GEOQUÍMICA E ASPECTOS PETROGENÉTICOS DO GRANITO XINGUARA, TERRENO GRANITO-GREENSTONE ARQUEANO DE RIO MARIA - CRÁTON AMAZÔNICO

Abstract

The Xinguara Granite (XingG), an Archean granitic pluton situated in the northern part of the Rio Maria Granite-Greenstone Terrain, southeast of Pará state, is intrusive in TTG granitoids and gneisses and in the Rio Maria Granodiorite. The XingG exhibits a remarkable compositional homogeneity, being composed essentially by metaluminous to slightly peraluminous leucogranite with high SiO 2 and alkali contents, high K 2 O/Na 2 O ratios, low MgO and moderate CaO contents. Subordinate, it also occurs pegmatoid granites and leucosyenogranites. The leucomonzogranite fácies was subdivided on basis of the geochemical data in the LMG1 and LMG2 types, whole silica contents are similar. However, the LMG2 type has lower Al 2 O 3 Cb, MgO, CaO and Na 2 O and higher K 2 O, Rb, Zr and Y contents compared to the LMG1 type. In geochemical diagrams the LMG1 and LMG2 types define contrasting evolutive trends, indicating that they crystallized from two different liquids. The behavior of lithophile elements suggests that a moderate fractionation of potassic feldspar, plagioclase and biotite controlled the small degree of differentiation of Xinguara pluton magmas. The fractionation of feldspar is confirmed by the deep Eu negative anomalies (Eu/Eu* ranging from 0.28 to 0.38 in the LMG1 type). The LMG1 type shows, in general, an acentuated depletion in the heavy rare earth elements (Lan/Ybn, ranging from 149.07 to 16.63), which is probably due to garnet and/or amphibole retention in the melt residue. The large extent of fractionation of the HREE, the depletion in Y, Ti and Nb parallel to the enrichment in P e Sr in comparison to the average composition of the upper continental crust, as well as the high values of the Rb/Y and Al 2 O 3 /TiO 2 ratios, indicate a similarity between the Xinguara leucogranites and Archean calc-alkaline leucogranite of type 2. The two types of Xinguara leucogranites were probably derived by different degrees of partial melting from a same crustal protolith, which is possibly similar to the TTG granitoids and gneisses or, alternatively, to the Rio Maria Granodiorite.