Article

K-metasomatism and the origin of Ba- and inclusion-zoned orthoclase megacrysts in the Papoose Flat pluton, Inyo Mountains, California, USA

In the Papoose Flat pluton in the Inyo Mountains of California the orthoclase megacrysts are suggested to form by K-metasomatism because microfracturing of a few normally zoned plagioclase crystals channeled introduced K-bearing fluids to these places, thereby promoting K-metasomatism and localized growth to form the megacrysts. Isochemical recrystallization to produce the megacrysts from preexisting stressed orthoclase and plagioclase crystals is ruled out because the original relatively undeformed granodiorite of the Papoose Flat pluton contained only 8-13 vol. % orthoclase that could not have been reorganized into more than 19 vol. % orthoclase in the groundmass and megacrysts and because original biotite was insufficient to supply enough K. An outside K source is required. Si-metasomatism of biotite to form quartz would have supplied some of the K. Concentrically oriented plagioclase inclusions that occur parallel to Ba-K growth zones in the megacrysts are shown to be formed from primary groundmass minerals, some of which were broken into fragments but others were not microfractured. The occurrences of myrmekite inside the megacrysts, scalloped edges on all surfaces of many plagioclase inclusions, sodic rims on irregular plagioclase inclusions against outer surfaces of former faces of the orthoclase, and disoriented inclusions that are not parallel to growth zones provide evidence for the primary origin of the inclusions. Rare trails of inclusions through the megacrysts indicate that the megacrysts are porphyroblasts and not residual phenocrysts (porphyroclasts). Microcline replacements of plagioclase which occur at lower temperature in cataclastically deformed granite (micro-aplites) produce abundant myrmekite and recrystallized sodic plagioclase. Pseudo-aplites in strongly sheared zones result where nearly all plagioclase is replaced by microcline, and most biotite is replaced by quartz. Progressive non-isochemical K-metasomatism occurred in the pluton with increasing deformation from the initial orthoclase appearance through the megacryst stage and then the final production of microcline. Orthoclase megacrysts in the wall rock Campito sandstone are shown to have a metasomatic origin similar to that found in the Papoose Flat pluton.

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