SIGNIFICANCE OF LAYERING IN THE MOUNT SHERIDAN GABBRO, OKLAHOMA

The Cambrian Mt. Sheridan Roosevelt Gabbro of the Southern Oklahoma Aulacogen exhibits repetitive phase layering and cryptic layering interpreted to form as a result of multiple replenishment of new magma into the chamber during crystallization. Samples were collected along a traverse spanning 714 vertical feet up the side of “little” Mt. Sheridan towards the contact with Mount Scott Granite. Petrographic observations document changes in modal abundance, size, shape, alteration, and zonation of plagioclase crystals throughout the vertical section. Variation in abundance of granophyre and particularly olivine aids in recognition of two distinct zones within the pluton: a large, multiply layered, lower zone overlain by a homogenous upper zone. Layers in the lower zone coarsen upward and display repetition in the modal abundance of olivine and 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 pegmatitic dikes and “blobs” are common to locally abundant in the lower chamber, and rare in the upper chamber. Sericitic alteration of plagioclase and alteration rims on oxides is more abundant in the lower zone. This 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 contact with the overlying granite 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 multiple cumulate layers, and the other producing homogenous gabbro. 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 through mixing of new magma and melts filter pressed from the lower cumulate pile.