IS PLIOCENE MAGMATISM IN THE NORTHERN SIERRA NEVADA RANGE LINKED TO LITHOSPHERIC DELAMINATION OR TO THE ANCESTRAL CASCADE RANGE?

Delamination has been invoked to explain high-potassium magmatism in the central and southern Sierra Nevada Range around 3.5 Ma (Pliocene), but magmatism of this age also occurs around and north of the Lake Tahoe area of the northern Sierra Nevada. Plate reconstructions indicate that the Mendocino Triple Junction was located well north of the southern and central Sierra during Pliocene volcanism there, but south of the Tahoe-Reno area. Therefore, Pliocene volcanism in the southern and central Sierra cannot be part of the ancestral Cascade arc, but Pliocene volcanism near and north of Lake Tahoe may be. We have undertaken a geochemical and geochronological study of volcanic rocks between Lake Tahoe and Mt. Lassen in order to determine whether they exhibit characteristics of the ancestral Cascade arc or of delamination. This study extends a similar study by Cousens (2008) in the Tahoe-Truckee area. Ar-Ar analyses are in progress.

Samples from the Portola and Tahoe-Truckee areas are largely basalts and andesites. Although they are not nearly as potassic as Pliocene rocks farther south, they exhibit chemical variations that are distinct from those of the Mt. Lassen volcanic field of the Cascade Range, which lies immediately to the north. Principal major-element differences are: (1) higher K₂O and lower MgO and CaO at a given value of SiO₂, and (2) a restricted range of SiO₂, with most samples <62 wt%. In contrast, the Lassen field ranges up to ~75 wt%. Passing north from Tahoe toward Mt. Lassen, the chemical differences between these rocks and the Lassen field lessen (K₂O decreases whereas MgO and CaO increase), although restricted silica variability remains.

Our analysis indicates that late Cenozoic volcanism near and north of Lake Tahoe is chemically distinct from Lassen volcanism, but the trends converge passing northward. Chemical differences may reflect changes in lithospheric composition (reflecting the general northward transition from N. American to accreted lithosphere), and the greater silica variability at Mt. Lassen may occur owing to the large central volcanic center there. Volcanism near and north of Lake Tahoe seems to have occurred in a discrete Pliocene pulse, similar to delamination magmatism to the south, but exhibits geochemical characteristics that are similar to Cascade volcanism.