Metamorphic Geology

Most of my field work has been either in low-pressure regional metamorphic terrains or in contact-metamorphic aureoles. The theme of the field-site studies is heat and fluid transport in the upper part of Earth’s crust. The field areas are closely associated with plutons, either as single or composite stocks or as large batholiths; the igneous activity represents the transport of heat from Earth’s interior. Intrusion of magma drives hydrothermal circulation, which results in chemical differentiation through fluid–rock reaction. The evident result is the formation of ore deposits, but a more subtle result is the transport of carbon and other elements during metamorphic reactions in the hydrothermal system.

Panamint Mountains, California

The metamorphic terrain in the Panamint Mountains, California, was developed in Late Precambrian sedimentary rocks during Mesozoic time, contemporaneous with emplacement of the Sierra Nevada batholith. The metamorphism is characterized by the presence of andalusite and cordierite in pelitic rocks, an assemblage that formed in a low-pressure, high-temperature metamorphic environment. I am now determining the petrogenesis of the rocks in Wildrose Canyon, which includes calcic schist in the Kingston Peak Formation, dolomitic marble in the Noonday Dolomite, and pelitic schist in the Johnnie Formation. Peter Nabelek and I are determining the stable isotope compositions of carbonate rocks and combining the results with petrologic information to determine the relation between fluid transport and metamorphism during batholith emplacement. A description of the geology of the Telescope Peak Quadrangle was given by
Labotka, Albee, Lanphere, and MacDowell (1980)

Toquima

Toquima Range, Nevada

Kelly Plummer studied the metamorphism around the Belmont Pluton, central Nevada, for his MS thesis. A group of granitic plutons intruded lower Paleozoic carbonate rocks during Cretaceous time, after thrust faulting during the Antler orogeny. Tremolite and diopside formed in the rocks during metamorphism through the breakdown of dolomite + quartz. The isotopic composition of calcite and dolomite indicate interaction with various amounts of aqueous fluid, independent of metamorphic grade. That interaction probably occurred during Tertiary time when several calderas formed and when gold was deposited in the nearby Manhattan district.

Eastern Blue Ridge

My students and I have been studying the complex metamorphism of the eastern Blue Ridge province, western North Carolina. The region consists of quartzofeldspathic and garnetiferous gneiss and schist, with several juxtaposed ultramafic complexes. Metamorphism appears to have occurred during early Paleozoic time when the rocks were accreted to the margin of North America. Assemblages in the rocks include kyanite, sillimanite, and K-feldspar. Many rocks are rich in garnet, and the garnet grains are complexly zoned in Ca. The compositions seem to indicate a high pressure during metamorphism and partial melting. We are piecing the metamorphic history together to place the ultramafic rocks in context. The dunite at Balsam Gap, North Carolina, is the starting material for experiments on the rate of serpentinization.

Student Theses

Milo Cameron, MS 2001, Garnet zoning, mineral inclusions, and pT paths; Evidence for a polymetamorphic history of the Cullowhee Gneiss.

G. Landon Davidson, MS 1994, Tectonometamorphic history of a portion of the eastern Blue Ridge, Cullowhee, North Carolina.

Thomas S. Helms, PhD 1991, Tectonometamorphism of the Proterozoic Rocks in the Southern Black Hills, South Dakota.

Randal L. Kath, MS 1986, Petrology of the Contact Metamorphic Rocks beneath the Stillwater Complex, Montana: Conditions and Assemblages of Metamorphism

© Theodore C. Labotka 2017