Geochemistry and paleomagnetism of Keweenawan basalt in the subsurface of Nebraska

• Published in: Precambrian research 1996, Vol.76 (1), p.47-65
• Main Authors: Marshall, Leland P., Lidiak, Edward G.
• Format: Article
• Language: English
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/0301-9268(95)00027-5

Basalts of Keweenawan age (Mesoproterozoic) extend from the Lake Superior region southwestward into the subsurface of Nebraska and Kansas. One of the best examples of these subsurface basalts occurs in the Williams #1 Radenslaben well in Saunders County, Nebraska. The well is located on the flank of a linear, magnetic high within the confines of the Midcontinent Rift System. A core from the well preserves 186 continuous feet (56.7 m) of Keweenawan basalt. Five lava flows are present in the core. The flows have a coarse ophitic texture and consist of labradorite + augite + FeTi oxides + olivine pseudomorphs (chlorite). The primary mineralogy is well preserved in the top three flows (A through C), which represent almost 75% of the total core interval. Sporadic small patches of low-grade secondary minerals are concentrated in the upper portions (flow tops) of flows A through C, but decrease to minimal amounts in the massive flow interiors. In contrast, the bottom two flows (D and E) and an alteration zone on the top of flow B show the effects of incipient burial metamorphism and contain chlorite, albitized plagioclase, prehnite ±, epidote ±, and K-feldspar ± (megacrysts). A paleomagnetic reversal is recorded in the middle of flow C. The rock below the reversal has reversed polarity, and the rock above normal polarity. There is no evidence that any part of the core has been faulted. The original stratigraphic sequence is preserved. The age of the reverse-normal polarity in these rocks is not constrained but is similar in sequence to the main Keweenawan reversal recorded at 1097 Ma. The geochemical data suggest, however, that the polarity reversal may be later and record an event that is not presently recognized in the exposed flows of the Lake Superior region. The overall tholeiitic character of these basalts is demonstrated by their primary mineralogy, CIPW-normative chemistry, and immobile trace element concentrations. Trace element abundances in flows A, B and C are primary and have not been modified by the burial metamorphism. When normalized to chondrites, the trace elements form in a tight, coherent array. However, flows D and E clearly show the chemical effects of the burial metamorphism, and some of the trace elements, for example Ba, Rb, Sr, Na and to a lesser extent K, have been mobilized. The Radenslaben basalts have geochemical similarities to E-MORB, but with an additional LREE and LILE enrichment component. They are similar in trace element and