FORMATION OF PALEOARCHEAN CONTINENTAL CRUST IN THE CENTRAL WYOMING PROVINCE

Various studies have suggested that modern plate tectonics may have begun around 3 billion years ago. Identification and characterization of pre-3 Ga 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.

Paleoarchean tonalitic and granitic orthogneisses of the Sacawee block crop out across a 70 km wide swath in the Granite Mountains in central Wyoming. Ten U-Pb SHRIMP ages on these gneisses define two age groups of gneisses. The oldest orthogneisses, preserved as banded tonalitic gneisses, range from 3.45 to 3.38 Ga. 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 Ga.

The older group of banded orthogneisses are calcic, magnesian, and peraluminous, and SiO₂ varies from 66-74%. They are HREE-depleted, like other Archean TTG suite rocks. The younger gneisses range from sodic to potassic, calcic to calc-alkalic, magnesian to ferroan, and are peraluminous, and SiO₂ varies from 70-77%. This group of gneisses have less HRE-depleted patterns akin to modern basalt-andesite-dacite-rhyolite suites. 22 samples define a very tight cluster of ε_Nd at 3.3 Ga of between -0.9 and -5; one additional sample has ε_Nd at 3.3 Ga of +1.8. The 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 3.8 Ga 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 tonalities 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.