CONSTRUCTION OF DETRITAL MINERAL POPULATIONS: INSIGHTS FROM MIXING OF U-PB ZIRCON AGES IN HIMALAYAN RIVERS

A common assumption in detrital mineral studies is that detrital mineral populations provide an accurate representation of their entire source region. Implicit in this assumption is the idea that all lithologic units have equal potential to contribute heavy minerals in proportion to their exposure area in the source region. However, some detrital mineral populations may be biased by variable concentrations of minerals in bedrock and differential erosion rates within the source region. We attempt to evaluate the relative importance of these two variables by using mixing of U-Pb zircon ages to trace zircon populations from source units, through the fluvial system, and into the foreland. Two statistical techniques are used to determine the proportion in which two or more known age distributions combine to create a known mixed age distribution. Likewise, the concentration of zircon in bedrock and river sediment is determined by both point counting and XRF analysis, and is included in mixing calculations. In the Marsyandi drainage in central Nepal, small tributaries are used to define the U-Pb age distributions of individual lithologic units and trunk river samples are used to evaluate the relative contributions from each unit. Observed mixing proportions are compared with proportions predicted by a simple model incorporating lithologic exposure area and zircon concentration. Next, U-Pb age distributions from four adjacent drainages spanning ~250 km along the Himalayan front are compared, and estimates made of the proportion of zircon derived from each upstream lithology. Results show that a granitic lithology with low zircon concentration is underrepresented in the detrital population. This, combined with the fact that zircon concentrations vary wildly in metasedimentary bedrock, supports the idea that heavy minerals can be locally concentrated in sedimentary rocks, thus dominating the detrital population when they are eroded into the modern fluvial system. Despite this observation, U-Pb age distributions along this part of the Himalayan front are remarkably similar, suggesting that populations are consistent and well mixed on large spatial scales.