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; ε Nd from -1.9 to -2.8), Hf (zircon, LAM-ICP-MS; ε Hf from -2.0 to -3.1), and O (zircon, SIMS; δ 18O 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.
- Isotope geology
- Rapakivi granites