Neoarchean magmatism and Rhyacian reworking in the northeast of the Amapá Block, southeastern Guiana Shield, Brazil: Geochemistry, U–Pb geochronology, and Hf–Nd isotopes

Abstract

This study investigates the age, geochemical nature, sources, and Rhyacian reworking of the Neoarchean magmatism at the northeastern portion of the Amapá Block, southeastern Guiana Shield (northeast Amazonian Craton). This block consists of a large Archean continental landmass strongly reworked during the Transamazonian orogeny (2.26–1.95 Ga). We provide the very first geochemical results for Archean basement units of the Amapá Block, which are the Mesoarchean Tumucumaque and Neoarchean Guianense complexes, and Neoarchean Mungubas Granite. The first two exhibited geochemical affinity with enriched TTG-type and hybrid granitoids, respectively, while the Mungubas Granite showed affinity with biotite granites. Such affinity with C-type granitoids indicates that the genesis of these units involves partial melting of crustal sources. LA-MC-ICP-MS zircon U–Pb and Lu–Hf, and whole-rock Sm–Nd data were obtained for the Neoarchean Mungubas Granite and Guianense Complex. In both units, zircon U–Pb dating provided a distribution of dates that span ∼50 Ma forming two groups with distinct mean 207Pb/206Pb ages of ∼2.70 Ga and ∼2.65 Ga, respectively. The older age is considered as the crystallization/emplacement age of the Mungubas Granite and the Guianense orthogneiss protolith, while the younger one is ascribed to concealed Pb loss. The recognition of Mesoarchean inherited zircon core and xenocryst with ages of 2.84 Ga and 2.99 Ga suggest that these units reworked older basement rocks from the Amapá Block. Subchondritic ԐHf(2.70 Ga) of zircon and ԐNd(2.70 Ga) of whole-rock, along with their respective Hf-TDMC (3.6–3.2 Ga) and Nd–TDM (3.6 Ga) model ages, for both units, reflect the substantial crustal reworking of predominant Paleoarchean sources during the Neoarchean in the northeastern Amapá Block. In addition, U–Pb dating on titanite crystals from the Guianense orthogneiss yielded an unprecedented ∼2.05 Ga age, arguing that a Late Rhyacian metamorphic event affected the Archean orthogneisses. This 2.05 Ga event coincides with the emplacement of Rhyacian granitoids in the area, as well as with the 2.06–2.04 Ga late orogenic HT granulite facies and amphibolite facies metamorphic events identified, respectively, at the northern border and in the central-southern portion of the Amapá Block.