Zircon in emplacement borders of post-collisional plutons compared to country rocks : a study on morphology, internal texture, U–Th–Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil).

Abstract

Zircon is a powerful tool to study the internal evolution of igneous bodies and their interactions with country rocks. At pluton borders, zircon may record the emplacement history from the crystallization onset to deuteric processes, as well as inheritance from country rocks. We present a detailed morphology and internal structure study coupled with isotopic analyses (UeThePb and LueHf) on a great number of zircon grains extracted from samples collected at the borders of three distinct post-collisional intrusions of the Araçuaí orogen: granites from the Arace^ e Pedra Azul and Vitoria plutons, and a tonalite from the Mestre Alvaro pluton. For comparison, we also present mineral and bulk-rock chemistry data from these samples of post-collisional intrusions, as well as zircon UePb-Hf data from their country rocks (the Nova Venecia migmatitic paragneisses and Atal eia granites) and a wide dataset compilation. Zircon saturation geothermometry suggests igneous temperatures above 800 C for pluton borders. Zircon geochronology resulted in crystallization ages for borders of the plutons at 523 ± 2 Ma (Arace^ e Pedra Azul), 505 ± 1 Ma (Vitoria), and 527 ± 2 Ma (Mestre Alvaro). Lu eHf data (Arace^ e Pedra Azul pluton: εHf(t) 18.6 to 23.8, TDM ages from 2.25 to 2.47 Ga; Vitoria pluton: εHf(t) 7.4 to 10.3, TDM ages from 1.58 to 1.71 Ga; Mestre Alvaro pluton: εHf(t) 0.7 to 8.8, TDM ages from 1.27 to 1.66 Ga; Nova Venecia migmatitic paragneiss: εHf(t) þ4.1 to 39.2, TDM ages from 1.20 to 3.47 Ga; and Ataleia granite: εHf(t) 3.2 to 8.1, TDM ages from 1.42 to 1.64 Ga) indicate involvement of country rocks in the petrogenesis of post-collisional intrusions. Together, new and compiled data suggest: i) magma hybridization at high temperature, involving country rocks; ii) rapid growth of zircon crystals probably at rapid cooling rates; and iii) in situ dissolutionerecrystallization and overgrowth processes in zircon crystals in response to interactions with residual (late-stage) melts and/or deuteric fluids.