Transition from magmatic to K-metasomatic processes in granodiorites and Pyramid Peak granite, Fallen Leaf Lake 15-Minute Quadrangle, California

Alden Loomis studied the geology of the Fallen Leaf Lake 15-minute quadrangle in eastern California for his 1961 Ph.D. thesis. Many different granitic and relatively-mafic plutons (hornblende-biotite quartz diorite) occur in this area. These plutons exhibit such igneous features as contact metamorphic aureoles, mafic enclaves, schlieren, comb layering, dikes, hypidiomorphic granular textures, and strongly zoned plagioclase. Subsequent to the solidification of the plutons, deformation allowed hydrous K-bearing fluids to enter the solid rocks and cause K-metasomatism. In some places metasomatic K-feldspar cuts the borders of mafic enclaves and preferentially replaces their fine-grained plagioclase. Although Alden Loomis observed this K-metasomatism, he concluded that in comparison to the volume of rocks crystallized from magma, the amount of solid-state recrystallization and metasomatism was negligible. Because he reported myrmekite as an accessory, this study was initiated to see if the metasomatism was more extensive than he thought. Thin section analyses of textures show that as much as 28.5 volume percent K-feldspar was added to former deformed quartz diorite plutons, converting them into granodiorites while preserving igneous structures, hypidiomorphic granular textures, and remnants of zoned plagioclase crystals. Where deformation was relatively weak, only grain boundary seals were broken, and interstitial K-feldspar bordered by rim myrmekite replaced the exteriors of plagioclase crystals. Where greater deformation occurred, breaking not only grain boundary seals, but also microfracturing the plagioclase, the K-feldspar replaced both interiors and exteriors of the primary plagioclase crystals. In these places wartlike myrmekite was formed. In some places the metasomatic anhedral K-feldspar crystals grew to become nearly-euhedral K-feldspar poikiloblasts. Some parts of each granodiorite pluton still have remnants of a former quartz diorite, but some quartz diorite plutons still remain in the area unmodified. The relatively mafic Keith Dome quartz monzonite has not been modified, nor have parts of the Pyramid Peak granite where it exhibits diorite-hybrid-granite magma mingling and mixing. However, major parts of the Pyramid Peak granite have resulted from Kmetasomatism of former quartz diorite which converted this rock into granite.