MAGMA TRAPS AND DRIVING PRESSURE - CONSEQUENCES FOR PLUTON SHAPE AND EMPLACEMENT IN AN EXTENSIONAL REGIME

The level of emplacement and final form of felsic and mafic igneous ro cks of the Wichita Mountains Igneous Province, southwestern Oklahoma, U.S.A. are discussed in light of magma driving pressure, lithostatic l oad, and crustal magma traps. Deposition of voluminous A type rhyolite s upon an eroded gabbroic substrate formed a subhorizontal strength an isotropy that acted as a crustal magma trap for subsequent rising fels ic and mafic magma. Intruded along this crustal magma trap are the A-t ype sheet granites (length/thickness 100:1) of the Wichita Granite Gro up, of which the Mount Scott Granite sheet is typical, and smaller plu tons of biotite bearing Roosevelt Gabbro. In marked contrast to the su bhorizontal granite sheets, the gabbro plutons form more equant stocks with flat roofs and steep side wails. Late Diabase dikes cross-cut al l other units, but accompanying basaltic flows are extremely rare in t he volcanic pile. Based on magmastatic calculations, we draw the follo wing conclusions concerning the level of emplacement and the shape of these intrusions. (1) Magma can rise to a depth at which the magma dri ving pressure becomes negligible. Magma that maintains a positive driv ing pressure at the surface has the potential to erupt. (2) Magma asce nt may be arrested at a deeper level in the crust by a subhorizontal s trength anisotropy (i.e. crustal magma trap) if the magma driving pres sure is greater than or equal to the lithostatic load at the depth of the subhorizontal strength anisotropy. (3) Subhorizontal sheet-intrusi ons form along crustal magma traps when the magma driving pressure gre atly exceeds the lithostatic load. Under such conditions, the magma dr iving pressure is sufficent to lift the overburden to create the neces sary space for the intrusion. (4) Thicker steep-sided stocks or bathol iths, with flat roofs, form at crustal magma traps when the magma driv ing pressure approximates that of the lithostatic load. Under these co nditions, the necessary space for the intrusion must be created by oth er mechanisms (e.g. stoping). (5) Subvertical sheets (i.e. dikes) form when the magma driving pressure is less than the lithostatic load at the level of emplacement. (C) 1998 Elsevier Science Ltd. All rights re served.