Relationships of Pliocene magmatism in the northern Sierra Nevada Range to lithospheric delamination and the ancestral Cascade Range

Delamination has been invoked to explain Pliocene (3.5 Ma) magmatic activity in the central and southern Sierra Nevada Range (SNR). However, the extent and timing of proposed delamination is unknown. By examining volcanic rocks of a similar age in the northern SNR the extent of delamination can be further characterized. Alternatively, Pliocene magmatism in the northern SNR could be a result of subduction, making the area part of an ancestral Cascade Range. The Mendocino Triple Junction was located just south of Lake Tahoe at the time of eruption of Pliocene magmatism in the northern part of the range. Although the geochemical and isotopic compositions of the samples from the northern SNR are closely related to Cascade magmatism the differences include: higher K2O and lower MgO and CaO at a given value of SiO 2, a restricted range of SiO2, and two distinct magmatic events at 4 and 11 Ma.