The Proterozoic Mucajaí complex, Guyana shield - AMCG status established!

The Mucajaí AMCG complex forms the southeasternmost termination of the ca. 900 km Mesoproterozoic rapakivi granite belt in the northern part of the Amazonian craton. The rock types of the complex comprise massif-type anorthosite and quartz-fayalite mangerite to -syenite (FMS) partially surrounded by a two-phase, dominantly pyterlitic, rapakivi granite intrusion with a predominant biotite-hornblende granite (BHG) and a portion of a more evolved biotite granite (BG). This rock assemblage shows that Mucajaí is akin to the classic AMCG complexes; moreover, Mucajaí is, in this regard, unique within the Precambrian of South America. We report detailed ID-TIMS U-Pb zircon geochronology and Nd isotope compositions on all the major rock types of the complex. Our results confirm the previously reported Mesoproterozoic crystallization age for the complex. Furthermore they refine the relations between the different rock types of the complex showing overlapping ages for the anorthosite (1527 ± 2 Ma), FMS (1528 ± 2 Ma), and BHG (1527 ± 3 Ma). The BG is marginally younger (1520 ± 3 Ma) and thus the latest magmatic phase of the complex. The overall age of the Mucajai complex is therefore similar to the youngest members of the locus classicus Fennoscandian rapakivi granite association in central Sweden. The Nd isotope systematics of the rocks also follow a pattern typical of the classic AMCG complexes. All rock types have slightly negative initial ɛNd values at 1525 Ma (-2.8 in the BG to -1.9 in the anorthosite), implying the importance of a juvenile crustal source component in the petrogenesis of the Mucajai complex. In the light of these new data, comparison of the Mucajaí-complex to other AMCG complexes worldwide is justified not only geochemically and petrologically, but also in the geochronological and tectonic sense. For example, correlating the AMCG magmatism in the northern Amazonian craton and the Fennoscandian shield lends support to previous tectonic models that imply a common tectonic history of these crustal blocks during the Mesoproterozoic.