Distribuição e origem dos minerais detríticos pesados das areias praiais holocênicas do litoral norte do Rio Grande do Sul

Abstract

This study discusses the primary sources of detrital minerals of the Holocenic beach sediments from southern Brazil, based mainly on their chemical composition. Five samples of beach sand were collected at each 40km along the northern littoral of southernmost Brazil. Trace elements were determined by LA-ICP-MS and major elements by microprobe and electron microscopy. Pyroxene compositions indicate the contribution of granulites and post-collisional basic rocks. High-grade metapelitic rocks and amphibolites are important sources suggested by staroulite, rutile, aluminium silicates, almandine, grossular, zoisite, Ca-rich tourmaline, and commingtonite, identified in all samples. A-type granitoids in the source area are indicated by zircons with high Nb and ETR contents, Nb/Ta ratios > 20, and amphiboles with Fe/(Fe+Mg) > 0.9. Peraluminous granitoids are indicated by Mn-rich garnet and Fe-rich tourmaline, whilst the occcurrence of shoshonitic and medium to high-K subalkaline granitoids is pointed out by magmatic epidote, titanite, calcic amphiboles with moderate Fe/(Fe+Mg) ratios and zircons with Th/U = 0.3–0.5 and Nb/Ta = 5 - 15. Magnesian amphiboles indicate ultramafic sources. Hydrothermal products are also indicated in the primary sources by Li-rich tourmaline, actinolite, and hydrothermal epidotes. The homogeneity of abundance and typology of detrital minerals in the northern littoral indicate a common source for the studied sediments. The northeastern part of the Pelotas Batholith, is the probable source region. This study confirms that the chemistry of detrital minerals, including trace elements in zircons, can yield detailed and accurate information about their primary source, and so be of high relevance for provenance studies.