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The present study of Lower Kimmeridgian biostratigraL ^ H phy in Poland is based on the following ammonites: Ataxioceras, Ne114 brodites, Katroliceras, Glochiceras, Streblites, Taramelliceras, Idoceras, Sutneria, Orthosphinctes, Lithacoceras, Aspidoceras, Amoeboceras, Ringsteadia, Rasenia, Pictonia, and Zonovia. The analysis of this fauna confirms that the Lower Kimmeridgian sediments in Poland, also in its northern parts, have rightly been subdivided into the following three zones: the lower one — Sutneria platynota, the middle one — Ataxioceras hypselocyclum, and the upper one — Katroliceras divisum (partly).
Charles Lyell was appointed to the Chair of Geology at King's College London in February 1831, and proceeded to present a series of 12 lectures based on his recently published book Principles of Geology. According to his biographer, Leonard G. Wilson (1972), Lyell took a lot of trouble to prepare illustrations and demonstrations in support of his geological arguments. The lectures were open to the general public and attracted large audiences, including a number of his peers in the Geological Society, their wives and friends, who outnumbered the College students.
The role played by plume-generated crustal magmatic complexes in the segmentation of volcanic margins is highlighted by a preliminary study of magma flow directions in shallow intrusives from the East Greenland volcanic margin. We investigate the magmatic texture using anisotropy of magnetic susceptibility for eight dykes of tholeitic affinity belonging to a dyke swarm associated with the Tertiary opening of the North Atlantic Ocean. The thickness of the sampled dykes ranges from 3 to 37 m. The dykes are of doleritic texture and contain up to 12% of opaque minerals and 35% of plagioclase laths. Dykes showing a magnetic foliation plane within the dyke plane, i.e. a magnetic fabric usually attributed to magmatic processes, represent 40% of the samples from the studied swarm. These dykes have a low degree of anisotropy and their ellipsoid of magnetic susceptibility is strongly oblate. Inverse magnetic fabrics, where the maximum principal susceptibility axis lies near the pole to the dyke account for 60% of the data. We interpret these inverse fabrics either as alteration minerals with highly prolate ellipsoids of magnetic susceptibility or primary titanomagnetites with oblate ellipsoid of low anisotropy. The flow direction is inferred for normal fabric dykes using the mirror imbrication of magnetic lineation. Analysis of thin sections shows a good agreement between magnetic fabric directions and phenocryst preferred orientations. The inferred flow directions are predominantly horizontal, throwing a new light on volcanic margin development.
The El Laco magnetite-apatite ore deposits in the Andean Cordillera of northern Chile occur as massive, tabular bodies, as stratified, pyroclastic ores, and as crosscutting dykes and vein complexes. The ore deposits and surrounding volcanic rocks, mainly andesites, are Plio-Pleistocene in age and preserve many of their original volcanic textures and structures. All the field and laboratory data are consistent with an origin by eruption and shallow intrusion of a high-temperature, volatile-rich, iron-oxide magma. A number of other iron-oxide-apatite deposits of Cenozoic age in the Andean Cordillera, and a belt of Cretaceous iron deposits in the Coastal Cordillera of Chile also have features that suggest a magmatic origin. Associated with these magmatic ore deposits are economic and sub-economic concentrations of Cu, Au, U, and REE.
At Benfontein, near Kimberley, South Africa, three sills of kimberlite intrude Dwyka shales and overlying Karroo dolerite. Each sill results from numerous injections of kimberlitethat have consolidated to give the sill a layered appearance. Many layers - show magmatic sedimentation features and cumulus textures, and, although some show in situdifferentiation, other layers result from pre-injection differentiations The transporting, intercumulus liquid was carbonate-rich and some layers have differentiated to form a carbonate rock composed of the intercumulus calcite; this, on trace element and isotopic data, shows strong affinities with carbonatite. In one of the sills 'one calcite layer has migrated diapirically into overlying layers in.the sill. These sedimentation features, combined with thermal metamorphism of country-rock shales and the presence of quench calcite and apatite, are interpreted as evidence that the kimberlite was injected as a highly mobile fluid, comprising megacrysts of olivine, garnet, pyroxene, mica and picroilmenite in a hot carbonatitic liquid from which olivine, magnetic spinel, perov-skite, apatite, calcite, dolomite, ankerite and quartz crystallised. The evidence that the transportation medium was a warm carbonatitic liquid is directly opposed to earlier hypotheses proposing that kimberlite is intruded as a cold or plastic paste, and also supports proposals of a genetic link between kimberlite and carbonatite.
Magmatic Nickel-Copper-Platinum-group element sulfide deposits form as the result of the segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma, and the partitioning of chalcophile elements into these from the silicate magma.
The size of the deposits, their grades and ratios of economic metals are very variable. This is illustrated in Table 1.1, which summarizes data on tonnes ofresources + production, grades ofNi, Cu, Co and PGE, tonnes of contained metal, and value of the ore and of the individual metals. It is also illustrated in Fig. 1.1, which shows the percentages that Ni+Co, Cu and the PGE contribute to the value of many magmatic sulfide deposits/camps. <...>
The association between fithospheric extension, continental break-up, mantle plumes and massive bursts of igneous activity is well recognized, but their causal relationship remains controversial. According to active mantle hypotheses, rifting is initiated by doming above a mantle plume. Alternative hypotheses consider magmatism as a passive response to lithospheric stretching and rifting with the chance unroofing of a plume only enhancing lithospheric failure and producing abnormally large volumes of basaltic magmatism. Some models combine aspects of both active and passive hypotheses and it is the arrival of a new plume beneath lithosphere already under tension that causes it to split and form a new ocean. The active and passive hypotheses highlight important differences in the relative timing of rifting, magmatism and uplift. Consequently, this debate should be resolved and the main aim of this volume is to integrate relevant tectonic, geochemical and geophysical data which will lead to a better understanding of the causal relationships between magmatism and continental break-up.
Fifty years have now passed since Graham (1954) published his seminal paper advocating the use of anisotropy of magnetic susceptibility (AMS) as a rapid and sensitive petrofabric tool. During these five decades, Graham's 'underexploited' method has become standard, and AMS and related techniques are now routinely applied to characterizing fabrics in a wide variety of geological materials (e.g. the GEOREF database lists over 500 journal publications with 'magnetic anisotropy' as keywords).
Magnetic polarity stratigraphy, the stratigraphic record of polarity reversals in rocks and sediments, is now thoroughly integrated into biostratigraphy and chemostratigraphy. For Late Mesozoic to Quaternary times, the geomagnetic polarity record is central to the construction of geologic time scales, linking biostratigraphies, isotope stratigraphies, and absolute ages.
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