41. K-metasomatism of plagioclase to produce microcline megacrysts in the Cathedral Peak granodiorite, Sierra Nevada, California, USA

Higgins (1999) proposed that microcline megacrysts in the Cathedral Peak granodiorite resulted from Ostwald ripening in which textural coarsening of earlier formed small K-feldspar crystals were sacrificed to feed the growth of a select few larger crystals. Textural evidence shows that Ostwald ripening did not occur, but, instead, the megacrysts resulted from K-replacement of broken plagioclase crystals in sites of strong cataclasis, where tiny microcline crystals (1 to 15 mm in mean height) grew to as much as 80 mm. The progressive growth of the microcline encompassed broken remnant fragments of ground mass minerals in concentric shells. The evidence for K-replacement consists of veins of microcline penetrating cores of broken zoned plagioclase, the alignment of one plane of the grid-twinning in the microcline with albite twinning in the plagioclase, the occurrence of myrmekite, and the replacement of biotite by quartz to provide the needed K.