up to 48 individual fragments.Among the samples. 30 gave Ar³⁹-Ar⁴⁰ release patterns that allowed the assignment of a high- or intermediate-temperature plateau age and the recognition of three age groups. Group I (10 fragments) are 4.12-4.21 AE, Group 2 (13 fragments) are 3.89-4.02 AE, and Group 3 (6 fragments) are <2.5 AE in age. Only one fragment (3.60 AE) falls outside this grouping and possibly represents Theophilus ejecta. The probability that the gap between 4.12 and 4.02 AE is a statistical fluctuation is only ～2%. The exposure ages cluster strongly around 50 × 10⁶y. the age of North Ray Crater.The oldest, Group 1 fragments are all anorthositic metamorphosed breccias of light-matrix type. The younger. Group 2 fragments are noritic anorthosite and anorthositic norite breccias with textures indicative of greater annealing (melted matrix), one totally melted sample being of KREEP-basalt texture. The very young. Group 3 fragments are chiefly of glass or devitrified glass. There is a marked distinction between Groups 1 and 2 in compositional as well as textural properties. The Group 2 breccias are generally enriched in Mg, K and REE relative to the aluminous Group I breccias (eg. K ≤ 400 ppm in Group 1 and mostly ≥ 600 ppm in Group 2). This difference is attributed to the introduction of KREEP and mafic ANT components during the formation of the Group 2 breccias.The results are interpreted as reflecting two magnitudes of cratering. The older craters (>4.1 AE) were of medium size (diameters up to a few hundred kilometers), large enough to reset the ages but not capable of excavating deeper than predominantly feldspathic (anorthositic) layers of the crust. The younger craters (～3.9-4.0 AE) were, in contrast, those ascribed to major basin-forming events and were therefore capable of excavating a deeper and wider spectrum of crustal lithologies. The latter resulted in admixture of KREEP and mafic ANT components with the feldspathic ANT, cover layer. KREEP was thus only excavated in abundance during the basin-forming events, from a sub-crustal layer formed initially at ～4.4 AE but incorporated in the breccias at ～4 AE.The KREEP-contaminated. Group 2 breccias have—except two fragments—ages between 3.95 and 4.02 AE. This group includes a crystallized melt (3.97 ± 0.04 AE), close in composition and texture to 14310 (3.87 ± 0.04 AE) which is generally attributed to the Imbrian basin-forming event (～3.88 AE). The pre-Imbrian. Group 2 breccias of Apollo 16 can best be attributed to the Nectaris basin-forming event, which according to the clustered ages probably occurred at ～3.98 AE. Our results support a multi-impact lunar cataclysm with the formation of Nectaris (3.98 AE). Humorum. South Serenitatis, Crisium and Imbrium (3.88 AE) within a 0.1 AE time interval.     

The Precambrian geotectonic evolution of Africa: plate accretion versus plate destruction

The classical model of continental accretion and progressive cratonization, starting with an ancient nucleus onto which younger orogenic belts are welded (onion skin tectonics), has dominated geotectonic research for many years and has received new impetus with the general acceptance of the theory of plate tectonics. It has also been applied to the geotectonic evolution of large Precambrian crustal segments such as Africa.It is demonstrated, however, that new structural, geochronological and paleomagnetic data from Africa provide strong evidence for the existence of large cratonic continental plates since at least the Early Proterozoic which were later transected by linear mobile belts and thus partly destroyed. Crustal reworking and rejuvenation in these belts now gives the impression of younger orogens surrounding ancient “nuclei”.The geotectonic evolution of Africa is therefore characterized by plate destruction rather than by plate accretion with progressive cratonization, and only the “nuclei” have escaped this process.Many phenomena in the African mobile belts seem to indicate processes involving little or no relative motion of crustal plates or crustal shortening between them, and there is as yet no evidence of former oceanic plates to have been generated or consumed. It is therefore suggested that large-scale dispersive movements of major continental fragments were uncommon or absent in the Early and Middle Precambrian.Widespread drift may only have begun during the Late Precambrian/Early Paleozoic Pan African Tectogenesis, eventually leading to a crustal evolution as envisaged by the modern theory of global tectonics.     

Thermals over the sea and gull flight behavior

The flight performance of Herring Gulls relative to specific atmosphere and ocean conditions over the western North Atlantic indicates that large groups of gulls are able, through cooperative flight maneuvers, to induce ascending convective flow (thermals) in which they make extended soaring flights. These group flights in gull-induced thermals are limited to winds of 0 to ~ 1 m s^?1 and to sea-minus-air temperature differences (δT) of ~3 to 6?C. As wind speed increases from ~ 2 to 5 m s^?1, thermals are naturally induced, and the minimum δT required for soaring is inversely related to wind speed. At higher winds (~5 to 13 ms^?1), the minimum positive δT and minimum wind speed required for thermal soaring are directly related, thus indicating an apparent maximum efficiency for the natural production of thermals at wind speeds of about 5 m s^?1 and δT of 1 to 2 ?C.     

Meteoritic trace elements in lunar rock 14321, 184

The 16 trace elements (Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Rb, Re, Sb, Se, Te, Tl and Zn) were measured by radiochemical neutron activation analysis in six samples of 14321, 184: microbreccia-2 (15), microbreccia-3 (14A, 16A and 19A), basaltic clast (1A), and light matrix material (9A). The 14321 microbreccias typically contain a siderophile-rich ancient meteoritic component, poor in volatiles, which is characterized by low $\text{Ir}\text{Au}$ and $\text{Re}\text{Au}$ ratios (0.25-0.38 and 0.34-0.50, respectively, normalized to Cl). This component also occurs in Apollo 12 KREEP glasses, norite fractions of Apollo 14 1–2 mm soils, Apennine Front breccias, and Cayley Formation material, and may represent ejecta from the Imbrian basin.The basaltic clast 14321, 184-1A closely resembles 14053 in trace element content, and both are 5–10 times higher than mare basalts in volatile trace elements (Br, Cd, Tl). The light matrix material contains 9.2 ± 0.5 per cent of microbreccias, judging from its siderophile content.     

Die Entstehung der Kontinente

Ohne ZusammenfassungDas folgende ist nur ein Auszug aus einer grösseren Arbeit gleichen Titels, die inPetermann's Mitteilungen erscheint. Der wesentliche Inhalt dieser Untersuchungen wurde am 6. Januar 1912 auf der Jahresversammlung der Geologischen Vereinigung in Frankfurt a. M. vorgetragen unter dem Titel: Die Herausbildung der Grossformen der Erdrinde (Kontinente und Ozeane), auf geophysikalischer Grundlage, und weiter am 10. Januar in der Ges. z. Beförd. d. gesamten Naturwiss. zu Marburg unter dem Titel: Horizontalverschiebungen der Kontinente.