Abstract
The Ibare Lineament is a shear zone that sections the Western portion of Sul-riograndense Shield, affecting the orthogneissic rocks of Santa Maria Chico Granulitic Complex, acting as a conduct for hydrothermal fluids. The studied hydrothermal mineralogy reflects a propylitic alteration paragenesis that is composed of Fe-clinochlore, epidote, albite, calcite and quartz, typical low-pressure paragenesis (< 2,000 bar). This mineral association reflects an equilibrium in a neutral to slightly alkaline pH. Using the new approach for chlorite geothermometry, an average formation temperature of 274°C was obtained, compatible with the stability of polytype IIb chlorite and with the irregular textures of twinning in calcites. The fluid inclusions in quartz establish a low-salinity system (2.45 wt.% NaCl eq.) and an average temperature of 175°C. The obtained temperatures are interpreted as a continuum cooling model with meteoric fluids entrance. The system pressure is evaluated from isochores, reaching an average value of 1,560 bar. This is the first temperature and pressure estimate to the Ibare Shear Zone, which enabled the establishment of a shallow hydrothermal system (epithermal) with fluid interaction from magmatic and meteoric sources, confirmed by the behavior of carbon and oxygen stable isotopes, which indicated an the origin from mixed sources.
Highlights
The Ibaré Lineament is in the Western sector of Sulriograndense shield as a NW-SE-oriented geostructure on the limit between Taquarembó (SW) and São Gabriel (NE) terranes
Intense chloritization is described on these planes, which is associated with carbonates and hematite, indicating pervasive fluid percolation parallel to banding (NE) (Figs. 2A and 2B) with calcites that caused brecciation in the rock portions (Figs. 2C and 2D)
The mineralogy identified in the hydrothermal alteration of the orthogneisses of Santa Maria Chico Granulitic Complex (SMCGC) fits as a result of a propylitic alteration, whose mineralogy reflects a typical paragenesis of low pressure greenschist facies (< 2,000 bar)
Summary
The Ibaré Lineament is in the Western sector of Sulriograndense shield as a NW-SE-oriented geostructure on the limit between Taquarembó (SW) and São Gabriel (NE) terranes It is the superficial expression of a subvertical brittle-ductile transcurrent shear zone, initially with dextral sense, which was late reactivated in successive episodes with sinistral kinematic at shallow crustal levels (Luzardo & Fernandes 1990, Fernandes et al 1992, Ruppel 2010, Laux 2017). Driven by the economic potential, a study was developed with techniques to better evaluate the pressure and temperature (P-T) conditions of the hydrothermal alteration in a specific sector of the ISZ, composed by orthogneissic rocks of the SMCGC. The P-T conditions and the Brazilian Journal of Geology, 48(4): 685-702, December 2018
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