Field and textural evidence of two migmatitic events in the Sierras de Córdoba, Argentina

GUERESCHI, ALINA BEATRIZ; MARTINO, ROBERTO DONATO

Mendoza (Argentina)

Conferencia; Gondwana 12 Conference; 2005

Academia Nacional de Ciencias

The central sector of the Sierra de Comechingones has a polymetamorphic basement constituted mainly by anatectic migmatites and high-grade gneisses (Guereschi and Martino, 2003). The migmatites are of two types: metatexites and diatexites. The Tala Cruz metatexites (TCM) have stromatitic structure, with leucosomes of medium to coarse-grained granoblastic texture, composed of Qtz-Pl-Kfs; melanosomes formed by Bt-Sil-Grt; and mesosomes with a gneissic fabric, medium to fine-grained granoblastic texture, composed of Qtz-Pl-Grt-Bt-Rt-Zrn-Op. The Cañada del Sauce diatexites (CSD) have nebulitic structure and they include rafts of stromatitic migmatites, cordierite and quartz nodules, and xenoliths of gneisses. The nebulite has medium to coarse-grained granoblastic texture and it is constituted by Qtz-Pl-Kfs-Bt-Grt-Crd-Zrn-Ap-Op-Tur-Rt. The rafts of stromatitic migmatites vary from schollen-type inclusions, through schlierens to small Bt-Sil-Grt spots that can be interpreted as dismembered and partially assimilated remains of the TCM in the CSD. In all these rafts the mesoscopic layered fabric, the mineralogical composition and, especially, the internal fabric of the garnet are well preserved, but they have a superimposed blastesis of K-feldspar and cordierite which is not found in the TCM. The paragenesis of the metamorphic peak in the TCM is: Qtz + Pl + Bt + Grt + Sil + Kfs, representing equilibrium in the melting reaction: Bt + Sil + Qtz (+ Pl) = Grt + Kfs + melt. Textural evidence is the instability of biotite in contact with K-feldspar and the synkinematic blastesis of garnet including sillimanite and biotite. Finally, the CSD would have been generated, with K-feldspar and cordierite produced by the reaction: Bt + Sil + Qtz = Crd + Kfs + melt. Cordierite is also formed from garnet, by the reaction: Grt + Sil + Qtz = Crd. Thus, in the metamorphic evolution of the area two migmatization events are recognized: (A) a main regional metamorphic event or thermal peak that produced the dominant regional foliation, the high-grade parageneses and a first partial melting or anatexis in pelitic rocks, generating the TCM. Using thermobarometry in Grt-Sil gneisses, a temperature of 776ºC and a pressure of 7.9 Kb were determined for the metamorphic peak in the area (Guereschi and Martino, 2002). (B) a static decompressional event, with minor decrease of the temperature, which produced the CSD by new melting of the TCM. The more highly melted portions would have been homogenized, producing the remnants of stromatitic migmatites included in the nebulitic portion. The P-T conditions of this event are being analysed at the moment. The existence of two migmatization events had already been outlined by Gordillo and Lencinas (1979), although they considered the second event as the metamorphic peak and did not carry out detailed studies in the stromatitic migmatites. However, their observations were overlooked in later works. Recently, we have found field evidence of the two migmatization events in the Sierra Chica and in the Macizo de San Carlos, which emphasizes the regional importance of these events. The CSD constitutes the southern end of the Macizo Migmático de Cerro Pelado and together they form the Grupo Yacanto, which is correlated with the Macizo de San Carlos to the north of the Achala batholith, thus defining one of the most important regionally thermal axes in the Sierras Pampeanas de Córdoba (Martino, 2003). The main metamorphism in the Sierra de Córdoba has been dated at 520–530 Ma (Early Cambrian; main references and discussion in Guereschi and Martino, 2003 and Martino, 2003). However, based on lithology and similar processes, a correlation might be established between the second migmatitic event caused by decompression in the Sierra de Comechingones and the dated Cambrian event. In this case, the first migmatization event or metamorphic peak would probably have been Neoproterozoic. The two recognized events of migmatization are related to the evolution of a collisional orogen developed between the western margin of Gondwana and the Pampia terrane.   Gordillo, C.E., and Lencinas, A.N., 1979, Sierras Pampeanas de Córdoba y San Luis, in Leanza, A.P., ed., Geología Regional Argentina v. 1, Academia Nacional de Ciencias, Córdoba, Argentina, p. 577-650.. Guereschi, A.B., and Martino, R.D., 2002, Geotermobarometría de la paragénesis cuarzo-plagioclasa-biotita-granate-sillimanita en gneises del sector centro-oriental de la Sierra de Comechingones, Córdoba: Revista de la Asociación Geológica Argentina, v. 57, p. 365-375. Guereschi, A.B., and Martino, R.D., 2003, Trayectoria textural de las metamorfitas del sector centro-oriental de la Sierra de Comechingones, Córdoba: Revista de la Asociación Geológica Argentina, v. 58, p. 61-77. Martino, R.D., 2003, Las fajas de deformación dúctil de las Sierras Pampeanas de Córdoba: una reseña general: Revista de la Asociación Geológica Argentina, v. 58, p. 549-571. Número especial dedicado a F. González Bonorino: Geología de las Sierras Pampeanas.