Grain size distribution and age population of detrital zircons from the Changjiang (Yangtze) River system, China

Abstract

This study investigated the correlation of the size of detrital zircons with their age populations in a fluvial environment. From 31 floodplain sediments that were isolated from the mainstream and ten major tributaries of the Changjiang River, 3526 detrital zircon grains were selected for size examination, and 1358 were chosen for U–Pb age measurements. The Equivalent Spherical Diameter (ESD) of most zircon grains falls within the range of 32–250μm, and the dominant size is 63–125μm. Small-sized zircons with ESDs of <63μm are found in higher proportions in the mid-lower mainstream than in the upper reaches. An overall decrease in the average ESD size of detrital zircons towards the mid-lower reaches reflects the hydraulic sorting effect of different river flow regimes along this river. Overall, the age populations of zircons between the different tributaries and mainstream are similar, and several age groups correspond well with major tectonic–magmatic events that happened in the river basin. This study confirms the notion that younger zircons have coarser and more variable sizes than the older zircons, indicating the potential influence of hydrodynamic fractionation on zircon size and ages. The problem of grain-size bias on age-data acquisition and interpretation of detrital zircons from large rivers is not negligible and deserves more attention when using zircon geochronology to decipher sediment provenance and tectonic evolution. Although the zircons with an ESD of 63–125μm dominate the Changjiang sediment and have similar age populations as the bulk zircons, whether this narrow-sized window can accurately represent the bulk zircon geochronology of the weathered and heterogeneous rocks in the large catchment awaits additional investigations.