HR: 0800h
AN: V41B-1442    [Abstracts]
TI: Ancestral Cascade Arc volcanism in the North-Central Sierra Nevada, California
AU: * Sharma, R
EM: rohits@csufresno.edu
AF: California State University Fresno, Department of Earth and Dept. of Earth and Environmental Sciences, 2576 E. San Ramon Ave., MS/ST24, Fresno, CA 93710 United States
AU: Putirka, K
EM: kputirka@csufresno.edu
AF: California State University Fresno, Department of Earth and Dept. of Earth and Environmental Sciences, 2576 E. San Ramon Ave., MS/ST24, Fresno, CA 93710 United States
AU: Busby, C
EM: busby@geol.ucsb.edu
AF: University of California, Santa Barbara, Department of Geological Sciences, Web Hall, 2034, Santa Baebara, CA 93106 United States
AB: Volcanic rocks from the northern and central Sierra Nevada show the existence of an ancestral Cascade volcanic arc, which was active between at least 14-6 Ma. Volcanism in this region is of interest because it provides a test of at least two models of Sierra Nevada uplift: 1) Passage of the Mendocino Triple Junction (MTJ) which, if effective, should yield an arc to post-arc geochemical signal. 2) Delamination of sub-continental lithosphere, which, in the southern Sierra, has been linked to potassic volcanism. Preliminary age dates show that the Sierra Nevada Ancestral Cascades Arc (SNACA) was active during passage of the MTJ; in addition, the SNACA contains the type locality of latite, a high K₂O volcanic rock. Our reconnaissance study has yet to determine whether passage of the MTJ is evident, but has important implications regarding alleged geochemical signals of lithosphere delamination, and the origin of Sierra Nevada granitoids. Field relationships and age dates allow us to divide rocks from the Sonora Pass region into three units, in decreasing order of age: Pre-Stanislaus Merhton Formation ("Relief Peak"), Stanislaus Formation, and Post-Stanislaus Merhton Formation ("Disaster Peak"). We also show data from Carson Pass, and the putative Little Walker Caldera. This work shows the lack of a relationship between K₂O and eruptive period: the latites of the Stanislaus Formation are no more K₂O-enriched than at least some flows from older and younger flows in the central Sierra Nevada (Sonora and Carson Pass regions). Interestingly, however, the SNACA volcanics define a SiO₂-K₂O trend that is intermediate between Lassen (low K₂O) to the north, and Pliocene ultrapotassic rocks to the south. Perhaps the temporal pulse in K₂O in the southern Sierra Nevada is yet to be recognized in the north, or delamination is not the cause of uplift in the northern Sierra Nevada. But another possibility is that elevated K₂O is unrelated to delamination and that SiO₂-K₂O systematics reflect other tectonic features, i.e., increased levels of cpx fractionation, which might occur if the thickness of the upper crust increases to the south. In addition, geochemical variations can be explained by fractional crystallization alone, and Sierra Nevada granitoids lie at the evolved end of fractional crystallization trends. This suggests that the SNACA volcanics may provide a viable model for the primitive magmas and differentiation processes that gave rise to the Sierra Nevada granitoids.
DE: 1020 Composition of the continental crust
DE: 1036 Magma chamber processes (3618)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3618 Magma chamber processes (1036)
DE: 3619 Magma genesis and partial melting (1037)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005