New geologic and isotopic data for the Iriri-Xingu Domain, Central Amazonian Province: implications for the Iriri Group age

Abstract

The Iriri-Xingu Domain, located in the southern Central Amazonian Province, is one of the less geologically known regions in the Amazonian Craton. It is dominated by Paleoproterozoic volcanic rocks and granitoids, and the regional basement rarely crops out. Petrographic and field data allowed distinguishing three rocks groups: A-type Granites (syenogranites and quartz-syenites), I-type Granites (monzogranites, quartz-monzodiorites, quartz-monzonites and granodiorites) and Iriri Group volcanic and volcaniclastic rocks (rhyolites and dacites, and dacitic ignimbrites, respectively). Pb-Pb zircon ages indicate at least two Orosirian magmatic events in this region: 1992-1986 Ma and 1889 Ma. Sm-Nd isotopic data provided Nd T_DM from 2.43 to 3.13 Ga, and ε_Nd(t) from –10 to –3, which confirm an old crustal source for these magmas, typical of the Central Amazonian Province. These results show that the 1.99 and 1.89 Ga magmatic events, which are common in the Tapajós Domain, also occur in the Iriri-Xingu Domain. However, their Archean crustal sources contrast with the Paleoproterozoic sources characteristic of the Tapajós Domain. Additionally, the zircon ages obtained in this study for the volcanic rocks of the Iriri-Xingu Domain are older than ca. 1880 Ma, which typically correlate with the Iriri Group. Thus, we propose a lithostratigraphic redefinition for this unit based on the occurrence of volcanic rocks of ca. 1988 Ma in the type area.