GÊNESE E EVOLUÇÃO DOS GRANITÓIDES METALUMINOSOS DE AFINIDADE ALCALINA DA PORÇÃO OESTE DO ESCUDO SUL-RIOGRANDENSE: GEOQUÍMICA E ISÓTOPOS DE RB-SR E PB-PB

Abstract

The mctaluminous granites with alkaline affinity or tendency occur in lhe west and east portions of the Sul-riograndense Shield. They are separated in three groups. Although ali three might be related to A-type granites, in the wide sense of the term, theirgeochemical and isotopic features reflect differences in the sources or in the generation processes. These granites, in the east, have highly fractionated calc-alkaline nature, with alkaline tendency and are derived from sources enriched in K 2 O, Rb, U and Th and depleted in Nb. In the west, they show more prominent alkaline affinity and are grouped in the Saibro Intrusive Suite. This suite has two metaluminous granite types and the compositional variations among them is strongly constrained by intensity and diversity in the crusta! contribution. The strongly alkaline ones have mantle signature and are related to the end of an early magmatic event (580 - 610 Ma), with dominant shoshonitic affinity. The metaluminous sensu strictu terms are associated to a later event (540-560Ma) with more prominent participation of crustal sources. The main differences between the two types, that allow related respectively to A-1 and A-2 granites, include variations in the values of CaO/alkalis, Ba/Sr, Y/Nb and Zr/Nb ratios; in the content and geochemical behavior of Zr, and in the composition of mafic phases. The compositional characteristics indicating source types are also common to the composition of the rest of the granites later in the Brasiliano Cycle, in the west portion, independem of their geochemical signature (high K calc-alkaline, shoshonitic and alkaline). This allow to establish two regional domains: I) north-northeast portion (Cacapava do Sul - Sao Sepe), where the granitoids have strong crustal influence, as shown by the high negative values of EMI, moderate to high ( 87 Sr/ 86 Sr)i and, in the alkaline ones, lower initial Pb isotopic ratios; and 2) center-south portion (Lavras do Sul), where compositions with important mantle signature predominate. These granites show lower values of ( 87 Sr/ 86 Sr) and e Nd similar to the primitive mantle. Although the two granite groups are distributed in distinct regional domains, the evolution in the magmatic events, characterized by the transition in the geochemical affinity (shoshonitic-alkaline) and by the increasing crustal contribution, also occur in time, as illustrated in the region of the granites Jaguari-Lavras.