Predominance of high Th/U, magmatic zircon in Brazilian Shield sandstones

Abstract

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.