DETRITAL ZIRCON AS A TRACER FOR TIME-TRANSGRESSIVE, REGIONAL-SCALE ARC PROCESSES: A CASE STUDY FROM MODERN SIERRA NEVADA SANDS

The formation of magma above subduction zones represents a first-order tectonic process with implications for how elements are partitioned between rising magmas and residua that are trapped near the Moho and/or in the mantle. Still, the details of magma evolution remain enigmatic as evidenced by competing models proposed to explain the spatial and temporal variations in magmatic flux commonly observed in arcs. Here, we explore the utility of coupled detrital zircon geochronology and geochemistry as a tool to investigate arc processes using zircon derived from the Cretaceous Sierra Nevada batholith.

To characterize zircon compositions across the Cretaceous Sierra Nevada batholith, we measured U–Pb age, Hf isotopes and trace element geochemistry on detrital zircons from modern sands in six streams with catchments dominated by granitic rocks of the western Fine Gold Intrusive Suite (FGIS), the axial Yosemite Valley Intrusive Suite (YVIS), and the eastern Tuolumne Intrusive Suite (TIS), respectively. Our results indicate: 1) progressive enrichment of incompatible elements relative to compatible elements (e.g., higher U/Yb) correlated with decreasing eHf (~+7 to -7) from older western portions of the FGIS into younger and/or more eastern portions of the FGIS and the YVIS; 2) further enrichment in incompatible elements and variable eHf (~+5 to -5) in zircon from the earliest TIS relative to zircon from the YVIS; and 3) even further enrichment of incompatible elements and low eHf (~-4) in young zircon from the interior of the TIS.

At the range scale, increasing incompatible element concentration and generally decreasing eHf in progressively younger and more eastern zircon is consistent with the interpretation that magmas forming the Cretaceous Sierra Nevada batholith migrated into progressively more enriched continental lithosphere from 120 to 85 Ma. These zircon analyses set a baseline geochemical signal characteristic of landward arc migration, and can be used in conjunction with detrital records to assess migratory patterns of other Cordilleran arcs. At the scale of individual intrusive suites, the observed variation in trace elements and eHf record the evolution of magmas with time, and suggest that for any given mantle source, rising magmas became increasingly contaminated by crustal materials.