Contents volume 124, 1996

Abstract

The Cuchilla Dionisio Terrane (CDT) of Uruguay is a tectonostratigraphic unit defined as the block to the east of the Sierra Ballena Shear Zone (SBSZ). It is composed of a Palaeo- to Mesoproterozoic metamorphic basement (Cerro Olivo and Chafalote Complexes) intensely reworked and intruded by syncollisional to late orogenic granites between 680 and 550 Ma. Dacitic and rhyolitic volcanism is recorded around 570–575 Ma (Cerros de Aguirre and Sierra de Ríos Formations). The CDT is correlated with the Pelotas Terrane of southern Brazil. The western boundary of the block in Brazil is defined by the Canguçú and Major Gercino shear zones, which are the northward extension of the SBSZ. The present position of the CDT is not a consequence of orthogonal collision after completion of a Wilson Cycle, but of lateral sinistral accretion along the above mentioned megashear zones. The allochthonous nature of the CDT is postulated on the grounds that: (a) magmatism in the CDT is ca. 120 Ma younger than metamorphism of neighbouring volcanosedimentary successions, (e.g., Porongos Group in Brazil); and (b) while intense volcanism occurred in the CDT in the late Vendian, represented by the Cerros Aguirre and Sierra de Ríos Formations, a passive continental margin deepening to the E existed on the eastern edge of the Río de la Plata Craton, represented by the Arroyo del Soldado Group. Therefore the CDT and Río del la Plata Craton were separated hundreds or thousands of kilometers by late Vendian times. Accretion of the CDT took place at 530 Ma by tangential collision, and was one of the last events in the complicated amalgamation of W-Gondwana. In Uruguay, the CDT collided with the Nico Pérez Terrane, generating the SBSZ and sinistrally reactivating the Sarandí del Yí-Piriápolis Shear Zone. In Brazil, the CDT-Pelotas Terrane collided with the Paraná Block and a series of parautochthonous terranes (Curitiba, Apiaí) characterized by a Transamazonian basement strongly reworked in the Neoproterozoic.The provenance of the CDT is still uncertain, considering that both Transamazonian and Namaqua-Natal (Kibaran) ages have been reported from its basement. Previously reported Nd model ages (TDM) strongly suggest an African affinity. The Dom Feliciano Belt is thus a collage of diachronous units formed in different geotectonic settings, and not the product of a single orogenic cycle. Terrane-docking at different stages mainly by lateral accretion was the dominant process. Therefore usage of the name “Dom Feliciano Belt” should be restricted only to descriptive purposes.