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Carbonatites and associated nephelinites from Sao Vicente, Cape Verde Islands

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de Ignacio, Cristina and Muñoz García, Mercedes and Sagredo Ruiz, Juana (2012) Carbonatites and associated nephelinites from Sao Vicente, Cape Verde Islands. Mineralogical magazine, 76 (2). pp. 311-355. ISSN 0026-461X

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Abstract

The island of Sao Vicente has the most abundant carbonatite outcrops in the Cape Verde Islands. A
field survey of the main outcrops has shown that they consist of extrusive carbonatites, carbonatite
dykes and small apophyses of intrusive carbonatite. These outcrops are spatially related to nephelinites.
The compositions of the extrusive carbonatites and dykes plot close to, and within, the
magnesiocarbonatite field. In contrast, the intrusive carbonatites are calciocarbonatites, with similar
average strontium contents to those of extrusive carbonatites and dykes (around 4000 ppm), but
remarkably low barium, niobium and total rare earth element concentrations. Whole-rock geochemistry
indicates a strong affinity between the nephelinites and intrusive carbonatites, such that the latter could
represent fractionation products of the same parental magma. This is in agreement with radiogenic
isotope geochemistry, which shows a very restricted range of compositions in the Sr, Nd and Pb
systems. Fractionation from a common parental magma occurred in two main steps: high-temperature
nephelinite crystallization and high-temperature carbonatite immiscibility. The carbonatitic melts
crystallized in two different environments, as follows: (1) a shallow intrusive environment, giving rise
to the early calciocarbonatite cumulates; and (2) a vapour-dominated, extrusive environment,
producing the later magnesiocarbonatites.


Item Type:Article
Uncontrolled Keywords:Carbonatites, Extrusive, Intrusive, Nephelinites, Cape Verde Islands
Subjects:Sciences > Geology > Petrology
Sciences > Geology > Geochemistry
ID Code:23865
Deposited On:11 Dec 2013 08:25
Last Modified:01 Jun 2014 23:01

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