Age of the Wyatt Formation

The Wyatt Formation was initially assigned a Neoproterozoic date of 633 13 Ma (k = 1.39 x 10 year 0 based on the Rb-Sr systematics of four samples from Metavolcanic Mountain (Faure et al. 1968a Montigny and Faure 1969). This date was later restated as 630 14 Ma by Faure et al. (1968b). After analyzing additional samples, Faure et al. (1977) reported a date of 543 48 Ma (Early Cambrian) when the decay constant of Rb is taken to be A. = 1.42 x 10 year h The analytical data of seven samples of the Wyatt Formation from Metavolcanic Mountain, the Cleveland Mesa, and the Wisconsin Plateau in Appendix 1.9.2.2 constrain a straight line in Fig. 7.12 which defines the time of crystallization of these rocks  

The date indicates that these rocks were erupted during the Middle Cambrian Epoch. This revision of the geologic history of the Transantarctic Mountains indicates that the volcanic rocks of the Wyatt Formation were erupted at the surface of the Earth or intruded at shallow depth during the active phase of the Ross Orogeny and that the rocks probably cooled rapidly compared to the deep-seated granitic plutons of the Wisconsin 

Discovery Ridge, Ohio Range Biotite 472 24 Treves(1965) 

Lonely Ridge, Nilsen Plateau Biotite 472 10 McLelland (1967) 

O Brien Peak, Q. Maud Mtns. Muscovite 460 20 Craddock et al. (1964) 

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Faure et al. (1979) analyzed five samples of the Wyatt Formation collected by Minshew between Mt. Wyatt and Mt. Gardiner in the upper reaches of the Scott Glacier. The results in Appendix 6.7.3.3 were originally used only to state an upper limit of 788 Ma (Neoproterozoic) for the age of the Wyatt Formation. A least-squares linear regression of these data (plus one duplicate) defines line A in Fig. 6.15 which results in a calculated date of 524 13 Ma (Early Cambrian) with an initial Sr/ Sr ratio of 0.711919 0.000547 for X = 1.42 X 10" year Three of the samples in Fig. 6.15 precisely define line B which corresponds to... 

Nevertheless, the slope of a straight line defined by their data yielded a date of 545 59 Ma with and initial "Sr/ Sr ratio of 0.7114 0.0030. Stump et al. (1986) concluded that the date had been reset and therefore is an underestimate of the crystallization age of the Wyatt Formation. Even the U-Pb method of dating zircon failed to provide a reliable date of the Wyatt Formation because the data points derived from the zircon fractions analyzed by Van Schmus et al. (1997) did not define a discordia curve. Even so, Van Schmus et al. (1997) inferred Early Cambrian dates between 520 and 540 Ma for this formation. The most precise age determination for the Wyatt Formation is a U-Pb date of 526 2 Ma reported by Encamacidn and Grunow (1996). Therefore, the age of the Wyatt Formation appears to be Early Cambrian. 

The data listed below are previously unpublished because they scatter on the isochron diagram and were judged to be unsuitable for dating the Wyatt Formation by the whole-rock Rb-Sr method. Faure et al. (1979) used this data set only to set an upper limit of 788 Ma for the age of the Wyatt Formation. 

Fig. 6.9 Rb-Sr age determination of metavolcanic rocks of the Wyatt Formation in the valley of the Blackwall Glacier, Nilsen Plateau (Fig. 6.7). The date and initial Sr/ Sr ratio were calculated by Eastin (1970) by the methods of York (1966,1969) and have been adjusted to = 1.42 x 10 year Samples 461 and 463 were excluded because they deviate significantly from the line fitted to the remaining data points. Plotted from data by Eastin (1970)...

The igneous rocks of the Wyatt Formation occur at many locations within the Queen Maud Mountains, as well as in the Horlick and Thiel Mountains. In spite of these extensive exposures, the origin of the Wyatt Formation is still uncertain because in some places it clearly intruded folded metasedimentary rocks of the LaGorce Formation, whereas in other places it over-lies them conformably. In addition, hydrothermal alteration of the rocks and the effects of regional metamorphism described by Minshew (1967) have disturbed the isotopic composition of strontium of the rocks, which has caused data points to scatter on Rb-Sr isochron diagrams and caused large uncertainties in age determinations of the Wyatt Formation listed in Table 6.5. 

Fig. 7.12 Whole-rock Rb-Sr age determinations of the Wyatt Formation discussed in the text (Data from Montigny and Faure unpublished in Appendix 7.8.2.3)...

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