Mineralogical
investigations on nephelinite-phonolite-carbonatite Cenozoic
associations from Southern Morocco (Saghro, Anti-Atlas) led
to the discovery of mineral species yet not discovered in
Morocco. The cristallisation of peralkaline melts and the
interaction of these melts with preexisting phenocrysts led
to the formation of hainite, delhayelite, lorenzenite and
eudialyte-group minerals. Minerals compositions and whole-rock
major-element modelling clearly show that nephelinites, phonolites
and carbonatites are linked by a fractional crystallisation
process involving the formation of a ultrabasic-syenitic-carbonatitic
intrusions at depth. The results will be soon published in
Mineralogical Magazine (in press).
Diopside
and pyrope megacrysts of the Mbuji-Mai and Kundelungu kimberlite
keep the memory of reaction between metasomatic agent and
cratonic mantle. The paper by Pivin et al (Lithos, in press)
demonstrate that this suite of megacryst is comparable to
minerals forming mantle xenoliths sampled by kimberlite magmas.
Their characterisitics can not be attributed to an origin
by fractional crystallisation of a basic-ultrabasic magma
at depth, as commonly advanced for other megacryst suites.
Variscan
Danubian ophiolite in Romania preserved the evidences for
oceanic fluid-rock interactions. Sampled hydrothermal sequence
show that gabbros from the lower oceanic crust are progressively
transformed into listvenites, i.e. metasomatic rocks dominated
by calcite, quartz, chlorite and Cr-muscovite. Plissart et
al (Canadian Mineralogist, 2009) have estimated the temperature
of formation for each step of hydrothermal recrystallisation
on the basis of chlorite compositions and thermodynamic dataset
of phyllosilicate minerals.
The Gourma
in NE Mali preserved a thick sequence of mafic-ultramafic
rocks formerly interpreted as continental lower crustal granulites.
New data presented by Berger et al (2009) show that the garnet
metagabbros, pyroxenites and qz-gabbros are part of an intra-oceanic
arc root. Melting in the garnet metagabbro is evidenced by
the presence of trondhjemite veins with garnet-clinopyroxene-rutile
residues. The intra-arc differentiation observed at Amalaoulaou
has generated ultramafic rocks, denser than common upper mantle
rocks. The basaltic bulk composition of the arc can thus be
driven to an intermediate composition by gravity-driven sinking
of ultrabasic residues at the base of the oceanic arc sequence.
