TUFFS OF THE GODDARD PENDANT: SNAPSHOTS OF VOLCANIC SOURCE DURING LOW-FLUX MAGMATISM IN THE SIERRA NEVADA ARC

The Goddard Pendant, in the central Sierra Nevada, contains Jurassic and Early-Cretaceous tuffs that erupted during magmatic “lulls” in the Sierra Nevada arc. Bulk U-Pb zircon dating of tuffs in the pendant showed sections of ca. 160, 143, and 130-136 Ma (Tobisch et al., 1986), but no further studies have examined the geochemical make-up of these tuffs in order to assess their sources. This study uses major and trace element geochemistry, U-Pb zircon dating, and Sr, Nd, and O isotopes to elucidate volcanic sources and to test existing models of Sierra Nevada arc progression and magma flux cyclicity (e.g., Decelles et al. 2013). In addition, newly recognized interbedded tuffs in the thick sedimentary sequence in the pendant are being dated to clarify timing of sedimentation relative to volcanism, and to screen for possible repetition of the section by faulting. Results thus far confirm that the tuffs in the section are primarily dacite to rhyolite and calc-alkaline but are overprinted by greenschist to upper amphibolite facies metamorphism. Three samples from the mid-Jurassic section of the pendant have ⁸⁷Sr/⁸⁶Sr₍₁₆₀₎ of 0.705167–0.705968, and εNd₍₁₆₀₎ of –1.848 to –1.450. The Early Cretaceous units (n = 3) have ⁸⁷Sr/⁸⁶Sr₍₁₃₅₎ of 0.705218—0.706213, and εNd₍₁₃₅₎ of –3.575 to –0.472. As a whole, the Sr-Nd values fall along the general trend of the Sierra Nevada Batholith, but are more crustal than is typical, and are positioned east of the Sr_i = 0.706 isopleth. Nd isotopes in particular indicate that the Jurassic section is more primitive than similarly aged plutonic rocks, while the Cretaceous section has a greater crustal input, which is common for similarly aged rocks in the arc. Thus, the Goddard pendant volcanics capture two instances of felsic volcanism that show increasing crustal input with time during magmatic lulls.