Diabasic sills from the 1.27by old Carrizo Mountain Group of W Texas have immobile element abundances similar to those of modern mid-ocean ridge basalts (MORB). The presence of thick terrigenous sedimentary rocks and peraluminous rhyolitic ash-flow tuffs within the Carrizo Mountain Group preclude the diabases forming in a typical MORB environment. Field, petrographic, and geochemical evidence suggests that the Carrizo Mountain Group was depositied in an incipient continental margin back-arc basin that may have extended NE into the Texas panhandle region. The overlying Allamoore Formation consists of sedimentary rocks and basalt accumulated within a mature marginal basin behind the tholeiite-dominanted island arc postulated for the Llano area.-AuthorLunar & Planetary Inst, 3303 NASA Rd One, Houston, TX 77058, USA.