TRACKING CRETACEOUS CORDILLERAN MAGMATIC ARC PETROGENESIS USING COUPLED DETRITAL ZIRCON GEOCHRONOLOGY, HF ISOTOPES, AND GEOCHEMISTRY FROM MODERN SIERRA NEVADA SAND

The formation of arc magmas represents a first-order tectonic process with implications for how elements are partitioned between mamas rising into the crust and residua trapped in the mantle. Still, competing models for the evolution of arc magmas vary regarding the relative contribution of mantle and crustal components to the magma, and the degree to which primary melts are mixed and/or fractionated. Here, we present U-Pb ages, Hf isotopes and trace element geochemistry from detrital zircon collected from modern sand to characterize the Cretaceous Sierra Nevada batholith.

Sand samples from six streams with limited catchments were selected to characterize portions of the batholith composed of 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) 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 incompatible enrichment relative to zircon from the YVIS and variable eHf (~+5 to -5) in zircon from the earliest TIS; and 3) even further incompatible enrichment and low eHf (~-4) in young zircon from the interior of the TIS.

Increasing incompatible element concentration and generally decreasing eHf in younger and more eastern zircon is consistent with the interpretation that magmas forming the Cretaceous Sierra Nevada batholith migrated into more enriched continental lithosphere from 120–105 Ma. However, higher incompatible element concentrations without a correlative decrease in eHf from 100–95 Ma may indicate an increased component of enriched slab fluids/melts at that time. Finally, the switch from variable, to relatively homogeneous, eHf associated with further incompatible element enrichment from 93–85 Ma suggests a transition from poorly-mixed early magmatism, to a well-mixed, fractionating magmatic column at the height of the Late Cretaceous high-flux event. This study sets a baseline geochemical signal for landward arc migration during a high-flux event, and can be used in conjunction with detrital records to assess petrogenesis in other Cordilleran arcs.