Complex Eocene-Oligocene hypabyssal intrusive systems associated with basaltic phreatomagmatic vents in the area east of Pe?a Mountain, Big Bend National Park, West Texas

Abstract

An episode of explosive Eocene basaltic phreatomagmatic volcanism has recently been discovered within the Trans-Pecos Magmatic Province in west Texas. The study area focuses on basaltic rocks formed partly during this episode east of Pe??a Mountain, within Big Bend National Park. Hypabyssal systems consisting of three units of diabase, Type I, II, and III, as well as outcrops of the regional Alamo Creek Basalt are present in the study area. Phreatomagmatic vent-fill outcrops are intruded by diabase and contain bombs geochemically related to the Type I intrusion and the Alamo Creek Basalt, which have 40Ar/39Ar dates of ~ 47 Ma. The Type II intrusion has not been dated, but is geochemically similar to other intrusive basalts in the region with ages of ~ 43-42 Ma. The voluminous Type III intrusion, with 40Ar/39Ar dates of ~ 34 Ma, has partly obscured the phreatomagmatic vents and was not involved in explosive volcanism.