Williams, Michael

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Professor, Department of Geosciences
Last Name
Williams
First Name
Michael
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Earth Sciences
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Metamorphic Petrology
Precambrian Geology
Structural Geology
Introduction
Mike Williams earned a B.A. in geology from Amherst College, an M.S. from the University of Arizona, and a Ph.D. from the University of New Mexico. He has been a faculty member at the University of Massachusetts since 1987 and now serves as Head of the Department of Geosciences. Mike's research is focused at the cross-roads of ductile structural geology, metamorphic petrology, igneous petrology, and tectonics. Much of his research has involved Precambrian rocks (southwestern U.S.A. or northern Canada) but he has been increasingly involved with rocks of western New England. Mike is particularly interested in finding better ways to "read" the P-T-t-D (i.e. Pressure-Temperature-time-Deformation) paths from deformed and metamorphosed rocks, and interpreting the paths in terms of the tectonic history that produced them. He loves field work but also loves working with the electron microprobe and computer models.
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    PublicationOpen Access
    Petrogenesis of the 91-Mile Peridotite in the Grand Canyon: Ophiolite or Deep-Arc Fragment?
    (2021) Seaman, S. J.; Williams, M. L.; Karlstrom, K. E.; Low, P. C.
    Recognition of fundamental tectonic boundaries has been extremely difficult in the (>1000-km-wide) Pro-tero-zoic accretionary orogen of south-western North America, where the main rock types are similar over large areas, and where the region has experienced multiple postaccretionary deformation events. Discrete ultramafic bodies are present in a number of areas that may mark important boundaries, especially if they can be shown to represent tectonic fragments of ophiolite complexes. However, most ultramafic bodies are small and intensely altered, precluding petrogenetic analysis. The 91-Mile peridotite in the Grand Canyon is the largest and best preserved ultramafic body known in the southwest United States. It presents a special opportunity for tectonic analysis that may illuminate the significance of ultramafic rocks in other parts of the orogen. The 91-Mile peridotite exhibits spectacular cumulate layering. Contacts with the surrounding Vishnu Schist are interpreted to be tectonic, except along one margin, where intrusive relations have been interpreted. Assemblages include olivine, clinopyroxene, orthopyroxene, magnetite, and phlogopite, with very rare plagioclase. Textures suggest that phlogopite is the result of late intercumulus crystallization. Whole-rock compositions and especially mineral modes and compositions support derivation from an arc-related mafic magma. K-enriched subduction-related fluid in the mantle wedge is interpreted to have given rise to a K-rich, hydrous, high-pressure partial melt that produced early magnetite, Al-rich diopside, and primary phlogopite. The modes of silicate minerals, all with high Mg#, the sequence of crystallization, and the lack of early plagioclase are all consistent with crystallization at relatively high pressures. Thus, the 91-Mile peridotite body is not an ophiolite fragment that represents the closure of a former ocean basin. It does, however, mark a significant tectonic boundary where lower-crustal arc cumulates have been juxtaposed against middle-crustal schists and granitoids.