Constraining crystallization conditions during the Cambro-Ordovician post-collisional magmatism at Araçuaí belt (SE Brazil): Zircon as key petrologic witness

Abstract

During the Neoproterozoic West-Gondwana assembly, the Araçuaí belt together with its African counterpart, the West Congo belt, formed a confined orogenic system between the São Francisco and Congo cratons. As a result, a widespread magmatism took place from ca. 630 to 480 Ma at the Araçuaí belt, finishing with a ca. 530 to 480Ma post-collisional period. The Alto Chapéu pluton is one of the numerous intrusions emplaced during this post-collisional event. In order to investigate the crystallization conditions involved in the evolution of this pluton, we conducted a study based on zircon chemistry, concerning the granitic composition of this magmatism at the early stages of the Araçuaí orogenic collapse. Coupled zircon saturation and Ti-in-zircon temperatures along with REE-in-zircon oxygen fugacity estimatives indicate high temperatures and relative reducing conditions intervening magmatic differentiation given by autocrystic zircon domains (maximum 814 ± 14 °C to average 749 ± 24 °C and ΔQFM -3.78 ± 1.02). Even more reducing and hotter isolated magma pulses at Alto Chapéu Pluton are indicated by antecrystic zircon domains (isolated peaks of 864 ± 28 °C and 920 ± 30 °C and ΔQFM reaching −5.11 ± 0.84 at antecrystic rim). Inherited grains interpreted to derive from host rock point to slightly lower temperatures, and also reducing conditions (maximum 756 ± 16 °C to averages of 679 ± 22 °C and ΔQFM -3.88 ± 1.09). Older inherited zircons are problematic to constraint and because of this zircon trace element chemistry can shed light to characteristics of lost melts. The chemistry of inherited zircon points out to the presence of former continental-like crust at least 70 Ma prior to the Araçuaí main granite production events. The wide time-lapse involving zircon inheritance and crystallization seems to be related to post-collisional nature in recording a variety of geochemical sources signature.