Provenance of Upper Pennsylvanian siliciclastic‐carbonate deposits from the Monte Alegre and Itaituba formations, North Brazil: An integrated study of sandstone petrography, heavy mineral analysis and garnet geochemistry

Abstract

The Monte Alegre and Itaituba formations record the transition from a fluvial‐aeolian to a carbonate platform system during the Pennsylvanian on the southern margin of the Amazonas Basin, North Brazil. Sandstones of these formations were analysed for their petrofacies, provenance, tectonic configuration and diagenetic history to determine their origin. The Monte Alegre sandstones are fine‐ to medium‐grained, well‐sorted, bimodal quartz‐arenites with a normal grain‐supported framework of straight and concavo‐convex contacts. They present 97% of ZTR (zircon‐tourmaline‐rutile) heavy minerals. The Itaituba sandstones are fine‐ to very fine‐grained, well‐sorted, sub‐arkoses with an open to normal framework supported by carbonate cement. The heavy minerals are represented by zircon, tourmaline, rutile, kyanite and garnet, with almandine and pyrope as the main types derived from high‐grade amphibolite‐ to‐ granulite‐facies metasediments, in addition to intermediate‐acidic igneous rocks. The petrofacies analysis revealed that the sandstones of these formations belong to the tectonic regime of the continental block, Interior Craton. The Amazonas Basin basement on the southern edge consists of intrusive suites and cratonic rocks of the Tapajós and Iriri‐Xingu provinces, which are represented by the Cuiú‐Cuiú Complex and the Iriri Group. These groups of rocks were the main sources of sandstones of these formations. This conclusion indicates that these rocks were uplifted, exposed and eroded during and/or after the Middle Carboniferous. The imprints of early and deep burial diagenesis of these sandstones, including different stages of compaction, cementation and heavy minerals elimination, produced differences in sandstones porosity and heavy minerals assembly.