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 <5 m long, do not cross intrusive contacts, and postdate the pervasive alkali alteration. Late 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 <300°C, probably in superposed marine hydrothermal systems associated with coeval volcanic centers. Metavolcanic rocks of the upper section evidently interacted with meteorie waters, probably in a hydrothermal system associated with the Cretaceous caldera; rocks of the lower section that were adjacent to the caldera were also affected by this alteration. The preservation of the signatures of these earlier events, the nature of the early veins, and results from numerical models of hydrothermal flow that include fluid production indicate that during prograde contact metamorphism, the rocks of the pendant primarily interacted with locally derived fluids. Fluid flow was predominantly upward and away from intrusive contacts and down-temperature. Permeabilities are estimated to have been between 0{dot operator}1 and 1μD, which is that necessary for maintenance of lithostatic fluid pressures. In hydrothermal models with such permeabilities, large-scale circulation of meteoric fluids develops after prograde metamorphism ceases. The nature of the late veins in the Ritter Range pendant suggests that such a flow pattern evolved only after the pendant and granitoids had cooled below 450-500°C. The long-term history of alteration documented in the Ritter Range pendant is probably typical of wall rocks in most batholiths.