Alteration and remelting of nascent oceanic crust during continental rupture: Evidence from zircon geochemistry of rhyolites and xenoliths from the Salton Trough, California

Rhyolite lavas and xenoliths from the Salton Sea geothermal field (Southern California) provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust. Salton Buttes rhyolite lavas contain xenoliths that include granophyres, fine-grained altered rhyolites ("felsite"), and amphibole-bearing basalts. Zircon is present in lavas and xenoliths, surprisingly even in the basaltic xenoliths, where it occurs in plagioclase-rich regions interpreted as pockets of crystallized partial melt. Zircons in the xenoliths are exclusively Late Pleistocene-Holocene in age and lack evidence for inheritance. U-Th isochron ages are: 20.5 _{- 1.2} ^{+ 1.2} ka (granophyres), 18.3 _{- 3.5} ^{+ 3.6} ka (felsite), 30.1 _{- 12.4} ^{+ 14.1} ka and 9.2 _{- 6.6} ^{+ 7.0} ka (basalts; all errors 1 σ). The dominant zircon population in the rhyolite lavas yielded U-Th ages between ∼ 18 and 10 ka, with few pre-Quaternary xenocrysts present. δ¹⁸O _zircon values are lower than typical crustal basement values, thus ruling out rhyolite genesis by melting of continental crust. Moreover, δ¹⁸O _zircon values are ∼ 0.5-1.0‰ lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid-ocean ridge basalt, suggesting that basaltic crust and silicic plutons in the subsurface of the Salton Sea geothermal field isotopically exchanged with meteoric waters. This is evidence for deep-reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma.