|
Kerstin Saalmann, Geological Survey of Finland (Finland)
Marcus V.D. Remus, Universidade Federal do Rio Grande do Sul (Brazil)
Léo A. Hartmann, Universidade Federal do Rio Grande do Sul (Brazil)
|
|
Numerous alternative configurations have been proposed for the supercontinent Rodinia. In most reconstructions, the Rio de la Plata craton (RPC) is attached to the Amazon craton and placed in the centre of the supercontinent and hence in the major Grenville orogenic belt. However, the RPC is made up of basement rocks of Archaean and Palaeopoterozoic age, partly reworked in the Neoproterozoic Brasiliano orogenic cycle, with the latter starting already 880 Ma ago. There is also no geologic evidence for magmatism and sedimentation related to the proposed 750 Ma Rodinia break-up as would be expected from a central position of the RPC in this supercontinent.
The Neoproterozoic tectonic history at the eastern margin of the craton, recorded in southernmost Brazil, in contrast indicates the existence of a broad oceanic realm to the (present-day) E with subduction-activity between 880 Ma and 700 Ma. The southern Brasilian Shield Basement comprises from W to E the RPC, the juvenile Neoproterozoic São Gabriel block and the Encantadas block containing the Porongos and Dom Feliciano belts. Relics of two juvenile Neoproterozoic magmatic arcs, an intra-oceanic arc, and a younger continental arc or active continental margin can be identified in the São Gabriel block.
First subduction occurred at about 0.88 Ga, leading to accretion of the intra-oceanic island arc to the passive margin of the RPC. At ca. 780-700 Ma W-directed subduction occurred beneath an active continental margin consisting of the RPC and the attached island arc. At the same time, the Porongos basin formed on stretched continental crust of the Encantadas continental block. The collision of the RPC and São Gabriel block with the Encantadas block occurred between ca. 700 to 670 Ma. Deformation migrated further to the E with folding and shearing in the Porongos belt at 670-630 Ma followed by 630-610 Ma sinistral faulting in the Porongos belt and ductile shearing and voluminous granite magmatism in the Dom Feliciano belt. The protracted subduction-activity demonstrates that there was a broad ocean to the E of the RPC that existed for min. 180 Ma and that subduction started soon after the final stages of the Grenville orogeny.
The geological data and the tectonic history are incompatible with a position of the RPC in the core of Rodinia. The craton might have instead been located at a peripheral position in a passive margin setting of this supercontinent apart from the central collisional Grenville belt during Rodinia assembly. Such a position could explain the change at ca. 0.88 Ga from a passive margin environment to a ca. 180 Ma long history of island arc accretion and active continental margin magmatism in the São Gabriel block marking the beginning of the Brasiliano orogenic cycle. An alternative explanation would suggest that RPC, like other cratons possibly as well, was not part of Rodinia.
|
