Geochemistry and petrogenesis of the Mesoarchean granites from the Canaa dos Carajas area, Carajas Province, Brazil: Implications for the origin of Archean granites

Four Mesoarchean (2.93 to 2.83 Ga) granite units, which encompass the Canaa dos Carajas, Born Jesus, Cruzadao and Serra Dourada granites, were recognized in the Cana dos Carajas area of the Archean Carajas Province. The Mesoarchean units are composed dominantly of biotite leucomonzogranites. They are compared with the Neoarchean Planalto suite (2.73 Ga) which encompasses biotite-hornblende monzogranites to syenogranites. The Canaa dos Carajas, Bom Jesus and the variety of the Cruzadao granite with higher (La/Yb)(N) are geochemically more akin to the calc-alkaline granites, whereas the other varieties of the Cruzadao granite are transitional between calc-alkaline and alkaline granites. The Serra Dourada granite has an ambiguous geochemical character with some features similar to those of calc-alkaline granites and other peraluminous granites. The Planalto granites have ferroan character, are similar geochemically to reduced A-type granites and show a strong geochemical contrast with the Mesoarchean studied granites. The Mesoarchean granites described in the Cana dos Carajas area are geochemically distinct to those of the Rio Maria domain of the Carajas Province. The Canaa dos Carajas and Born Jesus granites are similar to the high-Ca granites, whereas the Cruzadao and Serra Dourada are more akin to the low-CaO granites of the Yilgarn craton. The geochemical characteristics of the Mesoarchean studied granites approach those of the biotite granite group of Dharwar but the latter are enriched in HFSE and HREE compared to the Canaa dos Carajas granites. The Neoarchean Planalto suite granite has no counterpart in the Mesoarchean Rio Maria domain of the Carajas Province, neither in the Yilgarn and Dharwar cratons. Geochemical modeling suggests that partial melting of a source similar in composition to an Archean basaltic andesite of the Carajas Province could give origin to the Bom Jesus and Cruzadao granites. In the case of the Born Jesus granite the residue of melting contained, in similar proportions, plagioclase, clinopyroxene, amphibole, and garnet, with subordinate ilmenite. The variations in (La/Yb)(N) and Sr/Y of the Cruzadao granite are controlled by changes in the residual melting phases. Garnet and amphibole are abundant in the residue of the variety with higher (La/Yb)(N). whereas in the rocks with moderate to lower (La/Yb)(N) garnet is absent, magnetite appears in the residue and amphibole initially increases but is replaced by orthopyroxene in the rocks with the lowest (La/Yb)(N). In the Canaa dos Carajas and Serra Dourada granites garnet was probably an absent phase in the residue of melting and the influence of amphibole was also apparently limited. A crustal environment and a variable pressure from 10 to 7-8 kbar are estimated for the generation of the granite magmas. The Bom Jesus and Cruzadao granites of Carajas derived from basaltic andesite sources and not of TTG and their geochemical contrasts can be explained by variation in the pressure of melting. The sources of Archean granite magmas are more diversified and could be more mafic than generally admitted. (C) 2012 Elsevier B.V. All rights reserved.