ZONALIDADE REVERSA DOS GRANITOS DO COMPLEXO INTRUSIVO LAVRAS DO SUL, RS: PETROGRAFIA, SUSCETIBILIDADE MAGNÉTICA E ÓXIDOS DE FE-TI

Abstract

We evaluate the magma redox conditions of granites from the Lavras do Sul intrusive complex - LSIC (ca. 606-586 Ma). The main goal is to explain the zoning of magnetic susceptibility-k that shows a concentric pattern revealing the reverse zoning of petrographic facies. The granite pluton includes: (a) alkali-calcic, biotite granodiorite and hybrid amphibole-biotite granites in the core (k of 4.0-7.5 10 -3 SI), and (b) alkaline, biotite-amphibole syenogranite and perthite granite that occur around the core (k of 9-10 10 -3 SI). All facies are ferromagnetic, and the differences in k-values are due to the distinct nature of parental magmas and to the variations in redox path shown by mafic phases (amphiboles, biotite and Fe-Ti oxides). Divergent Fe/Mg ratios between mafic silicates and rock imply an oxidizing environment for the crystallization of core types, in which the two Fe-Ti oxides are common (Ti-Mt:Ilm of 3:1 to 10:1). Such conditions were ascribed to the magmatic fluid phase on distinct situations, of crystal-volatile equilibrium (monzogranite) and of volatile exsolution (granodiorite). This reinforces that the core-granites represent distinct pulses of a hydrous magma (3-4 wt. % H 2 O) evolving at different pressure (4-2 kbar). The parental magma of alkaline granites had lower H 2 O content (~2 wt. %), and would be intrinsically oxidizing as suggested abundant Ti-magnetite (Ti-mt:Ilm ≥ 20:1) and primary titanite. Convergent Fe/Mg ratios for mafic phases and rock imply the mineralogical control in the redox path also marked by the decrease of pressure with differentiation (2-1 kbar). We conclude that the pluton formed during two major episodes of resurgence in a magma chamber related to the volcano-plutonic subsidence system-LSIC: a central event responsible for the alkali-calcic granites, and a circumferential one for the alkaline types. So, petrographic and magnetic zoning, strengthened by magnetic and gravity data, show such differences. Despite the low k-values in the core granites, a magmatic environment with varied fO 2 , allows the genetic link with the Au-Cu ore occurrences.