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 the chemical make-up of the crust as elements are partitioned between rising magmas and residua that are trapped near the Moho and/or in the mantle. Still, the details of magma generation and evolution remain enigmatic as evidenced by multiple competing models proposed to explain the spatial and temporal variations in magmatic flux commonly observed in arcs. Here, we explore the utility of detrital zircon geochronology and geochemistry as a tool to investigate arc processes using zircon derived from the Cretaceous Sierra Nevada batholith.

To characterize the range of 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 four 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 light REE (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 light REE and widely variable eHf (~+5 to -5) in zircon from the earliest TIS relative to zircon from the YVIS; and 3) even further enrichment of light REE and low eHf (~-3) in young zircon from the interior of the TIS.

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