Arc-related sodic hydrothermal alteration in the western US

Sodium-rich hydrothermal alteration is widely developed in Permian to Jurassic arc igneous rocks of the western US, but it is rare in younger rocks. Na-rich alteration reflects a paleogeographic control on development. Two varieties, NaCa and Na, can be defined by mineralogic and compositional changes. The transition away from widespread Na ± Ca-rich alteration in the late Mesozoic corresponds to and is consistent with the change from an early Mesozoic marine and/or arid rifted arc setting to a fully emergent, continental arc environment in the Cretaceous. -from Authors