A perspective on potassic and ultrapotassic rocks : constraints on the Paleoproterozoic late to post-collisional event in the São Francisco paleocontinent.

Abstract

The late- to post-collisional stage in orogenic systems is characterized by the coeval existence of bimodal potassic to ultrapotassic magmatic activity related to partial melting of an enriched lithospheric mantle together with crustal derived melts. In this paper, we present new whole rock geochemical analyses combined with zircon and titanite U–Pb and zircon Hf isotopic data from potassic to ultrapotassic rocks from six plutons that occur

within the Archean Itacambira-Monte Azul block (BIMA), to discuss their petrogenesis and the tectonic implica- tions for the São Francisco paleocontinent. The new U–Pb ages range from ca. 2.06 Ga to 1.98 Ga and reveal long- lasting potassic magmatism within the BIMA, which is within the late- to- post-collisional stage of the São

Francisco paleocontinent evolution. The ultrapotassic rocks are compatible with a fluid-related metasomatized

mantle source enriched by previous subduction events, whereas the potassic rocks are bimodal and have a tran- sitional shoshonitic to A-type affinity. These rocks have a hybrid nature, possible related to the mixing between

the mafic potassic/ultrapotassic rocks and high temperature crustal melts of the Archean continental crust. Our results also show an increase of within-plate signature towards the younger potassic magmas. The participation

of an important Archean crustal component in the genesis of these rocks is highlighted by the common and oc- casionally abundant occurrence of Archean inherited zircons. The Hf isotopic record shows that most of the zircon

inheritance has dominantly subchondritic εHf(t) values, which fits a crustal reworking derivation from a similar Eo- to Paleoarchean precursor crust. However, the presence of juvenile 2.36 Ga zircon inheritance in an ultrapotassic sample reveal the existence of a hidden reservoir that is somewhat similar to the described for the Mineiro Belt in southern São Francisco paleocontinent.