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Geology; January 2004; v. 32; no. 1; p. 73-76; DOI: 10.1130/G20007.1
© 2004 Geological Society of America
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Predominance of high Th/U, magmatic zircon in Brazilian Shield sandstones

Léo Afraneo Hartmann1 and João Orestes S. Santos2

1 Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, 9500, 91501-970 Porto Alegre, Rio Grande do Sul, Brazil
2 Geological Survey of Brazil (CPRM), Rua Banco da Província, 105, Porto Alegre, Rio Grande do Sul, Brazil

The strong predominance of magmatic (Th/U >0.2) zircon grains in quartz sandstones from 10 locations in the Brazilian Shield, as measured by sensitive high-resolution ion microprobe (SHRIMP), introduces a strong bias in the tectonic interpretation of source terrains, because the metamorphic sources remain undetected in the detrital population. Zircon grains from many mature, and some immature, sedimentary terrains show Th/U ratios >0.2, ratios commonly considered magmatic, complemented by the near absence of metamorphic Th/U ratios <0.1. Metamorphic rims on magmatic zircon grains from the Brazilian Shield are nearly absent from quartz sandstones, only being present in shielded young cores of a few crystals. High-U, metamorphic portions of zircon grains are more defective and liable to undergo comminution by abrasion during weathering and transport, and they do not remain in the sand fraction. Ages obtained from detrital zircon in mature sandstones reflect magmatic sources, not the full spectrum of orogenic events. Only less mature sedimentary rocks preserve a full record of orogenic rocks in the source. Mostly magmatic events were characterized in previous zircon provenance studies. Brazilian Shield detrital zircon may be a proxy for equivalent investigations in other provinces of the world.

Key Words: zircon • provenance • Brazilian Shield • Th/U • magmatic




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