Long-Term Evolution of Fluid–Rock Interactions in Magmatic Arcs: Evidence from the Ritter Range Pendant, Sierra Nevada, California, and Numerical Modeling

A record of > 100 million years of fluid flow, alteration, and metamorphism in the evolving Sierra Nevada magmatic are is preserved in metavolcanic rocks of the Ritter Range pendant and surrounding granitoids. The metavolcanic rocks consist of: (1) a lower section of mostly marine volcaniclastic rocks, lavas, and intercalated carbonate rocks that is Triassic to Jurassic in age, and (2) an upper section comprising a subaerial caldera-fill complex of mid-Cretaceous age. Late Cretaceous high-temperature contact metamorphism (∼2 kbar, >450–500°C) occurred after renewed normal faulting along the caldera-bounding fault system juxtaposed the two sections. The style and degree of alteration and δ18O values differ among the rocks of the upper and lower sections and the granitoids. Rocks of the lower section show pervasive lithologically controlled alkali alteration, local Mn and Mg enrichment, and oxidation. Some ash flow tuffs now contain up to 10% K2O by weight. The rocks of the upper section show lesser extents of alkali alteration. Granitoids that cut both sections are generally unaltered. Most metavolcanic rocks of the lower section have high δ18O values (+ 11 to + 16%; whole rock and quartz phenocrysts); however, lower-section rocks within the caldera-bounding fault system have low δ18O values of + 4 to +7‰. The metavolcanic rocks of the upper section also have low δ18O values of + 2 to + 7‰. Granitoids have δ18O values of + 7 to + 10‰, typical of unaltered Sierran granitoids. The lower section contains discontinuous veins of high-temperature (450–500°C) calc-silicate minerals. These veins are typically 10 m long, formed at temperatures of less than 450–500°C, and cross intrusive contacts. Veins have similar δ18O values to those of the local host rocks. The nature of the alteration and the high oxygen isotopic values of the rocks of the lower section indicate that these rocks interacted extensively with seawater at temperatures