Mineral thermobarometry and its implications for petrological constraints on Mesoarchean granitoids from the Carajás Province, Amazonian Craton (Brazil)

Abstract

This paper describes the first investigation of the crystallization parameters of the Mesoarchean granitoids from Ourilândia do Norte, which is located in the midwestern part of the Carajás Province. These rocks are divided into three major groups: (i) sanukitoids composed of amphibole-bearing granitoids and associated intermediate to mafic rocks; (ii) potassic granites composed of biotite monzogranites and associated high-Ti granodiorite; and (iii) subordinated TTG-affinity granitoids (amphibole-bearing trondhjemites and biotite-bearing tonalitic xenoliths). These rocks were formed from different sources at different depths, including enriched mantle (sanukitoids), continental crust (monzogranite), mantle-crust mixture (high-Ti granodiorite) and oceanic crust (TTG-affinity granitoids). Potassic granites and porphyritic sanukitoid facies have high magnetite contents, and consequently high magnetic susceptibility (MS), compared to the equigranular sanukitoid facies and trondhjemite. Thermometers based on apatite saturation in whole-rock indicate liquidus temperatures above 800 °C for all granitoids, reaching 959 °C in the high-Ti granodiorite. Amphibole-based and Si-sensitive geothermometers indicate solidus temperatures from 785 to 738 °C for sanukitoids and 892 to 772 °C for trondhjemite, with estimated emplacement pressures varying from 297 to 80 MPa and 580 to 263 MPa, respectively. Because the potassic granites display an amphibole-free assemblage, no geothermobarometric estimate based on amphibole composition was possible. However, potassic granites are associated with coeval sanukitoids (2.88 Ga), which indicates similar pressure conditions and emplacement depths for these rocks. Crystallization constraints from the Ourilândia sanukitoids are comparable to those of the geochemically similar Rio Maria sanukitoids of the Carajás Province, although the pressure estimates and water content show differences. The Fe/(Fe + Mg) ratios of both biotite and amphibole indicate high to intermediate redox conditions for the sanukitoids and potassic granites, which are mostly above that of nickel-nickel oxide (+1.0 < NNO < + 1.9), although less oxidized conditions are estimated for the trondhjemite (– 0.5 < NNO < + 1.0). High water content was estimated for the sanukitoid-related granitoids (> 5%) relative to the TTG-affinity granitoids (< 4%) using amphibole-based water calibrations. Compared with sanukitoids from other Archean cratons, the Ourilândia sanukitoids crystallization constraints show similarities with the more oxidized members from the Baltic Shield, the Closepet-type granites and the Matok-type sanukitoids but differ from the reduced redox conditions of the Elan-type granitoids from the East Sarmatian orogen.