The Mt. Sheridan Roosevelt Gabbro of the Southern Oklahoma Aulacogen displays evidence of multiple impulses of new magma into the chamber, resulting in mineralogical layering within the crystallizing magma chamber. Twenty-eight samples were collected along the southern part of “little” Mt. Sheridan, encompassing 714 vertical feet.  Modal mineralogical analysis and petrographic observations document changes in the size, shape, alteration, and zonation of plagioclase crystals throughout the vertical section.  In addition, abundance of granophyric textures and presence of olivine in select slides helps to define two distinct zones within the chamber.  The chamber itself appears to be stacked, with a large, layered lower zone, and a homogenous upper zone.  The lower zone coarsens upward in grain size and displays repetitions in the modal abundance of plagioclase.  Plagioclase in the lower chamber grade from unaltered to heavily altered and display zonation that changes from poorly developed, patchy zones to sharp, oscillatory zones.  These gradations in plagioclases are repeated at least three times in the lower chamber.  Felsic dikes and “blobs” are common in the lower chamber, while they are extremely rare in the upper chamber.  Sericitic alteration is more abundant in the lower zone.  Alteration rims on oxides occur in the lower zone as well.  These alteration tendencies may indicate that the magma “stewed in its own juices” before crystallization was complete, resulting in the deuteric alteration seen throughout the lower chamber. The upper chamber is a homogenous gabbro that increases in the modal abundance of plagioclase as the top of the chamber is approached.

The presence of two zones in the single magma chamber indicates that there were two different conditions of crystallization occurring within the chamber, one producing layers, and the other producing a homogenous zone.  It is possible that the lower chamber was formed by several impulses of magma, with time enough between each impulse to allow crystallization and accumulation from gravity settling to take place before the next impulse occurred. The large upper zone of the chamber may have formed from either a final large volume impulse of magma into the chamber or by homogenization of new magma and melts filter pressed from the lower cumulate pile by convection.