The Proterozoic to Triassic crustal evolution of central South America from Hf and O isotopes

The erosional debris of the poorly exposed Paleo- and Mesoproterozoic mobile belts of Amazonia (Terra Amazonica Orogen, 2-1 Ga, TAO-1) eventually accumulated in the orogenic basins of the central proto-Andean Terra Australis Orogen (TAO-2, 0.65-0.2 Ga), offering an abundant indirect source of information.

From newly constructed U-Pb, Hf and O isotope zircon data bases we derived that in TAO-1 eHf(t) values define 3 cycles between 2-1 Ga from strongly unradiogenic to radiogenic values. After the dispersal of Rodinia, TAO-2 registers one large similar cycle. In accretionary orogens, such trends indicate the progressive removal of lower crust and lithospheric mantle of the upper plate during subduction and their replacement by new radiogenic crust.

d¹⁸O data show a flat d¹⁸O trend at 6.3‰ over the first 800 Myr of TAO-1, increasing to elevated values around 7.3‰ during collision with Laurentia. Contrastingly, d¹⁸O of TAO-2 trends from 8‰ to more mantle-like values just below 7‰. The different trends show that anatectic intracrustal recycling played only a subordinate role in the generation of new crust during TAO-1. However, there is a correspondence between the O and Hf isotope trends in TAO-2 towards more juvenile compositions through time.

Global d¹⁸O data showed gradually increasing d¹⁸O after 2.5 Ga indicating the progressive hydration and intracrustal recycling of the continental crust after the Archean. Our data register the sudden appearance of d¹⁸O values up to 10‰ at the Archean-Proterozoic transition indicating that Amazonia had experienced intracrustal recycling at an accretionary margin already in the Late Archean.