EARLY CRUSTAL EVOLUTION OF NORTH AMERICA IN THE NORTHERN WYOMING PROVINCE

The eastern part of the Beartooth Mountain Range in Montana and Wyoming contains Early to Middle Archean rocks of the TTG association, which is the most common rock association in the Archean worldwide. Rock ages are from 3.1-3.5 Ga and detrital zircon ages are as old as 4.0 Ga. These rocks are enveloped by Late Archean (2.8 Ga) igneous intrusions. A few recent papers have suggested that the Earth underwent a major change in mantle melting and plate tectonics at ~3.2-3.3 Ga. The samples from the Beartooth area are within this age-range, and so provide a look at crust and mantle interaction during this proposed time of transition. Major element data suggest two groups: mafic and felsic. In mafic samples (<56% SiO2) there is a generally positive relationship between TiO2 and FeO/MgO. In felsic samples (>56% SiO2) there is a negative relationship between SiO2 and FeO, but a less well-defined correlation between SiO2 and MgO and TiO2. In intermediate-felsic samples (>52% SiO2) there is a positive relationship between Na2O/K2O and Ca2O. In the felsic samples there is a positive relationship between SiO2 and K2O, which suggests that K has not been significantly affected by metamorphism. Rare earth element (REE) patterns suggest at least two major groupings based on Yb values, those with high Yb values (>10x Chondrite) and those with low Yb values (<10x Chondrite). Both the mafic and felsic samples contain both high and low Yb value samples. Those samples with high Yb values have a relatively smooth negatively sloping heavy pattern, whereas samples with low Yb values show less of a smooth heavy Rare Earth Element pattern. Patterns showing depleted heavy REE are typical of Archean TTGs and are commonly attributed to melting of garnet amphibolites or eclogite. In addition, the high Yb samples show a negative Eu anomaly, and the low Yb value samples show both positive and negative Eu anomalies. These Eu anomalies are significant because positive ones mean accumulation of plagioclase, negative ones mean plagioclase fractionally crystallized or was left in the source when the magma finally separated from it. The difference in REE patterns between the high and low Yb samples likely means that these samples have at least two distinct sources. Whole rock analyses of Nd and Pb isotopes and U/Pb zircon chronology further constrain the origin of these rocks.