Dynamic magma systems, crustal recycling, and alteration in the Central Sierra Nevada batholith: The oxygen isotope record

Abstract

Values of δ18O of zircon from the central Sierra Nevada batholith (SNB), California, yield fresh insight into the magmatic evolution and alteration history of this classic convergent margin batholith. Direct comparison of whole-rock and zircon (Zrc) δ18O provides evidence for modest (0.5‰), but widespread, alteration, which has complicated interpretation in previous whole-rock δ18O studies. Four discrete belts of δ18O values are recognized in the central Sierra. A small belt of plutons with relatively low δ18O(Zrc) values (5.2-6.0‰) intrudes the foothills, with a sharp increase of δ18O revealing the concealed Foothills Suture; high δ18O(Zrc) values (7.0-8.5‰) dominate the rest of the western SNB. East of the axis of the Sierra, δ18O is distinctly lower (6.75-5.75‰), and decreases monotonically to the Sierra Crest. A sharp 1‰ increase of δ18O in the eastern Sierra reveals a second crustal boundary, with the fourth belt hosted in high-δ18O North American crust in the White Mountains and Owens and Long Valleys. Correlated O, Sr, and Pb isotope ratios reveal differences in magma generation between the western and eastern Sierra. The western Sierra experienced massive crustal recycling, with substantial melting and mobilization of accreted oceanic and volcanic arc rocks; crustal contamination affects many western SNB plutons. In contrast, the eastern Sierra was dominated by voluminous recycling of the lithospheric mantle and lower crust, with minimal crustal contamination. Batholith-wide shifts in δ18O occur between pulses of Cretaceous magmatism that may be linked to tectonic reorganizations of magma sources. Within intrusive suites, δ18O may be unchanged (Tuolumne); increase (Sonora and Whitney); or decrease (Sequoia and John Muir) with time. These trends show stable long-lived sources, or those where recycling and contamination may increase or decrease with time. Overall, δ18O reveals diverse magma system behavior at a range of scales in the Sierran arc. © The Author 2008. Published by Oxford University Press. All rights reserved.