Remagnetization and Clay diagenesis in Jurassic Sediments of Skye, Scotland

The thrust of this study is to test the hypothesized connection between magnetite authigenesis and diagenetic reactions forming illite, an idea based on the results of paleomagnetic, rock magnetic, geochemical, and petrographic/SEM studies on Jurassic sedimentary rocks of Skye, Scotland. The Jurassic rocks in southern Skye contain a dual polarity magnetization residing in magnetite that is interpreted as a chemical remanent magnetization (CRM) and has directions similar to those of early Tertiary igneous rocks on Skye. The presence-absence test and the timing of acquisition for this CRM suggest that magnetite authigenesis is related to the smectite-to-illite conversion, and that clay diagenesis is a viable remagnetization mechanism. The clay fractions consist of illite-smectite (I-S), kaolinite, and trace amounts of chlorite. The I-S exhibits Kalkberg order (IISI) and typically contains ∼ 85% to 90% illite layers based on the positions of the 001 reflection. The 2M₁, 1M and 1M_d illite polytypes are present in variable proportions depending on size fraction. The K-Ar dates of I-S in the finest fraction range from 100 to 131 Ma while the K-Ar dates of I-S of the coarse fractions range from 160-210 Ma. In all cases, the dates of the finest I-S are much greater than the Tertiary CRM ages recorded in these rocks (65 Ma). The K-Ar dates decrease with decrease in particle size. At first approximation, the decrease is thought to reflect smaller amounts of detrital illite in the finer fractions. The youngest K-Ar dates of I-S are significantly greater than the CRM age recorded in these sediments. The most likely explanation of the discordance between K-Ar dates and CRM is that the finest fractions contain small amounts of detrital illite.