LOW-K CALC-ALKALINE GRANITIC SERIES OF SOUTHEASTERN TOCANTINS STATE: CHEMICAL EVIDENCE FOR TWO SOURCES FOR THE GRANITE-GNEISSIC COMPLEXES IN THE PALEOPROTEROZOIC ALMAS-DIANÓPOLIS TERRANE

Abstract

Tonalites, trondhjemites and granodiorites (TTG) compose the major part of Archaean granite-gneiss terranes. These low-K calc-alkaline granitic rocks are divided into low- and high-Al (sub 2) O (sub 3) groups. This paper presents whole-rock chemical data displaying that peralumious and metaluminous low-K calc-alkaline granitoids that constitute the granite-gneiss complexes of Almas-Dianopolis Terrane, central Brazil. These complexes comprise several isotropic to weakly foliated granitoid bodies intruding narrow supracrustal belts. Recent SHRIMP U-Pb geochronology indicates that the granitoid bodies were intruded during the Paleoproterozoic (2.2 to 2.45 Ga). Amphibole-rich granitoids of Suite 1 (2.2 Ga) are dominantly tonalite, but include units of granodiorite, trondhjemite, quartzdiorite and quartz-monzodiorite with lower contents of Al (sub 2) O (sub 3) (metaluminous), high #Mg values, high Cr, Ni and V contents, and low Sr/Y and (La/Yb)n ratios. Mantle partial melting or fractional crystallization of mantle derived magmas yielded these Suite 1 melts. The Ribeiro das Areias Complex (2.45 Ga) and Suite 2 (2.2 Ga) are composed of tonalite, trondhjemite, granodiorite and minor monzogranite units with high-Al (sub 2) O (sub 3) contents (peraluminous), low #Mg values; low Cr, Ni, Y and Ybn contents, and high Sr/Y and (La/Yb)n ratios, which resulted from partial melt of metabasalt, leaving garnet and hornblende as restite. This metabasalt belongs either to subducted oceanic slab or to newly underplated basaltic crust. Oceanic lithosphere subduction is the most suitable geological process to yield both types of low-K calc-alkaline granitic series. If this is so, subduction related processes were active during the building up of the northwestern margin of the Sao Francisco Craton during Paleoproterozoic times.