Textural analysis and emplacement conditions of well-preserved Orosirian felsic volcanic rocks of Northern Amazon Craton, Brazil

Abstract

Located in the Amazon Craton, the Uatumã magmatism (1.89-1.87 Ga) consists in one of the oldest Silicic Large Igneous Provinces (SLIPs) on Earth. For a long time, the access to these deposits in the northern Amazon Craton (Erepecuru–Trombetas Domain) has been set back for volcanological studies due to dense vegetation cover and the absence of roads. Recent studies identify two Orosirian volcanic units in the region: the Iricoumé Group (1.89-1.87 Ga) related to the Uatumã magmatism, and the Igarapé Paboca Formation (1.99-1.94 Ga), associated to an older magmatism. Both units are widespread in the Erepecuru–Trombetas Domain and include effusive and explosive deposits. In this paper, we apply textural analyses and rheological estimations to determine the eruption and emplacement conditions of these two volcanic sequences. Textural analyses were carried out through fieldwork and petrography, including a systematic classification of lavas and volcaniclastic rocks. Rheological parameters were determined using geochemistry data to obtain melt viscosity (η) and temperature, zircon saturation (TZr), liquidus (TL), and glass transition temperatures (TG), for anhydrous and hydrous compositions. Textural analyses indicate the predominance of volcaniclastic facies with abundant eutaxitic and parataxitic textures. Rheological estimations reveal TL of 1020ºC, TZr 650-905ºC, and TG 640-753ºC for anhydrous Iricoumé Group melts. Eruptive viscosity estimations range from 8.4 to 11.7 log η (Pa.s). Igarapé Paboca melts present higher temperatures, with TL of 1050ºC, TZr 710-880ºC, and TG 670-740 ºC. Modeling using hydrous compositions indicate that minute amounts of water can strongly affect the rheology of the studied melts, reducing η, TL, TZr, and TG. The petrographic features indicative of hydrous magma reinforces the role of H2O as a controlling agent in the fragmentation of Iricoumé and Igarapé Paboca melts. The pyroclastic samples are marked by elevated ∆TZr - TG relationships indicative of high emplacement temperatures above the TG. Our results indicate that the high temperatures and the presence of network-modifier cations in the studied melts favored the development of extensive welded ignimbrites associated with low-eruption columns, likely developed in fissural and/or caldera systems.