Search for: All records

Superlatives—whether tallest, longest, or fastest—are more interesting than averages. This characteristic applies to many aspects of the geosciences, where scales of time and space are beyond human experience. The deepest trench, the highest mountain, and the most expansive desert are much more interesting than average ones. Interest in superlatives also applies to the oldest rocks. In this essay, we show that the oldest rocks in the United States are 3.62–3.45 billion years old (Ga) and are found in three different states. These localities define an east-west−trending belt in the upper midcontinent that stretches ~3000 km from Wyoming through Minnesota and into the Upper Peninsula of Michigan. Complex U-Pb zircon systematics are observed in the oldest rocks from all three areas, complicating efforts to distinguish zircons that crystallized in the magma(s) that made the host rock from xenocrystic zircons incorporated by assimilating older rocks. Within these uncertainties, the oldest rock in the United States is 3.62 Ga (Eoarchean to Paleoarchean), but older, 3.8 Ga zirconbearing felsic crust existed and may be identified by future investigations. 

The record of the first two billion years of Earth history (the Archean) is notoriously incomplete, yet crust of this age is present on every continent. Here we examine the Archean record of the Wyoming craton in the northern Rocky Mountains, U.S.A., which is both well-exposed and readily accessible. We identify three stages of Archean continental crust formation that are also recorded in other cratons. The youngest stage is characterized by a variety of Neoarchean rock assemblages that are indistinguishable from those produced by modern plate tectonic processes. The middle stage is typified by the trondhjemite-tonalite-granodiorite (TTG) association, which involved partial melting of older, mafic crust. This older mafic crust is not preserved but can be inferred from information in igneous and detrital zircon grains and isotopic compositions of younger rocks in Wyoming and other cratons. This sequence of crust formation characterizes all cratons, but the times of transition from one stage to the next vary from craton to craton.