INSIGHTS INTO MIDDLE TO LATE JURASSIC MAGMATISM WITHIN THE SIERRA NEVADA ARC AS RECORDED BY TRACE ELEMENT COMPOSITIONS OF DETRITAL ZIRCON

We explore the hypothesis that trace element (TE) contents of detrital zircon from arc-adjacent basins provide a record of arc magmatism that is different from, but complementary to the exposed Sierra Nevada arc. We evaluate the TE geochemistry of 312 Mesozoic (256-97 Ma) detrital zircon grains separated from sandstones deposited ca. 145-80 Ma along the length of the Great Valley forearc basin and ca. 140 Ma within the Goldstein Peak intra-arc basin. Discrimination analysis demonstrates that the vast majority of detrital zircon derive from the Sierra Nevada arc. Variation in detrital zircon geochemistry through time reveals spikes in MREE and HREE between ca. 165 and 150 Ma, coincident with an apparent peak in zircon productivity recorded by the forearc-intra-arc sediments. These regionally-distributed detrital zircon have anomalously high Gd (>65 ppm) and Yb (>775 ppm), and span the entire observed range of Sc (2-710 ppm). Elevated Yb concentrations generate low Sc/Yb values for many of these grains, such that discrimination analysis attributes them to ocean island magmatism. Covariation of Yb and Yb/Sc subdivides the high-M-HREE zircon into two distinct sets: (1) ca. 164 Ma zircon, which we correlate to the Coast Range Ophiolite; and (2) ca. 152 Ma zircon that extend deep into the “ocean island” field. Trace-element modeling by Barnes et al. (2016) for the Klamath arc suggests that recrystallization within upwelling hornblende-bearing magmas can generate magmas with “gull-wing” REE patterns. Based on additional modeling, we propose that the ca. 152 Ma peak defined by Sierran forearc-intra-arc detrital zircon records a large-volume and/or zircon-fertile magmatic forerunner to the regionally extensive, but low-volume and zircon-poor ca. 150 Ma Independence dike swarm. In our model, high M-HREE zircon crystallized from upwelling magmas in which amphibole-magma reactions produced unusual and presumably transient REE signatures. This unexpected result demonstrates the utility of coupling exposed arc and detrital records to obtain a more complete record of arc magmatism through time. Moreover, our results provide insight into petrologic mechanisms underlying empirically-defined TE discrimination diagrams and suggest caution regarding interpretations of Gd-Yb-Sc with respect to mantle plume origins.