The growth of large mafic intrusions: Comparing Niquelândia and Ivrea igneous complexes

The Niquelândia Complex, Brazil, is one of the world's largest mafic–ultramafic plutonic complexes. Like the Mafic Complex of the Ivrea-Verbano Zone, it is affected by a pervasive high-T foliation and shows hypersolidus deformation structures, contains significant inclusions of country-rock paragneiss, and is subdivided into a Lower and an Upper Complex. In this paper, we present new SHRIMP U–Pb zircon ages that provide compelling evidence that the Upper and the Lower Niquelândia Complexes formed during the same igneous event at ca. 790 Ma. Coexistence of syn-magmatic and high-T subsolidus deformation structures indicates that both complexes grew incrementally as large crystal mush bodies which were continuously stretched while fed by pulses of fresh magma. Syn-magmatic recrystallization during this deformation resulted in textures and structures which, although appearing metamorphic, are not ascribable to post-magmatic metamorphic event(s), but are instead characteristic of the growth process in huge and deep mafic intrusions such as both the Niquelândia and Ivrea Complexes. Melting of incorporated country-rock paragneiss continued producing hybrid rocks during the last, vanishing stages of magmatic crystallization. This resulted in the formation of minor, late-stage hybrid rocks, whose presence obscures the record of the main processes of interaction between mantle magmas and crustal components, which may be active at the peak of the igneous events and lead to the generation of eruptible hybrid magmas.