AGE AND PETROGENESIS OF THE MATLOCK ‘KERATOPHYRE’ IN NORTHWEST IOWA

Mineral exploration cores drilled in 1963 into a magnetic anomaly by the New Jersey Zinc Company in northwest Iowa found the ~2.7 Ga Otter Creek layered mafic intrusion stratigraphically overlain by the ~1.78 Ga Matlock “Keratophyre” (MK). The MK was logged in two of the drilled Matlock cores: C-5 and C-6. The MK was described as a dark gray porphyritic rhyolite unit in the subsurface Precambrian basement of Iowa that grades downward from a crystal-poor tuff (core C-6 and top of core C-5) into a volcanic flow breccia that contains mafic clasts from the underlying 2.7 Ga Otter Creek Layered Igneous Complex (core C-5). This project involved gathering new petrographic and compositional data to investigate the nature and petrogenesis of this highly altered, mixed rhyolite-basalt unit. A PXRF was used to survey compositional variations in the cores to aid sample selection for whole rock geochemistry and U-Pb zircon geochronology (high-Zr core sections). Zircon was recovered from two Matlock C-5 core sections, but the sample of crystal-poor tuff (C-6 core section) was barren. Zircons analyzed by LA-ICP-MS at the Arizona LaserChron Center yielded concordia ages of 1803 ± 4 Ma (sample C5488-9) and 1810 ± 9 Ma (sample C51067), suggesting the MK is older than the previous estimate (~1.78 Ga). The ICP-MS data on 24 whole rock samples shows that the MK is not a keratophyre unit but instead a high potassium rhyolite that has been highly altered and variably leached in alkali elements, and that there are three distinct rhyolite units within the MK unit based on incompatible and immobile element ratios, such as Zr/Nb and La/Sm. The PXRF and whole rock geochemical data highlight an unusual amount of mafic clasts. These whole rock geochemical data and new in-situ zircon U-Pb ages provide additional context about the southern margin of the Superior Craton.