STRUCTURAL AND PETROLOGICAL CHARACTERIZATION OF THE ARROIO DOS RATOS COMPLEX METATONALITES AND METADIORITES AT ITS TYPE-SECTION, REGION OF QUITÉRIA, RS

Abstract

The Arroio dos Ratos Complex is part of the Pelotas Batholith basement in the Sul-rio-grandense Shield. Geological and structural mapping of its type-section, in the region of Quitéria, RS, has led to the discrimination of the so-called Association 1 (A1), consisting of foliated tonalitic to dioritic rocks discordantly intruded by compositionally similar ones. Their basement comprises calc-silicate and tonalitic gneisses, as well as metavolcano-sedimentary rocks. The A1 rocks are medium- to coarse-grained, garnet-bearing biotite metatonalites, and mafic-rich, fine-grained metadiorites to metatonalites, where garnet is restricted to the most differentiated varieties. Field relations indicate that these fine-grained varieties have intruded the coarser ones as synplutonic dykes. The granoblastic texture found in both rock groups is attributed to their recrystallization under temperatures compatible with upper amphibolite to granulite facies conditions. The main foliation, S₁ , is NW- to EW-striking, and its variation is attributed mainly to the 1 effect of F₂ folding, whose axes plunge with low to moderate angles towards WSW, and coincides with  the stretching lineation position. The original orientation of S₁ , later fold effects being suppressed, is  interpreted to have been sub-horizontal, with shallow dip angles towards WSW. Sense of movement determined from macro- and micro-scale structures indicates top-to-E. The A1 metatonalites and metadiorites are meta- to peraluminous rocks and belong to the medium to high-K calc-alkaline series. The behavior of major and trace elements, together with moderately fractionated REE-patterns, with enrichment of LREE in relation to HREE, indicate that this magmatism took place in a mature continental arc environment, and that the A1 is comparable to other TTG associations. Preliminary data indicate a Paleoproterozoic age for the A1 rocks. The information gathered in this paper enables the comparison of this association with the Encantadas Complex orthogneisses, although the latter show stronger effects of regional, orogenic metamorphism. Thus, the A1 rocks are interpreted to represent the more mature phase of the same magmatic arc where the Encantadas Complex protoliths were formed.