IOWA'S OLDEST ROCKS

by
K.E. Windom, W.R. VanSchmus, K.E. Seifert, and R.R. Anderson

Iowa Academy of Science
103rd Session
April 19-20, 1991, Loras College, Dubuque, IA
1991 Program Abstracts, p. 20

ABSTRACT


Three spatially-related igneous bodies occur in extreme northwestern Iowa including a layered mafic/ultramafic complex, a deformed porphyritic quartz monzodiorite, and a volcanic tuff/breccia (Tvrdik, 1983). The volcanic tuff/breccia is keratophyrie in composition and overlies the layered complex, lying on an erosional surface; the contact relationships between the quartz monzodiorite and the other rock bodies are unknown. The quartz monzodiorite was previously determined to be 2523 Ma (Van Schmus et al., 1987) and Wallin and Van Schmus (1988) determined an age of 1775 Ma for the keratophyre. We have determined Nd-Sm isotopic relations in the layered complex and the keratophyre. The layered complex yields an age of 2.9 Ga with Nd, i.e., the magma was derived from an essentially undifferentiated mantle. The keratophyre has a model mantle age of 2.9 Ga, indicating that it was ultimately derived from the same mantle as the layered complex. No evidence was found of 3.6 Ga mantle, corresponding to rocks exposed farther north in the Minnesota River Valley. Thus, these data indicate a emplacement of 2.9 Ga mantle along the southern margin of the 3.6 Ga Superior Province.


REFERENCES:

Tvrdik, T.N., 1983, Petrology of the Precambrian basement rocks of the Matlock Project cores, northwestern Iowa: Iowa City, The University of Iowa, unpublished M.S. thesis, 113 p.

Van Schmus, W.R., Bickford, M.E., and Zietz, I., 1987, Early and middle Proterozoic provinces in the central United States, in A. Kroner, ed., Proterozoic Lithosphere Evolution: Geodynamics Series Volume 17, American Geophysical Union, Washington, D.C., and Geological Society of America, Boulder, Colorado, p. 43-68.

Wallin, E.T., and Van Schmus,W.R.,1988, Geochronological studies of the Archaen-Proterozoic transition, north-central United States, Geological Society of America, Abstracts with Programs, v. 20, p.131.