Abstract

Detrital-zircon U-Pb geochronology is commonly used to constrain the depositional age and provenance of ancient sedimentary systems. The efficacy of using the maximum depositional age of a stratigraphic unit to tightly constrain a deposit’s true depositional age is contingent on the sample having contemporaneous zircon grains, such as those supplied from an active volcanic arc. However, studies that use a deposit’s maximum depositional age to constrain its true depositional age rarely consider the specific nature of arc-derived sediment in terms of plutonic versus volcanic origins. Due to crystallization dynamics, among other factors, we hypothesize that plutonic and volcanic zircon characteristics may fundamentally differ, affecting the relative influence of volcanic (undissected) versus plutonic (dissected) arc provenance. To test this hypothesis, we used high-resolution X-ray computed tomography to characterize detrital-zircon size, shape, and abundance from forearc strata of the Cretaceous Great Valley Group (GVG), which temporally tracks progressive unroofing of the Sierra Nevada magmatic arc, and modern river sediments in central California that represent spatial variations in arc unroofing. Results of relative mass fraction and X-ray computed tomography analysis show that the total yield of zircon grains increases substantially in samples derived from a dissected magmatic arc, with an order-of-magnitude greater volumetric zircon abundance in comparison to those derived from undissected-transitional-arc sources. In contrast, detrital-zircon grain size and shape are only weakly correlated with arc-dissection provenance trends. Zircon grain-size distributions in the Cretaceous do not show a relation with arc dissection, whereas modern sediments show subtly increasing zircon grain size with increasing plutonic contribution and decreasing volcanic contribution. Zircon grains from the GVG show a subtle increase in the proportion of elongate, acicular grains during arc unroofing, and modern river samples exhibit zircon grain-shape distributions that vary non-systematically with respect to arc source. The zircon abundance trends found in this study suggest that when both plutonic and volcanic sources are available, plutonic zircon may overwhelm the comparatively rare volcanic zircon that provides depositional ages. These findings have implications for detrital-zircon studies that aim to: 1) estimate relative sediment supply between arc sources, 2) derive the relative intensity of arc activity (i.e., magmatic “flare-ups” and “lulls”), and 3) identify sufficient contemporaneous zircon to constrain a sample’s depositional age.