Combining detrital zircon shape and U–Pb–Hf isotope analyses for provenance studies – An example from the Aquiri region, Amazon Craton, Brazil

Abstract

Provenance studies are commonly based on in situ U–Pb dating and Hf isotope analyses of detrital zircon grains, while leaving shape parameters unconsidered, even though these can provide valuable information about the hinterland and detritus transport. This is demonstrated herein by presenting a comprehensive data set derived from approximately 950 detrital zircon grains from three quartzite samples of the Cuxiú Formation of the Aquiri region, which is part of the eastern realm of the Amazon Craton. The data set combines results of zircon U–Th–Pb–Hf isotope analyses with zircon shape, and some physical parameters, including length, width, aspect ratio, roundness, typology, alpha-decay dose and density. Zircon U–Pb ages and field relationships indicate that pebbly sandstones of the Cuxiú Formation were deposited during the Neoarchaean between 2705 and 2665 Ma, likely in a fluvial or near-shoreline environment of a rift basin, with zircon grains supplied from igneous rocks of the surrounding Amazon Craton. The detrital zircon grains record crystallisation ages of 3100–2800 Ma (74%), and subordinately of 2800–2700 Ma (13%) and 3600–3100 Ma (3%). Hafnium isotope analyses further suggest crust formation in the hinterland at 4.01–3.65 Ga (14%), 3.60–3.10 Ga (85%) and 2.95 Ga (1%). Well preserved euhedral shapes of more than 50% of the detrital zircon grains, independent of U–Pb ages, suggest fast, direct supply from heterogeneous Archaean basement sources to the sedimentary basin, with a short-distance transport, plausibly in a fluvial setting. Correlation of zircon roundness with width in the size range from 50 to 150 µm indicates more intense abrasion of larger zircon grains compared to smaller ones, reflecting difference in transport mechanisms – i.e., abrasive-rolling transport vs. non-abrasive suspension transport. Similar average values for length (~120 µm), width (~66 µm), aspect ratio (1.9) and roundness (~2.5), obtained from zircon grains of very different ages (3600–2700 Ma), as well as log-normal distribution of the width parameter, suggest thorough hydrodynamic sorting during transport. Zircon typology analysis reveals a wide range of combinations of {100}:{110} prism faces with{101}:{301}:{211} pyramids, pointing to zircon crystallisation from 900 to 600 °C (average 755 °C). Uranium–Th contents indicate low α-decay doses of ≪1 × 10−15 decay events/mg for all zircon grains by the time of deposition, providing an additional explanation for the excellent zircon preservation.