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

Values of delta O-18 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) delta O-18 provides evidence for modest (0.5), but widespread, alteration, which has complicated interpretation in previous whole-rock delta O-18 studies. Four discrete belts of delta O-18 values are recognized in the central Sierra. A small belt of plutons with relatively low delta O-18(Zrc) values (5.2-6.0 parts per thousand) intrudes the foothills, with a sharp increase of O-18 revealing the concealed Foothills Suture; high delta O-18(Zrc) values (7.0-8.5 parts per thousand) dominate the rest of the western SNB. East of the axis of the Sierra, delta O-18 is distinctly lower (6.75-5.75 parts per thousand), and decreases monotonically to the Sierra Crest. A sharp 1 parts per thousand increase of delta O-18 in the eastern Sierra reveals a second crustal boundary, with the fourth belt hosted in high-delta O-18 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 delta O-18 occur between pulses of Cretaceous magmatism that may be linked to tectonic reorganizations of magma sources. Within intrusive suites, delta O-18 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, delta O-18 reveals diverse magma system behavior at a range of scales in the Sierran arc.