Syenites

Abstract

Syenites are felsic plutonic igneous rocks characterized by the lack of quartz (<5 percent) or feldspathoids (<10 percent), although quartz syenites have 5–20 percent quartz. Typically they consist of feldspars, predominantly alkali feldspar with minor ferromagnesian minerals, usually pyroxene, amphibole or biotite. Syenites are overwhelmingly rocks of extensional environments and generally occur in areas of continental rifting, such as the African Rift Valley, and also on oceanic islands, such as the Canary archipelago. The extrusive equivalent of syenite is trachyte. As examples, we will look at two plutonic complexes containing syenites: the little‐known Kangerlussuaq intrusion in East Greenland and the more familiar Larvik Plutonic Complex in the Oslo region of Norway. Syenites occur in association with both oversaturated (quartz syenites and granites) and foid‐bearing rocks (typically nepheline syenites). Agpaitic syenites are highly evolved types with titano‐zircono‐silicates such as eudialyte. Syenites find use as dimension stones and countertops.