Universidade Federal do Espírito Santo, Departamento de Geologia, Alto Universitário s/n, 29500-000 Alegre, ES, Brazil.
Universidade Federal de Ouro Preto, Applied Isotope Research Group, Departamento de Geologia, Morro do Cruzeiro s/n, 35400-000 Ouro Preto, MG, Brazil.
An Acad Bras Cienc. 2021 Oct 29;93(3):e20200639. doi: 10.1590/0001-3765202120200639. eCollection 2021.
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 fluid-absent reactions that produced the first 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 significant 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.
巴西东南部阿拉萨伊造山带的碰撞花岗岩记录了岩石学和年代学证据，表明在造山演化过程中发生了多次地壳熔融。U-Pb 锆石数据表明，这些花岗岩结晶于 586±2Ma。高级变质作用（M1）涉及无流体反应的部分熔融，在约 750°C 的温度下产生第一代石榴石。矿物组合 A1（石榴石-黑云母-斜长石-钾长石-石英-钛铁矿-熔体）的保存表明，大部分熔体在这些岩石中或在接近变质峰条件下从岩石中逸出。第二次变质事件（M2）的特征是在保留的 A2 组合（石榴石-矽线石-黑云母-斜长石-钾长石-石英-钛铁矿-熔体）中生长第二代石榴石。矿物平衡模拟将 M2 变质条件约束在 713-729°C 和 6.2-7.3kbar。退变质组合（A3）记录了 610-660°C 的平衡条件。Hf 同位素组成表明，碰撞花岗岩的成因存在显著的地壳贡献。加厚地壳中的高地温为 562±2Ma 的 M1 事件提供了足够的热量。随后的加热可能与地幔热向地壳的传递有关，这发生在造山带的伸展变薄和重力坍塌过程中，导致了 526±4Ma 的 M2 事件。这次事件与造山带后碰撞岩浆的侵位同时发生。
