THE MESOPROTEROZOIC MUCAJAI ANORTHOSITE - MANGERITE - RAPAKIVI GRANITE COMPLEX, AMAZONIAN CRATON, BRAZIL

Abstract

The Mesoproterozoic Mucajai anorthosite – mangerite – rapakivi granite (AMG) complex, in northern Brazil, comprises rapakivi granites, fayalite–pyroxene quartz mangerite and syenite, and a massif-type anorthosite forming an asymmetrically zoned igneous complex without counterpart in the Amazonian craton. The rapakivi granites consist of biotite–hornblende granite, including pyterlites and subordinate wiborgites, and porphyritic biotite granite, and exhibit strong similarities with the classic rapakivi granites of Finland. The presence of a fayalite + ilmenite + quartz assemblage in the mangerites indicates low f (O 2 ) conditions during crystallization. The mangerites and rapakivi granites exhibit geochemical characteristics of A-type granites, including high contents of HFS elements (especially Zr), high Ga/Al, Rb/Ba and Rb/Sr values, high K 2 O contents and high FeO t /(FeO t + MgO), typical of reduced rapakivi granites. The biotite granite and the biotite–hornblende granite are related to each other by fractional crystallization, but the mangeritic rocks probably have an independent origin. A 1538 ± 5 Ma Pb–Pb evaporation age has been obtained for a fayalite–pyroxene mangerite. This age overlaps within analytical errors with the 1527 ± 7 Ma (U–Pb SHRIMP) and 1544 ± 42 Ma (U–Pb conventional method) ages previously reported for the anorthosite and rapakivi granites, respectively. The Nd data, including T DM model ages, in the range of 2.07–2.01 Ga, and e Nd (t) values between −2.4 and −1.3 suggest a juvenile crustal source for the granitic rocks of the complex. The e Nd (t) value calculated for the anorthosite (−2.9) probably reflects crustal contamination. Prominent bodies of rapakivi granite coeval with the Mucajai AMG complex are distributed along a NW–SE belt in the western part of the Guyana shield. The Mucajai AMG complex intruded 1.94 Ga country rocks at least ca . 400 my after the stabilization of this part of the Guyana shield, indicating an intraplate setting, in a geological scenario similar to those of the classical rapakivi complexes of the Fennoscandian shield.