The Goias greenstone belt: geochronologic studies

Goias Velho Group of the Greenstone Belt is represented by an Archean sequence of volcanic and sedimentary rocks. These rocks are encrusted in others of the Granite-Gneiss Complex and the contact between these two units is made through N 70 W thrust faults. Rb/Sr geochronological analysis in gneiss showed isochronic ages of about 2.670 Ma. Isochronic Rb/Sr ages gotten on Bugre Creek Gneiss show values of 1.900 Ma, perhaps related to reactivations of the Transamazonic Tectonic-Thermal Event. A dike of metamorphosed basic rock cutting the Granite-Gneiss Complex also show geochronological age of about 1.275 Ma gotten by K/Ar analysis on actinolite. This age is probably related to reactivations of the Uruacuano Cycle. Cataclastic gneiss in contact with rocks of the Greenstone Belt was analysed by K/Ar method on muscovite and showed ages of 1.470 Ma, probably related with reactivations of the N 70 W faults which form the lateral boundaries of this unit. (author). 16 refs., 4 figs., 2 tabs