STRUCTURAL AND METAMORPHIC HISTORY IN THE VICINITY OF PURPLE LAKE, MOUNT MORRISON ROOF PENDANT, CALIFORNIA

Purple Lake lies within the Mount Morrison Roof Pendant in the Sierra Nevada, California. This study examines the geologic history of a series of metavolcanic and metasedimentary rocks near Purple Lake, focusing on the structural and metamorphic events prior to and during the emplacement of two Cretaceous granitic plutons. The study reveals important implications for the deformational history of this area of the Sierra Nevada.

The rock units at Purple Lake consist of metalatite and metarhyolite tuffs with metasedimentary interbeds, including marbles and meta-sandstones. The rocks lie in contact with two Cretaceous granitic plutons. While mineral assemblages throughout the section do not precisely constrain metamorphic grade, they are consistent with greenschist facies metamorphism. The metavolcanic rocks do not coarsen at the pluton contacts, nor do their mineral assemblages change significantly. The presence of abundant quartz + anorthite, +/- epidote/thulite, +/- garnet in both veins and pods throughout the section indicates water-rich fluids infiltrated the volcanic section, causing extensive Ca-metasomatism towards the end of pluton emplacement.

After deposition in a shoreline-marine environment during the Late Triassic to Early Jurassic, compressional events caused multiple generations of deformation in the metavolcanic and metasedimentary rocks in the vicinity of Purple Lake. The predominant bedding-parallel foliation in the section averages N53W/73SW; foliations vary from this trend at pluton contacts and in folded units. After formation of the bedding-parallel foliation, subsequent compression created at least two distinct generations of major and minor folds. Crenulations on the bedding-parallel foliation occur alongside tension gashes in a folded metalatite. Pluton emplacement created additional foliations at intrusive contacts.