Estudo comparativo das rochas sanukitóides da área de Xinguara com outras ocorrências no domínio

Abstract

The Rio Maria sanukitoid Suíte outcrops in several areas of the Rio Maria domain (Rio Maria, Redenção, Bannach, Mata Geral and others), in the southern portion of the Carajás Province. Despite the existence of works describing the rocks of the Rio Maria Suíte in the Xinguara area, the present research discusses additional field, petrographic and magnetic susceptibility data of these rocks, as well as points their particularities in relation to the other rocks of this Suíte. Two stocks were the target of this research. One is located northwest and the other southwest from Xinguara. These bodies have similar features, being composed mainly of light gray rocks with green tones and medium-grained equigranular texture. Mafic enclaves with ellipsoidal forms with "dripped" feldspar crystals, oriented parallel to the foliation of the rock, are typical features of these rocks. The foliation varies from NW-SE to WNW-ESSE and dips around 60° SW. The petrographic descriptions allowed to identify three groups: (1) intermediate and ultramafic rocks, represented by quartz-dioritic and cumulative rocks; (2) granodioritic rocks, which are largely dominant and based on the variations of amphibole and biotite, it was possible to individualize them in hornblende-biotite granodiorites and biotite granodiorites; and 3) Tonalite rocks of very restricted occurrence. The amphibole and biotite are the main ferromagnesian minerals, and the most frequent accessory minerals are zircon, epidote, titanite, apatite, allanite, opaque and muscovite. The secondary minerals are chlorite, sericite-muscovite, epidote and carbonate. The alkali/plagioclase feldspar ratios increase and the M values decrease towards cumulative rock  Quartz-diorites  Tonalite  Granodiorite. The magnetic susceptibility (SM) of the studied rocks is low for granitoids, varying from 1.084 to 0.045 10-3 (SIv) and the distribution of values is unimodal with a peak in the range of log -3.9 to -3.7. This research revealed the following particularities about the studied rocks in relation to the other occurrences of Sanukitoid granitoids of the Rio Maria Domain: a) The NW-SE or WNW-ESE structural trend; b) granodiorites comparatively more enriched in quartz; c) relative abundance of rocks of quartz-dioritic and tonalitic compositions; d) the absence of monzogranitic facies; e) calcium-alkaline low-K trend, different from the striking K-mean trend in the other occurrences of the Rio Maria Suíte; f) the absence or low amount of amphibole in granodiorites; g) comparatively lower values of MS. It is considered that the processes that generated the various occurrences of the Rio Maria domain Sanukitoid rocks were similar, but not identical. In addition, local geology and diachronic processes could explain the structural differences observed in the studied rocks.