Sm–Nd isotope geochemistry of metamorphic volcano-sedimentary successions in the São Gabriel Block, southernmost Brazil: evidence for the existence of juvenile Neoproterozoic oceanic crust to the east of the Rio de la Plata craton

Abstract

Juvenile Neoproterozoic dioritic, tonalitic, trondhjemitic and granodioritic gneisses in the São Gabriel block, southern Brazil, have been identified by geochronologic studies. Age proposals for associated (ultra-)mafic metavolcanic and metasedimentary rocks, however, range from Archean to Neoproterozoic. Whole rock Sm–Nd analyses presented here support a Neoproterozoic age for these rocks. TDM model ages of the (ultra-)mafic metavolcanic rocks range between 0.65 and 1.35 Ga with ɛNd( t) positive values between 3.16 and 6.87; TDM model ages of metasedimentary and metavolcanoclastic rocks vary between 0.77 and 1.19 Ga with ɛNd( t) values between 1.2 and 6.23; tonalitic calc-alkaline gneisses show ɛNd( t) values of 4.34 and 6.3 and TDM model ages of 0.89 and 0.72 Ga, respectively. A late-kinematic granite (Santa Zélia granite) display slightly negative ɛNd( t) values (−1.6) and a higher TDM model age of about 1.4 Ga. These data support the existence of Meso/Neoproterozoic juvenile oceanic crust and island arc rocks during the Brasiliano orogenic events. The main source rocks of the metasedimentary units are previously formed juvenile rocks. The data also indicate minor assimilation of older crustal material and/or contamination of the melts by radiogenic Nd released from older rocks on the subducting slab. Existence of widespread old sialic crust in the subduction zone environment, however, can be ruled out indicating important orogenic accretion between 0.9 and 0.7 Ga. A geotectonic model for the São Gabriel block and the eastern margin of the Rio de la Plata craton comprises eastward subduction and following accretion of an intra-oceanic island arc between 0.9 and 0.8 Ga and a subsequent westward subduction with formation of an active continental margin at the eastern margin of the Rio de la Plata craton between 0.8 and 0.7 Ga. We postulate that the juvenile rocks of São Gabriel block represent relics of a Neoproterozoic ocean between the Rio de la Plata craton and a continental block (Encantadas block) possibly derived from the Kalahari craton. Subduction and arc accretion began roughly coeval with the initial stages of the break-up of Rodinia (0.9 Ga) and indicate a peripheric Rio de la Plata craton in relation to the Rodinia supercontinent with evolution from a passive margin to an active margin in the beginning of the Neoproterozoic Brasiliano orogenic events.