Sierra Blanca Region

Apart from Pajarito Mountain (No. 100) lying to the east, alkaline rocks in the Sierra Blanca region include the Three Rivers composite alkaline stock, the Black Mountain composite stock, the Sierra Blanca volcanics and a northeast-trending dyke swarm concentrated between the Three Rivers and Black Mountain intrusions. Much of the area is underlain by Triassic, Permian and Cretaceous strata which were domed in the vicinity of Black Mountain by the intrusion of syenite and quartz syenite. These rocks in turn were intruded by biotite alkali gabbro, and this was followed by emplacement of a dyke swarm including at least ten varieties of alkaline dykes, the great majority gabbroic in composition. The Sierra Blanca volcanics of flows, flow breccias and welded tuffs, including trachyphonolites and lesser volumes of trachyandesite, trachyte, phonolite, phonotephrite and trachybasalt, were then extruded. The volcanics have a minimum thickness of 540 m and originally probably reached 1500 m in thickness. The Three Rivers complex intrudes the volcanics and consists of amphibole-biotite syenite, syenite porphyry, quartz syenite and granophyric alkali granite. Nepheline syenite and sodalite-nepheline syenite occur on the southeastern side of the complex.