CHARACTERIZING DETRITAL ZIRCON POPULATIONS THROUGH THE LENS OF ARC UNROOFING, CALIFORNIA, USA

Detrital zircon applications are commonly used to constrain a deposit’s age, especially in sedimentary basins with an active magmatic arc detrital source. The efficacy of using a deposit’s maximum depositional age (MDA) to tightly constrain its true depositional age (TDA) is contingent on the deposit having contemporaneous (e.g., volcanic) zircon. However, applications using an MDA to constrain its TDA rarely consider the specific nature of arc-derived sediment (i.e., plutonic versus volcanic). We hypothesize that zircon grain characteristics such as size, shape, and abundance are linked with the deposit’s relative influence of undissected (volcanic) versus dissected (plutonic) arc provenance. To test this hypothesis, we evaluated detrital zircon populations from the Cretaceous Great Valley Group stratigraphy, which temporally reflect progressive arc-unroofing, as well as modern sediments in Central California that spatially reflect arc-unroofing.

Detrital zircon populations were characterized by their grain size, shape, and abundance using X-ray micro-computed tomography (microCT), bulk geochemistry, and relative weight fractions. Relative weight fraction and microCT results show that the total volume of zircon grains per sample increases substantially in dissected source samples. MicroCT results from the GVG samples show more than a 3,000 percent increase in volumetric abundance of zircon per sample from undissected to dissected sources. Within modern sediments, the samples yield a greater than 7,500 percent increase. With respect to volumetric grain size distributions, results vary between the modern and ancient case studies. The Cretaceous deep-water GVG samples show a greater relative abundance of smaller-volume grains in dissected arc sediments; whereas the modern fluvial samples show greater relative abundance of small zircon in the undissected arc derived sands. These results suggest that studies seeking to approximate relative source contribution and the depositional age of arc-derived sediment using an MDA, should account for relative fertility in contemporaneous volcanic zircon versus plutonic zircon.