The Western Amazonia Igneous Belt

Abstract

The Western Amazonia Igneous Belt (WAIB) occupies the northern part of South American continent, and a total area close to 125,000 km2. The belt is composed dominantly by volcano-plutonic felsic rocks (Juruena supersuite, Teles Pires suite and Colíder group) and has dominant alkali-calcic, metaluminous to peraluminous, ferrous, geochemical characteristics, similar to A-type granites. The Silicic members are represented by granites and rhyolitic-rhyodacitic volcanic rocks, mafic members by gabbroic rocks and diabase dykes. Intermediate rocks are rare. Local magma mingling and hybridization are present in several areas. The assemblage of silicic and basic rocks of WAIB was formed between c. 1825 and 1757 Ma. The presence of inherited zircons, from 1875 to 2050 Ma, as well as some Archean ages in the granitic and volcanic rocks from the WAIB, are suggestive of derivation by melting of pre-existing crustal basement (Ventuari-Tapajós Province). This hypothesis is also corroborated by bimodal magmatism and by several model-ages of granites, felsic volcanic rocks and mafic plutonic rocks, with TDM ranging from 2.0 to 2.3 Ga, and εNd value, varying from −3.90 to +2.52. The preferred model of formation of the WAIB envisages the emplacement of large volumes of hot mantlederived melts into the lower crust. These processes resulted in partial melting of lower-crustal material, mixing with the mantle-derived melts and formation of granitic melts. The timing and spatial features and interpreted petrogenetic mechanisms of WAIB are suggestive of a continental rift setting. In addition, this volcano-plutonic province is similar to silicic igneous provinces (Silicic Large Igneous Province-SLIP) in areal extent, eruptive volume and petrogenetic characteristics