Minerais pesados das porções média e superior do Grupo Guaritas (Eocambriano, RS): considerações sobre a proveniência sedimentar

Abstract

The Guaritas Group comprises sediments from alluvial fan, fluvial and eolian depositional systems. This study aimed with the heavy mineral analysis of sandstone samples from Varzinha, Pedra Pintada, Pedra da Arara and Serra do Apertado formations, units from the middle and upper portions of the Guaritas Group. For the heavy mineral analysis, it was used the very fine sand fraction of 29 samples collected in outcrops. This analysis included the identification and counting of the non-opaque and non-micaceous heavy minerals as well as the determination of the ZTi (zircon/(tourmaline+zircon)) and ZRi (zircon/(rutile+zircon) indexes and morphological types of zircon grains. The studied samples correspond to subarkoses, sublitharenites and litharenites. The identified heavy minerals were: zircon (82-13%), apatite (64-0%), tourmaline (41-3%), anatase (22-0%), garnet (29-0%), rutile (10-0%), sillimanite (5-0%), monazite (4-0%), epidote (2-0%), kyanite (1-0%) and fluorite (1-0%). There were recognized four morphological types of zircon grains: very well/well rounded short prism (47-6%), subrounded/subeuhedral short prism (92-42%), euhedral short prims (13-0%) and euhedral long prims (3-0%). The huge quantity of zircon, apatite and tourmaline grains (common accessory minerals in nearby granitic rocks) and the reduced proportion or absence of typically metamorphic heavy minerals like rutile, sillimanite, kyanite, staurolite, amphiboles and pyroxenes, are compatible with source area dominated by igneous acid rocks (granites). Very well rounded grains of zircon still indicate sediments from ancient sedimentary rocks. Similar values of the ZRi (0.92-0.96) and ZTi (0.71-0.82) indexes suggest, respectively, maintenance of the source area configuration and minor variation of the sedimentary reworking degree during the deposition of the middle and upper portions of the Guaritas Group. The Serra do Apertado Formation slightly differs from the other units by its relative higher ZTi and proportion of euhedral zircon grains. This is indicative of higher degree of sedimentary reworking and increasing contribution of sediments derived from igneous rocks during the final phase of sedimentary fill of the Camaqua basin.