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Sept. 2020: 
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Petrology, Geochemistry, and Structural
Geology of Pegmatites

     Pegmatites are coarse-grained intrusive igneous rocks that have very unique compositions. Generally, pegmatites are granitic or felsic (high silica content) and are enriched in elements that are typically found in low concentrations in normal granites (such as: Li, B, Be, rare earth elements, etc.). The unique compositions and coarse-grained nature of pegmatites create beautiful gemstones and an important source of typically scarce elements. Gemstones often found in pegmatites are tourmaline, beryl, spodumene, among others.
     A series of pegmatite intrusions are present in southeastern Pennsylvania in the Piedmont Province. Very little research has been conducted on these pegmatite dikes and the source of the melt for the dikes has not been clearly defined. Our research group has begun investigating the petrology, geochemistry, and origin of the Piedmont pegmatites and how they interact with the rocks they intrude.   

Field Mapping
     Above is a map showing the intrusion of pegmatite dikes at the Unionville Serpentine Barrens in Chester County PA. Our research has determined that the mapped dikes were improperly placed (white) and a forthcoming publication (Kerrigan et al., 2019) through the PA Geological Survey revises the locations of the pegmatites (gray).

Mineralogy and Textures
      Mapping of the pegmatites also included sample collection of pegmatites and the reaction rocks adjacent to the pegmatites that document the reaction between the pegmatite and the country rock serpentinite. After sample collection, our research group create thin-sections (thin slices of rock to be analyzed by polarizing microscopy) and send samples out for geochemical analyses. Using these data sets we have determined that the pegmatites at the Unionville Serpentine Barrens are classified as Muscovite-Rare Element pegmatites.

    Above are images of the pegmatite and reaction rocks adjacent to the pegmatites (top row) and thin-section images of those rocks (lower row). Using this mineralogical and textural analytical techniques, we are able to identify gradients across the contact between the pegmatite and serpentinite.  The reaction rocks vary in their mineralogy and texture depending on their proximity to the intruding pegmatites

     We also examine the geochemistry of the pegmatites, reaction rocks, and serpentinites to identify specific trends in the rocks. The intrusion of the pegmatites into the serpentinites is accompanied by a high amount of heat, pressure, and fluid flow. The fluid flow around these intrusions encourages component exchange and the adjacent rocks become severally altered, as seen in the textures above. .

     Above shows the geochemical trends of major and trace elements across the contact between the pegmatite (left) and the serpentinite (right). Clear trends can be seen across this gradient in specific major and trace elements that are sensitive to this fluid driven exchange. Further analyses are planned to examine the diffusion and flow of elements to determine the conditions and the period in which this hydrothermal system was active.
Origin of the Melt
     Pegmatites can be created in a number of different geologic environments. Often pegmatites are considered to be the last gasp of a magma chamber, however, they can be generated by partial melt accumulation in metamorphic settings. We have examined trace element geochemistry using petrogenetic discrimination diagrams to determine the source of the melt.

 The discrimination diagrams shown use the relationships of trace elements to determine the source of granitic melts (Pearce et al., 1984; Pearce, 1996). The pegmatites mainly plot in the Volcanic Arc Granite (VAG) and Post-Collisional Granite (Post-COLG) fields (other fields: ORG - Ocean Ridge Granite; Syn-COLG - Syn-Collisional Granite; and WPG - Within Plate Granite). These data suggest that the pegmatite may have been part of the island arc that collided with the Piedmont ~470 Ma during the Taconic Orogeny or produced shortly thereafter. 

Portions of this research has been published:
Also, components of this research have been presented at regional GSA:

I'm always looking for interested students to take on more projects!!!

Future Projects on Pegmatites:
Other Projects:
Top images (left to right): Geologic Map of the Youngsford Road ultramfic body; field work at Bells Mill Road with Ryan Kerrigan, Loring Simboli, and Sam Louderback; XPL image of orthopyroxene altering to anthophyllite; Secondary electron image of relict olivine altering to serpentine; field contact between the Bells Mill granodiorite and the Wissahickon schist; trace element chemical discrimination diagram for serpentinites. Copyright © 2020 Ryan Kerrigan (last updated Sept 2020)