Rapakivi microstructure in a 3.3 Ga rhyolite

The rapakivi microstructure has been widely studied in granites, but few investigations have focused on volcanic rocks, especially Paleoarchean types. The Contendas Rhyolite (CR), approximately 3.3 billion years old, is a rare example of a Paleoarchean volcanic rock with preserved rapakivi microstructure. This work provides a microstructural description, results of energy-dispersive X-ray spectrometry (EDS) analyses, and electron-backscatter diffraction (EBSD) studies of the rapakivi feldspars in the CR. Petrographic analysis shows that the microstructure features a single, thick rim of euhedral to subhedral albite surrounding a core of variable-shaped orthoclase. Unlike rapakivi in other volcanic rocks, which often have dendritic or cellular plagioclase rims, the CR displays well-developed plagioclase rims, indicating that they formed before reaching the surface, during rapid cooling. EBSD confirms that the albite rim shares crystallographic orientation with the orthoclase core, with parallel planes such as (001) Ab || (001) Or , (010) Ab || (010) Or , and (110) Ab || (110) Or , as well as continuity in twinning planes, evidencing epitaxial nucleation. Microstructural and chemical data suggest that sub-isothermal decompression is the primary mechanism of formation for rapakivi feldspars. This study is among the first to describe rapakivi microstructure in a Paleoarchean volcanic rock and to confirm EBSD evidence of crystallographic continuity between rim and core feldspars.