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PHYLLITES AND META-CONGLOMERATES OF THE SAN LUIS FORMATION AT CAÑADA HONDA: KINEMATICS AND TIME CONSTRAINTS Roberto Martino1, André Steenken2, 3, Mónica López de Luchi3, Klaus Wemmer2, Siegfried Siegesmund2 1 Cátedra de Geología Tectónica, Facultad de Ciencias Exactas, Físicas y Naturales. Universidad Nacional de Córdoba. Av. Vélez Sarsfield 1611. X 5016 GCA-Córdoba, rdmartino@com.uncor.edu. 2 Geoscience Centre of the University of Göttingen (GZG), Goldschmidtstr. 3, 37077 Göttingen, Germany, ssieges@gwdg.de, kwemmer@gwdg.de, asteenk@gwdg.de. 3 Instituto de Geocronología y Geología Isotópica (Ingeis),Ciudad Universitaria,1428 Buenos Aires, Argentina, deluchi@ingeis.uba.ar. On the Cañada Honda river, in the eastern belt of the San Luis Formation (Prozzi and Ramos 1988) phyllites and interlayered meta-conglomerates are recognized (Prozzi 1990). The phyllites are concordantly interlayered with an approximately 300m layer of poorly selected matrix-supported polymitic meta-conglomerates. Clasts are dominantly phyllitic, with minor quarzites and pure quartz that are intensely deformed. The matrix is a quartz rich phyllite composed of slightly irregular granoblastic quartz aggregates in a mica-rich matrix composed of quartz, sericite, chlorite, opaques and scarce zircons. Two sets of quartz veins crosscut the sequences coeval or pre-dating compressive deformation. The quartz veins are boudinated with necks almost perpendicular to the foliation. The very fine grained matrix occupies the rectilinear to slightly curved necks. The most common structure is a N 20°/ 80°-90° W S1 foliation that is defined by a combination of cleavage and banding in the phyllites. A lineation defined by sericite-chlorite trends N 215°-210°/52°-62° on S1. The clasts of the conglomerates are elipsoidal with their major axis parallel to the lineation. At the microscopic scale a second poorly developed S2 foliation cutting S1 of the phyllites is observed. Both S1 and S2 are defined by chlorite and sericite, i.e. at green-schist facies conditions. The assimetry of the (Chl1) chlorite blasts suggest west-up reverse kinematics with dextral strike-slip. On the basis of the boudinage of the quartz aggregates stretchting parallel to S1 is inferred. An elongation e = (lf - li)/ li = 0.43 is calculated. The poorly developed S2 foliation that is oblique to S1 and dips ~20° W results from a later extensional event that is indicated by the wrapping of the S1 foliation around Chl2 blasts. Due to the uniform metamorphic conditions and the location of the S2 foliation in the rheological weak phyllites it is considered that the extensional readjustment is post-compressional. Although a common deposition for the sediments of the San Luis Formation within the Puncoviscana Trough has been suggested (e.g. Rapela et al. 1998, Söllner et al. 2000), Nd isotope signatures do not support this correlation (Steenken et al. 2004). Age constraints on the sedimentation age of the phyllites are provided by the 478-468 Ma (e.g. Stuart-Smith et al., 1999) syn-kinematic intrusion of tonalites. The maximum age for the sedimentation is provided by a 529 ± 12 Ma zircon age of metavolcanic layers intercalated in both phyllite belts (Söllner et al. 2000). The metamorphic overprint and the west-up reverse kinematic can be related with the metamorphic peak of the Famatinian orogeny whereas the recorded extensional movements are more difficult to constrain. The time interval between 330 Ma and 290 Ma that is recorded by K/Ar mica fine-fraction dating technique (Wemmer 1991) is consistent with the formation of extensional graben structures in the easternmost parts of the Sierra de San Luis that appear as the basins for the Early Permian Bajo de Veliz Formation (i.e. the Paganzo Group, Azcuy et al. 1999). The lower limit, i.e. 290 Ma corresponds to the San Rafael orogenic phase (Azcuy et al. 1999) and may indicate the control of this orogenic phase on the latest displacements registered by the San Luis Formation. REFERENCES CITED Azcuy, C.L., Carrizo, H.A., and Caminos, R., 1999. Carbonífero y Pérmico de las Sierras Pampeanas, Famatina, Precordillera, Cordillera Frontal y Bloque de San Rafael: in Caminos, R.., ed., Geología Regional Argentina: Anales 29, p. 261-318. Prozzi, C.R., 1990. Consideraciones acerca del basamento de San Luis, Argentina: Actas XI° Congreso Geológico Argentino V. 1, p. 452-455. Prozzi, C.R., and Ramos, G., 1988. La Formación San Luis: Primeras Jornadas de Trabajo de Sierras Pampeanas: Abstract, p. 1. Rapela C.W., Pankhurst R.J., Casquet C., Baldo E., Saavedra J., Galindo C., and Fanning C.M., 1998. The Pampean Orogeny of the southern proto-Andes: Cambrian continental collision in the Sierras de Córdoba, in Pankhurst R.J., and Rapela C.W., eds., The Proto-Andean margin of Gondwana: Geological Society of London Special Publications 142, p. 181-217. Söllner, F., de Brodtkorb, M.K., Miller, H., Pezzutti, N.E., and Fernández, R.R., 2000. U-Pb zircon ages of metavolcanic rocks from the Sierra de San Luis, Argentina: Revista de la Asociación Geológica Argentina 55, p. 15-22. Steenken, A., López de Luchi, M.G., Siegesmund, S., Wemmer, K., and Pawlig S., 2004. Crustal provenance and cooling of the basement complexes of the Sierra de San Luis: An insight into the tectonic history of the proto-Andean margin of Gondwana: Gondwana Research 7, p. 1171-1195. Stuart-Smith, P.G., Camacho, A., Sims, J.P., Skirrow, R.G., Lyons, P., Pieters, P.E., and Black, L.P., 1999. Uranium lead dating of felsic magmatic cycles in the southern Sierras Pampeanas, Argentina: implications for the tectonic development of the Proto-Andean Gondwana margin, in Ramos V.A., and Keppie J.D., eds., Laurentia Gondwana connections before Pangea: Geological Society of America Special Paper 336, p. 87-114. Wemmer, K., 1991, K/Ar-Altersdatierungsmöglichkeiten für retrograde Deformationsprozesse im spröden und duktilen Bereich - Beispiele aus der KTB-Vorbohrung (Oberpfalz) und dem Bereich der Insubrischen Linie (N-Italien): Göttinger Arbeiten zur Geologie und Paläontologie 51, p. 1-61.

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