Detrital Minerals of Modern Beach Sediments in Southern Brazil: A Provenance Study Based on the Chemistry of Zircon

• Published in: Journal of coastal research 2010-01, Vol.26 (1), p.80-93
• Main Authors: de Barros, Carla Ennes, Nardi, Lauro V. S, Dillenburg, Sergio R, Ayup, Ricardo, Jarvis, Kym, Baitelli, Ricardo
• Format: Article
• Language: English
• Subjects: Beaches; Coastal sediments; Coasts; Geochemistry; Geology; Grains; Granite; Hafnium; heavy minerals; Holmium; Holocene; Lanthanum; Metamorphic rocks; mineral chemistry; Mineralogy; Minerals; Multivariate analysis; Niobium; Petrology; Provenance; Rare earth elements; Research Papers; Rocks; s; Sand; Scanning electron microscopy; sedimentary sources; Sediments; Streams; Studies; Tantalum; Thorium; Trace elements; Uranium; Yttrium; Zircon; Zirconium
• ISSN: 0749-0208; 1551-5036
• DOI: 10.2112/06-0817.1

The trace element content of zircon grains from Holocene beach sands from the Rio Grande do Sul state in southern Brazil was investigated in order to discuss their probable primary source. Zircon grains from 10 samples were separated and analysed by laser ablation inductively coupled plasma mass spectrometer (LA ICP-MS) for rare earth elements (REE), niobium (Nb), uranium (U), thorium (Th), yttrium (Y), hafnium (Hf), and tantalum (Ta) and by microprobe and scanning electron microscopy (SEM) for silicon (Si), zirconium (Zr), and phosphorus (P). The results obtained are similar to those suggested by the zircon classification developed by Belousova et al. (2002, Igneous zircon: trace element composition as an indicator on source rock type. Contributions to Mineralogy and Petrology, 143, 602–622), nevertheless, additional information and a more detailed characterisation of zircon sources were yielded. The probable geotectonic setting and geochemical affinity of zircon igneous sources were predicted mainly based on thorium/uranium (Th/U), yttrium/holmium (Y/Ho), and niobium/tantalum (Nb/Ta) ratios; lanthanum (La) and niobium (Nb) contents; and REE patterns of zircons. The statistical groups identified by multivariate analysis were an additional useful tool for identifying compositional groups of zircons. Zircon grains from the studied sediments are probably derived from an association of subalkaline to alkaline granitic rocks with their mafic counterparts and metamorphic rocks of amphibolite to granulite facies. The metamorphic sources were more important in the southern sector of the studied coastal segment, whilst in the northern sector zircons from A-type granites are more abundant. The so-called Pelotas Batholith, situated in the eastern part of the Sul-rio-grandense Shield, which is composed mainly of a postcollisional granitic association of Neoproterozoic age and Paleo to Neoproterozoic high-grade metamorphic sequences, is proposed as the probable source of the studied zircons. The compositional variations of zircons from the southern to the northern sector are coherent with the hypothesis that the studied sediments came from relatively near continental source areas, probably transported by several small streams and rivers during phases of sea-level low stands. The chemical composition of zircons, particularly trace element contents and ratios, can be a powerful tool for determining the primary sources of detrital zircons.