Diatreme

Abstract: 1 Introduction.- 2 Distribution and Tectonic Setting of Kimberlites.- 2.1 Africa.- 2.1.1 West Africa.- 2.1.2 Central Africa.- 2.1.3 East Africa.- 2.1.4 Southern Africa.- 2.2 U.S.S.R..- 2.2.1 Siberia (Yakutia).- 2.2.2 Western Russia.- 2.3 India.- 2.4 Australasia.- 2.4.1 Borneo.- 2.4.2 Solomon Islands.- 2.4.3 Australia.- 2.5 Western Europe and Greenland.- 2.6 North America.- 2.7 South America.- 2.7.1 Guyana.- 2.7.2 Brazil.- 2.7.3 Argentine.- 2.8 Indirect Evidence for Kimberlite Intrusions.- 2.9 Summary.- 3 Geology of Kimberlite Intrusions.- 3.1 Diatremes.- 3.2 Dykes.- 3.3 Sills.- 3.4 Diatreme-Dyke-Sill Relationships.- 3.5 Extrusive Kimberlites.- 3.6 Effects of Kimberlite Intrusions on Wall Rocks.- 3.6.1 Physical Effects.- 3.6.2 Metasomatic Effects.- 3.6.3 Thermal Effects.- 3.7 Emplacement of Kimberlite Diatremes.- 4 Petrography of Kimberlite.- 4.1 Introduction.- 4.2 Diatreme Facies Kimberlite.- 4.3 Hypabyssal Facies Kimberlite.- 5 Geochemistry of Kimberlites.- 5.1 Major Elements.- 5.2 Trace Elements.- 5.3 Rare-Earth Element Chemistry.- 5.4 Isotope Chemistry.- 6 Mineralogy of Kimberlites.- 6.1 Introduction.- 6.2 Native Elements.- 6.3 Sulphides.- 6.4 Halides.- 6.5 Oxides and Hydroxides.- 6.6 Carbonates.- 6.7 Sulphates.- 6.8 Phosphates.- 6.9 Silicates.- 6.9.1 Nesosilicates.- 6.9.2 Sorosilicates.- 6.9.3 Inosilicates.- 6.9.4 Phyllosilicates.- 6.9.5 Tektosilicates.- 6.10 Summary.- 7 Xenoliths in Kimberlite.- 7.1 Wall-Rock Fragments.- 7.2 Fragments from Earlier, Eroded Formations.- 7.3 Blocks Derived from Recognizable Underlying Formations.- 7.4 Granulites.- 7.4.1 Petrography.- 7.4.2 Phase Chemistry.- 7.4.3 Bulk Chemistry.- 7.4.4 Conditions of Formation.- 7.5 Mantle-Derived Xenoliths.- 7.5.1 Introduction.- 7.5.1.1 Peridotite-Pyroxenite Suite.- 7.5.1.2 Eclogites.- 7.5.1.3 Metasomatised Peridotites.- 7.5.1.4 Glimmerites and MARID-Suite Rocks.- 7.5.1.5 Miscellaneous Xenoliths.- 8 The Megacryst Suite.- 8.1 Petrography and Phase Chemistry.- 8.2 Trace Element and Isotope Chemistry.- 8.3 Conditions of Formation.- 9 The Sub-Continental Mantle and Crust - Evidence from Kimberlite Xenoliths.- 9.1 Distribution of Rock-Types Within the Upper Mantle.- 9.2 Processes in the Upper Mantle.- 9.3 The Deep Crust.- 9.4 Summary.- 10 Kimberlite Genesis.- 10.1 The Three Hypotheses.- 10.2 Relationship with Other Rock Types.- 10.3 The Kimberlite-Carbonatite Relationship.- 10.4 Kimberlite and Diamond.- 10.5 Relationship of Kimberlite Magmatism to Major Earth Movements.- 10.6 Outstanding Problems.- References.

Abstract: The Pleistocene maars in the Eifel region of Germany, and Massif Central in France, formed when fissures opened at the bottom of older valleys allowing stream water to pour down them and come into contact with rising magma. The resulting phreato-magmatic eruptions gave rise to both base surge and air-fall deposits. Spalling of wall rock at depth enlarged the fissure into an eruption chamber. Subsidence along a ring fault into the eruption chamber accounts for the larger crater cut into the country rocks. The volume relationship between the crater excavated, the ejected pyroclastic debris of the rim and the volume below the floor of the crater, indicates that the volume of the maar ejecta is always larger than the volume of the crater. The relationships between maars and tuff-rings are described; the distinctive features of the two depend on density differences between the pyroclastic debris and country rocks, on the distribution ratio between ejected material and debris remaining within the underlying diatreme, and most importantly on the total amount of juvenile material produced. Larger contents of juvenile material result in the formation of tuff-rings instead of maars, and in most cases also indicate a shallower eruption source of the former. As a result of these many variables, large diatremes, which display subsidence structures bounded by ring-faults, may produce either maars or tuff-rings at the surface.