INVESTIGADORES
OTAMENDI Juan Enrique
artículos
Título:
Amphibolite to granulite transition in aluminous greywackes from the Sierra de Comechingones, Córdoba, Argentina
Autor/es:
OTAMENDI, J.E., PATIÑO DOUCE, A.E. Y DEMICHELIS, A.H
Revista:
JOURNAL OF METAMORPHIC GEOLOGY
Editorial:
WILEY-BLACKWELL PUBLISHING, INC
Referencias:
Año: 1999 vol. 17 p. 415 - 434
ISSN:
0263-4929
Resumen:
We discuss upper-amphibolite- to granulite-facies, early Paleozoic
metamorphism and partial melting of aluminous greywackes from the Sierra de
Comechingones, SE Sierras Pampeanas of Central Argentina. Consistent
pressure-temperature estimates, obtained from equilibria involving Al and Ti
exchange components in biotite and from more traditional thermobarometric
equilibria, suggest that peak metamorphism of the exposed section took place at
an essentially constant pressure of 7 - 8 kbar, and at temperatures ranging from
650
ºC to 950 ºC. Mineral compositions record an initial decompression, after peak
metamorphism, of ~ 1.5 kbar, that was
accompanied by a cooling of ~100 ºC. Upper-amphibolite-facies gneisses consist
of the assemblage Qtz + Pl + Bt + Grt + Rt / Ilm. The transition to the
granulite facies is marked by the simultaneous appearance of the assemblage Kfs
+ Sil and of migmatitic structures, suggesting that the
amphibolite-to-granulite transition in the Sierra de Comechingones corresponds
to the beginning of melting. Rocks with structural and / or chemical
manifestations of partial melting range from metatexites, to diatexites, to
melt-depleted granulites, consisting of the assemblage Grt + Crd + Pl + Qtz +
Ilm ± Ath. The melting stage overlapped at least partially with decompression,
as suggested by the occurrence of cordierite, in both the migmatites and the
residual granulites, of two distinct textural types: idiomorphic porphyroblasts (probably
representing peritectic cordierite) and garnet-rimming coronas. Metapelitic
rocks are unknown in the Sierra de Comechingones. Therefore, it appears most
likely that the Al-rich residual assemblages found in the migmatites and
residual granulites was formed by partial-melting of muscovite- and
sillimanite-undersaturated metagreywackes.We propose a mechanism for this that relies on the subsolidus stabilization
of garnet and the ensuing changes in the octahedral Al content of biotite with
pressure and temperature.