Zircon U-Pb geochronology and Hf isotopes of the Luís Alves Terrane: Archean to Paleoproterozoic evolution and Neoproterozoic overprint

Abstract

Abstract The Luis Alves Terrane in southern Brazil is one of the largest expositions of the Archean to Paleoproterozoic units that acted as basement for the development of the Neoproterozoic Pan-African/Brasiliano orogenic belts in the Mantiqueira Province, in South America. Combined field observations, petrography, Hf and U-Pb LA-ICP-MS zircon and titanite isotopic data are presented for the basement of this terrane. The zircon U-Pb dataset spans a large time range with concordant ages from 3.2-1.8\xa0Ga. Several magmatic and metamorphic events are recorded by the U-Pb concordant ages reflecting the complex evolution of this crustal block. The oldest inherited zircon grains have Mesoarchean ages (3.2–3.1\xa0Ga), while two other magmatic populations yield ages of 2683\xa0±\xa070\xa0Ma and 2498\xa0±\xa011\xa0Ma. Most zircon grains crystallized during two high-grade metamorphic events at 2352\xa0±\xa023\xa0Ma and 2183\xa0±\xa017\xa0Ma. Titanite from two samples crystallized at 2.02–1.99\xa0Ga, probably during a retrograde metamorphic trajectory that re-equilibrated the studied samples in amphibolite-facies conditions. A late pegmatite dyke was emplaced between ca. 1.96 and 1.79\xa0Ga. Cathodoluminescence images show that the analyzed zircon samples, including those in the pegmatite, had their internal structure modified by hydrothermal processes. The localization of the outcrop close to the rim of the Campo Alegre Basin affected by an important Neoproterozoic hydrothermal event as well as normal and reverse discordant U-Pb ages in our dataset indicate that this event took place during the Brasiliano orogenic cycle at the end of the Neoproterozoic. The Hf isotope data show predominantly crustal signatures with eHf(t) values ranging from 1.3 to −16.8. Hf TDM model ages cluster in two groups (4.32–3.48 and 3.38–2.27\xa0Ga) indicating two pulses of magma differentiation from the mantle with subsequent mixing of the material during the Paleoproterozoic high grade metamorphic events.