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

Y 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 Cuxiu 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 Cuxiu 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 mu m 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 (similar to 120 mu m), width (similar to 66 mu m), aspect ratio (1.9) and roundness (similar to 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 degrees C (average 755 degrees C). Uranium-Th contents indicate low alpha-decay doses of << 1 x 10(-15) decay events/mg for all zircon grains by the time of deposition, providing an additional explanation for the excellent zircon preservation.

Automated summary

Provenance studies using in situ U-Pb dating and Hf isotope analyses typically overlook shape parameters of detrital zircon grains despite their potential value in understanding hinterland and detritus transport. This study in the Aquiri region of the eastern Amazon Craton presents a comprehensive data set combining zircon U-Th-Pb-Hf isotope analyses with zircon shape and physical parameters, revealing specific environmental conditions and source information during Neoarchaean deposition of the Cuxiu Formation. Well-preserved euhedral shapes of detrital zircon grains, combined with correlation of zircon roundness with width and consistent values of size and typology parameters across different ages, suggest fast, direct supply from local basement sources and thorough hydrodynamic sorting during transport.