THE AREQUIPA-ANTOFALLA BASEMENT (AAB): AN ORPHANED CRUSTAL BLOCK?

The Arequipa-Antofalla Basement (AAB), a Proterozoic crustal block along the central Andean margin, is a key for interpreting the pre-Andean history of South America. The AAB and adjacent Amazonia have opposing crustal growth directions, suggesting that the AAB did not form in situ with respect to Amazonia. Opposing arguments propose that the AAB is either: 1) a parautochthonous block of Amazonia, or 2) an allochthonous terrane transferred from a previously adjacent craton. The parautochthonous model proposes that the AAB was emplaced along a margin-parallel strike-slip fault, whereas the allochthonous model envisions transfer after a collision between northeastern Laurentia and southwestern Amazonia during the Sunsas-Grenville Orogeny. In this latter scenario, Laurentia was the “parent craton” of the AAB.

Given that a prospective “parent craton” should have corresponding pre-collisional and collisional tectonic histories and similar isotopic compositions as the AAB, new U-Pb geochronology and Pb isotope geochemistry in the AAB lead to the following conclusions: 1) the AAB accreted onto Amazonia during a Grenville-aged collision, 2) dissimilar tectonic histories preclude correlation of the AAB with Amazonia, and 3) different Pb isotopic signatures preclude correlation of the AAB with Laurentia. These conclusions require re-evaluation of Rodinia models that juxtapose northeastern Laurentia and western Amazonia in a Grenville-aged collision.