Aspects of Crystallization History in Spanish Peaks, Colorado

Abstract

<p> The opening of the Rio Grande Rift of Southern Colorado resulted in a wide variety of igneous features including the Spanish Peaks dual stocks, their surrounding radial dyke system and several proximal enigmatic plugs.</p> <p> Aspects of the crystallization history of two magmas have been studied by using petrographic observations and Crystal Size Distribution (CSD). These were used to examine opaques and feldspars from a rhyolitic dyke in the Spanish Peaks swarm and augite, olivine, and opaques from a gabbroic dyke within Huerfano Butte.</p> <p> Augite and opaques from Huerfano Butte reveal a history of no crystal fractionation during emplacement, no size dependent growth, and constant, continuous nucleation and growth.</p> <p> Huerfano Butte olivine and Spanish Peaks opaques have been subjected to resorption and depletion of nucleii. The resorbed olivine material was probably redeposited as augite. This could explain why Huerfano Butte augite has a much higher nucleation density than the other minerals.</p> <p> The phenocrystic feldspars from Spanish Peaks also have a deficit of nucleii, but they lack resorption features. Xenocrystic skeletal feldspars throughout the dyke lack a fines population and show resorption. The absence of fines is probably due to sorting of grains by density during magma convection before eruption.</p>