TTG Granitoids from the Água Azul do Norte (PA, Brazil): tectonic implications for the Carajás Province

Abstract

The TTG crust from the Água Azul do Norte, inserted in the geological context of the Carajás Province, located in the southern portion of the Carajás Domain, near the limit with the Granite-Greenstone Terrain of Rio Maria, is dominantly trondhjemites and are strongly akin to others TTG occurrences in Carajás domain, and differing from those of the Rio Maria domain due the presence of evolved terms, lower primary accessory minerals contents and for being essentially peraluminous. It exhibits moderate to high La/Yb and Sr/Y ratios, and low Nb/Ta ratio, which indicates that they wereit was produced from partial melt of metamafic source, at high to intermediate pressure conditions (~1.0–1.5 GPa), but still in the garnet stability field. The large tonalitic enclaves indentifiedidentified have particular mineralogical and geochemical features that do not allow associating them with the typical TTG associations. The flat Rare Earth Elements (RREs) pattern (low La/Yb ratio) indicates that they werethese rocks produced out the garnet stability field, in lower pressure conditions than TTG granitoids (≤1.0 GPa), and they could represent magmas with transitional behavior between typical TTG and sanukitoids. The less-evolved character of the enclaves, regarding that of the São Carlos tonalite of 2,93 Ga, associated with their chaotic structural patterns, suggests that these enclaves can represent preserved parts of an even older crust. The TTG crust studied in the region Água Azul do Norte region records at least two important deformational moments played in the area with coincident principal shortening axis (N10-20ºE). The first moment (D1) is evidenced for the compositional banding (E-W) formed by a strong pure shear component during the TTG granitoids emplacement in 2,93 Ga. The late phase of ductile deformation (D2), under medium amphibolite metamorphism facies conditions (550ºC), is marked by the transposition of simple and pure shear of the ancient structures, which originated the NW-SE, N-S, NE-SW and E-W foliation patterns. This deformational moment is registered to late intrusions of sin to post-tectonic leucogranites of 2.87 Ga, amphibolite bodies and the host rocks next to these intrusions. The transpressive strain identified in the area suggests that the plates convergence was active between 2.93 and 2.87 Ga, and, during the last phase of transpression, the strain was partitioned with the total stress of simple shear being accommodated in shear zones and shear bandings. As consequence, there is the preservation of regions with low deformation among shear zones controlled by pure shear. The existence of a TTG crust in Água Azul do Norte region compositionally distinct from Rio Maria domain, in addition to a complex deformational pattern that is usually associated with migmatization processes, suggest that this portion of Carajás domain does not represent an extension of the Rio Maria domain, but an independent Mesoarchean crust block without records of Neoarchean events, which would not be in compliance to the definition attributed to Carajás domain.