USING ZIRCON TRACE ELEMENT SIGNATURES TO EVALUATE THE NATURE OF MIDDLE TO LATE JURASSIC MAGMATISM WITHIN THE SIERRA NEVADA ARC

Middle-Late Jurassic magmatism in the Sierra Nevada arc was potentially complicated by forearc magmatism related to Coast Range ophiolite (CRO) formation. We use zircon trace element (ZTE) data to distinguish between the eroded products of coeval continental margin arc and CRO magmatism by comparing Jurassic detrital zircon deposited in two basins, both at ~36°N lat.: (1) ~100 Ma forearc turbidites from the basal Great Valley Group (GVG, deposited ~145-100 Ma); and (2) ~140-135 Ma fluvial sandstones from the Goldstein Peak intra-arc basin. We then compare ZTE characterizing the arc during CRO magmatism to both older and younger arc zircon to assess whether arc magmagenesis was affected by CRO magmatism.

Mesozoic grains were selected for ZTE analysis using the Stanford-USGS SHRIMP on the basis of U-Pb dates measured by LA-ICPMS at Arizona LaserChron Center. Ratios of Nb-Ce-Gd-Yb-Hf-Th-U are compared on 9 discrimination plots (Barth, Wooden and Tani, unpub.). 43 of 47 GVG forearc zircon grains plot as “oceanic” on at least one plot, but only 19 grains are classified as “oceanic” on 5 or more of the ZTE discrimination plots. These 19 “robust” oceanic grains have LA-ICPMS U-Pb crystallization ages spanning 176 ± 21 to 152 ± 15 Ma, roughly matching CRO formation (~170-155 Ma). Excluding 5 grains having errors >8.5 m.y. yields a weighted average date of 164 ± 2 Ma for this “oceanic” zircon source, corresponding closely with the CRO Stoneyford seamount in the eastern Klamath Mountains (164.8 Ma; Shervais et al., 2005).

None of the 92 Mesozoic detrital zircon analyzed from the Goldstein Peak intra-arc basin display “robust” oceanic ZTE. Though some grains have elevated U contents (>750 ppm), arc zircon crystallized during the Middle-Late Jurassic CRO event display typical arc REE, indistinguishable from both Early Jurassic and Early Cretaceous zircon. Comparably U-rich, 153-144 Ma arc zircon, however, display unusually high [Gd], [Yb] and Gd/Yb ratios at low Hf contents relative to both older and younger arc zircon. These unusual latest Jurassic-to-earliest Cretaceous arc zircon indicate either a short-lived change in the composition of the mafic parental magma, or an earlier onset of zircon crystallization relative to hornblende, immediately following CRO magmatism and preceding the start of the voluminous Cretaceous arc surge.