From Jurassic Shores To Cretaceous Plutons: Geochemical Evidence For Paleoalteration Environments Of Metavolcanic Rocks, Eastern California

Abstract

Volcanic and plutonic rocks exposed in east-central California record a long history of metasomatism and/or metamorphism within the Mesozoic Cordilleran continental arc. We use whole-rock and mineral elemental compositions, along with standard and cathodoluminescence petrography to characterize alteration histories of Late Triassic to Middle Jurassic metavolcanic rocks in the Ritter Range, White-Inyo Mountains, and Alabama Hills. Although alkali-metasomatism is widespread and pervasive, ratios and abundances of Ce, Th, Tb, and Ta suggest that mafic protoliths from the White-Inyo Mountains were shoshonitic, whereas those from the Ritter Range were calc-alkaline. Alkali exchange apparently modified the compositions of many metavolcanic rocks. Much of this metasomatism may have occurred at low-temperature (T) conditions, and attended or shortly postdated deposition of the volcanic protoliths. High δ18O values for K-rich metatuffs from the Ritter Range suggest that the K-metasomatizing fluid was low-T seawater. In contrast, low δ18O values for K-rich metatuffs from the Alabama Hills and Inyo Mountains seem to reflect rock interaction with meteoric water prior to contact metamorphism. Jurassic metatuffs deposited in marine (Ritter Range) and nonmarine (Alabama Hills, Inyo Mountains) settings display similar degrees of K for Na (or Ca) exchange that were affected by isotopically distinct fluids. Some alkali-metasomatism of Jurassic metavolcanic rocks is related to Cretaceous plutonism. In the Ritter Range and Alabama Hills, these effects are localized around pluton contacts, appear to be more vein related than pervasive, and overprint K-metasomatized assemblages.