As the picture of carbonatite magmatism continues to improve, so the bimodal distribution of calcio-and magnesio-carbona tites becomes more apparent. Calcite carbonatites are much more abundant: some of these may be differentiates from silicate parent magmas. But carbonatite volcanism requires that calciocarbonatit e can be sourced in the mantle, without involvement of silicate melts. Phase relations in the Ca-Mg carbonate system impose the requirement for separate sources for calcitic and dolo miti c carbonatites. Dolomite is the expected product of incipient mantle melting. Calciocarbonatit e may arise from melting of a higher level CaC03 rich stockwork metasome, in which higher pressure polymorphs, possibly including aragonite, might be the controlling influence on the melt composition.
Abstract The known carbonatite eruptive centres in Northern Rhodesia fall within a linear belt trending N.E.-S.W. the most conspicuous elements of which are the colinear troughs of the mid-Zambezi valley and the Luangwa rift. This lineament is an important and longstanding zone of crustal weakness. Its extensions outside Northern Rhodesia are briefly indicated and its ramifications inside the Territory are examined, particularly in reference to the localization of carbonatite activity around rift intersections.
Abstract The Rockeskyll complex in the north, central part of the Quaternary West Eifel volcanic field encapsulates an association of carbonatite, nephelinite and phonolite. The volcanic complex is dominated by three eruptive centres, which are distinct in their magma chemistry and their mode of emplacement. The Auf Dickel diatreme forms one centre and has erupted the only known carbonatite in the West Eifel, along with a broad range of alkaline rock types. Extrusive carbonatitic volcanism is represented by spheroidal autoliths, which preserve an equilibrium assemblage. The diatreme has also erupted xenoliths of calcite-bearing feldspathoidal syenite, phonolite and sanidine and clinopyroxene megacrysts, which are interpreted as fragments of a sub-volcanic complex. The carbonate phase of volcanism has several manifestations; extrusive lapilli, recrystallized ashes and calcite-bearing syenites, fragmented during diatreme emplacement. A petrogenetic link between carbonatites and alkali mafic magmas is confirmed from Sr and Nd isotope systematics, and an upper mantle origin for the felsic rocks is suggested. The chemistry and mineralogy of mantle xenoliths erupted throughout the West Eifel indicate enrichment in those elements incompatible in the mantle. In addition, the evidence from trace element signatures and melts trapped as glasses support interaction between depleted mantle and small volume carbonate and felsic melts. This close association between carbonate and felsic melts in the mantle is mirrored in the surface eruptives of Auf Dickel and at numerous alkaline-carbonatite provinces worldwide.
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