Through a glass darkly: the Wyoming record of Early Earth

May 21 (Tuesday), 6 p.m., Teton Co. Library Auditorium– Open to Public. Presentation: “Through a glass darkly: the Wyoming record of Early Earth”, Presented by Carol Frost, University of Wyoming.

It is fully accepted today that the theory of plate tectonics explains how our earth works, how continents have formed, moved across our globe, and been the creator of mountain ranges, earthquakes small and gigantic, and even a requisite basis for life on earth. But when did plate tectonics begin, how did the very first continents form? Possible answers to these questions will be addressed by Dr. Frost from data found here in Wyoming.

Various studies have suggested that modern plate tectonics may have begun around 3 billion years ago. Identification and characterization of pre-3 billion-year old continental crust is prerequisite to determining the tectonic processes prevailing during this earlier part of Earth’s history. Although important information is preserved in Paleoarchean and Hadean detrital zircons in younger sedimentary rocks, more direct information derives from study of Paleoarchean crust itself.

This talk describes the largest expanse of Paleoarchean rocks exposed in Wyoming: gneisses that crop out across a 70 km wide swath in the Granite Mountains. Ten U-Pb age determinations on these gneisses define two age groups. The oldest orthogneisses, preserved as banded tonalitic gneisses, range from 3.45 to 3.38 billion years. These banded gneisses are intruded by extensive, coarse-grained, undeformed to foliated, tonalitic to granitic orthogneiss that ranges in age from 3.30 to 3.33 billion years. The two groups of gneisses have different geochemical characteristics. Nd-isotope evidence suggests that none of the Sacawee block orthogneisses represent first-generation continental crust, rather that they formed from older felsic crust.  This finding is consistent with the silica-rich, peraluminous nature of these rocks, with the presence of 3.82 billion years zircon cores in several of the gneiss samples, and with Hf and O isotopic data from dated zircon areas.

We propose a model in which a thick crust formed over a zone of upwelling mantle. Continued input of mantle heat at the base of this crust caused partial melting and formation of peraluminous tonalite. Subsequent cycles of partial melting created tonalites and granites that are more silica-rich. Our study reinforces the suggestion that early continental crust may have formed in an oceanic plateau or island-like tectonic setting rather than one closely resembling a modern plate tectonic environment.