Granitóides TTG de Água Azul do Norte: implicações tectônicas para a Província Carajás

Abstract

The rocks that compose the TTG crust from the Água Azul do Norte area are dominantly trondhjemites and are strongly akin to others TTG occurrences in Carajás domain (Rio Verde and Colorado trondhjemites and related rocks from Nova Canadá area), differing from those of the Rio Maria domain (Caracol, Arco Verde, Mariazinha tonalities and Mogno, Água Fria trondhemites) due the presence of more evolved terms, lower primary accessory minerals contents and for being essentially peraluminous, without strongly decalcified plagioclase. They exhibit moderate to high La/Yb and Sr/Y ratios, and low Nb/Ta ratio, which indicate that they were produced from partial melt of metamafic source, at high to intermediate pressure conditions (~1.0-1.5 GPa). The large tonalitic enclaves indentified have particular mineralogical and geochemical features that do not allow associating them with the typical TTG associations. They exhibit significant amphibole contents (>5%), high Fe2O3, MgO, CaO, Ni e Cr contents, and a flat RRE pattern (low La/Yb ratio), which reflect their less evolved character compared with TTG granitoids. The low La/Yb and Sr/Y ratios indicate that they were 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 compared with the São Carlos tonalite, which represent the oldest fragment of Mesoarchean crust from Carajás domain, associated with their chaotic structural patterns, suggest that these enclaves can represent preserved parts of an even older crust. Structural data suggest that the TTG crust records at least two important deformational moments played in the area with coincident principal shortening axis (N10-20°E). The first moment (D1 ~2,93 Ga) is evidenced for the compositional banding (E-W) formed by a strong pure shear component during the TTG granitoids emplacement. The late phase of ductile deformation (D2 ~2,87 Ga), under medium amphibolite metamorphism facies conditions (550°C), 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 strongly linked to late intrusions of sin to post-tectonic Mesoarchean leucogranites. 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 fragment without records of Neoarchean events, which would not be in compliance to the definition attributed to Sapucaia subdomain.