Carbonatites and associated rocks from the Cape Verde Islands.

摘要

Carbonatite magmas are alkali-bearing, ionic melts, analogous to synthetic carbonate melts. REE complexing suggests that carbonato-complexes dominate the melt, and these are more stable for the LREE than the HREE, leading to the extreme LREE/HREE enrichments characteristic of carbonatites. Crystal settling is viable in static magma chambers. however in thin carbonatite dykes as seen on San Vicente, turbulent flow velocities preclude vertical particle transfer against the flow of liquid, and favour fractionation by rapid crystal growth in a boundary layer close to the dyke margins, and particle accretion to the dyke walls. The Vale de Cavaleiros sovite on Fogo, displays mineralogical variation as a result of adcumulation, gravitationally controlled sedimentalogical sorting and post-lithification deformation. Minor and trace element variations are explained partly as a function of the heterogeneity of mineralogy, but require chemical modification of the magma by fractional crystallization and auto-metasomatizm. O18o and O13C of the carbonatites on San Vicente are decoupled, however calcite and dolomite may have equilibrated with seawater at temperatures close to 100°C. Fractionation of calcite and apatite from a carbonatite magma, produces cumulate microsovite and conjugate evolved carbonatite liquids which form the ferrocarbonatite suite. Trace element variations within the ferrocarbonatite suite indicate that the unstained ferrocarbonatites (unaltered 'quench' carbonatite) are heterogeneously metasomatized and mineralized by evolved (contaminated) orange ferrocarbonatites to produce brown-black ferrocarbonatites. The degree of dolomitization of the Camile dyke on San Vicente, relates to the activity of Ba2+ released during recrystallization of the original carbonate minerals. The arrays on plots of 87Sr/86Sr vs 143Nd/144Nd vs. 206Pb/204Pb for rocks from the Cape Verde Islands are interpreted as a two component mixing lines between recycled pelagic sediment and recycled altered MORB. The melilitites and nephelinites of the Malhada Pedra formation on Maio defines an array from CSn = 157. to CSN = 07 (in CSn-(Or+Ab) n-Din space), which may be interpreted as describing the loci of migration of the peritectic melt during the melting episode. There is little evidence for the genesis of carbonatite by ultra - fractionation of carbonated, alkalic, silicate magma. Neither major or trace element evidence can discriminate between liquid immiscibility, or direct partial melting of carbonated peridotite at pressures greater than 20kbar as mechanisms for generating carbonatite magmas.