Petrology of the Uatumã Magmatism in the Serra dos Magalhães region : implications to the extension of the southeastern edge of the Amazonian Craton

Abstract

The Amazonian Craton (AC) is one of the largest Precambrian crustal segment in the world and host several metallogenetic provinces as the Carajás, Tapajós and Alta Floresta. The volcanoplutonic association of the Serra dos Magalhães region (SE of the AC) is part of the largest magmatic event of the craton (Uatumã Magmatism) and consists of volcanic rocks of the Iriri Group and granitoids of the Rio Dourado Intrusive Suite. In the study area, Quaternary alluvial terraces of the Araguaia Formation and alluvial deposits partially cover these units. The volcanic rocks comprise massive rhyodacites and rhyolites and flow-banded rhyolites. The rhyodacite and the rhyolite yield crystallization ages of 1871±11 Ma and 1898±10 Ma, respectively. Rhyodacites and most of the rhyolites show massive groundmass whereas some rhyolites show flow foliation in the highlands outcrops. The subordinate flow foliation and the lack of pyroclastic fragments suggest an effusive nature to the volcanic facies. The volcanic rocks associate to massive hornblende-biotite monzogranites of 1881±9 Ma. These rocks show porphyritic textures with alkali-feldspar megacrysts, plagioclase, hornblende, biotite and xenomorphic quartz. Both volcanic and granitic rocks are metaluminous to peraluminous with A-type affinity. The REE pattern of rhyodacites and granitoids are similar and indicates comagmaticity. The rhyolites present enrichment of HREE. Petrographic and geochemical characteristics attest a post-teconic character to the volcano-plutonism. These results imply in existence of a Paleoproterozoic or Archean basement beneath the Uatumã Magmatism units in the study area, underlying the Quaternary sedimentary cover and indicating that the AC extends beyond the occurrences of the Confresa town region. Geomorphological features and the drainage pattern, allied to the geological information, allow the extension of the SE edge of the AC for around 150 km southward, from what is currently delimited in regional geological maps. Regional linements suggest that the AC also extends underneath a broad area of the Parecis Basin.