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

GUERESCHI, A.B.; MARTINO, R.D.

Mendoza

Simposio; Gondwana 12: Geological and Biological Heritage of Gondwana; 2005

Varias instituciones argentinas

Field and textural evidences of two migmatitic events in the Sierra de Córdoba, Argentina Alina B. Guereschi y Roberto D. Martino Conicet y Facultad de Ciencias Exactas, Físicas y Naturales,Universidad Nacional de Córdoba. Av. Vélez Sársfield Nº1611 (X5016GCA), Córdoba, Argentina. aguereschi@efn.uncor.edu, rdmartino@com.uncor.edu In the central sector of the Sierra de Comechingones crops out a polimetamorphic 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 by Qtz-Pl-Kfs; melanosomes formed by Bt-Sil-Grt; and mesosomes with gneissic fabric, medium to fine-grained granoblastic texture, composed by 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, schlierens until 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 mineralogic composition and especially the internal fabric of the garnet are well preserved, but they have a superimposed blastesis of K-feldspar and cordierite which it is not found in the TCM. The paragenesis of the metamorphic peak in the TCM is: Qtz + Pl + Bt + Grt + Sil + Kfs, equilibrium of the melting reaction: Bt + Sil + Qtz (+ Pl) = Grt + Kfs + melt. The textural evidences are the instability of biotite in contact with K-feldspar and the synkinematic blastesis of garnet including sillimanite and biotite. Lately, the CSD would have been generated, with K-feldspar and cordierite produced by the reaction: Bt + Sil + Qtz = Crd + Kfs + melt. It is also formed cordierite starting 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, the physical conditions of 776ºC of temperature and 7,9 Kb of pressure for the metamorphic peak of the area were determined (Guereschi and Martino, 2002). (B) an static decompressional event, with minor decrease of the temperature that gave origin to the CSD from a new melting of the TCM. The more melted portions would have been homogenized, being remains of the stromatitic migmatites included in the nebulitic portion. The P-T conditions of this event are analyzing in these moments. The existence of two migmatization events had already been outlined by Gordillo and Lencinas (1979), although considering to the second event like the metamorphic peak and without having carried out detailed studies in the stromatitic migmatites. However, this observations were overlooked in later works. Recently, we have found field evidences of the two migmatization events in the Sierra Chica and in the Macizo de San Carlos, which emphasize the regional importance of these events. The CSD constitutes the southern end of the Macizo Migmático de Cerro Pelado and they integrate the Grupo Yacanto which is correlated with the Macizo de San Carlos to the north of the Achala batholith, defining in this way one of the most important regionally thermal axes in the Sierras Pampeanas de Córdoba (Martino, 2003). The main metamorphism for the Sierra de Córdoba has been dated in 520-530 Ma (lower Cambrian; main references and discussion in Guereschi and Martino, 2003 and Martino, 2003). However, based on lithology and similar processes, a correlation could be established between the second migmatitic event taken place by decompression in the Sierra de Comechingones and the event dated as Cambrian. Of being this way, the first migmatization event or metamorphic peak would probably be upper Proterozoic. The two recognized events of migmatization are related to the evolution of a collisional orogen developed between the western margin of Gonwana and the Pampia terrane. REFERENCES CITED Gordillo, C.E. and Lencinas, A.N., 1979, Sierras Pampeanas de Córdoba y San Luis. In: Geología Regional Argentina (ed. Leanza, A.P.), v. 1, p. 577-650. Academia Nacional de Ciencias, Córdoba, Argentina. 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.