Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 25-12
Presentation Time: 9:00 AM-6:00 PM


SZYMANSKI, Mark E. and BROWNE, Brandon, Department of Geology, Humboldt State University, 1 Harpst Street, Arcata, CA 95521

Quaternary volcanism within the Sierra Nevada block, like in the Golden Trout volcanic field (GTVF), is characterized by an eruption rate and eruption volume that are an order of magnitude less than at neighboring and contemporaneous Basin and Range volcanic fields, such as the Big Pine volcanic field (BPVF) and Coso volcanic field (CVF). Another unusual characteristic of the GTVF compared to Basin and Range volcanic fields is that GTVF volcanoes are not aligned along linear chains nor are they displaced by faulting. Examining atypical examples of mafic volcanic fields like the GTVF provides insight into an integrated suite of processes related to magma petrogenesis, ascent, and storage, which are fundamental controls on where mafic volcanoes form and how they evolve. This study presents measured whole rock 87Sr/86Sr and 143Nd/144Nd ratios of mafic lavas from the four Quaternary vents in the GTVF via HR MC-ICP-MS. 87Sr/86Sr ratios range from 0.705420 to 0.706026 and 143Nd/144Nd ratios range from 0.512492 to 0.512604 (ƐNd: -0.65381 to -2.85682). These values plot within the mantle array and overlap with published values of mafic lavas erupted contemporaneously from the BPVF and the most differentiated CVF mafic lavas. In contrast, Sr and Nd isotope ratios of GTVF mafic lavas differ from Pliocene volcanic fields located nearby within the Sierra Nevada block. These results affirm previous work indicating that Quaternary mafic volcanism in the western Basin and Range and in the southeastern Sierra Nevada share an origin of extensional stresses promoting magma generation through decompression melting of the lithospheric and asthenospheric mantle. An additional insight from this study, however, is that extensional stresses characteristic to the Basin and Range likely extend into the southeastern Sierra Nevada block as well. Moreover, similar mantle source compositions between the GTVF and BPVF combined with previously published compositional data from whole-rock as well as stable oxygen and major element analyses of olivine in both volcanic fields suggests that intact crust (e.g., sparsely or unfaulted, at least in the shallow crust and surface) in regionally extensional tectonic settings does not appear to inhibit magma genesis in the mantle but does likely hinder magma ascent while enhancing crustal contamination.