Petrology of the anorogenic, oxidised Jamon and Musa granites, Amazonian Craton: implications for the genesis of Proterozoic A-type granites

The 1.88 Ga Jamon and Musa granites are magnetite-bearing anorogenic, A-typ e granites of Paleoproterozoic age. They intrude the Archaean rocks of the Rio Maria Granite-Greenstone Terrain in the eastern part of the Amazonian C raton in northern Brazil. A suite of biotite +/- amphibole monzogranite to syenogranite, with associated dacite porphyry (DP) and granite porphyry (GP ) dykes, dominates in these subalkaline granites that vary from metaluminou s to peraluminous and show high FeO/(FeO + MgO) and K2O/Na2O, In spite of t heir broad geochemical similarities, the Jamon and Musa granites show some significant differences in their REE patterns and in the behaviour of Y. Th e Jamon granites are related by fractional crystallisation of plagioclase, potassium feldspar, quartz, biotite, magnetite +/- amphibole +/- apatite +/ - ilmenite. Geochemical modelling and Nd isotopic data indicate that the Ar chaean granodiorites, trondhjemites and tonalites of the Rio Maria region a re not the source of the Jamon Granite and associated dyke magmas. Archaean quartz diorites, differentiated from the mantle at least 1000 m.y. before the emplacement of the granites, have a composition adequate to generate DP and the hornblende-biotite monzogranite magmas by different degrees of par tial melting. A larger extent of amphibole fractionation during the evoluti on of the Musa pluton can explain some of the observed differences between it and the Jamon pluton. The studied granites crystallised at relatively hi gh fO(2) and are anorogenic magnetite-series granites. In this aspect, as w ell as concerning geochemical characteristics, they display many affinities with the Proterozoic A-type granites of south- western United States. The Jamon and Musa granites differ from the anorthosite-mangerite-charnockite-r apakivi granite suites of north-eastern Canada and from the reduced rapakiv i granites of the Fennoscandian Shield in several aspects, probably because of different magmatic sources. (C) 1999 Elsevier Science B.V. All rights r eserved.