Abstract
The mafic/ultramafic Ipanema Layered Complex (ILC), Minas Gerais, Brazil, consists of seven individual bodies. These units crosscut polyphase orthogneisses and interlayered paragneisses of the Paleoproterozoic Juiz de Fora Complex. Intrusive granitoids tectonically related to the Neoproterozoic Araçuaí orogen are also present in the study area. A Sm-Nd whole-rock linear array for seven samples (metapyroxenites, metaperidotites, metagabbro, and meta-anorthosite) from the Santa Cruz massif, the largest body of the ILC, suggest that it was emplaced at 1104 ± 78 Ma; the original magma was derived from a depleted mantle source (εNdt = +3.8). U-Pb single-grain zircon study of a meta-anorthosite yields an upper intercept age of 1719 ± 4 Ma, which is interpreted to represent inheritance. The lower intercept at 630 ± 3 Ma indicates that a Neoproterozoic tectonothermal episode overprinted the ILC; this event occurred under upperamphibolite- to granulite-facies conditions. The 630 Ma episode is consistent with the timing of regional metamorphism and deformation of the adjacent Araçuaí orogen (Brasiliano collage). Emplacement of the ILC and other coeval metamafics and meta-ultramafics (of alkaline affinity) in the region is attributed to early extension tectonics, accompanying accretion of the Rodinia supercontinent during the Mesoproterozoic—Neoproterozoic time boundary.
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