THE GEOLOGY AND TECTONIC EVOLUTION OF THE TOCANTINS GRANITE-GREENSTONE TERRANE: ALMAS-DIANÓPOLIS REGION, TOCANTINS STATE, CENTRAL BRAZIL

Abstract

The geology of Almas-Dianopolis region consists of a granite-greenstone terrane partially covered by Proterozoic metasedimentary units. The greenstone belt can be subdivided in two sequences. The first sequence, named Corrego do Paiol Formation, is composed of high-Fe basaltic flows, locally pillowed, and includes rare occurrences of komatiitic basalts. The second sequence, named Morro do Carneiro Formation, comprises a monotonous sequence of sericitic phyllites, as well as beds of chemical and clastic sediments. Both sequences were subjected to medium- to low- pressure metamorphism and variable temperatures in greenschist to amphibolite facies (Ml). Metamorphism was accompanied by a deformation event (Dn) that resulted in vertical sedimentary strata and contacts. Granitoid emplacement enhanced thermal conditions during Dn event, as suggested by amphibole exchange vectors - edenite and tschermakite substitutions are dominant. These granitoid bodies form large granite gneiss complexes that intruded the greenstone belts during or at the close of the Dn event. The granitoids are grouped in two suites: Suite 1 comprises amphibole-bearing tonalite, granodiorite, trondhjemite, quartz-monzodiorite and quartz-diorite; and Suite 2 is composed of tonalite, trondhjemite, granodiorite and monzogranite with biotite. Both suites have calc-alkaline tonalitic-trondhjemitic chemical affinities. Contact relationships, deformation style of wall rocks, fabric development and different source rocks illustrate a complex history of granite-gneiss terrane evolution. Later Dn+i strike-slip shear zones partially obliterated Dn structural features in the greenstone belts, but their effects were restricted in the granite-gneiss domains.