Insights into the late Archean evolution of the Carajás Province: Geochemical and geochronological constraints from the Santana do Araguaia complex, Amazonian craton, Brazil

Abstract

To the south of the Carajás Province, Amazonian Craton, 2.85–1.88 Ga old rock units underlie the Santana do Araguaia Domain (SAD), bounded by the Iriri-Xingu and Rio Maria domains and the Tocantins Province. The SAD differs from the Rio Maria Domain, as indicated by the scarcity of TTGs and the dominance of both high-K to weakly sodic granites, sodic granodiorites, and monzodiorite/quartz monzodiorite with high-Ba + Sr contents. Therefore, the SAD is a particular case to assess tectonic settings in the Archean, continental crust generation and the timing/nature of the Meso-Neoarchean transition from TTG-type to calc-alkaline high-K and sodic granitoid magmatism to the south of the Rio Maria Domain. The magmatic evolution of the SAD started at 2.85 and extended up to ∼1.88 Ga with a significant contribution from the pre-existing crust, differing from that of the Rio Maria Domain (3.0–2.86 Ga). The Santana do Araguaia Complex (SAC) comprises deformed to non-deformed rocks divided into (i) tonalite (TTG with high Al2O3 and medium La/Yb ratio), (ii) sodic granodiorite group (K2O/Na2O < 1) with Ba- and Sr-rich varieties, which can be divided into type 1 displaying high Sr/Y and La/Yb ratios, and type 2 with low Sr/Y and low to intermediate La/Yb, both resembling the sanukitoids low-Ti, (iii) hybrid granite group (high-K calc-alkaline), composed of two sub-types of high-Ba + Sr monzogranites, one of high Sr/Y that shares many features with the CA2-type of Archean calc-alkaline granites and HSG, and a second type, with low Sr/Y ratio and akin chemistry with the CA1 and normal granite (NG). Group (iv) is of biotite or low Ba–Sr granites classified as syenogranites CA1/NG strongly fractionated calc-alkaline rocks. Group (v) consists of rocks with high Ba–Sr but with moderate Mg# (mean = 44.8), low contents of transition elements, and moderate K2O/Na2O ratios (mean = 0.72); the group is divided into monzodiorite with high Sr/Y, La/Yb ratios, and low Y, and quartz monzodiorite with low Sr/Y, moderate La/Yb ratio and Y values, both with sanukitoids high-Ti characteristics. The rocks that constitute the studied complex show arc-like geochemical signatures, suggesting emplacement during the end of phase B (collisional) and the start of phase c (thermal relaxation/colappse) of stage 2 in the two-stage tectonic-magmatic model (subduction/collision-post-collision). In the Carajás Province the first stage of the model is marked by the genesis of TTG (3.0–2.94 Ga) –juvenile magmatism of the Rio Maria Domain, and the second stage, collisional phase, between 2.89 and 2.85 Ga with sanukitoid rocks, hybrid and biotite granites as well as significant occurrences of high Ba + Sr rocks and post-collisional granitic suites (∼2.74 Ga). However, the elemental whole-rock geochemistry and geochronology datasets show that the Santana do Araguaia Complex (SAC) is marked by high-K and Ba + Sr rocks such as abundant sanukitoid suites, hybrid and biotite granites, but with very scarce TTGs. The ages between 2.85 and 2.81 Ga obtained from SAC suggest the final phase “a” and beginning of phase “b” of the second evolutionary stage, collisional phase, and slab-breakoff/post-collisional, probably the terminal tectonic-thermal episode of Archean accretion and differentiation in the Amazonian Craton. The SAC rocks' contrasting Sr/Y and La/Y ratios cannot be explained exclusively by variation in the melting pressure (melts of mafic crust at >50 km) in a thickened continental crust. Multiple crustal petrogenetic processes or heterogeneous sources can produce the heterogeneous geochemical signatures that characterize the rocks of the complex. The admitted sources of these Archean granitoid magmas include basaltic andesite, sanukitoids, and metagreywackes, subordinate TTGs, at low to moderate pressures melting as they suggest low values of (Dy/Yb)n below 1.9 and relatively high-Sr contents. The mesostructure data indicate that the studied rocks were affected by high-temperature deformation (>600 °C) with partial melting.