Diabasic sills from the 1.27-b.y.-old Carrizo Mountain Group of west 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 deposited in an incipient continental margin back-arc basin that may have extended northeastward into the Texas panhandle region. The overlying Allamoore Formation consists of sedimentary rocks and basalt accumulated within a mature marginal basin behind the tholeiite-dominated island arc postulated for the Llano area.

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