Cambrian paleogeography at the western Gondwana margin: U-Pb ages and provenance areas of detrital zircons of the Mesón Group (Upper Cambrian), Northwest Argentina.

ADAMS, C.J., MILLER, H., ACEÑOLAZA, G.F., AND TOSELLI, A.J

Niza, Francia

Simposio; International Symposium on Andean Geodynamics - ISAG; 2008

Institut de Recherche pour le Développement (IRD)

In northwest Argentina, sedimentation of the Puncoviscana Formation (uppermost Neoproterozoic-early Cambrian) finished with folding, metamorphism, and granitoid magmatism of the Pampean Orogeny in mid-Cambrian times. Above a pronounced angular unconformity, the turbidites of the Puncoviscana Fm. are overlain by siliciclastic sedimentary rocks of mostly sandstone, partly conglomerate, siltstone and mudstone grain size, the Mesón Group that is divided into 3 formations: Lizoite, Campanario, and Chalhualmayoc. The Mesón Group is the basal unit for the sedimentation of the Famatinian (Ordovician-Devonian) orogenic cycle in northwest Argentina. The siliciclastic rocks of the Mesón Group were deposited in shallow, coast-near tide-dominated environments in the form of sand bars (Sánchez & Salfity, 1999, Aceñolaza, 2003, 2005).   Generally, the age of the Mesón Group has been considered Cambrian. On paleontological evidence, Sánchez & Salfity (1999) and Aceñolaza (2003, 2005 ) restricted the age to “Middle to Upper Cambrian”. The presence of late Early Cambrian zircons in part of the underlying Puncoviscana Formation (Adams et al., 2008), and the early to mid Cambrian zircon ages of the Santa Rosa de Tastil and Cañaní granitoids intruding the Puncoviscana Formation (513 Ma: Adams, oral com.; 514 -519 - 536 Ma: Bachmann et al., 1991), indicate that sedimentation of the Mesón Group did not begin before the Middle Cambrian. The Mesón Group as a lithological unit ends-up (finishes? – your choice) before the Ordovician, whereas quite simmilar siliciclastic facies continues through the Cambrian/Ordovician boundary into the lowermost part of the Tremadocian Santa Rosita Fm. (Aceñolaza, 2005).   Recently, zircon geochronology studies have been published from the underlying Puncoviscana Formation by Adams et al. (2008), and to the south and southeast from metamorphic equivalents of it (Schwartz & Gromet, 2004, Rapela et al., 2007, Escayola et al., 2007). Detrital zircon studies on Ordovician units of the Central Andes have been carried out by Chew et al. (2007) and Augustsson et al. (2007). In order to define the geotectonic position of the Mesón Group within the Gondwana Pacific margin, its relation to the underlying Puncoviscana Formation and the Pampean and Famatinian orogeny, and to recognize the relation of the provenance area of its sediments to those of the Puncoviscana Formation, samples have been taken from outcrops close to the Puncoviscana Formation. The aim of this work was to know, if the provenance areas of sediments of  the Gondwana margin have changed since deposition of the Puncoviscana Formation and the Pampean orogeny, if erosion of the Puncoviscana Formation and its metamorphic equivalents have contributed much to the sediments of the Mesón Group, or, if there was an important new input from the Brazilian shield, similar to that of the time of deposition of the Puncoviscana Formation, or from anywhere else.