Download A combined study of melt inclusions and classical petrology on the

A combined study of melt inclusions and classical petrology on the Ronda metatexites (S Spain)
Por: Omar Bartoli
Dipartimento di Geoscienze, Università di Padova
Viernes, 4 de Mayo de 2012, 12.30 horas
Sala de Audiovisuales, Facultad de Ciencias
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Melt inclusions hosted in peritectic minerals of
migmatites represent a novel and powerful
small-scale tool to investigate the anatexis
of the continental crust. In this study, using
as an example the Ronda metatexites from
the Betic Cordillera (S Spain) and taking
advantage of a new experimental approach
for remelting crystallized melt inclusions, a
combined study of classical petrology and
melt inclusions in migmatites was performed
to characterize in detail the composition and
the physical properties of anatectic melts,
the fluid regimes, and the melting
mechanisms and conditions during the
anatexis of the metasedimentary crust
located below the Ronda peridotite.
Doctor Omar Bartoli got his Bachelor
degree in Geology (2008) at the
Università di Parma (Italy). During his
Master thesis he worked on the
petrology and geochronology of
magmatic enclaves hosted in
trachytes, and on melt inclusions in
migmatites from Ronda (S Spain)
during his PhD thesis (Università di
Parma, 2012). At present, he is a
research fellow at the Università di
Padova (Italy), working on the
characterization of the early crustal
melts produced in different anatectic
terranes through the geochemical
study of melt inclusions and
nanogranites in migmatites and
granulites.
The quartzo-feldspathic metatexites are mainly
composed of Qtz + Pl + Kfs + Bt + Sil + Grt
and probably derived from a greywacke
protolith. Muscovite is very rare. The former
presence of melt is recorded by melt
inclusions and melt pseudomorphs at the
microscale, and by peraluminous
leucogranitic leucosomes at the mesoscale.
Melt inclusions have been found in peritectic
garnets of the investigated quartzofeldspathic migmatites. They are very small
in size, mostly ≤ 10 µm, and typically show a
well-developed negative crystal shape. Three
types of inclusions were identified: totally
crystallized (nanogranites), partially
crystallized and preserved glassy inclusions.
Crystallized melt inclusions contain a
granitic phase assemblage with quartz,
feldspars and micas. The remelting
experiments were conducted with a piston
cylinder apparatus, an approach that has
never been attempted before, until the
complete homogenization of crystal-bearing
melt inclusions at conditions (T=700 °C and
P=5 kbar) close to those inferred for
anatexis. The compositions of quenched
glasses are leucogranitic and peraluminous.
H2O content measured by Raman
spectroscopy ranges from 3.1 to 7.6 wt %.
The pseudosection constructed using the
bulk composition of the fully rehomogenized melt inclusions constrained the
melt entrapment at temperature close to the
minimum re-homogenization temperature (T
~700 °C). The composition of those melt
inclusions re-homogenized at 700 °C is
comparable to that of preserved glassy
inclusions in the same rocks.
Combining information from the melt inclusions
and from the classical petrology allowed a
better understanding of the melting
processes occurred in the crustal sequence
below the Ronda peridotite. The data
collected in this study suggest that the
crustal melting in Ronda metatexites
occurred initially and locally at the fluidsaturated solidus owing to the presence of
H2O-rich intergranular fluids, and likely
progressed by mica breakdown melting. Melt
inclusions record the composition of the
melt during the earliest stages of anatexis;
on the other hand, leucosomes in the same
rocks reflect the composition of melt at, or
closer to, the peak metamorphic conditions.
The anatectic melts at Ronda have viscosity
values greater than those commonly
considered for granitic melts formed at the
same P-T conditions, implying much longer
timescales for melt extraction and ascent
through the metasedimentary crust at
Ronda, as well as much greater strength of
the migmatites.