Studies of the Precambrian Geology of Iowa: Part 3. Geochronologic data for the Matlock drill holes

U-Pb ages have been determined for quartz monzodiorite (2,535 ± 5Ma) recovered from drill hole ClO and for keratophyre (1,782±10 Ma) recovered from drill hole C5 of the Matlock exploration project. Sm-Nd whole-rock analyses for several rock types from the Otter Creek layered mafic complex recovered from several drill holes of the Matlock project yield an isochron age of 2,890 ± 90 Ma, with an ϵNd(t) of -0.9 ± 2.4. These ages document crustal evolution in northwest Iowa, including the southern margin of the Superior Province, over more than one billion years from 2.9 to 1.8 Ga. Isotopic data also show that older crustal components were involved in genesis of the keratophyre, indicating that it was derived wholly or in part from older crust. Data are not sufficient to determine if the keratophyre was derived wholly from mafic crust like the Otter Creek complex or from a mixture of more felsic Archean crust and juvenile 1.8 Ga magmas.