Petrogenesis of the Afonso Cláudio Intrusive Complex: implications for the post-collisional evolution of the Araçuaí-West Congo orogen

Abstract

The Afonso Cláudio Intrusive Complex (ACIC) is a post-collisional intrusion from Araçuaí-West Congo orogen (AWCO), which represents an important tectonic segment from southeast Brazil and west Africa. The ACIC intrusion is associated to AWCO collapse stage, which ranges from 530 to 480 Ma. This pluton intruded gneisses from AWCO pre-collisional stage and Nova Venécia Complex and it is mainly composed of two off-centered monzogabbroic/monzodiorotic cores surrounded by quartz monzonite, with intense magma mingling and mixing widespread between these domains. Crustal xenoliths related to the enclosing country rocks occur widespread in the quartz monzonite domain, while are occasional in the monzogabbro/monzodiorite domain. In addition, syenogranite dykes commonly crosscut ACIC rocks. The typical AWCO post-collisional intrusions have been extensively studied, however the crystallization conditions of these intrusions have been occasionally investigated, while the commonly hosted crustal xenoliths have never been investigated in such detail. The examination of the chemistry of principal phases of the ACIC main rocks and the crystallization conditions of these rocks revealed that this pluton is formed by the interaction between mantle and crustal magmas which crystalized between middle to deep crustal levels. The commonly found xenoliths hosted by ACIC and enclosing country rocks show similar geochemical features, however both ones show different geochemical normalized patterns when compared to the ACIC main rocks. These results suggest that similar xenoliths and enclosing rocks are cogenetic, however both could not represent the main sources of magmas that generated the ACIC. Geochemical modeling of igneous processes of previous ACIC dataset considering investigated xenoliths and enclosing rocks as the main assimilant material suggests that mixing was responsible for the similar geochemical patterns between ACIC main rocks. The modeling also indicates that fractional crystallization with minor influence of assimilation from enclosing rocks controlled the magmatic evolution of monzogabbro and monzodiorite related to an enriched mantle source, while coupled assimilation and fractional crystallization ruled the evolution of quartz monzonite associated to lower crust magmas mainly contaminated by the enclosing rocks. Monazite dating of xenoliths showed that these ones preserved similar main ages related to different stages of ACIC evolution. The xenoliths showed main older ages of 478 and 477 Ma which are related to ACIC intrusion and crystallization during AWCO post-collisional stage, while younger ages of 431 and 427 Ma are probably related to the intrusion of syenogranite dykes, representing an extension of post-collisional magmatism or a later unrelated thermal event. Due to the similarities between typical post-collisional intrusions, these insights could be considered to other similar intrusions from this orogenic system.