Geochronological and geochemical evidences for extension-related Neoarchean granitoids in the southern São Francisco Craton, Brazil

Abstract

"New LA-SF-ICP-MS U-Pb zircon dating of high-K granitoids from the Campo Belo metamorphic complex, southern Sao Francisco Craton (Brazil), reveals a long period (ca. 100 My) of Neoarchean granitic magmatism that post-date the TTG magmatism. The oldest studied pluton is a highly porphyritic biotite orthogneiss emplaced at 2748 +/- 5 Ma, followed by a hornblende-biotite orthogneiss (2727 +/- 7 Ma). Both granitic bodies were affected by a deformation event prior to the emplacement of the Rio do Amparo, Bom Sucesso and Lavras granitoid plutons at 2716 +/- 6 Ma, 2696 +/- 6 Ma and 2646 +/- 5 Ma, respectively. The Neoarchean granitic magmatism ended with the intrusion of peraluminous leucogranitic dikes at 2631 +/- 4 Ma.

The 2.73-2.65 Ga Campo Belo granitoids share chemical features of A-type granites, such as high apatite- and zircon-saturation temperatures (mostly > 800 degrees C), relatively high Fe-number, high total alkalis and characteristic enrichment in LREE and HFSE although most samples of the Rio do Amparo granite have lower HFSE and LREE content that typical A-type granites but very high Th. The high Th content of the Rio do Amparo and Bom Sucesso granites may suggest involvement of Th-orthosilicate in their sources. The trace element composition permits to classify the Campo Belo granitoids as A(2)-type granites, suggesting derivation from partial melting of TTG-crustal sources likely in an extensional setting.

Significant reworking of Mesoarchean crust is suggested by mostly negative epsilon Nd-i, values (Rio do Amparo: -2.0 and +3.1; Bom Sucesso: -3.6, -3.1 and +0.9; Lavras: -2.5 and -0.2) and old Nd model ages (T-DM close to 3.1 Ga), although with probable involvement of juvenile material (T-DM of 2.7-2.9 Ga). This contrasts with Neoarchean granites of the northern Sao Francisco and Congo cratons characterized by negligible juvenile imprint.

The 2.75-2.63 Ga Campo Belo granitoids witness the thermal stabilization of the Archean lithosphere through a major episode of high-K granitoid magmatism between 2760 and 2600 Ma, which affected the whole Sao Francisco Craton and the northern Congo Craton. (C) 2017 Elsevier B.V. All rights reserved."