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Pipe G3b is part of the Upper Cretaceous carbonatitic Gross Brukkaros Volcanic Field in southern Namibia. The pipe represents
the root zone of a diatreme and is located 2800 m west of the rim of Gross Brukkaros, a downsag caldera. The pipe is exposed
approximately 550 m below the original Upper Cretaceous land surface. It cuts down into its own feeder dyke, 0.3 m thick.
The pipe coalesced from two small pipes and in plan view is 19 m long and 12 m wide. It consists of fragmented Cambrian Nama
quartzites and shales of the Fish River subgroup. Despite intensive brecciation, the stratigraphic sequence of the country
rocks is almost preserved in the pipe. In addition, the feeder dyke became fragmented too and can be traced in a 2- to 3-m-wide
zone full of carbonatite blocks along the southern margin of the pipe. The void space of the breccia is 30–50% in volume.
Finally, after the disruption of country rocks and feeder dyke, a little carbonatite magma intruded some of the void space.
The breccia of pipe G3b is considered to represent a root zone at the transition from the feeder dyke into a diatreme above.
Formation of the breccia required a shock wave thought to have been associated with a last explosion of the diatreme immediately
above the present level of exposure. The explosion can be shown to have been phreatomagmatic in origin.
Received: 11 October 1996 / Accepted: 6 March 1997 相似文献
105.
Oliver Ritter Ute WeckmannTim Vietor Volker Haak 《Physics of the Earth and Planetary Interiors》2003,138(2):71-90
The Namibian margin is dominated by the late Proterozoic to early Cambrian fold belts of the Damara Orogen, which wrap around and separate the Congo and Kalahari Cratons. This mosaic of relatively ‘soft’ fold belts and ‘hard’ cratons apparently controlled the path for the opening of the South Atlantic in the early Cretaceous. The continents split along the coast-parallel fold belts of the Damara Orogen while the inland fold belt (Damara Belt) was effected by extension and widespread igneous intrusion but never developed to the rift stage. This paper is concerned with the interpretation of magnetotelluric (MT) data along a 200 km NW-SE profile across the Damara Belt in NW Namibia. The regional, two-dimensional electrical resistivity model and the induction vector data exhibit three distinctive zones: (i) a generally very resistive upper crust which is typical for the granites and metasediments of the Damara Belt, (ii) two subvertical conductors in upper to mid-crustal levels which correlate with major tectonic zone boundaries and (iii) a highly conductive middle to lower crust in the southern part of the profile. The geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone in the middle crust. We interpret the high electrical conductivity in terms of graphite (or other forms of mineralization) enrichment along the shear planes. This zone of crustal weakness may have originated in Pre-Damara times and had probably experienced several episodes of crustal reactivation before the intrusion of basaltic dike swarms during the Cretaceous rifting and magmatism associated with the opening of the South Atlantic. 相似文献
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Stephan Königer Volker Lorenz Harald Stollhofen Richard A. Armstrong 《International Journal of Earth Sciences》2002,91(2):341-356
Thin, widespread, fallout tuff layers interbedded within fluvio-lacustrine successions of the Carboniferous-Permian Saar-Nahe Basin provide important tephrostratigraphic markers. In addition, radiogeochronometric data derived from the tuffs serve as calibration points for the adjustment to regional chronostratigraphy and to numerical time scales. The Pappelberg-Tuff in the Meisenheim Formation (Glan Group) has been dated by U/Pb zircon SHRIMP technique at 297.0Dž.2 Ma. Taking the Carboniferous/Permian boundary at 296 Ma, the Meisenheim Formation coincides approximately with this boundary. Consequently, underlying strata, lithostratigraphically regarded as the basal part of the 'Rotliegend', chronostratigraphically belong to the Upper Carboniferous. Bed thicknesses, grain size and sorting characteristics of the tuffs and the absence of contemporaneously emplaced volcanics within the Saar-Nahe Basin point to an extrabasinal derivation of the wind-drifted volcanic ash. Decreasing grain sizes of juvenile pyroclastic particles towards the north suggest source areas south of the basin within 300 km distance. The majority of the tuffs are rhyolitic to rhyodacitic and indicate petrographic and geochemical affinities to Moldanubian S-type granitoids, in particular to highly differentiated two-mica granites, and related volcanic effusives. Within the time frame considered here, such potential source rocks were emplaced in the northern and central Black Forest (SW Germany) and the northern Vosges (E France) at 100-150 km distance south of the Saar-Nahe Basin. 相似文献
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Ocean Dynamics - Mesoscale eddies in the open ocean are mostly formed by baroclinic instability, in which the available potential energy from the large-scale slope of the isopycnals is converted... 相似文献