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| Titel |
Remelting of nanogranites in peritectic garnet from granulites of Jubrique, Betic Cordillera, Southern Spain |
| VerfasserIn |
Amel Barich, Omar Bartoli, Antonio Acosta-Vigil, Stefano Poli, Bernardo Cesare, Carlos J. Garrido |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250096781
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| Publikation (Nr.) |
 EGU/EGU2014-12300.pdf |
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| Zusammenfassung |
| Peritectic minerals in migmatites can trap droplets of melt that form via incongruent melting
reactions during crustal anatexis [1]. In most cases, these melts crystallize and form
nanogranites upon slow cooling of the anatectic terrane [2]. To obtain information on the
primary compositions of crustal melts, including volatile concentrations in melt and
information on fluid regimes, nanogranites must be remelted and rehomogenized before
analysis [3].
A new occurrence of nanogranites was recently reported in granulitic gneisses at the
bottom of the prograde metamorphic sequence of Jubrique, located on top of the
Ronda peridotite slab (Betic Cordillera, S Spain) [4]. These nanogranites are trapped
in garnet porphyroblasts. They show partially irregular to well facetted negative
crystal shapes, and have variable size from ~5-10 μm to several tens of μm or even
~200 μm in diameter. They appear at the core and rim of large Grt crystals, and
are composed of rare glass, daughter Qz, Pl, Kfs, Bt and Ms, and solid inclusions
of Ky and less frequently Gr, Hc, Rt, Ilm, Zrn and Mnz. Ky was the main solid
phase that favored the trapping of melt inclusions (MI) by poisoning crystal surfaces
during Grt growth. The presence of Ky+Rt within MI, and their occurrence in the
high-P cores of Grt, suggest that partial melting in the gneisses initiated at relatively
high P conditions, and that most Grt in these rocks crystallized in the presence of
melt.
To recover the major and trace element composition of the primary melt during anatexis,
we performed remelting experiments on chips of MI-bearing Grt separated from a sample
of gneiss at Jubrique, using a piston cylinder apparatus. Experiments were run at
800, 825 and 850°C, 15 Kbar and dry conditions (i.e. no added H2O) for 24 hrs.
Preliminary results of the first experiment at 850º show that nanogranites have been
successfully remelted; they all contain glass regardless of their textural position,
i.e. at the core and rim of large Grt or in small single Grt. Glass is leucogranitic
(SiO2 -65wt%; FeO+MgO-2 wt%), potassic (K#-12), high in ASI (close to
peraluminous) and hydrous (H2O, estimated by deficiency from 100% in the EMP analysis,
-11wt.%.). The degree of remelting, however, varies among inclusion regardless
of the textural position; some inclusions only show glass whilst others have, in
addition, residual Als±Qz±Sp±Bt±Gr plus new St, which overgrows on Ky. The
presence of St, decrepitation cracks, Grt reaction rims and irregular boundaries in some
inclusions suggest interaction between melt and Grt and/or overheating during the
experiment.
[1] Cesare et al. (2011) J. Virt. Expl., 40, paper 2. [2] Cesare et al. (2009) Geology, 37,
627-630. [3] Bartoli et al. (2013) Geology, 41, 115-118. [4] Barich et al. (2013) GSA
Abstracts with Programs. |
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