Petrogenesis of the igneous Mucajaí AMG complex, northern Amazonian craton — Geochemical, U–Pb geochronological, and Nd–Hf–O isotopic constraints

Abstract

The ca. 1525Ma igneous Mucajaí anorthosite–monzonite–granite (AMG) complex in northern Brazil is a rare manifestation of Mesoproterozoic intraplate magmatism in the northern Amazonian Craton. The complex comprises a two-phase rapakivi granite batholith with subordinate quartz–fayalite monzonites and syenites and the closely associated Repartimento anorthosite. Zircon U–Pb (ID–TIMS) geochronology reveals that the anorthosite (1526±2Ma), monzonite (1526±2Ma), and the main-phase biotite–hornblende granite (1527±2Ma) of the complex intruded the Paleoproterozoic (~1.94Ga) country rocks simultaneously at ~1526Ma and that the more evolved biotite granite is marginally younger at 1519±2Ma. Intraplate magmatism in the Mucajaí region was relatively short-lived and lasted 12million years (1529–1517Ma) at maximum. The Nd (whole-rock, ID–TIMS; εNd from −1.9 to −2.8), Hf (zircon, LAM-ICP-MS; εHf from −2.0 to −3.1), and O (zircon, SIMS; δ18O from 6.1 to 7.0‰) isotopic compositions of the studied rocks are fairly uniform but still reveal a small degree of isotopic heterogeneity in the Paleoproterozoic crust enclosing the complex. The small isotopic differences observed in the two types of rapakivi granites (biotite–hornblende granite and biotite granite) may result either from an isotopically heterogeneous lower crustal source or, more likely, from contamination of the granitic magma derived from a lower crustal source during prolonged residence at upper crustal levels.