Department or Program




The purpose of this study was to produce a model of the petrogenesis of the Mt. Cabot hastingsite-riebeckite granite located in the northern half of the Northern Jefferson 7.5' quadrangle. This study aims to build on the structural information and the map produced by Hillenbrand (2017) as part of the USGS/NHGS StateMap program.

The Mt. Cabot area contains older surrounding bed rock of the Middle-Upper Ordovician Ammonoosuc Volcanics, the Lost Nation group, Jurassic igneous intrusives of the Pliny ring dike complex, and xenoliths of Conway Granite. The hastingsite-riebeckite granite is the largest pluton in the area. The pluton is part of the White Mountain Magma series and was formed after the tectonic events involving the Early-Mid Mesozoic opening of the Atlantic Ocean and part of the magmatic melting associated with ring dike formation and caldera collapses.

I analyzed samples of the hastingsite-riebeckite granite pluton of Mt. Cabot, NH, for their mineralogy, textures, and geochemistry. Discrete zones of hastingsite and riebeckite bearing granites suggest varying oxygen fugacity conditions across the magma body during crystallization or that distinct Na-rich or Ca-rich magmas makeup the pluton. Modal analysis shows two different rock types within the pluton, syenogranite, and quartz syenite. Exsolved alkali feldspar indicates crystallization at low pressure and depth, around 1.5- 2.1 kbar and 5.3- 7.6 km. Intergranular albite and albite rims on alkali feldspars indicate late stage albitization and replacement and unmixing of alkali feldspar at 410 ºC and 370ºC during crystallization. Sericitization of alkali feldspar and hydration of amphiboles suggest post magmatic hydrothermal alteration. XRF bulk-rock geochemistry and trace element data suggest that the Mt. Cabot granite is chemically similar to other riebeckite granites of the New Hampshire region. The discrete mineralogical zones were most likely caused by the influx and movement of melt that are controlled by the structural nature and instability of ring dike complexes.

Level of Access

Open Access

First Advisor

Genevieve Robert

Date of Graduation


Degree Name

Bachelor of Science

Number of Pages


Open Access

Available to all.