Comparing the Cana Brava and Niquelândia complexes: different contamination and fractionation processes in coeval intrusions

Contamination processes during the intrusion in the crust of mantle-derived melt which produced huge layered complexes are recognized in almost all the layered complexes (e.g. Val Sesia magmatic system). However, the contamination does not always occur (e.g. Finero mafic complex) or occurred in different intrusions with different modalities. The Niquelândia and Cana Brava complexes (Goiás, central Brazil) are part of a 300 km long, North-trending belt of mafic-ultramafic massifs outcropping in the Brasilia Belt. Among the three complexes forming the belt (together with the Barro Alto one), the Cana Brava complex is the less known while the Niquelândia complex is the better known. The intrusion of the complexes occurred during a continental rift in the lower crust and the parent melt compositions were estimated to be MORB-like. Notwithstanding these, the stratigraphy of the two complexes is different: the Niquelândia complex shows anorthositic rocks forming the so-called Upper Sequence while the Cana Brava complex is similar to the Niquelândia Lower Sequence. New U-Pb SHRIMP-II analyses on zircons from 4 samples from the Cana Brava complex provided for concordia ages between 798.7±2.2 Ma and 779.3±1.3 Ma. These ages constrain the Cana Brava intrusion at 800-780 Ma, similarly to the intrusion ages estimated in literature for the Barro Alto and Niquelândia complexes. Literature data suggests that the Niquelândia complex suffered crustal contamination as a late event during its growth only locally and in the Lower Sequence. The contamination enriched the melt in incompatible elements (e.g. LREE and Ba)and affected the Rb-Sr and Sm-Nd isotopes. New bulk-rock major and trace elements analyses from the Cana Brava complex show strong enrichments for themost incompatible elements at the top of the complex which suggest, together with the occurrence of xenoliths, that the parent melt was affected by crustal contamination. Rb-Sr and Sm-Nd isotopic analyses confirm this hypothesis, showing an increasing contamination trend along the stratigraphy (87Sr/86Sr(790) between 0.708243-0.736590 and ɛNd(790) between 1.71 and -8.47 from the bottom to the top). This suggests a continuous contamination process during the complex growth. The comparison of the two complexes evidenced a different development of the contamination processes. AlphaMELTS models for the Niquelândia and Cana Brava complexes provide evidences of different fractionation of the parent melts, thus suggesting that the different development of crustal contamination is led to the fractionation processes and a different melt compositions.