Basement evolution of the Sierra de la Ventana fold belt: New evidence for Cambrian continental rifting along the southern margin of Gondwana

Journal of the Geological Society, Jul 2003 by Rapela, C W, Pankhurst, R J, Fanning, C M, Grecco, L E

Abstract: U-Pb sensitive high-resolution ion microprobe data together with geochemical and Nd isotope analyses obtained in the basement complex of the Sierra de la Ventana Fold Belt indicate that the Early Palaeozoic passive margin history of the basin followed Cambrian magmatism related to rifting in a 600 Ma Neoproterozoic crust. The Cambrian episode started with intrusion of 531 + or - 4 and 524 + or - 5 Ma A- and I-type granites derived from a dehydrated infracrustal source ([epsilon]Nd^sub 530^ -3.1 to -5.9), and culminated with eruption of high-Zr peralkaline spherulitic rhyolites derived from an undepleted lithospheric mantle (509 + or - 5 Ma; [epsilon]Nd^sub 509^ +0.5 to +1.0). These rift-related magmatic rocks were covered by shelf sediments deposited along a once-continuous passive margin, encompassing the Sierra de la Ventana Fold Belt, the Cape Fold Belt, the Falkland/Malvinas microplate and the Ellsworth Mountains block in Antarctica. The Cambrian rifting event defined the outline shape of the southern part of Gondwana, and can be regarded as the initiation of the supercontinent stage, which lasted until Jurassic break-up. The conjugate continental fragments separated from Gondwana during the Cambrian rifting could be the source for microcontinents with c. 1000 Ma basement rocks that collided with the proto-Andean margin during Ordovician-Silurian times.

Keywords: U-Pb SHRIMP, Gondwana, geochronology, granites, rifting.

The mountain chain of the Sierra de la Ventana (also known as the Sierras Australes of Buenos Aires) represents a curved fold and thrust belt, 160 km long, located at 38[degrees]00'S, near the Atlantic margin of South America (Fig. 1). Its folded Palaeozoic sedimentary sequence has been a landmark for Gondwana reconstruction since the pioneering observations made by Keidel (1916), who established the close connection with similar sequences in South Africa, and Du Toit (1927, 1937), who included the Sierra de la Ventana and the Cape Fold Belt in his 'Samfrau geosyncline'. Most previous studies of the Sierra de la Ventana belt have focused on the stratigraphy, sedimentology and penetrative deformation that affected the Late Cambrian? to Permian sedimentary pile (see von Gosen et al. (1990) and Limarino et al. (1999) for a comprehensive list of references on these topics). The role of the igneous-metamorphic basement of the basin in the history of Gondwana is loosely constrained because of the lack of reliable radiometric ages of the igneous rocks, partly as a result of resetting of the K-Ar and Rb-Sr systems (Varela et al. 1990 and references therein). These basement rocks, deformed by the penetrative Late Palaeozoic folding and shearing event(s) of the Gondwanian orogeny affecting the whole basin, are seen as the counterpart of the Saldania belt, which is the basement of the sedimentary sequence deformed in the Cape Fold Belt (Fig. 2). The Saldania belt is in turn the southernmost part of a system of Neoproterozoic-Cambrian mobile belts that welded older cratons onto the African sector during the construction of Gondwana (Fig. 2) (Rozendaal et al. 1999).

We have investigated the magmatic history of the basement of the sedimentary sequence deformed in the Sierra de la Ventana Fold Belt, to make direct comparison and correlation with neighbouring basement terranes in NW Argentina, Uruguay, SE Brazil, and the postulated Gondwana counterparts in South Africa and Antarctica. These rocks are geographically located in a key area for identification of (1) the processes leading to formation of the proto-Pacific margin in SW Gondwana, and (2) the Neoproterozoic collision associated with the closing of the proto-Atlantic Adamastor ocean, which amalgamated the Rio de la Plata and the Kalahari pre-Mesoproterozoic cratons (e.g. Campos Neto 2000). This study includes the first U-Pb geochronological data for the Sierra de la Ventana basement. This method is considerably less susceptible to disturbance by deformation and low-grade thermal events than those used in previous studies of this area. Precise U-Pb sensitive high-resolution ion microprobe (SHRIMP) crystallization ages, together with geochemical and Nd-isotope signatures determined for the main magmatic units of the basement, have been used to reconstruct the geological and geodynamic evolution of the area before the protracted sedimentation of the basin, which lasted almost throughout the Palaeozoic era. An important conclusion is that initiation of the long-lived marine basin was in fact closely associated with the latest igneous events registered in the Sierra de la Ventana basement and in other sectors of the proto-Pacific margin.

Geological setting

Stratigraphy and age

The Palaeozoic fold belt of the Sierra de la Ventana is surrounded by a plain underlain by Late Tertiary and Quaternary sediments. Geophysical data (gravity, magnetism, heat flux and refraction seismicity; Kostadinoff 1993), as well as information from boreholes, show that the folded Palaeozoic sediments form a large subsurface basin, extending eastwards as far as the Middle Proterozoic Tandilia belt, and reaching a maximum thickness of 9 km, with the widest sector extending towards the Atlantic Ocean (Fig. 2).