Undergraduate Honors Theses

Thesis Defended

Spring 2015

Document Type


Type of Thesis

Departmental Honors


Geological Sciences

First Advisor

Charles Stern

Second Advisor

Lang Farmer

Third Advisor

Faan Tone Liu


The Spanish Peaks intrusive suite in south-central Colorado includes various mid-Tertiary intrusions, which includes the plutonic stocks of the Spanish Peaks themselves, as well as various dikes, sills, and volcanic plugs. This study specifically focuses on the relationship between the plutonic stock of the West Spanish Peak and a group of radial dikes centered on this mountain. The goal of this thesis is to determine if the radial dike swarm and the West Spanish Peak stock potentially share a common magma source based on their temporal, geochemical, and isotopic relationships.

The West Spanish peak is a holocrystaline quartz monzanite plutonic stock, and its surrounding hypocystalline radial dikes range from mafic to felsic in composition. Contained within the radial dikes surrounding the peak are cognate xenoliths and miarolitic cavities, features which have not been described in this geologic setting before. The holocrystalline cognate xenoliths in the radial dikes have gabbroic compositions. The miarolitic cavities are composed of a variety of both mafic and felsic minerals in concentric rims towards the center of the cavities. From the petrographic, major and minor oxide, trace element, and 87Sr/86Sr isotopic analysesI conducted on these samples, two major conclusions have been drawn. First, the West Spanish Peak stock, its surrounding radial dikes, and the cognate xenoliths included into the radial dikes are likely to be co-genetic, most probably forming from a single parent-magma body. Secondly, the miarolitic cavities in the radial dikes formed during a paleo-groundwater event, or series of events, which occurred after the emplacement of the radial dike swarm, but probably soon afterwards when the local groundwaters were still heated.