Modeling of in-situ crystallization processes in the Permian mafic layered intrusion of Mont Collon (Dent Blanche nappe, western Alps)


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Modeling of in-situ crystallization processes in the Permian mafic layered intrusion of Mont Collon (Dent Blanche nappe, western Alps)
Monjoie P., Bussy F., Lapierre H., Pfeifer H.R.
Statut éditorial
Date de publication
The Mont Collon mafic complex is one of the best preserved examples of
the Early Permian magmatism in the Central Alps, related to the
intra-continental collapse of the Variscan belt. It mostly consists (>
95 vol.%) of ol+hy-nonnative plagioclase-wehrlites, olivine- and
cpx-gabbros with cumulitic structures, crosscut by acid dikes.
Pegmatitic gabbros, troctolites and anorthosites outcrop locally. A
well-preserved cumulative, sequence is exposed in the Dents de Bertol
area (center of intrusion). PT-calculations indicate that this layered
magma chamber emplaced at mid-crustal levels at about 0.5 GPa and 1100
degrees C. The Mont Collon cumulitic rocks record little magmatic
differentiation, as illustrated by the restricted range of clinopyroxene
mg-number (Mg#(cpx)=83-89). Whole-rock incompatible trace-element
contents (e.g. Nb, Zr, Ba) vary largely and without correlation with
major-element composition. These features are characteristic of an
in-situ crystallization process with variable amounts of interstitial
liquid L trapped between the cumulus mineral phases. LA-ICPMS
measurements show that trace-element distribution in the latter is
homogeneous, pointing to subsolidus re-equilibration between crystals
and interstitial melts. A quantitative modeling based on Langmuir's
in-situ crystallization equation successfully duplicated the REE
concentrations in cumulitic minerals of all rock facies of the
intrusion. The calculated amounts of interstitial liquid L vary between
0 and 35% for degrees of differentiation F of 0 to 20%, relative to
the least evolved facies of the intrusion. L values are well correlated
with the modal proportions of interstitial amphibole and whole-rock
incompatible trace-element concentrations (e.g. Zr, Nb) of the tested
samples. However, the in-situ crystallization model reaches its
limitations with rock containing high modal content of REE-bearing
minerals (i.e. zircon), such as pegmatitic gabbros. Dikes of
anorthositic composition, locally crosscutting the layered lithologies,
evidence that the Mont Collon rocks evolved in open system with mixing
of intercumulus liquids of different origins and possibly contrasting
compositions. The proposed model is not able to resolve these complex
open systems, but migrating liquids could be partly responsible for the
observed dispersion of points in some correlation diagrams. Absence of
significant differentiation with recurrent lithologies in the cumulitic
pile of Dents de Bertol points to an efficiently convective magma
chamber, with possible periodic replenishment, (c) 2005 Elsevier B.V.
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Création de la notice
01/10/2012 20:07
Dernière modification de la notice
31/10/2018 8:08
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