Mineral chemistry and crystallization conditions of the anorogenic Gradaús batholith from the Carajás Province, Amazonian craton (Brazil)

Abstract

The Gradaús batholith (1.88 Ga) is part of the intense anorogenic granitic magmatism that occurred during the Paleoproterozoic in the Carajás Province. It has a subround shape, with an outcrop area of 900 km2, and is intrusive into metasediments of the Rio Fresco Group. The batholith is composed of four petrographic facies with varying mafic contents and sizes. Its amphiboles are calcic, with predominance of Fe-edenite and Fe-pargasite, having Fe/(Fe + Mg) ratios ranging from 0.78 to 0.90. Hastingsite and Fe-hornblende are present but in subordinate amounts. The biotite is ferrous and plots close to the annite composition field, with Fe/(Fe + Mg) ratios varying between 0.77 and 0.86. Temperature estimates obtained from the zirconium and apatite saturation geothermometer range from 870 to 790 °C and are interpreted as being close to the liquidus temperature. In contrast, temperatures obtained from the amphibole geothermometer range from 800 to 750 °C and are considered to be close to the solidus temperature. The emplacement pressure of the Gradaús batholith is estimated to be between 2.4 and 3.6 kbar, based on comparison with similar anorogenic batholiths in the Carajás Province. The high Fe/(Fe + Mg) ratios of amphibole, the dominance of ilmenite over magnetite, and the absence of magmatic titanite indicate that the Gradaús batholith crystallized under relatively reduced conditions, probably between the NNO and FMQ buffers. The biotites exhibit alkaline-subalkaline compositions and consistently plot in the A-type granite field. Comparative data on mineral chemistry and crystallization conditions between the Gradaús batholith with other anorogenic batholiths in the Carajás Province highlight strong similarities between the Gradaús and the Cigano from the Serra dos Carajás suite, as well as the nonmineralized facies of the Antônio Vicente from the Velho Guilherme suite.