USING EMPLACEMENT POSITION TO HELP CONSTRAIN ORIGIN OF GRANITES

The granite exposures in the Wichita Mountains of southwestern Oklahoma are some of the uppermost outcrops of the Cambrian Southern Oklahoma Aulacogen (SOA). This aulacogen is characterized by a close interplay between rifting and magmatism, so that each magmatic pulse is taken to be coincident with a new pulse of extension. The granites were emplaced near the end of the rifting event. The rift zone retains enough stratigraphic and structural history so that portions of its magmatic filling can be documented and leads to the following interpretive scenario.

Eleven mappable granites exist over an area of about 30 x 100km. They have all been shallowly intruded along an unconformity between underlying layered mafic complex(es?)(e.g., Glen Mountains Layered Complex) and an overlying rhyolitic sequence (Carlton Rhyolite). The granites are all A-type sheet granites which generally do not overlay but abut one another. Isotopic data indicate that some granites are primarily differentiates of mafic mantle magmas while others have heterogeneous crustal sources.

However, because the rising granitic magmas stopped at the same upper crustal discontinuity, their driving pressures must have been comparable whatever the source. This necessitates either 1)that the crustal zones from which differeniation took place and where adjacent crustal melting occurred were at the same crustal level, or 2)that wherever the magmas formed, they ascended and ponded at the same mid-crustal level before finally moving on upward to their emplacement positions.

Thus, in such situations it would make sense to search for petrologic/mineralogic indicators on source depth, or geophysically for discrete horizons of magma sourcing or ponding. For the SOA there are clues that help constrain the granite source levels to mid-crust.