A trama do monzogranito itã: Escudo das Guianas, RR

Abstract

The Itã Granite is composed mainly of granodiorite and monzogranite facies whose limits indicate that they were formed by pulses that interacted while still in a magmatic state, and that after crystallization were intruded by syenogranite in the form of dykes and apophyses. These lithofacies are metaluminous, exhibit high-K calc-alkaline geochemical signature and higher ETRL distribution in relation to heavy elements, which suggests moderate fractionation as the dominant petrogenetic process. Evidence of incipient assimilation was locally observed. The plutonic lodging of the Itã Granite in the lower crust occurred under a regional transpressive tectonic domain, imposing an elliptical shape with a major axis oriented to azimuths 220°-230° to 050°-060° and an internal Petro fabric record characterized by magmatic foliation with an attitude varying in azimuths 230°-260°/50°-70°. Mechanisms linked to chamber convective processes were preserved in the petrofabric of the center of the pluton. The organization, spatial distribution and orientation of this petrofabrics are consistent with the position of the regional tectonic shortening vector in the azimuths from 320°-340° to 140°-160°. The origin and orientation of its petrofabric into domains, and its high dip in the center of the pluton, similar to the petrofabrics of its host rocks, is suggestive of a plastic (high temperature) rheology of the crust during plutonic emplacement. Simple-shear solid-state deformation with low rake lineation and dominant northward droop and dextral rotation was recorded at high temperature. This deformation occurs arranged in bands where the rock exhibits mylonitic texture with attitude parallel to the magmatic foliation. The Itã Granite crystallized at temperatures from 843 °C to 827 °C, was hosted in two main pulses that interacted with each other, at pressures between 1043, 690 and 275 MPa consistent with the lower crust environment in a magmatic arc. During it is emplacement, the pluton recorded dominant submagmatic and subordinate solid-state petrofabrics under sin-kinematic to late-kinematic conditions. Considering that the host rocks of the Itã Granite are deformed charnockite and syenogranite (wiborgite type) and that they have petrofabric with organization and orientation similar to that of the Itã Granite, it is interpreted that all these rocks recorded their respective petrofabric under the influence of the same tectonic event and by both would be temporally close and cinematically coherent.