T92 Sigma Gamma Epsilon Undergraduate Research (Posters)

Monday, 5 November 2012
9:00 AM - 6:00 PM
Hall B

COMPARATIVE PETROLOGY AND MINERALOGY OF PEGMATITES FROM THE PIKES PEAK BATHOLITH, COLORADO

Paper number: T92-43
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

In the Pike’s Peak Batholith beneath Sentinel Rock, just west of Colorado Springs, Colorado, are many pegmatites of varying mineralogy. Most of these pegmatites are amazonite-bearing; however, a non-amazonite bearing pegmatite occurs at a higher elevation. Samples were first collected in 2010 from an amazonite pegmatite and the non-amazonite pegmatite. Sampling continued for the amazonite-bearing pegmatite for 2011 and 2012; however, the non-amazonite pegmatite was no longer present. The non-amazonite bearing pegmatite is composed of feldspar, quartz, and mica. This pegmatite is located on the Southeast base of Sentinel Rock at an elevation of 8900 feet and has a nearly vertical strike. It forms out of graphic granite and no aplite was observed. This pegmatite has abundant euhedral microcline and quartz developed in open pockets. Mica is found in the graphic granite and is almost completely weathered. The amazonite-bearing pegmatite that we sampled is at an elevation of 8200 feet. The strike of this lower pegmatite is nearly horizontal, and it contains amazonite, albite, fluorite, quartz, and mica.

The fact that the pegmatite with no amazonite occurs at a higher elevation than the amazonite-bearing pegmatite poses a problem: amazonite is favored nearer the top of the magma chamber, and therefore at higher elevations than at lower. These pegmatites are separated by 700 vertical feet, a relatively short distance when the size of the batholith is considered. If the simple pegmatite is structurally related to the amazonite, and in place, one would expect their elevations to be reversed. Since this paper will only compare their structural relationships, age dates and melt interactions of the pegmatites will be obtained and compared at a later stage in this project.

Perthite is present throughout the microcline in the pegmatite, with the bulk of it being albitic at the pocket and nearly equal Na and K perthite in the lower portions. This provides an opportunity for thermobarometry of this pegmatite based on its perthite content; the same will be done for the amazonite pegmatites for comparison. This will help test whether faulting may have displaced the pegmatites after their formation.