Amphibolite to granulite transition in aluminous greywackes from the Sierra de Comechingones, Cordoba, Argentina

We discuss upper-amphibolite to granulite facies, early Palaeozoic metamorp hism and partial melting of aluminous greywackes from the Sierra de Comechi ngones, SE Sierras; Pampeanas of Central Argentina. Consistent P-T estimate s, obtained from equilibria involving Al and Ti exchange components in biot ite and from more traditional thermobarometric equilibria, suggest that pea k metamorphism of the exposed section took place at an essentially constant pressure of 7-8 kbar, and at temperatures ranging from 650 to 950 degrees C. Mineral compositions record an initial decompression, after peak metamor phism, of c. 1.5 kbar, which was accompanied by a cooling of c. 100 degrees C. Upper-amphibolite facies gneisses consist of the assemblage Qtz + P1 Bt + Grt + Rt/Ilm. The transition to the granulite facies is marked by the simultaneous appearance of the assemblage Kfs + Sil and of migmatitic struc tures, suggesting that the amphibolite to granulite transition in the Sierr a de Comechingones corresponds to the beginning of melting. Rocks with stru ctural and/or chemical manifestations of partial melting range from metatex ites, to diatexites, to melt-depleted granulites, consisting of the assembl age Grt + Crd + Pl + Qtz + Ilm +/= Ath. The melting stage overlapped at lea st partially with decompression, as suggested by the occurrence of cordieri te, in both the migmatites and the residual granulites, of two distinct tex tural types: idiomorphic porphyroblasts (probably representing peritectic c ordierite) and garnet-rimming coronas. Metapelitic rocks are unknown in the Sierra de Comechingones. Therefore, it appears most likely that the Al-ric h residual assemblages found in the migmatites and residual granulites were formed by partial melting of muscovite- and sillimanite-undersaturated met agreywackes. We propose a mechanism for this that relies on the sub-solidus stabilization of garnet and the ensuing changes in the octahedral Al conte nt of biotite with pressure and temperature.