Petrology of volcanic rocks of the Serra Sul mine, Carajás Province, Pará

Abstract

The Parauapebas Formation and Igarapé Cigarra Formation constitute two Neoarchean extrusive units inserted in the metavolcanosedimentary sequence (~2.76 Ga) of the Grão Pará Group, Itacaiúnas Supergroup, being represented by basaltic seeps underlain and overlain by jaspillites, rhyolites, volcanoclastic rocks, and gabbro dikes/sills, thus being inserted in the Serra Sul district in the Mineral Province of Carajás, Pará. This study aimed to understand the igneous processes involved in the genesis of these basalts from both geological formations, supported by field, petrographic, and geochemical data. Inserted in the metavolcanosedimentary sequence of the Grão Pará Group (Carajás Domain), the basalts and basalt andesites, the most abundant types of the Parauapebas and Igarapé Cigarra Formations, occur as successions of massive and amygdaloidal lava flows. The study of core samples from nine stratigraphic holes that intersected basaltic rocks of the two geological formations in body S11D (Serra Sul) showed that these rocks reach thick layers with different characteristics, ranging from aphanitic to phaneritic. The basalts are dark green, aphanitic, phaneritic, thin to medium, and hypocrystalline. They show preserved igneous texture being commonly seriated as inequigranular, intersertal, ophitic, subophytic, intergranular, branching, and swallow tail. Its essential primary constituents are plagioclase, orthopyroxene, and clinopyroxene, and its accessory constituents are titanite and apatite. Chlorite, epidote, quartz, and calcite occur as secondary phases and are interpreted as the products of ocean-bottom hydrothermal alteration and/or incipient metamorphism. In general, the studied volcanic rocks stand out for SiO₂ content between 47.00 and 56.04%, high alkali contents (4.70–7.50%), K₂O contents between 0.40 and 3.97%, TiO₂ (< 1.0%), and MgO between 4.90 and 8.16%. In the classification diagrams, the samples are situated in the fields of basalts, basalt andesites, trachyandesite, and trachybasalt basalts, in the transition of the toleitic and calc-alkaline series. The basalts of the Parauapebas and Igarapé Cigarra Formations were probably formed in an intraplate continental environment influenced by subduction zones. Therefore, the Grão-Pará Basin was probably formed in a convergent regime related to a subduction-type environment. The geochemical data indicate that the studied rocks of the Parauapebas Formation and the Igarapé Cigarra Formation correspond to basalts of calc-alkaline affinity. Two facies were identified in the Parauapebas Formation, namely, the Amendoim and Violão facies. In the Igarapé Cigarra Formation as well, the following facies were identified: Jacaré and Três Marias. Based on the geochemical interpretation, it is assumed that the rocks studied are arc-like continental basalts, which are formed in a subduction environment with influence from a possible asthenospheric source and modified by crustal contamination.

KEYWORDS:
Serra Sul; Carajás; Brazil; high-pressure melting; petrology; geochemistry