HR: 1340h
AN: T33A-1146    [Abstracts]
TI: High K volcanism in the Sierra Nevada: A signal for the initiation of Walker Lane Faulting, and range uplift, not lithosphere delamination
AU: * Putirka, K D
EM: kputirka@csufresno.edu
AF: California State University, Fresno, Department of Earth and Environmental Sciences, 2345 E. San Ramon Ave., MS/MH24, Fresno, CA 93720, United States
AU: Busby, C J
EM: busby@geol.ucsb.edu
AF: University of California, Santa Barbara, Dept. of Earth Science, University of California, Santa Barbara, CA 93106, United States
AB: K₂O contents have long been recognized as a potential indicator of tectonic processes, and based upon models developed for the Andes (Kay and Kay, 1993) and Tibet (Turner et al., 1996), high-K volcanism has been related to lithosphere delamination, by partial melting of a K-metasomatized lower crust or upper mantle (Feldstein and Lange, 1999; Manley et al., 2000). However, new data from the central Sierra Nevada cast doubt on this K₂O-delamination link. Instead, high-K volcanism is better explained as low degree partial melts (F), where low F magmas are preferentially erupted over thick crust, under conditions of high tensile stress. In the central Sierra, a high tensile stress regime was imposed at the onset of Walker Lane transtension, at the eastern edge of the Basin and Range province. We surmise that high K volcanism is similarly controlled by the onset of tensile stresses throughout the Sierra, recording the initial phase of Sierra Nevada uplift. These conclusions stem from several observations. First, K₂O contents are highly correlated with Th (R=0.82), Ba (R=0.83), U (R=0.85), Rb (R=0.88) and Pb (R=0.83), and other highly incompatible elements, suggesting a general enrichment mechanism, such as low degree partial melitng. Second, volcanic rocks with the highest K have the highest La/Nb and the lowest ¹⁴³Nd/¹⁴⁴Nd, indicative of a mantle lithosphere source - inconsistent with delamination. Third, maximum K contents increase from north (near Lassen) to south, following in increase in crustal thickness and the (⁸⁷Sr/⁸⁶Sr) _i of basement granitoids, suggestive of a crustal control on volcanism. Finally, field evidence in the central Sierra shows that the pulse of high K₂O volcanism there was synchronous with the development of a pull-apart, along a series of right-stepping dextral transtensional faults, at the onset of Walker Lane faulting. Partial melting calculations verify that primitive magma compositions from Lassen to the southern Sierra, can all be explained by partial melting of a single mantle source, with Cordilleran-type enrichments, but no special K enrichments in any particular region. Moreover, just as low F melts are enriched in K, Pb, or U, they will also be enriched in water, which greatly reduces magma density (Ochs and Lange, 1999). Such differences in water contents provide a mechanism for regional variations in volcanic compositions: depth-integrated density models show that dry mafic magmas have insufficient buoyancy to erupt from beneath thick crust, but low F (water-enriched) melts are sufficiently buoyant to allow eruption. Theoretical models (Takada, 1994) further indicate that tensile stress regimes favor the transport of low F melts. Thus, where the crust is thick, such as in the southern Sierra Nevada, only low F magmas can erupt, due to their natural water enrichments and added buoyancy, and even then are probably only erupted when tensile stresses favor their segregation from their source region. In our alternative interpretation then, high-K volcanism reflects the inception of transtensional stresses, recording the birth of a plate boundary.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 3610 Geochemical modeling (1009, 8410)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8414 Eruption mechanisms and flow emplacement
SC: Tectonophysics [T]
MN: 2007 Fall Meeting