GEOLOGIA, PETROGRAFIA E GEOQUÍMICA DO GRANITO ANOROGÊNICO BANNACH, TERRENO GRANITO-GREENSTONE DE RIO MARIA, PARÁ

Abstract

The Bannach Granite is intrusive in to Archean rocks of the Rio Maria Granite- Greenstone Terrane, located at the southeastern border of the Amazonian craton. Eigth varieties of monzogranites have been recognized in this Paleoproterozoic batholith: amphibole + biotite ± clinopyroxene coarse-grained monzogranites: clinopyroxene- biotite-amphibole monzogranite, biotite-amphibole monzogranite, and amphibole-biotite monzogranite; Porphyritic biotite monzogranite; leucomonzogranites (coarse-grained, early and late medium-even-grained monzogranite and fine, even-grained monzogranite). The massif is zoned with the less evolved facies situated along the border and the more evolved facies in the center. The different facies are subalkaline, metaluminous to peraluminous, display K 2 O/Na 2 O ratios between 1 and 2 and FeOt/(FeOt + MgO) between 0.86 and 0.97. They show weak or moderate fractionation of heavy REE and negative europium anomalies that increase from the less evolved to the more evolved facies. The different facies display geochemical affinities with within-plate granites and A-type granites of the A2-subtype and are classified as ferroan granites. The Bannach granite evolved through fractional crystallization of titanomagnetite + ilmenite + zircon + apatite + hornblende ± clinopyroxene and feldspars minerals. The diferentiation trend was in the sense: biotite-amphibole monzogranite amphibole- biotite monzogranite Worphyritic biotite monzogranite leucomonzogranites. The late medium-even-grained leucomonzogranites facies is interpreted as a separate intrusion derived from strongly evolved liquids. The compositional gap between the clinopyroxene-biotite-amphibole monzogranite and the biotite-amphibole monzogranite facies suggests that the biotite-amphibole monzogranite liquid was not derived from the clinopyroxene-biotite-amphibole monzogranite by a simple fractional crystallization process. The clinopyroxene-biotite-amphibole monzogranite had a particular magmatic evolution, involving the participation of cumulatic processes. The Bannach granite displays a remarkable similarity with the Jamon, Musa and Redencao granites which constitute the Jamon Suite, justifying the attribution of the former to this suite.