TESTING A MODEL OF 2.8 GA ARC MAGMATISM WITH TRACE ELEMENTS

The trace- and major-element geochemistry of Archean rocks from the interior of the southeastern Beartooth Mountains indicates that these rocks were formed in a volcanic-arc tectonic setting. Thirty two of the same rock samples used in the study by Wooden and Mueller et al. (1982) were analyzed by an inductively coupled plasma emission mass spectrometer (ICP-MS) for trace element concentrations. Wooden and Mueller et al. (1982) previously analyzed these samples in the late 1970’s with an X-ray fluorescence spectrometer (XRF) for major- and trace-element concentrations. The trace element data acquired by the ICP-MS was used to provide additional support to the convergent plate boundary hypothesis, proposed in the paper by Wooden and Mueller et al. (1982), for the paleo-tectonic setting in the southeastern Beartooth Mountains.

Originally, Wooden and Mueller et al. (1982) proposed three prominent rock types within the field area on the basis of compositional characteristics. These rock types were informally named; the Long Lake granite, the Long Lake granodiorite and the dioritic amphibolite (Wooden and Mueller et al., 1982). An ICP-MS analysis confirmed these three rock types and identified an additional rock type informally called the Long Lake cumulate. The Long Lake cumulate samples display a large positive Eu anomaly, unique only to this unit, as well as a depletion in HREE and a relatively low abundance of LREE when compared to the other rock units.

The Th/U and U/Pb ratios defined by the samples of the Beartooth Mountain field area further support a volcanic-arc paleo-tectonic setting. Collectively these samples display a very high Th/U ratio (an average concentration of about 13.06) and a low U/Pb ratio (an average concentration of about 0.07). According to Weaver and Tarney (1984) the concentration value of Th/U for the average continental crust is about 4.38. The higher concentration of immobile Th to a lower concentration of U implies that U had left the system and became mobile, probably due to a redox reaction. In this case, the redox reaction was likely initiated by the fluids involved in subduction of the slab. The lower U/Pb ratio further suggests that U was removed from the system.