POLYMETASOMATIC HISTORY OF ASHFLOW TUFF AND GEOCHEMICAL EVOLUTION DURING CONTACT METAMORPHISM, MAMMOTH CREST, CALIFORNIA

Metavolcanic rocks can be powerful tools for documenting the complex histories of multiple episodes of fluid-rock interaction in long-lived arc crust. The Duck Lake area of the Mammoth Crest, eastern California, is part of a lithotectonic assemblage exposed along the eastern front of the Sierra Nevada. Meta-rhyolite tuffs at Duck Lake show complex K, Na, and Ca systematics and disequilibrium feldspar phase relations that reflect a superposed history of contrasting metasomatic systems. We have reconstructed parts of that history using cathodoluminescence and mineral and whole-rock compositions.

Cathodoluminescence petrography shows igneous phenocryst and ground mass feldspar completely replaced by near-end-member orthoclase (Or>95 after sanidine) and or albite (Ab>95 after plagioclase). These textures resemble those of nearby meta-tuffs in the Ritter Range Pendent (on strike, 20 km NW) and of unmetamorphosed Tertiary tuffs (Creede, CO, Socorro, NM) that have undergone low-temperature K- or K/Na- metasomatism. In those rocks alkali alteration occurred soon after deposition. These early features are overprinted by Ca-metasomatism that is manifested by calcite veins and grossular + epidote veins with calcic plagioclase (An80-100) vein envelopes. The early pseudomorphs of phenocrysts and groundmass feldspar are replaced by calcic plagioclase in this event. Whole-rock geochemistry shows concomitant gains of Ca and Sr and loss of Na. CaO/Na₂O for tuff samples ranges to 13, a value >6 times that of unaltered ashflow tuffs. K/Na values for Duck Lake tuffs are similar to those seen in the Ritter Range, Creede, and Socorro, but display distinct slopes on plots of K/Na versus Rb/Sr. Textures indicate that Ca was sourced by hydrothermal fluid derived from carbonates, probably locally.

The Ca-rich silicate veins cut metamorphic fabrics that Tobisch et al. (2000) regionally attributed to batholith emplacement, indicating Ca-metasomatism occurred during retrograde phases of contact metamorphism. Evidently, as seen in many ore deposits, cooling contact metamorphic systems can be accompanied by large amounts of mass transfer.