Phyllites and Meta-Conglomerates of the San Luis Formation at Cañada Honda: Kinematics and time constraints

MARTINO, R.D., ; STEENKEN, A.; LÓPEZ DE LUCHI, M. G.; SIEGESMUND, S.; WEMMER, K.

Mendoza

Simposio; Gondwana 12 Geological and biological heritage of Gondwana; 2005

Academia Nacional de Ciencias

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 S 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 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 reverse kinematics with dextral strike-slip. On the basis of the boudinage of the quartz aggregates stretchting parallel to S1 is inferred. 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 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.