Petrologic and geochronological constraints on the polymetamorphic evolution of the collisional granites, Araçuaí Orogen (SE Brazil).

Abstract

Collisional granites of the Araçuaí Orogen, southeastern Brazil, record petrological and geochronological evidence for multiple crustal melting during the orogeny evolution. U-Pb zircon data indicate that these granites crystallized at 586 ± 2 M.y. High-grade metamorphism (M1

) involved partial melting by fl uid-absent reactions that produced the fi rst generation of garnet in temperatures of approx. 750°C. Preservation of the mineral assemblage A1

(garnet-biotite-plagioclase-K-feldspar-quartz-ilmenite-melt) indicates that most of the generated melt was lost from these rocks at or near peak metamorphic conditions. A second metamorphic event (M2

) is characterized by growth

of a second generation of garnet in preserved A2

assemblage (garnet-sillimanite-biotite- plagioclase-K-feldspar-quartz-ilmenite-melt). Mineral equilibria modeling constrains

conditions of M2

metamorphism to 713-729 °C and 6.2-7.3 kbar. Retrograde assemblage

(A3 ) records equilibrium conditions at 610-660 °C. The Hf isotope composition indicates signifi cant crustal contribution to the genesis of the collisional granites. The elevated geotherms in thickened crust provide enough heat for the M1

event at 562 ± 2 M.y. Subsequent heating probably associated to the transfer of mantle heat to the crust during the extensional thinning and gravitational collapse of the orogen lead to the M2 event at 526 ± 4 M.y. This event is concomitant to the emplacement of the post- collisional magmas in the orogen.