UNDERSTANDING MAGMA EMPLACEMENT USING MAGMA DRIVING PRESSURE

Hogan, Price, and Gilbert (1998) developed an argument using magma driving pressure to explain crustal positions of plutons, and their shapes, in the extensional Cambrian Southern Oklahoma Aulacogen (SOA). Further discussion of this approach is useful in thinking about the structural evolution of the that rift and speculating on depths of magma sources. This rift cut the Proterozoic crust which extended across southern Oklahoma and northwest Texas. A brief analysis of the SOA situation may suggest applications to the formation of the "granite-rhyolite" Proterozoic crust.

Combining information on the emplacement level from a number of related granites, as in the SOA (Wichita Mountains), with magma driving pressure arguments, indicates that source conditions change with time. All eleven granites in the Wichitas intruded at the same stratigraphic level, but not at the same depth. Since the driving pressure for an intrusion must be high enough to lift the overburden, and overburden depths vary, then source depths must also be changing. Because finer-grained granites (shallow overburden) are earlier, and coarser-grained granites (more overburden) later in time, it appears that effective source depths are getting deeper. This is not unexpected, as silicic magmas seem to be the last extensive magmas generated, and probably represent the last of the extension process, and the beginning of rift cooling.

In those parts of the igneous Midcontinent Proterozoic where some stratigraphy can be ascertained, it may be possible to provide preliminary explanations about history and behavior of sources.