GEOLOGY, GEOCHEMISTRY AND MAGMATIC EVOLUTION OF THE PALEOPROTEROZOIC, ANOROGENIC OXIDIZED A-TYPE REDENCAO GRANITE OF THE JAMON SUITE, EASTERN AMAZONIAN CRATON, BRAZIL

Abstract

The anorogenic oxidized A-type Redencao granite, of Paleoproterozoic age, is located in the Amazonian craton to the south of the Serra dos Carajas, in Brazil. The granitic pluton is comprised essentially of monzogranites disposed in near-concentric zones. Except for clinopyroxene-bearing monzogranites generated by accumulation of mafic minerals, the least-evolved facies is a coarse biotite–hornblende monzogranite that evolved by fractional crystallization to the dominant facies, a coarse (hornblende) –biotite monzogranite. The central portion of the pluton consists of leucogranites, which were probably derived from an independent magma. Seriate and porphyritic biotite monzogranite varieties cross-cut the coarse (hornblende)–biotite monzogranite, forming annular structures. The porphyritic monzogranites are at least in part generated by mingling between the magmas of the coarse biotite monzogranite and leucogranites. The mixing of magmas is probably more common in granitic plutons than generally admitted. The different varieties of the Redencao pluton are ferroan alkali-calcic A-type granites [FeO t /(FeO t + MgO) values of 0.83 to 0.94; K 2 O/Na 2 O 1 to 2]. The oxidized character of the Redencao pluton is revealed by the ubiquitous occurrence of magnetite and titanite, and relatively lower FeO t /(FeO t + MgO) values and higher magnetic susceptibility compared to those observed in typical reduced A-type granites.