Reddish Metagranites from the Gennargentu Igneous Complex (Sardinia, Italy): insight into metasomatism induced by magma mingling

The mineralogy of thermometamorphic granites is relatively simple,
making it possible to track the spatial distribution of chemical and
mineralogical variations in these rocks and investigate the processes
that underpin these metamorphic reactions.We have undertaken a
detailed investigation of metagranites from the contact aureole that
fringes a quartz diorite intrusion of Late Permian age, emplaced
into Carboniferous peraluminous granites of the Gennargentu
Igneous Complex (Sardinia, Italy). New data are presented including
the petrography of metagranites within a 500 m zone adjacent to
the quartz diorite intrusion, the compositions of minerals and
bulk-rocks, and the oxygen isotope compositions of separated minerals.
We have used these data to assess the mobility of elements,
expressed as oxide, in the aureole, and the physical conditions of
fluid-assisted thermometamorphism. Modal variations and the
oscillatory zoning of plagioclase demonstrate that the shallow
(P 200MPa) quartz diorite intrusion was emplaced through a
number of magmatic injections.The border zone of the quartz diorite
intrusion presents evidence of two main processes: hybridization between
andesite and rhyolite magmas and volatile saturation of the
mingled magma. Modal differences in the contact zone with respect
to the protolith (i.e. peraluminous granite), variations in mineral
composition, temperature constraints and K2O, Na2O, SiO2
and Al2O3 indicate that a relatively large volume of the host granite
(up to 400 m from the contact) was metasomatized by
high-temperature (650^3508C) fluids derived from the mingled
zone of the quartz diorite intrusion. In detail, the metasomatic
K2O-rich fluid reacted with albite to form K-feldspar, and triggered the recrystallization of quartz and plagioclase to higher calcium concentrations.
The progressive increase in the MgO/(MgOþFeO)
of chlorite closer to the contact indicates that this phase also recrystallized.
The iron released during chlorite recrystallization was buffered
by hematite formation in the pores of metasomatic K-feldspar. The
Gennargentu metagranites provide evidence that metasomatic fluids
can play a major role in driving metamorphic reactions in contact
aureoles. For instance, the expected increase of Ca in plagioclase
owing to thermal equilibration was not achieved in the high-T zone
of the aureole because of fluid-assisted removal of cations.We conclude
that caution should be taken when interpreting the processes
that underpin contact metamorphism in terms of thermally driven,
ionic diffusion alone, because the role of fluids may be significant,
if not overwhelming, in the domains closest to the magmatic source.