LOW-δ18O GRANITES OF THE SIERRAN ARC—RECYCLING OF ALTERED VOLCANIC ROCKS

Although crustal recycling is widely recognized in the SNB by elevated δ¹⁸O(Zircon) values, some sub-mantle values are recorded in Sierran granitoid rocks: δ¹⁸O(Zircon) =4.7‰ Mt. Kaweah Granite; 4.6‰ Independence Granite; 4.8‰ Dragon Pluton, and a number of other high-silica granites have δ¹⁸O values of 5.0–5.3‰ (Lackey et al. 2008). Because zircon is highly resistant to secondary alteration, these subtly lower δ¹⁸O values require incorporation of low-δ¹⁸O melts from sources or wall rocks into the original granitic magmas. Low-δ¹⁸O rocks typically represent hydrothermally altered by heated meteoric water or seawater near Earth’s surface. Several lines of evidence point to the cause of the lowered δ¹⁸O values of these granites as recording melting of altered volcanic rocks from higher levels in the Sierran arc. These include: 1) the low-δ¹⁸O plutons are from shallow (<1 kbar based on hornblende barometry and porphyritic textures) depths in the batholith near Kearsarge Pass, Independence, and the Kaweah Peaks; 2) the plutons contain abundant partially melted xenoliths of metavolcanic rock; 3) garnet in skarns at Mineral King (to –4‰) and Strawberry Mine (~4‰) reveal low-δ¹⁸O hydrothermal systems extending to similar depths in the batholith; and 4) low measured whole rock-δ¹⁸O values of Mesozoic metavolcanic wall rocks (as low as –6‰, Kistler, 1994; this study), especially those of Cretaceous age, confirm widespread alteration of volcanic rocks by heated surface water. In addition, the correspondence of the Sierran low-δ¹⁸O granite zone with the Kern Canyon Fault, which may be a persistent structural weakness throughout the Cretaceous, suggests structural weaknesses promote fluid flow into the arc crust to further alter volcanic rocks. Displacive drawdown of wall rocks during pluton construction would have inserted the altered volcanics into the middle crust zones where conditions for partial melting would be more favorable. Because δ¹⁸O(Zrc) values in the granites are not exceptionally low (i.e., no negative values as at Yellowstone caldera), this assimilation mechanism is modest, although studies and models that attempt to link coeval plutonic and volcanic systems in arcs must also factor in the top-down influence of altered volcanic sections modifying later magmas, both plutonic and especially volcanic.