Direct dating of carbonate remagnetization by 40Ar/39Ar analysis of the smectite–illite transformation

The occurrence of remagnetizations in carbonate rocks is a longstanding issue, the result of a cryptic process that affects rocks in both fold-thrust belts and undisturbed continental interiors. This paper examines the clay mineralogy of insoluble material from remagnetized carbonates taken from the Cantabrian–Asturian belt, northern Spain. Detailed X-ray diffraction patterns indicate that mixed-layer clays from these rocks are mostly illitic, with an increasing proportion of authigenic illite polytype 1Md in finer grain size fractions. The 40Ar/39Ar age data from the different grain size fractions, coupled with quantitative polytype modeling, indicates an authigenic age that is coeval with the established late Paleozoic remagnetization age of these rocks. We propose that the transformation of Fe-rich smectite to Fe-poor illite created a population of authigenic magnetite that is responsible for the remagnetization of these rocks. The irreversible, kinetic dependence of clay mineral transformations appears to account for the irreproducibility and unpredictability of carbonate remagnetization, and may explain such vexing phenomena as synfolding magnetizations. This technique provides an independent assessment of the presence and timing of rock remagnetization with numerous potential applications in future studies of carbonate sequences.