Carbonate assimilation in open magmatic systems: the role of melt-bearing skarns and cumulate-forming processes

The geochemical characteristics of volcanic products in a variety of
tectonic settings demonstrate that incorporation of crustal material
into magmas is a relatively common process. Contamination of
magmas by crustal components, in turn, can have a significant
effect on magma composition and rheology. Despite this, the mechanism
by which contamination occurs is still not well established and
its efficacy is denied by some. In this study we focus on magma^
carbonate interaction and on the rock shells (cumulates and
skarns) formed at the contact between a magma chamber and its
wall-rocks.We deduce that previous, unsuccessful attempts at carbonate
assimilation^fractional crystallization (AFC) modelling can
be related to the paucity of information about the cumulate zone in
contact with skarns.We use one of the best examples of a magmatic
plumbing system emplaced within a thick carbonate substratum (the
Colli AlbaniVolcanic District in Central Italy) to demonstrate that
a ‘skarn environment’can act as a source of CaO-rich silicate melts,
and that the assimilation of these melts into the primitive magma is
the main process responsible for magma contamination, rather
than the ingestion of solid carbonate wall-rocks. In particular, by
means of microtextural observations, mineral chemistry, whole-rock
geochemical data and MELTS simulations we highlight the effect
of high Ca-Tschermaks (CaAl2SiO6) activity in the melt on the
stability of Cr-spinel, olivine, and clinopyroxene in cumulate rocks,
define a reaction-cumulate zone where clinopyroxene crystallization
is favoured, and model the magmatic differentiation processes active
in this zone.

INGV-DPC Project V3_1 Colli Albani
MIUR ‘Caratteristiche vulcanologiche e geomeccaniche
dei prodotti recenti dei Colli Albani’.

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