Petrografia, geoquímica e geocronologia de rochas vulcano- plutônicas orosirianas do SE do Cráton Amazônico: um estudo da fronteira dos domínios Tapajós e Iriri-Xingu

Abstract

The central part of the Amazonian Craton encompasses the Tapajós (TJD) and Iriri-Xingu (IXD) tectonic domains, which record volcano-plutonic events from the Orosirian times. The boundary between both domains was proposed from Nd isotopes and is marked by the predominance of juvenile paleoproterozoic sources or with little participation of the archaean crust for the TJD, whereas the rocks of the IXD present archaean crustal sources. However, the boundary between these domains is still subject of discussion due to the lack of isotopic data, the homogeneity of the lithotypes and the absence of tectonic structures that could outline this frontier. The study area is in the municipality of Trairão, southwestern Pará state, the probable boundary region between TJD and IXD, where there are occurrences of paleoproterozoic volcano-plutonic associations. We performed in situ U-Pb on zircon by plasmainduced mass spectrometry with laser ablation (LA-ICP-MS), Sm-Nd whole-rock by thermal ionization mass spectrometry (TIMS), together with geochemical analyses and petrographic data aiming to reconstruct the tectonic and chronological emplacement of the magmatic rocks, as well as to better establish the boundary between TJD and IXD. In the study area, the volcanic sequences are represented by the Moraes Almeida, Salustiano and Aruri formations, and the plutonic units correspond to the Creporizão, Parauari (granodioritic and granitic units) and Maloquinha suites. The effusive and volcanoclastic rocks are rhyolites, dacites, andesites, rhyolitic ignimbrites, and tufts. The plutonic units are granodiorites, quartz monzonites, monzogranites, and syenogranites. The geochemical results showed that the rocks of the Creporizão and Parauari intrusive suites (granodioritic and granitic units) have high-K calcalkaline to shoshonitic signature, a meta to peraluminous character, with enrichment in LILE (K, Rb, Ba, and Sr), moderate fractionation of heavy ETR and weak negative Eu* anomalies. Distinctively, the granitoids of the Maloquinha Intrusive Suite and the Salustiano and Moraes Almeida formations have a high-K calc-alkaline signature, from peraluminous to peralkaline character. These rocks present enrichment in HFSE (Zr, Hf, and Th), high ETR contents and pronounced negative Eu* anomalies. In tectonic environment classification diagrams, the granitoids of the Creporizão and Parauari (granitic and granodioritic units) intrusive suites and, Salustiano and Aruri formations showed geochemical affinities with volcanic arc granites (VAG), related to magmatic arc, while the rocks of the Maloquinha Intrusive Suite and the Moraes Almeida Formation are related to intraplate anarogenic environment. The U-Pb zircon dating allowed to identify the oldest plutonic and volcanic rocks, belonging, respectively, to the Intrusive Creporizão Suite, with crystallization age of 1980±6 Ma, and to the unit mapped xi as Salustiano Formation, aged of 1975±11 Ma. The rocks of the Maloquinha Intrusive Suite were generated at 1880±9 Ma, with a volcanic correspondent of the Moraes Almeida Formation of 1877±14 Ma, followed by the plutonic rocks of the Parauari Intrusive Suite (granodioritic unit, aged of 1876±9 Ma, and the granitic unit, of 1867±15 Ma), and the volcanic rocks of the Aruri Formation, generated at 1867±7 Ma. The geochemical characteristics combined with the geochronological data allowed to define a geodynamic evolution involving an context related to magmatic arc environment at ca. 1.98 Ga, which favored the formation of the rocks of the Intrusive Suite Creporizão and Salustiano Formation, followed by an intraplate extensional environment (ca. 1.88 Ga), marked by the coexistence of the types A and I rocks of the Maloquinha and Parauari (granodioritic and granitic units) suites, and Moraes Almeida and Aruri formations. Model ages Nd-TDM (2.31-2.64 Ga), with slightly to strongly negative values of εNd(t) (-1.39 to -7.11), indicate magmas derived from the melting of crustal sources of the Paleoproterozoic and Archaean. These results allowed to draw an NW-SE line, where mainly rocks of paleoproterozoic (<2.5 Ga) and archaean (> 2.5 Ga) model outcrop, belonging, respectively, to the TJD and IXD.