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Petrology, Geochemistry, and Structural
Geology of Ultramafic Rocks


    Ultramafic rocks are high in magnesium and iron content and low in silica content in comparison to other rocks. Ultramafic rocks are typically created in the lower levels of the crust or in the mantle and only under special circumstances are they able to make it to the surface.  Much of my recent research has focused on the ultramafic bodies found in the Piedmont Province of the Appalachian Mountain in southeastern Pennsylvania.

     Above is a generalize regional map of southeastern Pennsylvania showing the location of ultramafic rocks (green).  There are approximately 75 ultramafic bodies mapped in the Pennsylvanian Piedmont, however, much of the outcrops are now covered by urban development and sprawl. Our group has examined 15 of the ultramafic bodies that are accessible and have completed detailed studies. In the This research aims to resolve how these ultramafic rocks were created, their alteration history, and how they became exposed in the Pennsylvanian Piedmont.

Tectonics

The rocks of the Pennsylvanian Piedmont owe much of their history to the Taconic Orogeny. Approximately 470 Ma in the Ordovician Period, a series of island arcs, created by eastward dipping subduction, collided with the eastern edge of Laurentia (Paleo North America). This collision created a large mountain belt. During the collision over-thickening of the crust metamorphosed, folded, and faulted the rocks. After subsequent phases of orogenesis (the Alleghenian Orogeny), the mountains eroded and eventually expose the rocks seen in the Piedmont today. The image to the left is a paleographic map from Ron Blakey. 


Some of the main questions about the Piedmont ultramafic rocks have been: where exactly did these rocks come from and how were they transported from lower crustal to their current position?

Ophiolites
Some geologists believe these rocks were part of the oceanic crust that was caught up in the collision. top image

Arc Differentiate
Others believe the rocks were the lower ultramafic component of the arc magma chambers that collided with North America. middle image

Mantle Diapirs
Lastly, some other geologists believe that trans-tensional shear zones were present that allowed for mantle material to rise into the crust. lower image

Geochemistry
     To examine these questions and test these hypotheses, we turn to trace element geochemistry. Most of these rocks are significantly altered by metamorphism, which involve the exchange of chemical components. However, some trace elements are relatively immobile and are retained, at the original compositions, in the rocks. Examining these trace elements, geologists have identified specific signatures that are inherited from the original rock at proportions that reveal their original crystallization setting. These types of diagrams are typically referred to as Petrogenetic Discrimination Diagrams.  

The figure to the right plots the concentrations of ytterbium (Yb) and barium (Ba) of the rocks found in the Pennsylvanian Piedmont. Deschamps et al. (2013) determined specific ratios of these elements in serpentinite samples that were indicative of the setting where these rocks were created. The regions correspond to the following setting: Abyssal  - MORB or ophiolite; Mantle Wedge - mantle derived rocks; Refertilized and Subduction - refer to rocks involved in subduction zone volcanism or island arc settings. The majority of samples plotted on the Yb-Ba discrimination fall within the regions that would suggest an arc setting for the Piedmont ultramafics.
Since not all of our rocks are serpentinites, we look towards other systems for confirmation of our results. Shervais (1981) plotted vanadium (V), titanium (Ti) and scandium (Sc) in basaltic systems to determine the original setting that created the rocks. The fields plotted on the diagram are as follows: IAB - Island Arc Basalt; MORB - Mid-Ocean Ridge Basalt; and OIB - Ocean Island Basalt. In both diagrams the overwhelming signature is that of an island arc setting. 

Alteration History
     Another problem addressed by this research is the alteration history and mechanism of the ultramafic bodies have given them complex internal structures. Many of the ultramafic bodies of the Piedmont show blackwall alteration. Blackwall alteration is seen when ultramafic or mafic bodies experience incomplete alteration preserving a relatively unaltered core grading into zones of increasing hydration approaching the country rock. Country rock is a source of fluid and silica which alters the bodies into an onion skin morphology. 

A simplified view of a blackwall zone after Sanford (1982).

     While many of the ultramafic bodies exhibit blackwall alteration, there are a variety of styles of alteration found in the Piedmont. These difference in alteration styles could have to do with: the specific prototlith lithology of the ultramafics, the nature of the country rock, the availability of fluids, proximity to shear zones, or additional variables.


Some rocks at the ultramafic bodies
are undeformed and retain their
original igneous textures
Most rocks at the ultramafic bodies have been metamorphosed showing complete replacement and shear textures

     This research is on-going and with many problems still left to solve. 

Portions of this research has been published:
Also, components of this research have been presented at a number of venues:
I'm always looking for interested students to take on more projects!!!

Future Projects on ultramafics: 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 © 2019 Ryan Kerrigan (last updated Sept 2019)